Neutron Multiplicity Counting Research Articles

The measurement of passive neutron multiplicity counter gate utilisation factors and comparisons with theory

In a recent paper, a new method of calculating the doublets and triplets gate utilisation factors f 2 and f 3 for passive neutron multiplicity counters has been developed resulting in the MCF computer code. In this paper we arrange the basic multiplicity equations and generate nuclear data parameter combinations to enable f 2, f 3, f 2/ f 3 and f 2 2/ f 3 to be derived from measurements taken using a calibrated 252Cf source and multiplicity shift-register electronics. The two approaches have been compared with reference to Harwell Instruments Ltd. N95 neutron coincidence counting measurement chamber operated both with and without an internal cadmium liner. The Monte Carlo code MCNP™ was used to calculate neutron capture time distributions and these were fed into the MCF code to produce theoretical gate utilisation factors to compare with the experimental data. Pre-delays of 2 and 4 μs and coincidence gate widths between 16 and 240 μs were investigated. The predicted values agreed with those measured within 1.5% and 5% for the f 2 and f 3 results, respectively. These results confirm the power of the new theoretical treatment in accurately predicting gate utilisation factors in an absolute sense and with confidence at the design stage. Relative predictions can be made with greater accuracy.

Quantification of transuranic elements by Time Interval Correlation Spectroscopy of the detected neutrons

The non-destructive quantification of transuranic elements in nuclear waste management or in safeguards verifications is commonly performed by passive neutron assay techniques. To minimise the number of unknown sample-dependent parameters, Neutron Multiplicity Counting (NMC) is applied. We developed a new NMC-technique, called Time Interval Correlation Spectroscopy (TICS), which is based on the measurement of Rossi-alpha time interval distributions. Compared to other NMC-techniques, TICS offers several advantages.

A generalized multiplication correlation model for the description of fast and thermal neutron multiplication in neutron coincidence counting and time interval analysis

In safeguard verification measurements, neutron coincidence and multiplicity counting are commonly applied for the non-destructive assay of large amounts of plutonium. A model to describe the multiplication effects in the sample is needed to obtain unbiased measurement results. Assay of plutonium containers with a neutron coincidence collar lined with a cadmiumsheet generally involves only fast multiplication, since the moderating power of oxygen in plutonium oxides or mixed oxides is too low to generate a thermal neutron population in the sample. In this situation, Böhnel's model for fast, instantaneous multiplication is adequate to describe neutron multiplication and its effects on coincidence counting. When however fresh or spent fuel is assayed under water, thermal multiplication plays an important role. In these measurements conditions Böhnel's model is no longer valid. In this paper a new model, called the generalized multiplication correlation model, is presented. This model allows to describe the combined effect of fast and thermal multiplication in neutron coincidence and multiplicity counting techniques. The generalised multiplication correlation model is based on the identification of different time correlation schemes, which are involved in the detection of correlated neutrons leaking from a multiplying medium. The proposed model allows to describe the singles-, doubles- and triples-rates of fast and thermal neutrons in a medium with fast and thermal multiplication. It is also shown how these count rates can experimentally be obtained from the one- and two-dimensional Rossi-alpha distribution in time interval analysis and how the measurement of these distributions for fast and thermal neutrons allows to obtain the 240 Pu eff -mass.

Neutron detection and applications using a BC454/BGO array

Neutron detection and multiplicity counting has been investigated using a boron-loaded plastic scintillator/bismuth germanate phoswich detector array. The boron-loaded plastic combines neutron moderation (H) and detection ( 10B) at the molecular level, thereby physically coupling increasing detection efficiency and decreasing die-away time with detector volume. Separation of the phoswich response into its plastic scintillator and bismuth germanate components was accomplished on an event-by-event basis using custom integrator and timing circuits, enabling a prompt coincidence requirement between the BC454 and BGO to be used to identify neutron captures. In addition, a custom time-tag module was used to provide a time for each detector event. Time-correlation analysis was subsequently performed on the filtered event stream to obtain shift-register-type singles and doubles count rates.

An EPLD-based time-of-arrival acquisition board for neutron coincidence counting

A PC acquisition board for a new generation of passive assay neutron multiplicity counters is described. The board acquires the time-of-arrival of each digital pulse from a set of neutron counters together with the relevant detector identification number and stores these data in the PC memory. By statistical processing of the data safeguards verifications of plutonium bearing materials or nuclear wastes characterizations can be done. Simultaneous events from different detectors are processed sequentially (derandomized) without any data loss. A FIFO (First In First Out) bank on the board makes it possible to acquire data at a peak rate much higher than the PC bus bandwidth. All the logic circuits (input section with sequencer and channel encoder, 32-bit system clock, FIFO controller and PC bus interface) are implemented with EPLDs (Erasable Programmable Logic Devices), making it possible to fit up to 64 input channels in an AT full size board and up to four boards in the PC bus. As a general rule the processing time increases with the number of input channels; exploiting the internal architecture of the EPLDs several solutions with different trade off between speed and number of inputs were analyzed. The 36 input, 100 ns speed version was built, fully characterized and integrated in a multiplicity counting system. With an assembly driver up to 0.5 Mevents/s can be recorded with a 286 or higher based PC; the total number of events that can be continuously collected at maximum speed is limited only by the size of the PC extended memory. The board is also suited for time correlation analysis in other fields e.g. photon counting.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Development of a portable neutron-multiplicity counter and metrological controls

