Development of portable CdZnTe spectrometers for remote sensing of signatures from nuclear materials

Abstract

Room temperature cadmium zinc telluride (CZT) gamma-ray spectrometers are being developed for a number for years for medical, space and national security applications where high sensitivity, low operating power and compactness are indispensable. The technology has matured now to the point where large volume (several cubic centimeters) and high energy resolution (approximately 1% at 660 eV) of gamma photons, are becoming available for their incorporation into portable systems for remote sensing of signatures from nuclear materials. The straightforward approach of utilizing a planar CZT device has been excluded due to the incomplete collection arising from the trapping of holes and causing broadening of spectral lines at energies above 80 keV, to unacceptable levels of performance. Solutions are being pursued by developing devices aimed at processing the signal produced primarily by electrons and practically insensitive to the contribution of holes, and recent progress has been made in the areas of material growth as well as electrode and electronics design. Present materials challenges are in the growth of CZT boules from which large, oriented single crystal pieces can be cut to fabricate such sizable detectors. Since virtually all the detector grade CZT boules consist of several grains, the cost of a large, single crystal section is still high. Co-planar detectors, capacitive Frisch-grid detectors and devices taking advantage of the small pixel effect, are configurations with a range of requirements in crystallinity and defect content and involve variable degrees of complexity in the fabrication, surface passivation and signal processing. These devices have been demonstrated by several research groups and will be discussed in terms of their sensitivity and availability.

Vertical Bridgman growth and characterization of CdMnTe crystals for gamma-ray radiation detector

“…The so-called ''hole tailing'' is a serious impediment of parallel planar detector, degrading the energy resolution and peak efficiency, which is caused by the trapping of holes causing an incomplete collection of the total charge, or an asymmetry in the transport properties of electron and hole charge carriers [17]. In addition, the larger FWHM of photopeak and lower (mt) h can be probably caused by Te inclusions, which could posit either a reduced electron lifetime due to enhanced trapping at inclusions, or an altered carrier speed due to a distorted electric field at inclusions, or both [10].…”

Component Overpressure Growth and Characterization of High-Resistivity CdTe Crystals for Radiation Detectors

“…There have been increasing demands for real-time high-resolution detection and isotope identification in a variety of environments, especially for Homeland security and medical imaging applications. [1][2][3] Despite the excellent promise shown by CdTe, the quality of the material has not been fully optimized. In particular, there are defects related to Te inclusions, which have recently been shown to significantly affect device performance.…”

Study of ammonium fluoride passivation time on CdZnTe bulk crystal wafers

“…The advantages of the CZT material can be derived from the high average atomic number (Z=50) and density (d = 5.9 g/cm 3 ) yielding CZT detectors that are both efficient and compact. Cd 0.9 Zn 0.1 Te has an energy gap of 1.6 eV and is available in its high resistivity form (10 10 -10 11 Ωcm) as a highly compensated semiconductor where the carrier density is significantly lower than the concentration of deep levels [4].…”