Enormous effort has been exerted on research and development of CdZnTe (CZT) over the past two decades, as well as the pursuit of an alternative material to mitigate the disadvantages in today’s CZT material or provide comparable device performance at a lower cost of production. Although the quality of CdZnTe crystals has been improved drastically over the past few years and the material cost has steadily decreased, the yield of large-volume high-quality detector-grade CZT continues to be an issue due to its poor thermo-physical properties. TlBr was found to be a promising material to compete with CZT, but the contact degradation and device stability are still big issues and severely hinder the deployment of commercial TlBr-based devices for nonproliferation and national security applications. At BNL, we are developing a new compound Cd1-xZnx Te1-y Sey (CZTS) that holds promise as a potentially viable crystal for the replacement of CZT for some radiation detection and imaging applications. The addition of Se in the CZT compound has been found to be very effective in a drastic reduction of the sub-grain boundary network, leading to better compositional and charge-transport homogeneity. The new material has tremendous potential to increase the yield of high-quality detectors at a much lower cost of production. The reduction of the sub-grain boundary network can result in detectors with a lower voltage operation and increased detector thickness. Our efforts to develop CZTS for X- and gamma-ray radiation detector applications will be discussed in detail.
The large Halo orbit in L2 of the ATHENA mission will expose the spacecraft (SC) to a significant flux of charged particles which is expected to overlap with the energy range of the instruments. This is a source of measurement background that needs to be minimized as much as possible to achieve the strict requirements of the mission. The need to know and mitigate this type of background has been identified as critical, and has led to a number of technology development activities which are progressing in parallel to the Phase A activities. Particularly, this paper details the status of the on-going activities to develop a set of charged particle diverters whose goal is to reduce the background generated by soft-protons which are focused by the Silicon Pore Optics (SPO) mirror modules towards the instrument detectors. This paper explains the considerations leading to an accommodation of the charged particle diverters close to the instruments in the Science Instrument Module (SIM), and details the analytical approach followed to choose the massoptimal location for the case of a uniform magnetic field Halbach design. The case of graded (non-uniform) magnetic fields is also explained in an effort to further decrease the mass. Preliminary magnetic field maps are presented as a proxy to compare the mass from different options. Finally, the first engineering models, manufacturing and test plans are presented which are the focus of a technology development activity aiming at the validation of the technologies involved up to TRL5.
Alloying of CdZnTe (CZT) with selenium has been found to be very promising and effective in reducing the overall concentration of secondary phases (Te precipitates/inclusions) and sub-grain boundary networks in the crystals. These two types of defects are the main causes for incomplete charge collection, and hence they affect the yield of high-quality CZT, resulting in a very high cost for large-volume, high-quality detector-grade CZT detectors. The addition of selenium was also found to very effective in increasing the compositional homogeneity along the growth direction of the CdZnTeSe (CZTS) ingots grown by the traveling heater method (THM) technique. The compositional homogeneity along the growth direction can enhance the overall yield of detector-grade CZTS, which should therefore be possible to produce at a lower cost compared to CZT. The electrical properties and detector performance of the CZTS crystals will be presented and discussed.
Our prior investigations showed that alloying CdTe with selenium results in improved material characteristics, such as a reduction in the concentration of secondary-phase particles, better compositional uniformity and less sub-grain boundary networks, as compared to CdTe/CdZnTe. However, by alloying with Se, the band-gap of CdTeSe is significantly reduced from the value for CdTe, which is the main drawback for high-resistivity CdTeSe compounds useful for radiation detection. In order to increase the band-gap, we are now growing Cd1-xZnxSeyTe1-y crystals for detector applications. The effect of Se alloying with CdZnTe will be discussed in terms of the concentration of secondary phases, stress-related defects such as sub-grain boundaries and their networks. Characterization results for the transport properties of the as-grown materials will also be discussed.