Characterization of nuclear sources via two-neutron intensity interferometry

The neutron energy spectrum and the two-neutron correlation function have been measured for the E/A=45 MeV Ni + Al reaction in order to assess the space-time characteristics of the neutron emitting source. When comparing the data to a statistical model, the kinetic energy spectra, the integrated correlation function as well as the longitudinal correlation function are reproduced by one single source. However, only the inclusion of a short-lived pre-equilibrium component can account for the stronger correlation exhibited by neutron pairs emitted with high total momentum. The correlation function from events defined as peripheral by constraints on the highest charge of the projectile-like fragment does show a significantly weaker correlation than the minimum bias sample.     

裂变中子角关联分布的数值模拟

为了实现对自发裂变中子源能量、多重性和中子角关联分布的数值模拟,开发了根据裂变过程,逐步给出裂变中子多重性、能量和角分布等参数的NESF数值模拟程序,并将该程序集成到现有中子输运软件中.重点介绍我们开发的用于模拟自发裂变中子源能量、多重性和中子角关联分布的程序的背景、物理依据、程序流程,以及基于该程序开展的中子角关联分布特征与核部件特征关联的数值模拟.     

Effects of pairing correlation on neutron drop

In this work, the effects of the pairing correlation on the properties of neutron drops N=6-50 trapped in a harmonic oscillator potential with ω = 10 Me V are investigated by comparing the results given by the Skyrme Hartree-Fock and Hartree-FockBogoliubov theories with the density-dependent delta interaction(DDDI) pairing force. The results showed that the pairing correlation slightly made the neutron drops more bound, and increased the central neutron density, the spin-orbit and pseudo spin-orbit splittings. Thus, the pairing correlation must be accounted for to improve the Skyrme functional compared with the ab initio calculations. Furthermore, although the single-particle energy gaps with or without pairing were similar, the shell closures varied due to pair scattering. Here, the shell closures in neutron drops using the Sk M* parameter set and DDDI pairing force were found at N=8, 16, and 32.     

A revisit to the neutron skin thickness of neutron-rich nuclei in the statistical abrasion ablation model

The cross sections of the fragments produced in the projectile fragmentation reactions of the even calcium isotopes from A=36 to A=52 are calculated using the statistical abrasion ablation model.The neutron skin thickness are studied by investigating the fragments isotopic cross section distributions.The neutron-skin thicknesses of the calcium isotopes have a good linear correlation to the peak positions of their fragment isotopic cross section distributions.The correlation between the neutron skin thickness and the neutron density distributions of 48Ca is investigated by introducing a parameter to adjust the diffuseness parameter in the fermi-type density distribution.     

Spin correlation function of neutrons emitted by a compound nucleus with high angular momentum

The spin correlation function of two neutrons evaporated consecutively from a compound nucleus is calculated in the framework of a statistical model. The correlation function varies only weakly with the neutron emission angle, typically not more than 10%. This allows to use a simple formula to calculate the neutron spin correlation in good approximation.     

Neutron skin thickness and nuclear matter properties

Linear correlations are found among the isovector nuclear matter properties in both the Skyrme-Hartree-Fock (SHF) and the relativistic mean-field (RMF) models. In addition, we found a kind of correlation between the isovector nuclear matter properties and the incompressibility in the SHF model. The Skyrme parameters are related analytically to nuclear matter properties with the Thomas—Fermi approximation. By using a linear correlation between the neutron skin thickness and the pressure of the neutron matter in the SHF model, we show that the neutron skin thickness of ²⁰⁸Pb gives crucial information about not only the neutron equation of state but also the isovector nuclear matter properties and the parametrization of Skyrme interaction. The text was submitted by the authors in English.     

Analysis of the relationship between neutron dose and Cerenkov photons under neutron irradiation through Monte Carlo method

To theoretically explore the feasibility of neutron dose characterized by Cerenkov photons, the relationship between Cerenkov photons and neutron dose in a water phantom was quantified using the Monte Carlo toolkit Geant4. Results showed that the ratio of the neutron dose deposited by secondary electrons above Cerenkov threshold energy to the total neutron dose is approximately a constant for monoenergetic neutrons from 0.01 eV to 100 eV. With the initial neutron beam energy from 0.01 eV to 100 eV, the number of Cerenkov photons has a good correlation with the total neutron dose along the central axis of the water phantom. The changes of neutron energy spectrum and mechanism analysis also explored at different depths. And the ratio of total neutron dose to the intensity of Cerenkov photons is independent of neutron energy for neutrons from 0.01 eV to 100 eV. These findings indicate that Cerenkov radiation also has potential in the application of neutron dose measurement in some specific fields.     

