Measurements of Argon-39 at the U20az underground nuclear explosion site

Journal of Environmental Radioactivity 178-179 (2017) 28e35

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Measurements of Argon-39 at the U20az underground nuclear explosion site J.I. McIntyre a, *, C.E. Aalseth a, T.R. Alexander a, H.O. Back a, B.J. Bellgraph a, T.W. Bowyer a, V. Chipman b, M.W. Cooper a, A.R. Day a, S. Drellack b, M.P. Foxe a, B.G. Fritz a, J.C. Hayes a, P. Humble a, M.E. Keillor a, R.R. Kirkham a, E.J. Krogstad a, J.D. Lowrey a, E.K. Mace a, M.F. Mayer a, B.D. Milbrath a, A. Misner a, S.M. Morley a, M.E. Panisko a, K.B. Olsen a, M.D. Ripplinger a, A. Seifert a, R. Suarez a a b

Pacific Northwest National Laboratory, Richland, WA, USA National Security Technologies, LLC (NSTec), North Las Vegas, NV, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 19 May 2017 Received in revised form 18 July 2017 Accepted 19 July 2017

Pacific Northwest National Laboratory reports on the detection of 39Ar at the location of an underground nuclear explosion on the Nevada Nuclear Security Site. The presence of 39Ar was not anticipated at the outset of the experimental campaign but results from this work demonstrated that it is present, along with 37Ar and 85Kr in the subsurface at the site of an underground nuclear explosion. Our analysis showed that by using state-of-the-art technology optimized for radioargon measurements, it was difficult to distinguish 39Ar from the fission product 85Kr. Proportional counters are currently used for high-sensitivity measurement of 37Ar and 39Ar. Physical and chemical separation processes are used to separate argon from air or soil gas, yielding pure argon with contaminant gases reduced to the parts-permillion level or below. However, even with purification at these levels, the beta decay signature of 85Kr can be mistaken for that of 39Ar, and the presence of either isotope increases the measurement background level for the measurement of 37Ar. Measured values for the 39Ar measured at the site ranged from 36,000 milli- Becquerel/standard-cubic-meter-of-air (mBq/SCM) for shallow bore holes to 997,000 mBq/ SCM from the rubble chimney from the underground nuclear explosion. © 2017 Published by Elsevier Ltd.

Keywords: Nuclear explosion monitoring Argon-39 Argon-37 On-site inspection

1. Introduction Fission product and neutron activation gases created in the subsurface from an underground nuclear explosion (UNE) may be transported to the surface via forcing mechanisms. These include thermally induced pressure caused by the initial explosion, and later by diffusion and barometric pumping (Carrigan et al., 1996). However, a number of transport uncertainties associated with the movement of gases from the subsurface to the surface remain because of the wide variety of physical conditions that the gases may encounter. Some of the variables include the burial depth of the nuclear explosion, the presence or absence of subsurface water, the porosity of the subsurface rocks and soils, the creation or preexistence of fractures, and other local geological and emplacement

* Corresponding author. E-mail address: [email protected] (J.I. McIntyre). http://dx.doi.org/10.1016/j.jenvrad.2017.07.013 0265-931X/© 2017 Published by Elsevier Ltd.

conditions. In experiments conducted at the Nevada Nuclear Security Site (NNSS), formerly known as the Nevada Test Site, Pacific Northwest National Laboratory (PNNL) and National Security Technologies (NSTec) simulated the radioactive noble gases that would be created during a nuclear explosion and measured surface concentrations over a period of months. These experiments were conducted to study the flow of subsurface gases to better understand and quantify the subsurface transport of noble gases following an UNE. The experiments provided the bounds for several of the important observable variables: the time after the explosion that gases reach the surface, the concentrations of these gases, and the expected isotopic ratios at the surface. In addition, the tests helped identify conditions that could alter the expected measurement outcomes that might occur at a test location a few weeks to a few years following an UNE. The radioxenon isotopes (131mXe, 133Xe, 133mXe, and 135Xe) are

