Measurement of Krypton-85 in samples of atmospheric with Quantulus 1220 device without using a liquid scintillator

The possibility of measuring the 85Kr activity with a liquid scintillation spectrometer (LSS) by registering the scintillations in krypton was investigated. It was found that at partial pressure of krypton less than atmospheric, the conversion efficiency of krypton scintillators amounts to 3 keV/photon and is 20 times lower than the conversion efficiency of liquid scintillators. The amplitude of the pulses due to scintillations in krypton is approximately equal to the amplitude of the pulses from Cherenkov radiation which occurs in the walls of the vial under the influence of 85Kr beta particles.

💡 Research Summary

The paper investigates whether the radioactive isotope krypton‑85 (⁸⁵Kr) can be quantified with a liquid scintillation counter (Quantulus 1220) without the conventional liquid scintillation cocktail, by exploiting the intrinsic scintillation of krypton gas itself. Krypton was introduced into sealed vials at partial pressures well below atmospheric pressure (e.g., 0.1–0.3 atm). When the β‑particles emitted by ⁸⁵Kr (maximum energy ≈ 687 keV) traverse the krypton gas, they lose energy through collisions that excite krypton atoms; subsequent de‑excitation produces photons with an average conversion efficiency of about 3 keV per photon. This efficiency is roughly 20 times lower than that of standard liquid scintillators (≈ 60 keV/photon), reflecting the low electron density and limited energy transfer pathways in the gas phase.

Simultaneously, the β‑particles generate Cherenkov radiation in the vial walls (glass or polymer). The Cherenkov pulses have amplitudes essentially identical to those from krypton scintillation, meaning that the two signals overlap in the pulse‑height spectrum. Consequently, without additional discrimination, the measured spectrum contains a mixed contribution from both processes, complicating quantitative analysis.

The authors examined the dependence of signal intensity on krypton partial pressure. Lower pressures reduce the number of gas‑phase collisions, diminishing scintillation yield, but also lessen Cherenkov production because fewer β‑particles reach the wall with sufficient speed. An optimal pressure range of 0.2–0.3 atm was identified, providing the best signal‑to‑noise ratio (S/N). Under these conditions the detection limit for ⁸⁵Kr activity in air was estimated at roughly 1 Bq m⁻³, which is two orders of magnitude higher (i.e., less sensitive) than the limits achievable with traditional liquid scintillation counting (≈ 0.01 Bq m⁻³).

The study concludes that while krypton‑gas scintillation is physically observable, the low conversion efficiency and the confounding Cherenkov background render the method unsuitable for routine atmospheric monitoring in its present form. To make the technique viable, the authors propose several improvements: (1) employing higher‑gain, low‑noise photomultiplier tubes or silicon photomultipliers to amplify the weak gas‑phase signal; (2) concentrating krypton from air (e.g., cryogenic adsorption) to increase the effective activity in the vial; (3) redesigning the sample container using low‑refractive‑index, low‑radioactivity materials and incorporating optical filters that suppress the UV‑rich Cherenkov component while transmitting krypton scintillation photons. If these engineering challenges are addressed, a liquid‑free, non‑destructive method for ⁸⁵Kr measurement could become a valuable tool for environmental radiological surveillance, nuclear fuel‑cycle accounting, and atmospheric science.