User`s Manual Digital Gamma Finder (DGF) Model Polaris
Contents
1. in this interval then there is no trailing edge pileup PEAKSEP is usually set to a value close to L G 1 Pulse 1 passes this test as shown in Figure 6 6 Pulse 2 however fails the PEAKSEP test because pulse 3 follows less than 1 0 us Notice by the symmetry of the trapezoidal filter if pulse 2 is reyected because of pulse 3 then pulse 3 is similarly rejected because of pulse 2 6 5 Filter decimation To accommodate the wide range of filter rise times from 0 1 us to 44 us the filters are implemented in the RTPUs in FPGA configurations with different clock decimation filter ranges The ADC sampling rate is always 25ns but in higher clock decimations several ADC samples are averaged before entering the filtering logic In decimation 1 2 samples are averaged 2 samples in decimation 2 and so on Since the sum of rise time and flat top is limited to 31 decimated clock cycles filter time granularity and filter time limits are as follows Decimation Filter granularity max Tris T nat min T rise Min Tiat 1 0 05us 1 55us 0 15us 0 15us 2 0 1us 3 1us 0 5us 0 3us 3 0 2us 6 2us 0 4us 0 6us 4 0 4us 12 4us 0 Sus 1 2us 5 0 Sus 24 8us 1 6us 2 4us 6 1 6us 49 6us 3 2us 4 8us Table 6 RTPU clock decimations and filter time granularity As the decimations are implemented in different FPGA configurations different files have to be downloaded to the FPGA to change decimation 6 6 Count Rates a2. DSP variables might change and shift it is important to always refer to the ASCII file to match a variable with its value The DSP variable values are downloaded directly to the Polaris DSP and converted into user values in the Polaris viewer 3 6 User customization The Polaris Viewer provides all necessary functions to set up and run the Polaris and a set of basic analysis tools However the user might be interested in using the numerous tools and functions available in IGOR to perform custom curve fits calculate results or even perform macros or scripts for routine tasks 3 6 1 Igor menus and command line Without giving a full introduction into IGOR which can be found in the WaveMetrics documentation we list a few useful tools and features below Help for IGOR is available through IGOR s help browser located under Help in the top menu bar Tools to modify graphs are available from the Graph menu in the top menu bar if the graph is the front window You can modify symbols trace appearance axes etc Most items axes traces labels etc in a graph can also be modified by double clicking on the item Useful keyboard shortcuts are Ctrl I to show or hide cursors on a graph and Ctrl A to rescale a graph to the full size The full range of IGOR s analysis tools are available in the Analysis menu in the top menu bar This includes curve fits wave statistics and various smoothing functions Curve fits can be customized
3. Sustained count rate into spectrum up to 750 000 cps with scintillator Excellent pile up inspection double pulse resolution of 100ns Automatic optimization of instrument settings to match detector characteristics Digital oscilloscope and FFT for health of system analysis Digital gain stabilization Triggered waveform acquisition for advanced R amp D 14 bit 40 MSPS 100 us Contact XIA for 14 bit 65 MSPS and even 80 MSPS option e Compton suppressor input accepts photomultiplier tube input Includes preamplifier power and high voltage supply 1 2 Specifications Inputs Analog Signal Input Gate Input Inputs Digital Gate Input Sync Input HV Inhibit Interface USB EPP OEM Digital Controls Gain Shaping Data Reported Spectrum Other Selectable input impedance 500 900 2500 and 10kQ 10V pulsed 3V DC Selectable input attenuation 1 21 1 12 1 5 and 1 1 Dual purpose see below Input for photomultiplier tube signal from Compton shield Impedance 50Q 10V pulsed 2V DC Dual purpose see above TTL logic input for specialty applications TTL logic input to control time resolved data collection including scanning and phase locked loop applications TTL logic input Selectable logic HI or LO for HV shut down Serial interface Enhanced Parallel Port IEEE 1284 Auxiliary 25 pin programmable bidirectional I O connector for specialty applications 80 1 gain range
4. USB interface is implemented in a separate microcontroller chip It is configured by a separate on board PROM The USB microcontroller also reads temperature data from an on board thermometer which the DSP can use to detect and compensate gain drifts 22 6 Theory of Operation 6 1 Digital Filters for y ray detectors Energy dispersive detectors which include such solid state detectors as Si Li HPGe Hglz CdTe and CZT detectors are generally operated with charge sensitive preamplifiers as shown in Figure 6 1a Here the detector D is biased by voltage source V and connected to the input of preamplifier A which has feedback capacitor Cf and feedback resistor Ry The output of the preamplifier following the absorption of an y ray of energy E in detector D is shown in Figure 6 1b as a step of amplitude V on a longer time scale the step will decay exponentially back to the baseline see section 6 3 When the y ray is absorbed in the detector material it releases an electric charge Q Ex g where e is a material constant Qx is integrated onto Cs to produce the voltage Vx Q C ExM eC Measuring the energy E of the y ray therefore requires a measurement of the voltage step V in the presence of the amplifier noise o as indicated in Figure 6 1b 4 gt g 2 E a 5 0 O E g S 2 Aa 4 0 00 0 02 0 04 0 06 a b Time ms Figure 6 1 a Charge sensitive preamplifier with RC feedback b Output on absorption of an
5. filtering to perform essentially the same action as a shaping amplifier The important difference is in the type of filter used In a digital application is easy to implement finite impulse response filters and we use a trapezoidal filter The flat top will typically cover the rise time of the incoming signal and makes the pulse height measurement less sensitive to variations of the signal shape Secondly the RTPU contains a pileup inspector This logic ensures that if a second pulse is detected too soon after the first so that it would corrupt the first pulse height measurement both pulse are rejected as piled up The pileup inspector is however not very effective in detecting pulse pileup on the rising edge of the first pulse i e in general pulses must be separated by their rise time to be effectively recognized as different 20 pulses Therefore for high count rate applications the pulse rise times should be as short as possible to minimize the occurrence of pileup peaks in the resulting spectra If a pulse was detected and passed the pileup inspector a trigger may be issued That trigger would notify the DSP that there are raw data available now Ifa trigger was issued the data remain latched until the RTPU has been serviced by the DSP The third component of the RTPU is a FIFO memory which is controlled by the pile up inspector logic The FIFO memory is continuously being filled with waveform data from the ADC On a trigger it is
6. is off the optimum peaks in the spectrum will broaden and if t is very wrong the spectrum will be significantly blurred 10 The first and usually sufficiently precise estimate of t can be obtained from the Find routine in the System Configuration tab see item 3 in section 3 1 Measure the decay time several times and settle on the average value Fine tuning of t can be achieved by exploring small variations around the fit value 2 3 This is best done at high count rates as the effect on the resolution is more pronounced The value of t found through this way is also valid for low count rates Manually enter t in the System Configuration tab take a short run and note the value of t that gives the best resolution 3 3 5 Dynamic range In most cases the dynamic range should be set not much larger than the region of interest for example to 1 5 2 0 MeV for the 1 332 MeV y rays of Co This is not a very critical setting though since with the 64k channels in the Polaris MCA there is still sufficient detail at lower energies even if the dynamic range is set higher than necessary For very high count rates however the situation is somewhat different The architecture of the Polaris is such that the full range of the preamplifier output is mapped to the input range of the ADC not simply the step height of a single pulse As a result at high count rates when pulses sit on the falling slope of one or even several previous pu
7. light output in order to achieve high count rates and at the same time simplify the system The current output from the PMT traces the scintillation light intensity and can be fed directly to the Polaris inputs If the PMT is operated at negative high voltage its anode is at ground potential and we can pick off the current directly If the PMT is powered by positive high voltage its anode is at high potential and the current has to be picked off through a coupling capacitor In order to avoid the introduction of unwanted time constants it is advisable to couple the anode current capacitively into a current to voltage converting preamplifier Some manufacturers sell PMTs that are powered with positive high voltage with a base that includes an integrating preamplifier This preamp can be converted into a current to voltage converter by removing its integrating capacitor It may also be necessary to improve the local RC filtering of high voltage inside the PMT base With these modifications the preamplifier output will trace the scintillation light in time and its integral will be proportional to the energy deposited in the scintillator 12 3 5 Polaris data structure 3 5 1 IGOR data In the Polaris viewer a number of output variables contain data that might be useful for calculations and or custom displays They are listed in Table 1 IGOR variable or wave name Description root polaris LiveTime Polaris live t
