Large-space cluster model calculations for the 3He(3He,2p)4He and 3H(3H,2n)4He reactions

The ³He(³He,2p)⁴He and ³H(³H,2n)su4He reactions are studied in a microscopic cluster model. We search for resonances in the ³He+³He and ⁴He + p + p channels using methods that treat the two- and three-body resonance asymptotics correctly. Our results show that the existence of a low-energy resonance or virtual state, which could influence the ⁷Be and ⁸B solar neutrino fluxes, is rather unlikely. Our calculated ³He(³He,2p)⁴He and ³H(³H,2n)⁴He cross sections are in a good general agreement with the experimental data.     

Scattering of He and He from polarized He between 7 and 18 MeV

Polarization analyzing power of ³He³He and ³He⁴He elastic scattering is reported for bombarding energies between 7.5 and 17.9 MeV. A phase-shift analysis for ³He⁴He elastic scattering incorporating this and other polarization data is presented. The analyzing power of ³He³He elastic scattering is consistent with zero at θ_c.m. = 66°.     

On the quark structure of the lightest nuclei

It is shown that in the 9q- and 12q-systems, i.e.,³He-³He and⁴He nuclei, the most energetically favourable colour magnetic types of symmetry are [f]_cs=[621] and [f]_cs=[82²], which corresponds to the orbital configurationsS ⁵ P ⁴ [52²] _X andS ⁶ P ⁶ [62³] _X . The colour magnetic nucleon attraction associated with these configurations increases as the square of the nucleon number in the sequence²H,³He,⁴He, becoming quite strong in⁴He. This may provide a basis for explaining the “mysterious properties” of³He-³He and⁴He. However, with the further increase in A, the above effect drastically decreases. A qualitative calculation on the quark excitation spectrum for the³H-³He and⁴He nuclei is presented. The⁴He nucleus seems to possess some properties of a “quark magic nucleus”. The experimental situation is discussed.     

Studying the ejection of light charged particles induced by 50 MeV <Superscript>3</Superscript>He ions upon interacting with a <Superscript>112</Superscript>Sn nucleus

Experimental double-differential and integral spectra of (³He, xp), (³He, xd), (³He, xt), (³He, x³He) and (³He, xα) reactions on ¹¹²Sn nuclei induced by 50 MeV 3He ions are presented. Theoretical calculations of the experimental inclusive spectra of the reactions are performed using the exciton model of preequilibrium decay. The corresponding mechanisms of reactions are determined. The experimental results can be used to develop new approaches in the theory of nuclear reactions, and to design safe and wasteless hybrid nuclear power plants.     

A study of 3He-4He tunneling motion in solid 3He-4He mixtures by means of T1ϱ measurements

³He spin relaxation time in the rotating frame (T_1?) has been measured for 2% ³He in ⁴He solid samples of different molar volume at exchange plateau region (0.5 K). From the data, the characteristic frequency by which a ³He atom exchanges its position with a neighboring ⁴He atom is determined.     

Manifestation of dissipation and compressibility in the experiments on quantum cavitation

Cavitation in the liquid helium isotopes of ³He and ⁴He is considered. It is shown that the dissipative processes play an important role in the growth of the stable phase nucleus in the normal liquid ³He. This leads to the lack of the quantum behavior of cavitation in this system down to 2 mK, which is in contrast to the thermal-quantum crossover in the cavitation of the superfluid ⁴He at 600 mK. Below 180 mK, the dissipative ³He kinetics is of the Knudsen type. The high value (600 mK) for the transition into the quantum kinetic behavior in ⁴He is related to the compressibility of a liquid, which leads to a noticeable emission of sound with cavitation. The recent experiments on quantum cavitation in the liquid helium isotopes ³He and ⁴He are discussed.     

4He fragmentation induced by 46–400 GeV Protons

Inclusivep, d and³He cross sections have been obtained using experimental data on p⁴He collisions over an incident proton energy range of 46-400 GeV. Considering the slopes of spectra, the effective nuclear temperature has been found to be T₀=11 MeV, which is in good agreement with experimental results for lower energies and heavier targets. The shape of the³He inclusive spectrum suggests the existence of two different³He production mechanisms. The experimental³He cross section is compared with theoretical predictions assuming fragment formation as a result of the spectator and cluster knockout mechanisms. The⁴He→³He +n vertex constant has been found to be G²=10.9±0.2 fm.     

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.     

Atomic impact experiments with free helium-3 and helium-4 clusters

Free helium clusters with of the order of 10³ to 10⁷ atoms have been predicted to reach final temperatures of about 0.4 K in the case of⁴He, and of 0.15 K in the case of³He. In the first case, the clusters have to be expected to be superfluid, in the latter case to be normalfluid. Impact experiments with cesium atoms, which are calculated to be strongly repelled from inside liquid⁴He, and slightly from liquid³He, indicate cesium embedding into or onto⁴He clusters. Xenon atoms, calculated to be strongly bound to both quantum liquids, show incomplete momentum transfer to⁴He as well as to³He clusters at higher impact speeds.     

The low solar neutrino flux: A search for a resonance in 6Be

A study of neutron time-of-flight spectra from the ⁴He(³He, n)⁶ reaction at E_3He = 38.61 MeV, sets a one standard deviation upper limit of 7 μb/sr for the cross section to a possible narrow level near the ³He + ³He threshold.     

