Elsevier

Radiation Measurements

Volume 46, Issue 12, December 2011, Pages 1426-1431
Radiation Measurements

Effect of neutron irradiation on dosimetric properties of TLD-600H (6LiF:Mg,Cu,P)

https://doi.org/10.1016/j.radmeas.2011.06.066Get rights and content

Abstract

Ideally, dosimeters should measure the dose without their dosimetric properties being affected by the radiation type being measured. Industry-wide occupational radiation workers that can be potentially exposed to neutron radiation fields are routinely monitored by thermoluminescent (TLD) dosimeters. The neutron dose measured by these devices is obtained from the interaction products of thermal (slow) neutrons and 6Li. The number of alpha particle and 3H ions (tritium) released in the (n, 6Li) reaction is proportional to the dose received by the dosimeter. It is a common practice to evaluate the neutron dose indirectly from these products. In this work we present for the first time direct evidence that the neutron exposure to LiF:Mg,Cu,P (TLD-600H) can have deleterious effects on the dosimetric properties of one of the most commonly utilized dosimeters in the world. The interstitial non-negligible tritium dose contribution to the dosimeter is a result of the beta decay of the tritium nuclei interstitially present in the material. If not accounted for the effect of this self irradiation can directly affect the measured occupational dose to workers. In order to estimate the significance of these effects, tritium dose buildup was measured in the TLD-600H irradiated to different neutron doses from a reactor and a 252Cf source and then calculated in terms of a daily dose buildup rate. The effect of the self irradiation on these dosimeters resulting from tritium decay was extensively studied. The TLD dosimeters that were exposed to neutron doses show a significant dose buildup even after they have been annealed, i.e. even after the signal produced by the incident neutron dose is completely erased from the lower energy trap states of the dosimeter. The other discovered effect of neutron irradiation on TLD-600H is a significant loss in the detection sensitivity (up to 15%).

Introduction

Neutron albedo dosimetry of most passive dosimeters is based on following nuclear reactionL6i+n1H3(2.73MeV)+α4+(2.05MeV)

The neutron dose-response results from the α particles and tritium ions released as a result of the reaction (1). However there is a possible second contribution to the total neutron dose from the dosimeter readout, which is neglected in common practice. That is, the contribution from the self irradiation of the dosimeter material resulting from the radioactive decay of the tritium atoms that were created in the neutron–lithium interaction. Tritium atoms created in the dosimeter material are in a radioactive state and decay with a half-life of 12.33 years by ejecting a beta particle with an average energy of 5.7 keV and an associated antineutrino. The radioactive tritium atoms created in the dosimeter then act as an internal source of beta particle radiation. A conservatively negative consequence of the self irradiation is the over-estimation of the neutron dose reported to the individual worker.

While the effect has been studied in other TLD materials, no peer reviewed study of this effect has been demonstrated for 6LiF:Mg,Cu,P (such as the Harshaw model 8840/41 dosimeter1) dosimeter Burgkhardt and Piesch, 1982a, Burgkhardt and Piesch, 1982b. It was estimated in Burgkhardt and Piesch (1982a) that “TLD-600 can be reused as long as the total accumulated dose in terms of a gamma equivalent reading due to neutron irradiation is lower than about 10 Gy” In reference Burgkhardt and Piesch (1982b), special software was developed to correct neutron dose for tritium buildup dose.

The aim of this paper is to report and discuss the effects of neutron irradiation of 6LiF:Mg,Cu,P dosimeters.

Section snippets

Materials and methods

The Harshaw dosimeter 8841 consists of a total of 4 elements: three are made of 7LiF:Mg,Cu,P (TLD-700H) and the fourth element is made of 6LiF:Mg,Cu,P (TLD-600H). The Harshaw dosimeter has been described in detail elsewhere (Cassata et al., 2002, Moscovitch et al., 2006, St. John et al., 2006, Romanyukha et al., 2008).

The materials and methods of this report are based on two operational scenarios where personnel dosimeters can be exposed to a relatively high neutron dose. For example, they can

Results and discussion

Fig. 1 shows the measured values of the 6LiF:Mg,Cu,P dose buildup resulting from five different storage periods of the same dosimeters. The storage periods were seven months, four months, six weeks and two months respectively. The control dose measured on TLD-600H was averaged over 14 dosimeters and subsequently subtracted.

Fig. 2 shows a typical glow curve for TLD-600H of a TLD obtained as follows: Immediately after the TLD was irradiated to a neutron dose of 16.1 Gy, it was annealed to a zero

Conclusions

In this work we present clear evidence that 6LiF:Mg,Cu,P thermoluminescent dosimeters irradiated with neutrons show two main effects: one is the dose buildup generated from the beta decay of the tritium atoms created and bonded inside the dosimeter material; the other effect is the loss of sensitivity of the TLDs as indicated by the increased presence of a high temperature peak of the glow curve. In the case of our laboratory, NDC, with a declared neutron lower limit of detection (LLD) of

Disclaimer

The study was funded through U.S. Department of Defense operational and maintenance budget. The funding source did not play any role in this study design, the collection, analysis and interpretation of data. The views expressed in this paper are those of the authors and do not reflect necessarily the official policy or position of the Department of the Navy, Department of Defense, or the U.S. Government.

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