8. the sums in real time and in the complexity of the electronics required to generate usually from stored coefficients normalized w sets on a pulse by pulse basis The Polaris takes a different approach because it was optimized for very high speed operation It implements a fixed length filter with all w values equal to unity and in fact computes this sum afresh for each new signal value k Thus the equation implemented is k L G k LV NY NY 6 2 i k 2L G 1 i k L 1 where the filter length is L and the gap is G The factor multiplying Y arises because the sum of the weights here is not normalized Accommodating this factor is trivial While this relationship is very simple it is still very effective In the first place this is the digital equivalent of triangular or trapezoidal if G 0 filtering which is the analog industry s standard for high rate processing In the second place one can show theoretically that if the noise in the signal is white i e Gaussian distributed above and below the step which is typically the case for the short shaping times used for high signal rate processing then the average in Eqn 6 2 actually gives the best estimate of V in the least squares sense This of course is why triangular filtering has been preferred at high rates Triangular filtering with time variant filter lengths can in principle achieve both somewhat superior resolution and higher throughputs but comes at the co
9. use EPP connect the Polaris EPP port to the host computer s parallel port printer port The connection should be made with an IEEE 1284 compliant cable On the host computer the BIOS setting for the parallel port has to be EPP usually the case on modern computers The EPP address will typically be 0x378 and sometimes 0x278 To use USB connect the Polaris USB port to the host computer s USB connector Make sure the EPP port is disconnected Whenever you plug in the USB cable or switch on the Polaris your computer will take a few seconds to recognize the new USB device Avoid attempts to communicate with the Polaris during that time it might cause Windows to lock up When you connect the Polaris for the first time Windows will recognize a new device and want to install a driver for an EZ USB controller The USB driver is located in the drivers directory of the software distribution 2 2 2 Detector Signal Input The detector signal from the preamplifier connects to the BNC connector labeled INPUT on the Polaris back panel The termination of the signal line can be set to 50Q 90Q or 250Q as well as 10kQ using jumpers on the circuit board see section 7 1 2 in the appendix The signal input must fall in the range of 3V unless jumpers are set for signal attenuation 2 2 3 Preamplifier power and HV bias Preamplifier power 12V and 24V is provided at the DB9 connector labeled PREAMP POWER High voltage bias for th
10. y rays arriving separated by various intervals Because the triangular filter is a linear filter its output for a series of pulses is the linear sum of its outputs for the individual members in the series The fast filter has a rise time of 0 1 us with a gap of 0 1 us The slow filter has a rise time of 1 2 us with a gap of 0 35 us Pileup occurs when the rising edge of one pulse lies under the peak specifically the sampling point of its neighbor Thus in Figure 6 6 peaks 1 and 2 are sufficiently well separated so that the leading edge of peak 2 falls after the peak of pulse 1 Because the trapezoidal filter function is symmetrical this also means that pulse 1 s trailing edge also does not fall under the peak of pulse 2 For this to be true the two pulses must be separated by at least an interval of L G Peaks 2 and 3 which are separated by less than 1 0 us are thus seen to pileup in the present example with a 1 2 us rise time This leads to an important point whether pulses suffer slow pileup depends critically on the rise time of the filter being used The amount of pileup which occurs at a given average signal rate will increase with longer rise times Because the fast filter rise time is only 0 1 us these y ray pulses do not pileup in the fast filter channel The Polaris can therefore test for slow channel pileup by measuring the 30 fast filter for the interval PEAKSEP after a pulse arrival time If no second pulse occurs
11. 2000 Run Active 0x4000 LAM Request cleared by reading the word count register 0x8000 Polaris Live Table 8 Map of the Control and status register CSR 35 7 3 Troubleshooting 7 3 1 7 3 2 7 3 6 IGOR reports Function compilation error at startup Check if the driver file Polaris xop is located in the Igor Extension folder by default in C Program Files Wavemetrics IgorPro Folder If not copy the file from the drivers directory in the Polaris software distribution IGOR reports missing DLL file Make sure DLPortI O is installed on your computer It can be installed by running the program port95nt exe located in the drivers subdirectory USB communication does not work Check if USB driver is installed Verify that EPP port is not connected Power cycle Polaris give system enough time to initialize new USB device Igor reports missing files at system startup Check if the file names point to existing folders in your directory structure especially if you installed the Polaris software in a custom directory Check the filename of the settings file make sure the file exists You can create a new file by clicking on the Create button in the System Configuration tab Igor reports FPGA download unsuccessful at system startup Verify EPP cable or USB cable is connected properly but not both Verify Polaris is plugged in and switched on If using EPP verify that EPP or EPP ECP was selec
12. 4 3 3 4 Parameter tiles E E EE OE 16 3 6 User CUSCOMIZATION AAA 16 3 6 1 Igor menus and command Line eee eceesceesceseeeeccesecesecaecanecaeecaeeeaeeeeeeeeeeeeesreeeeeeeneenaees 16 30 2 Teorprocedureswis 0 cies atest O heen a a a wae RGa es eine ae 17 3 63 DSP CuStomiZattOm ica cetonas 17 Programmers DUI O ios ri A ada rd 18 Hardware CESCHIPION dci 20 5 1 Analog signal conditioning sesssesssesssesssesssesssessseosseossessscossocssosssseossecssesssesssosssosso 20 5 2 Real time processing unit eessesssessseosseossessseossoessoossocssossoossoosssossoosssosssosssosssosssssssss 20 5 3 Digital signal processor DSP sessseosseossessseosseossocssosssoosoossoossoossoosssosssosssosssosssssssos 21 5 4 Spectrum MEM ONY sisscssssciccccscscnscsocssovedseennscecasosstescousadeeasesceasesnesdbeaseccvaccsenesseasessencesens 22 5 5 Host UNtOY PAO cc ccsceccsscaieccoscersencscesiecssrenceessdescossdestsesastecdcoossseuseassececsesdessenssdeatecsecereues 22 6 Theory Of OD CVA a di ciaci n 23 6 1 Digital Filters for y ray detectors sscccscsscsrcccsccsccscssccesssscessecsesccsssccessssssssessese 23 6 2 Trapezoidal Filtering in the Polaris sscccsssccssscsscecssscecssscsccssesccesssseessessesees 25 6 3 Baselines and preamplifier decay times cscccccscsscsecsssecsscsescessescersscseessceeese 26 6 4 Thresholds and Pile up Inspection sccscsccssscscccsscsecssscecsccsescccss
13. FPGA download unsuccessful at system startup ooooooonccnocnnocnconcoonconoconocononnnoos 36 LIG Tgor can not open Mlese E beds EENE E eee shea tees 36 7 3 7 No traces or only flat lines in Oscilloscope cescceseeseceseceseececeeeeneeeseeeeceeeeseeeeseeeeeneeaees 37 7 3 8 Very high input count rate during run oo ec eceeceeeceeeceseceecseecaeecaeeeeeeeeceeeeseesseeeeeeseenaees 37 139 Nerylow livetime during Tunsori dee ieee ceeteee eset ee ence Senta 37 7 3 10 Large peak at low end of spectrum 0 eecceecceseceseceseceecseeeseeeaeeaeecaeeeeeeneeseseeeeeseeeseeneeenaees 37 7 3 11 Spectrum has very wide and blurred peaks oooonooonnccnnconocnononononnnonnnon nono nonnononcnnn no nono nono nono noo 37 7 3 12 Igor reports need to have at least as many data points as fit parameters ce eee 37 11 1 Overview The Digital Gamma Finder DGF family of digital pulse processors features unique capabilities for measuring both the amplitude and shape of pulses in nuclear spectroscopy applications The DGF architecture was originally developed for use with arrays of multi segmented HPGe gamma ray detectors but has since been applied to an ever broadening range of applications The DGF Polaris formerly the Gamma200 is a high precision ultra fast all digital spectrometer comprising a single DGF processing channel a preamplifier power supply and a detector bias supply up to 5 000V in a compact package
14. IVETIME RUNTIME OCR ICRm 6 6 In the Polaris Viewer FTDT is used to compute a fractional Pileup Content This parameter is defined as FTDT RUNTIME The Pileup Content is thus an estimate of the pulses not counted in the FASTPEAKS due to pile up in the trigger Note that in the Polaris Viewer runs are usually set to last for a given real time which is different from RUNTIME Real time is the laboratory time Windows time used to poll the Polaris RUNTIME is the actual time the Polaris is ready to take data not counting time for setting up the run and reading out the data The difference is usually small 32 7 Appendix This section contains hardware related information 7 1 Jumpers 7 1 1 Input JP1 JP9 and JP10 In the typical configuration power for the detector preamplifier is supplied by the Polaris via the NIM standard DB 9 Preamplifier Power interface The output signal and its reference return via BNC coaxial cable to the Polaris input connector Some manufacturers instead route the signal back through the preamplifier power cable in order to save space The Polaris accommodates either configuration using the INPUT jumpers JP9 and JP10 If set to DSUB9 they connect pins 8 and 3 of the DB9 connector to the BNC shield and inner conductor respectively If set to BNC only the BNC connector is available for input signals The default is BNC JP1 connects the shield of the preamplifier po
