J. Vetter

Development of a low-momentum “surface” muon beam for LAMPF☆

Abstract A low momentum positive muon beam has been developed at LAMPF, utilizing muons which originate from pion decays at rest near the surface of the pion production target and are transported by the LAMPF stopped muon channel. The muon beam has a total flux of 2.5 × 104μ+ per s per μA of a proton beam (8 × 106μ+ s−1 for the present operating current at LAMPF of 300 μA), an extremely narrow range spread of 50 mg/cm2 in CH2, and a positron contamination (e+/gm+) of 60%.

Hyperfine structure measurement of the9Be+ 22s1/2 ground state by optical pumping

Thermal9Be+ (I=3/2) ions are polarized by direct optical pumping in He buffer gas. Observation of⩼gDF=± 1 transitions yields for the hyperfine splitting frequencyvhfs(9Be+, 1s2 2s2S1/2)=1250018(5) kHz. The result is discussed with regard to predictions of recent theoretical calculations and of the Fermi-Segré formula for the ground state hfs of the Li isoelectronic sequence.

Muon location and mobility in high-purity metals

Positive muons are expected to be easily trapped by imperfections including impurities in metals (Seeger 1975). Therefore it is essential to study muon diffusion and localization on well-characterized high-purity materials. The sensitivity to impurities has been confirmed by recent ~SR studies on Nb (Birnbaum et al. 1978, Borghini et al. 1978). In this note we report results of DSR experiments on high-purity monocrystals of beryllium, tantalum, and niobium. Starting from high-purity material, the Be target was vacuum melted and zone-refined i0 times resulting in a residual resistivity ratio F ~ R2g3K/R4.2K = 400, corresponding to a purity of 99.993 wt% (Aldinger 197~).0f the remaining 70 wt.ppm impurities, the principal constituent was iron. The preparation of the Ta (F = 5000, interstitial C+O+N<5 at 9 and Nb (F = 3000, N<2 at.ppm, 0 < 1 at.ppm, Ta ~ 5 at.ppm) targets is described elsewhere (Gladisch et al. 1978). The experimental setup used in our measurements is similar to that of Dorenburg et al. (1978)._The ~SR data were analyzed by computing the mean polarization P for each precession period of the muon spin and assigning it to the time t at the centre of the period. In order to obtain the depolarization rate A e ~ t~l(~(t e) Z P(O)/e),) suitable theoretical functions were adapted to the experimental ~(t in the cases of Be and Ta. In the case of Nb, however, the rapid decrease of polarization allowed t e to be found by mere interpolation. The temperature dependence of A e in Be was measured in 24.6 mT external magnetic field transverse to the muon polarization and at angles of O, 50,and 90 degrees with respect to the c-axis of the hcp lattice structure. No significant orientation dependence of A e was found at any of the investigated temperatures. Therefore, in Fig.1 only averaged values are shown, except for temperatures around lOOK, where the maximum A e values are found. Here the results for individual orientations are given separately. They are also presented in Table I, together with results of calculations using van Vlecks theory under the assumption that muons are located either on tetrahedral or octahedral interstitial sites in an unrelaxed Be lattice 9 For completeness, we also include polycrystalline averages of the theoretical values for these types of interstitial sites. These results are in accordance with the assumption that quadrupole coupling (Hartmann 1977) suppresses the orientation dependence of A e at lOOK in a similar way as in Cu, where comparably weak orientation and field dependences were found up to ~50 mT (Camani et al. 1977). The magnetic field strength B_ above which dipolar coupling (nuclear spln I, gyromagnetlc ratlo YI) starts to determine the orientation dependence of A e is given by

Effect of colour centres (F′-centres) on the depolarization of positive mouns in KCl

First results are reported of a project to study the depolarization- and diffusion mechanisms of positive muons in ionic crystals by observing the effect that the presence of charged defect centres has on the residualμ+-polarization. It is shown that theμ+-polarization in KCl is significantly increased by the presence ofF′-centres. The muons “find” theF′-centres at concentrations of a few ppm at temperatures as low as 10 K.

Hyperfine pressure shift of 137Ba+ ions in noble gas buffers

Abstract Fractional hyperfine pressure shifts for 6 2 S 1 2 137 Ba + (I= 3 2 ) ions were measured with optical pumping, yielding: ( 1 ν o ) ∂ν ∂p = +7(7) for He and −825(20) for Ar in 10 −9 /torr at 0°C. These shifts for 137 Ba + were considerably more negative than those measured for the isoelectronic 133 Cs atom.

Nuclear magnetic resonance of 9Be atoms by charge exchange with optically pumped 9Be+ ions

Abstract NMR of free 9 Be (I = 3 2 ) 1 S 0 atoms is observed via resonant charge exchange with optically pumped 9 Be + 2 S 1 2 ions in He buffer gas, yielding μ( 9 Be ) = −1.1778 (9) μ n . Applications of the method suitable for all group II elements and Yb include: precision measurements of nuclear moments, chemical shifts, hfs-anomalies, and investigation of charge exchange processes.