- Email: [email protected]
beta low-level counting systems for solid samples
501
Nuclear Instruments and Methods in Physics Research B17 (1986) 501-505 North-Holland, Amsterdam
PERFORMANCE
OF ALPHA/BETA
LOW-LEVEL
COUNTING
SYSTEMS
FOR SOLID
SAMPLES
R. MAUSHART Luboratorium
Prof. Dr. Be&old,
D-7546 Wdbad
I, FRG
The characteristics of currently available low-level systems for beta and alpha/beta
1. The development so far
Low-level counting systems for solid samples have been an indispensable tool for monito~ng the radioactivity in the environment for decades. The task is generally to detect smallest absolute amounts of radioactivity, and this frequently in very tow concentration in inactive carrier material and with a large number of samples. One of the variables determining the detection limit to be achieved by a system is the background count rate. Low-level systems are therefore characterized by three measures intended to achieve a low background count rate: shielding against external gamma radiation; anticoincidence screen to reduce the hard cosmic radiation component; and selection of construction materials according to inherent contamination. The detection limit can also be improved to a certain degree by counting more material per sample. This leads to the requirement for large-area detectors. The large-area proportional counter tube was developed in Karlsruhe primarily to solve this problem as far back as 1958 [l] and then improved in subsequent years [2]. This type of detector is still used predominantly in low-level systems, and exclusively for large-diameter samples (standard up to 200 mm). This was joined later by the dual scintillation (“Phoswich”) detector [3] as an alternative. Apart from technical improvements in details, however, low level counting systems have not changed to any great extent for a long time. It is only with the progress and radical change that has occurred in the field of electronics in the last few years that new possibilities have been opened up to low level counting technology. The main achievements are - decisive improvements in simultaneous, separate counting of the alpha and beta components [4]; - multichannel systems for simultaneous counting of 4, 8 or 10 samples as an alternative to conventional automatic sample changers [S]; - the use of micropr~essors and minicomputers offers Ol~S-583X/86/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
measurements are described.
the opportunity for improved data analysis and statistical checks, semi-automatic calibration and, not to be underestimated, for data reduction with low level counting systems. In the following sections the essential characteristics of currently available low level systems for beta or alpha/beta measurements will be described. Pure alpha counting systems are not included since almost all of them are not gross activity counters but spectroscopy systems and therefore subject to quite different conditions and requirements.
2. Mechanics, shielding and detectors in low level counting systems There is a basic distinction between manual and automatic sample changing systems. Manual systems can be subdivided into systems for single and multiple samples whereas the automatic changer systems are distinguished by the type of transport system used and subdivided into chain-type changers and stack-type changers. Because of space requirements, chain-type changers are restricted to small planchet diameters whereas stack-type changers are available for planchet diameters up to 200 mm and for 100 and more samples. Overall, manual changers tend to have a lower background count rate because the lead shielding can be arranged to form a more solid enclosure (there are no ducts for the mechanical drive, the opening through which the planchets are inserted can be shut off more easily). In practice, however, this difference can get lost due to the effect of different designs on the background count rate (see also the section on background count rate). The shielding may consist of lead or steel. Standard lead bricks are mainly used. The shielding thicknesses lie between 50 an 100 mm, typical weights for the overall units area are 100-200 kg for small planchet diameters and 600-900 kg for large planchet diameters. It is worth noting that a heavier lead shielding does not V. COUNTIN~/SPECTROMETRY
R. Maushart
502
/ Low-level counting systems for solid samples
Table 1 Detectors used in low level counting systems Scintillation detector (Fhoswichf
up to a planchet diam. of 125 mm
Proportional counter tube Sealed
Throu&-flow
up to a planchet diam. of 50 mm
all sizes
up to a plan&et diam. of 200 mm
