Ultra-low to Moderate Radiation Level Neutron Dosimetry Measurements with H*10-TMFD vs. ROSPEC, Eberline, and Ludlum Detector Systems.

Abstract: H*10 neutron dosimetry (unlike gamma dosimetry), requires consideration of neutron energy spectra due to the 20× variation of the weight factor over the thermal-to-fast energy range, as well as the neutron radiation field dose rates ranging from cosmic, ~.01 μSv h -1 levels to commonly encountered ~10-200 μSv h -1 in nuclear laboratories/processing plants, and upwards of 10 4 Sv h -1 in nuclear reactor environments. This paper discusses the outcome of the comparison of spectrum-weighted neutron dosimetry covering thermal-to-fast energy using the novel H*-TMFD spectroscopy-enabled sensor system in comparison with measurements using state-of-the-art neutron dosimetry systems at SRNS-Rotating Spectrometer (ROSPEC), and non-spectroscopic Eberline ASP2E ("Eberline") and Ludlum 42-49B ("Ludlum") survey instrumentation. The H*-TMFD was validated for gamma blindness using a 2.48×10 10 Bq 137 Cs source. The background dose rate in Savannah River Nuclear Solutions' (SRNS) low-scatter facility with all neutron sources withdrawn was estimated at 0.005 μSv h -1 . From moderately high radiation field tests conducted with the high intensity (1.4 × 10 9 n s -1 ) 252 Cf source and a total data collection time of ~0.15 h, the predicted dose rates from Eberline (non-spectroscopic), Ludlum (non-spectroscopic), and spectroscopic H*-TMFD instruments were found to be: ~170 μSv h -1 , ~200 μSv h - , and ~ 120 μSv h -1 , respectively. The equivalent spectroscopic (SRNS measured) H*10 dose rate from ROSPEC value is 130 μSv h -1 , within 10% of H*10-TMFD measurement. Tests conducted for ultra-low intensity radiation field used a ~ 1.6 × 10 3 n s -1 252 Cf bare neutron source for which over a collection time of ~18 h, the Eberline meter measured an instantaneous dose/count rate of 0 μSv h -1 and a pulse-integrated dose rate of 0.034 μSv h -1 at ~1 m. In contrast, the H*-TMFD panel located 0.22 m in direct line of sight of the 252 Cf source spectroscopically measured ~0.4 μSv h -1 (within +/- 5%) over 1.8 h collection live time-with which spectrum matched perfectly to that of a bare 252 Cf source. The H*TMFD predicted value of ~0.4 μSv h -1 was cross-checked and found to be within 10% of LLNL's published value of ~0.37 μSv h -1 (intensity/distance corrected via 1/r 2 law of 25.5 μSv h -1 at 1 m for a 1 μg 252 Cf source); as well as from use of ICRP 74 conversion coefficients and MCNP code simulations. As expected, for a bare 252 Cf source, H*TMFD measured epithermal neutron energy-related dose rates are well below 1% of the total dose rates. For ~0.01 μSv h -1 neutron radiation fields, ROSPEC measurements for H*10 dose rates are estimated to take 7+ d, vs. under 2 h with the H*TMFD. The feasibility of using a single CTMFD in survey mode for H*10 dose rate (nSv h - to μSv h -1 ) measurements within 2-3 min is demonstrated.