15. The Polaris provides unparalleled spectral accuracy with up to 64K channels spectrum length and can on the other hand sustain count rates of up to 750 000 counts per second into the spectrum Connection to the host computer is by USB or EPP Extended Parallel Port IEEE 1284 or an auxiliary 25 pin programmable bidirectional I O connector for specialty applications The Polaris can accept signals from virtually any radiation detector Signals with decay times as fast as 230ns from Nal Tl for instance to as slow as 10ms can be processed without the need for external electronics The Polaris has built in support for HPGe detectors with a Compton shield the photomultiplier signal from the shield can be fed directly into the Gate input of the Polaris No external electronics is necessary For specialty applications the Polaris can perform pulse shape analysis for instance for neutron gamma discrimination and can also report data as a list of entries containing energy time of arrival and even waveforms 1 1 Features e Designed for high precision y ray spectroscopy with HPGe detectors e Directly compatible with scintillator PMT combinations Nal CsI BGO and many others e Input signal decay time as fast as 230 ns and up to 10ms exponentially decaying e Wide range of filter rise times from 50 ns to 45 us equivalent to 22 ns to 20 us shaping times e Selectable spectrum length from 1K to 64K channels 4 3x10 counts per channel
16. User s Manual Digital Gamma Finder DGF Model Polaris Version 3 0E June 2004 X Ray Instrumentation Associates 8450 Central Ave Newark CA 94560 USA Phone 510 494 9020 Fax 510 494 9040 http www xia com Disclaimer Information furnished by XIA is believed to be accurate and reliable However XIA assumes no responsibility for its use or for any infringement of patents or other rights of third parties which may result from its use No license is granted by implication or otherwise under the patent rights of XIA XIA reserves the right to change the POLARIS product its documentation and the supporting software without prior notice 1 DIE A a ii 1 1 1 AA AS 1 1 2 SPECIICALIONS os lt sccceccsvesvasdsccssecaste scones suasosebeesenses cose sssseoncagas cessoscvessessescensesstoscopascseanccesess 2 DENTAL UD iio 3 2 1 Scope OF document cecccooiccconocinnniccinnenivoncnanacc anedasoninoniacacc s ddoccoocincieces cadein cio coci ec ssori 3 2 2 Hardware installation csccccsccssscccscesccesssseressscccccccesceesssceesssssescceseseesssssesescssesees 3 2 2 1 Host Computer TO is 3 222 Detector Signal Input a ha ee RAS 3 2 2 3 Preamplifier power and HV Dias cccccecsseessesseeeeceeeceeceseceaeceeceseceseceaecseecseecaeeeaeeeseeneeeeenes 4 PAE ENC NS occas E RE ES A E E AEE R ER E 4 VIA A AN elias hts Blo E E bea ee 4 2 3 Software Installation ccccscsscssesssccccccessccsssssessecc
17. User_Name is the name of the user variable which is being communicated between the Polaris Viewer and the Polaris library Direction is an Igor numerical variable which determines whether this is a Write Direction 0 or Read Direction 1 operation Acquire _Data Run_Type User_Data File_Name Where Run_ Type is a bit mask bit 0 is Get_Traces 18 bit 1 is MCA run bit 2 is List Mode run bit 4 is Start new run bit 5 is Start resume run bit 6 is Stop a run bit 7 is Poll the run status All other bits should be set to 0 So for example if user wants to start a new MCA run the Run_Type should be 0x12 User_Data is an Igor unsigned 32 bit integer wave to transfer data between the Polaris Viewer and the Polaris library The size of this wave is defined in Polaris Viewer File Name is the name of the file which stores either the MCA spectrum or the list mode run data So it is only needed when calling Acquire _Data to stop a run In other cases File Name can simply be an empty string Set_Current_Module ModNum Where ModNum is an Igor variable specifying the current Polaris module number since Polaris library supports multiple modules For a single Polaris operation ModNum should be set to 1 19 5 Hardware description The Polaris is a single channel unit designed for Gamma ray spectroscopy and waveform capturing It incorporates five functionally different building blocks which we describe below This sectio
18. ary form unsigned 16 bit integer One or more readouts of the Polaris output buffer are saved in a single file The buffer length is 8K words each word is a 16 bit unsigned integer A parameter file with the same name but extension itx is saved together with the data file The output data can be written in a number of formats though currently only one format is actually used The Polaris Viewer has built in functions to parse the files and display event data and waveforms If user code is used to read the files it should access the three variables BUFHEADLEN EVENTHEADLEN and CHANHEADLEN in the parameter file of a particular run to navigate through the data set 14 The buffer content always starts with a buffer header of length BUFHEADLEN Currently BUFHEADLEN is six and the six words are Word Variable Description 0 BUF NDATA Number of words in this buffer 1 BUF MODNUM Module number 2 BUF FORMAT Format descriptor 3 BUF_TIMEHI Run start time high word 4 BUF TIMEMI Run start time middle word 5 BUF TIMELO Run start time low word Table 3 Buffer header data format Following the buffer header the events are stored in sequential order Each event starts out with an event header of length EVENTHEADLEN Currently EVENTHEADLEN 3 and the three words are Word Variable Description 0 EVT PATTERN Hit pattern 1 EVT_TIMEHI E
19. bes from a Compton rejection shield can be connected to the GATE input to veto events from the main detector No functions are currently implemented for the SYNC input However it can be customized by XIA through software for example to signal the Polaris that a sample is ready or to advance internal counters 2 3 Software Installation The Polaris Viewer XIA s graphical user interface to set up and run the Polaris is based on WaveMetrics IGOR Pro To run the Polaris Viewer you have to have IGOR Pro Version 4 0 or higher installed on your computer The software resides in a folder Polaris with 7 subfolders configuration dsp doc drivers firmware MCA and pulseshape The IGOR control program and the online help files are not in any of the subfolders but are placed one level up in Polaris Make sure you keep this folder organization intact as the IGOR program and future updates rely on this Feel free however to add folders and subfolders at your convenience To install the Polaris Software run the program Setup exe from the CD ROM and follow the dialog instructions The setup program will install all necessary drivers for the Polaris Only if you exit the setup program before the installation is complete will you have to install the following drivers manually 1 On Windows 98 and later the parallel port can no longer be addressed directly Even if you use only the USB port for I O communication with the Polaris
20. ds out the spectrum accumulated so far After each data taking run the spectrum is saved automatically in the data file specified in the System tab See section 3 5 2 for the data format of the spectrum file 3 2 5 MCA Analysis The MCA Analysis tab is used to investigate the spectrum You can perform Gauss fits on peaks to find the resolution and calibrate the energy spectrum by entering a known energy value for a fitted peak Using the Show ROIs button you can define several regions of interest which are summed or fitted for the number of counts in the region On this tab you can also save spectra and read them back from file IGOR text format by pressing the Save or Read button Additionally you can import spectrum from and export spectrum to CHN ORTEC files using the Import or Export button 3 3 Optimizing Parameters Optimization of the Polaris s run parameters for best resolution depends on the individual system and usually requires some degree of experimentation The Polaris Viewer includes several diagnostic tools and settings options to assist the user as described below 3 3 1 Noise For a quick analysis of the electronic noise in the system you can view a Fourier transform of the incoming signal by selecting Oscilloscope gt Show FFT in the System Configuration tab The graph shows the FFT of the untriggered input sigal of the Oscilloscope By adjusting the dT control in the Oscilloscope and clicking the Refresh bu
