Beta-gamma counting system for Xe fission products

A beta-gamma coincidence counting system has been developed for automated analysis of Xe gas samples separated from air. The Xe gas samples are contained in a cylindrical plastic scintillator cell located between two NaI(T1) scintillation detectors. The X-ray and gamma spectra gated by coincident events in the plastic scintillator cell are recorded for each NaI(T1) crystal. The characteristic signatures of the^131mXe,^133gXe,^133mXe, and^135gXe isotopes of interest for nuclear test-ban verification as well as the procedures and results of absolute efficiency measurements are described. A NaI(T1) crystal with provision for 4 sample cells has been implemented for the system to be deployed in the field. Examples of data on ambient air samples in New York City obtained with the field prototype are presented.     

Geant4 Monte Carlo radioxenon beta-gamma coincidence efficiency simulation for a phoswich detector

In this work, a Monte Carlo (MC) simulation model is established to accurately characterize a phoswich beta-gamma coincidence detector system. This model can be easily used to predict the beta-gamma coincidence efficiencies of xenon radioisotopes at various stable xenon concentrations in the counting cell. The results demonstrate that there is a significant inverse correlation between beta-gamma coincidence efficiency and stable xenon concentration. The influence of stable xenon concentration on beta-gamma coincidence counting efficiency has been investigated for each individual xenon radioisotope. The results indicate that the effect of stable xenon concentration on beta-gamma coincidence efficiency depends on the xenon radioisotope and its decay modes. The coincidence efficiency of ¹³³Xe with 31.0-keV X-ray decay mode is the most affected one; and then followed by ^131mXe, ¹³³Xe with 81.0-keV gamma-ray decay mode, ^133mXe and finally ¹³⁵Xe. The study also indicates that the gamma absorption by xenon gas plays more of a role in the decrease of beta-gamma coincidence efficiency for ¹³³Xe and ¹³⁵Xe, and that the conversion electron spectrum shifting and broadening plays more of a role in the reduction of beta-gamma coincidence efficiency for the metastable radioxenon of ^131mXe and ^133mXe.     

Concentration independent calibration of β–γ coincidence detector using 131mXe and 133Xe

Absolute efficiency calibration of radiometric detectors is frequently difficult and requires careful detector modeling and accurate knowledge of the radioactive source used. In the past we have calibrated the β–γ coincidence detector of the Automated Radioxenon Sampler/Analyzer (ARSA) using a variety of sources and techniques which have proven to be less than desirable (Reeder et al., J Radioanal Nucl Chem, 235, 1989). A superior technique has been developed that uses the conversion-electron (CE) and X-ray coincidence of ^131mXe to provide a more accurate absolute gamma efficiency of the detector. The ^131mXe is injected directly into the beta cell of the coincident counting system and no knowledge of absolute source strength is required. In addition, ¹³³Xe is used to provide a second independent means to obtain the absolute efficiency calibration. These two data points provide the necessary information for calculating the detector efficiency and can be used in conjunction with other noble gas isotopes to completely characterize and calibrate the ARSA nuclear detector. In this paper we discuss the techniques and results that we have obtained.     

Modeling β-γ coincidence spectra of <Superscript>131m</Superscript>Xe, <Superscript>133</Superscript>Xe, <Superscript>133m</Superscript>Xe,and <Superscript>135</Superscript>Xe

In support of the Comprehensive Nuclear-Test-Ban Treaty (CTBT), improvements have been made to the model of the Automated Radioxenon Sampler/Analyzer (ARSA) β-γ coincidence detector for radioxenon monitoring. MCNPX is used to simulate the detector response for all the electrons and photons emitted from ^131mXe, ¹³³Xe, ^133mXe, ¹³⁵Xe, and ¹³⁷Cs signals. A MatLab code was written to incorporate the MCNPX results in the calculation of β-γ coincidence spectra. These will aid in the development of the Spectral Deconvolution Analysis Tool (SDAT)¹ and to calibrate β-γ coincidence systems. The models developed for this work include improvements over previous models in their ability to address Compton scattering in the β-cell, and the β-distribution offset in the 31 keV γ-ray region for ¹³³Xe.     

