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Alternative treaty monitoring approaches using ultra-low background measurement technology
ARTICLE IN PRESS Applied Radiation and Isotopes 67 (2009) 746–749
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Alternative treaty monitoring approaches using ultra-low background measurement technology H.S. Miley , C.E. Aalseth, T.W. Bowyer, J.E. Fast, J.C. Hayes, E.W. Hoppe, T.W. Hossbach, M.E. Keillor, J.D. Kephart, J.I. McIntyre, A. Seifert Pacific Northwest National Laboratory, P.O. Box 999, Richland, USA
a r t i c l e in f o
Keywords: CTBT IMS Ultra-low-background Rasa Aerosol
a b s t r a c t The International Monitoring System (IMS) of the Comprehensive Test Ban Treaty includes a network of stations and laboratories for collection and analysis of radioactive aerosols. Alternative approaches to IMS operations are considered as a method of enhancing treaty verification. Ultra-low background (ULB) detection promises the possibility of improvements to IMS minimum detectable activities (MDAs) well below the current approach, requiring MDAp30 mBq/m3 of air for 140Ba, or about 106 fissions per daily sample. & 2009 Published by Elsevier Ltd.
1. Introduction The International Monitoring System (IMS) is a verification arm of the Comprehensive Nuclear Test Ban Treaty (CTBT). This system is chartered to detect nuclear testing via phenomena that include seismic, hydroacoustic, atmospheric infrasound waves, and radionuclides in the atmosphere. Collection and analysis of radioactive material is slower because of the travel time of air parcels, but provides the only certain evidence that an event was nuclear in nature. Of the 321 field sites under development for the various sensors needed for the IMS, 80 are designated for aerosol monitoring. IMS aerosol systems are required to have a very sensitive detection limit for 140Ba, a key fission product at a peak of the fission yield curve. To accomplish this, the stations require a large volume of air to be filtered every 24 hours, then decayed for 24 hours, then measured for 24 hours. The decay period improves the MDA by allowing a large portion of the background-inducing radon progeny to decay before the sample is measured. By sampling more than 12 000 m3 of air, mechanically concentrating the filter to improve sample geometry efficiency, and employing a large Ge detector, minimum detectable concentrations (MDC) as defined by Currie’s LD (Currie, 1968) with a 5% false positive and false negative probability can be obtained at or below 30 mBq of 140 Ba per cubic meter of air. This sensitivity level has been determined, through atmospheric transport modeling, to be capable of detecting a 1 kt atmospheric explosion anywhere on the Earth’s surface within two weeks at the 90% confidence level.
Corresponding author.
E-mail address: [email protected] (H.S. Miley). 0969-8043/$ - see front matter & 2009 Published by Elsevier Ltd. doi:10.1016/j.apradiso.2009.01.069
Samples for which the IMS station detects two or more Treatyrelevant isotopes are sent to one of 16 IMS-certified laboratories for confirmatory analysis, where the sample is re-measured for up to seven days.
2. Application of ultra-low background (ULB) approaches for the IMS 2.1. Rationales for and against ULB monitoring for aerosols Historical atmospheric monitoring pursued fission product releases for decades after the atmospheric testing era began to decline in 1962 with the Atmospheric Test Ban. Key isotopes like 140 Ba were observed as high as 10 Bq/m3 in Richland, WA, from leakages from partially contained Chinese tests, which were at least one order of magnitude less than concentrations seen from true atmospheric tests, as shown among others by Perkins et al. (1990). These reached Richland an average of about 12 days after announced explosion times (Miley et al., 1999), commensurate with the 10 000 km downwind air travel distance. The systems used for this type of detection were manually operated blowers and dual NaI coincidence radiation measurement systems and achieved MDC’s of 1 103 Bq/m3. By comparison, the IMS have a sensitivity of about 3 105 Bq/m3. It might appear obvious that the IMS sensitivity is already greater than needed to detect above ground nuclear tests and that lower background radiation detection is futile. The conclusion that IMS sensitivity need not be improved is unsound due to the current undetectability of underground testing using the aerosol approach. It seems unlikely that a future nuclear test done in violation of a test ban would be conducted in the atmosphere. The
ARTICLE IN PRESS H.S. Miley et al. / Applied Radiation and Isotopes 67 (2009) 746–749
