Room-temperature performance of 3 mm-thick cadmium–zinc–telluride pixel detectors with sub-millimetre pixelization

Abstract

Cadmium–zinc–telluride (CZT) pixel detectors represent a consolidated choice for the development of room-temperature spectroscopic X-ray imagers, finding important applications in medical imaging, often as detection modules of a variety of new SPECT and CT systems. Detectors with 3–5 mm thicknesses are able to efficiently detect X-rays up to 140 keV giving reasonable room-temperature energy resolution. In this work, the room-temperature performance of 3 mm-thick CZT pixel detectors, recently developed at IMEM/CNR of Parma (Italy), is presented. Sub-millimetre detector arrays with pixel pitch less than 500 µm were fabricated. The detectors are characterized by good room-temperature performance even at high bias voltage operation (6000 V cm−1), with energy resolutions (FWHM) of 3% (1.8 keV) and 1.6% (2 keV) at 59.5 keV and 122.1 keV, respectively. Charge-sharing investigations were performed with both uncollimated and collimated synchrotron X-ray beams with particular attention to recovering the charge losses at the inter-pixel gap region. High rate measurements demonstrated the absence of high-flux radiation-induced polarization phenomena up to 25 × 106 photons mm−2 s−1.

Energy Recovery of Multiple Charge Sharing Events in Room Temperature Semiconductor Pixel Detectors

“…Intense investigations have been made on the effects of charge-sharing and cross-talk phenomena in CZT and CdTe pixel detectors [ 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 ]. Charge-sharing events are generated by the splitting of the charge cloud created by a single interacting event and collected by two or more pixels.…”

“…To avoid this, the rejected events after CSD can be recovered through the charge-sharing addition (CSA) technique, which consists of summing the energies of the coincidence events [ 16 , 17 , 18 ]. Unfortunately, as documented in the literature [ 17 , 18 , 19 , 20 , 21 ], the energy recovered after CSA is often lower than true photon energy, which is due to the presence of charge losses near the region of inter-pixel gap. These losses are related to the presence of distorted electric field lines at the inter-pixel gap [ 14 ].…”

“…Two key features characterize this technique: (i) first, the modeling does not depend on the photon energy, but it is only related to the physical and geometrical characteristics of the electrode layout; (ii) second, it can be easily obtained even with uncollimated photon beams. This approach was successfully applied to several CZT and CdTe pixel detectors [ 20 , 21 , 22 , 23 , 24 , 25 ] with improvements in energy resolution and counting efficiency. However, since this technique is applied to shared events involving only two adjacent pixels ( m = 2), multiple coincidences with m > 2 are still rejected from the measured energy spectra.…”

“…Different CZT crystals were used. Some detectors are based on the traveling heater method (THM) [ 19 , 20 , 21 ] and boron oxide encapsulated vertical Bridgman (B-VB) [ 23 , 27 , 29 ] CZT crystals. In addition to the standard or low flux LF-THM CZT crystals, we also tested high-flux HF-THM CZT crystals [ 20 , 30 ], which were recently produced by Redlen Technologies and characterized by enhanced hole charge transport properties to minimize radiation-induced polarization at high fluxes.…”

“…However, as widely shown in both CdTe and CZT pixel detectors, charge losses at the inter-pixel gap often create incomplete energy recovery after CSA [ 17 , 18 , 19 ]. Recently, an energy-recovery technique for double charge shared events ( m = 2) was proposed by our group [ 20 ] and successfully applied to several CZT and CdTe pixel detectors [ 20 , 21 , 22 , 23 , 24 , 25 ]. Despite the benefits on detector performance, this technique can only be applied to shared events involving only two adjacent pixels ( m = 2) and, therefore, multiple coincidences with m > 2 and coincidences with diagonal pixels are rejected from the measured energy spectra.…”