Optimised adiabatic fast passage spin flipping for He neutron spin filters

We describe here a method of performing adiabatic fast passage (AFP) spin flipping of polarized ³He used as a neutron spin filter (NSF) to polarize neutron beams. By reversing the spin states of the ³He nuclei the polarization of a neutron beam can be efficiently reversed allowing for the transmission of a neutron beam polarized in either spin state. Using an amplitude modulated frequency sweep lasting 500 ms we can spin flip a polarized ³He neutron spin filter with only 1.8×10⁻⁵ loss in ³He polarization. The small magnetic fields (10-15 G) used to house neutron spin filters mean the ³He resonant frequencies are low enough to be generated using a computer with a digital I/O card. The versatility of this systems allows AFP to be performed on any beamline or in any laboratory using ³He neutron spin filters and polarization losses can be minimised by adjusting sweep parameters.     

Anisotropy of the electric susceptibility in superfluid 3He A

A technique is proposed for determining the orientation of the orbital anisotropy axis in ³He A. It relies upon the anisotropy of the dielectric tensor which is estimated to be 3×10^?7 (1?T/T_c) in ³He A.     

A high-pressure polarized <Superscript>3</Superscript>He gas target for nuclear-physics experiments using a polarized photon beam

Following the first experiment on three-body photodisintegration of polarized ³He utilizing circularly polarized photons from High-Intensity Gamma Source (HIg \gamma S) at Duke Free Electron Laser Laboratory (DFELL), a new high-pressure polarized ³He target cell made of pyrex glass coated with a thin layer of sol-gel doped with aluminum nitrate nonahydrate has been built in order to reduce the photon beam-induced background. The target is based on the technique of spin exchange optical pumping of hybrid rubidium and potassium and the highest polarization achieved is ∼ 62% determined from both NMR-AFP and EPR polarimetries. The phenomenological parameter that reflects the additional unknown spin relaxation processes, X , is estimated to be ∼ 0.10 and the performance of the target is in good agreement with theoretical predictions. We also present beam test results from this new target cell and the comparison with the GE180 ³He target cell used previously at HIg \gamma S. This is the first time that the sol-gel coating technique has been used in a polarized ³He target for nuclear-physics experiments.     

Preparation of isotopically pure superfluid 4He suitable for constructing a high density neutron source

We have developed an isotopic purification technique by which the ⁴He/³He ratio may be raised beyond 2 × 10¹⁵. This purity is sufficient for an experimental realisation of the neutron bottle recently proposed by Golub and Pendlebury.     

Large nonlinear optical properties of Fe2O3 nanoparticles

Nonlinear optical properties of Fe₂O₃ nanoparticles were investigated by the signal-beam Z-scan technique with Ar⁺ and Ne–He lasers. The largest reported effective nonlinear coefficient, n₂=−8.07×10⁻⁷ cm²/W, was obtained. It is demonstrated that the nonlinear optical response originals from quantum confinement effect.     

Cold and ultracold collisions of MgH (<Superscript>2</Superscript>Σ<Superscript>+</Superscript>) with helium

The cross sections for rotational, fine-structure and projection-change transitions in collision of MgH (²Σ⁺) molecule with ³He and ⁴He atoms were computed at cold and ultracold temperatures using the ab initio potential energy surface. The significant suppression feature and resonance pattern caused by spin-rotation interaction were found and discussed. The collisions exhibit a strong isotope effect in the ultracold regime. The rotational relaxations of the fine-structure excited energy levels tendentiously preserve the F index. The ratio of the cross sections for elastic and spin-flipping collision with ³He partner is always greater than seven orders of magnitude for the energy range 10^-6−1cm^-1 and suggests the MgH molecule is a good candidate for ³He buffer gas cooling.     

