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January 28, 2022

APL finds new way to detect radioactivity

By TONY WU | March 26, 2015

Radioactive decay happens naturally to all materials. However, some elements decay much faster than others, allowing scientists to detect and identify certain materials. The importance of radioactivity detection is not often understated. These detection methods have a variety of uses, including monitoring nuclear power plants and screening cargo for potential terrorist activities. Some methods use helium-3, a material that is traditionally hard to obtain, but a new idea from the Applied Physics Laboratory (APL) may have solved the problem of its rarity.

Current technologies for detecting radioactive decay utilize detectors filled with helium gas. When a neutron is released from the material, it impacts the detector and the helium molecule absorbs the thermal neutron. The helium molecule then breaks down into hydrogen ions. The detectors are then subjected to gamma rays, allowing the software to measure the count rate of the radioactive decay. By using helium gas, the resulting hydrogen ions produce low sensitivity to gamma rays, raising the efficiency of the detectors. Though effective, helium-3 is produced in low quantities, mostly as a byproduct of hydrogen-3 decay.

Other detection materials also exist such as boron trifluoride (BF3) enriched with boron-10. The mixture is necessary because boron does not exist as a gas. As a result, BF3 is used to suspend elemental boron in a gaseous phase. However, BF3 is highly toxic. Therefore, a safer method of neutron detection that relies on more abundant materials is necessary to meet the demands of radioactive detection.

A novel method created by a team of scientists, led by Christopher Lavelle at the APL, involves noble gas scintillation, a process in which particles produce light when they impact noble gases. In the experiment, a converter is set up before the detectors to generate ions from radioactive decay materials. For detection, the detectors were filled with xenon gas, a noble gas that has a high scintillation yield.

In a typical detection process, the product of radioactive decay enters the chamber of the machine. The neutron impacts the converter, producing energetic ions that head in different directions. One of the ions impacts the detector filled with xenon gas and ionizes the noble gas. The excited xenon atoms then undergo decay, producing photons in the process. The amount of photons are then detected, and radioactive decay is said to have occurred when the amount of photons exceeds a minimum threshold.

In this setup, a coating of foam, enriched with boron-10, surrounds the detectors. The foam is the converter since the boron isotope absorbs neutrons readily and emits ions during the process. The photons that are generated from the detectors will then pass through the foam to reach photon detectors that are placed outside. Because of the placement of the foam between the detectors, photons might be blocked from reaching the outside detectors. To test the photon’s ability to penetrate the foam, scientists allow neutrons to impact the foam, measuring the threshold of detection. The results suggest that the photons emitted by xenon gas are large enough to pass through the foam and reach the outside detectors.

While the discovery offers a new method for safe radioactivity detection, there are some aspects that need to be addressed such as the use of multiple boron foam layers or the use of transparent foam that display the same properties as boron.

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