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The feasibility of large-geometry Metal Oxide Semiconductor Field Effect Transistor (MOSFET) devices has been assessed for both active and passive neutron dosimetry and use in radiotherapy environments. Neutron sensitivity has been enhanced with the use of polymeric cement surrounding the gate region. Neutron activation via nuclear interaction processes is a potential problem with conventionally packaged and fabricated devices. To overcome this problem, a unique low-activation device design is described. Standard Dual in-Line devices, modified with polymeric cement and boron loaded cement have been exposed to gamma rays (60Co) and neutrons (gamma-ray shielded 252Cf) to provide neutron sensitivity estimates. The results show that the neutron sensitivity can be increased by a factor of approximately three by the use of a thin layer of polymeric cement over the gate region. Essentially zero activation is observed in the activation-reduced design compared with 1000 cps in the conventional design MOSFET when both are exposed under identical conditions to a neutron field from a gamma-ray shielded 252Cf isotopic source.

The results of a recent study on the limiting uncertainties in the measurement of photon radiation dose with MOSFET dosimeters are reported. The statistical uncertainty in dose measurement from a single device has been measured before and after irradiation. The resulting increase in 1/f noise with radiation dose has been investigated via various analytical models. The limit of uncertainty in the ubiquitous linear trend of threshold voltage with dose has been measured and compared to two nonlinear models. Inter-device uncertainty has been investigated in a group of 40 devices, and preliminary evidence for kurtosis and skewness in the distributions for devices without external bias has been observed.

This paper compares and contrasts the use of MOSFET dosimeters and extended rem counters for neutron detection in cosmic radiation fields. These technologies have been exposed to a reference field and data are presented that demonstrate the contrasting sensitivities of these approaches. The MOSFETs are observed to be insensitive to dose levels associated with cosmic fields whilst the extended rem counters give a good response, demonstrating that copper has potential as a replacement for lead in these systems. ).Publisher Item Identifier S 0018-9499(00)11181-5.Authorized licensed use limited to: Lancaster University Library. Downloaded on December 23, 2008 at 04:46 from IEEE Xplore. Restrictions apply.

The response functions for two cosmic neutron detection systems have been calculated using Monte-Carlo computational methods. The detection systems that form the focus of this research are modified Leake detector designs in which a central thermal neutron detector is surrounded by a sphere of high-density polyethylene. In this arrangement, the surrounding polyethylene moderates the incident fast neutrons that are then detected by the central detector; in this case a 3He-filled proportional counter. In order to extend the response of these detector systems to cater for cosmic neutron environments, a shell of high-Z material has been included in each to promote (n, xn) reactions in the polyethylene moderator. We have used shells of lead and copper for this purpose to bring the high-energy component of the cosmic field, extending up to several Ge V, within the capability of the detector systems. In particular, copper has been used in comparison with lead since the former is easier and safer to machine and handle. The overall diameter of the instruments studied in this work is 208 mm. Calculations of the neutron response have been performed with MCNP4C, for the thermal-20 MeV energy range, and with MCNPX 2.1.5 / LA150N neutron libraries for the higher-energy cosmic region of the spectrum beyond 20 MeV. The results of these calculations are compared with experimental data that have been recorded with the instruments at the CERN Cosmic Reference Field Facility (CERF), Geneva, Switzerland. This comparison is discussed in respect of the likely applications of these detector systems to high-energy neutron field measurement on-board aircraft and in the vicinity of high-energy particle accelerators. The former application is gaining considerable research attention following the revised estimates of relative biological effectiveness of cosmic neutron fields and the related recommendation that aircrew be regarded occupationally-exposed radiation workers, on behalf of the International Commission on Radiological Protection (lCRP) and the European Union. The latter application is of interest to accelerator operators and users who can be exposed to neutron fields with 1 00 MeV components as a result of scattering reactions from the materials comprising the accelerator and associated shielding. This is of growing concern to the medical field where the use of cyclotrons is often a key aspect of radiation treatment and therapy, in which the control and estimation of administered doses is a key objective.