IMS includes a radioactive xenon gas monitoring component specifically for this reason, as xenon is the most likely fission product to leak from an underground test and be detected downwind. This concept was demonstrated in the detections reported by Becker et al. (2008) and Saey et al. (2007) after the nuclear test of 2006, announced by the Democratic People’s Republic of Korea (DPRK). However, no corroborating reports of aerosol debris were found. Improving the sensitivity to aerosols, particularly iodine isotopes, which are the most volatile and likely to escape, would add great confidence to the detection of small leaks, since Kalinowski and Pistner (2006) has shown that there is some anthropomorphic background of xenon isotopes due to reactor leakage and medical isotope production. Another rationale commonly cited against using more sensitive radiation detection is the natural radioactivity in the air and the resulting obscuring background activity in the filters. The decay products of 222Rn and 220Rn are ionized and rapidly attached to ambient aerosols and thus captured by the IMS filtration. For 222 Rn, these are dominated by 214Pb and 214Bi, which decay with a 26.8 minute half life. For 220Rn, the major isotopes are 212Pb, 212Bi, and 208Tl, which decay with a 10.6 hour half life. The 24 hour decay in the IMS is adequate to reduce the 222Rn decay products, but provides only a factor of 4 reductions in the 220Rn. The question is whether the remaining 220Rn products will overwhelm low background counting systems. To resolve this question, a simple experiment was carried out in which a typical IMS sample from Richland, WA, was collected and measured in the normal IMS sequence, compared to the background of the IMS station detector, and then re-measured on a modestly low background system at the surface of the Earth after eight days of decay: a decay factor of over 65 000. Fig. 1 shows that only the 478 keV gamma ray of 7Be (T1/2 ¼ 54 d) is observed significantly above background in the low background measurement. We conclude that even without chemical separation, the use of very low background detectors is effective on aged filters. It is not necessary to hypothesize what background levels may be achieved in germanium spectrometers for the purposes of treaty verification; many examples in the literature already exist, for example those of Miley et al. (1992) have been built and operated by PNNL. These include a range from a low-background surface system operated with a 4p veto system (17-A) to a pair of systems with electroformed copper cryostats (TWIN) operated previously in the Homestake Mine, at a depth of about
105 104 Counts per keV
IMS filter Decayed filter IMS Station BKG
Pb-212 Be-7 Tl-208 511 Bi-212
Tl-208
Tl-208 K-40
103
Tl-208
102 101 100
747
Table 1 List of real and hypothetical systems considered for IMS use. Detector
Type
Relative Eff
BKG comments
RASA BUL-p BUL-w TWIN
p-type p-type Well type p-type
90% 100% 100% 50% or 1 kg
about 100 17-A levels same as 17-A 10 lower than 17-A 1000 lower than 17-A
4000 m.w.e. (m.w.e. ¼ meter water equivalent, a shielding height of the material expressed as the water equivalent). 2.2. Potential ULB detection systems for the IMS: stations and labs Several possibilities exist for improving the sensitivity to fission products in the IMS without serious changes in the network. The two main possibilities are to employ the best possible ULB detection at the station in accord to the natural activity of the filter, but after some modest period of decay. The second is to greatly improve the background at the laboratories of the IMS. Options for improvement of IMS laboratory measurement have already been shown. The goal of these laboratories is to increase confidence in field detections by remeasuring the isotopes seen in the field, where possible, and perhaps to add additional isotopes that were below detectability at the station. ULB techniques are perfectly suited to this. It is also possible, but not current practice, to send aerosol samples to these laboratories when other phenomena (e.g. seismic, hydroacoustic, or infrasound) provide signals that may indicate a test. In this case, the IMS station may have detected nothing but natural background aerosol isotopes. By employing the most sensitive laboratory detection method available, the most sensitive possible Treaty detection threshold will be possible. A key issue is the rate of waveform triggers; the rate of triggers may be so high that many laboratory detector systems are required. To understand the opportunities available for IMS-type sample measurement at stations and in IMS laboratories, a variety of new and existing detector designs are considered. The reference case is a normal IMS system, denoted here as RASA or Radionuclide Aerosol Sampler/Analyzer, a particular type of IMS aerosol system designed at PNNL with a large Ge detector and minimal shielding. The RASA (Miley et al., 1998) and TWIN (Miley et al., 1992) detectors in Table 1 have been introduced above and in the literature. Two planned commercial detectors for the Brodzinski Underground Laboratory, currently under construction at PNNL, will be operated with muon veto shields and below 30 meters water equivalent. In reality, it is reasonable to expect these two detectors to have similar backgrounds to one another, but the BUL-p is given the same background as a 4p vetoed surface system to explore the sensitivity of a surface-based 4p veto shielded detector location with a RASA at an IMS station. These detector background levels are hypothetical, but attainable based on past experiences.
3. Estimate of impacts of various detectors on the IMS
0
500
1000 1500 2000 Energy in keV
2500
3000
Fig. 1. Comparison of normal (24 hours) IMS-type filter measurement with a 24 hours IMS station background and the filter itself decayed for eight days and remeasured on a low background detector for 24 hours. The low-level nature of the decayed sample shows that there is promise for measuring decayed IMS filters even without chemical separation of the natural isotopes present.