necessarily lead to a lower background count rate. This is largely determined by careful sealing of the openings and fed-tbrou~ holes for cables, gas pipes and sample slide. Table I shows the usual detector types, the dual scintillation detector (Phos~~h) and the proportional counter tube and their main areas of use. The performance data are compared in the following sections. In systems using the proportional gas flow detector, which should also or mainly be used for counting alpha radiation, one sometimes finds what are called “gastight” versions which enable the samples to be counted without a window. However, the advantage of higher efficiency with the alpha measurement is counterbalanced by several serious disadvantages of a technical nature such as increased gas consumption, increase of the overall counting time due to the fact that inital flushing is required, instabilities in the count due to possible gas or vapour escape from the samples and the risk of internal detector ~ont~nation by samples in powder form. In many cases it is possible to achieve an efficiency of approximately 30% and better even for alpha samples using low planchets and positioning the samples in a suitable manner as close to the detector window as possible, so that the advantage achieved with windowless systems hardly justifies the disadvantages mentioned. An undesirable feature of automatic changers and multi-sample systems is the possibility of “cross-talk’ from adjacent ~gher-a~ti~ty samples to the background count rate at the punting position. However, in practice the risk is much less than that known from gamma changers since it is possible to eliminate crosstalk from beta radiation with a suitable design, and since the inherent efficiency of the counter for gamma radiation is low. Brochures therefore only rarely give any indication as to the cross-talk factor; however, it should be known if high- and low-activity samples have to be expected in a test series. For m~ti-s~ple systems with through flow detectors the typical values lie around 10N3, for an automatic changer system for 50-mm planchets they lie around 10M4 j137Cs).
3. Background count rates, efficiencies and detection limits Table 2 gives a summary of the essential factors deterring the baekground count rate in a low level coun~g system. An analysis of the data given in brochures for different systems on the market shown in fig. 1 illustrates the high degree to which the individual possibilities for achieving a low background count rate have already been refined. In spite of widely different construction and types of detector the background count rates lie in a relatively narrow band which can roughly be described by 0.06 cpmcm-* of detector area. The systems using scintillation detectors, however lie at the upper edge of the band. The scatter becomes wider For systems for 2OO-mm planchets where the background count rates lie between approximately 35 min-’ and less than 10 mm-’ [9]. In order to obtain pa~c~arly low background count and rates, the three factors - shielding, anti~incidence material selection - need to be balanced very carefully. As also shown by commercially available systems, this enables the specific beta background count rate to be lowered to approx. 0.035 min-‘cmS2 for all planchet sizes. Optimum commercial values lie at 0.03-0.04 min - rem- *. The manufacturer can select the detector material but not the sample planchet material. However, when values are low, the background count rate of the sample planchets themselves becomes clearly felt. In one test series, for instance, the background deter~ned for empty planchet positions was 0.86 ruin-‘, for alu~~um planchets (0 60 mm) 2.63 mm-! and for steel planchets 1.14 min-‘. These may be haphazard values but they show the magnitude of the effects on the background count rate to be expected. It is more difficult to provide a comparison of the alpha background count rates for the various systems. They often exhibit a wide scatter from one unit to another even for the same type of equipment. With 50/60-mm plancbets the values range between 0.04 and 0.4 mm-‘, for 200~mm planehets between 0.5 and 3 mm-‘. When the detection limit is defined in accordance
Table 2 Beta background count rate components Cause
Remedy
Ambient gamma radiation Hard component of cosmic radiation Inherent conta~nation of the detector material Inherent contamination of sample pianchets
Lead shielding Anticoincidence circuit Material selection! Material selection?
503
R. Maushart / Low-level counting systems for solid samples
a.10
20
0 05
10
lm?
5
0.01 0.5
1
2
5
10
20
2 Fig.
2. Detection background
limit (30 accuracy) as a function of the count rate for a counting time of 1 h.
7
0,5
Planchet Diameter mm
I I 25 30
II I 50 60 80
II I 130144 200
Fig. 1. Beta background count-rates of different low level systems as a function of the planchet diameter. The background count-rates represented here have been taken from company literature. The top two curves span the range for single-channel manual and automatic sample changers. The lower curve which is near the values for the lo-channel manual
this score the systems with scintillation counters do considerably less well than those with proportional gas flow counter tubes (10% 14C efficiency with the Phoswith detector as compared with 30-35% with gas flow counter tubes). The background ranges encountered in practice for different planchet diameters have been entered in fig. 2 as shaded bars. If one also takes into account the effect of efficiency then one can see that the detection limits for equipment with 200-mm planchets can lie between 0.05 Bq and approximately 0.12 Bq, the difference being a factor of 2.5. With 50/60-mm planchets the detection limit drops to below 0.02 Bq, with 25/50-mm planchets to below 0.01 Bq.