Development of a portable neutron-multiplicity counter and metrological controls

Assay variance as a figure of merit for neutron multiplicity counters

This paper describes a procedure for estimating assay variance for the neutron multiplicity counters that are being developed to assay special nuclear materials. The procedure estimates the moments of the expected neutron multiplicity distribution and calculates the expected variance without requiring a sample measurement. Results of the procedure are compared with actual measurements made with existing prototype counters. Assay variance is used as a figure of merit to compare the counters and to identify the most important design parameters. In the future, this procedure can be used to optimize the design of new counters.

A Neutron Multiplicity Counter for Plutonium Assay

This paper reports on a new neutron multiplicity counter developed that can provide International Atomic Energy Agency inspectors a means of verifying plutonium samples by a full neutron multiplicity analysis where conventional neutron coincidence counters yield outliers. This prototype detector comprises an array of {sup 3}He and liquid scintillator neutron detectors and is one of a new generation of multiplicity counters under development for nuclear safeguards. A moments analysis solution to the plutonium mass, (a,n) rate, and multiplication is calculated using the measured neutron multiplicity distributions, and an expression for the uncertainty in the multiplication-corrected mass has been derived.

Direct measurements of neutron emission in complete and incomplete 12C-induced fusion reactions.

The multiplicity of neutron emission in $^{12}\mathrm{induced}$ nuclear reactions with $^{197}\mathrm{Au}$, $^{209}\mathrm{Bi}$, $^{232}\mathrm{Th}$, and $^{238}\mathrm{U}$ was measured directly on-line using a Gd-loaded scintillation neutron multiplicity counter with 1 m diameter recording single neutrons with a registration efficiency of about 80%. Using a well-focused carbon beam one could measure up to about 30 neutrons per nuclear interaction directly, when the neutron multiplicity counter was triggered with a prompt fission fragment. For $^{197}\mathrm{Au}$ and $^{209}\mathrm{Bi}$ targets, fission was observed to follow compound-nucleus formation, whereas in the cases of $^{232}\mathrm{Th}$ and $^{238}\mathrm{U}$, fission following incomplete fusion was seen to contribute also. An average multiplicity of \ensuremath{\nu}\ifmmode\bar\else\textasciimacron\fi{}=(4\ifmmode\pm\else\textpm\fi{}1) neutrons was measured for incomplete fusion, while for complete fusion, \ensuremath{\nu}\ifmmode\bar\else\textasciimacron\fi{} varied between 8 and 12, depending on target and bombarding energy. The results complement measurements of Viola et al. on fragment-angular correlations and agree with them.

Total photonuclear absorption cross section for Pb and for heavy nuclei in the Δ-resonance region

The partial sum σ (2)( E γ ) = Σ i⩾2 σ(γ, in…), representing the inclusive cross section for all reaction channels in which at least two neutrons are emitted, has been measured with a quasimonochromatic photon beam obtained by the in-flight annihilation of monoenergetic positrons, and neutron multiplicity counting. These experimental results, taken with photon energies E γ from 145 up to 440 MeV for Pb and with photon energies E γ = 235 MeV and 330 MeV for Al, Cu, Zr, Sn, Ho, Ta and U, are subsequently used to determine the total photonuclear absorption cross section σ(tot: E γ ) and to study the dependence upon the mass number A of the normalized cross section σ(tot: E γ )/ A. These results are then compared with other information on the total photonuclear absorption cross section.

Neutron Multiplicity Distributions and the Role of Transfer and Breakup in Quasielastic Heavy-Ion Reactions at 15 MeV/Nucleon

A new tool for the study of heavy-ion reactions is obtained by adapting a $4\ensuremath{\pi}$-neutron-multiplicity counter to accelerator experiments. The instrument registers almost all neutrons emitted in each single reaction event. For quasielastic reactions of 290-MeV $^{20}\mathrm{Ne}$+$^{197}\mathrm{Au}$ the measured neutron multiplicity distributions allow a quantitative decompsotion of the cross section into two contributions: one with higher $Q$ value from transfer reactions, one with lower $Q$ value due to the decay of the decay of the projectilelike fragments.