High temperature spontaneous ferromagnetism and other magnetic phenomena in liquid 3He and 3He4He dilute mixtures

Equilibrium ferromagnetic ordering in liquid ³He at temperatures T ? 0.3 K due to long-wavelength spin fluctuations is predicted. The magnetocaloric effect and other thermodynamic properties of ³He↑ are discussed. The properties of ³He↑⁴He solutions are considered in the case of a restricted geometry.     

Elastic scattering of 3He and 4He at incident energies near the coulomb barrier

The elastic scattering of ³He and ⁴He from Zr, Mo, Cd, and Te isotopes is studied at incident energies near the Coulomb barrier. Marked differences are observed between the excitation curves of ³He and ⁴He. These differences are shown to be due to a large surface absorption in the ³He scattering. A systematic study of the size parameters deduced from the present and other ⁴He experiments shows deviations from the $\text{A}\text{1}\text{3}$ law for nuclei near closed neutron shells.     

3He D-state effects in (d, 3He) reactions

The tensor analysing power of the ²⁷Al(d, ³He)²⁶Mg reaction has been measured at E_d = 12.4 MeV. The results are reproduced by predictions of the DWBA including D-state components in the ³He wave function. The magnitude of the tensor analysing power provides information on the asymptotic D-state to S-state ratio in the ³He wave function. The value obtained for ³He is in agreement with the corresponding value for ³H.     

Microscopic study of the low-energy3He(3He, 2p)4He and3H(3H, 2n)4He fusion cross sections

We have calculated the³He(³He, 2p)⁴He and³H(³H, 2n)⁴He reaction cross sections at low energies within the microscopic multichannel resonating group method. For both reactions, we find good agreement with experiment. For the³H(³H, 2n)⁴He reaction, our calculated energy dependence reproduces that of each individual low-energy experimental data set, except for a normalization constant. Using this fact, we derive at a low-energy³H(³H, 2n)⁴He rate by taking the averaged mean of these fits.This work has been supported in part by the National Science Foundation, Grants PHY86-04197 and PHY88-17296.     

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.     

Superfluidity of 3He in dilute solutions of 3He in liquid 4He

The possibility of superfluidity of ³He in dilute solutions of ³He in liquid ⁴He is investigated, and the corresponding critical temperature is estimated. The results indicate S-state pairing, i.e., for small concentrations of ³He, the highest transition temperature is obtained for L = 0, but the obtained critical temperatures are very low.     

Assessing the use of 3H–3He dating to determine the subsurface transit time of cave drip waters

³H–³He measurements constitute a well-established method for the determination of the residence time of young groundwater. However, this method has rarely been applied to karstified aquifers and in particular to drip water in caves, despite the importance of the information which may be obtained. Besides the determination of transfer times of climate signals from the atmosphere through the epikarst to speleothems as climate archives, ³H–³He together with Ne, Ar, Kr, Xe data may also help to give new insights into the local hydrogeology, e.g. the possible existence of a perched aquifer above a cave. In order to check the applicability of ³H–³He dating to cave drips, we collected drip water samples from three adjacent caves in northwestern Germany during several campaigns. The noble gas data were evaluated by inverse modelling to obtain recharge temperature and excess air, supporting the calculation of the tritiogenic ³He and hence the ³H–³He age. Although atmospheric noble gases were often found to be close to equilibrium with the cave atmosphere, several drip water samples yielded an elevated ³He/⁴He ratio, providing evidence for the accumulation of ³He from the decay of ³H. No significant contribution of radiogenic ⁴He was found, corresponding to the low residence times mostly in the range of one to three years. Despite complications during sampling, conditions of a perched aquifer could be confirmed by replicate samples at one drip site. Here, the excess air indicator ΔNe was about 10 %, comparable to typical values found in aquifers in mid-latitudes. The mean ³H–³He age of 2.1 years at this site presumably refers to the residence time in the perched aquifer and is lower than the entire transit time of 3.4 years estimated from the tritium data.     

Quasi-free processes in 6Li(3He,pα)4He reaction at low energies

The ⁶Li(³He, pα)⁴He reaction was studied experimentally at 3.5, 4.4 and 5.5 MeV in the quasi-free reaction kinematical region. The effects of a resonance in the virtual ²H(³He, p)⁴He reaction on the three-body reaction cross section were investigated.     

Near-barrier neutron transfer in reactions with <Superscript>3</Superscript>He nucleus

Experimental cross sections for the formation of ^196,198Au isotopes in the ³He + ¹⁹⁷Au reaction and ^44,46Sc isotopes in the ³He + ⁴⁵Sc reaction are analyzed. To calculate transfer probabilities and cross sections, the time-dependent Schrödinger equation is numerically solved for the external neutrons of the ³He, ⁴⁵Sc, and ¹⁹⁷Au nuclei. It is shown that the contribution from the fusion channel with subsequent evaporation is important for the ³He + ⁴⁵Sc reaction and negligibly small for the ³He + ¹⁹⁷Au reaction. Fusion–evaporation is taken into account using the NRV and PACE codes. Calculation results demonstrate overall satisfactory agreement with experimental data.     

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.