changer corresponds to a specific background count rate of 0.035 min-‘cm-2 X [7]; 0 [S]; + [IO]; 0 [ll]; 0 [12]; 0 [13]. 4. Low level counting systems for simultaneous separate alpha/ beta measurement with the known formula
then one can see immediately that the quality of the low level counting system depends to a greater extent on the efficiency 7 than on the background count rate no. Add to this the fact that the background is a fixed, as of were “built-in” quantity, while the efficiency can be affected to a certain degree by the system user by sensible sample preparation and positioning. The counting efficiency in fig. 2 has been varied as a parameter. The shaded bands mark a spread of the beta background count rates of different systems. It can be seen from fig. 2 that, for systems with 50-mm planchets, the same detection limit is obtained with a background of of 1 min-’ at 30% efficiency as with a background 1.9 n-tin-’ and 40% efficiency. For this reason it is also not very useful to compare theoretical efficiency data of various systems. However, even for samples which are not ideal, a good efficiency for the low energy components of beta radiation is always a precondition for optimum system efficiency; this can be recognised by a good efficiency for 14C. On
More recent developments in pulse shape discrimination in proportional counter tubes for the separation of alpha and beta radiation [4] have made this application of topical interest for low level systems, too. Table 3 lists the various possibilities and shows a comparison of the performance data for commercially available systems. Data based on in-house measurements and company literature [7-91 have been used. 90Sr/90Y (beta) and *i’Po (alpha) have been used as radioactive test sources. Systems with dual detector have been on the market for quite some time. While they exhibit virtually ideal separation factors, they suffer the severe disadvantage that beta radiation can be measured only with an initial absorption which corresponds to a complete absorption of the alpha radiation and thus a layer thickness of approximately 6-7 mg cm-*. This initial absorption virtually precludes the measurement of 14C beta radiation completely which means that, for beta measurements, this system is highly energy-dependent and exhibits a poor efficiency. This is why the dual detector was not successful. Alpha/beta V. COUNTING/SPECTROMETRY
Table 3 Separation factors for different low level counting systems for simultaneous alpha/beta Proportional gas flow detector
Beta in alpha Alpha in beta
measurement Phoswich scintillation detector
Dual detector
Pulse amplitude separation
Pulse shape separation
Pulse amplitude separation
0% <10-r%
1% 3%
1.5% 10%
separation by means of pulse arn~~~~dc analysis, both in the proportional detector and in the scintillation detector, is frequently used but suffers of the relatively high mutual cross-talk factors. In this context the scintillation detector provides the poorest results, another factor being that, unlike the proportional counter, it carmot be operated purely in the alpha region, i.e. when beta sources are present, an alpha measurement is virtually impossible. It is only with pulse shape separation in the proportional detector that results can be obtained which are useful for actual operating practice. The cross-talk from beta radiation into the alpha channel is virtually zero, which is of particular importance for low level measurements. On the other hand, it is difficult to define the cross-talk rate for alpha radiation into the beta channel. On the one hand most alpha sources such as pluto~um 239 or americium 241 also emit gamma or X-ray radiation which is absorbed in the detector with a probability which depends on their energy, and interpreted as beta radiation. This incorrectly suggests alpha cross-talk into the beta channel. Even with pure alpha sources, e.g. *“PO the degree of cross-talk into the beta channel depends on the initial absorption of aIpha particles. When cross-talk is critical, the user needs to determine the cross-talk factor with genuine sources, i.e. sources which correspond to his measuring conditions with regard to condition and nuclide type. When assessing the circumstances one needs to take into account that even with conventions alpha/beta measurement used so far, i.e. with subsequent measurements first in the alpha and then in the alpha f beta plateau, identical circumstances prevail, i.e. in this case also the values determined first in the alpha plateau had to be weighted by an appropriate factor if they were to be deducted correctly from the alpha i- beta count rate.
