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A rapid method for the In vivo monitoring of radiocaesium activity in sheep
J. Environ. Radioactivity 7 (1988) 209-214
A Rapid Method for the In vivo Monitoring of Radiocaesium Activity in Sheep R. C. K. Meredith, K. J. Mondon & J. C. Sherlock Food Science Division, Ministry of Agriculture, Fisheries and Food, Great Westminster House, Horseferry Rd, London SW 1P 2AE (Received 21 September 1987; accepted 17 December 1987)
ABSTRACT Following the accident at Chernobyl there was a need to determine caesium activity in individual sheep on farms. This need arose so that animals on farms within a restricted area could be released from restrictions provided the tissue caesium activity was below the recommended limit. This paper describes the development o f a rapid method for the in vivo monitoring of" sheep using a lead-shielded sodium iodide scintillation detector coupled to a portable single channel analyser. The method enables one sheep per minute to be monitored and the results showed a good correlation (R 2 = 93%) with laboratory measurements o f meat from sheep which had previously been monitored in vivo prior to sacrifice.
INTRODUCTION The fallout from Chernobyl resulted in persistent elevated levels of 137Csand 134Cs in herbage in many parts of the United Kingdom, the worst affected areas being Cumbria, north Wales, west Scotland and northern Ireland (Smith & Clark, 1986). Restrictions were placed on the movement of sheep within the worst affected areas in the UK. The method used to assess the suitability for derestriction of zones within the worst affected areas has already b e e n described in the previous paper (Sherlock et al., 1988). By late August, all the zones that could be released from restriction had been released. This left a core of zones which would not meet the criteria of derestriction but which, because of local variations in contamination levels, were likely to contain many sheep with less than 1000 Bq kg -~ in their flesh. 209 © 1988 Crown Copyright
210
R. C. K. Meredith, K, J. Mondon, J. C. Sherlock
It seemed that the only hope for allowing the movement of sheep out of the remaining restricted areas was to wait for activity levels to fall or monitor in v i v o each animal the farmers wished to move. The latter option was chosen since it seemed that activity levels were not falling and many farmers would need to move their sheep from September onwards.
INITIAL A P P R O A C H Work undertaken in June 1986 had demonstrated (Sherlock et al., 1988) that it was practicable to monitor caesium activity in live sheep. However, this work was undertaken using sophisticated equipment requiring highly trained operators and only about 25 sheep per day could be monitored. The remaining zones within the restricted areas contained several hundred thousand sheep. Clearly the existing methodology was inadequate to monitor all the sheep which the farmers wished to move out of the restricted areas. What was required was a cheap and very rapid method of in vivo monitoring which could be undertaken by non-specialist staff. A n initial assessment of portable radiation monitors indicated that e q u i p m e n t based on a 4.4 cm or 5 cm sodium iodide (thallium-doped) lead-shielded scintillation detector coupled to a portable single channel analyser (SCA) might be suitable for the purpose. Two portable instruments were obtained on loan from the manufacturer (John Caunt Scientific Ltd, Eynsham, Oxfordshire, UK); the instrument type was an SD1A SCA coupled to a lead-shielded DH 100 sodium iodide detector. The shielding was 3 m m in thickness and 25 cm in length and covered all the detector except for the face of the crystal. The single channel analysers were set to measure gamma emissions from 137Csand 134Csin the region of 520-700 kev.
METHOD DEVELOPMENT Much of the m e t h o d development was dictated by practical circumstances. The major requirements were for a speedy development of the method and a very rapid rate of measurement. Clearly the longer the activity in the animals could be counted the more accurate would be the measurement but the slower would be the throughput. In addition, the longer the count time the greater the chance of the animal moving and invalidating the measurement. Ideally the sheep would be held in a restricting device like a cattle crush but there was insufficient time to develop a simple portable restraining device which would also permit a rapid throughput of sheep. The simplest
In vivo monitoring of radiocaesium activity in sheep
211
way of restraining the sheep, namely the farmer holding the animal against the side of a pen, wall, or gate, proved effective. The counting time chosen was a compromise between a long time giving accuracy and a short time determined by how long the sheep could be kept still. The most effective counting procedure was to make three 10 second counts on the most fleshy part of the buttock of the sheep and to report the m e a n of the 3 counts. Clearly one 30 second count would have been easier but, whilst the sheep could be held still for 10 seconds on most occasions, it was far more difficult to hold them still for 30 seconds. Thus 10 second counts were less wasteful of the operators' time and, by averaging three separate applications of the detector, provided an additional means of reducing systematic errors.