21. e detector is provided at the SHV connector labeled DETECTOR BIAS For detectors with thermal shutdown protection connect the shutdown line to the BNC connector labeled L N INHIBIT Make sure the jumper settings for the shutdown logic matches your particular detector see section 7 1 6 in the appendix for details When the Polaris is switched on either the or the LED on the front panel is orange indicating the HV polarity currently set The polarity can be switched using an internal PCB polarity key Open the top cover pull out the small green circuit board near the front right corner and install it upside down The HV bias can be adjusted from 0 to 5000V on the front panel Use a small screwdriver to turn the potentiometer labeled ADJUST to set the voltage The set voltage is shown in the LCD display To turn on the high voltage push the red ENABLE button The polarity LED will change to red and the LCD display will now show the actual output voltage ramping up from zero to the set voltage Pushing the button a second time will ramp down the high voltage back to zero 2 2 4 AC power The Polaris can be powered either from 115VAC or 230VAC depending on the LINE SELECT switch It is rated for 200mA 60Hz 115VAC setting or 100mA 50Hz 230VAC setting 2 2 5 Auxiliary signal inputs The SYNC and GATE BNC connectors accept auxiliary timing or vetoing signals Photomultiplier tu
22. e fast filter is used to detect the arrival of y rays the slow filter is used for the measurement of Vx with reduced noise at longer rise times The fast filter has a filter length Ly 0 1us and a gap Gf 0 1us The slow filter has L 1 2us and G 0 35us The arrival of the y ray step in the preamp output is detected by digitally comparing the fast filter output to THRESHOLD a digital constant set by the user Crossing the threshold starts a counter to count PEAKSAMP clock cycles to arrive at the appropriate time to sample the value of the slow filter Because the digital filtering processes are deterministic PEAKSAMP depends only on the values of the fast and slow filter constants and the risetime of the preamplifier pulses The slow filter value captured following PEAKSAMP is then the slow digital filter s estimate of Vx 29 ADC units 56 58 60 62 64 66 68us Time Figure 6 6 A sequence of 3 y ray pulses separated by various intervals to show the origin of pileup and demonstrate how it is detected by the Polaris The value V captured will only be a valid measure of the associated y ray s energy provided that the filtered pulse is sufficiently well separated in time from its preceding and succeeding neighbor pulses so that their peak amplitudes are not distorted by the action of the trapezoidal filter That is if the pulse is not piled up The relevant issues may be understood by reference to Figure 6 6 which shows 3
23. ee the discussion in section 6 5 for further details 3 3 3 Threshold and Trigger Filter Parameters In general the trigger threshold should be set as low as possible for best resolution If too low the input count rate will go up dramatically and noise peaks will appear at the minimum and maximum edge of the spectrum If the threshold is too high especially at high count rates low energy events below the threshold can pass the pile up inspector and pile up with larger events This increases the measured energy and thus leads to exponential tails on the ideally Gaussian peaks in the spectrum Ideally the threshold should be set such that the noise peaks just disappear The settings of the trigger filter have only minor effect on the resolution However changing the trigger conditions might have some effect on certain undesirable peak shapes A longer trigger rise time allows the threshold to be lowered more since the noise is averaged over longer periods This can help to remove tails on the peaks A long trigger flat top will help to trigger on slow rising pulses and thus result in a sharper cut off at the threshold 3 3 4 Decay time The preamplifier decay time Tt is used to correct the energy of a pulse sitting on the falling slope of a previous pulse The calculations assume a simple exponential decay with one decay constant A precise value of t is especially important at high count rates where pulses overlap more frequently If t
24. esccsssecessscscsescescccssseeesssssssocsees 4 Polaris Viewer AR A ia 6 3 1 A ROA 6 3 2 Navigating the Polaris VieWeT mmoocommsmsm 7 DIM A A A T A elcid coe 7 3 2 2 System Configuration 2 s c scccicisdesiecedeeseeenedeadocodeenceesadesdonchocetodegdnesdsesscevedegdaesdecsovetedandoeeed 7 IAN RS O 8 SOA MEAR UI tr ent ende rd e ld ol 9 SDS MOA AA e e ti a o e 9 3 3 Optimizing Parameters ccsccccscscscsccsscesccscssscecssssesccssssccessseesscssssccesssseessessesceees 9 E Noisea ilived acs teyactuds tees Meesute vce onde en a e a wes el a ER 9 33 2 Energy Filter Pararneters vecindad dial eat Mireles ae 10 3 3 3 Threshold and Trigger Filter Parameters ccccescceseceseesseeseeeeeeeeeeeeceeenseeeeeeerenereneeeeeaees 10 IIA a DEC AA Ta a EE E e e Rented heel A lees evs bese needs Rect de tole a ll caco o 10 A iia k din ela A E N hort ones 11 3 4 Typical Applications sccccsssssscssscescccsssccesssceecsccsescccsssceessccsesccssescessssssessessesoes 11 SAM A NN 11 3 4 2 Spectroscopy with shield ooo iia tidad 12 3 4 3 Using the Polaris with scintillator detectors eccceccessceseceseeeeeceeeeseeeeeeeeeeeeeeeeeeeeeeeeseeaees 12 3 5 Polaris data structure sisscssccccccrsssssssssccosseceesassvassovsssosesesavassondssosesaedenacseessoseescdeasceesasose 13 B Dili TGOR A A ES E AE E AE AA E ATE 13 3 5 2 MCA data Files noia 13 3 5 3 P lseshap data les ei R E A R E RE A a 1
25. f above 100 or below 0 click Adjust to automatically set the DC offsets If the pulse amplitude is too large to fall in the display range increase the Dynamic Range in the Acquisition Settings tab of the main control panel Since the offsets might drift for example after changes in input count rate it is useful to leave the display open and check the offsets once in a while 3 In the System Configuration tab enter an estimate of the preamplifier RC decay time and then click on Find to determine the actual decay time 4 In the System Configuration tab click on Save Settings to save the system parameters found so far 5 Click on the MCA Run tab set Run Time to 30 seconds or so of preset real time then click Start During the run you can click the Update Spectrum button to view the accumulation of data into the spectrum 6 After the run is complete the Update Spectrum button grays out again click on the MCA Analysis tab Select a known peak from the spectrum and set Start Channel and End Channel as the limits for a Gauss fit You can also use the mouse to drag the cursors in the MCA graph to the limits of the fit Click Gauss Fit to perform the fit Enter the true energy value in the Peak Position field to calibrate the energy scale 7 Click on the System Configuration Tab then again click on Save Parameters The Polaris is now set up and you can take runs and modify parameters to adapt it best to your system 3 2 Navigat
26. his file is separate from the main IGOR experiment file to allow saving and restoring different settings for different detectors and applications Parameter files are saved and loaded with the corresponding buttons in the System Configuration tab After loading a settings file you have to click the Start System button to apply the settings to the module At module initialization the settings are automatically read and applied to the Polaris from the current file The System Configuration tab also has a number of buttons linking to the following diagnostic graphs e The Oscilloscope shows a graph of ADC samples read untriggered pulses from the signal input The time intervals between the samples can be adjusted for intervals greater than 0 275us the samples will be averaged over the interval The main purpose of the Oscilloscope is to make sure that the signal is in range in terms of gain and DC offset pulses fall between 10 and 80 on the right axis The Oscilloscope is also useful to estimate the noise in the system Clicking on the Show FFT button opens the FFTDisplay where the noise spectrum can be investigated as a function of frequency This works best if the Oscilloscope trace contains no pulses i e with the detector attached but no radioactive sources present e Inthe Pulse Shape panel you can acquire individual pulses with a time resolution of 25ns This is a useful tool to find out the characteristics of a given detector a
27. ime in sec root polaris RunTime Polaris run time in sec root polaris InputCountRate Input count rate in cps root polaris OutputCountRate Output count rate in cps root polaris ShieldCountRate Count rate at Gate input root polaris ComptonCountRate Coincidence rate of detector and Gate pulses root polaris MCA wave MCA spectrum wave Table 1 IGOR output variables The input variables shown below should only be changed in the control panel to make sure all dependencies are updated properly IGOR variable or wave name Description root polaris DynamicRange Dynamic range in MeV root polaris PreampGain Preamplifier gain in mV MeV root polaris TriggerThreshold Trigger threshold in keV root polaris BaselinePercent Default offset level in root polaris DetectorTau Preamplifier decay time root polaris HistogramLength Histogram length number of bins root polaris TraceLength Trace length of pulse shape data root polaris TraceDelay Pre trigger time of pulse shape data root polaris XDT Time step of oscilloscope trace root polaris TriggerPeakingTime Rise time of trigger filter root polaris TriggerGapTime Flat top time of trigger filter root polaris EnergyPeakingTime Rise time of energy filter root polaris EnergyGapTime Flat top time of energy filter root polaris PresetRunTime Preset run time root polaris PresetRunType 0 infinite 1 preset real time 2 preset live time