To consider how these systems such as these could impact the IMS, and further to consider the most likely way they would be used, a rough calculation was executed using measured or reasonable estimates for efficiency, resolution, and background. In all cases, a 95% confidence level was sought, and computed using the approach of Currie for a well known background. A few
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key isotopes were considered, and all were compared to a computation of the sensitivity of an IMS station in the processing of a normal daily sample. Since any additional measurement would occur after the initial measurement, and since the 220Rn daughters need time to decay, a scenario was considered in which the sample decays three days after the conclusion of the IMS station measurement, then is subjected to a second measurement. In each case, these are compared to the original, unmodified IMS measurement. Because the relative proportions of isotopes detected after a fission event will be determined by their fission yield, half life, decay time, the gamma branching ratio, and the detector efficiency, the sensitivity of each isotope is computed in minimum detectable fissions. Literally, this is the number of fission atoms from the fission source that must be on the filter to provide 95% confidence detection. This unit allows the direct comparison of the sensitivity of various isotopes. For a measurement at a station, we compare each of the detectors in Table 1 for a modest three day decay period and a one day measurement, such that each sample from the station could be re-measured regularly (Table 2). For this application, only the surface-based BUL-p detector is practical. In fact, a similar installation (Schwaiger et al., 2002) has been demonstrated at the Austrian Research Center Seibersdorf (ARCS). Inspection of the relative sensitivity portion of Table 3 shows that a factor of 10–70 sensitivity increase would be possible in field applications of the BUL-p, perhaps even retaining the automatic nature of some IMS systems. The 99Mo and 131I
Table 2 Isotopes used for comparison and key nuclear data used. Isotope
140
140
99
131
Fission yield T1/2 (d) Gamma energy (keV) Gamma-ray emission probability
0.06 12.8 538 0.24
0.06 12.8 487 0.455
0.06 2.75 140 0.89
0.03 8.06 364 0.82
Ba
La
Mo
I
sensitivity may be slightly overstated due to the long half life of Be which can be seen as perhaps a factor of two increase in lowlevel activity below 478 keV. A more complex calculation is required to investigate this fully, but at an approximate level, Table 3 is indicative of a very substantial increase in sensitivity of an IMS station (Table 4). To consider a laboratory-based application, which could reasonably have at least modest underground shielding, sufficient decay is required to bring 220Rn decay products low enough to benefit from ULB detectors. A decay period of seven days and a counting time of seven days have been considered. All minimum detectable fission atom quantities have been computed back to the time of the original, unmodified RASA measurement so that the increase in sensitivity can be fairly compared. A longer measurement time also greatly improves the sensitivity of the measurement. The sensitivity of the three lowest background systems is impressive, between 50 and 1000 times as sensitive. The overstatement of the 99Mo and 131I impact is likely larger here than in the three-day decay case in Table 3, and in fact the improvement for these isotopes in the best three detectors may be not be much larger than the still respectable range of 30–600 times larger shown in Table 3. The standout result is the well type detector, BUL-w. A single, commercially obtainable ULB well detector at modest depth is extremely sensitive, but may not be employable in the IMS due to operational rules: the filters, even when compressed, are tens of cubic centimeters in size, and would require ashing to fit in the well. Destructive analysis, though highly sensitive, is not part of IMS protocol; the evidentiary nature of the samples must be maintained. The more exotic TWIN detectors perform extremely well in a non-destructive mode, and can deliver perhaps up to 600 times the sensitivity of the IMS station, or a few thousand fissions of the original explosion. Are levels this low useful? By providing unprecedented sensitivity for isotope detection, the ability of the network to detect both real signatures and local backgrounds is greatly enhanced. At somewhat higher sample activity levels, corresponding to current marginal detection by existing systems, the higher 7
Table 3 Sensitivity obtained from applying the detectors of Table 1 for a one day measurement of a three day aged IMS filter, compared to the normal IMS station sensitivity, represented by a RASA. Isotope
140
140
Detector
Fissions
Factor
Fissions
Factor
Fissions
Factor
Fissions
Factor
RASA (IMS) BUL-p BUL-w TWIN
642 103 62 103 37 103 61 103
1.0 10 18 11
316 103 31 103 16 103 27 103
2.0 20 41 24
67 103 9.2 103 1.0 103 4.0 103
9.5 70 641 162
269 103 28 103 10 103 18 103
2.4 23 62 36
Ba
99
La
131
Mo
I
Sensitivity is in minimum detectable fission (95% CL) back calculated to the start of the IMS station measurement.
