C. A. Scott

Measurement of the 1s-2s energy interval in muonium

The 1s-2s interval has been measured in the muonium (&mgr;(+)e(-)) atom by Doppler-free two-photon pulsed laser spectroscopy. The frequency separation of the states was determined to be 2 455 528 941.0(9.8) MHz, in good agreement with quantum electrodynamics. The result may be interpreted as a measurement of the muon-electron charge ratio as -1-1.1(2.1)x10(-9). We expect significantly higher accuracy at future high flux muon sources and from cw laser technology.

Commissioning of the Rutherford Appleton Laboratory pulsed muon facility

Abstract The pulsed muon channel of the ISIS facility at the Rutherford Appleton Laboratory has been successfully commissioned as a tool for μSR research using polarised surface muons and for experiments requiring negative cloud muons. The beam line is described and the present performance given. At the present time, the pulsed muon beam gives 105 μ+/s (2000 μ+/pulse) with very thin production targets (2.5 mm thick carbon) and 30% of the ISIS design proton current. μSR test spectra demonstrate the capability of the source and instrumentation, the performance relative to the continuous sources at the meson factories, and the potential for new science.

Positive muon data analysis at ISIS

The pulsed surface muon beam of the Rutherford Appleton Laboratory is well suited to performing μSR measurements in zero and low magnetic fields with the longitudinal set-up. In this paper we describe our data analysis procedure and the effect of the collimation on the spectra. The determination of the efficiency ratio of the telescopes is discussed. We point out that for some measurements it is important to take into account the muon beam structure properly.

HYPERFINE COUPLING CONSTANTS IN MUONIUM ADDUCTS OF THE CARBONYL BOND

A representative series is studies of organic radicals formed by the effective addition of muonium to the carbonyl oxygen atom of aldehydes, ketones, esters and amides. Low values of the muonelectron hyperfine coupling are measured which are particularly sensitive to radical structure and dynamics. The systematics of these values are discussed in terms of the opposing contributions of conjugation and hyperconjugation.

Fast E-field switching of a pulsed surface muon beam: The commissioning of the European muon facility at ISIS

Abstract The ISIS pulsed muon facility at RAL has been upgraded by the inclusion of a fast E -field kicker which simultaneously divides and distributes the muon pulses at surface momentum to the three experimental areas at a repetition rate of 50 Hz. This upgraded facility has been successfully commissioned in conjunction with a new μSR spectrometer. It has been shown that this new spectrometer can operate as expected with a figure of merit for μSR experiments similar to that of the original spectrometer, in spite of receiving only half of the relative muon intensity. This twofold increase in experimental capability will be further increased in the near future by the incorporation of experimental equipment in the third beamline. Such a facility will be capable of satisfying a European wide demand for μSR research with pulsed surface muons.

Rates of motion for free radicals in zeolites as directly measured by longitudinal field muon spin relaxation

The direct measurement of reorientational rates for hydrocarbon radicals in zeolites is presented, specifically cyclohexadienyl in zeolite X, using a muon spin relaxation method. The method allows discrimination between different adsorption sites and the determination of activation energies for surface motion of the radicals. Copyright

A μSR and neutron polarisation analysis study of Mn moment delocalisation in Fe substituted YMn2

Neutron polarisation analysis measurements reveal antiferromagnetic spin correlations persisting to temperatures of 120 K in Pauli paramagnetic Y(Mn1−xFex)2, 0.03≤x≤0.05. The mean moment at the Mn(Fe) site is found to be 0.2μB. Transverse field μSR is characterised by weak exponential damping with a rate of 0.02 μs−1 at 300 K increasing according to the power lawT−0.75 to only 0.16μS−1 at 12 K. It is suggested that these results are consistent with a slowing down of longitudinal spin fluctuations at the Mn site as temperature decreases.

Radio-frequency μSR experiments at the ISIS pulsed muon facility

This paper explores the use of pulsed radio-frequency (RF) techniques to remove the frequency limitations imposed on conventional transverse muon spin rotation (μSR) experiments at a pulsed muon source by the finite muon pulse width. The implementation of the 90° pulse technique is demonstrated by observing the free precession signal of diamagnetic muons implanted in polythene, the change in signal amplitude as a function of RF pulse length is plotted and the precise condition for a 90° pulse determined. The technique is evaluated by comparing measurements made using conventional spin rotation experiments to those employing pulsed RF methods. The potential for applying standard NMR multiple-pulse methods to the μSR experiment is considered and the use of two-pulse RF sequences (90°x−τ−90°x and 90°x−τ−180°x) to form a muon spin echo demonstrated.

Studies of muonium-substituted molecules in propan-2-one and in aqueous solutions of propan-2-one

Muonium-substituted molecules, both paramagnetic and diamagnetic, are formed when positive muons are implanted in pure propan-2-one and in binary aqueous systems. These have been studied by the technique known as muon spin rotation, or µ.s.r.Identification of the paramagnetic fraction as the muonic propan-2-ol radical (CH3)2ĊOMu and of the diamagnetic fraction in aqueous solutions as ‘light water’ MuOH is corroborated by the qualitive behaviour of the µ.s.r. parameters frequency, amplitude and linewidth. Comparison of the hyperfine coupling to the muon in (CH3)2ĊOMu with that to the hydroxy proton in (CH3)2ĊOH reveals an unusually large isotope effect, which is interpreted in terms of the preferred molecular conformation. The significant effect of solvation on the muonic hyperfine constant is interpreted in terms of hydrogen and muonium bonding.Muonium itself is detectable in dilute aqueous solutions. From measurements of its reaction rate, as well as other considerations, it is concluded that neither thermal nor epithermal muonium can be the major radical precursor. A production mechanism for the observed radical is proposed involving the encounter of radical-anion and muonic-cation states. However, thermal reactions of muonium can account for some loss of polarisation, as well as a corresponding enhancement of the radical and diamagnetic yields measured at sufficiently low magnetic fields.

Sorption of benzene in cation-exchanged zeolite X, as measured by longitudinal field muon spin relaxation (LF-MuSRx)

The direct measurement of reorientational rates for hydrocarbon radicals in zeolites is presented, specifically cyclohexadienyl in zeolite X, using a muon spin relaxation method. The method allows discrimination between different adsorption sites and the determination of activation energies for surface motion of the radicals. Copyright