The detection of neutrons in the presence of significant gamma-radiation is often required in arms control, material accountability, and nuclear smuggling scenarios as well as in basic nuclear research. The new scintillator material LiBaF3 offers the possibility of measuring neutron count rates and energy spectra simultaneously while measuring gamma-count rates and spectra using a single detector. These capabilities derive from the fact that LiBaF3 exhibits a very fast core-valence luminescence under gamma-irradiation whereas this component is missing under neutron irradiation. Relatively simple pulse shape analysis techniques can be used to obtain excellent neutron/gamma discrimination. We present our current results illustrating these capabilities.
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 the131mXe,133gXe,133mXe, and135gXe 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.
Authors:P. Reeder, T. Bowyer, J. McIntyre, and W. Pitts
The International Monitoring System for the Comprehensive Nuclear-Test-Ban Treaty will include measurements of Xe fission products. Pacific Northwest National Laboratory has developed an automated system for separating Xe from air which detects Xe fission products using a beta-gamma counting system for 131mXe, 133mXe, 133gXe, and 135gXe. Betas and conversion electrons are detected in a plastic scintillation cell containing the Xe sample. Gamma and X-rays are detected in a NaI(Tl) scintillation detector which surrounds the plastic scintillator sample cell. Two-dimensional pulse-height spectra of gamma-energy versus beta-energy are obtained. The plastic scintillator spectrum in coincidence with the 31-keV X-rays from 131mXe. 133mXe, and 133gXe is a complex mixture of conversion electrons and betas. A new technique to simultaneously measure the delayed coincidence (T1/2 = 6.27 ns) between beta-particles from 133gXe and conversion electrons depopulating the 81-keV state in 133 Cs is being developed. This technique allows separation of the 133gXe beta spectrum from the conversion electrons due to 131mXe and 133mXe and uniquely quantifies all three nuclides.
Authors:T. Bowyer, K. Abel, C. Hubbard, M. Panisko, P. Reeder, R. Thompson, and R. Warner
Pacific Northwest National Laboratory, with guidance and support from the U.S. Department of Energy's NN-20 Comprehensive
Test Ban Treaty (CTBT) Research and Development program, has developed and demonstrated a fully automatic sampler-analyzer
(ARSA) for the collection and quantitative measurement of the four xenon radionuclides,131mXe (11.9 d),133mXe (2.19 d),133Xe (5.24 d), and135Xe (9.10 h), in the atmosphere. These radionuclides are important signatures in monitoring for compliance to a CTBT, and may
have applications in stack monitoring and other areas where xenon radionuclides are present. The activity ratios between certain
of these radionuclides permit discrimination between radioxenon originating from nuclear detonations and that from nuclear
reactor operations, nuclear fuel reprocessing, or from medical isotope production and usage. With the ARSA system, xenon is
continuously and automatically separated from the atmosphere at flow rates of about 100 lpm by sorption-bed techniques. Samples
collected in 8 hours are automatically analyzed by electron-photon coincidence spectrometry to provide detection sensitivities
as low as 100 μBq/m3 of air. This sensitivity is about 10-fold better than achieved with reported laboratory-based procedures1 for the short time collection intervals of interest. Gamma-ray energy spectra and gas analysis data are automatically collected.
Authors:J. McIntyre, K. Abel, T. Bowyer, J. Hayes, T. Heimbigner, M. Panisko, P. Reeder, and R. Thompson
The Pacific Northwest National Laboratory has developed an Automated Radioxenon Sampler/Analyzer (ARSA) in support of the Comprehensive Nuclear-Test-Ban-Treaty (CTBT) to measure four radioxenon isotopes: 131mXe, 133mXe, 133gXe, and 135gXe. This system uses a beta-gamma coincidence counting detector to produce two-dimensional plots of gamma-energy versus beta-energy. Betas and conversion electrons (CE) are detected in a cylindrical plastic scintillation cell and gamma and X-rays are detected in a surrounding NaI(Tl) scintillation detector. The ARSA has been field tested at several locations to measure the radioxenon concentrations. Most recently it has been deployed at the Institut für Atmosphärische Radioaktivität in Freiburg, Germany. During the first 4 months of 2000 the measured 133Xe oncentrations have varied between 0.0±0.1 and 110±10 mBq/m3 air. The longer lived 131mXe (T1/2 = 11.9 d) and short lived 135Xe (T1/2 = 9.1 h) have also been detected in small quantities, while 133mXe concentrations have been consistent with zero. Minimum detectable concentration (MDC) calculations for 133gXe fell well below the 1 mBq per standard-cubic-meter of air requirement adopted by the CTBT Preparatory Commission.1 A description of the radioxenon detector, the concentration and MDC calculations and preliminary results of the field test in Germany are presented.
Authors:T. Bowyer, K. Abel, C. Hubbard, A. McKinnon, M. Panisko, R. Perkins, P. Reeder, R. Thompson, and R. Warner
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.