Estimating 10B and 14N doses using a pair of LiF-based TLD-600 and TLD-700 as an alternative to gold activation method

Abstract

Purpose: A new method using TLD-600 and TLD-700 was proposed to measure a thermal neutron flux, which is used to evaluate 10B and 14N doses for boron neutron capture therapy (BNCT). Material and method: TLD-600 and TLD-700 were enriched by 6Li (95.6%) and 7Li (99.99%), respectively. The TLDs were calibrated using a 252Cf neutron source within a 30 cm-diameter D2O sphere to produce a thermal neutron flux calibration factor. The calibration factor was obtained by dividing the neutron-induced TLD readings by the delivered thermal neutron fluence. In addition, a neutron self-shielding correction was calculated to compensate for the perturbed neutron fluence between adjacent TLDs. To validate the procedure involved in this new method, TLDs were irradiated by thermal neutron fluxes from the research/educational reactor. The results were compared with the thermal neutron fluxes by the conventional gold activation method. Results and discussion: The calibration factor from TLDs irradiated by the thermal neutron flux from the 252Cf neutrons was determined 3.80 × 104 cm−2 nC−1. Using this factor, the thermal neuron flux at the reactor was measured 2.28 × 105 cm−2 W−1 s−1 by the TLD method. The one measured by the gold activation method was 2.17 × 105 cm−2 W−1 s−1 showing a difference of 5%. With the thermal neutron fluence measured by TLDs, a Li-6 dose of TLD-600 (Dn,αTLD) can be calculated. Then the two major components of BNCT dosimetry, a B-10 dose (D(n,α)B−10) and an N-14 dose (D(n,p)N−14) can be calculated by multiplying Dn,αTLD with constants due to a strong 1/v behavior of the three reactions. Conclusions: The TLD method developed in this study was successfully validated against an unknown mixed neutron/gamma field from the reactor, showing about 5% difference in measured thermal neutron fluences when compared to the conventional gold activation method. Therefore, we concluded that the two major dose components of BNCT, 10B and 14N doses can be accurately evaluated by the neutron-induced TLD dose.