High sensitivity MOSFET-based neutron dosimetry M. Fragopoulou a , V. Konstantakos a , M. Zamani a,n , S. Siskos a , T. Laopoulos a , G. Sarrabayrouse b,c a Aristotle University of Thessaloniki, Physics Department, 54124 Thessaloniki, Greece b CNRS; LAAS; 7 avenue du colonel Roche, F-31077 Toulouse, France c Universite´ de Toulouse; UPS, INSA, INP, ISAE; LAAS; F-31077 Toulouse, France article info Article history: Received 24 October 2009 Received in revised form 10 June 2010 Accepted 11 June 2010 Keywords: MOSFET’s Neutron dosimetry abstract A new dosemeter based on a metal-oxide-semiconductor field effect transistor sensitive to both neutrons and gamma radiation was manufactured at LAAS-CNRS Laboratory, Toulouse, France. In order to be used for neutron dosimetry, a thin film of lithium fluoride was deposited on the surface of the gate of the device. The characteristics of the dosemeter, such as the dependence of its response to neutron dose and dose rate, were investigated. The studied dosemeter was very sensitive to gamma rays compared to other dosemeters proposed in the literature. Its response in thermal neutrons was found to be much higher than in fast neutrons and gamma rays. & 2010 Published by Elsevier B.V. 1. Introduction Metal-oxide-semiconductor field effect transistor (MOSFET) dosemeters have numerous advantages, such as low cost, small size and weight, robustness, accuracy of measurement, real-time or delayed direct read-out, information retention with small fading, possibility of integration with other sensors and/or circuitry. In addition, they can be used without power supply during irradiation, i.e. as passive dosemeters for dose measure- ments of large ranges, which is very important for a dosemeter. These advantages demonstrate that MOSFETs can be successfully applied in dosimetry. Although this kind of dosemeters is not significantly influenced by neutrons [1] and the response, if any, was mainly due to the interaction of neutrons with the packaging [2,3], they have been used in several application fields [4,5] especially in gamma and neutron dosimetry. However, they can be used as a neutron dosemeter when an appropriate converter is applied [6–8]. In addition, using two dosemeters, with and without converter, gamma rays can be easily separated by neutrons. In the literature, thermal neutron doses measured by MOSFETs are rarely presented. The published data show also that the response to fast neutrons is very low (of the order of 0.01 mV/mGy) due to the large distance between the converter and the detector [3,8]. Also, the lower detectable dose of these dosemeters is very high, about 1 mSv. Especially for personal dosimetry, the lower detectable dose is a very important parameter, which needs improvement to become as low as possible in order to lead to practical applications. Therefore, the characteristics of MOSFETs need further studies, even for lower doses, in order to be applied in gamma and neutron dosimetry. In this work, LAAS MOSFETs were equipped with a thick gate insulator in order to increase the sensitivity and was covered by a layer of 3 mm 6 LiF converter for the detection of both thermal neutrons (up to 1 eV) and intermediate-fast neutrons. The effect of neutron irradiation on the threshold voltage of the transistors was studied in order to determine the sensitivity of these devices. 2. Experimentals The p-type MOSFETs with 1.6 mm thick gate insulator studied in the present work, Fig. 1, were developed at LAAS-CNRS. Above the gate insulator a Cr layer with 0.1 mm thickness was placed. The channel of the MOS dosemeter has 9.5 mm width and 37 mm length. Due to the large gate area, the MOSFET dosemeter presents a very low electronic noise level. As is known from previous studies in the literature, thick SiO 2 p-MOSFETs operate with a high negative threshold voltage of the order of several Volts [9]. The studied p-MOSFETs were fabricated following a process designed for improving both sensitivity to radiation dose and stability. They operate with a negative threshold voltage, about  5 V. In order to measure neutron doses a 3 mm thick 6 LiF converter was deposited on the surface of the MOS gate, upon the Cr layer. The estimation of the appropriate thickness of the Cr layer on the gate insulator and of the Li deposit layer on Cr surface, Monte Carlo calculations have been performed, as mentioned in previous publication [10]. According to these results, above 3 mm of 6 LiF the mean deposited energy in gate insulator is constant. Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/nima Nuclear Instruments and Methods in Physics Research A 0168-9002/$ - see front matter & 2010 Published by Elsevier B.V. doi:10.1016/j.nima.2010.06.095 n Corresponding author. Tel.: + 30 2310998176; fax: + 30 231 0998058. E-mail address: zamani@physics.auth.gr (M. Zamani). Please cite this article as: M. Fragopoulou, et al., Nucl. Instr. and Meth. A (2010), doi:10.1016/j.nima.2010.06.095 Nuclear Instruments and Methods in Physics Research A ] (]]]]) ]]]–]]]