A study on non-polarized CdZnTe detector under high-flux X-ray irradiation (#4105)
F. Yang1, 2, W. Jie1, 2, T. Wang1, 2, S. Xi1, 2, P. Zhu1, G. Zha1, 2
1 Northwestern Polytechnical University, Shool of Material Science and Engineering, xi'an, Shaanxi, China
Over the past decade years, Cd0.9Zn0.1Te(CZT) had became the most promising high-flux radiation detector material on medical imaging field, because of the high photoelectric conversion efficiency and working at room temperature. However, no reports of high photon counting rate of about (1~10)×107/(mm2*s) was published, while the mechanism of the limited counting rate was not clear yet. Polarization effect according to space charge in the detector was regarded as a main reason which caused low counting rate. This study focuses on another mechanism that limited the counting rate of CZT detectors. Photocurrent characterization was applied in our home-designed X-ray testing system. In this test procedure, the linear array CZT detector’ photocurrent was measured in a bias about -450V under X-ray irradiation. The result showed that for both high and low counting rate detectors the photocurrent was found to be linear related with the X-ray tube current, which proved that the low counting rate detector was not polarized. Comparing the two different detectors’ current-time (I-t) curves we found that the stability of the photocurrent was strongly related with the limitation of the detectors. We considered that high frequency noise in the unstable photocurrent detector might cause pile-up effect in ASIC. After a period of study, the electrode injection in CZT detector was determined to be a main factor to the time stability of photocurrent. Some fabrication optimization and electrode structure were attempted to prevent electrode injection in the detectors. After that the counting rate of our CZT photon counting detectors successfully increased from 1×105/(mm2*s) to 5×106/(mm2*s), and photon counting imaging was presented. Our work illustrated that in addition to polarization, the stability of photocurrent was another important factor in CZT photo counting detectors.
Keywords: CdZnTe, high flux, photon counting detector, electrode injection
Effects of Tellurium Oxide on Surface Current and Performance of CdZnTe Nuclear Radiation Detectors (#3959)
S. U. Egarievwe1, 2, E. D. Lukosi2, I. O. Okwechime1, A. Hossain3, R. B. James4
1 Alabama A&M University, Nuclear Engineering and Radiological Science Center, Normal, Alabama, United States of America
Cadmium zinc telluride (CdZnTe) radiation detectors have found applications in medical imaging and in the detection of radiological and nuclear threats. While there has been success in developing CdZnTe detectors with less than 1% FWHM energy resolution at the 662 keV, charge collection can be improved through optimization of surface treatment techniques. The dominant surface species on CdZnTe are cadmium, tellurium, and tellurium oxide. The tellurium oxide is expected to produce a more chemically stable surface. This paper presents the quantification of the tellurium oxide species using X-ray electron spectroscopy (XPS), and the correlation of XPS results to surface leakage current and detectors performance. The effects of the tellurium oxide on the aging of the detector will also be presented.
Keywords: Nuclear radiation detectors, CdZnTe, X-ray electron spectroscopy, Surface treatment, tellurium oxide
High-spatial-resolution in-operando current mapping and X-ray fluorescence measurements: revealing nano-scale defects in CdZnTe crystals (#3695)
G. Yang1, H. Yan1, A. E. Bolotnikov1, G. S. Camarda1, Y. Cui1, A. Hossain1, U. N. Roy1, R. Gul1, Y. S. Chu1, E. Nazaretski1, X. Huang1, P. Vanier1, R. B. James2
1 Brookhaven National Laboratory, Upton, New York, United States of America
CdZnTe (CZT) has been recognized as one of the leading materials for fabricating room-temperature X-ray and gamma-ray detectors. However, the presence of material defects is still a challenging problem, which affects the availability of high-quality CZT crystals. Especially little work has been done to understand the effects of nano-scale defects of CZT on device performance. In this work, we used a nanofocused X-ray beam with a 50-nm focus size to illuminate CZT detector and collected the X-ray Beam Induced Current (XBIC) and X-ray Fluorescence (XRF) signals simultaneously. Our observation shows a strong correlation between the XBIC mapping and the XRF mapping. Detailed results and analysis will be presented. Our efforts offer a better understanding in terms of material defects, impurities, and electrical properties of CZT radiation detectors at an unprecedented nano-scale level.