SDAT implementation for the analysis of radioxenon β–γ coincidence spectra

The standard deconvolution analysis tool (SDAT) was developed for analysis of radioxenon β–γ coincidence spectra measured as part of the international monitoring system as defined in the comprehensive nuclear-test-ban treaty. The SDAT software analyzes each β–γ coincidence spectrum by fitting library vectors of each radionuclide of interest: ^131mXe, ^133mXe, ¹³³Xe, and ¹³⁵Xe. Detector background and radon are incorporated as optional components of the sample solution. Results are reported in mBq m^?3. A new graphical user interface has been developed to facilitate ease of use and improve the data visualization. Automated energy versus channel calibration algorithms were developed and implemented based on ¹³⁷Cs β–γ coincidence spectra. Details on the user tool and testing are included.     

Degradation of 81 keV <Superscript>133</Superscript>Xe gamma-rays into the 31 keV X-ray peak in CsI scintillators

Pacific Northwest National Laboratory uses beta-gamma coincidence detectors in a number of xenon sampling and measurement systems to enable simultaneous, sensitive measurements of ¹³¹Xe, ¹³³Xe, ^133mXe, and ¹³⁵Xe for treaty monitoring applications. In recent years, a new style of beta–gamma detector was developed to improve upon the detector module used in the Automated Radioxenon Sampler/Analyzer. The results of an MCNP5 Monte Carlo simulation of the new detector cell are presented, with particular emphasis on the identification of an energy deposition sequence with the potential to introduce significant error into the detector efficiency calibration. This sequence occurs when an 81 keV gamma from ¹³³Xe is absorbed in an inactive region of the CsI(Na) scintillator, followed by emission of a 31 keV X-ray from cesium (or possibly a 28.5 keV X-ray from iodine). These X-rays add excess counts into the 31 keV peak observed in the decay of ¹³³Xe. The impact of this effect on different efficiency calibration techniques is discussed.     

Energy resolution of liquid scintillator for α-particles and internal conversion electrons at lower temperatures

The energy resolution of -particles from²⁴¹Am,²²²Rn,²¹⁸Po and²¹⁴Po and internal conversion electrons from^131mXe with a liquid scintillation system has been studied at temperatures from 13 °C to –65 °C. At lower temperatures the liquid scintillation system has shown to give better energy resolutions both for -particles and internal conversion electrons compared with the values obtained at ordinary temperature. The phenomena were explained by the increase in light output of liquid scintillator at lower temperatures.     

Radioxenon production through neutron irradiation of stable xenon gas

The Spectral Deconvolution Analysis Tool (SDAT) software was developed to improve counting statistics and detection limits for nuclear explosion radionuclide measurements. SDAT utilizes spectral deconvolution spectroscopy techniques and can analyze both β–γ coincidence spectra for radioxenon isotopes and high-resolution HPGe spectra from aerosol monitors. The deconvolution algorithm of the SDAT requires a library of β–γ coincidence spectra of individual radioxenon isotopes to determine isotopic ratios in a sample. In order to get experimentally produced spectra of the individual isotopes, we have irradiated enriched samples of ¹³⁰Xe, ¹³²Xe, and ¹³⁴Xe gas with a neutron beam from the TRIGA reactor at The University of Texas. The samples were counted in an Automated Radioxenon Sampler/Analyzer (ARSA) style β–γ coincidence detector. The spectra produced show that this method of radioxenon production yields samples with very high purity of the individual isotopes for ^131mXe and ¹³⁵Xe and a sample with a substantial ^133mXe to ¹³³Xe ratio.     

