Sergey V. Naydenov

High efficiency fast neutron detectors based on inorganic scintillators

The possibility was studied of using highly efficient heavy inorganic oxide solid-state scintillation detectors for the detection of mixed gamma-neutron radiation. In the detection of gamma-neutron radiation, the gamma detection efficiency for such detectors reaches 70-80%, with neutron detection efficiency not less than 40%. Detection efficiencies were measured for the heavy oxide scintillator crystals CWO (CdWO₄), ZWO (ZnWO₄), BGO (Bi₄Ge₃O₁₂), GSO (Gd₂SiO₅), YSO:Ce (Y₂SiO₅:Ce) and LuAG:Ce (Lu₃Al₅O₁₂:Ce). For comparison purposes, the scintillators NaI:Tl, LiI:Eu, ZnSe(Te), CsI(Tl) were also considered. The dependencies of detection efficiency on Zeff, thickness and area of the detector were obtained. To eliminate the effects of accompanying gamma-radiation in the detection of neutrons, suppression of gamma-background was studied using lead protection of 2-40 mm thickness as well as a protection screen consisting of a cylindrical ring of BGO with an internal diameter of 40 mm and an external diameter 60 mm surrounding a cylindrical CWO scintillator detector of dimension 40×40 mm. Suppression of the gamma background by factors of 2-10 was achieved with passive protection. With the BGO screen in an active mode, gamma background suppression reached 10³. Also, an original “windows” method [1] of data analysis was proposed for mathematical processing of gamma background, with appropriate software, allowing us to reach neutron/gamma ratios from 10⁵ to 10⁷.

Detection of gamma-neutron radiation by novel solid-state scintillation detectors

In this work, we present results of our experimental and theoretical studies on the detection efficiency of fast neutrons from 239Pu-Be and 252Cf sources by the heavy oxide scintillators BGO, GSO, CWO and ZWO, as well as ZnSe(Te, O). We have investigated the efficiency of registration of the combined γ and neutron radiation from Pu-Be and Cf-252 neutron sources and have explored the possibility of suppressing the γ-radiation by several methods - passive protection by lead shielding, active defense by detector design and the use of special software for gamma discrimination. The last method is the most promising, but it requires more detailed development. In addition, we provide examples of practical applications of these results, as well as the results of our search for new ways to develop large-sized detectors of lower cost by creating a new multilayer structure from composite (flexible) scintillator panels alternated with transparent plastic scintillator layers that serve as light guides.

Theoretical analysis of physical limits of energy resolution for detectors of scintillator-photodiode type and ways to improve their spectrometric characteristics

The development prospects of a scintillator-photodiode type detector with an improved energy resolution attaining few per cent, about 1.5 to 2.5%, are considered. The main resolution components have been analyzed theoretically, their theoretical and physical limits have been established. Empirical data on the properties of novel scintillators have been considered confirming the possibility of the energy resolution improvement. Ways have been proposed to optimize the detector statistical fluctuations and the scintillator intrinsic resolution. A specific importance of the intrinsic resolution is shown as a limiting threshold factor at the ionizing radiation energy values from 662 keV to 10 MeV and over.

Radiographic spectroscopy of atomic composition of materials: a multi-energy approach

Theoretical model of multi-energy radiography (MER) for reconstruction of the atomic structure is proposed. It is shown that, using multi-channel absorption and detection of radiation, effective atomic number and quantitative chemical composition of the materials can be readily reconstructed. This approach opens prospects for improvement of efficiency of X-ray techniques in non-destructive testing, nuclear and safety monitoring, security customs control, and others.

Looking for new possibilities to improve properties of two-energy detectors of scintillator-photodiode type for inspection systems of international security: Multi-energy radiography against terrorism. Theory and experiments

Multi-energy radiography is a new direction in nondestructive testing. Its specific feature is separate detection of penetrating radiation in several energy channels. Multi-energy radiography allows quantitative determination of atomic composition of the objects. This is its principal advantage over conventional radiography. In particular, dual-energy radiography allows determination of the effective atomic number of a material with accuracy up to 80-90%. Development of three-energy radiography and radiography of higher multiplicity makes it possible to reconstruct the exact chemical composition also. This means, for example, detection of explosives and other illegal objects in luggage with reliability close to 100%. Thus, these developments can find application in anti-terrorist activities, in industrial testing and nuclear medicine

