Abstract Ultra-pure construction materials are required for the next generation of neutrino physics, dark matter and environmental science applications. These new efforts require materials with ...purity levels at or below 1 uBq/kg 232Th and 238U. Yet radiometric analysis lacks sensitivity below ~10 uBq/kg for the U and Th decay chains. This limits both the selection of clean materials and the validation of purification processes. Copper is an important high-purity material for low-background experiments due to the ease with which it can be purified by electrochemical methods. Electroplating for purification into near-final shapes, known as electroforming, is one such method. Continued refinement of the copper electroforming process is underway, for the first time guided by an ICP-MS based assay method that can measure 232Th and 238U near the desired purity levels. An assay of electroformed copper at 10 uBq/kg for 232Th has been achieved and is described. The implications of electroformed copper at or better than this purity on next-generation low-background experiments are discussed.
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.
Pacific Northwest National Laboratory (PNNL) is developing a capability to measure the absolute activity concentration of gaseous radionuclides using length-compensated proportional-counting. This ...capability will enable the validation of low-level calibration standards for use in PNNL's new shallow underground laboratory. Two sets of unequal length proportional counters have been fabricated; one set has been fabricated using ultra-low background (ULB) electroformed copper and a second set fabricated from Oxygen-Free High-Conductivity Copper (OFHC).
•Length-compensated internal proportional counting is discussed.•We described the development of low-background unequal-length proportional counters.•We described the development of unequal-length OFHC proportional counters.•A gas mixing and transfer system is described.•An uncertainty analysis, following the ISO GUM guidelines, is presented.
Over the past several years, the Pacific Northwest National Laboratory (PNNL) has developed an ultra-low-background proportional counter (ULBPC) technology. The resulting detector is the product of ...an effort to produce a low-background, physically robust gas proportional counter for applications like radon emanation measurements, groundwater tritium, and
37
Ar. In order to fully take advantage of the inherent low-background properties designed into the ULBPC, a comparably low-background dedicated counting system is required. An ultra-low-background counting system (ULBCS) was recently built in the new shallow underground laboratory at PNNL. With a design depth of 30 m water-equivalent, the shallow underground laboratory provides approximately 100× fewer fast neutrons and 6× fewer muons than a surface location. The ULBCS itself provides additional shielding in the form of active anti-cosmic veto (via 2-in-thick plastic scintillator paddles) and passive borated poly (1 in.), lead (6 in.), and copper (~3 in.) shielding. This work will provide details on PNNL’s new shallow underground laboratory, examine the motivation for the design of the counting system, and provide results from the characterization of the ULBCS, including initial detector background.
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.
A Noble Gas Migration Experiment injected
127
Xe,
37
Ar, and sulfur hexafluoride into a former underground nuclear explosion shot cavity. These tracer gases were allowed to migrate from the cavity to ...near-surface and surface sampling locations and were detected in soil gas samples collected using various on-site inspection sampling approaches. Based on this experiment we came to the following conclusions: (1) SF
6
was enriched in all of the samples relative to both
37
Ar and
127
Xe. (2) There were no significant differences in the
127
Xe to
37
Ar ratio in the samples relative to the ratio injected into the cavity. (3) The migratory behavior of the chemical and radiotracers did not fit typical diffusion modeling scenarios.
.
Building on the successful experience in operating the DarkSide-50 detector, the DarkSide Collaboration is going to construct DarkSide-20k, a direct WIMP search detector using a two-phase Liquid ...Argon Time Projection Chamber (LAr TPC) with an active (fiducial) mass of 23 t (20 t). This paper describes a preliminary design for the experiment, in which the DarkSide-20k LAr TPC is deployed within a shield/veto with a spherical Liquid Scintillator Veto (LSV) inside a cylindrical Water Cherenkov Veto (WCV). This preliminary design provides a baseline for the experiment to achieve its physics goals, while further development work will lead to the final optimization of the detector parameters and an eventual technical design. Operation of DarkSide-50 demonstrated a major reduction in the dominant
39
Ar background when using argon extracted from an underground source, before applying pulse shape analysis. Data from DarkSide-50, in combination with MC simulation and analytical modeling, shows that a rejection factor for discrimination between electron and nuclear recoils of
>
3
×
10
9
is achievable. This, along with the use of the veto system and utilizing silicon photomultipliers in the LAr TPC, are the keys to unlocking the path to large LAr TPC detector masses, while maintaining an experiment in which less than
<
0
.
1
events (other than
ν
-induced nuclear recoils) is expected to occur within the WIMP search region during the planned exposure. DarkSide-20k will have ultra-low backgrounds than can be measured
in situ
, giving sensitivity to WIMP-nucleon cross sections of
1
.
2
×
10
-
47
cm
2
(
1
.
1
×
10
-
46
cm
2
) for WIMPs of 1 TeV/c
2
(10 TeV/c
2
) mass, to be achieved during a 5 yr run producing an exposure of 100 t yr free from any instrumental background.
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 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.
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