Neutron moderation in a bulk sample and its effects on PGNAA setup geometry

In a prompt gamma ray neutron activation analysis (PGNAA) setup, the neutron moderation in the bulk sample also plays a key role. This can even dominate the thermalization effects of the external moderator in some cases. In order to study the neutron moderation effect in the bulk sample, moderators with two different sizes of the sample were tested at the King Fahd University of Petroleum & Minerals (KFUPM) PGNAA facility. In these tests, the thermal neutron relative intensity and prompt gamma ray yield from the two moderators were measured using nuclear track detectors (NTDs) and NaI detector, respectively. As predicted by Monte Carlo simulations, the measured intensity of thermal neutron inside the large sample cavity due to the external moderator was smaller than that from the smaller sample cavity. Due to its larger size, additional thermalization of neutrons will take place in the larger sample. In spite of smaller thermal neutron yield from the external moderator at the large sample location, higher yield of the prompt gamma ray was observed as compared to that from the smaller sample. This confirms the significance of neutron moderation effects in the bulk sample and can thereby affect the PGNAA geometry size. This allows larger samples in conjunction with smaller moderators in the PGNAA setup.     

A Monte Carlo simulation and setup optimization of output ef f iciency to PGNAA thermal neutron using ~(252)Cf neutrons

We present the design and optimization of a prompt γ-ray neutron activation analysis(PGNAA) thermal neutron output setup based on Monte Carlo simulations using MCNP5 computer code. In these simulations, the moderator materials, reflective materials, and structure of the PGNAA252 Cf neutrons of thermal neutron output setup are optimized. The simulation results reveal that the thin layer paraffin and the thick layer of heavy water moderating effect work best for the252 Cf neutron spectrum. Our new design shows a significantly improved performance of the thermal neutron flux and flux rate, that are increased by 3.02 times and 3.27 times, respectively,compared with the conventional neutron source design.     

Application of a cryogenic moderator in the REMUR neutron reflectometer

Examples of the application of a cryogenic moderator in the REMUR time-of-flight neutron reflectometer of the IBR-2 pulsed reactor (Dubna) are given. The results of two experiments are presented: spatial beam splitting upon reflection from a magnetically noncollinear film and the recording of a microbeam formed by a layered waveguide. A conclusion concerning the efficiency of the cryogenic moderator for experiments of this type is made.     

Improved slow-positron yield using a single crystal tungsten moderator

A well-annealed W(110) single crystal was used as a fast-to-slow positron moderator. The measured moderator efficiency at room temperature using a⁵⁸Co positron source in the backscattering geometry is =(3.2±0.4)×10^–3, roughly a factor of three better than for the best previously reported Cu(111)+S moderator. We find a stable positron moderation efficiency over a period of several weeks when maintained at pressures around 10^–9 Torr and an energy spreadE = 0.7 eV of the emitted slow positrons. An initial attempt was made to fabricate a hybrid Cu on W(110) moderator, which yielded of about 1.2×10^–3 after annealing.     

Experimental neutron flux measurements with a diamond detector at the QUINTA setup

The operational capability of a diamond detector used to measure the neutron spectrum by the response function on the QUINTA setup [1] installed at the proton beam of the phasotron [2] (Laboratory of Nuclear Problems, Joint Institute for Nuclear Research) was demonstrated in the energy interval of 2.1–20 MeV. The neutron-flux count rate was measured. The energy of neutrons was estimated at 7.4–25.7 MeV based on the diamond-detector response spectrum. The dependence of the diamond-detector response spectra on the angle between the proton beam and the line going through the detector and the center of the QUINTA setup was investigated. The angular anisotropy of the neutron flux was demonstrated. Measurements at different distances from the detector to the QUINTA setup were performed.     

Analysis of experimental data on the neutron yield from muons

Experimental data accumulated over 60 years of studying the yield of cosmogenic neutrons in a liquid scintillator (Y _LS), iron (Y _Fe), and lead (Y _Pb) were analyzed. This analysis revealed that the main part of the results on the yield Y _LS were overestimated by about 30%. With allowance for this circumstance, all experimental data can be described by the dependence Y(ē _µ, A) = b _n A ^β ē _µ ^α , where the product b _n ē _µ ^α stands for the energy spent by a muon on neutron production. The exponents of α = 0.78 and β = 0.95 are determined by the properties of the medium and by neutron production in showers.     

Measurement of cold neutron spectra using a model cryogenic moderator of the IBR-2M reactor

The method and results of an experiment to determine the cold neutron spectrum from solid mesitylene at moderator temperatures of 10–50 K are presented. This study was performed at the DIN-2PI spectrometer of the IBR-2 reactor. The objective of the study was to verify the system of constants used in the Monte Carlo simulation of cryogenic neutron moderators of the IBR-2M reactor and to obtain the cold neutron yield as a function of the moderator temperature. Satisfactory agreement between the experimental and calculated neutron spectra at a mesitylene temperature of 20 K has been obtained; the ratio of cold neutron intensities at 10 and 50 K is ∼1.8.     

Monte-Carlo calculations for neutron yield from photonuclear reactions following bremsstrahlung in tungsten target

The photonuclear reactions of (γ, xn) or (γ, xnp) types can be used to produce high-intensity neutron sources for research and applied purposes. In this work a Monte-Carlo calculation has been used to evaluate the production yields of neutrons from the (γ, n) and (γ, 2n) reactions following the bremsstrahlung produced by 100 and 200 MeV electron beams on the tungsten target. Our calculations indicated that the neutron yield is an increasing function of the target thickness in a considered range from 1.5 to 2.5 mm.     

