Performance evaluation of neutron detectors incorporating intrinsic Gd using a GEANT4 modeling approach
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Performance evaluation of neutron detectors incorporating intrinsic Gd using a GEANT4 modeling approach Abigail A. Bickley1, Christopher Young1, Benjamin Thomas1, John W. McClory1, Peter A. Dowben2, James C. Petrosky1 1 Department of Engineering Physics, Air Force Institute of Technology, 2950 Hobson Way, Wright-Patterson Air Force Base, OH 45433-7765, U.S.A. 2 Department of Physics and Astronomy, University of Nebraska - Lincoln, 855 North 16th St, Lincoln, NE 68588-0299, U.S.A. ABSTRACT Solid-state neutron detectors from heterostructures that incorporate Gd intrinsically or as a dopant may significantly benefit from the high thermal neutron capture cross section of gadolinium. Semiconducting devices with Gd atoms can act as a neutron capture medium and simultaneously detect the electronic signal that characterizes the interaction. Neutron capture in natural isotopic abundance gadolinium predominantly occurs via the formation of 158mGd, which decays to the ground state through the emission of high-energy gamma rays and an internal conversion electron. Detection of the internal conversion electron and/or the subsequent Auger electron emission provides a distinct and identifiable signature that neutron capture has occurred. Ensuring that the medium responds to these emissions is imperative to maximizing the efficiency and separating out other interactions from the radiation environment. A GEANT4 model, which includes incorporation of the nuclear structure of Gd, has been constructed to simulate the expected device behavior. This model allows the energy deposited from the decay of the metastable state to be localized and transported, providing for analysis of various device parameters. Device fabrication has been completed for Gd doped HfO2 on n-type silicon, Gd2O3 on p-type silicon and Gd2O3 on SiC for validation of the code. A preliminary evaluation of neutron detection capabilities of these devices using a GEANT4 modeling approach is presented. INTRODUCTION Strong motivation exists for developing new technologies capable of detecting neutron sources. The detection of neutrons has received renewed interest due to concerns about the proliferation of special nuclear materials (SNM) and global terrorism. Of particular interest is the detection of isotopes of uranium and plutonium. Plutonium has a measurable neutron emission rate owing to its unique spontaneous fission rate, while uranium neutron emission can be enhanced using active interrogation methods. However, neutron emission rates are low, thus detectors possessing a large neutron capture cross section and high efficiency are required. Furthermore, the neutron energy spectra of SNM peaks in the MeV region while the neutron capture cross sections of traditional detection media are strongly biased towards thermal energies. An example of this is provided by the 3He proportional counter, which can be used to detect both thermal and fast neutrons via the 23 He+ 01n→13 H+11p reaction. Recent shortages in the global supply of 3He have led to rationing of this isotope [1
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