Neutron Optical Potential Research Articles

Vertical time-of-flight spectroscopy of ultracold neutrons

This paper describes the mathematical basis for analysis of vertical time-of-flight (TOF) measurements with ultracold neutrons (UCN). In contrast to conventional horizontal TOF, this enables spectroscopy of UCN incident on the chopper with kinetic energies below the neutron optical potential of the detector entrance window. Much of the treatment presented here is general, and may be applied to horizontal and vertical TOF data of stationary and time-dependent spectra. Transformation rules are given for the conversion of vertical TOF spectra into other spectra of interest, and some aspects of detection efficiency are discussed that are common to all vertical TOF measurements using detectors with entrance windows. Throughout the paper, examples are provided using experimental data from measurements at the SUN-2 prototype superthermal UCN source. The technical details of these experiments and a comprehensive characterization of the source will be published by the present authors in a later paper.

Ultracold neutron storage in a bottle coated with the fluoropolymer CYTOP

The fluoropolymer CYTOP was investigated in order to evaluate its suitability as a coating material for ultracold neutron (UCN) storage vessels. Using neutron reflectometry on CYTOP-coated silicon wafers, its neutron optical potential was measured to be 115.2(2) neV. UCN storage measurements were carried out in a 3.8 l CYTOP-coated aluminum bottle, in which the storage time constant was found to increase from 311(9) s at room temperature to 564(7) s slightly above 10 K. By combining experimental storage data with simulations of the UCN source, the neutron loss factor of CYTOP is estimated to decrease from 1.1(1)times 10^{-4} to 2.7(2)times 10^{-5} at these temperatures, respectively. These results are of particular importance to the next-generation superthermal UCN source SuperSUN, currently under construction at the Institut Laue-Langevin, for which CYTOP is a possible top-surface coating in the UCN production volume.

Neutron interferometry and tests of short-range modifications of gravity

We consider tests of short-distance modifications of gravity based on neutron interferometry in the scenario of large extra dimensions. Avoiding the non-computability problem in the calculation of the internal gravitational potential of extended sources, typical of models with zero-width brane, we determine the neutron optical potential associated with the higher-dimension gravitational interaction between the incident neutron and a material medium in the context of thick brane theories. Proceeding this way, we identify the physical quantity of the extra dimension model that the neutron interferometry is capable of constraining. We also consider interferometric experiments in which the phase shifter is an electric field, as in the test of the Aharanov-Casher effect. We argue that this experiment, with this non-baryonic source, can be viewed as a test of the short-range behavior of Post-Newtonian parameters that measure the capacity of the pressure and the internal energy for producing gravity.

A new approach to search for free neutron-antineutron oscillations using coherent neutron propagation in gas

Coherent forward neutron propagation in gas is discussed as a new approach to search for neutron-antineutron oscillations (n−n¯), which violate both B and B−L conservation. We show that one can increase the probability of neutron - antineutron transitions in the presence of a nonzero external magnetic field to essentially free neutron oscillation probability by tuning the density of an appropriate mixture of gases so that the neutron optical potential of the gas cancels that from the magnetic field.

Internal consistency of neutron coherent scattering length measurements from neutron interferometry and from neutron gravity reflectometry

Many theories beyond the Standard Model postulate short-range modifications to gravity which produce deviations of Newton's gravitational potential from a strict $1/r$ dependence. It is common to analyze experiments searching for these modifications using a potential of the form $V^{\prime}(r)=-\frac{GMm}{r} [1+\alpha \exp{(-r/\lambda)}]$. The best present constraints on $\alpha$ for $\lambda <100$\,nm come from neutron scattering and often employ comparisons of different measurements of the coherent neutron scattering amplitudes $b$. We analyze the internal consistency of existing data from two different types of measurements of low energy neutron scattering amplitudes: neutron interferometry, which involves squared momentum transfers $q^{2}=0$, and neutron gravity reflectometry, which involves squared momentum transfers $q^{2}=8mV_{opt}$ where $m$ is the neutron mass and $V_{opt}$ is the neutron optical potential of the medium. We show that the fractional difference $\frac{\Delta b}{|b|}$ averaged over the 7 elements where high precision data exists on the same material from both measurement methods is $[2.2 \pm 1.4] \times 10^{-4}$. We also show that $\frac{\Delta b}{|b|}$ for this data is insensitive both to exotic Yukawa interactions and also to the electromagnetic neutron-atom interactions proportional to the neutron-electron scattering length $b_{ne}$ and the neutron polarizability scattering amplitude $b_{pol}$. This result will be useful in any future global analyses of neutron scattering data to determine $b_{ne}$ and bound $\alpha$ and $\lambda$. We also discuss how various neutron interferometric and scattering techniques with cold and ultracold neutrons can be used to improve the precision of $b$ measurements and make some specific proposals.