Systems like this which have come onto the market reccmly and which permit the simultaneous and separate measurement of several samples offer an interesting alternative to conventional automatic changers [5].
Un the one hand this reduces the totai measuring time in which 10 samples are counted - by the factor 10 while retaining the same accuracy. Compared to other systems this permits 10 times the sample thro~gbput during working hours. On the other hand it is possible to recognise samples of markedly higher activity-outliers - either immediately or shortly after starting the measurement without having to wait for the entire measuring cycle of an automatic sample changer to be completed. Both features can be of very great advantage, in particular in the case of incidents, where saving time is important. These systems also obviate the need for m~h~ically moving components which means improved reliability and freedom from maintenance for the entire system. These systems can be built with very thin detectors which results in a good efficiency both with regard to the shielding against external gamma radiation and as regards the anticoincidence effect. By using low-activity material in the const~ction of the system, back~ound count rates of less than I rnin-’ are obtained for 60-mm planchets and of less than 0.3 rnin-’ for 30-mm planchets; compared to conventional systems these are very good levels, Fig. 3 shows the detection &nits which can be obtained as a function of cubing time.
Fig. 3. Detection limit obtained with a bac~~ound of I tin-’ and an efficiency of 44% (beta) and 0.05 ruin-r and 26% (alpha), respectively, plotted as a function of counting time.
R. Mawhart
505
/ Low-level counting systems for solid samples
6. Connection of microprocessors and minicomputers Nowadays almost all low-level systems on the market use microprocessor-controlled counter/timer systems which enable at least the background to be deducted automatic~ly and the raw data to be computed into absolute or specific activity units by using calibration
factors. With an acceptable amount of engineering effort one can go further and use freely programmable minicomputers, for instance to calculate the statistical reliability of individual counts, to compare samples with each other or to form histograms from larger numbers of samples or mean values of certain sample groups. The possibility of including semi-automatic calibration is also of particular interest. For this known samples are brought to the detector and the computer then automatically determines the calibration factor from these counts and applies them to later samples. Depending on the requirements of each individual problem set, the number of possible variations for the evaluation is virtually unlimited. Even with sample identification - which has always been somewhat problematic - there are promising developments on the horizon. The samples are coded individually and the counting system contains a reader for this code which is transmitted to the computer together with the count. The code can simultaneously carry data relating to further sample characteristics; for instance with air filters they can relate to location and
duration of exposure to dust or air throughput. With such systems it no longer matters in which sequence the samples were introduced into the changer and mix-ups due to incorrect allocation are therefore impossible. References [l] H. Kiefer and R. Maushart, Nukleonik 1 (1958) 103. [2] H. Kiefer, Preprint, KfK 411 (1966). [3] M.R. Mayhugh, A.C. Lucas and B.K. Utts, The IEEE Nuclear Science Symposium (1977). [4] H. Kiefer, B. Reinbardt, H.G. Rober and S. Ugi, 16 Jahrestagung des Fachverbands ftir Strahlenschutz (Mtinthen 1982) FS-83-30-T, GSF-Bericht A 4/83, [5] R. Maushart, Fachtagung Nuclex (Basel, 1981). [6] Richtlinien zur Emissions-und Immissionstiberwachung kemtechnischer Anlagen, GMBl Nr. 32 (1979). 171 Passeur d’&chantillons NU 15, Firmenschrift Numelec, Versailles Cedex, France (without data). [8] Task-12 Alpha-beta counting system (Harshaw Comp., Cleveland, 1982). [9] Low level counting system LB 761 GD for simultaneous, separate alpha-beta measurement (Berthold Comp., Wildbad, 1983). [lo] Automatischer Probenwechsler FHT 770 M (FAG Kugelfischer Comp., Erlangen, 1979). [ll] Automatic low level sample changer for planchets LB 100 L, (Berthold Comp., Wildbad, 1979). [12] Low level manual sample changer for planchets LB 750 LB 760 GD (Berthold, Wildbad, 1979). 1131 IO-Channel low level planchet counting system, LB 770, (Berthold Comp., Wildbad, 1981).