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produced in large quantities as fission products from an UNE. The longest-lived of these isotopes, 131mXe, has an 11.9 day half-life, which is too short to allow radiometric measurement over a yearlong study given the limitations on the activity that could be produced and released (Browne and Firestone, 1986). Instead, 127Xe (t1/ 2 ¼ 36.4 days) was used as a radioxenon stand-in for the fission product radioxenon isotopes. Thirty-seven TBq of 127Xe were generated via neutron capture during reactor irradiation of 126Xe at the University of Texas, Austin (Klingberg et al., 2015). Argon-37 can be produced via the 40Ca(n,a) 37Ar reaction and is likely to be produced in most UNEs because calcium is ubiquitous throughout the earth's crust. Like 127Xe, 37Ar has a long enough half-life (t1/ 2 ¼ 35.0 days) for use during a year-long period, and was produced via neutron capture on 36Ar at the University of Texas at Austin. Frequently SF6 has been used as a stable surrogate for the radioactive noble gases (radioxenon in particular) to avoid the half-life decay issues (Carrigan et al., 1996). However, from previous experiments, the use of SF6 as a surrogate for radioxenon and radioargon isotopes was found to introduce errors in interpretation because SF6 has a significantly different adsorption coefficient in water than the noble gases (Olsen et al., 2016), and the amount of subsurface water along the transport pathways is not well known. Krypton-85 is another fission product noble gas that is produced in large quantities in an UNE and its presence was anticipated during sampling. Because these experiments used a former UNE site, the amount of 85Kr (t1/2 ¼ 10.7 years), see Table 1 [Singh and Cameron, 2006; Singh and Chen, 2014]) could be considerable even decades after the UNE. Initial analysis of several subsurface gas samples found an unknown contaminant present after purification of argon, which was later identified as 39Ar. The identification of this isotope was non-trivial as it was difficult to distinguish 39 Ar from 85Kr; the latter was originally considered the most likely contaminant. In the rest of the paper we describe: the U20az test bed used for the subsurface transport, our measurement techniques to distinguish between the isotopes, how these isotopes can interfere with the measurement of other nuclear explosion signature radioisotopes, and general conclusions.

2. U20az test bed site In 2013, a noble gas migration experiment was conducted at the U20az site at NNSS to study the detection of an UNE using noble gas signatures (Olsen et al., 2016; Carrigan and Sun, 2014). The U20az site was the location of an underground nuclear test (Barnwell) conducted during 1989 with a yield between 20 and 150 kt (DOE, 2000,p. 86; Schoengold et al., 1996, p. 231). The site was identified as a testbed for gas migration experiments in 2012 and has had several deep and shallow boreholes drilled from 2014 to 2016 for sampling campaigns that will last over the course of month to years. The experiment involved the injection of 9.21  1010 Bq of 127 Xe, 4.48  1010 Bq of 37Ar, and 121 kg of SF6, into the U20az chimney through an 1800-foot drill-back hole. These tracer gases migrated from the U20az chimney to near-surface and surface sampling locations and were detected in soil gas samples collected using multiple on-site inspection sampling approaches. The results of this experiment were:  Subsurface-to-surface dilution factors calculated for SF6, 37Ar, and 127Xe demonstrated that SF6 was enriched in all of the samples relative to both 37Ar and 127Xe.  When 127Xe and 37Ar were present in a soil gas sample, there were no significant differences in the 127Xe-to- 37Ar ratio in the samples relative to the ratio injected into the chimney.

29

 Migratory behavior of the chemical and radiotracers did not fit typical diffusion modeling scenarios, which predicted different arrival times and dilution factors for the three tracers. Results of this experimental effort suggested the need for additional studies to confirm the initial observations and be incorporated into subsurface transport models to account for the observed migration behaviors of the noble gases as they migrate from an UNE to surface sampling locations. Recent work has begun to address this issue by incorporating thermally driven effects from the UNE that are then coupled to barometric transport in the modeling efforts (Sun et al., 2015; Sun and Carrigan, 2016). 2.1. Background

37

Ar measurements

In 2015, a second set of experiments was conducted at the U20az test bed location to further understand noble gas migration in the subsurface environment. A shallow well (10 m deep) was drilled and segmented into three zones as part of a study to measure the naturally occurring 37Ar background produced by cosmicray-generated neutrons. Several previous studies by the University of Bern Noble Gas Laboratory have shown that the 37Ar activity can vary greatly depending on the calcium content of the soil. Typical 37 Ar concentrations from shallow subsurface gas sampling ranged from a few milli-becquerels per standard cubic meter1 (mBq/SCM) to a few tens of mBq/SCM (Riedmann and Purtschert, 2011). 2.1.1. Description of borehole NG-6 The shallow bore hole (NG-6) was drilled in 2014. In October 2015, PNNL staff segmented the well into three zones and Fig. 1 shows the segmentation and completion of NG-6 to create the three isolated sampling points. The 37Ar and 127Xe from the June 2013 injection had decayed away to well below the 1mBq/SCM detection levels of the argon and xenon systems used for quantification. The zones were sealed for several months to allow the ingrowth of 37Ar. In March 2016, four 2-m3 whole air samples were taken, one from each of the three zones and one atmospheric sample. The 147 days of intervening time allowed the 37Ar produced in the soil to reach greater than 97% of the final equilibrium value. During sample collection, pressure transducers measured the pressure at each of the three depths and barometric pressure. These data were used to estimate the formation permeability. Based on the measured response, the permeability calculated for the three zones were within an order of magnitude. The middle zone had the lowest measured permeability, and the shallow zone had the highest measured permeability. The results are also consistent with images taken of the borehole prior to segmentation and completion; the shallow zone has the largest fractured area and the middle zone the smallest fractured area. For all zones, there was no discernible decrease in pressure when sampling was occurred in an adjacent zone. This indicates that the borehole zones were adequately sealed. 3. Experimental PNNL processed the NG-6 samples for 37Ar and found them to contain a contaminant that decayed via a high-energy beta and had a half-life that was much greater than the 35-day half-life of 37Ar. There are two potential gases that this contaminant could be, based on the previous UNE at the site: 85Kr from fission of uranium or