28. in fine steps Digital trapezoidal filter Rise time and flat top set independently 0 050 45 us in small steps 1024 65536 channels 32 bit deep 4 294 967 295 counts per channel Real time live time input and throughput count rates and Compton shield statistics Control I O via OEM Port Control Signals Other Specifications Detector Supply Preamp Supply Sends or receives TTL CMOS control signals via optional OEM connector to create flexible custom interfaces to external instruments or industrial equipment Custom on board software facilitates integration of the Polaris processor core into dedicated spectroscopy applications High voltage 5000 V SHV connector push button on off front panel adjust 60 seconds on off ramp 24 V and 12 V each rated at 100 mA 2 Setting up 2 1 Scope of document This document covers Polaris devices with serial numbers 100 150 2 2 Hardware installation On the front panel of the Polaris spectrometer are controls for detector HV bias as well as the main power switch All connections are made on the back panel They include host computer I O detector signal input and preamplifier power detector HV bias AC power as well as auxiliary signal inputs Some settings need adjustment of internal jumpers which can be accessed by removing the top cover of the chassis 2 2 1 Host Computer I O Host Computer I O is made either through the EPP port or the USB port To
29. ing the Polaris Viewer 3 2 1 Overview The Polaris Viewer consists of a number of graphs and control panels linked together by the main Polaris Control Panel The Polaris Control Panel is divided into 4 tabs corresponding to the 4 topics summarized below The System Configuration tab contains controls used to initialize the module and the file and directory settings The Acquisition Settings tab contains controls to adjust parameters such as dynamic range filter rise time and flat top and trigger threshold The MCA Run tab is used to start and stop runs and in the MCA Analysis tab are controls to analyze save and read spectra Below we describe the concepts and principles of using the Polaris Viewer Detailed information on the individual controls can be found in the online help for each panel 3 2 2 System Configuration The Polaris Viewer comes up in exactly the same state as it was when last saved to file using File gt Save Experiment However the Polaris modules itself loses all programming when switched off When the Polaris is switched on again only the Host I O interface is initialized automatically All the other programmable components need code and configuration files to be downloaded to the module Clicking the Start System button in the System Configuration tab or the Continue button in the I O panel performs this download The Polaris being a digital system all parameter settings are stored in a settings file T
30. lses the dynamic range has to be high enough to accommodate the combined height of the overlapping pulses For example if at high count rates up to 3 pulses of 1 332 MeV overlap within the say 50us decay of the first pulse the dynamic range is best set to about 3 x 1 332 MeV 3 99 MeV or higher Otherwise the signal will go out of range often and especially the peaks at the high energy end of the spectrum will lose counts You can use the Oscilloscope graph to verify if the Dynamic Range is appropriate 3 4 Typical Applications In the following section we outline a few typical application examples and give the parameter settings that may be used as a starting point These example settings are included on the Polaris software distribution 3 4 1 Spectroscopy The Polaris is a high precision ultra fast all digital spectrometer It provides unparalleled spectral accuracy with up to 64K channels spectrum length and can on the other hand sustain count rates of up to 750 000 counts per second into the spectrum at an input count rate of over 2 1 million counts per second The Polaris can accept signals from virtually any radiation detector Signals with decay times as fast as 230ns from Nal T1 for instance to as slow as 10ms can be processed without the need for external electronics 11 3 4 2 Spectroscopy with shield In many applications a shielding detector surrounds the sensitive detector The shield is used to provide a veto when it fi
31. ly be ignored but check the settings and maybe reload from file 31
32. m Eqn 6 2 When the slope is not zero the mean values of the two sums will differ because they are taken over regions separated in time by L G on average 27 With a RC type preamplifier since pulses are not the simple step functions described above the slope of the preamplifier is rarely zero Every step decays exponentially back to the DC level of the preamplifier During such a decay the baselines are obviously not zero This can be seen in Figure 6 4 where the filter output during the exponential decay after the pulse is below the initial level Note also that the flat top region is sloped downwards Using the decay constant t the baselines can be mapped back to the DC level This allows precise determination of y ray energies even if the pulse sits on the falling slope of a previous pulse The value of t being a characteristic of the preamplifier has to be determined by the user and host software and downloaded to the module 6 4 Thresholds and Pile up Inspection As noted above we wish to capture a value of V for each y ray detected and use these values to construct a spectrum This process is also significantly different between digital and analog systems In the analog system the peak value must be captured into an analog storage device usually a capacitor and held until it is digitized Then the digital value is used to update a memory location to build the desired spectrum During this analog to digital conversio
33. n concentrates on the functionality aspect Technical specification can be found in section 1 2 5 1 Analog signal conditioning Each analog input is first fed into a signal conditioning unit The task of this circuitry is to adapt the incoming signal to the input voltage range of the ADC which spans 1 00V The input signal is adjusted for offset and there is a computer controlled gain stage This helps to bring the signal into the ADC s voltage range and set the dynamic range of the channel The ADC is not a peak sensing ADC but acts as a waveform digitizer In order to avoid aliasing we remove the high frequency components from the incoming signal prior to feeding it into the ADC The anti aliasing filter a 3rd order active Sallen Key filter cuts off sharply at the Nyquist frequency namely half the ADC sampling frequency Though the Polaris can work with many different signal forms best performance is to be expected when sending the output from a charge integrating preamplifier directly to the Polaris without any further shaping 5 2 Real time processing unit The real time processing unit consists of a field programmable gate array FPGA and a FIFO memory The data stream from the ADCs is sent to this unit at the full ADC sampling rate Using a pipelined architecture the signals are also processed at this high rate without the help of the on board digital signal processor DSP The real time processing unit RTPU applies digital
34. n process the system is dead to other events which can severely reduce system throughput Even single channel analyzer systems introduce significant deadtime at this stage since they must wait some period typically a few microseconds to determine whether or not the window condition is satisfied Digital systems are much more efficient in this regard since the values output by the filter are already digital values All that is required is to take the filter sums reconstruct the energy Vx and add it to the spectrum In the Polaris the filter sums are continuously updated by the RTPU see section 6 2 and only have to be read out by the DSP when an event occurs Reconstructing the energy and incrementing the spectrum is done by the DSP so that the RTPU is ready to take new data immediately after the readout This usually takes much less than one rise time so that no system deadtime is produced by a capture and store operation This is a significant source of the enhanced throughput found in digital systems 28 32x10 7 ADC Output x Fast Filter Output a 31 7 o Slow Filter Output a ES H Do i f o H S88 nee ce RD H D S 2 a E 29 5 O fal Arrival Tima 4 27 26 44 45 46 47 48us Time Figure 6 5 Peak detection and sampling in the Polaris The peak detection and sampling in the Polaris is handled as indicated in Figure 6 5 Two trapezoidal filters are implemented a fast filter and a slow filter Th
35. nd optimize the parameters accordingly For example the flat top of the energy filter should ideally be only slightly larger than a typical rise time of a pulse You can also investigate non ideal behavior such as preamplifier overshoots Pulses are saved to a binary file see section 3 5 3 for a format description It is also possible and has been implemented in other models of XIA s DGF product line to perform pulse shape analysis in the Polaris during data acquisition and discriminate events accumulated into the spectrum for example removing events with too long and or too short rise times Contact XIA for details 3 2 3 Acquisition Settings Internally the module parameters are handled as binary numbers and bitmasks The Acquisition Settings tab gives access to user parameters in meaningful physical units Values entered by the user are converted by the Polaris Viewer to the closest value in internal units You can change rise times of the digital filters modify the dynamic range set the trigger threshold etc Refer to the online help for detailed descriptions of the parameters 3 2 4 MCA Run The MCA Run tab is used to start and stop runs You can set the run time to either a preset live time or preset real time or run unlimited On the right side of the panel a summary of run statistics is periodically updated during a run including real time and live time and various count rates Clicking the Update Spectrum button rea
