Linear x-ray detector array made on bulk CdZnTe for 30~100 keV energy

Journal of Electronic Materials, Jun 1997 by Yoo, S S, Jennings, G, Montano, P A

Similarly, energy spectra were measured for the test 16 element array and uniformity of the FWHM of the 122 keV photopeak was observed. As shown in Fig. 7, the photopeak FWHMs of the test elements were consistently better than that of the large area detector, and excellent uniformity of the FWHM was obtained as shown in Fig. 7. The average FWHM at 122 keV is 5.78% with 0.18% standard deviation, which corresponds to 7 keV FWHM with 230 eV deviation in the energy resolution. The improved energy resolution over the large area detector is due to the small pixel effect.18 When the array element pitch size is small, an enhanced electric field occurs near the small electrodes. When the bias is applied, such that electrons are collected to the electrodes, the electron charge contribution to the total signal becomes significant, whereas the contribution of the relatively slow holes, which tend to be trapped, becomes negligible.

The charge sharing, which is usually observed in small size array detectors, can be reduced by providing a ground path for the unwanted charges. Additional metal contacts, called guard rings, can be placed between each element and around the array detectors. Therefore, the charge created in between elements are drifted to the nearest ground guard ring contacts before they are collected to the element detectors, thus providing the electrical isolation between elements. In addition to the AC blocking resistors of the adjacent elements, capacitors were connected in parallel, so that the charges created in the neighboring elements pass through the capacitors to the ground. Compared with the spectrum measured without the capacitors, low energy counts were greatly improved as shown in Fig. 8. However, slightly increased counts are observed below 37 keV. This seems to be due to the photons projecting in the slanted angle to the element from the planar source.

Because of the planar shape of the Co^sup 57^ source used in this study, the photon flux impinging onto the test element is difficult to measure. Therefore, the source was encapsulated in lead block with a 1 mm diameter collimator on one side. Using the collimator, the photon flux will be greatly reduced but the radiation direction will be well defined. Therefore, charge sharing should be greatly reduced in the small pitch array. The energy spectrum was measured with a reference CdZnTe detector of the same material quality but of large area, 1 x 1 cm^sup 2^. In Fig. 9, the measured photon counts were greatly reduced compared with the case ofthe open source. The Co^sup 57^ photopeaks, escape peaks, and lead fluorescence peak can be distinguished. Similarly, using an element detector with ground capacitors to the adjacent elements, the spectrum measurement was made after the element was carefully aligned to the center of the collimator using a micro-translation stage. It is noticed that the low energy spectrum was further reduced. However, during the alignment, a great number of counts below 16 keV were recorded, which are due to the noise generated by induction current of the micro-motors. It was found later that the noise can be decreased by reducing the velocity and acceleration of the motor. Therefore, counts below 20 keV from both spectra were neglected. The average count rates were calculated to be 1.63 and 0.32/s, for the large area detector and the test element detector, respectively. Assuming a uniform photon flux distribution within 1 mm diameter area, the ratio of the count rates should agree with the ratio of the beam area and the linear detector element area. We obtained good agreement between these ratios, 17.8 and 18% of the area and count rate ratio, respectively.