A new sensitization technique of CR-39 track detectors for improved neutron dosimetry

In the past decades, the need has been growing for accurate neutron and charged particle dosimetry (Bos and d’Errico, 2006). This is due to increasing exposure levels in the nuclear industry, deriving from more frequent in-service entries at commercial nuclear power plants, and from increased plant life-extension or decommissioning activities. Medical activities also cause increasing neutron exposures, mainly traceable to the increasing availability of radiotherapy facilities using proton beams with energies between 50 and 250 MeV. Etched-track detectors, particularly polyallyl-diglycol carbonate (PADC) or CR-39, have almost replaced albedo thermoluminescent detectors as the most widely used passive neutron personal dosimeters. This is due to great improvements occurred since the 1980s in etching procedures, read-out equipment, and quality of the detector materials (d’Errico and Bos, 2004). A standard size CR-39 neutron dosimeter can record hundreds of individually detectable charged particle tracks. Their characteristic geometric parameters contain information on the energy distribution of incident neutrons and can be used for spectral fluence determinations or for improved dosimetric assessments (Luszik-Bhadra et al., 1997). However, imaging these tracks and determining their statistics presents a formidable challenge (d’Errico et al., 1997). Particularly difficult is counting automatically the smallest recoil proton tracks, which can be erased by chemical etching procedures due to their limited dimensions (Hulber and Selmeczi, 2005). In this context, detector processing techniques yielding tracks of larger sizes and thus enhancing readability are extremely valuable and warrant further investigation. Two of the main sensitization techniques are the post-irradiation treatment with carbon dioxide under pressure, first reported by Fujii et al., (1995), and the UV irradiation treatment, reported by Wong and Hoberg (1982). While both techniques were further studied separately, this study proposes an original approach, combining both treatments to obtain a “superadditive” effect. The idea is based on an analysis of how each technique works, suggesting they could potentially complement each other. Although the fundamental mechanism of the sensitivity enhancement of the CO2 treatment is, yet, not fully understood, it is known that for the sensitization to effectively work, the PADC detectors have to contain a certain amount of carbon dioxide diffused inside, during the chemical etching (Hassan et al., 2013). Therefore, the gas plays a role during the etching process, but does not physically affect the latent tracks, before it. In the case of the UV irradiation treatment, it works by breaking the chemical bonds of the polymer, which can occur through smaller energy transfers than are typically required for ionization (2-3 eV as opposed to 10-15 eV) (Saad et al., 2015). The shorter molecular chain makes the polymer dissolve more rapidly, affecting both, bulk etch and track etch responses. Therefore, as each technique enhances the detectors’ sensitivity in a different way, their combination could potentially work