Table 4 Sensitivity obtained from applying the detectors of Table 1 for a seven day measurement of a seven day aged IMS filter compared to the normal IMS station sensitivity, represented by a RASA. Isotope
140
Detector
Fissions
RASA (IMS) BUL-p BUL-w TWIN
140
Ba Factor 3
642 10 29 103 11 103 11 103
1.0 22 56 60
99
La
Fissions
Factor 3
316 10 15 103 4.9 103 4.8 103
2.0 44 130 135
131
Mo
I
Fissions 3
67 10 9.5 103 0.7 103 1.6 103
Sensitivity is in minimum detectable fission (95% CL) back calculated to the start of the IMS station measurement.
Factor 9.5 67 928 413
Fissions
Factor 3
269 10 15 103 3.7 103 3.6 103
2.4 44 174 177
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sensitivity of the new systems described translate into quantification with smaller error bars than currently available. By providing isotope quantification where current methods can only detect, civilian activity and other innocent local background phenomena may be better distinguished from Treaty violation activity.
4. Conclusions It has been shown by approximate calculations that substantial sensitivity enhancement of the IMS network of aerosol stations is possible without radical changes in the network. Given that systems such as the BUL-p and BUL-w are available commercially and at costs of perhaps 25% the original cost of an IMS station, the factor of 10–70 available at the IMS station within four days of the original IMS aerosol measurement is a practical opportunity to extend IMS capability at a time when the filters would simply be waiting to be shipped to an archive. An extra benefit of this approach is that maintenance downtime of Ge detectors currently dominates system downtime. Having a second system as part of the IMS station means that the loss of one detector does not render a station non-functional. A second, and potentially staggering increase in sensitivity could come from performing IMS aerosol laboratory measurement in highly capable ultra-low background detection systems, possibly underground. If triggered by a wider range of events, potentially including Xe detections or waveform events, more laboratory systems may be needed than one per laboratory. The sensitivity improvements considered here would provide the IMS with capability unprecedented in the science of atmospheric monitoring, rapidly increasing knowledge of sources of atmospheric radioactivity of interest to treaty verification and environmental science, while the heightened sensitivity would also reveal previously un-observed anomalies that would require study. Gaining a full understanding of the source of such non-test
749
related radionuclide signatures would allow further increase in the confidence assigned to any future positive IMS collection.
Acknowledgments This research was sponsored by the Office of Nonproliferation Policy (NA-241), National Nuclear Security Administration. Pacific Northwest Laboratory is operated for DOE by Battelle Memorial Institute under Contract DE-AC06-76RLO 1830. References Becker, A., Wotawa, G., Ringbom, A., Saey, P., 2008. Backtracking of noble gas measurements taken in the aftermath of the announced October 2006 event in North Korea by means of the PTS methods in nuclear source estimation and reconstruction. Geophysical Research Abstracts, vol. 10, SRef-ID: 1607-7962/ gra/EGU2008-A-11835. Currie, L.A., 1968. Limits for qualitative detection and quantitative determination. Analytical Chemistry 40 (3), 568–593. Kalinowski, M., Pistner, C., 2006. Isotopic signature of atmospheric xenon released from light water reactors. Journal of Environmental Radioactivity 88 (3), 215–235. Miley, H., Brodzinski, R., Reeves, J., 1992. Low background counting systems compared. Journal of Radioanalytical and Nuclear Chemistry 160 (2), 371–385. Miley, H., Bowyer, S., Hubbard, C., McKinnon, A., Perkins, R., Thompson, R., Warner, R., 1998. A description of the DOE Radionuclide Aerosol Sampler/Analyzer for the Comprehensive Test Ban Treaty. Journal of Radioanalytical and Nuclear Chemistry 235 (1–2), 83–87. Miley, H., Arthur, R., Lepel, E., Pratt, S., Thomas, C., 1999. Evaluation of laboratory detection systems for fission product detection. IEEE Nuclear Science Symposium Conference Record, Seattle, WA, vol. 2, pp. 786–790. Perkins, R., Robertson, D., Thomas, C., Young, J., 1990. Comparison of nuclear accident and nuclear test debris. Proceedings of the International Symposium on Environmental Contamination Following a Major Nuclear Accident, Vienna, Austria, IAEA1990-SM-306/125. Saey, P., Bean, M., Becker, A., Coyne, J., d’Amours, R., De Geer, L.-E., Hogue, R., Stocki, T., Ungar, R., Wotawa, G., 2007. A long distance measurement of radioxenon in Yellowknife, Canada, in late October 2006. Geophysical Research Letters 34, L20802. Schwaiger, M., Steger, F., Schroettner, T., Schmitzer, C., 2002. A ultra low level laboratory for nuclear test ban measurements. Applied Radiation and Isotopes 56 (1–2), 375–378.
