E. A. Meleshko

A combined spectrometric detector of fast neutrons

A combined spectrometric detector consisting of two subdetectors for fast and thermal neutrons has been developed. The fast neutron detector is a total absorption detector with a polystyrene-based scintillator generating a signal proportional to the absorbed energy of a fast neutron. The thermal neutron detector based on a 6LiF/ZnS(Ag) scintillator in the form of a thin screen detects neutrons that have lost almost all their energy in the first detector and have been thermalized. The electronics of the combined detector operates in coincidence: if a neutron has been detector by the thermal neutron detector, the signal from the fast neutron detector participates in the formation of the amplitude spectrum within a predetermined time interval (fast neutron moderation time). The methods for reconstructing the neutron spectrum by the spectrum recorded using the combined detector are considered. The measured neutron spectra from various sources are presented.

An experimental setup for studying spectra of β particles emitted by mixtures of 235U and 239Pu thermal neutron fission products

An experimental setup on the thermal neutron beam of the IR-8 reactor at the National Research Centre Kurchatov Institute is described. This setup has been designed to precisely measure the ratio of spectra of β particles emitted by mixtures of 235U and 239Pu fission products. The experiment is based on simultaneous measurements of β spectra from fissile isotopes 235U and 239Pu and the background spectrum in the same neutron beam. Measurements of the β spectra are taken by a high-selectivity β spectrometer in the time interval when the neutron beam is intercepted by a mechanical chopper. The evolution of the measurement technique is considered.

EFFECT/BACKGROUND CORRELATIONS IN NANOSECOND NEUTRON ANALYSIS

Nanosecond Neutron Analysis (NNA) technology is used for detection, identification, and localization of explosives. The main source of background in NNA are gamma-rays induced by fast tagged neutrons in non-explosives materials, and gamma-rays produced by non-tagged neutrons slowed down or emitted at the large angles to the trajectories of tagged neutrons. The results of experimentations and numerical simulation of NNA carried out in Russian Research Center “Kurchatov Institute” are reported. This work has been implemented for the geometry of experimental setup for explosives detection. The energy and time dependencies of the detected gamma-rays were obtained due to the selection of events by NNA technology. The comparison of theoretical and experimental data is considered, and the ways of improvement of their convergence are discussed. The basic components of background induced by neutron interaction with various objects (neutron generator units, shielding, gamma-detector) as well as sources of time fluctuations affecting the total time resolution are examined. The experimental and theoretical results show that the NNA technology provides effective (by 2-4 orders of magnitude) suppression of background by spatial and time discrimination of events.

A monitoring and measuring module for tagged neutron experiments

A tagged neutron technology for remote neutron monitoring is used to reduce the background counting rate in recording of useful events. The key elements of the technology are a neutron generator with a built-in α-particle detector, nanosecond electronic system, and fast detectors of secondary radiation produced by emitted neutrons. A module for monitoring and measuring the pulse-height and time parameters of the secondary radiation detector with a time reference to the α-particle detector is described. A prototype of the experimental setup comprising the main components of a standard system based on the tagged neutron technology has been designed to test the module. Numerical calculations have been performed to simulate the γ-ray and neutron transport and the event recording procedure as applied to the geometry of the experimental setup. The time errors and the main sources of background events in measurements using the tagged neutron technology have been investigated. Background counts are shown to affect only slightly the useful data acquired in this geometry.

A high-selectivity β spectrometer

Measurements of β spectra from fission fragments are complicated by high background of γ rays and neutrons. A scintillation β spectrometer capable of precisely reconstructing the β particle spectra in the energy range of >1 MeV has been developed to reduce the effect of this background without deteriorating the high detection efficiency for β particles. The functional diagram of the spectrometer is described, and the results of spectrometer testing using radioactive electron sources are presented.

A study of the possibility of improving the time resolution of the PHOS spectrometer

Experimental investigations have shown that a timing device based on a constant-fraction discriminator is capable of providing a time resolution of approximately 0.5 ns and a timing error of approximately ±0.25 ns in the energy range of 0.8−1.8 GeV. In the region of “low” energies (<0.6 GeV), the time resolution is decreased by the influence of detector noise, whose level is ~3–5 mV for most scintillation detectors under investigation.

A multichannel spectrometer for detecting γ rays in a wide range of time intervals during pulse neutron irradiation

A versatile spectrometer has been developed for measuring the nuclear radiation spectrum in discrete time intervals. The design and operating principle of the spectrometer are described. Since the main logical part is based on a field programmable gate array, time intervals may be tuned in a wide range without changing the spectrometer hardware. The test experiments have demonstrated the spectrometer applicability for measuring the amplitude and timing parameters of the γ response when an analyzed sample is irradiated with a pulsed neutron flux. This device can be used to solve many problems of nondestructive testing, e.g., spectrometric neutron logging, detection of explosives, identification and characterization of fissile materials.

High-speed electronics of the T0 detector for the ALICE experiment (CERN)

The design and special features of the main units of high-speed electronics for the trigger subsystem of the T0 detector of the ALICE experiment are considered. Its characteristic time resolution is 50 ps. The dead time does not exceed 25 ns.

Characteristics of the detecting equipment for the nanosecond tagged-neutron technology

Detecting equipment for the nanosecond tagged-neutron technology has been developed, and its characteristics have been studied. The principles of arrangement and operation of the readout electronics based on the selection of useful events according to specified criteria and data accumulation by a buffer-memory unit with subsequent transfer of data arrays to a remote computer for processing and visualization. The main selection criterion is the presence of signals from α- and γ detectors within the time gate and amplitude ranges in the absence of overlapped events. A prototype of a setup for testing the developed equipment was assembled and experimental studies of its characteristics were performed. The time resolution attained in the recording of α-γ coincidences is 1.0 ± 0.1 ns at an amplitude resolution of the γ detector of 3.6–3.8%.

An experimental setup for measuring neutron yield from nuclear materials under exposure to bremsstrahlung (a photonuclear method)

An experimental setup developed on the basis of the У-28 electron accelerator for investigating the applicability of a photonuclear method to detection of nuclear materials is described. The photonuclear method is based on quantitative analysis of delayed and prompt neutron fluxes from a sample after its irradiation with bremsstrahlung. Using results of numerical simulation of electron-photon and γ-ray-neutron transport, the key elements of the setup—the converter, the beam filter, the bremsstrahlung collimator, and the multilayer neutron detector—are optimized. Electronic modules have been developed for the data acquisition system and for switching a high voltage applied to the neutron counter for the bremsstrahlung emission time. The yields of prompt and delayed neutrons from nuclear material simulators (natural uranium and beryllium) shielded with various materials have been measured. In our experiments, it is shown that the photonuclear method allows reliable detection of highly shielded nuclear materials with masses of tens or hundreds of grams at a distance of >1 m.