We have experimentally determined the production rate of 39 Ar and 37 Ar due to cosmic ray neutron interactions in argon at sea level. Understanding these production rates is important for argon-based dark matter experiments that plan to utilize argon extracted from deep underground because it is imperative to know what the ingrowth of 39 Ar will be during the production, transport, and storage of the underground argon. These measurements also allow for the prediction of 39 Ar and 37 Ar concentrations in the atmosphere which can be used to determine the presence of other sources of these isotopes. Through controlled irradiation with a neutron beam that mimics the cosmic ray neutron spectrum, followed by direct counting of 39 Ar and 37 Ar decays with sensitive ultra-low background proportional counters, we determined that the production rate from cosmic ray neutrons at sea-level is expected to be (759 ± 128) atoms/(kg Ar day) for 39 Ar, and (51.0 ± 7.4) atoms/(kg Ar day) for 37 Ar. We also performed a survey of the alternate production mechanisms based on the state-of-knowledge of the associated cross-sections to obtain a total sea-level cosmic ray production rate of (1048 ± 133) atoms/(kg Ar day) for 39 Ar, (56.7 ± 7.5) atoms/(kg Ar day) for 37 Ar in underground argon, and (92 ± 13) atoms/(kg Ar day) for 37 Ar in atmospheric argon.

We have developed a wearable radiation sensor using Cs 2 LiYCl 6 :Ce (CLYC) for simultaneous gamma-ray and neutron detection. The system includes two ø2.5×2.5 cm 3 crystals coupled to small, metal-body photomultiplier tubes. A custom, low-power electronics base digitizes the output signal at three time points and enables both pulse height and pulse shape discrimination of gamma rays and neutrons. The total counts, anomaly detection metrics, and identified isotopes are displayed on a small screen. Users may leave the device in unattended mode to collect long-dwell energy spectra. The system stores up to 18 hours of one-second data, including energy spectra, and may transfer the data to a remote computer via a wired or wireless connection. The prototype is 18×13×7.5 cm 3 , weighs 1.3 kg, not including the protective pouch, and runs on six AA alkaline batteries for 29 hours with the wireless link active, or 41 hours with the wireless link disabled. In this paper, we summarize the system design and present characterization results from the detector modules. The energy resolution is about 6.5% full width at half maximum at 662 keV due to the small photomultiplier tube selected, and the linearity and pulse shape discrimination performance are very good.

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