Project of the ultracold and cold neutron source at the WWR-M reactor with superfluid helium as a moderator

The WWR-M reactor of the Petersburg Nuclear Physics Institute provides a unique opportunity for creating conditions of low radiative heat release (～4 × 10^?3 W/g) at a sufficiently high neutron flux (～3 × 10¹² neutrons/(cm² s)). This opportunity can be implemented in the reactor thermal column, which represents a 1-m-diameter channel adjacent to the reactor core. This diameter of the channel allows the arrangement of the core gamma shielding made of bismuth (15 cm thick), a graphite premoderator (300 dm³) at a temperature of 20 K, and a converter with superfluid helium (35 dm³) at a temperature of 1.2 K. Calculations show that the heat release in the source (20 W) can be removed by pumping helium vapor, and the density of ultracold neutrons in an experimental trap will be ～10⁴ neutrons/cm³, which is higher than that of existing sources of ultracold neutrons by two to three orders of magnitude.     

On the theory of pneumotransport of beads in cold neutron moderator of the IBR-2 reactor

The paper presents some aspects of pneumotransport theory of solid ball motion along a pipeline of complex geometry having application for the pelletized cold moderator of the IBR-2 reactor. Motion equations for a ball carrying by gas flow along a round tube both smooth and rough, are obtained. Aerodynamics forces and momentum of forces, acting on the ball depending on its velocity are defined on the base of numerical modeling of ball movement, and analytical simplified model of pneumotransport of balls along round tube is created. This model meets the demand for practical application as it shows satisfactory agreement with experimental modelling of ball movement in a glass tube. One of important results of the work is giving proofs of irregular movement of ball along a tube stipulated by roughness of the tube side.     

Statistical theory for the ratio of isomer to prompt and neutron to prompt cross sections

A new method for the calculation of the ratios between several cross sections in nuclei in which prompt fission, isomeric fission and neutron evaporation dominate is presented. The theory gives very good agreement with experiment.     

Elastic neutron scattering from 40Ca for neutron energies to 2 MeV

Total and differential elastic neutron scattering cross sections for Ca are analysed for resonance parameters below 2 MeV. A detailed reproduction of the total neutron cross section up to 1.2 MeV is given with multilevel R-matrix formulas. Using the theoretical expressions of Blatt and Biedenharn, shape features of the scattering data versus angle have been explored for spin and parity assignments. Strength functions and average level spacings are determined for s-, p- and d-waves. Reduced widths and the R∞ coefficients have been obtained from the analysis. Using these quantities and the sum rules, the intermediate coupling picture of Lane, Thomas and Wigner is applied to the study of s-, p- and d-wave strength in the energy region up to 2 MeV above the neutron threshold energy.     

D-D neutron yield in the 125 J dense plasma focus Nanofocus

We present here a very small transportable dense plasma focus with 125 J of energy able to be used mainly as an intense fast neutron source. The aim of this work was to design, construct and experimentally study a very compact nuclear fusion apparatus, at the lower energy limit, useful for multiple applications, such as soil humidity measurements, inspection of several materials metallic inclusions, medical neutron-therapies, etc. Besides, the possibility of using the same device as X-rays emitter has been explored. In a narrow range of deuterium filling pressure around 1 mbar, peaked Rogowski dips are observed. Correspondingly, strong neutron and hard X-ray pulses are measured. The neutron pulses last, in average, 50 ns, being about 10⁶ the amount of neutrons per pulse. The performance of this device has shown to be higher than any other plasma focus apparatus, compar ed on the empirical scaling law of neutron yield vs. pinch current.Received: 29 April 2003, Published online: 5 August 2003PACS: 52.58.Lq Z-pinches, plasma focus and other pinch devices - 52.70.Ds Electric and magnetic measurements - 52.70.Nc Particle measurements     

Effect of cathode structure on neutron yield performance of a miniature plasma focus device

In this Letter we report the effect of two different cathode structures - tubular and squirrel cage, on neutron output from a miniature plasma focus device. The squirrel cage cathode is typical of most DPF sources, with an outer, tubular envelope that serves as a vacuum housing, but does not carry current. The tubular cathode carries the return current and also serves as the vacuum envelope, thereby minimizing the size of the DPF head. The maximum average neutron yield of (1.82±0.52)×¹⁰⁵ n/shot for the tubular cathode at 4 mbar was enhanced to (1.15±0.2)×¹⁰⁶ n/shot with squirrel cage cathode at 6 mbar operation. These results are explained on the basis of a current sheath loading/mass choking effect. The penalty for using a non-transparent cathode negates the advantage of the smaller size of the DPF head.     

Interference effects in neutron scattering on protons in a double minimum potential

Based on the theoretical treatments by Stiller and Stamenkovich of the incoherent neutron scattering at a proton in a double minimum potential well experiments had been performed with KH₂PO₄ single crystals to prove the predicted interference effect.     

NMR quadrupolar effects in neutron irradiated LiF

Quadrupolar effects have been observed in the NMR spectrum of neutron irradiated LiF crystals. This is related to the presence of Li atom precipitates which are of two types - small platelets and bigger isotropic clusters.     

Correlation effects in small-angle multiple neutron scattering

The dependence of the spectra of small-angle multiple neutron scattering on the volume fraction occupied by scattering grains is considered. The concentration expansion is used to develop scattering theory in the eikonal approximation. The leading term of the expansion reproduces the standard low-concentration theory (Mollier). Some properties of the first correction term are analyzed, and it is shown that the angular distribution narrows with an increase in concentration, in qualitative agreement with the experimental data.