New compact neutron supermirror transmission polarizer

A new compact neutron supermirror transmission polarizer is suggested. The polarizer consists of a set of plates transparent to neutrons placed in the magnet gap. There are no air gaps between the plates. Polarizing supermirror coating without absorbing underlayer is deposited on the polished surfaces of the plates. Magnetic and nonmagnetic layers of the supermirror coating as well as the material of the plates have nearly equal neutron-optical potentials for spin-down neutrons. There is a considerable difference between neutron-optical potentials of layers in the supermirror structure for spin-up neutrons. As a result, spin-up neutrons reflect from the supermirror coating and deviate from their initial trajectories whereas spin-down neutrons do not practically reflect from the coating and, consequently, do not deviate from their initial trajectories. Thus, spin-down neutrons dominate near the axis of distribution of intensity on the angle for the beam transmitted through this polarizer, i.e., the beam is substantially polarized. Application is discussed of this polarizer in a research facility for small angle scattering of monochromatic neutrons with wavelengths λ = 4.5÷20 Å. The polarizing cross section of the beam of this facility is 30×30 mm2. Calculated parameters are presented of a polarizer on silicon plates with supermirror CoFe/TiZr (m = 2) coating. The suggested polarizer is compared with solid state bender, S-bender and widely known transmission neutron polarizer V- cavity in the same spectral range. Two polarizers are used to cover the wavelength range λ = 4.5 ÷20 Å: the first one whose length is 50 мм covers the range λ = 4.5 ÷10 Å and the second one whose length is 21.2 мм covers the range λ = 10 ÷20 Å. The length of each of these polarizers is more than 30 times smaller than that of V-cavity! On the other hand, basic parameters of the proposed polarizer, polarization of the beam falling on the sample P and transmission coefficient T-of the main spin component, exceed those of V-cavity. T- = 0.8 − 0.9 for both polarizers and for each wavelength range. Polarization P is very high. P is better than -0.99 for wavelength range λ = 12.5 ÷ 20 Å at the beam divergence of 24 mrad.

The single-particle characteristics of Pb isotopes near the drip lines calculated within the dispersive optical model

The neutron and proton dispersive optical potential for the 208Pb nucleus has been determined for the energy region from–70 to +60 MeV and used to calculate the differential elastic scattering, the total interaction and reaction cross sections, as well as the single-particle characteristics, the neutron and charge densities, rms radii, and the thickness of the nucleus skin. The calculated results are in good agreement with the experimental data. The proton dispersive optical model potential for the spherical and close to spherical Pb isotopes within the neutron and proton drip lines has been obtained by a similar method. The calculation predicts a trend towards the growth of the proton particle-hole gap, which corresponds to Z = 82 shell closure as Z approaches the proton drip line.

Parity-even and time-reversal-odd neutron optical potential in spinning matter induced by gravitational torsion

Recent theoretical work has shown that spin $1/2$ particles moving through unpolarized matter which sources torsion fields experience a new type of parity-even and time-reversal-odd optical potential if the matter is spinning in the lab frame. This new type of optical potential can be sought experimentally using the helicity dependence of the total cross sections for longitudinally polarized neutrons moving through a rotating cylindrical target. In combination with recent experimental constraints on short-range P--odd, T--even torsion interactions derived from polarized neutron spin rotation in matter one can derive separate constraints on the time components of scalar and pseudoscalar torsion fields in matter. We estimate the sensitivity achievable in such an experiment and briefly outline some of the potential sources of systematic error to be considered in any future experimental search for this effect.

Ultracold neutron accumulation in a superfluid-helium converter with magnetic multipole reflector

We analyze the accumulation of ultracold neutrons (UCNs) in a superfluid-helium converter vessel surrounded by a magnetic multipole reflector. We solved the spin-dependent rate equation, employing formulas valid for adiabatic spin transport of trapped UCNs in mechanical equilibrium. Results for saturation UCN densities are obtained in dependence of order and strength of the multipolar field. The addition of magnetic storage to neutron optical potentials can increase the density and energy of the low-field-seeking UCNs produced and serves to mitigate the effects of wall losses on the source performance. It also can provide a highly polarized sample of UCNs without need to polarize the neutron beam incident on the converter. This work was performed in preparation of the UCN source project SuperSUN at the Institut Laue--Langevin.

