Development of a segmented fast neutron spectrometer

We discuss the development of a spectrometer based on full energy absorption using liquid scintillator doped with enriched ⁶Li. Of specific interest, the spectrometer is expected to have good pulse height resolution, estimated to lie in the range 10–15% for 14-MeV neutrons. It should be sensitive to flux rates from 10⁻⁶ cm⁻² s⁻¹ to 10⁶ cm⁻² s⁻¹ above a threshold of 500 keV in an uncorrelated γ background of up to 10⁴ s⁻¹. We have constructed a pilot version of the detector using undoped liquid scintillator, and we report its present status. The detector’s efficiency is determined by the volume of the scintillator (∼1.21) and is estimated to be 0.2–0.5% for 3-MeV neutrons. The good pulse height resolution is achieved by compensation of the nonlinear light yield of the scintillator due to the use of optically separated segments, which collect scintillations from each recoil proton separately. We demonstrate here the response of the detector to neutrons from a Pu-α-Be source, whose energies range up to 10 MeV. Initial testing indicates a low threshold (≈600 keV) and good spectral response after requiring a multiplicity of three segments. Such a spectrometer has applications for low-background experiments in fundamental physics research, characterizations of neutron flux in space, and the health physics community. The text was submitted by the authors in English.     

Limits on a nucleon–nucleon monopole–dipole coupling from spin relaxation of polarized ultra-cold neutrons in traps

A new limit is presented on the axion-like monopole–dipole P, T-non-invariant interaction in a range (10⁻⁴–1) cm. The spin-dependent nucleon–nucleon potential between neutrons and nucleons of the walls of the cavity containing ultra-cold neutrons should affect the neutron depolarization probability at their reflection from the walls. The limit is obtained from existing data on the ultra-cold neutron depolarization probability per one collision with the walls.     

Effect of neutron irradiation on the IR absorption spectra of cupric oxide single crystals

A strong increase of the absorption coefficient with photon energy increasing from 0.1 to 1.0 eV is observed in the spectra of CuO single crystals irradiated with neutrons to a fluence of 5×10¹⁸ cm⁻². The difference of the absorption coefficients before and after irradiation depends on the wavelength as λ⁻². The effect of neutron irradiation on CuO is qualitatively similar to that of neutrons on other semiconductors (for example, GaAs) and differs from that obtained by irradiating CuO with charged particles. Zh. Tekh. Fiz. 69, 98–99 (December 1999)     

Atom trap trace analysis

A new method of ultrasensitive isotope trace analysis has been developed. This method, based on the technique of laser manipulation of neutral atoms, has been used to count individual ⁸⁵Kr and ⁸¹Kr atoms present in a natural krypton gas sample with isotopic abundances in the range of 10⁻¹¹ and 10⁻¹³, respectively. The method is free of contamination from other isotopes and elements, and can be applied to many different isotope tracers for a wide range of applications. This revised version was published online in August 2006 with corrections to the Cover Date.     

Energetic neutrons and gamma rays measured on the Aryabhata satellite

An experiment to measure energetic neutrons and gamma rays in space was launched in the first Indian scientific satellite,Aryobhata, on April 19, 1975. From this experiment, the first measurements in space of the Earth’s albedo fiux of neutrons of energy between 20 and 500 MeV have been made; the values obtained for two mean geomagnetic vertical cut-off rigidities of 5.6 and 17.0 GV are (6.3±0.4)×10⁻² and (1.4±0.3)×10⁻² neutrons cm⁻² sec⁻¹ respectively. These measurements confirm that protons arising from cosmic ray albedo neutron decay, can adequately account for the protons in the inner radiation belt. Observations on gamma rays of energy between 0.2 and 24 MeV have enabled the determination of the total background gamma ray flux in space as a function of latitude. This in turn has permitted useful information on the diffuse cosmic gamma rays. We have also observed four events that showed sudden increases in the gamma ray counting rates between 0.2 and 4.0 MeV. Observational details of these events are given.     