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Alternative treaty monitoring approaches using ultra-low background measurement technology H.S. Miley , C.E. Aalseth, T.W. Bowyer, J.E. Fast, J.C. Hayes, E.W. Hoppe, T.W. Hossbach, M.E. Keillor, J.D. Kephart, J.I. McIntyre, A. Seifert Pacific Northwest National Laboratory, P.O. Box 999, Richland, USA
a r t i c l e in f o
Keywords: CTBT IMS Ultra-low-background Rasa Aerosol
a b s t r a c t The International Monitoring System (IMS) of the Comprehensive Test Ban Treaty includes a network of stations and laboratories for collection and analysis of radioactive aerosols. Alternative approaches to IMS operations are considered as a method of enhancing treaty verification. Ultra-low background (ULB) detection promises the possibility of improvements to IMS minimum detectable activities (MDAs) well below the current approach, requiring MDAp30 mBq/m3 of air for 140Ba, or about 106 fissions per daily sample. & 2009 Published by Elsevier Ltd.
1. Introduction The International Monitoring System (IMS) is a verification arm of the Comprehensive Nuclear Test Ban Treaty (CTBT). This system is chartered to detect nuclear testing via phenomena that include seismic, hydroacoustic, atmospheric infrasound waves, and radionuclides in the atmosphere. Collection and analysis of radioactive material is slower because of the travel time of air parcels, but provides the only certain evidence that an event was nuclear in nature. Of the 321 field sites under development for the various sensors needed for the IMS, 80 are designated for aerosol monitoring. IMS aerosol systems are required to have a very sensitive detection limit for 140Ba, a key fission product at a peak of the fission yield curve. To accomplish this, the stations require a large volume of air to be filtered every 24 hours, then decayed for 24 hours, then measured for 24 hours. The decay period improves the MDA by allowing a large portion of the background-inducing radon progeny to decay before the sample is measured. By sampling more than 12 000 m3 of air, mechanically concentrating the filter to improve sample geometry efficiency, and employing a large Ge detector, minimum detectable concentrations (MDC) as defined by Currie’s LD (Currie, 1968) with a 5% false positive and false negative probability can be obtained at or below 30 mBq of 140 Ba per cubic meter of air. This sensitivity level has been determined, through atmospheric transport modeling, to be capable of detecting a 1 kt atmospheric explosion anywhere on the Earth’s surface within two weeks at the 90% confidence level.
Corresponding author.
E-mail address: [email protected] (H.S. Miley). 0969-8043/$ - see front matter & 2009 Published by Elsevier Ltd. doi:10.1016/j.apradiso.2009.01.069
Samples for which the IMS station detects two or more Treatyrelevant isotopes are sent to one of 16 IMS-certified laboratories for confirmatory analysis, where the sample is re-measured for up to seven days.
2. Application of ultra-low background (ULB) approaches for the IMS 2.1. Rationales for and against ULB monitoring for aerosols Historical atmospheric monitoring pursued fission product releases for decades after the atmospheric testing era began to decline in 1962 with the Atmospheric Test Ban. Key isotopes like 140 Ba were observed as high as 10 Bq/m3 in Richland, WA, from leakages from partially contained Chinese tests, which were at least one order of magnitude less than concentrations seen from true atmospheric tests, as shown among others by Perkins et al. (1990). These reached Richland an average of about 12 days after announced explosion times (Miley et al., 1999), commensurate with the 10 000 km downwind air travel distance. The systems used for this type of detection were manually operated blowers and dual NaI coincidence radiation measurement systems and achieved MDC’s of 1 103 Bq/m3. By comparison, the IMS have a sensitivity of about 3 105 Bq/m3. It might appear obvious that the IMS sensitivity is already greater than needed to detect above ground nuclear tests and that lower background radiation detection is futile. The conclusion that IMS sensitivity need not be improved is unsound due to the current undetectability of underground testing using the aerosol approach. It seems unlikely that a future nuclear test done in violation of a test ban would be conducted in the atmosphere. The
ARTICLE IN PRESS H.S. Miley et al. / Applied Radiation and Isotopes 67 (2009) 746–749
IMS includes a radioactive xenon gas monitoring component specifically for this reason, as xenon is the most likely fission product to leak from an underground test and be detected downwind. This concept was demonstrated in the detections reported by Becker et al. (2008) and Saey et al. (2007) after the nuclear test of 2006, announced by the Democratic People’s Republic of Korea (DPRK). However, no corroborating reports of aerosol debris were found. Improving the sensitivity to aerosols, particularly iodine isotopes, which are the most volatile and likely to escape, would add great confidence to the detection of small leaks, since Kalinowski and Pistner (2006) has shown that there is some anthropomorphic background of xenon isotopes due to reactor leakage and medical isotope production. Another rationale