Stability of muon-oxygen bond sites in RBa2Cu3O7

Relative energies of muon probe sites in the chain region of RBa₂Cu₃O₇ (RBCO) are investigated using a molecular quantum chemistry calculation for (Oμ)⁻ embedded in a cluster of point charges to simulate local charge distributions in RBCO. Partial Cu−O chain covalency results in a O-μ...O muon site between the chain and bridging oxygens. However, Cu-μ “hydride”-like sites are suggested by results for nominally ionic clusters.     

Negative muon hyperfine transition rate in aluminum

The residual polarization of theF ^– hyperfine state of ^–27Al has been investigated as a function of applied transverse magnetic field strength using standard TD- ^–SR techniques. TheF ^– precession frequency is –0.2623(5) [theoretical value: –0.2622] times that of the free muon in the same field. The observed muon decay electron asymmetry in theF ^– state decreases with increasing magnetic field strength, due to initial precession in the opposite direction of theF ⁺ state, which most muon initially populate, followed by a rapid transition to theF ^– state. A fit of the data to this model indicates a transition rateR=41(9)s^–1, in good agreement with theoretical predictions. This method can be used to determineR experimentally in other cases where it is too fast to be observed directly.     

Trapping by vacancies and mobility of positive muons in neutron-irradiated aluminium

By means of the muon spin rotation (µSR) technique the trapping at vacancies and the mobility of positive muons are investigated in the temperature range 2.5 K to 297 K in polycrystalline aluminium irradiated with fast neutrons at 4.6 K. The observed damping of the µSR signal at temperatures below 50 K is in good agreement with saturation trapping at monovacancies. The temperature dependence of the damping above 50 K indicates that the muon diffusivity exhibits a minimumD _min ^µ+ 110^–13 m² s^–1 near 125 K, which is explained by a transition from coherent to incoherent tunneling. With the help of an estimated trapping cross section a monovacancy concentration of ~3 × 10^–4 after 125 K annealing is deduced, in good agreement with measurements of the residual electrical resistivity. As expected, the vacancy recovery shows up in an irreversible decrease of the damping above ~ 175 K.Work supported by the Bundesminister für Forschung und Technologie and by Schweizerisches Institut für Nuklearforschung (SIN). Requests for reprints and correspondence should be addressed to A. Seeger, MPI für Metallforschung, Heisenbergstraße 1, D 7000 Stuttgart-80, Germany     

Muon lifetimes and weak interaction coupling constants

Improvements in determining muon lifetimes that are possible with the pulsed muon beam at the ISIS accelerator complex near Oxford, UK, are examined. The requirements of the standard model do not require an improvement in (µ⁺)[G_µ], which is fortunate as the low repetition rate of ISIS make such difficult. However, for stopping µ^± in liquid hydrogen the differences [(µ⁺) –(µ^–, observing decay e^–)] and [(µ^–, e^–) – (µ^–, observing capture n)] can be well improved to give the induced pseudo-scalar coupling constant,g _P, to ±2% and hence the first test of the chiral structure of the nucleon.On leave from Department of Physics, William & Mary, VA, USA.     

Delayed muon induced fission of209Bi and the role of meson-exchange currents

The probability for delayed muon induced fission of²⁰⁹Bi has been determined from a ( ^–,f ₁ f ₂) measurement. The measured fission probability P _f =(4.2±0.7)×10^–5 is compared with theoretical predictions. The high fission threshold reaction seems well suited for studying the influence of two-body meson-exchange currents in nuclear muon capture.We are indebted to the following institutes or organizations for financial support: Bundesministerium für Forschung und Technologie der Bundesrepublik Deutschland contract number 06 BN 271 (HP, PD, HH, FR, CR), Foundation for Fundamental Research on Matter (FOM) and the Netherlands Organization of the Advancement of Pure Research (NWO) (JK, CTAMdL, WL, AT) and the Schweizer Nationalfonds (LS).     

Anisotropic paramagnetic shift of μ−O in the highT c related materials LaSrCuO and LaCuO

The negative muon (^–) was used as a microscopic probe for the study of the electronic state at the oxygen site in highT _c related LaSrCuO materials. Using good single crystals, two types of signals are obtained corresponding to two different oxygen sites: one highly anisotropic, which shows a large (order of 1%) shift under the magnetic field inc-axis direction, the other less anisotropic and with a smaller amplitude (order of 0.2%). These results of ^–OSR show quit a difference with the result of¹⁷O-NMR, where the paramagnetic shift has axial symmetry along the Cu-O bond direction and a magnitude of the order of 0.2% and 0.05%.     

