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A multiple-channel 2- and 3-fold coincidence counting system for radioactivity standardization
436
Nuclear Instruments and Methods in Physics Research A263 (1988) 436-440 North-Holland, Amsterdam
A MULTIPLE-CHANNEL 2- AND 3-FOLD COINCIDENCE COUNTING SYSTEM FOR RADIOACTIVITY STANDARDIZATION B.R .S . SIMPSON and B.R . MEYER National Accelerator Centre, PO Box 72, Faure, 7131 South Africa
Received 18 September 1986 and in revised form 10 August 1987
The advantages of a multiple-channel coincidence unit for 47x/3-y liquid scintillation radioactivity standardization are discussed. A microcomputer-controlled counting system is described, and compared with an older 3-channel coincidence system, against which it has been tested. The multiple-channel system shows potential for greater precision.
1. Introduction During recent years the National Accelerator Centre of South Africa has made use solely of liquid scintillation counting for routine absolute radioactivity measurement. Usually the 41TP-y coincidence extrapolation method [1] with 15 data points is used, this number of points being considered sufficient for accurate polynomial fitting. The counting efficiency is varied electronically by setting different discrimination levels in one of the /3-channels . TSCA modules are used for this purpose. Until recently, only three data points could be measured simultaneously and the procedure had to be repeated five times to obtain the required 15 data points per sample . The ideal situation would be to measure all 15 data points simultaneously . The advantages of such a system over the previous one are: (i) Data collection is much quicker, making the system more convenient for routine counting. (ii) Individual point scatter is reduced, because the same gamma count is used for determining the efficiencies (coincidence to gamma count ratios) of the 15 data points, thus facilitating a more reliable selection of the trend of the data points to be fitted. (iii) Any minor systematic fault or timing problem in a particular TSCA/COINC UNIT channel will affect only a single point and not five, as with the 3-channel coincidence system . (iv) Reproducibility with respect to repeat and corresponding background measurements is enhanced, because the discrimination levels on the TSCA modules can remain fixed once set. (v) It will facilitate greater statistical accuracy for short-lived isotopes with half-lives of the order of hours 0168-9002/88/$03 .50 © Elsevier Science Publishers B.V . (North-Holland Physics Publishing Division)
(e.g.
1231)
because more sources can be counted in a relatively short period. This will ensure sufficiently high count rates without the need for more radioactive solution having to be added to the scintillator liquid, where problems such as dissolving, quenching and adsorption might occur. There will also be less dependence on decay corrections . The following is a description of the 15-channel coincidence counting system which is now in operation, with emphasis placed on the performance .
2. The counting system The counting system consists of two RCA8850 photomultiplier tubes viewing a cylindrical sample container, the flat ends being directly coupled to the tube faces [2]. These phototubes are used to detect charged particles and low energy X-rays. A 75 x 75 mm NaI(TI) crystal coupled to a photomultiplier detects the photons . The signals from the detectors are shaped in the usual way before being sent to the TSCA modules. The slow logic output signals provide the input to the coincidence unit as shown in fig. 1 . This 15-channel 2- and 3-fold coincidence unit was designed and built at the NAC. The output signals from the coincidence unit are interfaced to a 32-channel scaler module through a patch panel (fig. 2) . The scaler only counts signals when the timer has been activated. All communication between the desktop computer and the CAMAC modules is through a CAMAC-to-GPIB interface module. A computer program written in Basic activates the
437
B.R.S. Simpson, B.R . Meyer / A multiple-channel coincidence counting system 01
P2
PRE-AMPS
ORTEC 113
O TEC 113
ORTEC 113
LINEAR AMPS
ORTEC 410
OR TEC 10
ORTEC 410
INPUTS
TSCA LEVEL OISCRS
15-FOLD COINCIDENCE UNIT
FIXED B IN
Il
v 551
VARIABLE B IN
f __-_--
IN COINCIDEN E BF-B V COINC DENCES
OUTPUTS
DATA ACQUISITION SYSTEM Fig. 1 . Block diagram of the electronic configuration. software programmable timer and the 24-bit counters . The counters are read and reset by the program, which also checks for counter overflows and takes the appropriate action should these occur. The counter contents are transferred to data files and stored on flexible disc . When the counting system became operational, a test was carried out using a pulse generator . This provided signals through a linear amplifier and constant fraction TSCA modules to all the inputs of the 15-channel coincidence unit . The double and triple coincidence count rates were tested up to 75 000 s -t and were all exactly the same, as expected for fixed frequency pulses . A second test was made with random pulses from t39Ce mixed with a liquid scintillator in a counting cell . The outputs used were taken from the two photomultiplier tubes viewing the cell . One of these was fed into the inputs G and B t and the other into each of the B,, inputs . In principle the triple and B-B coincidence counting rates should be identical . In practice small differences up to a maximum of 0 .005% were found . These were possibly due to random losses or gains as a result of relative timing effects within the coincidence unit and were considered as negligible . Coincidence resolving times for the fifteen channels
were determined by mixing two independent -y-pulse trains, the one feeding the 4w inputs of the coincidence circuit and the other feeding only the y input . They were typically 0.57 tts .