36. nd Livetime During data acquisition y rays will be absorbed in the detector at some rate This is the true input count rate which we will refer to as ICR Because of pileup not all of these will be detected by the Polaris y ray pulse detection circuitry which will thus report a measured input count rate ICRm which will be less than ICR This phenomenon it should be noted is a characteristic of all y ray detection circuits whether analog or digital and is not specific to the Polaris Of the detected y rays some fraction will also satisfy pileup tests and have their values of V captured and placed into the spectrum This number is the output count rate which we refer to as the OCR The standard description of extending dead time gives 31 OCR ICR exp ICR DEADTIME 6 4 Where in the Polaris DEADTIME is usually very close to the values 2 PEAKSEP or approximately 2 L G The Polaris normally returns in addition to the collected spectrum the actual time RUNTIME for which data was collected together with the number FASTPEAKS of fast peaks detected and the number of V captured events NUMEVENTS It also returns a quantity FTDT the time the trigger filter spends above threshold and is not able to record further triggers From these values both OCR and ICR can be computed according to Equation 6 5 ICRm FASTPEAKS RUNTIME FTDT OCR NUMEVENTS RUNTIME 6 5 The LIVETIME can then be computed as follows L
37. nd some internal relays will click If no error messages appear the system is initialized In the IGOR window you will now see the main Polaris Control Panel from which all work is conducted The tabs in the Control Panel are arranged in logical order from left to right Detailed description of controls and panels can be found in the on line help from within the Polaris Viewer To view the help texts click the Help button in the lower left corner of the control panel In the help topics click on blue underlined links to jump to cross references You can also use IGOR s built in help browser to access the Polaris specific help file by selecting Help gt Help Topics from the top menu bar Choose Polaris Help in the popup menu on the left and select the appropriate help topic from the list on the right For an initial setup and data taking run you would typically follow the sequence below Count rates should be kept reasonably low at first about 5000 cps especially for determining the decay time If you encounter problems and strange effects see the troubleshooting section in the Appendix 1 In the System Configuration tab make sure the Polarity matches the polarity of pulses from your preamplifier 2 Click on the Oscilloscope button This opens a graph that shows the untriggered signal input Click Refresh to update the display The pulses should fall between 10 and 80 on the right axis If no pulses are visible or if they are cut of
38. outine works from the data in memory to generate the requested output data In this scheme the greatest processing power is located in the RTPU Implemented in a FPGA it processes the incoming waveforms from its associated ADC in real time and produces for each valid a event a small set of distilled data from which pulse heights and arrival times can be reconstructed The computational load for the DSP is much reduced as it has to react only on an event by event basis and has to work with only a small set of numbers for each event 21 5 4 Spectrum Memory Energy spectra are accumulated in a 64k x 32b1t memory chip allowing for 64k bins with more than 4 billion counts each The DSP passes energy values to a memory manager implemented in an FPGA which then increments the corresponding bin in the spectrum The host computer can read the spectrum via the DMA bus without interrupting the DSP operation This architecture further reduces the computational load for the DSP and allows for fast transfers of spectrum data For special applications for example accumulating several independent spectra in a single run this memory can be extended up to a total of 512k bins Contact XIA for details 5 5 Host interface The EPP interface through which the host communicates with the Polaris is implemented in its own FPGA The configuration of this gate array is stored in a PROM which is placed in the only DIP 8 IC socket on the Polaris board The
39. present The fluctuations in the baseline have a standard deviation O which is referred to as the electronic noise of the system a number which depends on the rise time of the filter used Riding on top of this noise the y ray peaks contribute an additional noise term the Fano noise which arises from statistical fluctuations in the amount of charge Q produced when the y ray is absorbed in the detector This Fano noise os adds in quadrature with the electronic noise so that the total noise in measuring Vx is found from Or sqrt of oe 6 3 The Fano noise is only a property of the detector material The electronic noise on the other hand may have contributions from both the preamplifier and the amplifier When the preamplifier and amplifier are both well designed and well matched however the amplifier s noise contribution should be essentially negligible Achieving this in the mixed analog digital environment of a digital pulse processor is a non trivial task however 3 33 10 a A ADC Output dE Filter Output e e ale e e a 31 ADC units 30 29 28 75 80 85 90 95us Time Figure 6 4 Ay ray event displayed over a longer time period to show baseline noise and the effect of preamplifier decay time In the general case however the mean baseline value is not zero This situation arises whenever the slope of the preamplifier signal it not zero between y ray pulses This can be seen fro
40. r and semi Gaussian filtering by stretching the triangular rise time a bit so that the true triangular rise time is typically 1 2 times the selected semi Gaussian rise time This also leads to an apparent advantage for the analog system when its energy resolution is compared to a digital system with the same nominal rise time One important characteristic of a digitally shaped trapezoidal pulse is its extremely sharp termination on completion of the basewidth 2L G This may be compared to analog filtered pulses which have tails which may persist up to 40 of the rise time a phenomenon due to the finite bandwidth of the analog filter As we shall see below this sharp termination gives the digital filter a definite rate advantage in pileup free throughput 3 33x10 32 ADC units 31 30 9 5 10 0 10 5 11 0 11 5 12 0 12 5us Time Figure 6 3 Trapezoidal filtering of a preamp step with L 1us and G 0 4pus 6 3 Baselines and preamplifier decay times Figure 6 4 shows an event over a longer time interval to show how the filter treats the preamplifier noise in regions when no y ray pulses are present As may be seen the effect of the filter is both to reduce the amplitude of the fluctuations and reduce their high 26 frequency content This signal is termed the baseline because it establishes the reference level from which the y ray peak amplitude V is to be measured The mean value of the baseline is zero if no pulses are
41. rence guide to those users who want to write their own Igor programs within the Polaris Viewer The Polaris library file Polaris xop has been designed such that the users only need to use the following five functions to build their own data acquisition routines For more details please contact XIA Hand_Down_File_Names All_Files Where All Files is an Igor text wave with six entries of file names the system FPGA MMU file FIPPI file DSP code file DSP I O parameter value file DSP I O parameter name file and DSP memory name file All these file names should contain the full file path name Boot_System Boot_Pattern Where Boot Pattern is a bit mask bit 0 Boot MMU System FPGA bit 1 Boot FIPPI bit 2 Boot DSP bit 3 Load DSP parameter values bit 4 Apply DSP parameters calls Set_ DACS and Program _FIPPI The next three bits are used for specifying I O types bit 5 OFFLINE bit 6 EPP bit 7 USB So if the user wants to download MMU FIPPI DSP code and parameter values and apply DSP parameters using a USB connection the Boot_Pattern should be Ox9F User_Par_IO User_Names User_Values User_Name Direction Where User Names is an Igor text wave containing the names of all the user variables communicating between the Polaris Viewer and the Polaris library User_Values is an Igor double precision wave containing the values of all the user variables communicating between the Polaris Viewer and the Polaris library
