Mitigation of Memory Effect in Plastic Scintillator-based Beta Cells

Monitoring radioactive releases from nuclear explosions is a major component of the International Monitoring System (IMS) of the Comprehensive-Nuclear-Test-Ban-Treaty (CTBT). A highly compact and high-efficiency beta-gamma coincidence detector is used to detect
¹³³
Xe,
^131m
Xe,
^133m
Xe and
¹³⁵
Xe in the sampled air for treaty monitoring. The plastic scintillator-based beta cell used for sampling is known for trapping some amount of radioxenon giving rise to, what is known as, the memory effect. There have been several efforts to reduce the memory effect by coating the inside of the cell with various materials. In this paper, RMD reports on the reduction of memory effect through the application of a thin aluminum oxide (Al2O3) coating inside the beta cells via the conformal coating technique of Atomic Layer Deposition (ALD). These coatings were applied at low temperatures (45-65 °C) to prevent thermal damage to the plastic scintillator, which was made of polyvinyl toluene (PVT). The pinhole-free ALD coatings measuring <100 nm thickness proved to be effective barrier against the diffusion of gaseous radioxenon species.The memory effect characterizations were performed at the Pacific Northwest National Laboratory (PNNL) on the ALD-coated beta cells using
^131m
Xe and
¹³³
Xe sources. These beta cells indicated a ~99% reduction in memory effect when compared with an unprocessed beta cell without any coating. Furthermore, the ALD processing of the beta cells did not have any negative impact on the scintillator performance, whose measured energy resolution were in the range of 26-36% using the
^131m
Xe 129-keV beta signature peak. Thus, these results demonstrate a very promising thin film ALD technology towards the reduction of memory effect in beta cells, which are critical for CTBT verification using IMS.