3 . Comparison The viability of the new system has been demonstrated by standardizing a number of radioactive solutions . A standardized 17 Ga sample was sent to the International Reference System (SIR) of the Bureau International des Poids et Mesures (BIPM) for comparative measurement in their ionization chamber [3], where an excellent result was achieved . The recent international comparison of t°9 Cd activity measurements organised by the BIPM also afforded an opportunity for using the new system and again the results compared favourably with those of the other participating laboratories [4] . A direct comparison of the new system against the previous one had previously been undertaken using a 139Ce solution . The measured activity as given by the two counting systems agreed within experimental error, and the results indicated that the uncertainty given by
B. R. S. Simpson, B. R . Meyer / A multiple-channel coincidence counting system
438
OUTPUT FROM THE COINCIDENCE UNIT
N W J m Q V
Z0m m
O
1
2 O
3 O
4 O
O 17
O 18
O 19
O 20
32 CHANNEL SCALER IN VETO 17-32 IN 1-16
O
O O O O PATCH PANEL (BNC CO O O O O O 21 22 23 24 25
1 111213 O O O O NECTORS) O O O O 26 27 28 29
14 O
15 O
16 O
O 30
O 31
O 32
LEVEL ADAPTOR NIM/TTL COMPL .
STATION 1
STATION 2 CAMAC MODULES
DESKTOP COMPUTER AND DUAL DISC DRIVE
Fig. 2. Data acquisition hardware. the new system was a factor of 2 smaller than for the old system . To investigate further the indicated improvement in precision given by the new system, a 'Co solution was used to provide data for a detailed comparison. Eight sources were prepared, four providing counting rates of typically between 10000 and 19000 s - ' for the different discrimination settings in the 4?r channels and 1100 s - ' in the y channel. Four sources to provide low counting rates were also prepared . Here the 477 rate varied typically between 1500 and 2500 s- ' and the y channel rate was typically 160 s- '. For the higher activity sources, a single count of 800 s duration was made on each source using the new system, while five 800 s counts were made on each source with the previous system . Thus the counting statistics for each datum point was the same for each system. In the case of the low activity sources, the counting time was extended from 800 to 1800 s. Hence the statistical fluctuation of the experimental data was only a factor of 2 worse than for the higher activity sources. During the data analysis, plots of the 477 count rate versus counting efficiency were generated. An analysis of the efficiency formulae indicated that a first order
polynomial was most suitable for fitting the data. This was verified by the analysis of the new system data which indicated that a straight line did indeed provide the best fit. The extrapolated value corresponding to 100% efficiency was then used to calculate the activity of the 6° Co solution . The activity values, as measured by the various data sets, all agreed within experimental error, there being a 0.25% difference between the highest and lowest values . Table 1 Reduced chi-square values Previous system
New system
2 3 4
0.60 0.67 2 .82 2.16
0 .12 0.45 0 .20 0.18
5 6 7 8
0 .82 2 .03 0 .64 0 .47
0 .73 0.64 0 .33 1 .02
Source High rate
Low rate
1
B .R.S. Simpson, B. R . Meyer / A multiple-channel coincidence counting system
439
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NEW 5
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1.0
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2.0
1 .2
1.4
1.0
1.5
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Fig. 3. Residuals of a first order polynomial fit to the data for the old 3-fold coincidence unit (on the left), and the new 15-fold coincidence unit (on the right) . The top eight sets (numbered 1 to 4) are from the high activity sources and the bottom eight (numbered 5 to 8) are from the low activity sources (note the scale change). The data are plotted as a function of the inverse of the efficiency . A measure of the precision of the counting systems is provided by calculating the residuals between the data points and the linear fit. The residuals are shown graphically in fig. 3, where it is clear that for the new system there is a reduction in the spread of the residuals indicating less scatter in the data. This applies to both the high and low count rate measurements . The improved precision is demonstrated more clearly in table 1 which shows the reduced chi-square values given by the
