The scattering of slow neutrons by phosphorus and nitrogen

The coherent scattering lengthsb and the free cross sections for the scattering of slow neutrons by P and N were measured on powdered samples. From the results we derived data for the incoherent cross sections and for the spin state scattering lengths. We found for phosphorus:b=5.13±0.01 fm, σ_free=3.134±0.010b and σ_inc=0.006±0.016b, and for nitrogen:b=9.36±0.02 fm, σ_free=10.03 ±0.08 b and σ_inc=0.4±0.1 b. Derived values for resonance parameters of N are given.     

The scattering of thermal neutrons by crystals

The present article describes some experiments with thermal neutrons to investigate a variety of properties of crystals. Elastic scattering of the neutrons, like Bragg scattering of X-rays, helps to determine the atomic structure of crystals, and is particularly useful for locating light atoms, such as hydrogen, where X-rays fail. It also gives information on the arrangement of electron spins and the density distribution of unpaired electrons in magnetic atoms. Measurement of the change in energy of the neutrons in inelastic scattering gives the frequencies of the normal modes of vibration of the crystal, which are related to the interatomic forces. For magnetic crystals, inelastic scattering also gives the frequencies of spin waves, which depend on the magnetic interactions between the atoms. The article concludes with an account of the experimental techniques of thermal neutron scattering.     

Scattering of slow neutrons by bound nuclei

TheT-operator for scattering of slow neutrons by a system of bound nuclei is calculated up to quadratic terms in the scattering length. Binding effects as well as effects of multiple scattering have to be included in order to avoid inconsistencies. For the discussion of binding effects one can adopt methods developed by Dietze and Nowak [1] for treating scattering by an elastically bound nucleus. In particular the case of coherent elastic scattering is discussed: we show how the corrections can be expressed in terms of correlation functions and that binding effects are most important for scattering by light nuclei.     

Spin-dependent scattering of slow neutrons on polarized 17O

The method of pseudomagnetism has been applied to measure the spin-dependent scattering length b_N of slow neutrons on ¹⁷O nuclei. A powder sample of CaO, enriched with ¹⁷O, has been used. In this sample, dynamic nuclear polarizations of ± 10 % have been achieved at 2.5 T and 0.2 K using paramagnetic impurities created by fast neutron irradiation (F-centers). b_N = 0.12 ± 0.04 fm has been found. This value is close to an earlier shell model calculation.     

The scattering of slow atoms by light

A kinetic equation has been obtained describing the motion of atoms in a resonant field of standing light wave. This equation is used to describe the scattering of atoms for conditions close to experimental ones [1]. It is shown that the dependence of atomic scattering has a dip at the resonance frequency of the field.     

Multiple scattering theory for slow neutrons (from thermal to ultracold)

The general theory of neutron scattering is presented, valid for the whole domain of slow neutrons from thermal to ultracold. Particular attention is given to multiple scattering which is the dominant process for ultracold neutrons (UCN). For thermal and cold neutrons, when the multiple scattering in the target can be neglected, the cross-section is reduced to the known value. A new expression for inelastic scattering cross-section for UCN is proposed. Dynamical processes in the target are taken into account and their influence on inelastic scattering of UCN is analyzed. Received 21 July 1999     

Scattering of slow neutrons by arsenic and selenium isotopes

Coherent neutron scattering lengths and free cross sections were measured for arsenic, for ordinary selenium and its isolated isotopes. By means of the Cristiansen filter technique the following scattering lengths for the bound atoms were obtained (in fm):b(As) =6.58(1) [b ₊=6.04(5) andb _?=7.47(8)],b(Se)=7.970(9),b(⁷⁶Se)=12.2(1),b(⁷⁷Se) =08.25(8),b(⁷⁸Se)=8.24(9),b(⁸⁰Se)=7.48(3) andb(⁸²Se)=6.34(8). Transmission measurements with neutrons of 1.26 eV and 5.19 eV resulted in an energy independent free scattering cross section for arsenic ofσ _{s, t}=5.40(3)b (in the eV-region). For ordinary selenium energy dependent free cross sections ofσ _{s, t}(1.26 eV)=7.9(1)b andσ _{s, t} (5.19 eV)=7.55(3)b were found. The results were compared with the resonance parameters of the nuclei and with different sets of potential scattering radii. Thus it could be concluded that there is no evidence for a bound state of the neutron-nucleus compound⁷⁵As+n but a clear evidence for strong bound levels of⁷⁶Se+n and⁷⁷Se+n.     

Small angle critical scattering of neutrons by nickel

The critical magnetic scattering of neutrons by nickel was studied as a function of temperature and of the scattering vector κ in the range 0.008?0.04 Å^?1 for various specimens ranging from a single crystal to heavily deformed polycrystals. The shift of the temperature where the maximum of the scattered intensity is found as a function of scattering vector κ depends on the degree of crystalline imperfection. For the single crystal the observed shift is ΔT/T? 5 · 10^?3. On the basis of the Ornstein-Zernike theory the observed scattering by the single crystal yields the critical index \(\gamma = 2v = 1.32 \pm _{0.14}^{0.20} \) as predicted by most of the theories.     