Site of the positive muon in YBa2Cu3O7

A combination of μSR and¹⁷O nuclear quadrupolar μLCR has been used to obtain detailed information on the muon site in YBa₂Cu₃O₇. The rms magnitude of the nuclear dipolar fields in YBa₂Cu₃O₇ enriched with 38%¹⁷O is about twice that in the natural¹⁶O sample. In addition the¹⁷O μLCR spectrum shows that the electric field gradient tensor at the oxygen closest to the muon is almost equal to that measured by NMR for the CuO₂ plane sites O(2,3). These results suggest that the muon in fully oxygenated YBa₂Cu₃O₇ is located at a single site 1.0 Å from an oxygen ion, probably O(2,3).     

On the change of polarization of positive muons in alkali halides

Positive muons implanted in nonconducting solids form with high probability hydrogenlike muonium atoms (µ ⁺ e ^–) with properties similar to those ofU ₂-centers. The influence of superhyperfine interactions with neighbor nuclei on the evolution of the polarization of the muon is investigated theoretically. The resulting muon polarization in longitudinal magnetic fields is calculated for muonicU ₂-centers in some alkali halides.     

The intense neutron generator of 14 MeV neutrons

An innovative intense neutron generator of 14 MeV neutrons for the irradiation of future reactor materials is presented. Negative pions are produced inside a 5–10 T magnetic field by an intense deuteron beam interacting with a carbon target. The pions and the muons from pion decay in flight are collected in the backward direction and stopped in a deuterium-tritium-hydrogen target of high density. Using an 18 MW deuteron beam at 1.5 GeV (12 mA=7.5 × 10¹⁶d/s), circa 10¹⁶gt^– /s can be generated, decaying to muons of which up to 10¹⁵ µ^–/s stop in the D/T/H mixture. Assuming X_c=100 fusions per muon, the µCF source produces 14 MeV neutrons with a source strength of up to 10¹⁷ n/s, i.e. a neutron power of 200 kW. The environment of the second target, the neutron source itself, can be made to resemble part of the Tokamak ring to be simulated for irradiation test samples.     

μ −SR-Spectroscopy on light neutral muonic atoms

In the past few years ^–SR-spectroscopy has been successfully applied to the simple muonic atoms, namely muonic helium: He ^–e^–. Here the theoretical and experimental progress in understanding this atomic system consisting of a nucleus, a muon and an electron is reviewed. The various effects contributing to the ground state hyperfine structure are explained. The experiments are described and are compared with the theoretical calculations.     

Mechanism of self-trapping of electron excitation in alkali-halide crystals

A dynamic method is used to calculate the equilibrium positions of ions near heavy and light homologous impurities for KCl:J and KJ:Cl crystals. The most probable sites of self-traps are shown.Lattice ralaxation near an excited anion is modeled. The localization of electron excitations (EE) in alkali-halide crystals is examined from the viewpointof the formation of a covalent bond between anions as a relust of their approach toward each other. The EE localization time is determined by the lifetime of local vibrations in the crystal near the excited anions (5-10^–13–10^–11 sec).Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 35–38, January, 1986.     

Construction of Riken-ral muon facility at ISIS and advanced μSR

By utilizing the intense pulsed proton beam available at the ISIS facility of RAL, the new muon facility project of an advanced superconducting muon channel funded by the RIKEN is now under construction. The new facility, by adopting the superconducting solenoid system, will produce the strongest backward decay pulsed ⁺ or ^–in the momentum range from 20 MeV/c to 120 MeV/c. Also, by adopting the pulsed magnetic kicker, each one of two muon pulses will be supplied to two extraction channels simultaneously. Various important muon science experiments including advanced pulsed ^–SR andmu ⁺SR experiments will be realized.     

Probing oxygen in high Tc superconductor LaSrCuO by negative muons

A novel microscopic method of muon spin rotation for negative muon (μ⁻) attached to oxygen is applied on highT _c materials LaSrCuO. The paramagnetic shift and relaxation rate are measured as a function of Sr concentration and temperature. Analysis shows the large difference in shift and relaxation between two different sites. A large anisotropy was also observed by preliminary measurements along different crystalline axes.     

Estimation of Sea Level Muon Energy Spectrum at High Latitude from the Latest Primary Nucleon Spectra Near the Top of the Atmosphere

The vertical muon energy spectra at sea level have been estimated from directly measured primary cosmic-ray nucleon spectrum. The hadronic energy moments have been calculated from the CERN LEBC EHS data on the Lorentz invariant cross-section results on pp ^±X and pp K^±X inclusive reactions and are duly corrected for A–A collisions. Finally, the sea level muon energy spectra have been calculated from the decay of conventional mesons, using standard formulation. The estimated muon spectra are found to be in good agreement with the directly measured muon spectra obtained from the different experiments.     