1 SCM is a standard cubic meter of air at a temperature of 0  C and a pressure of 100 kPa.

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Table 1 Relevant nuclear decay parameters for the isotopes under discussion. Isotope

Decay Mode

T1/2

Daughter

Major Emission (keV)

Average Energy (keV)

Probability (%)

37

Electron capture bbb-,g

35.0 d 269 yr 10.7 yr

37

Auger e 2.82 565 687 137, 514

219 272

100 100 99.56 0.43

Ar 39 Ar 85 Kr

Cl 39 K 85 Rb

proportional counters (PCs), high-purity germanium HPGe gamma spectrometers, and plastic-scintillation beta detectors. The PC measurements were used to determine 37Ar concentrations, and then the PC energy range was set to a higher dynamic range to measure the high-energy beta distribution. The gamma measurements were used to examine the sample for the presence of the low-intensity gamma emission from 85Kr. High-fidelity beta energy measurements were used to determine the beta endpoint energy of the samples and then compared to known standards. 3.1. Physics decays for

37

Ar,

39

Ar, and

85

Kr

Table 1 shows the nuclear decay mechanisms, half-lives, and other relevant physics parameters for the three isotopes under consideration. Without careful beta endpoint energy determinations or the ability to detect very low branching ratio gammas, as in the case for the gamma from 85Kr, it was difficult to distinguish the difference between 39Ar and 85Kr. Argon-37, unlike the other isotopes of interest, is a more challenging isotope to measure. It decays 100% of the time into 37Cl by electron capture with very low energy X-rays. The majority of the time a K-shell Auger electron is emitted with an energy of 2.82 keV. X-rays with energy of ~2.6 keV and ~2.8 keV are also emitted, but with a much lower probability. This low-level electron is possible to measure via internal-source proportional counting, but requires a lowthreshold, low-background detector (Aalseth et al., 2011). 3.2. Argon gas separation system

Fig. 1. A schematic of the completion of U20az NG-6 hole with three sampling zones (shown in yellow). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

plutonium, or 39Ar via the 39K(n, p)39Ar reaction. Krypton-85 was identified as one of several isotopes released from the Barnwell UNE at U20az (Schoengold et al., 1996, p. 276). The fission product 85 Kr is produced in very large quantities from nuclear explosions, has a long half-life (t1/2 ¼ 10.72 yrs), is a high-energy beta emitter, and is likely to be present as a post-purification radioactive contaminant if a high decontamination factor is not achieved. Argon-39 was also considered a potential radioactive contaminant after modeling efforts confirmed that it could be produced in significant quantities in an UNE as a neutron activation product. Argon-39 is also a high-energy beta emitter, has a long half-life (t1/ 37 Ar by the argon 2 ¼ 269 yrs), and would not be separated from processing system. In addition, several other NNSS UNEs indicate that 39Ar was released as part of drill-back operations (Schoengold et al., 1996, p. 276). To determine which of these contaminant isotopes caused the large background, a series of measurements were made with