<Emphasis Type="Italic">T</Emphasis>-odd angular correlations in the emission of prompt gamma rays and neutrons in nuclear fission induced by polarized neutrons

Study of the T-odd three-vector correlation in the emission of prompt neutrons from ²³⁵U fission by polarized cold neutrons has been continued at the facility MEPHISTO of the FRM II reactor (Technical University of Munich). The sought correlation was not found within experimental error of 2.3 × 10⁻⁵. The upper limit for the asymmetry coefficient has been set to |D _n | < 6 × 10⁻⁵ at 99% confidence level, whereas for ternary fission correlation coefficient D _α = (170±20) × 10⁻⁵. This limit casts doubt on a model that explains the three-vector correlation in ternary fission by the Coriolis mechanism. At the same time, five-vector correlation in the emission of prompt fission neutrons has been measured, which describes the rotation of the fissioning nucleus at the moment it breaks (ROT effect). At the angle 22.5° to the fission axis, the correlation coefficient was found to be (1.57 ± 0.20) × 10⁻⁴, while at the angle of 67.5° it is zero within the experimental uncertainty. The existence of ROT effect in the emission of prompt fission neutrons can be explained by the anisotropy of neutron emission in the rest frame of the fragment (fission fragments are aligned with respect to the axis of deformation of the fissioning nucleus), similar to the mechanism of ROT effect in the emission of prompt γ-rays.     

Model of metal surface erosion under irradiation by high-power pulsed ion beams

The mechanisms of erosion of metal surfaces under the action of submicrosecond (10⁻⁹−10⁻⁶ s) ion beams in the power density range of P = 10⁶−10⁹ W/cm² with a particle energy of 1–2000 keV are considered. It is shown that the collective processes associated with the radiation heating of the surface are of great importance. A model for the erosion is proposed. In accordance with this model, the flow of atoms of the target leaving the surface being irradiated consists of two independent components caused by collisional sputtering and evaporation, respectively. The influence of the irradiation parameters on the erosion coefficient and the ratio between the sputtering and evaporation factors is analyzed.     

Studies of parity and time reversal symmetries in neutron scattering from165Ho

We describe searches for parity and time reversal violations in the scattering of polarized neutrons from polarized and aligned¹⁶⁵Ho targets. We have completed a search with 7.1 and 11.0 MeV neutrons for P_oddT_odd terms in the elastic scattering forward amplitude of the form s. (I×K), wheres is the neutron spin,I is the target spin andk is the neutron momentum vector. The target was a single crystal of holmium, polarized horizontally along itsb axis by a 1 Tesla magnetic field. The neutrons were polarized vertically. Differences in the neutron transmission were measured for neutrons with spins parallel (antiparallel) toI×k. The P,T violating analyzing powers were found to be consistent with zero at the few 10⁻³ level: ρ_P,T(7.1 MeV)=−0.88 (±2.02) x 10⁻³, ρ_P,T(11.0 MeV)=−0.4 (±2.88) x 10⁻³. We have also attempted to find enhancements with MeV neutrons in P-violation due to the term s\k. We are preparing an aligned target cryostat for investigations of P_evenT_odd terms {bd(I\k)(I×k)\s} in neutron scattering. The target will be a single crystal cylinder of¹⁶⁵Ho cooled to 100 mK in a bath of liquid helium and rotated by a shaft from a room temperature stepping motor. The cylinder will be oriented vertically and the alignment (c) axis oriented horizontally. Warming or rotation of the sample allows one to separate effects that mimic the sought-after time reversal violating term.     

The characteristic X-ray spectra of free atoms of metals

The article presents the results of a study of characteristic X-ray spectra of free atoms by means of a new simple technique. A pulsed electron beam was used for evaporation and to create inner-shell vacancies in free atoms of metals. The spectra were obtained with the help of an X-ray monochromator which allowed precise comparison between the free-atoms spectra and corresponding solid-state spectra. The shifts of the peaks were measured and found to be in the range Δλ/λ≈10⁻⁵–10⁻⁴. The K-, L- and M-series spectra were studied. A number of the free-atoms spectra revealed structure which was not resolved in the solid-state spectra and which is of interest for atomic structure calculation applications. This electron beam technique for the investigation of X-ray characteristic spectra of free atoms can be used for the refinement of X-ray wavelength standards.     

