3He Polarization Research Articles

NMR calibration of optical measurement of nuclear polarization in 3He.

We have performed precise NMR measurements in order to calibrate a standard optical technique for measuring the polarization of $^{3}\mathrm{He}$ nuclei. This optical technique requires knowledge of the pressure-dependent relationship between the nuclear polarization and the degree of circular polarization of the 668-nm light emitted from a $^{3}\mathrm{He}$ discharge. We determined the nuclear polarization by comparing the adiabatic fast-passage NMR signal from samples of polarized $^{3}\mathrm{He}$ at pressures between 0.1 and 5 torr to an identical sample of water. For water, the polarization is known from statistical mechanics. The $^{3}\mathrm{He}$ is polarized using the metastability exchange technique for optical pumping of $^{3}\mathrm{He}$. The accuracy of the calibration is \ifmmode\pm\else\textpm\fi{}2%, which is required for applications in nuclear physics.

First test at ILL of a gaseous 3He neutron polarization filter

Polarized 3He, a promising method for the filtering of the neutron spin, is becoming a reality owing to the development of optical pumping methods and new solid state lasers. We have used a fully polarized neutron beam of 0.84Å wavelength to monitor the 3He polarization in a Rb/ 3He gas mixture pumped with a titanium-sapphire laser.

A high power lamp-pumped LNA laser with thermally tuned etalon

This report describes arc-lamp pumped, tunable, continuous-wave LNA laser with improved output efficiency, suitable for optical pumping of helium. Using thermal tuning of intracavity etalons instead of angle tuning reduces losses caused by walk-off effects and provides continuous frequency scanning without a significant decrease in laser power. We obtain an output power of 4 W in a linewidth of 2.5 GHz with the laser wavelength tuned to the 1.083 μm absorption line of metastable helium. This result represents a fourfold gain in efficiency compared with conventional tuning by tilting. The laser provides efficient polarization of helium by optical pumping in various applications, such as nuclear spin polarization of 3He and spin-polarized electron sources.

Neutron polarization with a polarized 3He spin filter

We report the first use of a polarized 3He spin filter to polarize epithermal neutrons. The 3He was polarized to 70% by spin exchange with optically pumped Rb vapor and had a cross sectional area of 0.65 cm 2 and a thickness of 3 × 10 20 atoms cm −2 of 3He. Neutron polarization up to 20% at 0.734 eV was produced in an epithermal neutron beam at the Los Alamos Neutron Scattering Center and measured by observing the change in neutron transmission produced by the 3He polarization and also the helicity dependent transmission for a parity-nonconserving resonance in 139La.

Neutron polarization with polarized 3He

An effective polarizer for thermal and epithermal neutrons is possible with polarized, high density, gaseous 3He. We describe the technique of 3He polarization by spin exchange with laser optically pumped Rb vapor which can provide 6 cm 3of > 70% polarized 3He at a density of 3×10 20 atoms cm −3 (10 atm at STP). The 3He polarization can be rapidly reversed, a requirement for sensitive symmetry tests.

Nuclear spin polarization of 3He in 23Na(5s) 3He spin-exchange collisions

The formation of bound, excited sΣ states of alkali- 3He and alkali- 21Ne molecules leads, in spin-exchange collisions, to significant enhancement of the nuclear spin polarization of gaseous 3He and 21Ne compared to that produced in ground state collisions. Calculations for Na(5s)- 3He spin-exchange collisions are presented.

Surface Transfer of Polarization: A New Method of Polarization of Liquid 3He

We have performed a new method of production of enhanced liquid 3He nuclear magnetization. Liquid is confined by fluorocarbon microspheres. Magnetization of nuclear spins in the substrate(19F,S=½) is enhanced by Dynamic Nuclear Polarization. At the surface of the substrate magnetization interaction between 19F and 3He spins transmits 19F polarization to 3He solid-like surface layer. Since magnetization transfer between solid 3He and liquid 3He is much faster than all relaxation processes, the magnetization enhancement is transmitted to liquid 3He. Peculiar consequence of fast magnetization transfer between the two 3He phases will be also discussed: a sudden lattice warming produces an increase of liquid magnetization.