To make the 37Ar measurement, argon was separated from the 400-L whole air grab sample and purified using PNNL's Argon Separation and Purification System (Aalseth et al., 2016). This system used several separation and purification processes to produce a high-purity argon sample required for the PC measurement. The first step was to determine the gas composition. For the U20az samples, the composition was similar to air but with elevated levels of CO2. The composition was analyzed to ensure the Pressure Swing Adsorption (PSA) process used to remove N2 from the sample functioned correctly. Elevated CO2 levels were not a concern, as long as the CO2 was less than a few percent by volume. To separate and purify argon, the sample passed through a multicolumn PSA apparatus that uses a lithium exchanged zeolite (LiLSX) to separate nitrogen from oxygen and argon in the gas sample. This subsystem produced a gas stream containing around 5% Ar in oxygen. After the PSA nitrogen removal step, oxygen was removed from the permeate stream by passing the gas through a CoO bed at 700  C. The cobalt oxide was oxidized to Co2O3, removing oxygen from the gas stream. The CoO bed removed the majority of the oxygen and a high-temperature electrochemical pump tube held at 700  C was used to remove any remaining oxygen. The electrochemical pump tube used an oxygen transporting ceramic, such as is used in solid oxide fuel cells, and electrical energy to remove oxygen from the gas stream. The resulting gas contained less than 1 ppm oxygen. After oxygen removal, the gas stream passed through a small LiLSX adsorption trap held at 120  C to remove any nitrogen and

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high molecular weight noble gases not removed during the PSA process. After passing through the cold LiLSX trap, the resulting argon stream was frozen in a small collection volume held at less than 200  C. At the end of processing, the head space of the collection volume was evacuated to remove non-condensable gases such as helium and neon. This resulted in high-purity argon suitable for use in the PCs used to make the 37Ar measurement. The PCs were filled with a gas consisting of 10% methane and the high-purity argon gas sample. The collection volume was heated and the collected argon expanded into the proportional counter. The detector valves were closed when the desired load pressure was reached. 3.3. Proportional counter data analysis of NG-6 samples for

37

Ar

PNNL has developed an ultra-low-background measurement capability centered around a Shallow Underground Laboratory (Aalseth et al., 2012). This facility accommodates the production of ultra-clean radiopure materials that are used in the creation of ultra-low-background proportional counters (ULBPCs) (Hoppe et al., 2014; Aalseth et al., 2009, 2016). These detectors can be used for the measurement of low-activity gas samples in the UltraLow-Background Counting System (Seifert et al., 2013). Each of the NG-6 samples were processed at PNNL and then counted in the Shallow Underground Laboratory until sufficient counting statistics were achieved. Fig. 2 shows an example of an argon sample (FP005) calibrated at the 37Ar region of interest (ROI) 0e15 keV. A constrained fit on an exponential background is shown in magenta where the 37Ar peak would be located (2.82 keV). The background activity in this sample was greatly elevated above normal air, so the sensitivity in the 37Ar ROI is diminished. Given the elevated background, this sample was recalibrated to look at higher energy events. Fig. 3 shows the same argon sample (FP005) now calibrated at 0e400 keV. The spectrum in black is the argon sample data, the spectrum in blue is the detector background (subtracted out during analysis), and the spectrum in red is a PC efficiency measurement of 39Ar. The efficiency and the sample data have very similar shapes, which indicate that the additional analyte present in the argon sample may be 39Ar as illustrated in Fig. 2. Further discussion of 39Ar measurements is described in Hall et al. (2015). Table 2 shows the results for the 37Ar measurements made in the shallow borehole NG-6. Typical background levels at the depths measured range from no 37Ar up to 40 mBq/SCM. The measured values had large uncertainties because of the 39Ar interference but corresponded to the approximate ranges expected. Values within 1 m of the surface fall to zero because of barometric cycling effects. Samples 1e3 m below the surface were expected to have the highest levels of 37Ar as the neutron fluence was greatest from 0 to 3 m. Below 6 m37Ar values were expected to fall off from decreased

31

neutron fluence caused by increased overburden (Riedmann and Purtschert, 2011). The 39Ar activity of the argon samples was analyzed using an 39 Ar detector efficiency reference spectrum and the results are presented in Table 3. The sample taken from the U20az chimney had activity that was higher than the reference spectrum, most likely because of the very good containment of the site with few, if any, large fractures that extend from the chimney to the surface.

3.4. Beta endpoint energy measurements of

85

Kr and

39

Ar

Both 85Kr and 39Ar were measured in a beta/gamma detector system to determine if the endpoint energies could be measured. The beta detector consisted of a plastic BC-404 beta cell optically cemented to a photomultiplier tube and placed in a NaI(Tl) wellstyle detector to measure gamma and X-rays (Cooper et al., 2007). Data were collected with an XIA Pixie-4 Multichannel Digital Gamma Finder with in-house software. The cells were connected to a gas manifold to transfer gas out of and into the system via vacuum and volumetric expansion. The cells were put under vacuum and a background measurement was performed. The beta and the gamma spectra were measured for 24 h. A sample of 85Kr was then put into the cells and measured for 24 h. A background subtraction on the measured 85Kr beta spectrum was performed to see the 85Kr beta continuum. To determine the endpoint energy of the beta continuum, a Fermi-Kurie plot was produced by plotting the function:

vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u nðEÞ u FðEÞ ¼ tqffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  ffi 2 E þ 2mc2 E E þ mc2 where E is the energy detected by the beta cell, n is the number of counts, and mc2 is the mass of the electron (511 keV) (Cooper et al., 2008; Krane, 1988). A straight line was fit to the results and the intercept with the x-axis was determined to be the endpoint energy. For the 85Kr data, the endpoint energy was 687 keV as seen in Fig. 4. The low-energy and high-energy region were excluded from the fit. The results of the measurements can be seen in Fig. 4 and clearly indicate that the beta cell measurements can distinguish among the beta energy spectrum from 85Kr and the beta energy spectra from 39Ar. A measurement of a known 39Ar beta spectra would provide the most convincing evidence for comparison if such a source were available. Instead a careful study was made using gamma-ray analysis of a known 85Kr sample, the whole air from the U20az chimney sample (no gas separations), and the purified U20az chimney sample. As illustrated in Fig. 4, the purified argon sample collected from U20az chimney is clear evidence that the additional contaminant was 39Ar and not 85Kr.

Fig. 2. Argon sample (FP005) collected on March 14, 2016 from the NG-6 shallow borehole and analyzed at the 37Ar ROI (0e15 keV). A constrained fit on an exponential background is shown in magenta where the 37Ar peak would be located. The background activity in this sample was greatly elevated (860x “normal air”) so the sensitivity in the 37Ar ROI was diminished. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Fig. 3. Argon sample (FP005a) recalibrated from 0 to 400 keV to look at higher energy events and compared to an erences to colour in this figure legend, the reader is referred to the web version of this article.)

Table 2 Argon samples analyzed at the

39

Ar efficiency spectrum (red). (For interpretation of the ref-

37

Ar ROI.

Sample ID

Sample Description

Bkg Elevation Factor over atm. air

Bkg Count Rate (cpd) in the 5e15 keV ROI

Constrained Fit Analysis of SCM)

FP005

NG-6, Shallow, 3/14/ 16 NG-6, Mid, 3/14/16 NG-6, Deep, 3/15/16

860x

22,276.2

31.2 ± 10.6

1,020x 676x

26,433.2 17,502.2

43.6 ± 11.6 20.3 ± 14.8

FP006 FP007

Table 3 Argon samples analyzed at the hole casing and chimney.

39

Ar ROI with two additional samples for the bore-

Sample ID

Sample Description

39

FP005a FP006a FP007a FP010 FP011

NG-6, Shallow, 3/14/2016 NG-6, Mid, 3/14/2016 NG-6, Deep, 3/15/2016 U20az, Casing, 5/18/2016 U20az, Chimney, 5/18/2016

40,400 ± 360.0 48,900 ± 420.0 36,600 ± 388.0 99,900 ± 789.0 952,700 ± 7720

Ar Activity (mBq/SCM) whole-air equivalent

Fig. 4. Fermi-Kurie Plot of 85Kr (known) and the purified argon sample collected from U20az chimney. The Fermi-Kurie line for the 85Kr function is fit between 150 and 625 keV and for the argon sample, the fit is between 190 and 450 keV. In both cases, the lines are extended to the x-axis to determine the endpoint energy.

3.5. Gamma-ray analysis for

85

Kr

Using gamma-ray spectroscopy, it is possible to determine the Kr content of a whole-air sample. However, 85Kr is difficult to measure with high sensitivity via gamma-ray spectroscopy because of the low 0.434% gamma emission intensity of its primary line at 514 keV and the proximity of this primary line to the 511 keV annihilation peak. To use this measurement technique for quantification of both the processed argon samples and a whole-air sample (non-processed), an efficiency calibration of the 85