Photoneutron cross sections for the silicon isotopes

The photoneutron cross sections for $^{28}\mathrm{Si}$, $^{29}\mathrm{Si}$, and $^{30}\mathrm{Si}$ have been measured up to 33 MeV with monoenergetic photons from the annihilation in flight of fast positrons, using neutron multiplicity counting. Average neutron energies were obtained simultaneously with the cross-section data by the ring-ratio technique. The giant dipole resonances for $^{28}\mathrm{Si}$ and $^{30}\mathrm{Si}$ exhibit appreciable fragmentation; that for $^{29}\mathrm{Si}$ does not. The ($\ensuremath{\gamma}, 2n$) cross section for $^{30}\mathrm{Si}$ is large; that for $^{29}\mathrm{Si}$ is consistent with zero. The ($\ensuremath{\gamma},1n$) cross section for $^{30}\mathrm{Si}$ decreases sharply with energy to values near zero as the ($\ensuremath{\gamma}, 2n$) cross section grows, then increases to appreciable values as the ($\ensuremath{\gamma}, 2n$) cross section diminishes; this extreme behavior, although never seen before, is attributable to the competition between the ($\ensuremath{\gamma}, n$), ($\ensuremath{\gamma}, 2n$), and ($\ensuremath{\gamma}, \mathrm{pn}$) decay channels. Some properties of the isospin components of the giant resonance are inferred. Other features of the data, including the integrated cross sections, are found to be similar in many respects to corresponding results for the oxygen and magnesium isotopes. The $^{28}\mathrm{Si}$ nucleus is found to be a better core for $^{29}\mathrm{Si}$ and $^{30}\mathrm{Si}$ than might have been expected from previous descriptions of its open-shell character.NUCLEAR REACTIONS $^{28}\mathrm{Si}(\ensuremath{\gamma}, n)$, $^{29}\mathrm{Si}(\ensuremath{\gamma}, n)$, $^{30}\mathrm{Si}(\ensuremath{\gamma}, n)$, $^{30}\mathrm{Si}(\ensuremath{\gamma}, 2n)$, ${E}_{\ensuremath{\gamma}}=13.4\ensuremath{-}33.1$ MeV; measured $4\ensuremath{\pi}$ neutron yields for monoenergetic photons, average photoneutron energies, integrated cross sections; isospin splitting of giant dipole resonance; comparisons of results with oxygen and magnesium isotopes; $^{28}\mathrm{Si}$ as a core nucleus.

Prompt Neutron Multiplicity Distribution in the Spontaneous Fission of Curium-242

The prompt neutron multiplicity distribution for the spontaneous fission of 242Cm has been measured. The measurement has been carried out with a neutron multiplicity counter using hydrogen moderati...

Photoneutron cross sections forMn55andCo59

Photoneutron cross sections, including sigma((..gamma..,n) + (..gamma..,pn)), sigma (..gamma..,2n), and sigma (..gamma..,3n), were measured for /sup 55/Mn and /sup 59/Co from threshold to 36.5 MeV, with a photon energy resolution which varied from 80 keV at the lowest to 170 keV at the highest energies measured. The source of radiation was the monoenergetic photon beam obtained from the annihilation in flight of fast positrons. The partial photoneutron cross sections were determined by neutron multiplicity counting, and the average neutron energies for (..gamma..,1n) and (..gamma..,2n) events were determined simultaneously with the cross-section data by the ring-ratio technique. The cross sections exhibit considerable but weak structure. Other nuclear information extracted from the data includes parameters of the giant dipole resonance, integrated cross sections and their moments, and nuclear symmetry energies. A comparison is made with previous experimental data for these nuclei as well as with theoretical predictions based upon hydrodynamic, vibrational, and dynamic collective models. None of these models fits the data for these odd-even nuclei satisfactorily; more theoretical work is needed for this nuclear mass region.

Giant resonance in transitional nuclei: Photoneutron cross sections for osmium isotopes

Photoneutron cross sections, including $\ensuremath{\sigma}[(\ensuremath{\gamma},n)+(\ensuremath{\gamma},pn)]$, $\ensuremath{\sigma}[(\ensuremath{\gamma},2n)+(\ensuremath{\gamma},p2n)]$, and $\ensuremath{\sigma}(\ensuremath{\gamma},3n)$, were measured for $^{188}\mathrm{Os}$, $^{189}\mathrm{Os}$, $^{190}\mathrm{Os}$, and $^{192}\mathrm{Os}$ from 7 to 30 MeV and for $^{186}\mathrm{Os}$ from 11 to 20 MeV, with a photon energy resolution of about 300 keV. The source of radiation was the monoenergetic photon beam obtained from the annihilation in flight of fast positrons. The partial photoneutron cross sections were determined by neutron multiplicity counting, and the average neutron energies for each multiplicity were determined simultaneously with the cross-section data by the ring-ratio technique. Nuclear information extracted from the data includes parameters of the giant dipole and giant quadrupole resonances, integrated cross sections and their moments, nuclear symmetry energies, and nuclear deformation parameters and intrinsic quadrupole moments. No fewer than eight kinds of evidence point to a sudden change of behavior between $^{189}\mathrm{Os}$ and $^{190}\mathrm{Os}$, which could be interpreted as a phase transition from a statically deformed prolate nucleus to a $\ensuremath{\gamma}$-unstable one, in general (but not detailed) agreement with the prediction of a dynamic-collective-model calculation.