Basic connection between superconductivity and superfluidity

A basic and inherently simple connection is shown to exist between superconductivity and superfluidity. It is shown that the author's previously derived general equation, which agrees well with the superconducting transition temperatures for the heavy-electron superconductors, metallic superconductors, oxide superconductors, metallic hydrogen, and neutron stars, also works well for the superfluid transition temperature of 2.6 mK for liquid³He. Reasonable estimates are made from 10^–3 to 10⁹ K — a range of 12 orders of magnitude. The same paradigm applies to the superfluid transition temperature of liquid⁴He, but results in a slightly different equation. The superfluid transition temperature for dilute solutions of³He in superfluid⁴He is estimated to be l–10K. This paradigm works well in detail for metallic, cuprate, and organic superconductors.     

Coulomb predissociation rates of He dµ molecular ions

The predissociation or particle emitting decay rates of muonic molecular ions³He dµ and⁴He dµ are estimated by means of the complex rotation method. We use a highly accurate variational three-bodyP-wave function based on random-tempered Slater-type exponential expansion. A resonance eigenvalue ofE=–48.420 89 eV and =0.348 × 10^–3 eV for the³He dµ ion and a resonance eigenvalue ofE=–58.225 303 eV and =0.118 × 10^–3 eV for the⁴He dµ ion have been obtained. These results are in a good agreement with the ones previously obtained by Kino and Kamimura by scattering calculation with the non-adiabatic coupled-rearrangement-channel method.     

Kinetics of the sorption of <Superscript>3</Superscript>He by C<Subscript>60</Subscript> fullerite. The quantum diffusion of <Superscript>3</Superscript>He and <Superscript>4</Superscript>He in fullerite

The kinetics of the sorption and subsequent desorption of gaseous ³He in a C₆₀ fullerite powder has been studied in the temperature range of 2–292 K. The temperature dependences of the diffusion coefficients of ³He and ⁴He impurities in fullerite have been plotted using the measured characteristic times of filling of octahedral and tetrahedral interstices, as well as previous data. These temperature dependences of the diffusion coefficients of ³He and ⁴He impurities in fullerite are qualitatively similar. A decrease in the temperature from 292 to 79 K is accompanied by a decrease in the diffusion coefficients, which corresponds to the dominance of the thermally activated diffusion of helium isotopes in fullerite. A further decrease in the temperature to 8–10 K leads to an increase in the diffusion coefficients by more than an order of magnitude. The diffusion coefficients of ³He and ⁴He are independent of the temperature below 8 K, indicating the tunnel character of the diffusion of helium in C₆₀ fullerite. The isotope effect is manifested in the difference between the absolute values of the diffusion coefficients of ³He and ⁴He atoms at the same temperatures.     

Angular distribution measurements of Li(p,α)He reaction at 140 keV proton energy using nuclear track detectors

Angular distributions of a ⁶Li(p,α) ³He reaction were measured at six angles for 140 keV proton energy using nuclear track detectors (NTDs). The measurements were carried out over 60°–160° lab. angles in 20° increments using a scattering chamber of 80° beam line of the 350 kV accelerator. A semiconductor silicon surface barrier (SSB) detector was placed at +160° and was used as a monitor. The results have shown that the CR-39 detector has excellent capabilities to distinguish 1.4–2.7 MeV α+ ³He particles from the ⁶Li(p,α) ³He reaction and 8–9.4 MeV α-particles from the ⁷Li(p,α) ⁴He reaction through their track diameters. However, it was not possible to distinguish between the 2.3 MeV ³He ions and the 1.7 MeV ⁴He ions from the ⁶Li(p,α) ³He reaction from their track diameter measurements, but it was possible to differentiate between the two, from the darker contrast of the ³He particles caused by its deeper tracks as compared to those of ⁴He.     

The He scattering and reactions on C and cluster states of C

The ⁶He+¹²C elastic and inelastic scattering and the ⁶He+¹²C→α+¹⁴C reaction have been measured using a 18.0 MeV ⁶He beam. Experimental results for the elastic scattering are in fair agreement with optical model predictions, using the potentials found in the analysis of ⁶Li scattering on ¹²C at similar energies. In triple coincidences, the ⁶He+¹²C→¹⁰Be+2α reactions were clearly seen, with the ¹⁰Be nucleus left in ground and several excited states. The dominant mechanism of this reaction is sequential decay through cluster states of ¹⁴C.     