V. CGUNTING/SPE~ROMETRY
Nuclear Instruments and Methods in Physics Research B17 (1986) 501-505 North-Holland, Amsterdam
PERFORMANCE
OF ALPHA/BETA
LOW-LEVEL
COUNTING
SYSTEMS
FOR SOLID
SAMPLES
R. MAUSHART Luboratorium
Prof. Dr. Be&old,
D-7546 Wdbad
I, FRG
The characteristics of currently available low-level systems for beta and alpha/beta
1. The development so far
Low-level counting systems for solid samples have been an indispensable tool for monito~ng the radioactivity in the environment for decades. The task is generally to detect smallest absolute amounts of radioactivity, and this frequently in very tow concentration in inactive carrier material and with a large number of samples. One of the variables determining the detection limit to be achieved by a system is the background count rate. Low-level systems are therefore characterized by three measures intended to achieve a low background count rate: shielding against external gamma radiation; anticoincidence screen to reduce the hard cosmic radiation component; and selection of construction materials according to inherent contamination. The detection limit can also be improved to a certain degree by counting more material per sample. This leads to the requirement for large-area detectors. The large-area proportional counter tube was developed in Karlsruhe primarily to solve this problem as far back as 1958 [l] and then improved in subsequent years [2]. This type of detector is still used predominantly in low-level systems, and exclusively for large-diameter samples (standard up to 200 mm). This was joined later by the dual scintillation (“Phoswich”) detector [3] as an alternative. Apart from technical improvements in details, however, low level counting systems have not changed to any great extent for a long time. It is only with the progress and radical change that has occurred in the field of electronics in the last few years that new possibilities have been opened up to low level counting technology. The main achievements are - decisive improvements in simultaneous, separate counting of the alpha and beta components [4]; - multichannel systems for simultaneous counting of 4, 8 or 10 samples as an alternative to conventional automatic sample changers [S]; - the use of micropr~essors and minicomputers offers Ol~S-583X/86/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
measurements are described.
the opportunity for improved data analysis and statistical checks, semi-automatic calibration and, not to be underestimated, for data reduction with low level counting systems. In the following sections the essential characteristics of currently available low level systems for beta or alpha/beta measurements will be described. Pure alpha counting systems are not included since almost all of them are not gross activity counters but spectroscopy systems and therefore subject to quite different conditions and requirements.
2. Mechanics, shielding and detectors in low level counting systems There is a basic distinction between manual and automatic sample changing systems. Manual systems can be subdivided into systems for single and multiple samples whereas the automatic changer systems are distinguished by the type of transport system used and subdivided into chain-type changers and stack-type changers. Because of space requirements, chain-type changers are restricted to small planchet diameters whereas stack-type changers are available for planchet diameters up to 200 mm and for 100 and more samples. Overall, manual changers tend to have a lower background count rate because the lead shielding can be arranged to form a more solid enclosure (there are no ducts for the mechanical drive, the opening through which the planchets are inserted can be shut off more easily). In practice, however, this difference can get lost due to the effect of different designs on the background count rate (see also the section on background count rate). The shielding may consist of lead or steel. Standard lead bricks are mainly used. The shielding thicknesses lie between 50 an 100 mm, typical weights for the overall units area are 100-200 kg for small planchet diameters and 600-900 kg for large planchet diameters. It is worth noting that a heavier lead shielding does not V. COUNTIN~/SPECTROMETRY
R. Maushart
502
/ Low-level counting systems for solid samples
Table 1 Detectors used in low level counting systems Scintillation detector (Fhoswichf
up to a planchet diam. of 125 mm
Proportional counter tube Sealed
Throu&-flow
up to a planchet diam. of 50 mm
all sizes
up to a plan&et diam. of 200 mm