37

Ar on Exponential Bkg (mBq/

CASCADES germanium array was performed (Keillor et al., 2011). On May 23, 2016, the efficiency calibration was performed using 316 ± 6 cc of pure krypton gas containing 0.765 ± 0.015 Bq/cc. The reference gas was contained in a 53 ± 0.01 cc Swagelok® bottle for measurement on the CASCADES system. Fig. 5 shows the 514 keV 85 Kr peak, along with the system background. This measurement established the 514 keV peak detection efficiency as 6.87% ± 1.37% for this counting geometry. Fig. 6 shows the results from a 14-day count on the CASCADES detector of the U20az sample after it was processed to extract the argon and counted in a proportional counter. The peak analysis at 514 keV indicated that the sample was at or very near the detector background caused by the 511 keV annihilation peak. The gamma ray spectra shown in Fig. 6 also include a spectrum measured using 500 cc of unprocessed whole air from the chimney sample. The whole air spectrum clearly indicated a substantial peak at 514 keV that was well above the detector background and was consistent with the 85Kr reference peak. The argon present in the processed chimney sample represented more than 100 times the air present in the sample of whole air from the chimney. As illustrated in Fig. 6, the whole-air sample from the U20az chimney (prior to argon processing) had a clear 85Kr peak at 514 keV and was determined to be 0.131 ± 0.0045 Bq/cc of whole air. However, the purified argon sample from the U20az chimney did not appear to have 85Kr present and was consistent with the empty cylinder background spectra. Based on the positive result for 85 Kr in the whole-air sample and the limit value for the postpurification sample, these measurements established a factor of greater than 10,000 for the separation of krypton from argon for the argon processing system.

3.6. Model of

39

Ar production from np reaction on

39

K

Having established that the interference in the 37Ar measurements is from 39Ar, efforts were undertaken to better understand were the 39Ar came from. The most likely cause was from neutron capture from the actual UNE. Other likely production mechanisms were 39Ar produced via cosmic-ray interactions in the soil and in the air. However, at the levels seen across the U20az site, it was clear that these mechanisms were insufficient in strength to account for the ~1000 fold increase in 39Ar activity. This left neutron activation from an UNE as the most likely production mechanism of

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Fig. 5. Krypton-85 peak at 514 keV measured in reference krypton gas, along with system background showing the annihilation peak (511 keV).

39

the observed Ar. Argon-39 is primarily produced through an incident neutron reaction on potassium, 39K(n,p)39Ar. Like the 40Ca (n, a) 37Ar reaction, this is a threshold reaction; plots of the ENDF cross sections for these reactions are shown in Fig. 7 (Chadwick et al., 2011). For potassium the estimated average elemental composition in the upper continental crust around the world is 28,650 ppm (Wedepohl, 1995). With a natural abundance of about 93%, 39K is very likely to be substantially neutron-activated in natural geology surrounding an underground nuclear explosion. As an estimate of possible 39Ar activity following an UNE, a calculation was made of a generic 1 kt-equivalent energy nuclear fission burst surrounded by an isotropic shell of geologic material representative of the average elemental composition of the upper continental crust. This calculation made use of the Monte Carlo Nparticle (MCNP) transport code to estimate the resulting flux of neutrons from a fissioning bare sphere of highly enriched uranium fuel and subsequent attenuation through geologic material (Hendricks et al., 2000). Generation of isotopic inventories, including the 39K(n,p) 39Ar reaction of specific interest, resulting from the neutron fluence was calculated using ORIGEN 2.2 (Croff, 1983). The resulting inventory of 39Ar from neutron activated 39K was found to be 2.57  1012 Bq. Fig. 8 depicts a plot of the estimated 37 Ar, 39Ar, and 85Kr inventories per kt of nuclear yield decayed out to 15 years. This is a rough estimate of the activation inventory for a generic 1-kt UNE since the true nuclear explosive yield is not known

33

Fig. 7. Plots of the primary 37Ar and 39Ar producing reaction cross-sections taken from ENDF.

beyond the published 20e150 kt range. In addition, the true geologic makeup and scenario-specific characteristics of the explosion would have a substantial effect on the activated nuclide inventories that result in the surrounding rock. Still, barring dilution in the subsurface environment and outright loss to the atmosphere, the 39Ar resulting from an UNE is likely to remain significant and potentially detectable for an extended period of time (years to decades). From Fig. 8, it is clear that both argon isotopes can be produced in greater quantities than the fission product 85Kr, which is directly related to the UNE.

4. Results and discussion The initial assumptions when setting up the test bed activities around the U20az site were that the subsurface concentrations from previous injections (127Xe and 37Ar) had decayed away and that other contaminants such as 85Kr would not present an issue. The purpose of the 37Ar studies was to determine the background levels of this important on-site inspection isotope across a range of subsurface geological types at the NNSS. Measurements of 37Ar at naturally occurring concentrations levels are important to identify the effect of different subsurface processes such as retention times, production rates, and the role of atmospheric pumping. The discovery of a large interfering background from another argon isotope, 39Ar, was not foreseen at the outset of the measurement campaign and had the potential to dramatically affect the sensitivity for measurement of 37Ar when used as an indicator of a recent underground nuclear explosion.

