Pacific Northwest National Laboratory has recently opened a shallow underground laboratory intended for measurement of low-concentration levels of radioactive isotopes in samples collected from the ...environment. The development of a low-background liquid scintillation counter is currently underway to further augment the measurement capabilities within this underground laboratory. Liquid scintillation counting is especially useful for measuring charged particle (e.g., β and α) emitting isotopes with no (or very weak) gamma-ray yields. The combination of high-efficiency detection of charged particle emission in a liquid scintillation cocktail coupled with the low-background environment of an appropriately designed shield located in a clean underground laboratory provides the opportunity for increased-sensitivity measurements of a range of isotopes. To take advantage of the 35m-water-equivalent overburden of the underground laboratory, a series of simulations have evaluated the scintillation counter's shield design requirements to assess the possible background rate achievable. This report presents the design and background evaluation for a shallow underground, low background liquid scintillation counter design for sample measurements.
•Graded-shielding can produce an ultra-low-background liquid scintillation counter.•Location in a shallow underground cleanroom further enhances background reduction.•A novel light collection design and selected low background materials are utilized.•The background is predicted to be 10–100 times below typical commercial systems.•Simulations tentatively predict a background rate of order 10 counts per day.
The Ultra-Low Background Liquid Scintillation Counter developed by Pacific Northwest National Laboratory will expand the application of liquid scintillation counting by enabling lower detection ...limits and smaller sample volumes. By reducing the overall count rate of the background environment approximately 2 orders of magnitude below that of commercially available systems, backgrounds on the order of tens of counts per day over an energy range of ~3–3600keV can be realized. Initial test results of the ULB LSC show promising results for ultra-low background detection with liquid scintillation counting.
•Observed background is within a factor of 2 of the predictions from simulation.•Background achieved is 2 orders of magnitude below commercially available systems.•Backgrounds are in tens of counts per day over an energy range of ~3– 3600keV.•Initial results show promise for ultra-low background detection with the ULB LSC.
A new ultra-low-background proportional counter was recently developed with an internal volume of 100cm3 and has been characterized at pressures from 1–10atm with P-10 (90% Ar, 10% methane) gas. This ...design, along with a counting system providing event digitization and passive and active shielding, has been developed to complement a new shallow underground laboratory (30m water-equivalent). Backgrounds and low-level reference materials have been measured, and system sensitivity for 37Ar has been calculated.
•A new PNNL shallow underground laboratory is operational.•A low-background gas proportional counting system for argon has been prepared.•First background data has been collected relevant to an Ar-37 signature.•First calibration measurements of a low-level standard have been made.•Detector response to Ar-37 has been calculated and Ar-37 sensitivity projected.
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.
•We report on the detection of 39Ar at the location of an underground nuclear explosion on the Nevada Nuclear Security Site.•Argon-39 was detected, along with 37Ar and 85Kr in the subsurface at the Barnwell underground nuclear explosion.•These isotopes are produced through fission (85Kr) through the 40Ca(n, alpha) 37Ar reaction or 39K(n, p) 39Ar reaction.•Measured values for the 39Ar ranged from 36,000 (mBq/SCM) for shallow bore holes to 997,000 mBq/SCM from the rubble chimney.•Argon-39 and 85Kr, could interfere with gas sampling and detection equipment used in an on-site inspection.
The measurement of 37Ar has been proposed as a method of detecting underground nuclear testing. The isotope 37Ar is generated by neutron activation of calcium by the reaction, 40Ca(n, a)37Ar, and, as ...a noble gas, is able to migrate more freely underground. Pacific Northwest National Laboratory has developed a high throughput 37Ar collection and measurement system using modified Ultra-Low Background Proportional Counters (ULBPCs). This system is capable of collecting, purifying, and measuring radioactivity in argon from either atmospheric or soil gas samples. This process is automated, with minimal operator intervention. This paper describes quantification of 37Ar and how we account for varying background conditions. An example is provided in which 39Ar has been shown to be a significant background in some sample sets, created by process 39K(n,p)39Ar during an underground nuclear explosion that would also create 37Ar. To account for the large background of 39Ar, we fit the data with a constant plus exponential background model with Gaussian signal model and use the results of a constrained fit to calculate 37Ar activity. We discuss the methods used to purify and count samples. We discuss the impact of increased 39Ar backgrounds on the measurement of 37Ar.