commonly cited against using more sensitive radiation detection is the natural radioactivity in the air and the resulting obscuring background activity in the filters. The decay products of 222Rn and 220Rn are ionized and rapidly attached to ambient aerosols and thus captured by the IMS filtration. For 222 Rn, these are dominated by 214Pb and 214Bi, which decay with a 26.8 minute half life. For 220Rn, the major isotopes are 212Pb, 212Bi, and 208Tl, which decay with a 10.6 hour half life. The 24 hour decay in the IMS is adequate to reduce the 222Rn decay products, but provides only a factor of 4 reductions in the 220Rn. The question is whether the remaining 220Rn products will overwhelm low background counting systems. To resolve this question, a simple experiment was carried out in which a typical IMS sample from Richland, WA, was collected and measured in the normal IMS sequence, compared to the background of the IMS station detector, and then re-measured on a modestly low background system at the surface of the Earth after eight days of decay: a decay factor of over 65 000. Fig. 1 shows that only the 478 keV gamma ray of 7Be (T1/2 ¼ 54 d) is observed significantly above background in the low background measurement. We conclude that even without chemical separation, the use of very low background detectors is effective on aged filters. It is not necessary to hypothesize what background levels may be achieved in germanium spectrometers for the purposes of treaty verification; many examples in the literature already exist, for example those of Miley et al. (1992) have been built and operated by PNNL. These include a range from a low-background surface system operated with a 4p veto system (17-A) to a pair of systems with electroformed copper cryostats (TWIN) operated previously in the Homestake Mine, at a depth of about
105 104 Counts per keV
IMS filter Decayed filter IMS Station BKG
Pb-212 Be-7 Tl-208 511 Bi-212
Tl-208
Tl-208 K-40
103
Tl-208
102 101 100
747
Table 1 List of real and hypothetical systems considered for IMS use. Detector
Type
Relative Eff
BKG comments
RASA BUL-p BUL-w TWIN
p-type p-type Well type p-type
90% 100% 100% 50% or 1 kg
about 100 17-A levels same as 17-A 10 lower than 17-A 1000 lower than 17-A
4000 m.w.e. (m.w.e. ¼ meter water equivalent, a shielding height of the material expressed as the water equivalent). 2.2. Potential ULB detection systems for the IMS: stations and labs Several possibilities exist for improving the sensitivity to fission products in the IMS without serious changes in the network. The two main possibilities are to employ the best possible ULB detection at the station in accord to the natural activity of the filter, but after some modest period of decay. The second is to greatly improve the background at the laboratories of the IMS. Options for improvement of IMS laboratory measurement have already been shown. The goal of these laboratories is to increase confidence in field detections by remeasuring the isotopes seen in the field, where possible, and perhaps to add additional isotopes that were below detectability at the station. ULB techniques are perfectly suited to this. It is also possible, but not current practice, to send aerosol samples to these laboratories when other phenomena (e.g. seismic, hydroacoustic, or infrasound) provide signals that may indicate a test. In this case, the IMS station may have detected nothing but natural background aerosol isotopes. By employing the most sensitive laboratory detection method available, the most sensitive possible Treaty detection threshold will be possible. A key issue is the rate of waveform triggers; the rate of triggers may be so high that many laboratory detector systems are required. To understand the opportunities available for IMS-type sample measurement at stations and in IMS laboratories, a variety of new and existing detector designs are considered. The reference case is a normal IMS system, denoted here as RASA or Radionuclide Aerosol Sampler/Analyzer, a particular type of IMS aerosol system designed at PNNL with a large Ge detector and minimal shielding. The RASA (Miley et al., 1998) and TWIN (Miley et al., 1992) detectors in Table 1 have been introduced above and in the literature. Two planned commercial detectors for the Brodzinski Underground Laboratory, currently under construction at PNNL, will be operated with muon veto shields and below 30 meters water equivalent. In reality, it is reasonable to expect these two detectors to have similar backgrounds to one another, but the BUL-p is given the same background as a 4p vetoed surface system to explore the sensitivity of a surface-based 4p veto shielded detector location with a RASA at an IMS station. These detector background levels are hypothetical, but attainable based on past experiences.
3. Estimate of impacts of various detectors on the IMS
0
500
1000 1500 2000 Energy in keV
2500
3000
Fig. 1. Comparison of normal (24 hours) IMS-type filter measurement with a 24 hours IMS station background and the filter itself decayed for eight days and remeasured on a low background detector for 24 hours. The low-level nature of the decayed sample shows that there is promise for measuring decayed IMS filters even without chemical separation of the natural isotopes present.