Fig. 6. A HPGe gamma-ray spectrum taken of a small 500 cc sample of whole air from the chimney of U20az (solid line) along with a background count of an evacuated cylinder (dotted line), and a spectrum of the purified 39Ar sample. The purified argon and empty cylinder spectra are difficult to distinguish, and are both present as the small peak at 511 keV). The 85Kr reference sample spectrum (peak extending off-scale) is provided as a comparison to the whole air sample.

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Fig. 8. Estimated inventories of 37Ar and 39Ar activated in rock assuming a 1-ktequivalent highly enriched uranium fission source. Also shown is the activity of 85Kr produced via fission. Both the argon and krypton specific activities were modeled using MCNP and ORIGEN 2.2. The production of the two argon isotopes used average crustal abundances of calcium and potassium; therefore, they were likely to have wide dynamic range depending upon the environment of the UNE.

Fig. 9. Expanded view to focus on the positive detections of 37Ar with low to moderate 39 Ar activities. Argon-39 activities for all samples are estimated using the rate observed in a 10e15 keV ROI available in the 37Ar analysis. A calibration factor relating the 10e15 keV ROI rate to total 39Ar activity was derived separately (see text for details). The green dashed line indicates the activity of 39Ar in the atmosphere; this is considered to be a relatively constant value (Benetti et al., 2007). Note the logarithmic scale on the horizontal axis. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

4.1. Argon-39 interference Natural soil gas backgrounds of 37Ar vary from less than 1 mBq/ SCM to 125 mBq/SCM of whole-air depending on the geographical location and soil characteristics (Riedmann and Purtschert, 2011); the activity of 39Ar is 1.01 Bq/kg of natural Argon, which corresponds to a39Ar concentration of 16.6 mBq/SCM in whole-air equivalent (Benetti et al., 2007). This value provides a useful reference point for considering the elevated levels of 37Ar seen in this work as well as the effect of elevated 39Ar levels. Positive detections of 37Ar (red circles) shown in Fig. 9 are from soil gas samples measured at PNNL from a range of environmental sampling locations that included the U20az site. To determine the effect of interference from 39Ar, these samples were recalibrated from the 37Ar energy scale (0e15 keV) to directly measure the total 39 Ar activity from 0 to 400 keV. A calibration factor was determined which related the observed background rate in the 10e15 keV window to the total measured 39Ar activity in the sample. This factor was calculated for five different samples with varying levels of 39Ar activity and then averaged together. This calibration factor was used to estimate the 39Ar activity for all 37Ar measurements in Figs. 9 and 10. The blue diamond markers in Figs. 9 and 10 represent the minimum detectable 37Ar concentration (at 1-sigma) that would need to be present for 37Ar to be detectable using the current proportional counting system at PNNL. The analysis presented in Fig. 10 uses a constrained fit, which includes an exponential background term driven largely by 39Ar and a Gaussian peak for the 37Ar signal (see Figs. 2 and 3). The uncertainties on the 37Ar value are increased as higher 39Ar concentrations contribute more to the backgrounds under the 37Ar peak. The upper limits shown are estimates of the concentration of 37Ar that could be present in the sample without being detected at the 68% confidence level (1sigma). Fig. 9 shows that as 39Ar background levels increase, the sensitivity for 37Ar is reduced because of interference. This sensitivity has been parametrized to determine the 37Ar minimumdetectable-concentration as a function of the concentration of 39 Ar. Not included in this analysis is the reduction of the 37Ar activity from half-life decay. It is clear from Fig. 8 that at times greater than one year, the concentration of 37Ar falls below the activity of both 39Ar and 85Kr. The results of this study have implications for the local

monitoring of nuclear tests such as subsurface gas samples from an on-site inspection. Two long-lived noble gas isotopes that could cause backgrounds for decades in gas sampling and detection equipment used in an on-site inspection have been measured (Xie et al., 2014). The potential for presence of 39Ar is particularly significant as it will interfere with 37Ar measurements and could easily obscure the 37Ar signal after one year. Measurement of 39Ar and 85 Kr signatures in the subsurface following an UNE could be advantageous if an on-site inspection occurs years to decades after the nuclear test.

5. Summary The worldwide crustal abundance of calcium and potassium are both in the range of a few percent, but locally variable. In addition, the energy-dependent (n,p) cross sections for the production of 37 Ar and 39Ar (see Fig. 7) above about 1 MeV are within a factor of approximately two of each other, where the peak of the fission neutron energy lies for a nuclear explosion. Hence, the expected concentrations of these two argon isotopes generated by an UNE should be approximately the ratio of their half-lives (35 days vs. 269 years). The concentration of 39Ar at the time of the UNE could

Fig. 10. Activity in whole-air equivalent (mBq/SCM) for 37Ar as a function of elevated 39 Ar backgrounds (where the 39Ar activity is in units of Bq/SCM).