CALIBRATION Two S D I A instruments and detectors were calibrated by using them to determine in vivo activity in sheep which were due to be sacrificed on the same day as the measurements were made as part of the muscle tissue monitoring programme. The measurements were made over two consecutive days, 40 sheep being measured on each day. A kilogram of muscle tissue from the hind leg of each sheep was monitored for activity in the laboratory using a high purity germanium detector and a counting time of 1 hour. The limit of detection was 10 Bq kg -~ for 137Csand at 500 Bq kg -1 the error (2o-) was +__30Bq kg -1. The activity picked up by the detector used for the in vivo measurements would have two components, namely that actually present in the sheep and that emanating from the background. Correction for background would ideally be made on sheep free from intrinsic activity. The ideal was unattainable since the intention was to use this method on farms in the worst contaminated areas in the UK. Consequently the background activity was determined by the operator making measurements with the detector pressed against his own stomach. Background readings will vary from location to location but, where these measurements were made, a typical background reading was 60 ___4 counts 10 s -1.
RESULTS Figure 1 is a plot of the background-corrected activity (counts 10 s -1) in live sheep vs the laboratory measurements on muscle tissue from the sheep for one of the instruments. Visual inspection of the curve indicated that there
212
R. C. K. Meredith, K. J. Mondon, J. C. Sherlock
was a linear relationship between the two sets of measurements. The best fit regression lines for the two instruments are given below. C o u n t s 10 s -1 = 5 + 0.151 (Bq kg -1) (instrument 1) S t a n d a r d deviation of the intercept = 4.1 Standard deviation of the slope = 0.004 6 R 2 = 93% N = 80 and C o u n t s 10 s -~ = 4 + 0-152 (Bq kg -1) (instrument 2) Standard deviation of the intercept = 3.7 Standard deviation of the slope = 0.004 6 R 2 = 93% N = 80 B o t h calibration curves are remarkably similar and have intercepts which are not significantly different from zero. This indicates that the method of correcting for background activity was adequate and that the two instruments were capable of yielding effectively the same results.
DISCUSSION The m e a s u r e m e n t s described in this paper were made on two consecutive mornings in S e p t e m b e r 1986. Two staff were required to make the m e a s u r e m e n t s , one to hold the detector and one to operate the instrument and record the readings. A third person was needed to move the sheep about and hold them still whilst the measurements were made. Practical experience on farms has since demonstrated that the method can be used routinely to monitor sheep at the rate of 1 animal per minute during a working day including 6-7 hours of measurements. Clearly, with a short counting time and the possibility of inter-operator variation, the in vivo method will never be as precise nor as accurate as laboratory measurements made on muscle tissue. Nevertheless, the regression equations indicate that the results from the in vivo method correlate reasonably well with laboratory measurements. In addition, measurements can be m a d e at a speed sufficient to enable one team of two staff with one instrument to monitor up to 1500-2000 animals in a 5 day working week. The regression equations indicate that, on average, an in vivo reading of a b o u t 155 c 10 s -1 above background would correspond to a muscle caesium activity of 1000 Bq kg -1. By setting a 'rejection' limit well below 155 c 10 s -~ it should be possible to determine whether or not an animal may be m o v e d from the restricted area with a high degree of confidence of not falsely 'passing' an animal which should fail the check. In practice the rejection
I n vivo
monitoringof radiocaesium activity in sheep
213
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1500
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L A B O R A T O R Y M E A S U R E M E N T OF C A E S I U M ACTIVITY IN SHEEP MUSCLE
(Bqkg-' fresh weight)
Fig. 1. Calibration graph for SD 1.
limit chosen by the Ministry was 110c 10s -1 above background. This corresponds to a 97.5% level of confidence that the flesh of an animal just below the rejection limit does in fact contain less than 1000 Bq kg-l: the limit was derived from a consideration of the variability of individual measurements. The in vivo monitoring method described in this paper has been in operation since mid-September 1986 and, by the end of March 1987, more that 40 000 animals had been monitored in Cumbria alone. Performance of the instruments in the field was checked on a day to day basis by making measurements against a 137Cs source of known activity with each instrument having its own source.
ACKNOWLEDGEMENTS We acknowledge the Ministry of Agriculture, Fisheries and Food for permission to publish this paper. We thank Mr J. Barrow, Mr E. Gander, Mr S. Hattie and Mr B. Suttie (all MAFF staff) for their invaluable assistance in conducting the work described in this paper. Thanks are also due to Mr A. Lally and his staff at the Central Veterinary Laboratory, Weybridge for making the measurements of caesium activity in muscle tissue.
214
R. C. K. Meredith, K. J. Mondon, J. C. Sherlock
REFERENCES Sherlock, J. C., Andrews, D., Dunderdale, J., Lally, A. & Shaw, P. (1988). The in vivo measurement of radiocaesium in lambs. J. Env. Radioactivity, 7,215-220. Smith, F. B. & Clark, M. J. (1986). Radionuclide deposition from the Chernobyl cloud. Nature, 322,690-1.
