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Register a new userAn Investigation on Cooling of CZT Co-Planar Grid Detectors
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The effect of moderate cooling on CdZnTe semiconductor detectors has been studied for the COBRA experiment. Improvements in energy resolution and low energy threshold were observed and quantified as a function of temperature. Leakage currents are found to contribute typically $sim$5 keV to the widths of photopeaks.
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The characterization of detectors fabricated from home-grown crystals is the most direct way to study crystal properties. We fabricated planar detectors from high-purity germanium (HPGe) crystals grown at the University of South Dakota (USD). In the fabrication process, a HPGe crystal slice cut from a USD-grown crystal was coated with a high resistivity thin film of amorphous Ge (a-Ge) followed by depositing a thin layer of aluminum on top of the a-Ge film to define the physical area of the contacts. We investigated the detector performance including the $I$-$V$ characteristics, $C$-$V$ characteristics and spectroscopy measurements for a few detectors. The results document the good quality of the USD-grown crystals and electrical contacts.
Compact multi-channel radiation detectors rely on low noise front-end application specific integrated circuits (ASICs) to achieve high spectral resolution. Here, a new ASIC developed to readout virtual Frisch-grid cadmium zinc telluride (VFG CZT) detectors for gamma ray spectroscopy is presented. Corresponding to each ionizing event in the detector, the ASIC measures the amplitude and timing at the anode, the cathode and four pad sense electrodes associated with each sensor in a detector array. The ASIC is comprised of 52 channels of which there are 4 cathode channels and 48 channels which can be configured as either anode channels with a baseline of 250 mV or pad sense channels to process induced signals with a baseline of 1.2 V. With a static power dissipation of 3 mW, each channel performs low-noise charge amplification, high-order shaping, peak and timing detection along with analog storage and multiplexing. The overall channel linearity was better than $pm$ 1 % with timing resolution down to 700 ps for charges greater than 8 fC in the 3 MeV range. With a 4 x 4 array of 6 mm x 6 mm x 20 mm virtual Frisch-grid bar sensors connected and biased, an electronic resolution of $approx$ 270 rms $e^{-}$ for charges up to 100 fC in the 3.2 MeV range was measured. Spectral measurements obtained with the 3D correction technique demonstrated resolutions of 1.8 % FWHM at 238 keV and 0.9 % FWHM at 662 keV.
The COBRA collaboration operates a demonstrator setup at the underground facility LNGS (Laboratori Nazionali del Gran Sasso, located in Italy) to prove the technological capabilities of this concept for the search for neutrinoless double beta-decay. The setup consists of 64 $(1times!1times!1)$ cm$^{3}$ CZT detectors in CPG configuration. One purpose of this demonstrator is to test if reliable long-term operation of CZT-CPG detectors in such a setup is possible. The demonstrator has been operated under ultra low-background conditions since more than three years and collected data corresponding to an exposure of 218 kg$cdot$days. The presented study focuses on the long-term stability of CZT detectors by analyzing the intrinsic, fourfold forbidden non-unique $^{113}$Cd single beta-decay. It can be shown that CZT detectors can be operated stably for long periods of time and that the $^{113}$Cd single beta-decay can be used as an internal monitor of the detector performance during the runtime of the experiment.
The vertex detectors are crucial detectors for future linear e+e- colliders since they must give the most accurate location of any outgoing charged particles originating from the interaction point. The DEPFET collaboration is developing a new type of pixel sensors which provide very low noise and high spatial resolution. In order to precisely determine the track and vertex positions, multiple scattering in the detector has to be reduced by minimizing the material in the sensors, cooling, and support structures. A new method of cooling by blowing air over the sensors has been developed and tested. It is applied in the design and construction of the Belle-II detector and may be used in the new generation of vertex detectors for linear colliders.
We have modeled laser-induced transient current waveforms in radiation coplanar grid detectors. Poissons equation has been solved by finite element method and currents induced by photo-generated charge were obtained using Shockley-Ramo theorem. The spectral response on a radiation flux has been modeled by Monte-Carlo simulations. We show 10$times$ improved spectral resolution of coplanar grid detector using differential signal sensing. We model the current waveform dependence on doping, depletion width, diffusion and detector shielding and their mutual dependence is discussed in terms of detector optimization. The numerical simulations are successfully compared to experimental data and further model simplifications are proposed. The space charge below electrodes and a non-homogeneous electric field on a coplanar grid anode are found to be the dominant contributions to laser-induced transient current waveforms.
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