Derivation of an optical potential for statically deformed rare-earth nuclei from a global spherical potential

The coupled-channel theory is a natural way of treating nonelastic channels, in particular those arising from collective excitations characterized by nuclear deformations. A proper treatment of such excitations is often essential to the accurate description of experimental nuclear-reaction data and to the prediction of a wide variety of scattering observables. Stimulated by recent work substantiating the near validity of the adiabatic approximation in coupled-channel calculations for scattering on statically deformed nuclei, we explore the possibility of generalizing a global spherical optical model potential to make it usable in coupled-channel calculations on this class of nuclei. To do this, we have deformed the Koning-Delaroche global spherical potential for neutrons, coupling a sufficient number of states of the ground-state band to ensure convergence. We present an extensive study of the effects of collective couplings and nuclear deformations on integrated cross sections as well as on angular distributions for neutron-induced reactions on statically deformed nuclei in the rare-earth region. We choose isotopes of three rare-earth elements (Gd, Ho, W), which are known to be nearly perfect rotors, to exemplify the results of the proposed method. Predictions from our model for total, elastic, and inelastic cross sections, as well as for elastic and inelastic angular distributions, are in reasonable agreement with measured experimental data. These results suggest that the deformed Koning-Delaroche potential provides a useful regional neutron optical potential for the statically deformed rare-earth nuclei.

Neutron scattering from208Pb at 30.4 and 40.0 MeV and isospin dependence of the nucleon optical potential

Analysis of data involving nuclei far from stability often requires optical potential (OP) for neutron scattering. Since neutron data is seldom available, while proton scattering data is more abundant, it is useful to have estimates of the difference of the neutron and proton optical potentials. This information is contained in the isospin dependence of the nucleon OP. Here we attempt to provide it for the nucleon-208Pb system. The goal of this paper is to obtain accurate n+208Pb scattering data, and use it, together with existing p+208Pb and 208Pb(p,n)208$Bi*_{IAS} data, to obtain an accurate estimate of the isospin dependence of the nucleon OP at energies in the 30-60 MeV range. Cross sections for n+208Pb scattering were measured at 30.4 and 40.0 MeV, with a typical relative (normalization) accuracy of 2-4% (3%). An angular range of 15 to 130 degrees was covered using the beam-swinger time of flight system at Michigan State University. These data were analyzed by a consistent optical model study of the neutron data and of elastic p+208Pb scattering at 45 MeV and 54 MeV. These results were combined with a coupled-channel analysis of the 208Pb(p,n) reaction at 45 MeV, exciting the 0^+ isobaric analog state in 208Bi. The new data and analysis give an accurate estimate the isospin impurity of the nucleon-208Pb OP at 30.4 MeV, caused by the Coulomb correction to the proton OP. The corrections to the real proton OP given by the CH89 global systematics was found to be only few percent, while for the imaginary potential it was over 20% at the nuclear surface. Based on the analysis of the measured elastic p+208Pb data at 40 MeV, a Coulomb correction of similar strength and shape was also predicted for the p+208Pb OP at energy around 54 MeV.

Choice of potential in the analysis of small-angle neutron scattering

The small-angle scattering of thermal neutrons from large-scale inhomogeneities of a material is considered. It is shown that the neutron-optical potential must be taken as the inhomogeneity potential both in the Born and eikonal approximations. The neutron-optical potential is the Fermi pseudopotential averaged over a volume containing many nuclei but with linear sizes much smaller than the sizes of the inhomogeneities measured.

Calculation of single-particle energies in the 28 56 Ni28 and 28 78 Ni50 nuclei within the mean field model with the dispersive optical potential

Parameters of the neutron and proton dispersive optical potentials were determined by the earlier developed method for the unstable double magic nuclei 2856Ni28 and 2878Ni50. They were used to calculate the neutron and proton single-particle energies of these nuclei near the Fermi energy. Good agreement with the experimental data is obtained.

Analysis of deuteron elastic scattering fromLi6,7using the continuum discretized coupled channels method

The continuum discretized coupled channels (CDCC) approach is applied to analysis of deuteron elastic scattering from {sup 6,7}Li in the energy range from 10 to 50 MeV. Phenomenological neutron and proton optical potentials that are essentially important in the CDCC calculation are determined from the present optical model analysis of differential cross sections of nucleon elastic scattering, neutron total cross sections, and reaction cross sections of {sup 6,7}Li for energies from 5 to 50 MeV. The CDCC result provides satisfactory agreement with experimental data, particularly at forward angles. The obtained nucleon optical model potentials are found to describe reasonably well both nucleon and deuteron elastic scattering from {sup 6,7}Li for energies up to 50 MeV.

