Direct conversion detectors: the effect of incomplete charge collection on detective quantum efficiency.

Direct conversion detectors offer the potential for very high resolution and high quantum efficiency for x-ray imaging, however, variations in signal can arise due to incomplete charge collection. A charge transport model was developed to describe the signal and noise resulting from incomplete charge collection. This signal to noise ratio (SNR) reduction was incorporated into the cascaded systems model for a simple x-ray detector. A new excess noise factor, A(c) (termed the collection noise factor) is introduced to describe the reduction in detective quantum efficiency (DQE). The DQE is proportional to the product of the quantum efficiency and the collection noise factor. If the trapping cross sections for electrons and holes are very different, and if the detector is biased improperly, the collection noise factor can drop to as low as 50%. In addition, the signal loss due to incomplete charge collection will reduce the DQE in the presence of added noise. Because of this, the DQE generally does not continue to improve with greater detector thickness. The collection noise factor and DQE are predicted for CdZnTe, PbI2, and Se. The optimization of detector thickness should be based not only on quantum efficiency but also on the charge collection statistics, which are influenced by bias field and polarity.

[1]  Harrison H. Barrett,et al.  The effect of pixel geometry on spatial and spectral resolution in a CdZnTe imaging array , 1995, 1995 IEEE Nuclear Science Symposium and Medical Imaging Conference Record.

[2]  Yael Nemirovsky,et al.  Study of the charge collection efficiency of CdZnTe radiation detectors , 1996 .

[3]  M J Yaffe,et al.  Model of the spatial-frequency-dependent detective quantum efficiency of phosphor screens. , 1990, Medical physics.

[4]  W. Heisenberg,et al.  On the structure of atomic nuclei , 1932 .

[5]  R. K. Swank.  Absorption and noise in x‐ray phosphors , 1973 .

[6]  K. Hecht.  Zum Mechanismus des lichtelektrischen Primärstromes in isolierenden Kristallen , 1932 .

[7]  W. Heisenberg,et al.  Über den Bau der Atomkerne. III , 1932 .

[8]  Y. Nemirovsky,et al.  Statistical models for charge collection efficiency and variance in semiconductor spectrometers , 1997 .

[9]  H. B. Barber,et al.  Charge transport in arrays of semiconductor gamma-ray detectors. , 1995, Physical review letters.

[10]  Majid Rabbani,et al.  Analysis of signal and noise propagation for several imaging mechanisms , 1989 .

[11]  Kanai S. Shah,et al.  Electronic transport in polycrystalline Pbl2 films , 1999 .

[12]  B G Fallone,et al.  Cascade analysis for medical imaging detectors with stages involving both amplification and dislocation processes. , 2001, Medical physics.

[13]  P. Luke.  Single-polarity charge sensing in ionization detectors using coplanar electrodes , 1994 .

[14]  Safa Kasap,et al.  X-ray sensitivity of photoconductors: application to stabilized a-Se , 2000 .

[15]  Eustace L. Dereniak,et al.  A 48/spl times/48 CdZnTe array with multiplexer readout , 1995, 1995 IEEE Nuclear Science Symposium and Medical Imaging Conference Record.

[16]  John A. Rowlands,et al.  Signal and noise analysis using transmission line model for larger-area flat-panel x-ray imaging sensors , 1999, Medical Imaging.

[17]  Eustace L. Dereniak,et al.  A 48/spl times/48 CdZnTe array with multiplexer readout , 1995 .

[18]  Rodney Shaw,et al.  Signal-to-noise optimization of medical imaging systems , 1999 .

[19]  Yael Nemirovsky,et al.  Methodology for evaluation of mobility-lifetime product by spectroscopy measurements in CdZnTe spectrometers , 1997 .