Measurement and simulation of specific heat for metastable liquid Ni80Fe10Cu10 alloy

The specific heat of undercooled liquid Ni₈₀Fe₁₀Cu₁₀ alloy was experimentally measured by electromagnetic levitation drop calorimeter, and also numerically simulated by the molecular dynamics method. The achieved maximum undercooling is up to 252 K (0.15 T _L) in the experiments, and the measured result is 41.67 J mol⁻¹ K⁻¹. The simulation provides calculated data within 0∼702 K undercooling range, which is much broader than the experimental regime. The simulated value is 37.02 J mol⁻¹ K⁻¹. Although there exists a difference of 4.65 J mol⁻¹ K⁻¹ between them, the result is quite acceptable for simulation. Furthermore, the liquid structure of undercooled Ni₈₀Fe₁₀Cu₁₀ alloy is studied in terms of the total and partial pair distribution functions, which display that the ordered degree of atoms enhances from a normal liquid to metastable state.     

Polarization of leading Λ hyperons produced by neutrons on target nuclei

The polarization of the leading Λ hyperons produced on carbon and lead target nuclei by 4–10 GeV neutrons in the angle range Θ<8.5° with respect to the beam and with only neutral-particle accompaniment is measured: 〈Π〉=−0.096±0.018 for C and 〈Π〉=− 0.128±0.047 for Pb. The dependence of the polarization on the transverse momentum and the Feynman variable is measured. The normalized invariant cross section as a function of p _⊥ ² is found to be approximated by the function A exp(−Bp _⊥ ² ), where the parameter B is independent of the kind of nucleus (B=8.71±0.09 (GeV/c)⁻² for carbon and B=8.83±0.18 (GeV/c)⁻² for lead). Pis’ma Zh. éksp. Teor. Fiz. 64, No. 4, 237–240 (25 August 1996)     

Collisions of alkali K and Rb atoms in ground state: Modification of interaction potentials

The interaction of alkali K and Rb atoms that reside in the ground state is considered in the range of collision energies E = 10⁻⁴ to 10⁻² au. The singlet (X ¹Σ⁺) and triplet (a ³Σ⁺) interaction potentials available in the literature are analyzed and modified. For the KRb dimer in the range of interatomic distances 15–21a ₀, we chose analytical representations of the singlet and triplet potentials that more accurately describe the interaction of alkali Rb and K atoms in the ground state. Complex cross sections of the spin exchange are calculated for the first time that permit one to calculate the processes of polarization transfer and relaxation times, as well as shifts in the magnetic resonance frequencies caused by K-Rb spin exchange collisions.     

Monitoring of the thermal neutron flux in the EDELWEISS II dark matter direct search experiment

The results from measurements of thermal neutron flux in the EDELWEISS II experiment aimed at the direct detection of WIMPs (weakly interacting massive particles) by means of cryogenic germanium bolometers are described. Detailed knowledge of the neutron background is of crucial importance for the experiment, since neutrons with the MeV energy range of scattering seem to be hard to distinguish from the expected WIMP signal within the bolometers. Monitoring of the thermal neutron flux is performed using a mobile detection system with a low background proportional ³He counter. The neutron flux measurements were performed both outside and inside the device’s shielding, in the direct proximity of a cryostat with built-in germanium detectors. The sensitivity of the created thermal neutron detection system is on the level of 10⁻⁹ neutron (cm² s)⁻¹.     

The evolution of roughness of supersmooth surfaces by ion-beam etching

The behavior of roughness on surfaces of Cr/Sc multilayer structure, crystalline silicon [100], and fused quartz upon ion beam etching in the region of middle and high frequencies of the spatial spectrum (10⁻²–10² μm⁻¹) is studied. The possibility of keeping surface roughness at level σ ∼ 0.3 nm upon etching by Ar atoms/ions to depths of up to 10 μm is demonstrated.     