The Neutron Spherical Optical Model Absorption

The energy-dependent behavior of the absorption term of the spherical neutron optical potential for doubly magic 208Pb and neighboring 209Bi is examined. These considerations suggest a model with an intuitively attractive energy dependence of the imaginary potential that provides a good description of the observed neutron cross-sections and that is qualitatively consistent with theoretical concepts. It is an alternative to arbitrary assumptions involved in many conventional optical-model interpretations reported in the literature.

Experimental study of neutron-optical potential with absorption using Fabry–Perot magnetic resonator

We have succeeded in measuring spin precession and transmission probability of neutron through Supersendust(SSD)-(Ge/Gd)-SSD Fabry–Perot magnetic resonator, and estimated with the optical potential model for magnetic multilayer system with absorption.

Analysis of 58Ni(n, α) Reaction Cross Sections with the Hauser-Feshbach Statistical Theory and the Bayesian Parameter Estimation Method

Experimental data of 58Ni(n, α) reaction cross sections are analyzed with the Hauser-Feshbach statistical model with the width fluctuation correction by Moldauer. A neutron optical potential which was derived from neutron elastic scattering data is used for an entrance channel in order to fix the total reaction cross sections at reliable values. Global optical potentials are used for the proton and α-particle emission channels, and level density parameters of the residual nuclei are determined from the cumulative plot of discrete levels. These parameters are adjusted to the experimental data of 58Ni(n, α) and (n, p) reaction cross sections, including the angular distribution of the emitted α-particles. The parameters thus obtained yield a good fit to the experimental cross sections of those reactions up to 8 MeV.

Estimation of Energy Dependence of the Optical Potential Parameters for 209Bi

A parameter estimation method based on Bayes’ theorem is applied to the parameters of the neutron optical potential for 209Bi. The potential parameters at a certain energy are determined according to an assumption that parameters vary gradually with the incident neutron energies, and it is demonstrated that the method can be used as an efficient tool for investigating the energy dependence of the optical potential parameters.The derived potential parameters are expressed by both a polynomial function and a Brown-Rho parameterization, and the energy dependencies are interpreted by a dispersion relation theory. Both expressions of the energy-dependent parameters give a good description of the experimental elastic scattering cross section and the total cross section.

Measurement and theoretical analysis of neutron elastic scattering and inelastic reactions leading to a three-body final state for6Liat 10 to 20 MeV

The neutron elastic and inelastic scattering double-differential cross sections of ${}^{6}\mathrm{Li}$ were measured at incident neutron energies of 11.5, 14.1, and 18.0 MeV. A phenomenological neutron optical model potential of ${}^{6}\mathrm{Li}$ was constructed to describe the total and elastic scattering cross sections from 5 MeV to several tens MeV, based on the present data together with information from other works. This potential was found to describe the inelastic scattering to the first excited state ${(E}_{x}=2.186 \mathrm{MeV})$ well via the distorted-wave Born approximation (DWBA) calculation with the macroscopic vibrational model. The continuum neutron energy spectra and angular distributions were then analyzed by the theory of final-state interaction extended to the DWBA form, with an assumption that the $d\ensuremath{-}\ensuremath{\alpha}$ interaction is dominant in the three-body final state consisting of $n, d,$ and \ensuremath{\alpha} particles. Such a calculation was found to be successful in explaining the major part of the low-excitation neutron spectra and angular distribution down to the Q-value region of $\ensuremath{-}9 \mathrm{M}\mathrm{e}\mathrm{V},$ except for the Q-value range where the $n\ensuremath{-}\ensuremath{\alpha}$ quasifree scattering will give a non-negligible contribution at forward angles.

中性子ポテンシャルの現状 光学ポテンシャル研究の新しい展開を目指して

Recent needs of nuclear data are found not only in the fields related to nuclear power plants but also in the other fields: medical applications, transmutation of nuclear waste, space development, non-destructive test, analysis, wide-range dosimetry, etc. To meet such wide needs, nuclear data are required for various types of incident particles and target nuclei in the wide energy range. Under these circumstances, “Research Committee on Neutron Optical Potentials” had been organized in the period from April, 1992 to March, 1994, and had investigated and discussed various problems related to neutron optical potentials which are indispensable for nuclear data evaluation. Their final report, “Present Status of Neutron Potential” was published in May, 1997. In the present report, we overview the results given in the committee.