Angular correlation of neutrons from thermal-neutron fission of 235U

The neutron-neutron angular correlation for neutrons with energies ? 1 MeV emitted in thermal-neutron-induced fission of ²³⁵U has been measured. The correlation could be adequately described by a simple model, assuming isotropic neutron emission in the fully accelerated fragment frames together with a 20% isotropic scission component and using binomial distributions to simulate the neutron multiplicity (instead of the usual gaussian form) for each fission fragment. The results give no indication of enhanced polar emission.     

On the detection of nuclear spin waves by inelastic neutron scattering

The possibility of measuring nuclear spin waves (NSW) by inelastic neutron scattering is discussed. The differential cross section and scattered state polarization for the scattering of thermal neutrons from systems described by the Suhl-Nakamura Hamiltonian are developed in the Van Hove correlation function formalism; the relevant correlation functions for the Suhl-Nakamura system are computed. The implications of these calculations for the feasibility of detecting nuclear spin wave modes in neutron scattering experiments are discussed.     

Correlation of x-ray and neutron emissions from an ion implosion system

The paper presents the results of space- and time-resolved studies of X-rays and neutrons emitted by the SOWA-150A ion-impulsion facility equipped with a coaxial set multi-rod electrodes. Proportionality and correlation of neutron and hard X-ray pulses favour the hypothesis of a beam-target mechanism of the neutron production.     

Influence of electrode and insulator materials on the neutronemission in a low energy plasma focus device

Insertion of internal impurities is one of the factors for the degradation of neutron output in plasma focus devices. In this context, neutron emission measurements were carried out in a 2.2 kJ (7.2 μF, 25 kV) Mather type squirrel cage plasma focus device with various combinations of electrode and insulator materials. The neutron yield was measured by silver activation detector. The results indicate that a central electrode of low erosion rate material produces maximum neutron yield with the highest anisotropy factor. A time integrated X-ray pinhole image shows that focus corresponding to the highest neutron yield material is the most compressed. The dielectric constant of the ceramic insulators has, at best, a very weak correlation with neutron output. The nonceramic insulators like perspex, nylon, or teflon neither produce focus nor neutrons     

Reduction of systematic uncertainty in the transmission measurement of iron using entropy based mutual information

Neutron cross-section determination by transmission measurement is essentially a data reduction process of obtaining the cross-section from the raw data of the neutron detector counts. The data reduction parameters are dominated by systematic uncertainty with a large coefficient of correlation. All the efforts have so far been devoted only to minimize the statistical uncertainty in neutron detector counts by repeated measurements but the dominant systematic uncertainty in the data reduction parameters has either been neglected or randomized. As the conventional central limit theorem cannot be applied to reduce the highly correlated systematic uncertainty, we propose a new approach called entropy based mutual information to set theoretical limits and minimize the systematic uncertainty using an algorithm we have developed. In our algorithm one can determine the upper and lower bounds of the non-diagonal elements of the correlation matrix for maximization of mutual information. We demonstrate the utility of our approach in reducing the systematic and hence the total uncertainty of neutron cross-section of iron in the transmission measurement.     

Systematics of neutron emission probabilities from delayed neutron precursors

A simple correlation based on the gross theory ofβ-decay is derived between the neutron emission probabilitiesP _n of delayed neutron precursors, theirβ-decay energies and the neutron binding energies of the daughter nuclei. The correlation is shown to be valid for delayed neutron precursors among the fission products. TheP _n-values of several expected but still unidentified neutron precursors are estimated together with their contributions to the delayed neutron groups in thermal-neutron induced fission of²³⁵U. Some aspects of theβ-strength function for transitions into highly excited states are discussed.     