fits . The increased spread in the residuals as given by the previous counting system can be ascribed to the fact that each set of three data points can be offset because a different gamma measurement is used in calculating the efficiency . This results in additional scatter over and above the statistical uncertainty of each point. To summarise, the improved precision of the new counting system reduces the uncertainty due to the fitting procedure which is often the main source of the
440
B.R.S. Simpson, B.R. Meyer / A multiple-channel coincidence counting system
category B uncertainty [5,6] in absolute activity measurements. Acknowledgement Thanks are due to Messrs . D.A. Raave and V.C. Wikner of the NAC for the design and construction of the coincidence unit and to Mr. J.V. Pilcher, also of the NAC, for his assistance in developing the control portion of the data acquisition program .
References [1] A.P . Baerg, Nucl . Instr. and Meth. 112 (1973) 143. [2] Monograph BIPM-3, The Application of Liquid-Scintillation Counting to Radionuclide Metrology, Bureau International Des Poids et Mesures (1980). [3] A. Rytz, Int. J. Appl . Radiat . Isot . 34 (1983) 1047 . [4] BIPM report CCEMRI (11)/87-7, to be published. [5] P. Giacomo, Metrologia 17 (1981) 69 . [6] J.W . Miller, Precision Measurements and Fundamental Constants, vol. 2, eds., B.N . Taylor and W.D . Phillips, Nad. Bur. Stand. (US) Spec . Publ . 617 (1984) p. 375.
Nuclear Instruments and Methods in Physics Research A263 (1988) 436-440 North-Holland, Amsterdam
A MULTIPLE-CHANNEL 2- AND 3-FOLD COINCIDENCE COUNTING SYSTEM FOR RADIOACTIVITY STANDARDIZATION B.R .S . SIMPSON and B.R . MEYER National Accelerator Centre, PO Box 72, Faure, 7131 South Africa
Received 18 September 1986 and in revised form 10 August 1987
The advantages of a multiple-channel coincidence unit for 47x/3-y liquid scintillation radioactivity standardization are discussed. A microcomputer-controlled counting system is described, and compared with an older 3-channel coincidence system, against which it has been tested. The multiple-channel system shows potential for greater precision.
1. Introduction During recent years the National Accelerator Centre of South Africa has made use solely of liquid scintillation counting for routine absolute radioactivity measurement. Usually the 41TP-y coincidence extrapolation method [1] with 15 data points is used, this number of points being considered sufficient for accurate polynomial fitting. The counting efficiency is varied electronically by setting different discrimination levels in one of the /3-channels . TSCA modules are used for this purpose. Until recently, only three data points could be measured simultaneously and the procedure had to be repeated five times to obtain the required 15 data points per sample . The ideal situation would be to measure all 15 data points simultaneously . The advantages of such a system over the previous one are: (i) Data collection is much quicker, making the system more convenient for routine counting. (ii) Individual point scatter is reduced, because the same gamma count is used for determining the efficiencies (coincidence to gamma count ratios) of the 15 data points, thus facilitating a more reliable selection of the trend of the data points to be fitted. (iii) Any minor systematic fault or timing problem in a particular TSCA/COINC UNIT channel will affect only a single point and not five, as with the 3-channel coincidence system . (iv) Reproducibility with respect to repeat and corresponding background measurements is enhanced, because the discrimination levels on the TSCA modules can remain fixed once set. (v) It will facilitate greater statistical accuracy for short-lived isotopes with half-lives of the order of hours 0168-9002/88/$03 .50 © Elsevier Science Publishers B.V . (North-Holland Physics Publishing Division)
(e.g.