Specific environmental applications can benefit from dual tritium and radiocarbon measurements in a single compound. Assuming typical environmental levels, it is often the low tritium activity ...relative to the higher radiocarbon activity that limits the dual measurement. In this paper, we explore the parameter space for a combined tritium and radiocarbon measurement using a natural methane sample mixed with an argon fill gas in low-background proportional counters of a specific design. We present an optimized methane percentage, detector fill pressure, and analysis energy windows to maximize measurement sensitivity while minimizing count time. The final optimized method uses a 9-atm fill of P35 (35% methane, 65% argon), and a tritium analysis window from 1.5 to 10.3keV, which stops short of the tritium beta decay endpoint energy of 18.6keV. This method optimizes tritium-counting efficiency while minimizing radiocarbon beta-decay interference.
•Use of a single compound (methane) for dual tritium and radiocarbon measurements.•Optimized analysis window for simultaneous tritium and radiocarbon measurement.•Allows for optimization of tritium counting in the presence of radiocarbon.
This paper describes the generation of 39Ar, via reactor irradiation of potassium carbonate, followed by quantitative analysis (length-compensated proportional counting) to yield two calibration ...standards that are respectively 50 and 3 times atmospheric background levels. Measurements were performed in Pacific Northwest National Laboratory's shallow underground counting laboratory studying the effect of gas density on beta-transport; these results are compared with simulation. The total expanded uncertainty of the specific activity for the ~50× 39Ar in P10 standard is 3.6% (k=2).
•39Ar is generated via reactor irradiation of potassium carbonate.•Low-level standards are produced to assist in ground water age-dating studies.•Quantification using length compensated proportional counting is performed.•Wall- and threshold-effects are studied and discussed in terms of uncertainty.
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/m
3 to as high as 120
mBq/m
3. Xe-133m and
135Xe were also detected occasionally during the test at concentrations of less than 1 to a few mBq/m
3.
Comparing a protein’s concentrations across two or more treatments is the focus of many proteomics studies. A frequent source of measurements for these comparisons is a mass spectrometry (MS) ...analysis of a protein’s peptide ions separated by liquid chromatography (LC) following its enzymatic digestion. Alas, LC−MS identification and quantification of equimolar peptides can vary significantly due to their unequal digestion, separation, and ionization. This unequal measurability of peptides, the largest source of LC−MS nuisance variation, stymies confident comparison of a protein’s concentration across treatments. Our objective is to introduce a mixed-effects statistical model for comparative LC−MS proteomics studies. We describe LC−MS peptide abundance with a linear model featuring pivotal terms that account for unequal peptide LC−MS measurability. We advance fitting this model to an often incomplete LC−MS data set with REstricted Maximum Likelihood (REML) estimation, producing estimates of model goodness-of-fit, treatment effects, standard errors, confidence intervals, and protein relative concentrations. We illustrate the model with an experiment featuring a known dilution series of a filamentous ascomycete fungus Trichoderma reesei protein mixture. For 781 of the 1546 T. reesei proteins with sufficient data coverage, the fitted mixed-effects models capably described the LC−MS measurements. The LC−MS measurability terms effectively accounted for this major source of uncertainty. Ninety percent of the relative concentration estimates were within 0.5-fold of the true relative concentrations. Akin to the common ratio method, this model also produced biased estimates, albeit less biased. Bias decreased significantly, both absolutely and relative to the ratio method, as the number of observed peptides per protein increased. Mixed-effects statistical modeling offers a flexible, well-established methodology for comparative proteomics studies integrating common experimental designs with LC−MS sample processing plans. It favorably accounts for the unequal LC−MS measurability of peptides and produces informative quantitative comparisons of a protein’s concentration across treatments with objective measures of uncertainties.
Simultaneous measurement of tritium and (14)C would provide an added tool for tracing organic compounds through environmental systems and is possible via beta energy spectroscopy of sample-derived ...methane in internal-source gas proportional counters. Since the mid-1960's atmospheric tritium and (14)C have fallen dramatically as the isotopic injections from aboveground nuclear testing have been diluted into the ocean and biosphere. In this work, the feasibility of simultaneous tritium and (14)C measurements via proportional counters is revisited in light of significant changes in both the atmospheric and biosphere isotopics and the development of new ultra-low-background gas proportional counting capabilities for small samples (roughly 50 cc methane). A Geant4 Monte Carlo model of a Pacific Northwest National Laboratory (PNNL) proportional counter response to tritium and (14)C is used to analyze small samples of two different methane sources to illustrate the range of applicability of contemporary simultaneous measurements and their limitations. Because the two methane sources examined were not sample size limited, we could compare the small-sample measurements performed at PNNL with analysis of larger samples performed at a commercial laboratory. These first results show that the dual-isotope simultaneous measurement is well matched for methane samples that are atmospheric or have an elevated source of tritium (i.e. landfill gas). However, for samples with low/modern tritium isotopics (rainwater), commercial separation and counting is a better fit.
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