The Polarization of Liquid 3He by Fast Melting of Polarized Solid: Present State of the Art

3He is a fermion, with a very small (nuclear) magnetic moment of 0.8 mK/Tesla. Because of its Pauli-spin susceptibility at T ≪ TF, large polarizations by brute-force would require magnetic fields of several hundred Tesla, which is impossible at present. In the last years two methods have been developed which allow large liquid polarization. The first used by the group of the Ecole Normale of Paris uses a laser to polarize the gas phase at room temperature followed by condensation. With this method liquid polarizations larger than 40% at 400 mK have been obtained and the results will be discussed by M. Leduc and co-workers. The second method which will be the subject of this paper is presently being used in 7 or 8 laboratories and was pioneered in Grenoble and Kopenhagen. It consists in quickly melting the highly polarized solid phase which, being Curie-like can be magnetized to more than 80% at a few mK in presently attainable fields. In this way liquid has been produced with polarization up to 60%, while the final temperatures range from 300 mK to 25 mK depending on the polarization. Relaxation times of 20 minutes have been obtained and several experiments are being carried out concerning viscosity, sound velocity, lowering of the melting curve, relaxation time etc. which will give important information on the nature of liquid 3He and the 3He-3He interaction.

Dynamic nuclear polarization of liquid 3He in powered charcoal.

By inducing electronic spin flips in a powdered sucrose charcoal immersed in liquid $^{3}\mathrm{He}$, we have produced a steady-state enhancement of the nuclear polarization in the liquid. In a magnetic field of 182 G and at a temperature of 1.8 K, the maximum positively enhanced polarization was 18% greater than the equilibrium value. Magnetic relaxation times of the liquid are unaffected by the enhancement. The results demonstrate the existence of a magnetic coupling between the liquid $^{3}\mathrm{He}$ and the charcoal substrate, similar to that found by other investigators for fluorocarbon substrates.

Optical nuclear polarization — Targets and sources

The nuclear polarization of3He by optical pumping using discharge lamps is described, as well as attempts (both successful and unsuccessful) to make a polarized3He target. Recent experimental results with a polarized3He target are presented. Experiments to polarize3He using a laser as a pumping light source are described, and the prospects for producing a dense polarized3He target are indicated. The polarization of other nuclei, especially Xe and Kr, by spin transfer from laser polarized alkalis is described.

The analyzing power Ay[( θ) for 12C(n, n 0,1) 12C between 8.9 and] 14.9 MeV neutron energy

The analyzing power A y( θ) for 12C(n,n) 12C elastic scattering and for inelastic scattering to the first excited state ( J π = 2 +, Q = −4.44 MeV) of 12C was measured in the energy range from 8.9 to 14.9 MeV in 1 MeV steps. A pulsed polarized neutron beam was produced via the su2rmH( d , n ) 3He polarization transfer reaction. Monte Carlo simulations were used to correct the data for finite geometry and multiple scattering effects. The A y data, together with published crosssection data, were analyzed in the framework of the spherical optical model and in the coupled-channels formalism. A good description of the data has been achieved.

Phase shift analysis of 3He(p, p) 3He elastic scattering

A phase shift analysis has been performed using all data available for 3He + p elastic scattering between 2.3 and 13.6 MeV. Recent 3He polarization data have been instrumental in decreasing the uncertainty of the phases. In addition a new set of phase shifts was found at all energies. Various solutions were used to predict those regions of angle and energy where efficiency correlation measurements are particularly sensitive to changes in the phases.

Scattering of polarised and unpolarised 3He by 3H and high excitation in 6Li

Angular distributions of the 3He polarization analysing power of the elastic scattering of 3He by 3H have been measured at 33.3, 29.9, 27.7, 25.3,23.5 and 19.9 MeV. Differential cross section angular distributions were also measured at 33.3, 29.9, 25.3 and 23.5 MeV, as well as excitation functions of both cross section and 3He polarization over the energy range from 20 to 33 MeV. The data have been analysed in terms of phase shifts. The presence of broad P- and F-levels in 6Li with ( 3He + 3H) structure predicted by the resonating group theory has been confirmed.