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be expected to be in the range of 103 to 104 of the concentration of 37Ar. However, after approximately 10 half-lives (~350 days) of 37 Ar, the concentrations of the two argon isotopes will begin to equate and the 39Ar created in an UNE could begin to cause an appreciable background to the measurement of 37Ar. This result is highly variable, since the concentration of potassium and calcium may vary around the UNE location by several orders of magnitude. Given typical ranges for these two elements, it can be expected that after approximately one year measurement of the 37Ar concentration may be still be significantly affected by the presence of 39Ar. An additional concern is the presence of 85Kr directly from the fission process. The presence of 85Kr will not generally affect detector operation, and so trace amounts of krypton in the detector could occur without notice. As demonstrated above, the presence of 85 Kr may not be easy to distinguish from 39Ar, especially with infield measurement conditions. Therefore, there is a possibility that there could be a misidentification between 39Ar from 85Kr. For this reason, care should be taken in the processing of air samples to collect only the argon fraction. By keeping the total contamination from krypton isotopes to less than 1% of the total activity fraction, then the interference on 37Ar measurements occurring a few months following a nuclear test would be minimal, and accurate 37Ar quantification should be possible. However, it may not be possible to determine whether the interference in the 37 Ar measurement was from 39Ar or 85Kr. The use of field quality stable gas measurements such as a gas chromatograph or a mass spectrometer should be considered as important diagnostic tools to aid in this determination. Acknowledgements The authors wish to acknowledge to the National Nuclear Security Administration, Defense Nuclear Nonproliferation Research and Development, the Underground Nuclear Explosion Signatures Experiment working group, and an interdisciplinary group of scientists and engineers from Pacific Northwest and Lawrence Livermore national laboratories and Nevada Security Technology. This work was performed by Pacific Northwest National Laboratory under award number DE-AC52-06NA25946. References Aalseth, C.E., Humble, P.H., Mace, E.K., Orrell, J.L., Seifert, A., Williams, R.M., 2016. Shielding concepts for low-background proportional counter arrays in surface laboratories. App. Radiat. Isot. 108, 92e99. Aalseth, C.E., Bonicalzi, R.M., Cantaloub, M.G., Day, A.R., Erikson, L.E., Fast, J., Forrester, J.B., Fuller, E.S., Glasgow, B.D., Greenwood, L.R., Hoppe, E.W., Hossbach, T.W., Hyronimus, B.J., Keillor, M.E., Mace, E.K., McIntyre, J.I., Merriman, J.H., Myers, A.W., Overman, C.T., Overman, N.R., 2012. A shallow underground laboratory for low-background radiation measurements and materials development. Rev. Sci. Instrum. 83 (11), 113503. Aalseth, C.E., Day, A.R., Haas, D.A., Hoppe, E.W., Hyronimus, B.J., Keillor, M.E., Mace, E.K., Orrell, J.L., Seifert, A., Woods, V.T., 2011. Measurement of 37Ar to support technology for on-site inspection under the comprehensive nucleartest-ban treaty. Nucl. Instrum. Methods Phys. Res., Sect. A 652 (1), 58e61. Aalseth, C.E., Day, A.R., Hoppe, E.W., Hossbach, T.W., Hyronimus, B.J., Keillor, M.E., Litke, K.E., Mintzer, E.E., Seifert, A., Warren, G.A., 2009. Design and construction of a low-background, internal-source proportional counter. J. Radioanal. Nucl. Chem. 282 (1), 233e237. Benetti, P., Calaprice, F., Calligarich, E., Cambiaghi, M., Carbonara, F., Cavanna, F., Cocco, A.G., Di Pompeo, F., Ferrari, N., Fiorillo, G., Galbiati, C., Grandi, L., Mangano, G., Montanari, C., Pandola, L., Rappoldi, A., Raselli, G.L., Roncadelli, M., Rossella, M., Rubbia, C., Santorelli, R., Szelc, A.M., Vignoli, C., Zhao, Y., 2007. Measurement of the specific activity of 39Ar in natural argon. Nucl. Instrum. Methods Phys. Res., Sect. A 574 (1), 83e88. Browne, E., Firestone, R.B., 1986. Table of Radioactive Isotopes, first ed. John Wiley & Sons, New York. Carrigan, C.R., Sun, Y., 2014. Detection of noble gas radionuclides from an underground nuclear explosion during a CTBT on-site inspection. Pure Appl. Geophys. 171 (3e5), 717e734.

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