To consider how these systems such as these could impact the IMS, and further to consider the most likely way they would be used, a rough calculation was executed using measured or reasonable estimates for efficiency, resolution, and background. In all cases, a 95% confidence level was sought, and computed using the approach of Currie for a well known background. A few
ARTICLE IN PRESS 748
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key isotopes were considered, and all were compared to a computation of the sensitivity of an IMS station in the processing of a normal daily sample. Since any additional measurement would occur after the initial measurement, and since the 220Rn daughters need time to decay, a scenario was considered in which the sample decays three days after the conclusion of the IMS station measurement, then is subjected to a second measurement. In each case, these are compared to the original, unmodified IMS measurement. Because the relative proportions of isotopes detected after a fission event will be determined by their fission yield, half life, decay time, the gamma branching ratio, and the detector efficiency, the sensitivity of each isotope is computed in minimum detectable fissions. Literally, this is the number of fission atoms from the fission source that must be on the filter to provide 95% confidence detection. This unit allows the direct comparison of the sensitivity of various isotopes. For a measurement at a station, we compare each of the detectors in Table 1 for a modest three day decay period and a one day measurement, such that each sample from the station could be re-measured regularly (Table 2). For this application, only the surface-based BUL-p detector is practical. In fact, a similar installation (Schwaiger et al., 2002) has been demonstrated at the Austrian Research Center Seibersdorf (ARCS). Inspection of the relative sensitivity portion of Table 3 shows that a factor of 10–70 sensitivity increase would be possible in field applications of the BUL-p, perhaps even retaining the automatic nature of some IMS systems. The 99Mo and 131I
Table 2 Isotopes used for comparison and key nuclear data used. Isotope
140
140
99
131
Fission yield T1/2 (d) Gamma energy (keV) Gamma-ray emission probability
0.06 12.8 538 0.24
0.06 12.8 487 0.455
0.06 2.75 140 0.89
0.03 8.06 364 0.82
Ba
La
Mo
I
sensitivity may be slightly overstated due to the long half life of Be which can be seen as perhaps a factor of two increase in lowlevel activity below 478 keV. A more complex calculation is required to investigate this fully, but at an approximate level, Table 3 is indicative of a very substantial increase in sensitivity of an IMS station (Table 4). To consider a laboratory-based application, which could reasonably have at least modest underground shielding, sufficient decay is required to bring 220Rn decay products low enough to benefit from ULB detectors. A decay period of seven days and a counting time of seven days have been considered. All minimum detectable fission atom quantities have been computed back to the time of the original, unmodified RASA measurement so that the increase in sensitivity can be fairly compared. A longer measurement time also greatly improves the sensitivity of the measurement. The sensitivity of the three lowest background systems is impressive, between 50 and 1000 times as sensitive. The overstatement of the 99Mo and 131I impact is likely larger here than in the three-day decay case in Table 3, and in fact the improvement for these isotopes in the best three detectors may be not be much larger than the still respectable range of 30–600 times larger shown in Table 3. The standout result is the well type detector, BUL-w. A single, commercially obtainable ULB well detector at modest depth is extremely sensitive, but may not be employable in the IMS due to operational rules: the filters, even when compressed, are tens of cubic centimeters in size, and would require ashing to fit in the well. Destructive analysis, though highly sensitive, is not part of IMS protocol; the evidentiary nature of the samples must be maintained. The more exotic TWIN detectors perform extremely well in a non-destructive mode, and can deliver perhaps up to 600 times the sensitivity of the IMS station, or a few thousand fissions of the original explosion. Are levels this low useful? By providing unprecedented sensitivity for isotope detection, the ability of the network to detect both real signatures and local backgrounds is greatly enhanced. At somewhat higher sample activity levels, corresponding to current marginal detection by existing systems, the higher 7
Table 3 Sensitivity obtained from applying the detectors of Table 1 for a one day measurement of a three day aged IMS filter, compared to the normal IMS station sensitivity, represented by a RASA. Isotope
140
140
Detector
Fissions
Factor
Fissions
Factor
Fissions
Factor
Fissions
Factor
RASA (IMS) BUL-p BUL-w TWIN
642 103 62 103 37 103 61 103
1.0 10 18 11
316 103 31 103 16 103 27 103
2.0 20 41 24
67 103 9.2 103 1.0 103 4.0 103
9.5 70 641 162
269 103 28 103 10 103 18 103
2.4 23 62 36
Ba
99
La
131
Mo
I
Sensitivity is in minimum detectable fission (95% CL) back calculated to the start of the IMS station measurement.