Quest for the 10He nucleus

The spectrum of ¹⁰He was studied by means of the ³H(⁸He, p)¹⁰He reaction at a laboratory energy of 25 MeV/A and small center-of-mass angles. Missing mass spectrum of ¹⁰He was derived from the obtained p-⁸He coincidence. A resolution of 0.7 MeV was achieved in this spectrum for the measured ¹⁰He energy. Most likely, a well isolated group of 10 events detected between 2 and 5 MeV and showing a maximum at about 3 MeV in the spectrum of the present work exhibits the ¹⁰He g.s. resonance.     

Neutron excess nuclei of hydrogen and helium at ACCULINNA

Attempt to observe a ⁷H resonance produced in the reaction ²H(⁸He,³He)⁷H resulted only in setting a limit dσ/dΩ≤ 20 μb/sr for the reaction exit channel which could populate a resonance lying between 0 and 3 MeV above the ⁷H decay threshold. The quasi-free scattering of the α core bound in ⁶He was explored keeping in mind the possible study of the cluster structure of this halo nucleus. For the first time coincident particles emitted in the ⁴He(⁶He, 2α)nn reaction were detected in wide angular ranges giving a wide kinematical range of the measured angular and momentum distributions. The contribution of processes, competing with QFS in the α+ α+n+n output channel, was considerably suppressed by the selection of events with E_α1(2)-nn>10 MeV. A number of experimental distributions, relevant to the reaction mechanism and to the ⁶He structure, were compared with the results of MC simulations based on the PWIA formalism. The PWIA predictions showed consistency with the experimental data.     

Measurement of the breakup channels in nuclear muon capture by3He and4He

The rates of the breakup reactions _B(³He) and _B(⁴He) in nuclear muon capture by³He and⁴He have been measured. The measurements were performed on the muon beam of PSI with a multi-anode high-pressure ionization chamber filled with isotopically pure³He or⁴He at 120 bar. The chamber was used as a target and detector for both the incoming muons and for the charged reaction products (tritons, deuterons and protons) of the breakup reactions _B(³He) and _B(⁴He). Our statistics, about 5 × 10⁵ of breakup events, provides a possibility to improve the precision of evaluated rates. The preliminary results: _B(³He)=720 ± 70 s^–1, _B(⁴He)=415 ± 40 s^–1 are compared to theoretical predictions and to previous experiments.     

Defect-related light emission from processed He-implanted silicon

Photoluminescence (PL) from He⁺-implanted Si (Si:He, He⁺ dose—2×10¹⁶ cm⁻², at 150 keV) is related to its microstructure; it has been tuned by processing at 720-1400 K under hydrostatic Ar pressure (HP, up to 1.2 GPa). Processing of Si:He at 720 K for 10 h results in an appearance of the D2 and D3 dislocation-related PL lines, these last of the highest intensity. Only the D1 dislocation-related line of intensity increasing with HP has been detected after processing at 920-1070 K. The D1 (of the highest intensity), D2 and D3 PL lines are observed after the treatment at 1270 K. No dislocation-related PL has been detected for Si:He processed at 1400 K. The treatment of Si:He at 720-1270 K under HP makes it possible to produce Si:He of specific microstructure resulting in strong PL at 0.81, 0.87 or 0.94 eV.     

The density and pressure of helium in bubbles in metals

Abstract

The energy shift of the He 1¹S₀?2¹P₁ transition, ΔE(n), can be used to determine the density, n, of He in bubbles in metals. A self-consistent band structure calculation for solid fcc He yields a linear relationship ΔE=C.n with C _th=22 × 10^?3 eV nm³. Systematic electron energy loss spectroscopy and transmission electron microscopy studies of He bubbles in Al for various He doses and temperatures result in C_exp=(24±8).10^?3 eV nm³ in agreement with theory. The analysis is consistent with the assumption that dislocation loop punching is the dominant bubble growth mechanism during high-dose room temperature implantation. The application to He bubbles in Ni indicates a maximum He density of n=0.2 × 10³ nm^?3 for which He should be solid at room temperature.     