Scattering of 33 MeV polarized3He particles by hydrogen and helium isotopes

Angular distributions of the 3He polarization analysing power were measured for elastic scattering of a 33 MeV polarized 3He beam by 4He, 3He, 3H, 2H and 1H targets. The analysing power data together with the differential cross sections were successfully interpreted in terms of a standard optical model. Two discrete sets of parameters were found for each nucleus. Some features of the optical-model phase-shifts are discussed and compared with other results.

Equality of Analyzing Power and Polarization in the ReactionH3(p,n)3He

The quantities A/sub y/ and P/sub y/ were remeasured for E/sub p/<4 MeV in the reaction /sup 3/H(p,n)/sup 3/He. Although our A/sub y/ data confirm previous data, our P/sub y/ values are appreciably larger than earlier results and in fact agree well with those for A/sub y/. Elimination of the previously reported A/sub y/-P/sub y/ difference has important consequences. Charge-symmetry-breaking effects must be small or nonexistent in this reaction; and the previously required f-wave admixture to the lowest J/sup pi/=2/sup -/ state of /sup 4/He is no longer necessary. (AIP)

Polarization effects in ( 3He, p) reactions on 3He and 7Li at 14 MeV

Angular distributions of the proton polarization and differential cross section of the ( 3He, p) reaction leading to the ground states of the nuclei 5Li and 9Be were measured at a 3He incident energy of 14 MeV. Sizeable polarization values were observed in both the cases. The experimental results are compared with a standard two-nucleon transfer DWBA theory. The effects of spin-orbit distortions in the 3He and proton channel are discussed.

The elastic scattering of 33 MeV polarized 3He particles by beryllium and carbon

Angular distributions of 3He polarization in elastic scattering from 9Be and 12C, and in inelastic scattering to the 4.44 MeV state of 12C were measured using a 33 MeV polarized 3He beam. The polarization and cross-section data were successfully analysed in terms of the standard optical model. Selectively of the 3He elastic polarization for various potential sets was established in both optical model and coupled-channel analyses. While it is possible to determine unambiguously the geometry of the spin-orbit term, its strength is strongly correlated with other optical model parameters.

Scattering of 31.5 MeV polarized 3He by protons

The angular distribution of 3He polarization was measured in the scattering of a polarized 3He beam by protons at a mid-target energy of 31.5 MeV. The results together with the available cross section and proton polarization data were analysed in terms of phase shifts and mixing parameters. The measured left-right asymmetry of recoil protons was used in the design of a 3He polarimeter.

Polarization of 3He elastically scattered from 12C at 28 MeV and the spin-orbit coupling

The polarization angular distribution of 3He elastically scattered from 12C at 28 MeV has been measured using the double scattering method. The 3He polarization reached −28 % at θc.m. = 47°. An optical model analysis of the polarization and differential cross section requires a spin-orbit potential of ≈ 4 MeV to fit the data over the angular range of 12.5° c.m. to 72.5° c.m. This spin-orbit potential is higher than the theoretical prediction for simple constituent-nucleon coupling.

Measurement of the proton polarization in the 3He(d, p) 4He Rection between 2.0 and 6.0 MeV

Angular distributions of the polarization of 3He(d, p) 4He protons have been measured for unpolarized deuterons with laboratory energies of 1.99, 2.81, 3.94 and 6.00 MeV. The polarizations were determined from the left-right asymmetry of elastic scattering through 60° in 4He gas using a vaned polarimeter whose analyzing power was computed by trajectory tracing from the known p-α polarizations. A contour diagram of proton polarization for deuteron energies of 1 to 12 MeV is presented. A Legendre polynomial expansion of four or five terms has been fitted to the products p( θ) σ( θ). The coefficients of the expansions of these and other measurements show resonance-like behaviour at 5Li excitations of 20.0 and 20.9 MeV. The vector-polarized beam and polarized-target analyzing powers are compared with the proton polarization. The proton polarizations are in good agreement with the 3H(d, n) 4He neutron polarizations when compared at the same entrance-channel energy but disagree when compared at equal exit-channel energies or compound-nucleus excitations.