C. F. Smith

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.

The neutron detectors based on oxide scintillators for control of fissionable radioactive substances

A large-area X-ray CMOS image sensor (LXCIS) is widely used in mammography, non-destructive inspection, and animal CT. For LXCIS, in spite of weakness such as low spatial and energy resolution, a Indirect method using scintillator like CsI(Tl) or Gd2O2S is still well-used because of low cost and easy manufacture. A photo-diode for X-ray imaging has large area about 50 ~ 200 um as compared with vision image sensors. That is because X-ray has feature of straight and very small light emission of a scintillator. Moreover, notwithstanding several structure like columnar, the scintillator still emit a diffusible light. This diffusible light from scintillator can make spatial crosstalk in X-ray photodiode array because of a large incidence angle. Moreover, comparing with vision image sensors, X-ray sensor doesn’t have micro lens for gathering the photons to photo-diode. In this study, we simulated the affection of spatial crosstalk in X-ray sensor by comparing optical sensor. Additionally, the chip, which was fabricated in 0.18 um 1P5M process by Hynix in Korea, was tested to know the effect of spatial crosstalk by changing design parameters. From these works, we found out that spatial crosstalk is affected by pixel pitch, incident angle of photons, and micro lens on each pixels.

Fast neutron detectors based on solid-state single crystalline and multilayer composite scintillators

We report on the development and analysis of two types of fast neutron and neutron-gamma detection media: (1) large volume inorganic oxide crystalline scintillators; and (2) large-area multilayer composite structures consisting of dispersed granules of small-crystalline scintillators contained in a transparent plastic matrix. The first type is based on solid-state single crystal CWO, BGO and ZWO scintillators of large size (diameter 30-50 mm, length up to 100 mm). For these large crystals, we have experimentally confirmed previous results demonstrating very high efficiency for fast neutron detection (up to 50%), which were obtained earlier on small-sized samples of smaller (about 10-12 cm3) volume. Such larger-size scintillators are needed in order to increase detector sensitivity for the detection of very small quantities of neutron-emitting radioactive materials. In our subsequent search for new efficient and cost effective solid-state neutron detectors, we have developed and studied the second type of detection media: large area multi-layer composite detectors (named ZEBRA). We measured the sensitivity of a 40×100×100 mm multilayer ZEBRA detector (with a total thickness 40 mm, and a scintillator cross sectional layer of area 100 cm2) comprised of dispersed GSO(Ce) and found it to be comparable to the normal sensitivity of a 3He-detector with an area of 4000 cm2. The efficiency of ZEBRA detectors is not lower than 50%, i.e., approximately at the same level of detection achieved by single crystal scintillators. The cost of such composite detectors is projected to be substantially lower than that of neutron/gamma detectors based on conventional single crystals.

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.

New neutron detectors based on inorganic scintillators using inelastic scattering

Detection efficiency of fast and thermal neutrons was studied for detectors based on scintillators LiI(Eu), CWO, BGO, GSO, ZnWO4, CsI(Tl), ZnSe(Te), used in systems for detection of fissionable substances. Results are presented of our studies aimed at practical application of an efficient method for detection of fast and thermal neutrons, which uses the process of inelastic scattering on atom nuclei present in inorganic scintillators. The most evident practical application field for this method is inspection systems for prevention of illegal transportation of radioactive substances.