A Rapid Method for the In vivo Monitoring of Radiocaesium Activity in Sheep R. C. K. Meredith, K. J. Mondon & J. C. Sherlock Food Science Division, Ministry of Agriculture, Fisheries and Food, Great Westminster House, Horseferry Rd, London SW 1P 2AE (Received 21 September 1987; accepted 17 December 1987)
ABSTRACT Following the accident at Chernobyl there was a need to determine caesium activity in individual sheep on farms. This need arose so that animals on farms within a restricted area could be released from restrictions provided the tissue caesium activity was below the recommended limit. This paper describes the development o f a rapid method for the in vivo monitoring of" sheep using a lead-shielded sodium iodide scintillation detector coupled to a portable single channel analyser. The method enables one sheep per minute to be monitored and the results showed a good correlation (R 2 = 93%) with laboratory measurements o f meat from sheep which had previously been monitored in vivo prior to sacrifice.
INTRODUCTION The fallout from Chernobyl resulted in persistent elevated levels of 137Csand 134Cs in herbage in many parts of the United Kingdom, the worst affected areas being Cumbria, north Wales, west Scotland and northern Ireland (Smith & Clark, 1986). Restrictions were placed on the movement of sheep within the worst affected areas in the UK. The method used to assess the suitability for derestriction of zones within the worst affected areas has already b e e n described in the previous paper (Sherlock et al., 1988). By late August, all the zones that could be released from restriction had been released. This left a core of zones which would not meet the criteria of derestriction but which, because of local variations in contamination levels, were likely to contain many sheep with less than 1000 Bq kg -~ in their flesh. 209 © 1988 Crown Copyright
210
R. C. K. Meredith, K, J. Mondon, J. C. Sherlock
It seemed that the only hope for allowing the movement of sheep out of the remaining restricted areas was to wait for activity levels to fall or monitor in v i v o each animal the farmers wished to move. The latter option was chosen since it seemed that activity levels were not falling and many farmers would need to move their sheep from September onwards.
INITIAL A P P R O A C H Work undertaken in June 1986 had demonstrated (Sherlock et al., 1988) that it was practicable to monitor caesium activity in live sheep. However, this work was undertaken using sophisticated equipment requiring highly trained operators and only about 25 sheep per day could be monitored. The remaining zones within the restricted areas contained several hundred thousand sheep. Clearly the existing methodology was inadequate to monitor all the sheep which the farmers wished to move out of the restricted areas. What was required was a cheap and very rapid method of in vivo monitoring which could be undertaken by non-specialist staff. A n initial assessment of portable radiation monitors indicated that e q u i p m e n t based on a 4.4 cm or 5 cm sodium iodide (thallium-doped) lead-shielded scintillation detector coupled to a portable single channel analyser (SCA) might be suitable for the purpose. Two portable instruments were obtained on loan from the manufacturer (John Caunt Scientific Ltd, Eynsham, Oxfordshire, UK); the instrument type was an SD1A SCA coupled to a lead-shielded DH 100 sodium iodide detector. The shielding was 3 m m in thickness and 25 cm in length and covered all the detector except for the face of the crystal. The single channel analysers were set to measure gamma emissions from 137Csand 134Csin the region of 520-700 kev.
METHOD DEVELOPMENT Much of the m e t h o d development was dictated by practical circumstances. The major requirements were for a speedy development of the method and a very rapid rate of measurement. Clearly the longer the activity in the animals could be counted the more accurate would be the measurement but the slower would be the throughput. In addition, the longer the count time the greater the chance of the animal moving and invalidating the measurement. Ideally the sheep would be held in a restricting device like a cattle crush but there was insufficient time to develop a simple portable restraining device which would also permit a rapid throughput of sheep. The simplest
In vivo monitoring of radiocaesium activity in sheep
211
way of restraining the sheep, namely the farmer holding the animal against the side of a pen, wall, or gate, proved effective. The counting time chosen was a compromise between a long time giving accuracy and a short time determined by how long the sheep could be kept still. The most effective counting procedure was to make three 10 second counts on the most fleshy part of the buttock of the sheep and to report the m e a n of the 3 counts. Clearly one 30 second count would have been easier but, whilst the sheep could be held still for 10 seconds on most occasions, it was far more difficult to hold them still for 30 seconds. Thus 10 second counts were less wasteful of the operators' time and, by averaging three separate applications of the detector, provided an additional means of reducing systematic errors.