Theory of hyperfine anomalies in muonic atoms

Negative muon spin precession experiments by Yamazaki et al. have found giant hyperfine anomalies in muonic atoms ranging from a few percent up to 36%. In order to understand their results, we present Breit interaction calculations based on atomic self-consistent unrestricted Dirac-Fock solutions which explicitly include all electrons and the negative muon. The Breit interaction results (including the relativistic correction for the bound muong-factor), vary from near zero for ^–O/N to –5% for ^–Pd/Rh; this latter is much larger than the calculated muonic or nuclear Bohr-Weisskopf anomalies and much smaller than the 36% measured value. For ^}Ni/Co we find a calculated range of results (depending on assumed electronic configurations) of –2.3 to –2.7% in excellent agreement with recent measurements of the Yamazaki group. This excellent agreement in ^}Ni/Co provides strong support for the earlier suggestions that the discrepancy in the case of ^–Pd/Rh is due to experimental factors.This work was supported by the U.S. National Science Foundation (DMR Grant No. 82-16543) and the U.S. Department of Energy.     

μ+ Localization and tunnelling in Sc

The angular and magnetic field dependence of the transverseSR relaxation rate in single crystal Sc shows unambiguously that, at low temperatures, the muon is localized along the-axis joining interstitial tetrahedral sites. The best agreement with the measured results, covering a complete range of high and low field data, is obtained on assuming a muon state with fast tunnelling between two adjacent sites at positionsz/c=±0.09 above and below thea–b plane. Both contributions from the muon-induced electric field gradient and that inherent to the crystal were considered.     

μ + sites and local moments in Van-Vleck paramagnet: PrNi5

Multi-frequencyμ ⁺ SR spectra were observed in PrNi₅ at 23 K in an external transverse field of 3010 G. Three types of interstitial sites are identified as the most likely muon stopping positions. A substantial amount of the magnetic moment sampled by the μ is found to be localized on the Ni sites.     

Polarization and thickness effects in Mössbauer spectroscopy

The lattice sites of positive pions in gold have been determined as a function of temperature by means of π⁺/μ ⁺ channelling experiments. A blocking pattern in the <100> direction at low temperatures indicates that only octahedral interstitial sites are occupied. The vanishing of this pattern and the reduction of muon flux enhancement observed in <110> ifT is raised above the Debye temperature (165 K) may be interpreted in terms of muon dechannelling owing to the thermal vibrations of the lattice atoms. Time-differential π⁺/μ ⁺ channelling studies on α-iron showed between 20 K and 70 K a strong dependence of the channelling profiles on the time the pions spent in the sample. The ‘young’ pions are well localized on O- or T-sites whereas the ‘old’ pions either become delocalized or occupy sites with strongly reduced symmetry. A model accounting for these observations is suggested. It is argued that there is a close relationship to earlierμ ⁺ SR experiments.     

Search for Higgs Boson in UHE Cosmic Rays

Considering the SM Higgs boson mass in the range of (95–235) GeV, we present here a mechanism for indirect searches of this scalar in UHE cosmic rays interactions. The mechanism is the decay of Higgs bosons which are produced through bubble formation due to vacuum excitation in an UHE cosmic rays interactions with air nuclei. We develop a model of hadronic interaction based on algorithms of the GENCL code of the UA5 experiment of CERN and some physics of CORSIKA code (Karlsruhe report), incorporating a fraction of energy transfer to bubble formation through vacuum excitation and subsequent multiparticle production via conversion of Higgs boson to heavy fermion pairs. Such events are expected to have high multiplicity and excess muons. This mechanism has significant effect starting from E _P 10¹⁸eV. It is found that the average muon number decreases gradually upto 175 GeV Higgs boson mass and remain practically constant thereafter for all primary energies (E _P) above 10¹⁸ eV and for all fractions of energy transfer (f _e 0.01–0.5). The fluctuation of muon multiplicity decreases with E _P and increases very slowly with Higgs mass upto 175 GeV, remaining practically invariant thereafter.     

Hadronic vacuum polarization contribution to the Lamb shift in muonic hydrogen

We report on the accuracy of the measurement of the total cross section of the process e⁺e^– ZH and of the Higgs boson mass that would be achieved in a linear collider operating at a centre-of-mass energy of 350 GeV, assuming an integrated luminosity of 500 fb^–1. For that we have exploited the recoil mass off the Z using its leptonic decays into electron and muon pairs. The Higgs mass is determined with 150 MeV accuracy, the recoil mass resolution is about 1.5 GeV and the cross section is obtained with a statistical error of 3%.