Determination of low level85Kr and133Xe concentrations in the environment

This study was performed under the joint TRMC/INER program for the determination of low level⁸⁵Kr and¹³³Xe concentrations in the environmental air samples. Based on cryogenic adsorption of krypton and xenon on charcoal followed by chromatographic separation from other gases, the⁸⁵Kr and¹³³Xe recovered from 200 liters of atmospheric air can be determined by either on-line gas flow proportional counter or liquid scintillation counting. The recovery yields of krypton and xenon examined by using⁸⁵Kr and¹³³Xe tracers were nearly 100%. The minimum detectable activity of⁸⁵Kr and¹³³Xe by gas flow proportional counting is about 7.40 Bq. The method is satisfactory for environmental monitoring applications under abnormal conditions of nuclear facilities. However, for lower level environmental⁸⁵Kr and¹³³Xe measurements, the liquid scintillation counting method can be applied due to their extremely low detection limits (i.e. 0.107 Bq and 0.093 Bq for⁸⁵Kr and¹³³Xe, respectively). Using this method, the measurable limits of concentrations are 0.535 Bq/m³ and 0.466 Bq/m³ for⁸⁵Kr and¹³³Xe, respectively.     

A radioxenon detection system using CdZnTe,an array of SiPMs,and a plastic scintillator

This paper presents the design and characterization of a prototype compact beta–gamma radioxenon detection system that utilizes a coplanar CdZnTe crystal, an array of SiPMs, and a plastic scintillator. The detector is directly mounted on a custom PCB. The system provides the advantage of room-temperature operation, while being compact, low noise, and with simple readout electronics. Preliminary measurements using ¹³⁷Cs, ¹³⁵Xe, and ^133/133mXe were conducted to optimize various system parameters to achieve optimal resolution of key photopeaks. The purpose of this research was to explore the potential of these radiation detection elements for use in beta–gamma coincidence applications.     

Sensitivity study on modeling radioxenon signals from radiopharmaceutical production facilities

As part of the Comprehensive Nuclear Test-Ban Treaty (CTBT), the International Monitoring System (IMS) was established to monitor the world for nuclear weapon explosions. As part of this network, systems are in place to monitor the atmosphere for radioxenon. The IMS routinely detects radioxenon from sources other than nuclear explosions. One of these radioxenon sources is radiopharmaceutical production facilities. This is a sensitivity study on the nuclear forensic signals possible from such facilities. A fission process model was produced to calculate the activity of ^131mXe, ^133mXe, ¹³³Xe and ¹³⁵Xe in the process utilized to produce ⁹⁹Mo and ¹³¹I for medical applications through high enriched uranium fission. The computer model accounts for fractionation of radionuclides within a decay chain that may result from filtering or chemical procedures. Ratios of the radioxenon isotopes are calculated as a function of decay time after the release. The ratios are then compared to those expected from nuclear explosions. The main conclusion from this work is that the two main factors that affect the nuclear forensic signal from radiopharmaceutical production facilities are the sample irradiation time and the use of emission gas storage tanks.     

Nuclear charge distribution in the spontaneous fission of252Cf: Determination of fractional cumulative yields of133mTe and133gTe

The fractional cumulative yields (FCY) of^133mTe and^133gTe in the spontaneous fission of²⁵²Cf were measured for the first time by a radiochemical method. The values ofFCY are 0.533±0.014 and 0.291±0.042 for^133mTe and^133gTe, respectively. The isomeric state to ground state fractional independent yield (FIY) ratio of¹³³Te,R, was found to be 3.5. The root-mean-square angular momentum of the primary fragment corresponding to the fission product¹³³Te, Jr.m.s.=8.8h, was estimated according to a simple one-parameter statistical model. The fractional cumulative yields from this work together with other literature data in the mass region A=131–141 are compared with the normal yields given by the empiricalZ _p model by Whhl. It suggests that both theN=82 neutrons shell and nucleus pairing effects are not apparent for the spontaneous fission of²⁵²Cf.     

