High-level Neutron Coincidence Counter Research Articles

Design and performance of A 3He-free coincidence counter based on parallel plate boron-lined proportional technology

Thermal neutron counters utilized and developed for deployment as non-destructive assay (NDA) instruments in the field of nuclear safeguards traditionally rely on 3He-based proportional counting systems. 3He-based proportional counters have provided core NDA detection capabilities for several decades and have proven to be extremely reliable with range of features highly desirable for nuclear facility deployment. Facing the current depletion of 3He gas supply and the continuing uncertainty of options for future resupply, a search for detection technologies that could provide feasible short-term alternative to 3He gas was initiated worldwide. As part of this effort, Los Alamos National Laboratory (LANL) designed and built a 3He-free full scale thermal neutron coincidence counter based on boron-lined proportional technology. The boron-lined technology was selected in a comprehensive inter-comparison exercise based on its favorable performance against safeguards specific parameters. This paper provides an overview of the design and initial performance evaluation of the prototype High Level Neutron counter—Boron (HLNB). The initial results suggest that current HLNB design is capable to provide ~80% performance of a selected reference 3He-based coincidence counter (High Level Neutron Coincidence Counter, HLNCC). Similar samples are expected to be measurable in both systems, however, slightly longer measurement times may be anticipated for large samples in HLNB. The initial evaluation helped to identify potential for further performance improvements via additional tailoring of boron-layer thickness.

6Li foil scintillation sandwich thermal neutron detector

Increasing needs for neutron detection and limited supply of 3 He have led to the need for replacement neutron detection technology. This paper presents the design and initial results for a neutron detector ( 6 Li foil scintillator sandwich) that uses lithium metal foil to detect thermal neutrons . The reaction products, primarily triton, deposit most of their energy in thin scintillator films and create light pulses. Gamma rays can deposit only a small amount of energy in the thin films and so produce only very small light pulses. Lithium is preferable to boron in this application because triton escapes from lithium more easily than does the alpha particle from boron, allowing the use of thicker films and hence greater efficiency. In addition, triton has a higher light output in the scintillator than the boron alpha particle. Lithium metal is preferable to a lithium compound, such as lithium fluoride, because the number of tritons that escape from the metal is greater for the same amount of lithium. Monte Carlo simulations show that good efficiency values can be achieved with reasonably sized detectors, values that are greater than that of comparable 3 He systems for portal monitors. In addition, simulations of a neutron coincidence counting concept show high counting efficiency and short die-away time (16 μs), which imply better performance than that of the high-level neutron coincidence counter (HLNCC)-II. Initial experimental measurements on a prototype detector using alpha particles (having similar light output to the expected tritons) show good light collection and transport properties.

A novel electronic chain for high level neutron coincidence counters

This paper describes a counting chain (charge preamplifier, shaping amplifier and threshold discriminator) devoted to /sup 3/He neutron detectors. The preamplifier adopts an innovative circuit with the gate of the input JFET floating and a DC feedback loop that stabilizes the output voltage acting on the input cascade second transistor. Static and dynamic analysis, including the effects of the detector bias network, is developed and reported. The shaping amplifier is a fifth-order quasi-gaussian OPAMP active filter that minimizes the number of components. Shaping time constants ranging from 1 /spl mu/s down to 100 ns are explored in order to maximize the count rate capability, maintaining nevertheless neutron discrimination. Experimental signals and spectra show that 100 ns shaping time is a lower limit for the usual /sup 3/He proportional detector. Since this counting chain is characterized by single 5V supply operation, high counting rate, small size and low cost, it is well suited for high efficiency neutron well detectors where a large number (10-100) of counting tubes are used (High Level Neutron Coincidence Counter HLNCC).

The measurement and calculation of the correlated neutron production rate from [formula omitted] sources

The production of time-correlated neutrons from six 241 AmO 2 Be(α, n) neutron sources has been measured using a high level neutron coincidence counter (HLNCC-II). Five of the sources were contained in standard Amersham X.2 capsules while the sixth was in the larger X.3 format. The results quantitatively support the hypothesis that the predominant source of time-correlated neutrons arises from the Be(n, 2n) process initiated by the primary 241AmO 2/Be(α, n) neutrons. On this basis, analysis of the experimental data yields an (n, 2n) (α, n) ratio of ≈ 1.8% for the X.2 sources and ≈ 2.1% for the X.3 source. This work suggests that passive neutron coincidence counters could be used to good effect in the measurement and testing of integral (n, 2n) cross section data. Tentative conclusions are that the EFF-2 evaluation of the Be(n, 2n) double differential cross section is to be preferred over the ENDF/B-VI evaluation and that the integrated-over-angle Be(n, 2n) cross sections appearing in these evaluations may be approximately 20% low.