Keywords: nano-scale defects, CdZnTe, X-ray Beam Induced Current, X-ray Fluorescence
Laser Induced Transient Current Technique as a Powerful Tool to Determine Hole Transport Properties in CdZnTe Detectors (#2989)
A. Zappettini1, A. Santi2, M. Bettelli1, M. Zanichelli2, L. Abbene3, F. Principato3, M. Pavesi1, 2
1 IMEM, CNR, Parma, Italy
We have recently shown that , by the use of the laser Induced Transient Current Technique (LI-TCT), it is possible to determine, at the same time, charge transport properties (namely carrier lifetime and mobility) and the electric field distribution in the direction parallel to the applied electric field direction, in CZT detectors.
Also, we showed that the method is applicable to situations where a more complex contact geometry makes the induced charge affected also by the so-called weighing field. In these conditions, the current transients result to be affected not only by the transport parameters and by the electric field distribution, but also by the weighing field distribution.
In this work, we show that the LI-TCT method can also be used for the determination of holes transport properties. As a result, mobility and lifetime of holes, together with the electric field distribution, can be obtained at once.
This result can be achieved by adopting Pt contact on both detector surfaces. Pt contacts are deposited by a new electroless deposition process based on the use of methanol and dimethylformamide (DMF) as solvents. This contact is mechanically stable and withstands the standard tape test.
In this work, we show the procedure to obtain Pt contacts by this new procedure, electrical characterization of the contact, and LI-TCT characterization of CZT different detectors.
Keywords: CdZnTe, mobility, carrier lifetime, electric field distribution
Quad-grid implementation of the Virtual Coplanar Grid for a 11x11 pixelated CZT detector (#3199)
R. Espagnet1, A. Frezza1, L. - A. Hamel2, J. - P. Martin2, P. Després1, 3
1 Université Laval, Department of Physics, Engineering Physics and Optics and Cancer Research Center, Université Laval, Quebec city, Québec, Canada
The development of a blood gamma counter for quantitative studies in PET and SPECT imaging has required the design of a virtual coplanar grid (vCPG) to use a commercially available CZT detector of 20x20x1 mm3 having 121 pixels and read with only two channels. This was done by connecting these 121 pixels to form intercalated bands and to reproduce a CPG design. The vCPG characterization was previously conducted and yielded conclusive results. However, the energy resolution of 8% achived was worse than the 4% reported in the case of lithographically obtained CPG. The purpose of this work is to study new geometries of pixel connections to get a better energy resolution. A circuit with two charge-sensitive preamplifiers was designed to connect the detector and to relay each 121 pixels to a PCB where they can be connected by small wires to form the desired vCPG geometry. The tested geometry, named virtual Coplanar quad-Grid (vCPqG), consists in a virtual version of the four quadrant lithographic CPG rotated by 90 degrees used by the COBRA collaboration. This geometry gives a symmetric anode pattern and allows to decrease the bulk-leakage current on each CPG to improve the energy resolution and the intrinsic efficiency. The bias and the weighting factors were optimized from vCPG selected parameters for each coplanar grid of the vCPqG. Parameters leading to the best results were a cathode voltage of -2500V, a bias between the anodes of 100V and weighting factors of 0.53, 0.8, 1 and 0.66 to compensate for electron trapping. With the optimized parameters, the achieved energy resolution ranged from 4.2% to 5.5% at 662 keV. The stability of the measurements over time and the channel-energy linearity were the same as the vCPG while the energy resolution was improved by more than 40%.
Keywords: vCPG, CoPlanar Grid, CZT, CPqG, CdZnTe, Pixel detector, vCPqG
Study of Transient Current Waveforms and Electric-field Profile in Room Temperature-Radiation Detectors (#3447)
R. Gul1, 2, U. N. Roy1, R. B. James1, 3
1 Brookhaven National Laboratory, CFN, Upton, New York, United States of America
We will present characterization data for the electric field and electrical transport properties of Bridgman- and THM- (travelling heater method) grown CZT, CMT, and CZTS detectors by using a transient current technique (TCT). The TCT system has an excellent position resolution and focused beam width; it has been routinely used for detailed study of the transient waveforms and the electric-field profile in II-VI radiation detectors. Two planar detectors from each type of detectors were characterized along the thickness of the detector extending from the top contact (cathode) to the bottom contact (anode). Transient current waveforms were collected for different applied bias conditions, and they were analyzed to determine the electron charge-carrier mobility and lifetime. In addition the data are used to calculate and plot the internal electric-field profile. The measured electric-field profile shows variations indicating creation of a positive space charge induced by the laser illumination. The electric-field analysis indicates that the field strength decreases from the cathode towards the anode, which shows the strong positive space-charge region near the cathode and weaker space charge region towards the anode of the detector. A model based on the transient current waveforms will also be presented.
Keywords: radiation detectors, Transient current, wave form, electric field, space charge, TCT