Innovative concept for a major breakthrough in atmospheric radioactive xenon detection for nuclear explosion monitoring

The verification regime of the comprehensive test ban treaty (CTBT) is based on a network of three different waveform technologies together with global monitoring of aerosols and noble gas in order to detect, locate and identify a nuclear weapon explosion down to 1 kt TNT equivalent. In case of a low intensity underground or underwater nuclear explosion, it appears that only radioactive gases, especially the noble gas which are difficult to contain, will allow identification of weak yield nuclear tests. Four radioactive xenon isotopes, ^131mXe, ^133mXe, ¹³³Xe and ¹³⁵Xe, are sufficiently produced in fission reactions and exhibit suitable half-lives and radiation emissions to be detected in atmosphere at low level far away from the release site. Four different monitoring CTBT systems, ARIX, ARSA, SAUNA, and SPALAX? have been developed in order to sample and to measure them with high sensitivity. The latest developed by the French Atomic Energy Commission (CEA) is likely to be drastically improved in detection sensitivity (especially for the metastable isotopes) through a higher sampling rate, when equipped with a new conversion electron (CE)/X-ray coincidence spectrometer. This new spectrometer is based on two combined detectors, both exhibiting very low radioactive background: a well-type NaI(Tl) detector for photon detection surrounding a gas cell equipped with two large passivated implanted planar silicon chips for electron detection. It is characterized by a low electron energy threshold and a much better energy resolution for the CE than those usually measured with the existing CTBT equipments. Furthermore, the compact geometry of the spectrometer provides high efficiency for X-ray and for CE associated to the decay modes of the four relevant radioxenons. The paper focus on the design of this new spectrometer and presents spectroscopic performances of a prototype based on recent results achieved from both radioactive xenon standards and air sample measurements. Major improvements in detection sensitivity have been reached and quantified, especially for metastable radioactive isotopes ^131mXe and ^133mXe with a gain in minimum detectable activity (about 2 × 10^?3 Bq) relative to current CTBT SPALAX? system (air sampling frequency normalized to 8 h) of about 70 and 30 respectively.     

Research reactors as sources of atmospheric radioxenon

Radioxenon emissions of the TRIGA Mark II research reactor in Vienna were investigated with respect to a possible impact on the verification of the Comprehensive Nuclear Test-Ban-Treaty. Using the Swedish Automatic Unit for Noble Gas Acquisition (SAUNA II), five radioxenon isotopes ¹²⁵Xe, ^131mXe, ^133mXe, ¹³³Xe and ¹³⁵Xe were detected, of which ¹²⁵Xe is solely produced by neutron capture in stable atmospheric ¹²⁴Xe and hence acts as an indicator for neutron activation processes. The other nuclides are produced in both fission and neutron capture reactions. The detected activity concentrations ranged from 0.0010 to 190 Bq/m³. The source of the radioxenon is not yet fully clarified, but it could be micro-cracks in the fuel cladding, fission of ²³⁵U contaminations on the outside of the fuel elements or neutron activation of atmospheric Xe. Neutron deficient ¹²⁵Xe with its highly complex decay scheme was seen for the first time in a SAUNA system. In many experiments the activity ratios of the radioxenon nuclides carry the signature of nuclear explosions, if ^131mXe is omitted. Only if ^131mXe is included into the calculations of the isotopic activity ratios, the majority of the measurements revealed a “civil” signature (typical for a NPP). A significant contribution of the TRIGA Vienna to the global or European radioxenon inventory can be excluded. Due to the very low activities, the emissions are far below any concern for human health.     

Automated separation and measurement of radioxenon for the Comprehensive Test Ban Treaty

A fully automatic radioxenon sampler/analyzer (ARSA) has been developed and demonstrated for the collection and quantitative measurement of the four xenon radionuclides,^131mXe(11.9 d),^133mXe(2.2 d),¹³³Xe(5.2 d), and¹³⁵Xe(9.1 hr), in the atmosphere. These radionuclides are important signatures in monitoring for compliance to a Comprehensive Test Ban Treaty (CTBT). Activity ratios of these radionuclides permit source attribution. Xenon, continuously and automatically separated from the atmosphere, is automatically analyzed by electron-photon coincidence spectrometry providing a lower limit of detection of about 100 μBq/m³. The demonstrated detection limit is about 100 times better than achievable with reported laboratory-based procedures for the short-time collection intervals of interest.     

