Basic Metrology: Gas Counting

Summary

Radioactive gases such as ³H, ⁸⁵Kr, ¹³³Xe, and ¹³⁵Xe are important in applications including fusion and fission reactors, environmental monitoring, treaty verification, medical imaging, and radiation exposure evaluation. Different industries that generate and monitor radioactive gases rely on continued availability of calibrations for gas volumetric activity. In 2025 NIST is re-establishing its gravimetric Gas Counting capabilities to realize the Becquerel for radioactive gases, with an eventual goal to provide standards and calibrations services.

Description

Credit: NIST

Gas-filled detectors operate by measuring electrical current produced by radioactive particles interacting with an inert counting gas inside a volume. Depending on the applied voltage in the detector, different multiplication factors are achieved, which are used by specialized instruments such as ionization chambers, proportional counters, and Geiger-Müller counters. For radioactive gas counting, the radioactive gas is mixed with inert counting gas, and the mixture is used to fill the detector volume.

A set of 3 tubular gas-filled detectors, each of a different calibrated volume, is used to extrapolate the count rate of an extended volume of gas from the volumes inside each of the tubes. The detectors are carefully designed to control end-effects at either end of the tube, where the counting rate differs from the bulk rate at the center of the tube. Additional calibrated volumes, thermometers, pressure meters, and quantitative gas transfer techniques, allow for accurate determination of radioactive gas volume.

As of early 2025, the Gas Counting system is undergoing construction and preliminary testing.