42. res This helps to reject events in which energy scattered out of the sensitive detector or background radiation penetrated from the outside Such a veto signal can be connected to the Gate BNC connector on the backside of the Polaris If the Polaris module comes equipped with the Compton rejection circuitry the Gate accepts signals directly from a photomultiplier tube The Compton Shield Veto popup menu in the System Configuration tab of the Polaris Viewer controls the Gate When the veto is disabled the Gate input is ignored Otherwise the event is rejected if a gate pulse is detected within 1 us of a trigger from the detector input 3 4 3 Using the Polaris with scintillator detectors For semiconductor detectors signals are invariably picked up by a charge integrating preamplifier with a relatively long decay time 50us or longer On the contrary the light signal from scintillators is usually amplified by a photomultiplier PMT In this case a preamplifier will most likely not be necessary as the gain of a PMT will almost always be sufficiently high A second benefit of a charge integrating preamplifier is that longer filters can be used on its step like output to suppress the electronic noise and improve energy resolution In scintillator applications however the energy resolution is rarely limited by the electronic noise Hence we can take advantage of the often fairly short decay time constants for the scintillation
43. rmal operation the signal gain is combined from a coarse gain set by relays and a digital fine gain applied in the DSP An additional fine gain can be set by a variable gain amplifier VGA To do so set JP106 to on However this should rarely be necessary since the coarse gain is precise to about 2 Furthermore the VGA introduces additional electronic noise and thus is not recommended for optimal performance 7 1 5 Compton Veto Polarity JP110 JP111 The Gate input accepts both positive and negative pulses from photomultiplier tubes Set Jumpers JP110 and JP111 to if the pulses are positive to if they are negative 7 1 6 HV shutdown JP20 JP21 Detectors with thermal shutdown protection usually come in two variants Either logic high one indicates shutdown or logic low zero indicates shutdown To match the Polaris shutdown circuitry to either variant set JP20 to the logic level indicating the shutdown and JP21 to the logic level indicating normal operation This means that the Jumpers are always opposite to each other 34 7 2 Control and Status Register Bits CSR Modules 0x0001 Run Enable 0x0002 New Run 0x0004 Unused clear using CamacClear 0x0008 LAM Enable 0x0010 DSP Reset 0x0020 FPGA Reset 0x0040 Unused clear using CamacClear 0x0080 Synch Flag 0x0100 FPGA Download Error in channel 0 0x0200 Reserved 0x0400 Reserved 0x0800 Reserved 0x 1000 DSP Error 0x
44. root polaris RunTime Unit Time multiplier 60 for min 3600 for hours etc Table 2 Igor Input Variables 3 5 2 MCA data files MCA files are saved automatically after each run to the filename specified in the System Configuration tab as a binary file unsigned 4 byte integer words Additionally on the MCA analysis tab MCA data can also be saved to an IGOR text file itx in ASCII format as shown in the example below In the file header the most important operating conditions are summarized The user is prompted for entries to the Detector Condition and Operator keywords before saving the spectrum The header is 13 followed by the MCA data each line has the number of counts in one channel The last line of the file contains an IGOR command for scaling the MCA in the same energy scale as originally saved IGOR X XIA Polaris MCA data saved Fri Apr 12 2002 6 19 21 PM X Detector X Condition X Operator X Run Time sec 6360 92 X Live Time sec 5224 5 X Rise Time us 6 X Flat Top us 1 2 X Decay Time us 47 35 X Dynamic Range MeV 2 24534 X Trigger Threshold keV 15 1876 X Input Count Rate cps 9417 14 X Output Count Rate cps 7734 71 WAVES MCAchO BEGIN oooo o 0 0 END X SetScale P x 0 0 0379690920408061 MCAchO0 SetScale y 0 0 MCAch0 3 5 3 Pulse shape data files Pulse shape data is saved in bin
45. seessessesecsesees 28 6 5 Filter decimation sccssccsscsssssssevssessscssccsssscssscssscssessssscsescsssssssssessnessnessssssssessnees 31 6 6 Count Rates and LivetiM ooooocconnooonoooncnnnnonnnonanonanonononnnconnccnnccnnccono cono cono coro rocaconaccnnos 31 Zo APPEAR A E is 33 7 1 A A ON 33 7 1 1 Imput JP1 JP9 and PIU aeeai a a A aE A Aa a A Aia 33 7 1 2 Signal Termination and Attenuation JP108 JP109 JP112 JP113 seses 33 71 3 Mode JP103 TP TOA resien inne ne ESETE ENEE EE ETE 33 LEA VGA IP DO idad 34 7 1 5 Compton Veto Polarity JP110 JPLLL oo e cee cecccsccsseceseceeeceecaeeeneeeeeeeeeeeeesereeeeneeeeseenaees 34 7 1 6 HV shutdown JP20 JP 21 sc sscccccsscctesssseestcsesessescacasdvcsedeseeecacassecdasebsacaacassessedavseccacattesebunsasiess 34 7 2 Control and Status Register Bits oocooonooononononanonanononconaconnccnnccnncconocono cono conc cono cocnconoos 35 7 3 Troubleshooting ccsscccsssccccscescsesessceccccsesccesssccessccccsccssesceesesceessscssescesssssesssessees 36 7 3 1 IGOR reports Function compilation error at StartUP ooooooninnnnnnnncnnonnooncnnnnanonnnconono nono nonn neos 36 13 2 IGOR rep rts missing DEL file iia a aaa 36 7 3 3 USB communication does not Work cccessecssecseeseceeseeceseeeeesecseesecaevsecnaeeeceaecaeesecaeeeteneees 36 7 3 4 Igor reports missing files at system Startup cescesecseesceeseceeeceeeeseeeneeeeeeeeeeeeeseeeeeeseensees 36 7 3 5 Igor reports
46. st of a significantly more complex circuit and a rate dependent resolution which is unacceptable for many types of precise analysis In practice XIA s design has been found to duplicate the energy resolution of the best analog shapers while approximately doubling their throughput providing experimental confirmation of the validity of the approach 6 2 Trapezoidal Filtering in the Polaris From this point onward we will only consider trapezoidal filtering as it is implemented in the Polaris according to Eqn 6 2 The result of applying such a filter with Length L 1us and Gap G 0 4us to a y ray event is shown in Figure 6 3 The filter output is clearly trapezoidal in shape and has a risetime equal to L a flattop equal to G and a symmetrical falltime equal to L The basewidth which is a first order measure of the filter s noise reduction properties is thus 2L G 25 This raises several important points in comparing the noise performance of the Polaris to analog filtering amplifiers First semi Gaussian filters are usually specified by a shaping time Their rise time is typically twice this and their pulses are not symmetric so that the basewidth is about 5 6 times the shaping time or 2 8 times their rise time Thus a semi Gaussian filter typically has a slightly better energy resolution than a triangular filter of the same rise time because it has a longer filtering time This is typically accommodated in amplifiers offering both triangula
47. stopped and the read pointer is positioned such that it points to the beginning of the pulse that caused the trigger When the DSP collects event data it can read any fraction of the stored waveform up to the full length of the FIFO 5 3 Digital signal processor DSP The DSP controls the operation of the Polaris reads raw data from the RTPU reconstructs true pulse heights applies time stamps and prepares data for output to the host computer and increments spectra in the external memory The host computer communicates with the board via the EPP or USB interface using a direct memory access DMA channel Reading and writing data to DSP memory does not interrupt its operation and can occur even while a measurement is underway Note that EPP transfers introduce additional noise to the signal so it is best to avoid transfers while a run is in progress USB transfers do not show this problem The host sets variables in the DSP memory and then calls DSP functions to program the hardware Through this mechanism all gain and offset DACs are set and the RTPU are programmed The RTPU processes its data without support from the DSP once it has been set up When it generates a trigger an interrupt request is sent to the DSP The DSP responds with reading the required data from the RTPU and storing those in memory It then returns from the interrupt routine without processing the data to minimize the DSP induced dead time The event processing r
48. ted in your computer s CMOS or BIOS setup ECP is the abbreviation of Enhanced Capabilities Port If only ECP was selected the EPP connection for Polaris won t work Also verify the EPP address shown in the I O panel matches the address on your computer To do so open the System Control Panel of MS Windows In the Device Manager look for ports and double click on the entry printer ports to display the port properties The EPP address range is listed in the Resources tab Igor can not open files Make sure the directory structure is intake In the main Polaris folder there should be seven subfolders configuration dsp doc drivers firmware MCA and pulseshape Make sure all files are not marked as read only in the Windows operation system 36 7 3 7 No traces or only flat lines in Oscilloscope e Click on Adjust to bring the DC offset in range e Check input signal connection 7 3 8 Very high input count rate during run e Check trigger threshold and increase above noise level 7 3 9 Very low livetime during run e Check trigger threshold and increase above noise level 7 3 10 Large peak at low end of spectrum e Check trigger threshold and increase above noise level 7 3 11 Spectrum has very wide and blurred peaks e Check and remeasure decay time 7 3 12 Igor reports need to have at least as many data points as fit parameters e This message occurs if an automatic fit in Igor failed It can usual
