CHARACTERIZATION OF FLUORESCENT NUCLEAR TRACK DETECTORS AS CRITICALITY DOSIMETERS II

Fluorescent nuclear track detectors (FNTDs) as criticality dosimeters for both neutrons and gamma are further characterized in terms of angular dependence and quick dose assessment. The power spectrum integral depth profiles obtained from stacks of fluorescent images acquired within FNTDs exposed to a broad spectrum neutron field at various angles are analyzed to determine a calibration curve for angular dependence. MCNPX simulations were shown to be in good agreement with experimental results. A prototype triage reader was also designed and tested for quick assessment of dose. An unfolding technique incorporating both energy dependence and angular dependence is discussed. The advantages and shortcomings of using FNTDs in the event of a criticality excursion accident are analyzed.

COMPARISON BETWEEN PADC AND FNTD NEUTRON DETECTOR SYSTEMS IN BLIND TESTS

The objective of this study was to compare a neutron dosimetry system based on polyallyl diglycol carbonate (PADC) detectors with a new system based on Al2O3:C,Mg fluorescence nuclear track detectors (FNTD). The irradiations, performed as part of an intercomparison organized by the Physikalisch-Technische Bundesanstalt (PTB), Germany, were on a PMMA phantom with 252Cf or 241Am-Be source, usually with the phantom surface perpendicular to the radiation beam (0° angle), and with Hp(10) values between 0.3 and 7 mSv. One 252Cf irradiation was performed at 30° angle, and one with an additional 1 mSv gamma irradiation. The results showed an agreement between the two techniques with an average and maximum difference between PADCs and FNTDs of 1.5 and 22%, respectively, if one compares only cases of doses >1 mSv. For one of the irradiation conditions with dose of 0.9 mSv, use of the incorrect calibration factor for the FNTD (252Cf instead of 241Am-Be) led to reported values ~×2 larger than the given doses, due to low statistics in the determination of the ratio between 6Li-doped glass and polyethylene neutron converters. Although the FNTD track analysis algorithm may need further development, the results presented here demonstrate the feasibility of the FNTD technology and indicate areas requiring improvements.

ENERGY RESPONSE OF FLUORESCENT NUCLEAR TRACK DETECTORS OF VARIOUS COLORATIONS TO MONOENERGETIC NEUTRONS

The neutron-energy dependence of the track-counting sensitivity of fluorescent nuclear track detectors (FNTDs) at two ranges of Mg doping, resulting in different crystal colorations, was investigated. The performance of FNTDs was studied with the following converters: Li-glass for thermal to intermediate-energy neutrons, polyethylene for fast neutrons, and polytetrafluoroethylene (Teflon™) for photon- and radon-background subtraction. The irradiations with monoenergetic neutrons were performed at the National Physics Laboratory (NPL), UK. The energy range was varied from 144 keV to 16.5 MeV in the personal dose equivalent range from 1 to 3 mSv. Monte Carlo simulations were performed to model the response of FNTDs to monoenergetic neutrons. A good agreement with the experimental data was observed suggesting the development of a basic model for future MC studies. Further work will focus on increasing FNTD sensitivity to low-energy neutrons and developing a faster imaging technique for scanning larger areas to improve counting statistics.