Adsorption of He on He crystal surfaces

We have compared the surface properties of crystals respectively grown from normal ⁴He containing 130 ppb of ³He and from ultrapure ⁴He (0.4 ppb). Below 0.4 K, ³He impurities are found to decrease both the surface tension and the step energy. Our results are consistent with the existence of two-dimensional bound states for ³He atoms at the solid-liquid interface. Quantitative agreement with the data is found with a binding energy _s ≈ 4.3 K and a ³He density saturating around 0.4 monolayer. The presence of steps is found to increase the binding energy by about 10 mK.     

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°.     

Two-neutron elastic transfer 4He(6He,4He)6He at E = 151 MeV

Using the framework of the coupled reaction channels (CRC) the elastic scattering and the elastic transfer in the system ⁶He + ⁴He measured at E = 151 MeV have been analysed. It is shown that the structure observed in the backward range of the angular distributions is influenced by the interference of the elastic 2n-transfer with a two-step process passing through the 2⁺ excitation in ⁶He. The two-neutron transfer mechanism is studied in the microscopic approach and it is found that for the ground-state transition the one step dominates by a factor 10 over the two-step mechanism at this energy. Received: 29 October 2001 / Accepted: 4 December 2001     

Stability of 3He2 4He M and 3He3 4He M L = 0,1 Clusters

We have studied the stability of mixed ³He/⁴He clusters in L = 0 and L = 1 states by the diffusion Monte Carlo method, employing the Tang-Toennies-Yiu (TTY) He-He potential. The clusters ³He⁴He_M ( ) and ³He₂⁴He_M (L = 0, S = 0) are stable for M > 1, while to bind two ³He in a triplet state the minimum number of ⁴He is four. Considering clusters with three ³He, ³He₃⁴He₄ is the smallest stable system in the L = 1 state, while ³He₃⁴He₈ is the smallest stable system in the L = 0 state.     

氦原子与HD,HT,DT分子碰撞角分布的理论计算

采用Ｔａｎｇ－Ｔｏｅｎｎｉｅｓ势模型,通过坐标变换,得到了氦原子与氢分子的非对称同位素替代体系ＨＤ,ＨＴ,ＤＴ在质心坐标系中的势能函数,在此基础上计算了Ｈｅ－ＨＤ,ＨＴ,ＤＴ碰的角分布。     

X-rays from antiprotonic3He and4He

Antiprotonic X-rays from the helium isotopes have been observed at pressures of 36, 72, 375 and 600 mbar. The antiproton beam from LEAR with momenta of 309 and 202 MeV/c has been stopped at these pressures using the cyclotron trap. The X-rays were detected with Si (Li) and intrinsic Ge semiconductor detectors. Absolute X-ray yields were determined and the strong-interaction 2p shifts and the 2p and 3d broadenings measured to be _2p=(–17±4) eV, _2p=(25±9) eV and _3d=(2.14 ±0.18) meV for ¯p³He and _2p=(-18±2) eV, _2p =(45±5) eV and _3d=(2.36±0.10) meV for ¯p⁴He.The efforts of the LEAR staff and the help of P. Gauss from the CERN Cryogenic Group as well as the technical assistance of M. Dröge and M. Stoll are gratefully acknowledged. This work is part of the Ph.D. of one of us (M.S.), University of Karlsruhe (1987), KfK report no. 4222.     

Density matrix for3He/He II mixture

The hydrodynamic equations of a mixture of superfluid⁴He with normal fluid³He are derived by an extension of statistical mechanic techniques previously applied to pure superfluid⁴He.     

Scattering lengths of pionic 3He and 4He

Scattering lengths of pionic ³He and ⁴He and the charge exchange contribution to the 1s width of pionic ³He are calculated within the fixed scatterer approximation of the multiple scattering formalism. Particular attention is focussed on the nuclear physics part and on πN p-wave contributions. For the first time triple scattering and double-spin-flip contributions have been included. We find significant deviations from previous estimates and calculations. Good agreement is achieved with the experimental π^?3He scattering length, whereas in the case of ⁴He a repulsive dispersion contribution is clearly needed. We propose to use the measured 1s level shift of pionic ³He as a constraint to deduce a precise value of the isoscalar πN scattering length. Furthermore, we find that multiple scattering reduces the impulse approximation value for Γ_1s(π^?3He → π^{0 3}H) by more thsn 20 %. This result casts some doubt on impulse approximation calculations of radiative pion capture as well.     

Microscopic theory of3He—4He solutions

The ring diagram contribution to the grand partition function of a mixture of fermions and bosons is obtained from a rigrous formula which extends the single component formula of Montroll and Ward. Together with the contributions from some other relevant graphs, it is applied to a dilute solution of³He with⁴He. For a small mass difference parameter, the ground state energy is shown to be expressed as a linear function of³He concentration. The coefficients of this energy expression are obtained explicitly for soft and hard-sphere potential models. From the energy, the differential volume coefficient is obtained also.This work was supported by ONR under Contract No. N00014-79-C-0451     

The He He n coupling constant and the mesonic contribution from nucleon He forward dispersion relations

Forward dispersion relations for nucleon-⁴He scattering have been analyzed for 0 < E_lab < 600 MeV with the help of new experimental data. Improving over a previous analysis restricted to E < 50 MeV the ⁴He³He n coupling constant is determined unambiguously as r = 3.2 ± 0.4 in agreement with information from stripping reactions. Moreover the strength of uncorrelated three nucleon exchange and the summed strength of mesonic exchange is determined.