49. tton you can investigate different frequency ranges For best results remove any source from the detector and only regard traces without actual events If you find sharp lines in the 10 kHz to 1 MHz region you may need to find the cause for this and remove it If you click on the Filter button you can see the effect of the energy filter simulated on the noise spectrum 3 3 2 Energy Filter Parameters The main parameter to optimize energy resolution is the rise time of the energy filter Generally longer rise times result in better resolution but reduce the throughput Optimization should begin with scanning the rise time through the available range Try 2us 4us 8us 11 2us take a run of 60s or so and note changes in energy resolution Then fine tune the rise time The flat top usually needs only small adjustments For a typical coaxial Ge detector we suggest to use a flat top of 1 2us For a small detector 20 efficiency a flat top of 0 8us is a good choice For larger detectors flat top of 1 2us and 1 6us will be more appropriate In general the flat top needs to be wide enough to accommodate the longest typical signal rise time from the detector It then needs to be wider by one filter clock cycle than that minimum but at least 3 clock cycles Note that the filter clock cycle ranges from 0 05 to 1 6us depending on the filter time range so that it is not possible to have a very short flat top together with a very long rise time S
50. ust be individually normalized Preamp Output mV 20 22 24 26 28 30 Time us Figure 6 2 Digitized version of the data of Figure 6 1 b in the step region The primary differences between different digital signal processors lie in two areas what set of weights wi is used and how the regions are selected for the computation of Eqn 6 1 Thus for example when the weighting values decrease with separation from the step then Eqn 6 1 produces cusp like filters When the weighting values are constant one obtains triangular if the gap is zero or trapezoidal filters The concept behind cusp like filters is that since the points nearest the step carry the most information about its height they should be most strongly weighted in the averaging process How one chooses the filter lengths results in time variant the lengths vary from pulse to pulse or 24 time invariant the lengths are the same for all pulses filters Traditional analog filters are time invariant The concept behind time variant filters is that since the y rays arrive randomly and the lengths between them vary accordingly one can make maximum use of the available information by setting Length to the interpulse spacing In principal the very best filtering is accomplished by using cusp like weights and time variant filter length selection There are serious costs associated with this approach however both in terms of computational power required to evaluate
51. vent time high word 2 EVT_TIMELO Event time low word Table 4 Event header data format The LSB bit 0 of the hit pattern if set indicates that the channel has recorded an event The other bits are unused and reserved After the event header follows the channel information A channel header of length CHANHEADLEN which may be followed by waveform data For standard List Mode the only pulse shape data format currently supported CHANHEADLEN and the nine words are Word _ Variable Description 0 CHAN NDATA Number of words for this channel 1 CHAN TRIGTIME Fast trigger time 2 CHAN ENERGY Energy 3 CHAN XIAPSA XIA PSA value 4 CHAN USERPSA User PSA value 5 Reserved Raw data 6 Reserved Raw data 7 Reserved Raw data 8 Reserved Raw data Table 5 Channel header possibly followed by waveform data If CHAN_NDATA gt 9 there will be waveform data following this channel header Any waveform data for this channel would then follow this header An offline analysis program can recognize this by computing N WAVE DATA CHAN DATA 9 If 15 N__WAVE _DATA is greater than zero it indicates the number of waveform data words to follow 3 5 4 Parameter files Polaris Parameter files are saved as a list of 416 numbers in binary form signed 16 bit words The numbers correspond to a list of 416 DSP variable names stored in G200Ecode var an ASCII file in the dsp folder As the
52. wer cable to board GND if set to the GND symbol Otherwise the cable shield is unconnected 7 1 2 Signal Termination and Attenuation JP108 JP109 JP112 JP113 Near the input BNC connector are 4 jumpers to select the input impedance and attenuation Jumper JP108 bypasses a 1kQ attenuation resistor Jumpers JP112 JP109 and JP113 connect to ground via 250Q 50Q and 90Q respectively If neither of jumpers JP112 JP109 or JP113 is set the input is terminated with 10kQ Thus the jumper settings are as follows JP108 Remove only if you require attenuation Attenuation will be 1 5 if JP112 is set 1 21 if JP109 is set 1 12 if JP113 is set JP112 Set for input impedance of 250Q JP109 Set for input impedance of 50Q JP113 Set for input impedance of 900 Table 7 Input impedance selection jumpers 7 1 3 Mode JP103 JP104 The analog signal conditioning circuitry can accommodate both resistive feedback and pulsed reset preamplifiers JP103 and JP104 determine the digitally controlled signal to 33 be subtracted from the preamplifier signal In the default position labeled RC a low frequency offset suitable for resistive feedback preamplifier types is used This digitally controlled offset voltage is sufficient to eliminate the average decaying sum of prior signal pulses in resistive feedback preamplifiers The pulsed reset preamplifiers are not supported by the current software 7 1 4 VGA JP106 In no
53. with user defined fit functions Note that most Polaris data resides in the polaris subfolder and have to be addressed as root polaris MCAwave rather than simply MCAwave see section 3 5 1 for details Every IGOR Pro experiment file has a history window with command line for entering and logging commands and messages If you modify graphs or panels the modification commands are usually printed in the history window from where they can be copied to the command line to edit and or repeat or into a user procedure 16 The command line is also useful to issue commands such as to duplicate the current spectrum to compare it with other spectra from file or from subsequent runs or for simple calculations 3 6 2 Igor procedures All underlying functions and procedures of the Polaris Viewer are available in IGOR s procedure windows which can be accessed by clicking Windows on the top of the Igor window and then selecting Other Windows 3 6 3 DSP customization For demanding applications pulse shape analysis can be performed in the Polaris digital signal processor while data is taken Examples include rejecting events based on certain user defined criteria or calculating timing or energy quantities from the acquired waveforms The DSP code is set up with calls to user routines which can be modified by users and compiled into the main code Contact XIA for details 17 4 Programmers guide This is a short refe
54. y ray Reducing noise in an electrical measurement is accomplished by filtering Traditional analog filters use combinations of a differentiation stage and multiple integration stages to convert the preamp output steps such as shown in Figure 6 1b into either triangular or semi Gaussian pulses whose amplitudes with respect to their baselines are then proportional to Vx and thus to the y ray s energy Digital filtering proceeds from a slightly different perspective Here the signal has been digitized and is no longer continuous but is instead a string of discrete values such as 23 shown in Figure 6 2 Figure 6 2 is actually just a subset of Figure 6 1b which was digitized by a Tektronix 544 TDS digital oscilloscope at 10 MSA megasamples sec Given this data set and some kind of arithmetic processor the obvious approach to determining V is to take some sort of average over the points before the step and subtract it from the value of the average over the points after the step That is as shown in Figure 6 2 averages are computed over the two regions marked Length the Gap region is omitted because the signal is changing rapidly here and their difference taken as a measure of Vx Thus the value Vx may be found from the equation Vo gt WY Wy 6 1 i before i after where the values of the weighting constants W determine the type of average being computed The sums of the values of the two sets of weights m
55. you must run the program port95nt exe from the CD ROM located in the drivers subdirectory This will install DIPortlO on your computer a utility to enable direct addressing of the parallel port 2 Ifyou use the USB port for I O communication you have to install the Polaris USB driver on your system The driver xia2k inf is located in the drivers subdirectory When Windows detects new hardware direct it to look for drivers in that folder 3 Many functions of the Polaris Viewer are precompiled in an Igor xop file For Igor to be able to use these functions the file Polaris xop from the drivers directory must be copied into the Zgor Extensions folder usually located in C Program Files Wavemetrics Igor Pro Folder 3 Polaris Viewer 3 1 Getting Started After installing the software and connecting the Polaris to a pulser or detector double click on the Polaris pxp file in the Polaris folder to start the Polaris Viewer When the Viewer has been loaded it will prompt you to choose the I O type e USB using universal serial bus e EPP using enhanced parallel port e Offline working without a Polaris spectrometer attached If you use the EPP port for I O communication set the EPP Address to the value of the EPP port on your computer Typically the address is 0x378 sometimes 0x278 See section 7 3 5 for details Select the 1 O type you are using then click Continue The 1 O light on the Polaris will flash a
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