A study on the global detection capability of IMS for all CTBT relevant xenon isotopes Réal D'Amours1 and Anders Ringbom2 1- Canadian Meteorological Centre (CMC), Environment Canada, Dorval, QC, Canada 2- Swedish Defence Research Agency (FOI), SE-164 90, Stockholm, Sweden 1.Introduction 4. Dispersion model Detection limit (mBq/m3) 100 Previous studies of the measurement capability of 10 1 calculations the radioxenon part of the International Monitoring System (IMS) for CTBT monitoring have all been 0.1 It was impractical to calculate SRS fields to form a full made using simple assumptions with regard to the 0.01 climatological database. In order to sample a representative detection limits of the measurement systems and -90 -45 0 45 90 spectrum of meteorological conditions, it was decided to select without taking the existing background into account. Latitude (degrees) a sequence of 7 days, 3 days apart. The inverse model was Furthermore, all earlier studies have been done for Fig. 1: Measured 95:th percentiles of the atmospheric activity of executed for days 3-6-9-12-15-18-21 in July and December. 133 Xe only. The recent development of sensitive 133Xe as a function of latitude for the IMS sites used in this study. Calculations were repeated for 4 years 2005-2006-2007 and radioxenon systems and the build-up of the network The results illustrate the well known fact that the xenon background is higher at the northern hemisphere. The highest 2008. The simulations ran 14 days backward. within the International Noble Gas Experiment detection limits were obtained for stations close to the isotopic (INGE), has substantially increased the possibility to produciton facilities in Chalc River, Canada, and Fleurus, Belgium. This resulted in 56 14-day simulations to estimate the coverage perform a more realistic study of the network for July and December. The SRS fields were calculated for the performance in the case of a nuclear explosion. For the metastable states, and for 135Xe, typical critical 133 Xe, 131mXe, 133mXe, and 135Xe isotopes, for every 3-hour limits were used for the majority of stations. In a few interval, 14 days backwards in time. The goal of the study presented here was to: cases the same method as described for 133Xe was used. The detection limits for 131mXe and 133mXe were, The fields were combined with the assumed source term to • Use the data set presented in  to define detection with a few exceptions, in the range 0.06 -0.1 mBq/m3, calculate a concentration at the station. The first statistic thresholds for 133Xe, 131mXe, 133mXe, and 135Xe for while a typical detection limit for 135Xe was 0.3 mBq/m3. calculated was the frequency at which a given location would 131mXe, 133mXe, 133Xe Fig. 3: Average number of stations (including variance) detecting a release of and 135Xe assuming all IMS sites. The thresholds should be calculated For the remaining IMS sites, the detection limit was have been detected by that station using it's estimated the source term described in Section 3. taking the local radioxenon background into estimated based on the general site location. threshold during the span of the 14 days. Then the 39 account. frequency fields were combined into a single field showing the • Use the CMC global Lagrangian dispersion model in 3. The radioxenon number of stations that would have seen the location at least once during the span of the 14 preceding days. Finally the inverse mode to calculate Source Receptor Sensitivity (SRS) fields for each station in the IMS source term average and the variance of the number detecting stations was calculated for the 28 simulations in July and the 28 in network. December. The last calculations were also repeated for 133Xe The activities of the different radioxenon isotopes and 135Xe using a minimum frequency of 4 detections for each • Combine the SRS fields, the station dependent following a nuclear explosion are strong functions of station. thresholds, and a plausible radioxenon release term time (see Fig. 2), and the isotopic ratios for a prompt to estimate the global isotope dependent radioxenon detection capability of the network. The study should release will be very different compared to a release after one day. Here we assume a source term based 5. Results include seasonal variations, and be conducted for on both the 39 station noble gas (NG) network currently a 10% release of the activity produced by a 1 kt The results of the calculations are displayed in Fig. 3 and Fig. Fig. 4: Average number of stations (including variance) detecting a release of 133Xe and 135Xe four times, assuming the being built and the 79 IMS station network (only plutonium charge after 2.78 days, when 133Xe reaches 4. The coverages are quite similar for133Xe, 131mXe, and source term described in Section 3. 133m results for the NG network are reported here). maximum. This results in the following activities: Xe, while the network detection capability is, as expected, considerably less for 135X because of its very short half life. The detection capability is higher in winter for both Conclusions 2. Calculation of site- 131mXe 133mXe 7.22e+11 6.38e+13 Bq/kt Bq/kt hemispheres, because of increased meteorological activity in that season, and faster air streams. The detectability is dependent detection • An estimation of detection capability of the NG network has been done using 133Xe 1.06e+15 Bq/kt higher in the northern hemisphere likely because of the 135Xe 3.91e+14 Bq/kt higher number of stations, and very low in the equatorial realistic estimates of the detection levels at stations based on radioxenon limits regions, especially in the mid Pacific, because of fewer stations and reduced atmospheric transport in the tropics. backgrounds statistics. The detectability is also low on the Tibetan plateau in July. • The CMC Lagrangian dispersion model was used to calculate a detectability The observed 133Xe background at 17 IMS sites was statistic: the number of stations in the NG network detecting the location at least fitted to normal or lognormal distributions as described Interestingly the variance of the number of detecting stations once during the preceding 14 days. in . The lognormal distributions were folded with the seems to be higher over the regions were the gradient of the concentration uncertainty as a function of average is large, and tends to be lower over the regions • The statistic indicates that the network capability is higher in the mid latitudes concentration. In cases when the distribution obviously where the average reaches its maximum. This is more and the polar regions. There is a strong seasonal effect, detectabilty being higher not was lognormal, the result from data itself was apparent in December in the Northern Hemisphere, over the in winter. used, Atlantic. Again this is probably attributable to the nearly without fitting procedure. The resulting distribution was constant high meteorological activity over that area in • The coverage is very low in the equatorial regions, especially in the Pacific. used to define a detection limit at the 95th percentile December that ensures a rapid transport to the European NG for each site (see Fig. 1). Fig. 2: Radioxenon activities as a function of time following fission of 239Pu stations. 135 (PBq/kt). The calculations take into account the full decay chains .The • The coverage is very low for Xe (0-1 stations), but fairly similar for the other vertical green line marks the time chosen for the source term in this work. isotopes (0-15 stations). As expected, the detectability is lower when using a threshold References: frequency of 4 detections per stations (only 133Xe and 135Xe  A. Ringbom et. al., “Characterization of the global distribution of atmospheric radioxenons”, ISS09, Vienna, 2009. are shown) . However the patterns remain similar.  L.-E. De Geer, Rep. FOI-R—2350—SE.
Pages to are hidden for
"A study on the global detection capability of IMS"Please download to view full document