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The dependence of the modulation transfer function on the blocking layer thickness in amorphous selenium x‐ray detectors
Author(s) -
Hunter David M.,
Belev Gueorgi,
DeCrescenzo Giovanni,
Kasap Safa O.,
Mainprize James G.,
Rowlands J. A.,
Smith Charles,
Tümer Tümay,
Verpakhovski Vladimir,
Yin Shi,
Yaffe Martin J.
Publication year - 2007
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.2757002
Subject(s) - detector , optics , materials science , optical transfer function , amorphous solid , x ray detector , blocking (statistics) , physics , chemistry , statistics , mathematics , organic chemistry
Blocking layers are used to reduce leakage current in amorphous selenium detectors. The effect of the thickness of the blocking layer on the presampling modulation transfer function (MTF) and on dark current was experimentally determined in prototype single‐line CCD‐based amorphous selenium ( a ‐ Se ) x‐ray detectors. The sampling pitch of the detectors evaluated was 25 μ m and the blocking layer thicknesses varied from 1 to 51 μ m . The blocking layers resided on the signal collection electrodes which, in this configuration, were used to collect electrons. The combined thickness of the blocking layer and a ‐ Se bulk in each detector was ∼ 200 μ m . As expected, the dark current increased monotonically as the thickness of the blocking layer was decreased. It was found that if the blocking layer thickness was small compared to the sampling pitch, it caused a negligible reduction in MTF. However, the MTF was observed to decrease dramatically at spatial frequencies near the Nyquist frequency as the blocking layer thickness approached or exceeded the electrode sampling pitch. This observed reduction in MTF is shown to be consistent with predictions of an electrostatic model wherein the image charge from the a ‐ Se is trapped at a characteristic depth within the blocking layer, generally near the interface between the blocking layer and the a ‐ Se bulk.