Table 4 Sensitivity obtained from applying the detectors of Table 1 for a seven day measurement of a seven day aged IMS filter compared to the normal IMS station sensitivity, represented by a RASA. Isotope
140
Detector
Fissions
RASA (IMS) BUL-p BUL-w TWIN
140
Ba Factor 3
642 10 29 103 11 103 11 103
1.0 22 56 60
99
La
Fissions
Factor 3
316 10 15 103 4.9 103 4.8 103
2.0 44 130 135
131
Mo
I
Fissions 3
67 10 9.5 103 0.7 103 1.6 103
Sensitivity is in minimum detectable fission (95% CL) back calculated to the start of the IMS station measurement.
Factor 9.5 67 928 413
Fissions
Factor 3
269 10 15 103 3.7 103 3.6 103
2.4 44 174 177
ARTICLE IN PRESS H.S. Miley et al. / Applied Radiation and Isotopes 67 (2009) 746–749
sensitivity of the new systems described translate into quantification with smaller error bars than currently available. By providing isotope quantification where current methods can only detect, civilian activity and other innocent local background phenomena may be better distinguished from Treaty violation activity.
4. Conclusions It has been shown by approximate calculations that substantial sensitivity enhancement of the IMS network of aerosol stations is possible without radical changes in the network. Given that systems such as the BUL-p and BUL-w are available commercially and at costs of perhaps 25% the original cost of an IMS station, the factor of 10–70 available at the IMS station within four days of the original IMS aerosol measurement is a practical opportunity to extend IMS capability at a time when the filters would simply be waiting to be shipped to an archive. An extra benefit of this approach is that maintenance downtime of Ge detectors currently dominates system downtime. Having a second system as part of the IMS station means that the loss of one detector does not render a station non-functional. A second, and potentially staggering increase in sensitivity could come from performing IMS aerosol laboratory measurement in highly capable ultra-low background detection systems, possibly underground. If triggered by a wider range of events, potentially including Xe detections or waveform events, more laboratory systems may be needed than one per laboratory. The sensitivity improvements considered here would provide the IMS with capability unprecedented in the science of atmospheric monitoring, rapidly increasing knowledge of sources of atmospheric radioactivity of interest to treaty verification and environmental science, while the heightened sensitivity would also reveal previously un-observed anomalies that would require study. Gaining a full understanding of the source of such non-test
749
related radionuclide signatures would allow further increase in the confidence assigned to any future positive IMS collection.
Acknowledgments This research was sponsored by the Office of Nonproliferation Policy (NA-241), National Nuclear Security Administration. Pacific Northwest Laboratory is operated for DOE by Battelle Memorial Institute under Contract DE-AC06-76RLO 1830. References Becker, A., Wotawa, G., Ringbom, A., Saey, P., 2008. Backtracking of noble gas measurements taken in the aftermath of the announced October 2006 event in North Korea by means of the PTS methods in nuclear source estimation and reconstruction. Geophysical Research Abstracts, vol. 10, SRef-ID: 1607-7962/ gra/EGU2008-A-11835. Currie, L.A., 1968. Limits for qualitative detection and quantitative determination. Analytical Chemistry 40 (3), 568–593. Kalinowski, M., Pistner, C., 2006. Isotopic signature of atmospheric xenon released from light water reactors. Journal of Environmental Radioactivity 88 (3), 215–235. Miley, H., Brodzinski, R., Reeves, J., 1992. Low background counting systems compared. Journal of Radioanalytical and Nuclear Chemistry 160 (2), 371–385. Miley, H., Bowyer, S., Hubbard, C., McKinnon, A., Perkins, R., Thompson, R., Warner, R., 1998. A description of the DOE Radionuclide Aerosol Sampler/Analyzer for the Comprehensive Test Ban Treaty. Journal of Radioanalytical and Nuclear Chemistry 235 (1–2), 83–87. Miley, H., Arthur, R., Lepel, E., Pratt, S., Thomas, C., 1999. Evaluation of laboratory detection systems for fission product detection. IEEE Nuclear Science Symposium Conference Record, Seattle, WA, vol. 2, pp. 786–790. Perkins, R., Robertson, D., Thomas, C., Young, J., 1990. Comparison of nuclear accident and nuclear test debris. Proceedings of the International Symposium on Environmental Contamination Following a Major Nuclear Accident, Vienna, Austria, IAEA1990-SM-306/125. Saey, P., Bean, M., Becker, A., Coyne, J., d’Amours, R., De Geer, L.-E., Hogue, R., Stocki, T., Ungar, R., Wotawa, G., 2007. A long distance measurement of radioxenon in Yellowknife, Canada, in late October 2006. Geophysical Research Letters 34, L20802. Schwaiger, M., Steger, F., Schroettner, T., Schmitzer, C., 2002. A ultra low level laboratory for nuclear test ban measurements. Applied Radiation and Isotopes 56 (1–2), 375–378.