Stagnancy of the pygmy dipole resonance

The pygmy dipole resonance(PDR) of nickel isotopes is studied using the deformed random phase approximation method. The isoscalar character of the pygmy resonance is confirmed, and the correlation between the pygmy resonance and neutron skin thickness is discussed. Our investigation shows a linear correlation between PDR integral cross section and neutron skin thickness when the excess neutrons lie in pf orbits, with a correlation rate of about 0.27 fm~(-1). However, in more neutron-rich nickel isotopes, the growth of the pygmy dipole resonance is stagnant. Although the neutron skin thickness increases, the whole skin is not active. There is an inertial part in the nuclei~(70-78)Ni which does not participate in the pygmy resonance actively and as a result, contributes little to the photo-absorption cross section.     

Z箍缩中子编码诊断系统模拟平台搭建

开发了基于Geant4的Z箍缩中子编码成像系统模拟平台,实现聚变中子编码成像诊断系统各关键部件的完整模拟。获得了低中子产额（约1010量级）下,中子经编码孔编码后在闪烁体阵列中形成的发光分布图像。利用维纳滤波、Richardson-Lucy（RL）及遗传算法（GA）对低中子产额下获得的极低信噪比图像进行重建,并对信噪比、中子产额及重建效果进行了对比研究,结果表明:遗传算法对低信噪比中子编码图像的重建具有很强的鲁棒性;中子编码图像的信噪比与遗传算法重建结果的准确性呈正比。     

A method of space-time correlation functions for describing the quasifree knockout of valence neutrons from a halo nucleus

Expressions for the distribution functions of products of the valence neutron knockout reaction (p, p’n) for halo nuclei were obtained using a method of space-time correlation functions. An analysis of the efficiency of this reaction as a method for investigating the structure of a neutron halo is given. The fraction of events in which the internal motion in a neutron complex remains unperturbed in the case of the quasifree knockout of the complex from a halo nucleus by protons with an energy of several tens of MeV was calculated. An expression for the spectrum of outgoing protons was obtained and analyzed.     

The study of the thermal neutron flux in the deep underground laboratory DULB-4900

We report on the study of thermal neutron flux using monitors based on mixture of ZnS(Ag) and LiF enriched with a lithium-6 isotope at the deep underground laboratory DULB-4900 at the Baksan Neutrino Observatory. An annual modulation of thermal neutron flux in DULB-4900 is observed. Experimental evidences were obtained of correlation between the long-term thermal neutron flux variations and the absolute humidity of the air in laboratory. The amplitude of the modulation exceed 5% of total neutron flux.     

Measurements ofn−p correlations in the reaction of relativistic neon with uranium

We report a preliminary measurement of coincident neutron-proton pairs emitted at 45° in the interaction of 400, 530, and 650 MeV/A neon beams incident on uranium. Charged particles were identified by time of flight and momentum, as determined in a magnetic spectrometer. Neutral particles were detected using a thick plastic scintillator, and their time of flight was measured between an entrance scintillator, triggered by a charged particle, and the neutron detector. The scatter plots and contour plots of neutron momentum vs. proton momentum appear to show a slight correlation ridge above an uncorrelated background. The projections of this plane on then-p momentum difference axis are essentially flat, showing a one standard deviation enhancement for each of the three beams energies. At each beam energy, the calculated momentum correlation function for the neutron-proton pairs is enhanced near zero neutron-proton momentum difference by approximately one standard deviation over the expected value for no correlation. This enhancement is expected to occur as a consequence of the attractive final state interaction between the neutron and proton (i.e., virtual or “singlet” deuterons). The implications of these measurements are discussed.     

Resonance and background interference in 54Fe neutron capture

Asymmetric s-wave resonances observed in the neutron capture cross section of ⁵⁴Fe can be explained by interference between resonance valence components. An upper limit is obtained for the background capture cross section which is much smaller than that expected from the large correlation observed between s-wave reduced neutron widths and radiative widths.     

The lead nucleus as a miniature surrogate for a neutron star

The nucleus of ²⁰⁸Pb, a system 18 orders of magnitude smaller and 55 orders of magnitude lighter than a neutron star, may be used as a miniature surrogate to establish important correlations between its neutron skin and several neutron-star properties. Indeed, models with a thicker neutron skin in ²⁰⁸Pb generate larger neutron stars that have a lower liquid-to-solid transition density. Further, we illustrate how the correlation between the neutron skin in ²⁰⁸Pb and the radius of a 1.4 solar-mass neutron star may be used to place important constraints on the equation of state of neutron-rich matter and how it may help elucidate the existence of a phase transition at the core of the star.