G. V. Yakovlev

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 combined γ-neutron detector for fissile material monitoring using the pulse photonuclear technology

Prompt and delayed neutrons and secondary γ rays are the informational radiation in the photonuclear technology for recording fissile materials. Due to the high cost of detectors and difficulties in their placing around a container, it is desirable that the same detectors be used to record all kinds of informational radiation. The design of a multilayer γ-neutron detector is proposed, and the effect of the layer thickness on the recording efficiency is estimated. It is shown that background from prompt neutrons in recording of γrays can be reduced by proper selection of the time interval for γ-ray measurements.

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.

Optimization of Hardware for Tagged Neutron Technology

The possibility of background suppression by spatial and time discrimination of events stipulates the potentialities of the Nanosecond Tagged Neutron Technology (NTNT) for various application, e.g., for remote detection of wide range of explosives and flammable agents. For practical realization of NTNT the time resolution of pulses from gamma-detectors with respect to the associated pulses from alpha-detector should be close to 1 ns. The total intensity of signals can exceed 1 · 106 1/s from all gamma-detectors and 1 · 107 1/s from the alpha-detector. The processing of such stream of data without losses and distortion of information is one of challenging problems of NTNT. It is suggested to implement the preliminary “on-line” data processing by hardware. The architecture of data acquisition and control system and examples of realization are considered. Another important problem of NTNT is a choice of proper gamma-detectors to provide sufficient efficiency, amplitude and time resolution of measurements. With this aim, gammadetectors based on NaI, BaF2, LYSO, and BGO crystals were assembled; the crystals were coupled with fast photomultipliers of similar series for more unambiguous interpretation of measurements. The results of experiments and features of detectors as applied to the NTNT systems are discussed.

decrease in the dead time of proportional counters of neutrons after exposure to pulsed fluxes of ionizing radiation

During irradiation of a proportional counter with a high-power bremsstrahlung flux, a high density of charged particles arises in the interelectrode gap owing to primary and secondary ionization. Charge separation in the interelectrode gap induces an internal electric field that leads to a decrease in the gas-amplification factor to a level below the neutron-recording threshold. The delay time of neutron recording (dead time) by the counter after a bremsstrahlung burst may exceed 100 µs. A device developed for switching a high voltage applied to the counter for the bremsstrahlung emission time is described, as are experiments on detection of prompt fission neutrons emitted after irradiation of the fissile substance with a pulsed bremsstrahlung flux. The results show that, when this device is used, the neutron-recording delay time decreases severalfold.