Transfer, sequential decay, and quasi-free reactions induced by 18-MeV 6He beam on 6Li, 7Li, and 12C

The ⁶He + ^6,7Li and ⁶He + ¹²C scattering and reactions have been studied using an 18-MeV ⁶He beam. Experimental results for the elastic scattering on all three targets are in fair agreement with optical model predictions, using the potentials found in the analysis of the ⁶Li scattering on the same targets and at close beam energies. Several two-body exit channels show clear signatures of a direct reaction mechanism allowing extraction of spectroscopic information. The measured angular distribution for the ⁶He + ⁶Li → α + ⁸Li reaction indicated close similarity between the α + 2n configuration in ⁶He and the α + d configuration in ⁶Li. The obtained results for α-particle pickup from both ⁶Li and ⁷Li give large values of α spectroscopic factors for some ¹⁰Be states, indicating their well-developed α + ⁶He cluster structure. The exotic two-proton pickup reaction (⁶He, ⁸Be) was studied, as well as two-neutron and triton transfer reactions. Quasi-free scattering of ⁶He on deuteron and α particle in ⁶Li was also observed. The sequential decay reactions ⁶He + ⁶Li → ⁶He + α + d, ⁶He + ⁶Li → 2α + t + n, ⁶He + ⁷Li → ⁶He + α + t, and ⁶He + ¹²C → ¹⁰Be + 2α were clearly seen, and α clustering of some states in ^6,7Li, ^8–10Be, and ¹⁴C was thus studied. Several new spectroscopic results obtained for some ¹⁰Be states support the existence of a molecule-like rotational band in ¹⁰Be with a very large moment of inertia. The text was submitted by the authors in English.     

Absorption,emission and lifetime data on Cu+ in some alkali halides

The lifetime of the 3d⁹4s→3d¹⁰ transition of Cu⁺ in several alkali halides has been measured as a function of temperature from L.He.T. (?8 K) to R.T. Original instrumentation was developed allowing the automatic continuous recording, as a function of temperature, of single decay time constants even if varying at a rather high rate. Accurate fixed-point measurements by the single photon technique were also performed at R.T., L.N.T. and L.He.T. Two regions of variation of τ versus T have been observed. The interpretation which seems more plausible is that the Cu⁺ ion in its relaxed excited state is essentially in-center in the lower temperature region while for T >40 K it goes off-center even if not so strongly as in the ground state. In NaCl the behaviour of Cu⁺ is quite similar with the difference that in the ground state it is in-center over the whole temperature range from L.He.T. to R.T.     

Stopping power of low-energy deuterons in 3He gas

The stopping power of atomic and molecular deuterons in ³He gas was measured over the range E _d = 10 to 100 keV using the ³He pressure dependence of the ³He(d,p) ⁴He reaction yield. At energies above 30 keV, the observed stopping power values are in good agreement with a standard compilation. However, near 18 keV the experimental values drop by a factor 50 below the extrapolated values of the compilation. In a simple model, the behavior is due to the minimum 1s↦2s electron excitation of the He target atoms (= 19.8 eV, corresponding to E _d = 18.2 keV), i.e. it is a quantum effect, by which the atoms become nearly transparent for the ions.     

Relaxation of spin polarized 3He by magnetized ferromagnetic contaminants

In the first in a series of three papers on wall relaxation of spin polarized ³He we have reported on a breakdown of relaxation times which is observed after exposing the ³He containing glass cells to a strong magnetizing field. In this third paper we give a quantitative analysis of this phenomenon, based on magnetic signal detection by means of SQUIDs, on the pressure dependence of relaxation times in magnetized cells, as well as on Monte Carlo simulations of ³He-relaxation in a macroscopic dipole field. Our analysis allows to identify the contaminants as being aggregates of dust-like Fe₃O₄ particles (magnetite) with a radius R ? 10 mR \approx 10~\mu m and a remanent magnetic moment of the order of m ≈O(10 ^-10 ^{-10}~ A m²). The particles are located at or close to the inner glass surface.