| Abstrakt: |
Although direct-conversion solid-state neutron detection has been investigated for over five decades, propelling this technology beyond the basic research stage remains an outstanding challenge. This challenge is due to the very small selection of neutron-sensitive isotopes and therefore lack of mature semiconductor materials available for this technology. Given these constraints, there is a reason to investigate materials with less-than-optimal charge transport properties, which could include low charge carrier mobility/lifetime and/or single-carrier transport (i.e., order of magnitude or greater difference between electron and hole mobility). Such materials are potentially best-suited to a thin-film configuration, which provides not only leniency in terms of charge transport requirements, but also processing flexibility and integration advantages. Single-carrier transport in detectors with thicknesses less than or comparable to radiation penetration depth can lead to partial and position-dependent charge collection effects not treated in the general case of direct-conversion neutron detection. Here, we have developed a theory to include the effect of single-carrier charge collection and the possible mismatch between carrier transit time and integration time to study the performance of thin neutron detectors. Taking a boron carbide (B4C) direct-conversion thermal neutron detector as an example, we use custom Monte Carlo simulations to study the effects of a range of mobility, lifetime, thickness, and integration time values on detection efficiency and pulse height spectra. We discuss the interplay between the traditional mobility–lifetime product (μ τ) metric and the integration time to carrier transit time ratio (t i / t tr ) , which takes into account mobility (μ) specifically, and their effect on detection efficiency. We describe the effect of these parameters on pulse height spectra and show how, although single-carrier transport leads to a loss of spectral resolution when signal current is fully integrated, using integration times shorter than carrier transit time allows for recovery of spectral features. We additionally present two methods to extract the mobility–lifetime product of a single-carrier device, with the first being based on the steady-state current as a function of electric field under a steady-state radiation detection mode, and the second being based on the shift of spectral peaks as a function of electric field under a single-particle radiation counting mode, both using modified Hecht equations that do not require either surface or uniform radiation absorption conditions. Finally, we discuss the performance of a hypothetical single-carrier 5 μm thick B4C neutron detector, which can provide a maximum intrinsic neutron detection efficiency of 14% with a set lower level discriminator value of 25% of the total energy deposited. [ABSTRACT FROM AUTHOR] |