Quasielastic magnetic scattering of neutrons by heavy-fermion systems

A theory of quasielastic scattering by a spin liquid with resonating valence bonds is constructed. It is demonstrated that the dependence of the scattering cross section of the spin liquid on the energy transfer has the same shape as the quasielastic peak observed experimentally in heavy-fermion systems. It is shown that a consequence of fact that the excitations of the spin-liquid state obey Fermi statistics is that the total quasielastic scattering cross section oscillates as a function of the momentum transfer. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 6, 480–485 (25 September 1998)     

The scattering of fast neutrons from W184

Neutrons with energies of 0.3 to 1.5 MeV are scattered from W¹⁸⁴. Pulsed-beam fast time-of-flight techniques including a magnetic bunching system are utilized to resolve the elastically scattered neutrons from those inelastically scattered. The differential elastic cross section is measured at 50 keV intervals with an ≈20 keV incident neutron energy spread. The differential cross sections for inelastic scattering resulting in the excitation of residual nuclear levels at 111±5, 365±10, 690±40, 900±25, 1000±30, and 1120±30 keV are determined. In all instances the inelastically scattered neutrons are emitted, within experimental error, isotropically. The experimental results are compared with those obtained in previous work and with the predictions of theory.     

Personnel dosimetry of slow neutrons by solid-state nuclear track detector

A fission track solid-state detector, based on aluminophosphate or sodium-calcium glasses and foil of natural uranium, was investigated. The etching conditions and evaluation of detectors are discussed. In the given arrangement a sensitivity of 1 rem for aluminophosphate glasses and 5 rem for sodium-calcium glasses was attained with slow neutrons.     

Huang diffuse scattering of neutrons

Huang diffuse neutron scattering was measured for the first time on niobium with interstitially dissolved deuterium as well as on MgO after neutron irradiation and Li⁷F after-irradiation. With Huang diffuse scattering the strength and symmetry of the distortion field around lattice defects can be determined. Our results clearly demonstrate that this method is feasible with neutrons.The present results are compared with X-ray experiments and the advantages of using neutrons is discussed in some detail.     

Elastic scattering of 2.76 MeV neutrons by Ca40

The elastic scattering of 2.76 MeV neutrons by calcium has been studied using the nuclear emulsion technique. The angular distribution of the elastically scattered neutrons has been compared with optical-model calculations. A satisfactory agreement with the theory has been obtained. In addition, the total cross section for 2.76 MeV neutrons on calcium has been measured by the transmission technique.     

Inelastic incoherent scattering of neutrons by dislocations in crystals

A possibility of determination of the magnitude of the Peierls stress τ_p from the spectrum of neutrons, inelastically scattered by dislocations, is considered. The dislocation dynamic has been described by the flexible string model. It is shown, that in the case of multiphonon scattering one can find τ_p by measuring the temperature dependence of the half-width of the Gaussian curve (8). It is possible to determine τ_p also through the measurement of the width of a gap, which is present in the energetical distribution of neutrons, scattered by means of one-phonon exchange.     

The analysing power in the elastic scattering of 14 MeV neutrons by deuterons

The analysing power for the elastic scattering of 14.1 MeV neutrons by deuterons has been measured at 17 angles between 44.5° and 161.6° in the centre of mass. A polynomial fit to this data combined with that at the same energy by Preiswerk et al. and Fischer et al. has been made and compared with a similar fit to the p-d data of Duder et al. Significant differences are found over the full angular range. Similar comparisons have also been made on existing data at other energies between 8 and 50 MeV. The polynomial expansion coefficients have been determined as a function of energy and show differences between p-d and n-d scattering which vary smoothly with energy and appear to be greater than the experimental errors.     

Quasi-elastic scattering of neutrons by a water dispersion of nanodiamonds

Data on quasi-elastic neutron scattering by a water dispersion of nanodiamonds and a pure water as a supporting system has been analyzed. The observed difference in the quasi-elastic neutron scattering spectra is interpreted using two different approaches developed to date for describing the behavior of a solvent in the cases of low and moderate concentrations of carbon nanoparticles (for example, fullerenes, nanotubes, ultradispersed diamonds). Within the first approach, it is assumed that the influence of dissolved nanoparticles leads to an insignificant change in the properties of the solvent. Within the second approach, the properties of the solvent are assumed to be invariable as compared to those of the pure liquid, except for a narrow boundary region of the order of two to three atomic layers near the surface of nanoparticles, where the properties of the solvent change drastically. The explanation of the effect within the second approach seems to be more reliable despite the apparent complication of the mathematical treatment of the experiment.     

Inelastic scattering of slow electrons by lanthanum oxide

Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 55, No. 6, pp. 988–993, December, 1991.     

Spin orbit interaction and polarization of slow neutrons

It is shown that the spin orbit (LS) interaction of an anomalous magnetic moment is twice that of a Dirac moment. In neutron scattering this interaction leads to an imaginary spindependent scattering amplitude. Possible experiments are discussed, where interference ofLS scattering and nuclear scattering in crystals without inversion symmetry leads to detectable neutron polarization.