necessarily lead to a lower background count rate. This is largely determined by careful sealing of the openings and fed-tbrou~ holes for cables, gas pipes and sample slide. Table I shows the usual detector types, the dual scintillation detector (Phos~~h) and the proportional counter tube and their main areas of use. The performance data are compared in the following sections. In systems using the proportional gas flow detector, which should also or mainly be used for counting alpha radiation, one sometimes finds what are called “gastight” versions which enable the samples to be counted without a window. However, the advantage of higher efficiency with the alpha measurement is counterbalanced by several serious disadvantages of a technical nature such as increased gas consumption, increase of the overall counting time due to the fact that inital flushing is required, instabilities in the count due to possible gas or vapour escape from the samples and the risk of internal detector ~ont~nation by samples in powder form. In many cases it is possible to achieve an efficiency of approximately 30% and better even for alpha samples using low planchets and positioning the samples in a suitable manner as close to the detector window as possible, so that the advantage achieved with windowless systems hardly justifies the disadvantages mentioned. An undesirable feature of automatic changers and multi-sample systems is the possibility of “cross-talk’ from adjacent ~gher-a~ti~ty samples to the background count rate at the punting position. However, in practice the risk is much less than that known from gamma changers since it is possible to eliminate crosstalk from beta radiation with a suitable design, and since the inherent efficiency of the counter for gamma radiation is low. Brochures therefore only rarely give any indication as to the cross-talk factor; however, it should be known if high- and low-activity samples have to be expected in a test series. For m~ti-s~ple systems with through flow detectors the typical values lie around 10N3, for an automatic changer system for 50-mm planchets they lie around 10M4 j137Cs).
3. Background count rates, efficiencies and detection limits Table 2 gives a summary of the essential factors deterring the baekground count rate in a low level coun~g system. An analysis of the data given in brochures for different systems on the market shown in fig. 1 illustrates the high degree to which the individual possibilities for achieving a low background count rate have already been refined. In spite of widely different construction and types of detector the background count rates lie in a relatively narrow band which can roughly be described by 0.06 cpmcm-* of detector area. The systems using scintillation detectors, however lie at the upper edge of the band. The scatter becomes wider For systems for 2OO-mm planchets where the background count rates lie between approximately 35 min-’ and less than 10 mm-’ [9]. In order to obtain pa~c~arly low background count and rates, the three factors - shielding, anti~incidence material selection - need to be balanced very carefully. As also shown by commercially available systems, this enables the specific beta background count rate to be lowered to approx. 0.035 min-‘cmS2 for all planchet sizes. Optimum commercial values lie at 0.03-0.04 min - rem- *. The manufacturer can select the detector material but not the sample planchet material. However, when values are low, the background count rate of the sample planchets themselves becomes clearly felt. In one test series, for instance, the background deter~ned for empty planchet positions was 0.86 ruin-‘, for alu~~um planchets (0 60 mm) 2.63 mm-! and for steel planchets 1.14 min-‘. These may be haphazard values but they show the magnitude of the effects on the background count rate to be expected. It is more difficult to provide a comparison of the alpha background count rates for the various systems. They often exhibit a wide scatter from one unit to another even for the same type of equipment. With 50/60-mm plancbets the values range between 0.04 and 0.4 mm-‘, for 200~mm planehets between 0.5 and 3 mm-‘. When the detection limit is defined in accordance
Table 2 Beta background count rate components Cause
Remedy
Ambient gamma radiation Hard component of cosmic radiation Inherent conta~nation of the detector material Inherent contamination of sample pianchets
Lead shielding Anticoincidence circuit Material selection! Material selection?
503
R. Maushart / Low-level counting systems for solid samples
a.10
20
0 05
10
lm?
5
0.01 0.5
1
2
5
10
20
2 Fig.
2. Detection background
limit (30 accuracy) as a function of the count rate for a counting time of 1 h.
7
0,5
Planchet Diameter mm
I I 25 30
II I 50 60 80
II I 130144 200
Fig. 1. Beta background count-rates of different low level systems as a function of the planchet diameter. The background count-rates represented here have been taken from company literature. The top two curves span the range for single-channel manual and automatic sample changers. The lower curve which is near the values for the lo-channel manual
this score the systems with scintillation counters do considerably less well than those with proportional gas flow counter tubes (10% 14C efficiency with the Phoswith detector as compared with 30-35% with gas flow counter tubes). The background ranges encountered in practice for different planchet diameters have been entered in fig. 2 as shaded bars. If one also takes into account the effect of efficiency then one can see that the detection limits for equipment with 200-mm planchets can lie between 0.05 Bq and approximately 0.12 Bq, the difference being a factor of 2.5. With 50/60-mm planchets the detection limit drops to below 0.02 Bq, with 25/50-mm planchets to below 0.01 Bq.