CALIBRATION Two S D I A instruments and detectors were calibrated by using them to determine in vivo activity in sheep which were due to be sacrificed on the same day as the measurements were made as part of the muscle tissue monitoring programme. The measurements were made over two consecutive days, 40 sheep being measured on each day. A kilogram of muscle tissue from the hind leg of each sheep was monitored for activity in the laboratory using a high purity germanium detector and a counting time of 1 hour. The limit of detection was 10 Bq kg -~ for 137Csand at 500 Bq kg -1 the error (2o-) was +__30Bq kg -1. The activity picked up by the detector used for the in vivo measurements would have two components, namely that actually present in the sheep and that emanating from the background. Correction for background would ideally be made on sheep free from intrinsic activity. The ideal was unattainable since the intention was to use this method on farms in the worst contaminated areas in the UK. Consequently the background activity was determined by the operator making measurements with the detector pressed against his own stomach. Background readings will vary from location to location but, where these measurements were made, a typical background reading was 60 ___4 counts 10 s -1.
RESULTS Figure 1 is a plot of the background-corrected activity (counts 10 s -1) in live sheep vs the laboratory measurements on muscle tissue from the sheep for one of the instruments. Visual inspection of the curve indicated that there
212
R. C. K. Meredith, K. J. Mondon, J. C. Sherlock
was a linear relationship between the two sets of measurements. The best fit regression lines for the two instruments are given below. C o u n t s 10 s -1 = 5 + 0.151 (Bq kg -1) (instrument 1) S t a n d a r d deviation of the intercept = 4.1 Standard deviation of the slope = 0.004 6 R 2 = 93% N = 80 and C o u n t s 10 s -~ = 4 + 0-152 (Bq kg -1) (instrument 2) Standard deviation of the intercept = 3.7 Standard deviation of the slope = 0.004 6 R 2 = 93% N = 80 B o t h calibration curves are remarkably similar and have intercepts which are not significantly different from zero. This indicates that the method of correcting for background activity was adequate and that the two instruments were capable of yielding effectively the same results.
DISCUSSION The m e a s u r e m e n t s described in this paper were made on two consecutive mornings in S e p t e m b e r 1986. Two staff were required to make the m e a s u r e m e n t s , one to hold the detector and one to operate the instrument and record the readings. A third person was needed to move the sheep about and hold them still whilst the measurements were made. Practical experience on farms has since demonstrated that the method can be used routinely to monitor sheep at the rate of 1 animal per minute during a working day including 6-7 hours of measurements. Clearly, with a short counting time and the possibility of inter-operator variation, the in vivo method will never be as precise nor as accurate as laboratory measurements made on muscle tissue. Nevertheless, the regression equations indicate that the results from the in vivo method correlate reasonably well with laboratory measurements. In addition, measurements can be m a d e at a speed sufficient to enable one team of two staff with one instrument to monitor up to 1500-2000 animals in a 5 day working week. The regression equations indicate that, on average, an in vivo reading of a b o u t 155 c 10 s -1 above background would correspond to a muscle caesium activity of 1000 Bq kg -1. By setting a 'rejection' limit well below 155 c 10 s -~ it should be possible to determine whether or not an animal may be m o v e d from the restricted area with a high degree of confidence of not falsely 'passing' an animal which should fail the check. In practice the rejection
I n vivo
monitoringof radiocaesium activity in sheep
213
400 u~ w > '~a
z_~ 300
to
o
o
w "a
200
• ° ,." : °
0
•
~
°
a: ~
°
100-
i 500
°° e•
°
i
•
1000
1500
2000
2500
L A B O R A T O R Y M E A S U R E M E N T OF C A E S I U M ACTIVITY IN SHEEP MUSCLE
(Bqkg-' fresh weight)
Fig. 1. Calibration graph for SD 1.
limit chosen by the Ministry was 110c 10s -1 above background. This corresponds to a 97.5% level of confidence that the flesh of an animal just below the rejection limit does in fact contain less than 1000 Bq kg-l: the limit was derived from a consideration of the variability of individual measurements. The in vivo monitoring method described in this paper has been in operation since mid-September 1986 and, by the end of March 1987, more that 40 000 animals had been monitored in Cumbria alone. Performance of the instruments in the field was checked on a day to day basis by making measurements against a 137Cs source of known activity with each instrument having its own source.
ACKNOWLEDGEMENTS We acknowledge the Ministry of Agriculture, Fisheries and Food for permission to publish this paper. We thank Mr J. Barrow, Mr E. Gander, Mr S. Hattie and Mr B. Suttie (all MAFF staff) for their invaluable assistance in conducting the work described in this paper. Thanks are also due to Mr A. Lally and his staff at the Central Veterinary Laboratory, Weybridge for making the measurements of caesium activity in muscle tissue.
214
R. C. K. Meredith, K. J. Mondon, J. C. Sherlock
REFERENCES Sherlock, J. C., Andrews, D., Dunderdale, J., Lally, A. & Shaw, P. (1988). The in vivo measurement of radiocaesium in lambs. J. Env. Radioactivity, 7,215-220. Smith, F. B. & Clark, M. J. (1986). Radionuclide deposition from the Chernobyl cloud. Nature, 322,690-1.