D. R. Harshman

MSR measurement of the reaction rate of muonium with a supported platinum catalyst

Abstract Muonium spin rotation (MSR) studies of interactions between muonium and silica-supported platinum catalysts show: (1) two types of muonium traps intrinsic to hydrogen-reduced, unloaded silica surfaces, (2) removal of one type of Pt loading, and (3) chemical reaction between Mu and supported, oxygen-covered Pt, at the rate 3.5±0.15 μs −1 .

Diffusion and trapping of muonium on silica surfaces

Abstract The spin relaxation rate λ Mu of muonium atoms on fine silica powder surfaces was measured as a function of temperature and of the surface concentration of hydroxyl groups. Results indicate two-dimensional diffusion, trapping and detrapping of the muonium atoms on the silica surface. At low temperatures λ Mu decreases dramatically as the concentration of surface hydroxyls is reduced. A three-state model is used to extract the muonium adsorption energy and other physical parameters.

Thermal hopping ofμ+ between “FμF” centres in NaF

Strongly hydrogen-bonded diamagnetic “FμF” centres are formed by theμ+ in a wide variety of fluoride crystals. Hydrogen atoms are expected to form similar “FHF” complexes. Through the “motional narrowing” of the zero-field muon relaxation function in NaF, we have observed an Arrhenius temperature dependence of the dissociation rate of theFμF complex, yielding a binding energy of 1700 (200) K for theμ+ in theFμF centre.

POSITIVE MUONS IN ANTIMONY BISMUTH ALLOYS

StroboscopicμSR and TD-μSR techniques were used to measure theμ+ Knight shiftKμ, and relaxation rateλ inSbBi alloys as functions of magnetic fieldH, temperatureT, the angleθ betweenH and the crystallineĉ axis, and the concentration [Bi] of alloyed Bi. In pure Sb and inSbBi (6.5%),Kμ (θ=0) andKμ (θ=π/2) both decrease linearly withT up to about 100 K, but bothKμ and its anisotropy are smaller in the 6.5% alloy, indicating a “dilution” effect. With 15 at % Bi,Kμ is reduced further but itsT-dependence and that ofλ are dramatically altered. At low temperaturesKμ (θ=0) inSbBi(15%) actually becomes negative and the sign of the anisotropy is reversed. In the same sample,λ is proportional toH at both 20 K and 150 K; at 120 Kλ is proportional toKμ ifθ is used as an implicit variable, but at 36 K this is not the case. A consistent phenomenological description is offered.

μ +SR study of some magnetic superconductors

The relaxation of theμ+SR signal has been investigated for the three compounds YRh4B4, ErRh4B4 and SmRh4B4. In the non-magnetic superconducting (Tc ≈ 11 K) YRh4B4, the data display a Kubo-Toyabe (gaussian) shape for zero (transverse) magnetic fields. ErRh4B4 (superconducting below 8.7 K and ferromagnetic below 1 K) shows a dominant signal with very slow relaxation. In contrast SmRh4B4 (superconducting below 2.7 K and antiferromagnetic-superconducting below 0.87 K) shows a change in relaxation from gaussian above ≈ 60 K to exponential between 1 K and 4 K to two exponential signals (fast and slow) belowTN=0.9 K. In the region 0.9 K <T < 4.5 K, the relaxation time and the asymmetry both increase withT.

Hyperfine splitting of muonium in SiO2 powder

The hyperfine splitting of Mu in evacuated SiO2 powder has been measured over the temperature range 17–300 K using the two frequency method. Above 100 K it is consistent with the vacuum value (4463.3 MHz) indicating the Mu atoms are moving freely between the particle grains. At lower temperatures it drops rapidly to 4437±4 MHz at 17 K. The effect is attributed to thermal adsorption on the SiO2 surface. Measurements were also made on an Ar coated powder between 7–62 K. The hyperfine splitting is consistent with the vacuum value over this range.

Muon spin relaxation in ErRh4B4

Data on zero‐field μSR in ErRh4B4 are presented for temperatures from 4 to 300 K. Due to the large erbium moment, most of the muons have a relaxation time of less than 10 ns for temperatures below about 50 K. The data have been analyzed in terms of two distinct muon sites in the material. The deduced erbium relative fluctuation rate versus temperature is related to the crystalline electric field levels of erbium in the RERh4B4 structure.

Dynamical behavior of muonium on silica surfaces

The behavior of muonium on the surface of finely divided silica (amorphous SiO2) powder (mean grain diameter 70 Å) has been studied as a function of the surface concentration of hydroxyl groups. The temperature dependence of the Mu relaxation rate in transverse field was measured for samples prepared with 0%, 50% and 70% of the surface hydroxyl groups removed over the temperature range 4 K <T < 300 K. The relaxation rate shows a distinct maximum at about 25 K and a minimum at about 16 K for all three samples, and shows a dramatic decrease below 16 K as the concentration of surface hydroxyls is reduced. A three-state nonequilibrium model describing the diffusion and trapping of muonium on the silica surface is used to interpret the data.

The relaxation rate of muonium as a function of temperature and loading in silica-supported platinum catalysts

Abstract Recent results are presented for an experimental study of the temperature dependence of the relaxation rate of muonium in silica-supported platinum catalysts. An interpretation is proposed, with a model involving spin-relaxing collisions between Mu and the Pt particles of the catalysts, for which probabilities and rates are estimated.

Muonium reaction with ethylene on powder surfaces

The reaction rates of muonium with ethylene adsorbed on the surface of amorphous silica powder grains were measured between 6 K and 300 K. kf values characterizing the two-dimensional surface reactions and the 2 1/2 dimensional reactions with muonium in the gas phase were obtained.