Characterization of Cd0.9Zn0.1Te based virtual Frisch grid detectors for high energy gamma ray detection

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Abstract

Detector grade CZT single crystals have been grown from zone refined Cd, Zn, and Te (∼7 N) precursor materials using a tellurium solvent method. Infrared transmission imaging has shown an average tellurium inclusion size of ∼8 μm in the as-grown crystal. Radiation detectors were fabricated in planar and virtual Frisch grid geometry and characterized for their spectroscopic performance. Charge transport properties revealed a high drift mobility of∼1200 cm2/V s and a mobility–lifetime product of ∼2.8×10−3 cm2/V. A detector with planar configuration (dimensions 6.9×6.9×4.8 mm3) showed an energy resolution of 4.7% for 59.5 keV gamma rays. The detector in a Frisch grid configuration (dimensions 4.2×6.2×6.5 mm3) exhibited an energy resolution of 1.4% for 662 keV gamma rays. Digital spectroscopic measurements were carried out using a high-resolution digitizer card. A biparametric correlation scheme was employed to study the effect of charge loss on energy resolution of the planar detector. Based on this correlation scheme, a digital correction method was applied and improved pulse-height spectrum was obtained for 662 keV gamma rays using the planar detector with spectral features comparable to those of the virtual Frisch grid detector.

Introduction

CdZnTe (CZT) based devices have remained a topic of primary interest to detector scientists since the inception of the idea of using high Z materials for room temperature x-/gamma-ray detection. Properties like adequate gamma ray energy absorption even for small volume detectors, low leakage current, wide band gap at room temperature, high density and ease of detector fabrication have made it a very suitable candidate for nuclear radiation detection in the field of homeland security, medical imaging, infrared focal plane array, environmental monitoring, etc. [1], [2], [3], [4], [5], [6], [7]

Performance of large volume CZT detectors is limited by the poor charge transport properties like low drift mobility and lifetime especially for holes. Macroscopic defects such as cracks and twin/grain boundaries and microstructural defects such as mosaic structures, tilt boundaries, dislocations, point defects, impurities, and tellurium inclusions/precipitations lead to the poor charge transport properties in CZT materials [1].

Apart from the attempt of growing high-quality defect free crystals [8], [9], several special detector geometry e.g., virtual Frisch grid configuration [10], coplanar grid structure [11] and small pixel geometry [12] have been adopted in order to compensate for the poor hole transport properties in CZT crystals. Among various virtual Frisch collar configurations, non-contacting Frisch collar maintains the single-carrier nature of the detector with minimal leakage current flow between the anode and the grid as the Frisch collar is electrically isolated from the crystal surface which prohibits any flow of leakage current [13], [14]. Various offline digital correction schemes have also been adopted in order to compensate for the effect of charge loss on the detector performance [15], [16].

In the present work we have used a tellurium solvent method to grow high quality detector grade Cd0.9Zn0.1Te single crystals and fabricated detectors in planar and virtual Frisch grid configurations. The detectors were characterized with low leakage current under operating conditions. Electron drift mobility and mobility-lifetime (μτ) product have been measured using alpha ray spectroscopy and a time of flight technique respectively. High resistivity and electron drift mobility helped to obtain high energy resolution for high energy gamma rays. Digital spectroscopic measurements were carried out to study the depth dependence of charge loss in the planar detector and a digital correction scheme has been also adopted in order to compensate for the effect of charge loss in the planar detector.

Section snippets

Crystal growth

In-house zone refined (ZR) Cd, Zn, and Te precursors (∼7 N) were weighed in stoichiometric amounts for Cd0.9Zn0.1Te crystal growth with excess of 50% ZR Te and were vacuum sealed at 10−6 Torr in a carbon-coated quartz ampoule (∼4 mm wall thickness, ampoule ID 25 mm). The ZR precursors (5 N) were passed through a zone refining furnace at a rate of ∼22 mm/hr and were completed in an average of 35 cycles to achieve 7 N purity. The sealed ampoule was then loaded into a two-zone horizontal furnace for

IR transmission imaging

Te inclusions can be easily observed using IR transmission imaging. Te inclusions with diameters greater than 10 μm can act as potential charge trapping centers and significantly degrade the detector's performance [21], [22]. Fig. 2 shows an IR transmission image of a representative portion of the sample. The dark spots in the picture show the Te inclusions. The average size of the Te inclusions was estimated to be ∼8 μm. The small size and low density of Te inclusions in the crystals indicate

Conclusion

Gamma-ray detectors with two different configurations viz., planar and virtual Frisch grid were fabricated from CdZnTe single crystals grown using a Te solvent method. The crystals were seen to exhibit low leakage current at operating bias and room-temperature. The charge-transport property measurements revealed a high electron drift mobility of ∼1200 cm2/V s and an electron μτ product of 2.8×10−3 cm2/V which confirms that the crystals were of detector-grade quality. The superior crystal quality

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