V. Spassov’s research while affiliated with Medical University of Sofia and other places

The measurement system is designed to have a Si n+–p–p+ detector and current amplifier. It is examined in strong gamma fields. The detector works in a current-generation mode without applying reverse voltage. The amplifier measures currents in the range of 10−6–10−12A. The output current is proportional to the gamma-ray intensity. By integrating the output current, the system can measure the radiation dose for a preliminary chosen time. A maximum integral irradiation dose of 59.9Mrad is obtained for five consecutive periods. The I–V, C–V and α-spectrum measurements show that a self-annealing process of the gamma-ray generated defects occurs during the break time between the consecutive periods. Such an annealing can lead to an extended radiation tolerance in which the output current of the system is proportional to the gamma-ray intensity. Despite the density of the generated defects, a constant output current was observed up to the highest investigated dose of 59.9Mrad. Since the irradiations are performed at relatively high gamma-ray intensity (25krad/h), the current-generation rate is much higher than the recombination rate.

Two types of silicon detectors are reported (ion implantation and surface barrier detector) for the detection of neutrons in high gamma-background. The purpose is to find an optimal design of silicon detector combined with the neutron converter, which is efficient enough to neutron flux and with low sensitivity to gamma-rays. The experiments show that for fabrication of effective Si detectors for neutrons (using the (n,alpha) reactions) in the presence of an intensive gamma-field one must optimise the resistivity of the Si substrate and the active area of the detector, at which the detector has a maximum efficiency to the charged particles and is relatively insensitive to the gamma-rays.

The performance of n- and p-type silicon detectors manufactured by a recently developed planar technology for the detection of β-particles, conversion electrons and γ-rays is reported. Two structures are tested: an ion implanted detector with p+-n-n+ structure and a diffusion detector with n+-p-p+ structure.The experiments show that planar detectors can be used for high energy electron counting, for γ-ray detection and for identification of low energy β-sources.The phenomenon of total absorption of high energy conversion electrons in thin silicon detectors is observed and explained. It is demonstrated that the peaks of total absorption can be used for the estimation of the thickness of the depleted region of the detector.

Two types of detectors were developed by conventional planar technology: a p⁺-n-n⁺ ion-implanted detector and a n⁺p-p⁺ diffused detector. There was no observable difference in the quality of the detectors manufactured in this way. Both detectors were investigated for alpha and beta spectroscopy and exhibited good energy resolution. With an additional deposition of a ⁶LiF converter layer the detectors can be used for detection of neutrons

The note demonstrates the potential capabilities for X-ray analysis by avalanche detectors with relatively thick drift regions. The experiments showed that Si AD can be used in X-ray analysis of the K lines of all elements from the first half of the Periodic Table and of the L and M lines of almost all elements. The standard planar technology used for detector manufacturing can reduce their price.

Three types of silicon avalanche detectors for low energy gamma- and X-ray counting were manufactured using planar and planar-epitaxial technology. The detectors were tested at room temperature with 57Co and 241Am. Besides the characteristic lines of the sources additional peaks were observed due to X-rays of elements included in the matrices and holders of the sources.For 6.4 keV the energy resolution is approximately 10%.