changer corresponds to a specific background count rate of 0.035 min-‘cm-2 X [7]; 0 [S]; + [IO]; 0 [ll]; 0 [12]; 0 [13]. 4. Low level counting systems for simultaneous separate alpha/ beta measurement with the known formula
then one can see immediately that the quality of the low level counting system depends to a greater extent on the efficiency 7 than on the background count rate no. Add to this the fact that the background is a fixed, as of were “built-in” quantity, while the efficiency can be affected to a certain degree by the system user by sensible sample preparation and positioning. The counting efficiency in fig. 2 has been varied as a parameter. The shaded bands mark a spread of the beta background count rates of different systems. It can be seen from fig. 2 that, for systems with 50-mm planchets, the same detection limit is obtained with a background of of 1 min-’ at 30% efficiency as with a background 1.9 n-tin-’ and 40% efficiency. For this reason it is also not very useful to compare theoretical efficiency data of various systems. However, even for samples which are not ideal, a good efficiency for the low energy components of beta radiation is always a precondition for optimum system efficiency; this can be recognised by a good efficiency for 14C. On
More recent developments in pulse shape discrimination in proportional counter tubes for the separation of alpha and beta radiation [4] have made this application of topical interest for low level systems, too. Table 3 lists the various possibilities and shows a comparison of the performance data for commercially available systems. Data based on in-house measurements and company literature [7-91 have been used. 90Sr/90Y (beta) and *i’Po (alpha) have been used as radioactive test sources. Systems with dual detector have been on the market for quite some time. While they exhibit virtually ideal separation factors, they suffer the severe disadvantage that beta radiation can be measured only with an initial absorption which corresponds to a complete absorption of the alpha radiation and thus a layer thickness of approximately 6-7 mg cm-*. This initial absorption virtually precludes the measurement of 14C beta radiation completely which means that, for beta measurements, this system is highly energy-dependent and exhibits a poor efficiency. This is why the dual detector was not successful. Alpha/beta V. COUNTING/SPECTROMETRY
Table 3 Separation factors for different low level counting systems for simultaneous alpha/beta Proportional gas flow detector
Beta in alpha Alpha in beta
measurement Phoswich scintillation detector
Dual detector
Pulse amplitude separation
Pulse shape separation
Pulse amplitude separation
0% <10-r%
1% 3%
1.5% 10%
separation by means of pulse arn~~~~dc analysis, both in the proportional detector and in the scintillation detector, is frequently used but suffers of the relatively high mutual cross-talk factors. In this context the scintillation detector provides the poorest results, another factor being that, unlike the proportional counter, it carmot be operated purely in the alpha region, i.e. when beta sources are present, an alpha measurement is virtually impossible. It is only with pulse shape separation in the proportional detector that results can be obtained which are useful for actual operating practice. The cross-talk from beta radiation into the alpha channel is virtually zero, which is of particular importance for low level measurements. On the other hand, it is difficult to define the cross-talk rate for alpha radiation into the beta channel. On the one hand most alpha sources such as pluto~um 239 or americium 241 also emit gamma or X-ray radiation which is absorbed in the detector with a probability which depends on their energy, and interpreted as beta radiation. This incorrectly suggests alpha cross-talk into the beta channel. Even with pure alpha sources, e.g. *“PO the degree of cross-talk into the beta channel depends on the initial absorption of aIpha particles. When cross-talk is critical, the user needs to determine the cross-talk factor with genuine sources, i.e. sources which correspond to his measuring conditions with regard to condition and nuclide type. When assessing the circumstances one needs to take into account that even with conventions alpha/beta measurement used so far, i.e. with subsequent measurements first in the alpha and then in the alpha f beta plateau, identical circumstances prevail, i.e. in this case also the values determined first in the alpha plateau had to be weighted by an appropriate factor if they were to be deducted correctly from the alpha i- beta count rate.
