Characterization of Thermoluminescent Dosimeters for Neutron Dosimetry at High Altitudes

Abstract

Neutrons constitute a significant component of the secondary cosmic rays and are one of the most important contributors to natural cosmic ray radiation background dose. The study of the cosmic ray neutrons' contribution to the dose equivalent received by humans is an interesting and challenging task for the scientific community. In addition, international regulations demand assessing the biological risk due to radiation exposure for both workers and the general population. Because the dose rate due to cosmic radiation increases significantly with altitude, the objective of this work was to characterize the thermoluminescent dosimeter (TLDs) from the perspective of exposing them at high altitudes for longtime neutron dose monitoring. The pair of TLD-700 and TLD-600 is amply used to obtain the information on gamma and neutron dose in mixed neutron-gamma fields due to the present difference in 6Li isotope concentration. A thermoluminescence dosimeter system based on pair of TLD-600/700 was characterized to enable it for neutron dosimetry in the thermal energy range. The system was calibrated in terms of neutron ambient dose equivalent in an experimental setup using a 241Am-B radionuclide neutron source coated by a moderator material, polyethylene, creating a thermalized neutron field. Afterward, the pair of TLD-600/700 was exposed at the CERN-EU High-Energy Reference Field (CERF) facility in Geneva, which delivers a neutron field with a spectrum similar to that of secondary cosmic rays. The dosimetric system provided a dose value comparable with the calculated one demonstrating a good performance for neutron dosimetry.

Keywords: South Atlantic anomaly; TLD pair; charged particle; cosmic radiation monitoring; neutron detectors; neutron dosimetry; thermal neutron facility; thermoluminescent dosimeters calibration.

Figure 1

Irradiation set up at HOTNES facility: cylindrical cavity (a) with polyethylene support (15 × 15 grid) for thermoluminescent dosimeters (b).

Figure 2

Measured neutron spectrum in HOTNES reference point with polyethylene ceiling in place. Errors are smaller than graphical symbols [49].

Figure 3

Relative intrinsic sensitivity factor for all Thermoluminescent Dosimeters (TLDs-600 and -700). The dot lines limit the range of acceptability of S_i [0.9–1.1].

Figure 4

Flux-to-dose equivalent conversion coefficient on a log-log scale reported in ICRP-74 [47] and the interpolated curve in the energy values of HOTNES spectrum. The vertical solid black line indicates the energy level of 0.5 eV.

Figure 5

Ambient dose equivalent (H*(10)) as a function of the neutron energy. The vertical solid black line indicates the energy level of 0.5 eV.

Figure 6

Irradiation set up at CERN-EU High Energy Reference Radiation Field (CERF) facility: the thermoluminescent dosimeters (indicated by a yellow label) were exposed simultaneously with other neutron detectors on the concrete platform.