Simulation on intensity self-pulsation in cw semiconductor lasers

Intensity self-pulsation in cw semiconductor lasers has been analyzed by computer simulation assuming saturable absorbing characteristics in the loss term of the rate equation. A loss decrease of about 1% due to the saturable absorber is capable of generating self-pulsation. The theoretical pumping dependence of the self-pulsation frequency is consistent with the experimental data. Reflected light in the range of 10⁻⁵∼10⁻³ reduces the self-pulsation. Reflected light of larger than 10⁻², however, induces self pulsation whose frequency is locked with the round trip time. The contribution of spontaneous emission in the range of 10⁻⁴∼10⁻³ also reduces the self-pulsation, while 10⁻⁶∼10⁻⁵ is ineffective.     

Analysis of formation of primary radiation-induced point defects in graphite under action of secondary neutrons and π<Superscript>−</Superscript>-mesons generated in various channels of nuclear reactions upon graphite irradiation by 450-GeV protons

A theoretical model is constructed and the rates of formation are calculated for primary radiation-induced point defects in graphite by secondary neutrons and π⁻-mesons generated in various nuclear reaction channels upon irradiation of graphite by a 450-GeV proton beam. Calculations are performed using the ENDF-VII nuclear data base for energies of secondary particles lower than 150 MeV. It is shown that neutrons produce the highest generation rate of point radiation-induced defects in graphite (the rate of formation of defects under the action of neutrons is an order of magnitude higher than the corresponding value for π⁻-mesons).     

Neutron emission under particular nonequilibrium conditions from Pd and Ti electrolytically charged with deuterium

Summary We report on neutron emission in palladium and titanium electrolitically charged with deuterium. The detection of neutrons is observed after thermal treatment of the electrode. In the hypothesis that neutrons came from cold fusion processes, we estimate a fusion rate as high as 1.3·10⁻²¹ fusions/deuteron pair/second.     

Mixing of neutral atoms and lepton number oscillations

We discuss oscillations of two neutral atoms which proceed with the violation of lepton number. One of the neutral atoms is stable, the other one represents a quasistationary state subjected to electromagnetic deexcitation. The system of neutral atoms exhibits oscillations similar to those of the system of neutral kaons and neutron-antineutron oscillations in the nuclear medium. The underlying mechanism is a transition of two protons and two bound electrons to two neutrons p + p + e _b ⁻ + e _b ⁻ ↔ n + n. A signature of the oscillations might be an electromagnetic deexcitation of the involved unstable nucleus and atomic shell with the electron holes. A resonant enhancement of the neutrinoless double electron capture takes place when the atomic masses tend to be degenerate. Qualitative estimates show that in searches for lepton number violation oscillations of atoms might be a possible alternative to the conventional mechanism of the neutrinoless double β decay process with emission of two electrons. The article is published in the original.     

Experimental determination of the rate constant for spin exchange in collisions of polarized metastable helium atoms with ground-state cesium atoms

The rate constant for spin exchange in a system consisting of a metastable helium atom and an alkali-metal atom is determined. An experiment on optical orientation of atoms established that the rate constant for spin exchange in a collision of a metastable 2³ S ₁ helium atom with a cesium atom in the 6² S _1/2 ground state equals (2.8±0.8)×10⁻⁹ cm³ s⁻¹. The rate constant for chemoionization of cesium atoms by metastable helium atoms was determined at the same time to be (1.0±0.3)×10⁹ cm³s⁻¹. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 3, 145–148 (10 August 1997)     

π−-meson and nucleon yields from light targets irradiated by deuterium and tritium nuclei beams at energies of 1 GeV/nucleon

In this paper, we present a model of calculation with respect to the interactions of high-energy nuclei with matter. Based on this model, we obtain results on energy and angular spectra of the n- and π⁻-particles produced in collisions of deuterium and tritium nuclei at energiesT _d=1 GeV/nucleon with light targets such as Li, Be. We have also estimated the production yields of neutrons and π⁻-mesons in targets of various radii, as well as mean energies of these these particles. Summarizing, we find that the lithium target of radiusR=10−12 cm for which the energy cost ε_π to produce one π⁻-meson is estimated as 6.7 GeV/π for a d-beam and 5.3 GeV/π for a t-beam is the most preferred pion-production target.