Migration of noble gas tracers at the site of an underground nuclear explosion at the Nevada National Security Site.

As part of an underground gas migration study, two radioactive noble gases (37Ar and 127Xe) and two stable tracer gases (SF6 and PFDMCH) were injected into a historic nuclear explosion test chimney and allowed to migrate naturally. The purpose of this experiment was to provide a bounding case (natural transport) for the flow of radioactive noble gases following an underground nuclear explosion. To accomplish this, soil gas samples were collected from a series of boreholes and a range of depths from the shallow subsurface (3 m) to deeper levels (~160 m) over a period of eleven months. These samples have provided insights into the development and evolution of the subsurface plume and constrained the relative migration rates of the radioactive and stable gas species in the case when the driving pressure from the cavity is low. Analysis of the samples concluded that the stable tracer SF6 was consistently enriched in the subsurface samples relative to the radiotracer 127Xe, but the ratios of SF6 and 37Ar remained similar throughout the samples.

The potential detection of low-level aerosol isotopes from new civilian nuclear processes.

As the world faces a challenging future in maintaining the commercial availability of radioactive isotopes for medical use, new methods of medical isotope production are being pursued. Many of these are small in size and could effectively operate continuously. With the potential for much shorter retention times, a new suite of isotopes may soon be found in the environment. The authors estimate that many more aerosols containing low-level isotopes of gas/volatile origin could be detectable at short range and times, and a few at longer ranges and times as compared to those released in more common nuclear reactor operations.

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

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-per-million 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.

Development of a low-level (37)Ar calibration standard.

Argon-37 is an environmental signature of an underground nuclear explosion. Producing and quantifying low-level (37)Ar standards is an important step in the development of sensitive field measurement instruments. This paper describes progress at Pacific Northwest National Laboratory in developing a process to generate and quantify low-level (37)Ar standards, which can be used to calibrate sensitive field systems at activities consistent with soil background levels. This paper presents a discussion of the measurement analysis, along with assumptions and uncertainty estimates.

Detection and analysis of xenon isotopes for the comprehensive nuclear-test-ban treaty international monitoring system.

The use of the xenon isotopes for detection of nuclear explosions is of great interest for monitoring compliance with the comprehensive nuclear-test-ban treaty (CTBT). Recently, the automated radioxenon sampler-analyzer (ARSA) was tested at the Institute for Atmospheric Radioactivity (IAR) in Freiburg, Germany to ascertain its use for the CTBT by comparing its results to laboratory-based analyses, determining its detection sensitivity and analyzing its results in light of historical xenon isotope levels and known reactor operations in the area. Xe-133 was detected nearly every day throughout the test at activity concentrations ranging between approximately 0.1 mBq/m3 to as high as 120 mBq/m3. Xe-133m and 135Xe were also detected occasionally during the test at concentrations of less than 1 to a few mBq/m3.

Intercomparison experiments of systems for the measurement of xenon radionuclides in the atmosphere.

Radioactive xenon monitoring is one of the main technologies used for the detection of underground nuclear explosions. Precise and reliable measurements of (131m)Xe, (133g)Xe, (133m)Xe, and (135g)Xe are required as part of the International Monitoring System for compliance with the Comprehensive Nuclear-Test-Ban Treaty (CTBT). For the first time, simultaneous testing of four highly sensitive and automated fieldable radioxenon measurement systems has been performed and compared to established laboratory techniques. In addition to an intercomparison of radioxenon monitoring equipment of different design, this paper also presents a set of more than 2000 measurements of activity concentrations of radioactive xenon made in the city of Freiburg, Germany in 2000. The intercomparison experiment showed, that the results from the newly developed systems agree with each other and the equipment fulfills the fundamental requirements for their use in the verification regime of the CTBT. For 24-h measurements, concentrations as low as 0.1 mBqm(-3) were measured for atmospheric samples ranging in size from 10 to 80 m(3). The (133)Xe activity concentrations detected in the ambient air ranged from below 1 mBqm(-3) to above 100 mBqm(-3).

A shallow underground laboratory for low-background radiation measurements and materials development.

Pacific Northwest National Laboratory recently commissioned a new shallow underground laboratory, located at a depth of approximately 30 meters-water-equivalent. This new addition to the small class of radiation measurement laboratories located at modest underground depths houses the latest generation of custom-made, high-efficiency, low-background gamma-ray spectrometers and gas proportional counters. This paper describes the unique capabilities present in the shallow underground laboratory; these include large-scale ultra-pure materials production and a suite of radiation detection systems. Reported data characterize the degree of background reduction achieved through a combination of underground location, graded shielding, and rejection of cosmic-ray events. We conclude by presenting measurement targets and future opportunities.

Alternative treaty monitoring approaches using ultra-low background measurement technology.

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 MDA < or = 30 microBq/m(3) of air for (140)Ba, or about 10(6) fissions per daily sample.

Search for quadrupole strength in the electroexcitation of the delta+(1232).

High-precision 1H(e,e'p)pi(0) measurements at Q2 = 0.126 (GeV/c)2 are reported, which allow the determination of quadrupole amplitudes in the gamma*N-->Delta transition; they simultaneously test the reliability of electroproduction models. The derived quadrupole-to-dipole ( I = 3/2) amplitude ratios, R(SM) = (-6.5+/-0.2(stat+sys)+/-2.5(mod))% and R(EM) = (-2.1+/-0.2(stat+sys)+/-2.0(mod))%, are dominated by model error. Previous R(SM) and R(EM) results should be reconsidered after the model uncertainties associated with the method of their extraction are taken into account.