Speaker
Description
Many synchrotrons worldwide are undergoing upgrades to diffraction limited storage rings (DLSRs). With these planned upgrades, the average energies of many beam lines will increase to $>20 \mathrm{keV}$ and fluxes will also increase considerably to $10^{12} \mathrm{ph} \cdot \mathrm{s}^{-1} \mathrm{~mm}^{-2}$. These challenging specifications require new detector materials to be used as silicon has poor efficiency and is susceptible to radiation damage. Until recently, $\mathrm{CdZnTe}$ material was known to polarize at fluxes $>10^6 \mathrm{ph} . \mathrm{s}^{-1} \mathrm{~mm}^{-2}$, making it unsuitable for $\mathrm{DLSR}$ synchrotrons [1]. Preliminary measurements using high-flux-capable CdZnTe (HF-CdZnTe) developed by Redlen Technologies have shown considerable promise although its performance and limitations are still to be fully understood.
The material was shown to have good linearity for fluxes between $7.1 \times 10^7 \mathrm{ph.s}^{-1} \mathrm{~mm}^{-2}$ to $8.1 \times 10^9$ ph. $\mathrm{s}^{-1} \mathrm{~mm}^{-2}$ however transient effects have also been observed which need to be understood further [2]. The aim of this work is to understand the fundamental process behind this improvement of performance in $\mathrm{HF}-\mathrm{CdZnTe}$ compared to spectroscopic $\mathrm{CdZnTe}$ and quantify transient effects observed.
HF-CdZnTe was purchased from Redlen and refabricated by Due2Lab to produce a simple planar $\mathrm{Pt} / \mathrm{CdZnTe} / \mathrm{Pt}$ detector $1.8 \mathrm{~mm}$ thick. Measurements of fundamental material properties such as the charge carrier transport properties, band gap and resistivity were evaluated. The planar detector was irradiated with a $5.5 \mathrm{MeV}$ Americium-241 $\alpha$ source and the waveform recorded as a function of bias voltage. The mobility and lifetime of the charge carriers were found from analysis of the pulse height spectra and signal rise times. Preliminary measurements of
HF-CdZnTe were found to be $\mu_{e}\tau_{e}=3.94 \times 10^{-3} \mathrm{~cm}^2 \mathrm{~V}^{-1}, \mu_{\mathrm{e}}=1.05 \times 10^3 \mathrm{~cm}^2 \mathrm{~V}^{-1} \mathrm{~s}^{-1}, \mu_{\mathrm{h}} \tau_{\mathrm{h}}=$
$2.09 \times 10^{-4} \mathrm{~cm}^2 \mathrm{~V}^{-1}, \mu_{\mathrm{h}}=29.52 \mathrm{~cm}^2 \mathrm{~V}^{-1} \mathrm{~s}^{-1}$. This novel result shows an increase in hole
mobility-lifetime ($\mu_{\mathrm{h}} \tau_{\mathrm{h}}=$
$2.09 \times 10^{-4} \mathrm{~cm}^2 \mathrm{~V}^{-1}$ ) compared to spectroscopic CdZnTe ($\mu_{\mathrm{h}} \tau_{\mathrm{h}}= \sim 1.5 \times 10^{-5}.$ $\mathrm{cm}^2 \mathrm{~V}^{-1}$ ) which is consistent with the improved performance at higher fluxes that has been recorded by previous groups [1].
Having characterized the charge transport properties of this device we then studied its performance at high x-ray fluxes. Measurements were taken at the Diamond synchrotron at B16 beamline. The x-ray beam energy was monochromated to $12 \mathrm{keV}$ and $20 \mathrm{keV}$. The planar detector was irradiated using a $1 \mathrm{~mm}^{2}$ beam and intensity of the x-ray was varied using beam line attenuators, exposing the detector to flux levels ranging from $10^6 \mathrm{ph.s}^{-1} \mathrm{~mm}^{-2}$ to $10^8 \mathrm{ph.s}^{-1} \mathrm{~mm}^{-2}$. To test the high flux performance, current pulses were recorded as a function of flux with a sampling period of $5 \mu \mathrm{s}$.
Over the range of fluxes measured, the detector showed good linearity. However, when the X-ray shutter was closed a long lived ( $t \sim 30 s)$ decay of charge was observed from the sensor. This suggests the presence of trapped charge in the sensor, the cause of which will be explored further in this talk.
1)https://ieeexplore.ieee.org/abstract/document/4346740
2)https://doi.org/10.1088/1748-0221/17/11/C11008