The measurement and calculation of the correlated neutron production rate from 241AmO 2/Be (α, n) sources

The production of time-correlated neutrons from six 241AmO2Be(α, n) neutron sources has been measured using a high level neutron coincidence counter (HLNCC-II). Five of the sources were contained in standard Amersham X.2 capsules while the sixth was in the larger X.3 format. The results quantitatively support the hypothesis that the predominant source of time-correlated neutrons arises from the Be(n, 2n) process initiated by the primary 241AmO2/Be(α, n) neutrons. On this basis, analysis of the experimental data yields an (n, 2n)(α, n) ratio of ≈ 1.8% for the X.2 sources and ≈ 2.1% for the X.3 source. This work suggests that passive neutron coincidence counters could be used to good effect in the measurement and testing of integral (n, 2n) cross section data. Tentative conclusions are that the EFF-2 evaluation of the Be(n, 2n) double differential cross section is to be preferred over the ENDF/B-VI evaluation and that the integrated-over-angle Be(n, 2n) cross sections appearing in these evaluations may be approximately 20% low.

Measurement of Neutron Burst Production in Thermal Cycle of D2Absorbed Titanium Chips

This paper reports on a high-level neutron coincidence counter equipped with 18 {sup 3}He tubes and a JSR-11 shift register unit with a detection limit of 0.20 n/s for a 2-h run which is used to study the neutron signals in D{sub 2} gas experiments. Different material pretreatments are selected to review the changes in frequency and size of the neutron burst production. Experimental sequence is deliberately designed to distinguish the neutron burst from fake signals, e.g., electronic noise pickup, cosmic rays, and other sources of environmental background. Ten batches of dry fusion samples are tested, among them, seven batches with neutron burst signals that occur roughly from {minus}100{degrees}C to near room temperature. In the first four runs of a typical sample batch, seven neutron bursts are observed with neutron numbers from 15 to 482, which are 3 and 75 times, respectively, higher than the uncertainty of the background. The samples seem to be inactive after four or five temperature cycles, and the inactive samples could be reactivated by degassing and recharging of deuterium. The same anomalous phenomena were observed in the Mentou Valley Underground Laboratory situated 580 m below ground.

Experimental study of neutron multiplication in BeO by the shift-register coincidence technique

The multiplication of 14 MeV neutrons by Be, which is used as a neutron multiplier in the design of fusion reactor blankets, is of topical interest as T-breeding depends on this. Earlier investigations have shown that the experimental multiplication of 14 MeV neutrons in Be and BeO is 20–25% lower than the calculated values. Recently, a different method has been proposed to measure the leakage multiplication based on a statistical correlation technique. This note explores the applicability of such a technique to determine experimentally the leakage multiplication of Am-Be source neutrons of average energy 4.5 MeV in a BeO assembly. The experimental method uses a high-level neutron coincidence counter (HLNCC) equipped with a shift-register coincidence logic unit. The deduced value of neutron leakage multiplication for 5 and 10 cm thick BeO in near rectangular geometry is 1.12 ± 0.03 and 1.15 ± 0.03, respectively.

A High-Performance Neutron Time Correlation Counter

A new, high-performance neutron time correlation, or coincidence, counter has been developed for the measurement of nuclear materials. This detector is an improvement over the high-level neutron coincidence counter that was used previously in that it has better counting rate capability, electronic stability, noise resistance, uniform counting zone, and portability. The system uses /sup 3/He gas tubes coupled to the shift register coincidence electronics through a new hybrid charge-sensitive preamplifier/discriminator. Counting rates of 1.3 MHz have been obtained and a counting stability of 0.002% has been measured. The system has been applied to the assay of plutonium content in metal, PuO/sub 2/, and PuF/sub 4/, as well as in uraniumplutonium mixtures.