An improved apparatus for the determination of krypton-85 and xenon-133 in an emergency situation

A simple and portable apparatus was developed for measurements of⁸⁵Kr and¹³³Xe that would be released into the atmosphere in an emergency situation of nuclear facilities. The method is based on cryogenic adsorption of these gases on charcoal followed by chromatographic separation from other gases. The⁸⁵Kr and¹³³Xe recovered from atmospheric air are determined separately by liquid scintillation counting. It takes about 1 hour for the stepwise determination of⁸⁵Kr and¹³³Xe. The atmospheric concentration of 3·10^–3 Ci per m³ air (1.1·10² Bq/m³ air) is measurable for both nuclides with 20% counting error.     

Probabilistic assessment for a sample to be radioactive or not: application to radioxenon analysis

When a net count value is below the type 1 error critical limit it is customary to declare that the activity is “below the detection limit”. The content of this declaration is particularly impoverished, incapable for example of discriminating between a net measurement just below the critical limit, but positive, and a negative net measurement, two types of information that it is legitimate and intuitive to think do not have the same weight of information. In the case of a spectral measurement of ^131mXe and ^133mXe certain information is available according to the various X and gamma emissions, which might all be below their respective critical limits. We shall see that a Bayesian probabilistic approach can be used, without considering the critical limits, to obtain anti-correlated maximum likelihood values taking all the information into account jointly and to obtain powerful and pertinent information in the form of the absolute probability that the sample contains ^131mXe and/or ^133mXe, all possible activity values combined. Conversely, of course, this is used immediately to deduce the probability that the sample does not contain ^131mXe and/or ^133mXe. This information enables the customary critical limit to be ignored.     

Uranium determination by133Xe in meteorites; comparative study with fission track and/or239Np methods and thermal release character of133Xe

Uranium concentrations in three types of inclusions of the Allende (C III), a bulk sample of the Plainview (H5), and a bulk and 5 to 15 density separates of the Huckitta pallasite were determined and discussed by comparing the results from homogenized fission track and/or²³⁹Np analyses with the¹³³Xe results. Stepwise heating experiment revealed that the¹³³Xe release from the Plainview is bimordal and incomplete by a simple heating at 1600 °C, and that the¹³³Xe releases in the Huckitta bulk and metallic samples are multitudinous but can be extracted rather completely at 1600–1700 °C. In the Huckitta, uranium-rich phase (∼100 ppb) was in the lightest density fraction of d=2.9–3.3, but the bulk uranium was found to be mostly from the heavy metal-rich fraction of d>4.2.     

Chemical analysis of heavy elements by identification of fission fragments through X-ray coincidence measurements

A two-parameter coincident (X-ray, X-ray) measurement was made using a Si(Li) X-ray detector and a Ge(Li) X-ray detector to study the X-ray production in the²⁵²Cf fission process.K X-ray peaks from adjacentZ fission products for elements Y through Rh and I through Pr are well resolved in both detectors. The measurement yields the result that there is a large number of coincident events with X-rays from the sameZ element as well as with X-rays from the complementary fission product (e.g. CsK X-rays are in coincidence with both the complementary TcK X-rays and the CsK X-rays). The various possibilities one may consider are: (1)K X-ray production by the primary fission process followed by internal conversion, (2) multipleK X-ray production in the stopping process of the fission products, (3)K shell ionization resulting from β-decay of the fission fragments followed by internal conversion in the same fragment, and (4) multiple internal conversion processes from cascading transitions. Each of these four possible causes for self-coincident X-ray production is explored. Further two-parameter measurements were made of low-energy γ-rays in coincidence with characteristicK X-rays from the individual elements formed in the fission. Combined with previous mass determinations, it was possible to identify many of the observed γ-rays with individual isotopes. However, a large number of low-energy transitions were observed and identified as to elemental charge, but which had not been seen previously so that no mass determination was possible.