1231)
because more sources can be counted in a relatively short period. This will ensure sufficiently high count rates without the need for more radioactive solution having to be added to the scintillator liquid, where problems such as dissolving, quenching and adsorption might occur. There will also be less dependence on decay corrections . The following is a description of the 15-channel coincidence counting system which is now in operation, with emphasis placed on the performance .
2. The counting system The counting system consists of two RCA8850 photomultiplier tubes viewing a cylindrical sample container, the flat ends being directly coupled to the tube faces [2]. These phototubes are used to detect charged particles and low energy X-rays. A 75 x 75 mm NaI(TI) crystal coupled to a photomultiplier detects the photons . The signals from the detectors are shaped in the usual way before being sent to the TSCA modules. The slow logic output signals provide the input to the coincidence unit as shown in fig. 1 . This 15-channel 2- and 3-fold coincidence unit was designed and built at the NAC. The output signals from the coincidence unit are interfaced to a 32-channel scaler module through a patch panel (fig. 2) . The scaler only counts signals when the timer has been activated. All communication between the desktop computer and the CAMAC modules is through a CAMAC-to-GPIB interface module. A computer program written in Basic activates the
437
B.R.S. Simpson, B.R . Meyer / A multiple-channel coincidence counting system 01
P2
PRE-AMPS
ORTEC 113
O TEC 113
ORTEC 113
LINEAR AMPS
ORTEC 410
OR TEC 10
ORTEC 410
INPUTS
TSCA LEVEL OISCRS
15-FOLD COINCIDENCE UNIT
FIXED B IN
Il
v 551
VARIABLE B IN
f __-_--
IN COINCIDEN E BF-B V COINC DENCES
OUTPUTS
DATA ACQUISITION SYSTEM Fig. 1 . Block diagram of the electronic configuration. software programmable timer and the 24-bit counters . The counters are read and reset by the program, which also checks for counter overflows and takes the appropriate action should these occur. The counter contents are transferred to data files and stored on flexible disc . When the counting system became operational, a test was carried out using a pulse generator . This provided signals through a linear amplifier and constant fraction TSCA modules to all the inputs of the 15-channel coincidence unit . The double and triple coincidence count rates were tested up to 75 000 s -t and were all exactly the same, as expected for fixed frequency pulses . A second test was made with random pulses from t39Ce mixed with a liquid scintillator in a counting cell . The outputs used were taken from the two photomultiplier tubes viewing the cell . One of these was fed into the inputs G and B t and the other into each of the B,, inputs . In principle the triple and B-B coincidence counting rates should be identical . In practice small differences up to a maximum of 0 .005% were found . These were possibly due to random losses or gains as a result of relative timing effects within the coincidence unit and were considered as negligible . Coincidence resolving times for the fifteen channels
were determined by mixing two independent -y-pulse trains, the one feeding the 4w inputs of the coincidence circuit and the other feeding only the y input . They were typically 0.57 tts .
3 . Comparison The viability of the new system has been demonstrated by standardizing a number of radioactive solutions . A standardized 17 Ga sample was sent to the International Reference System (SIR) of the Bureau International des Poids et Mesures (BIPM) for comparative measurement in their ionization chamber [3], where an excellent result was achieved . The recent international comparison of t°9 Cd activity measurements organised by the BIPM also afforded an opportunity for using the new system and again the results compared favourably with those of the other participating laboratories [4] . A direct comparison of the new system against the previous one had previously been undertaken using a 139Ce solution . The measured activity as given by the two counting systems agreed within experimental error, and the results indicated that the uncertainty given by
B. R. S. Simpson, B. R . Meyer / A multiple-channel coincidence counting system
438
OUTPUT FROM THE COINCIDENCE UNIT
N W J m Q V
Z0m m
O
1
2 O
3 O
4 O
O 17
O 18
O 19
O 20
32 CHANNEL SCALER IN VETO 17-32 IN 1-16
O
O O O O PATCH PANEL (BNC CO O O O O O 21 22 23 24 25
1 111213 O O O O NECTORS) O O O O 26 27 28 29
14 O
15 O
16 O
O 30
O 31
O 32
LEVEL ADAPTOR NIM/TTL COMPL .