Multi-energy method of digital radiography for imaging of biological objects

This work has been dedicated to the search for a new possibility to use multi-energy digital radiography (MER) for medical applications. Our work has included both theoretical and experimental investigations of 2-energy (2E) and 3- energy (3Е) radiography for imaging the structure of biological objects. Using special simulation methods and digital analysis based on the X-ray interaction energy dependence for each element of importance to medical applications in the X-ray range of energy up to 150 keV, we have implemented a quasi-linear approximation for the energy dependence of the X-ray linear mass absorption coefficient μm (E) that permits us to determine the intrinsic structure of the biological objects. Our measurements utilize multiple X-ray tube voltages (50, 100, and 150 kV) with Al and Cu filters of different thicknesses to achieve 3-energy X-ray examination of objects. By doing so, we are able to achieve significantly improved imaging quality of the structure of the subject biological objects. To reconstruct and visualize the final images, we use both two-dimensional (2D) and three-dimensional (3D) palettes of identification. The result is a 2E and/or 3E representation of the object with color coding of each pixel according to the data outputs. Following the experimental measurements and post-processing, we produce a 3Е image of the biological object – in the case of our trials, fragments or parts of chicken and turkey.

A dual-energy medical instrument for measurement of x-ray source voltage and dose rate

An original dual-energy detector and medical instrument have been developed to measure the output voltages and dose rates of X-ray sources. Theoretical and experimental studies were carried out to characterize the parameters of a new scintillator-photodiode sandwich-detector based on specially-prepared zinc selenide crystals in which the low-energy detector (LED) works both as the detector of the low-energy radiation and as an absorption filter allowing the highenergy fraction of the radiation to pass through to the high-energy detector (HED). The use of the LED as a low-energy filter in combination with a separate HED opens broad possibilities for such sandwich structures. In particular, it becomes possible to analyze and process the sum, difference and ratio of signals coming from these detectors, ensuring a broad (up to 106) measurement range of X-ray intensity from the source and a leveling of the energy dependence. We have chosen an optimum design of the detector and the geometry of the component LED and HED parts that allow energy-dependence leveling to within specified limits. The deviation in energy dependence of the detector does not exceed about 5% in the energy range from 30 to 120 keV. The developed detector and instrument allow contactless measurement of the anode voltage of an X-ray emitter from 40 to 140 kV with an error no greater than 3%. The dose rate measurement range is from 1 to 200 R/min. An original medical instrument has passed clinical testing and was recommended for use in medical institutions for X-ray diagnostics.

A new multi-layer scintillation detector for detection of neutron-gamma radiation

We present the results of our experimental and theoretical studies of fast neutron detection efficiencies of a newly-developed multi-layer scintillation detector comprised of a series of alternating layers of a composite inorganic scintillator material and an optically transparent plastic scintillator material (brand ZEBRA). We investigated the response of this type of detector to neutrons from 239Pu-Be and 252Cf sources. We evaluated the sensitivity of such ZEBRA detectors as a function of different thicknesses of the alternating layers of composite scintillator and light guides to optimize the performance of these structures. We compared the detection efficiency of these detectors with detectors based on the single crystals ZnSe, ZWO and GSO. We also analyzed the detection sensitivity of these crystal scintillation detectors as a function of thickness and cross-sectional area of the detection material. Finally, we created software that permits the achievement of a high level (up to 104) suppression of gamma-radiation from both accompanying and other external gamma-radiation.

Multi-energy ZnSe-based radiography against terrorism: theory and experiments

Multi-energy radiography is a new direction in non-destructive testing. Its specific feature is separate detection of penetrating radiation in several energy channels. Multi-energy radiography allows quantitative determination of the atomic composition of objects. This is its principal advantage over conventional radiography. In particular, dual-energy radiography allows determination of the effective atomic number of a material with an accuracy of up to 80-90%. Development of three-energy radiography and radiography of higher multiplicity makes it possible to further improve the reconstruction of an objects chemical composition. This presents the possibility, for example, of detection of explosives and other illegal objects in luggage with a reliability approaching 95-98%. These developments can find application not only in anti-terrorist activities, but also in industrial testing and nuclear medicine.

Spectrometric universality and reduction of nonstatistical noises in detectors with regular light collection

Intrinsic resolution of the scintillator is one of the most important constituents of the full energy resolution of a detector. Intrinsic resolution contains components preserved at any ionizing radiation energy. A major role is played by the resolution of light collection. This is a component determined by geometric-optical non-uniformities of scintillation energy propagation and collection. In this work, theoretical studies of general light collection features have been carried out. A universal law has been predicted for light collection dispersion in detectors of regular beam dynamics. Such systems include scintillation blocks with mirror reflecting surface and regular geometry in the shape of cylinder, parallelepiped or sphere. An important regular collection feature is weak dependence of its dispersion on the scintillator material or shape. This allows to relate spectrometric efficiency and detection efficiency for any detector of the said type. The theoretically obtained law is confirmed by the available experimental data. The developed theory allows finding new ways to eliminate internal noises affecting the radiation measurements data. Such optimization is a necessary condition for creation of new detectors with improved characteristics.