The effects of large signals on charge collection in radiation detectors: Application to amorphous selenium detectors

Abstract

Analytical and numerical models for studying the effects of large signals on charge collection efficiency in radiation detectors are described by considering bimolecular recombination between drifting charge carriers, carrier trapping, and space charge effects. First, an analytical solution is obtained by assuming that the field remains relatively uniform. Then the continuity equations for both holes and electrons, and Poisson’s equation across the photoconductor for a short pulse irradiation are simultaneously solved by the finite difference method, without any assumptions. There is a very good agreement between the approximate analytical and numerical solutions for the charge collection efficiency. The numerical results are also compared with Monte Carlo simulations of carrier transport. The charge collection efficiency model is applied to amorphous selenium x-ray image detectors. The bimolecular-recombination-limited charge collection efficiency depends on the total photogenerated carrier density rather than on its spatial distribution. It is found that the recombination plays practically no role up to the total instantaneous carrier generation Q0 of 109EHPs∕cm2 at the applied electric field of 10V∕μm. The effect of recombination on charge collection increases with decreasing applied electric field strength. For high carrier generation (e.g., Q0 of 1012EHPs∕cm2 for an applied field of 10V∕μm), the electric field distributions vary widely across the photoconductor thickness during the travel of charge carriers towards the electrodes. However, the effect of bimolecular recombination on charge collection efficiency is almost independent of bias polarity and the field distribution. The numerical results are also compared with recent experimental data available in the literature.

Corrections to the Hecht collection efficiency in photoconductive detectors under large signals: non-uniform electric field due to drifting and trapped unipolar carriers

“…We need to find the spatial distribution of the field and carrier distribution from which the transient current and hence Q c and CE can be calculated. Some examples that consider a nonuniform field and use numerical computations or approximations, can be found in [13,[17][18][19][20][21][22][23][24]. A semi-analytical treatise and full numerical solutions for the x-ray sensitivity of a-Se under large signals were given in [19].…”

“…Some examples that consider a nonuniform field and use numerical computations or approximations, can be found in [13,[17][18][19][20][21][22][23][24]. A semi-analytical treatise and full numerical solutions for the x-ray sensitivity of a-Se under large signals were given in [19].…”

“…We assume that the bulk is initially space charge free but once carriers are injected, the bulk has space charge due to the drifting injected carriers and due to trapped carriers left behind as the packet drifts. We use both Monte Carlo (MC) simulation (as described in [18,25] but for a single level of trapping) and numerical solutions of the continuity and Poisson equations as a comparison (as described in [19,20] but without the electrons and trap filling effects).…”

Enterococcus spp. Cell-Free Extract: An Abiotic Route for Synthesis of Selenium Nanoparticles (SeNPs), Their Characterisation and Inhibition of Escherichia coli

“…In contrast, elemental selenium (Se 0 ) is naturally insoluble, displays a lower toxicological potential and can form selenium nanoparticles (SeNPs). SeNPs can be recovered and have potential uses, such as in the agricultural and pharmaceutical industries [14], for glass production and as detectors in mammographic instruments [15,16] Several methods have been developed over the years for the remediation of seleniumladen waters. The remediation methods are in three categories, namely physical, chemical and biological techniques.…”

Manganese and Selenium

“…Selenium fertilizers and dietary supplements are used to counteract selenium deficiency [2], and selenium sulfide is used in fungicides and in antidandruff shampoos [4]. An emerging application of selenium is the 'amorphous selenium detector' in mammographic instruments [5].…”