STATION 1
STATION 2 CAMAC MODULES
DESKTOP COMPUTER AND DUAL DISC DRIVE
Fig. 2. Data acquisition hardware. the new system was a factor of 2 smaller than for the old system . To investigate further the indicated improvement in precision given by the new system, a 'Co solution was used to provide data for a detailed comparison. Eight sources were prepared, four providing counting rates of typically between 10000 and 19000 s - ' for the different discrimination settings in the 4?r channels and 1100 s - ' in the y channel. Four sources to provide low counting rates were also prepared . Here the 477 rate varied typically between 1500 and 2500 s- ' and the y channel rate was typically 160 s- '. For the higher activity sources, a single count of 800 s duration was made on each source using the new system, while five 800 s counts were made on each source with the previous system . Thus the counting statistics for each datum point was the same for each system. In the case of the low activity sources, the counting time was extended from 800 to 1800 s. Hence the statistical fluctuation of the experimental data was only a factor of 2 worse than for the higher activity sources. During the data analysis, plots of the 477 count rate versus counting efficiency were generated. An analysis of the efficiency formulae indicated that a first order
polynomial was most suitable for fitting the data. This was verified by the analysis of the new system data which indicated that a straight line did indeed provide the best fit. The extrapolated value corresponding to 100% efficiency was then used to calculate the activity of the 6° Co solution . The activity values, as measured by the various data sets, all agreed within experimental error, there being a 0.25% difference between the highest and lowest values . Table 1 Reduced chi-square values Previous system
New system
2 3 4
0.60 0.67 2 .82 2.16
0 .12 0.45 0 .20 0.18
5 6 7 8
0 .82 2 .03 0 .64 0 .47
0 .73 0.64 0 .33 1 .02
Source High rate
Low rate
1
B .R.S. Simpson, B. R . Meyer / A multiple-channel coincidence counting system
439
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NEW 5
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e 1 .4
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1.5
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Fig. 3. Residuals of a first order polynomial fit to the data for the old 3-fold coincidence unit (on the left), and the new 15-fold coincidence unit (on the right) . The top eight sets (numbered 1 to 4) are from the high activity sources and the bottom eight (numbered 5 to 8) are from the low activity sources (note the scale change). The data are plotted as a function of the inverse of the efficiency . A measure of the precision of the counting systems is provided by calculating the residuals between the data points and the linear fit. The residuals are shown graphically in fig. 3, where it is clear that for the new system there is a reduction in the spread of the residuals indicating less scatter in the data. This applies to both the high and low count rate measurements . The improved precision is demonstrated more clearly in table 1 which shows the reduced chi-square values given by the
fits . The increased spread in the residuals as given by the previous counting system can be ascribed to the fact that each set of three data points can be offset because a different gamma measurement is used in calculating the efficiency . This results in additional scatter over and above the statistical uncertainty of each point. To summarise, the improved precision of the new counting system reduces the uncertainty due to the fitting procedure which is often the main source of the
440
B.R.S. Simpson, B.R. Meyer / A multiple-channel coincidence counting system
category B uncertainty [5,6] in absolute activity measurements. Acknowledgement Thanks are due to Messrs . D.A. Raave and V.C. Wikner of the NAC for the design and construction of the coincidence unit and to Mr. J.V. Pilcher, also of the NAC, for his assistance in developing the control portion of the data acquisition program .
References [1] A.P . Baerg, Nucl . Instr. and Meth. 112 (1973) 143. [2] Monograph BIPM-3, The Application of Liquid-Scintillation Counting to Radionuclide Metrology, Bureau International Des Poids et Mesures (1980). [3] A. Rytz, Int. J. Appl . Radiat . Isot . 34 (1983) 1047 . [4] BIPM report CCEMRI (11)/87-7, to be published. [5] P. Giacomo, Metrologia 17 (1981) 69 . [6] J.W . Miller, Precision Measurements and Fundamental Constants, vol. 2, eds., B.N . Taylor and W.D . Phillips, Nad. Bur. Stand. (US) Spec . Publ . 617 (1984) p. 375.