New (Cd,Mn)Te X-Ray Detectors for the OMEGA Laser System

July 2009

Photoconductor used in the new x-ray detector on OMEGA

Graduate student, Allen Cross, with the photoconductor used in the new x-ray detector on OMEGA. The curve representing the voltage response collected from an OMEGA target shot is shown in the background.

Standard x-ray detectors used on the OMEGA Laser System have typically been diamond photoconductive detectors (PCD's) and x-ray diodes (XRD's). The diamond PCD's are used because of their high sensitivity to low-energy (soft) x rays, their large Eg of ~5 eV, and their very high speed of operation, allowing one to time-resolve different emission events (from the hot corona and burning core) occuring during an ICF shot. Recently, (Cd,Mn)Te (CMT) has been closely studied as a viable material for radiation detection because it can be used for x-ray energies of up to 100 keV and is relatively easy to grow as large, high-quality (homogeneous) single crystals. Numerous other applications for the Cd1-xMnxTe range from magnetic-field sensors and Faraday isolators to solar cells. Simultaneously, new research has been initiated at LLE on such CMT applications as magneto- and electro-optic sampling and coherent acoustic phonon generation and detection.

A (Cd,Mn)Te photoconductive detector

An x-ray calorimetry array composed of three diamond and three (Cd,Mn)Te photoconductive detectors with independent spectral filters

Recent investigations conducted at the Omega Facility have employed CMT PCD's, compared to diamond PCD's, as a diagnostics x-ray detection tool to conduct inertial confinement fusion (ICF) experiments. Development of the CMT PCD's for measuring x-ray emissions from the fusion experiments will enhance the x-ray measurements. The properties of this material include a very large resistivity, good electron transport, and an inherently very high x-ray stopping power, for which the photoelectric effect shows a near-flat, approximately 100% absorption spectrum up to ~50 keV. Research conducted at LLE indicates that CMT PCD's are a viable option that will allow accurate, temporal characterization of x-ray transients, especially at medium to high spectral ranges, generated during the ICF event.

For detector preparation and experimental setup, Cd1–xMnxTe (x = 0.05) single crystals, doped with V, were grown using a vertical Bridgman method and, subsequently, annealed in Cd vapor for the highest resistivity (~1010 Ωcm) and a good mobility-lifetime product (several 10–4 cm2/ V). The 1- and 2.3-mm-thick detectors were placed in the same housing used for two 1-mm-thick diamond PCD's. Each device was preceded by a 7.6-Ám-thick Be x-ray filter with 37% x-ray transmission at 1-keV cutoff energy. The response amplitudes and rise times of the CMT PCD's were measured and proved to be comparable with the diamond detector's performance, while their decay times were an order of magnitude longer (in the 5- to 10-ns range).

Schematic of a (Cd,Mn)Te photoconductive detector

Schematic of a (Cd,Mn)Te photoconductive detector

The OMEGA shots designed to test the performance of the CMT PCD's employed empty plastic-shell targets, which resulted in x-ray emissions from both the corona and the imploded shell, recorded as two peaks in time. The incident-beam illumination was comprised of 1-ns-long, square-shaped pulses with total energies ranging from 2.3 kJ to 22.6 kJ. The initial laser heating of the target created a hot corona, causing the first temporal peak, while the second temporal peak was caused by x-ray emission from the core formed by the compressed plastic shell. For comparison purposes, testing was performed using x-ray photons with energies above 1 keV, optimal for the diamond PCD. According to simulations for 1-mm-thick crystals at x-ray energies >10 keV, diamond absorption efficiency drops to <50%. For CMT, however, the same absorption drop occurs at ~100 keV, and one can observe a near-perfect, 100% absorption, up to 50 keV.

Successful implementation of the new CMT PCD's, with their promising wide spectral absorption characteristics, seems to be a very viable upgrade to the diamond PCD's, currently implemented as the x-ray diagnostics in the OMEGA target chamber. In the future, CMT PCD's are also likely to be implemented as γ-ray sensors in the diagnostic toolkit of the OMEGA Laser.