Systems like this which have come onto the market reccmly and which permit the simultaneous and separate measurement of several samples offer an interesting alternative to conventional automatic changers [5].
Un the one hand this reduces the totai measuring time in which 10 samples are counted - by the factor 10 while retaining the same accuracy. Compared to other systems this permits 10 times the sample thro~gbput during working hours. On the other hand it is possible to recognise samples of markedly higher activity-outliers - either immediately or shortly after starting the measurement without having to wait for the entire measuring cycle of an automatic sample changer to be completed. Both features can be of very great advantage, in particular in the case of incidents, where saving time is important. These systems also obviate the need for m~h~ically moving components which means improved reliability and freedom from maintenance for the entire system. These systems can be built with very thin detectors which results in a good efficiency both with regard to the shielding against external gamma radiation and as regards the anticoincidence effect. By using low-activity material in the const~ction of the system, back~ound count rates of less than I rnin-’ are obtained for 60-mm planchets and of less than 0.3 rnin-’ for 30-mm planchets; compared to conventional systems these are very good levels, Fig. 3 shows the detection &nits which can be obtained as a function of cubing time.
Fig. 3. Detection limit obtained with a bac~~ound of I tin-’ and an efficiency of 44% (beta) and 0.05 ruin-r and 26% (alpha), respectively, plotted as a function of counting time.
R. Mawhart
505
/ Low-level counting systems for solid samples
6. Connection of microprocessors and minicomputers Nowadays almost all low-level systems on the market use microprocessor-controlled counter/timer systems which enable at least the background to be deducted automatic~ly and the raw data to be computed into absolute or specific activity units by using calibration
factors. With an acceptable amount of engineering effort one can go further and use freely programmable minicomputers, for instance to calculate the statistical reliability of individual counts, to compare samples with each other or to form histograms from larger numbers of samples or mean values of certain sample groups. The possibility of including semi-automatic calibration is also of particular interest. For this known samples are brought to the detector and the computer then automatically determines the calibration factor from these counts and applies them to later samples. Depending on the requirements of each individual problem set, the number of possible variations for the evaluation is virtually unlimited. Even with sample identification - which has always been somewhat problematic - there are promising developments on the horizon. The samples are coded individually and the counting system contains a reader for this code which is transmitted to the computer together with the count. The code can simultaneously carry data relating to further sample characteristics; for instance with air filters they can relate to location and
duration of exposure to dust or air throughput. With such systems it no longer matters in which sequence the samples were introduced into the changer and mix-ups due to incorrect allocation are therefore impossible. References [l] H. Kiefer and R. Maushart, Nukleonik 1 (1958) 103. [2] H. Kiefer, Preprint, KfK 411 (1966). [3] M.R. Mayhugh, A.C. Lucas and B.K. Utts, The IEEE Nuclear Science Symposium (1977). [4] H. Kiefer, B. Reinbardt, H.G. Rober and S. Ugi, 16 Jahrestagung des Fachverbands ftir Strahlenschutz (Mtinthen 1982) FS-83-30-T, GSF-Bericht A 4/83, [5] R. Maushart, Fachtagung Nuclex (Basel, 1981). [6] Richtlinien zur Emissions-und Immissionstiberwachung kemtechnischer Anlagen, GMBl Nr. 32 (1979). 171 Passeur d’&chantillons NU 15, Firmenschrift Numelec, Versailles Cedex, France (without data). [8] Task-12 Alpha-beta counting system (Harshaw Comp., Cleveland, 1982). [9] Low level counting system LB 761 GD for simultaneous, separate alpha-beta measurement (Berthold Comp., Wildbad, 1983). [lo] Automatischer Probenwechsler FHT 770 M (FAG Kugelfischer Comp., Erlangen, 1979). [ll] Automatic low level sample changer for planchets LB 100 L, (Berthold Comp., Wildbad, 1979). [12] Low level manual sample changer for planchets LB 750 LB 760 GD (Berthold, Wildbad, 1979). 1131 IO-Channel low level planchet counting system, LB 770, (Berthold Comp., Wildbad, 1981).
V. CGUNTING/SPE~ROMETRY