Photonuclear cross sections forO18

All the major photonuclear cross sections for $^{18}\mathrm{O}$, including $\ensuremath{\sigma}(\ensuremath{\gamma},p)$, $\ensuremath{\sigma}[(\ensuremath{\gamma},n)+(\ensuremath{\gamma},np)]$, and $\ensuremath{\sigma}(\ensuremath{\gamma},2n)$, were measured as a function of photon energy from threshold to 42 MeV. The photon energy resolution was between 200 and 300 keV. The source of radiation was the monoenergetic photon beam obtained from the annihilation in flight of fast positrons. The partial photoneutron cross sections were determined by neutron multiplicity counting, and the average neutron energies for both single- and double-photoneutron events were determined simultaneously with the cross-section data by the ring-ratio technique. The photoproton cross section was determined by counting the delayed neutrons from the $\ensuremath{\beta}$-decay of the residual $^{17}\mathrm{N}$ nuclei. Integrated cross sections were extracted from the data and compared with sum-rule predictions. Resonances in the pygmy-resonance region exhibit substantial decay branching to negative-parity states in $^{17}\mathrm{O}$. Significant differences in the branching of resonances in the giant-resonance region to the various partial cross sections, together with the photoneutron-energy data, allow isospin assignments to be made for all the resonances, with the consequent delineation of the isospin strength distribution for $^{18}\mathrm{O}$. Evidence was found for two weak but broad resonances above the giant-resonance region. Finally, comparison with the corresponding cross sections for $^{16}\mathrm{O}$ shows considerable differences between the two nuclei.

Superheavy element search in bismuthinite, Bi2S3

Abstract Samples of bismuthinite were examined by using a neutron multiplicity counter capable of detecting spontaneous fission events in any element. No events characteristic of spontaneous fission decay of superheavy elements were found. If a superheavy element exists in the cosmic abundance ratio estimated from the carbonaceous chrondrite measurements of Flerov et al. then it has not (even weakly) followed bismuth through its geochemical evolution into our bismuthinite samples.

Search for Superheavy-Element Decay in Samples of Madagascar Monazite

Two samples of Madagascar monazite from the same geological formation as the biotite studied by Gentry et al. were examined by using a neutron multiplicity counter capable of detecting binary or ternary spontaneous fission decay in any element. No events characteristic of spontaneous fission decay of superheavy elements were found. Derived limits indicate that if superheavy elements were present, then their spontaneous fission half-lives must be extremely long or their concentrations extremely small. (AIP)

How to detect the heaviest man-made isotopes

The average number of neutrons emitted per spontaneous fission is predicted to increase substantially for very heavy nuclei. This suggests the use of neutron-multiplicity counters for the early detection of heavy isotopes produced by prompt neutron capture. We discuss the possibilities and limitations of such a technique. (AIP)

A Search for Naturally Occurring Superheavy Elements

IN a continuing effort to measure neutron emission in natural samples for evidence of superheavy elements, several ores, minerals, concentrates, and special samples have been examined in our neutron multiplicity counter1. This counter, containing twenty3He detectors in a paraffin matrix, enables the evaluation of the emitted neutron multiplicity spectrum of large samples (volume 20 1) with little or no chemical processing. Such measurements provide an effective tool in the search for superheavy elements, since either their decay or the decay of daughter nuclides is expected to proceed by spontaneous fission. Furthermore, the parameter v (average number of prompt neutrons emitted in the fission act) has been estimated2,3 to be very high for superheavy nuclides; for example, v (298114 or296112)≃10 in contrast to v ≲ 4 for all known spontaneously fissioning nuclides. In addition to providing a unique indicator, this predicted high value of v enhances the sensitivity for detecting superheavy elements. A neutron multiplicity counter with a single neutron counting efficiency ɛ° = 0.30 would exhibit an efficiency of ɛ(>3)=0.62 for observing multiplicities of three or greater and ɛ(>4)=0.35 for observing multiplicities of four or greater in a fission event in which ten neutrons were emitted. On the basis of a capability for detecting 1.0 fission event per day for a nuclide with a half life of 108 yr (approximately the shortest half life for a nuclide to have survived geological times), the maximum sensitivity of the counter for such nuclides can be estimated to be ˜5 × 1010 atoms or ˜10−15 g g−1 in a 25 kg sample.