Measurement of the stopping power of Au and MgF2 for slow muons

The spectral flux densityn(T) of ^– emerging from a Au or MgF₂ moderator has been measured at low energiesT using time-of-flight. Fromn(T) the stopping powerS(T) of Au was determined for 2 keVT22 keV, and of MgF₂ for 2 keVT12 keV. For Au,S(T) is smaller than calculated values obtained from proton atomic data practically in the wholeT range (Barkas effect); at lowT S(T) approaches the calculated values. For MgF₂,S(T) agrees fairly with the calculated values above 5 keV and then drops below these values. We ascribe this dropping to the large energy gap of the MgF₂ insulator.We wish to thank H. Angerer, H. Plendl, G. Schmidt, and C.A. Schug for help with the data taking, J. Homolka for computational help, H. Hagn, and H. Weiss for technical assistance and P. Maier-Komor and R. Scherrer for manufacturing the windows and targets. The hospitality of PSI and financial support by the German Bundesministerium für Forschung und Technologie are acknowledged.     

Two-dimensional melting of physisorbed submonolayers of argon and nitrogen

We have studied two-dimensional argon and nitrogen physisorbed on grafoil by conducting positron lifetime and Doppler broadening measurements as functions of adsorbate coverage and temperature. Positron lifetime and Doppler broadening parameters show turnarounds near one-half monolayer coverages at 77 K. The s parameter of the Doppler broadening spectra measured for one-half monolayer coverages of argon and nitrogen increases with temperature across the melting phase transition. We discuss the observed coverage and temperature effects following models based on Kosterlitz-Thouless-Nelson-Halperin-Young theory of two dimensional melting and positron localization in surface defects.     

Absolute pionic x-ray intensities in Mg and Fe

Absolute pionic x-ray intensities have been measured for Mg and Fe. The accuracy achieved was 2% of the line intensity for the most prominent transitions. In the Balmer series of Mg and the Paschen series of Fe the crossover transitions were found to be even more attenuated than the transitions between circular states, as deduced, for example, by comparison with muonic atoms; no crossover transitions were observed in the Fe Balmer series. According to cascade calculations based on the experimental intensities more than half of the pions are absorbed in Fe already from levels with principal quantum numbern>17 by strong interaction.Dedicated to Prof. Dr. P. Kienle on the occasion of his 60th birthday     

Formation of antihydrogen in excited states

A two state approximation method used by author earlier for positron–hydrogen charge-exchange process is used to compute antihydrogen formation cross-sections in excited states. Cross-section results are compared with other existing calculations in the energy range 30 keV–500 keV. Total cross-section results for antihydrogen formation are also being reported.     

Experimental evidence for muonicX-rays from fission fragments

In the spectrum of muonic X-rays of²³⁸U measured in coincidence with prompt fission events a structure has been found which is attributed to 2p1s transitions of the muon attached to heavy fragments. The intensityI _s of this structure relative to the strengthI _pf observed for prompt fission has been determined to beI _s/I _pf=(6.0± 2.1) %. Although the experimental significance for the appeareance of this phenomenon is weak, this is the first experimental indication for its occuxence.We are indebted to the following institutes or organizations for financial support: Bundesministerium für Forschung und Technologie der Bundesrepublik Deutschland contract number 06BN271 (CR,PD,HH,FR,WS), Foundation for Fundamental Research on Matter (FOM) and the Netherlands Organization of the Advancement of Pure Research (NWO) (JK,CTAMdL,AT), the Swiss National Foundation (SNF) (LAS,LS), and the Heinrich-Hertz-Foundation (HH) (FFK,BS).     

Low-energy pion-nucleus potential in the extended tree-level model for the pion-nucleon interaction

Starting from the extended tree-level model for theπN interaction [1] (hereafter ETLM) we demonstrated that the observed local pion-nucleus repulsion arises from the reduction of baryon masses in nucleus due to strong scalar nuclear field [2]. This is rather convincing argument in favour of nuclear relativism because such repulsion has no explanation within the traditional nonrelativistic approach to nucleus. Communicated by V.V. Anisovich     

Muonic x-ray intensity measurements on a highT c superconductor

Muonic x-ray spectra from the high-T _c superconductor YBa₂Cu₃O_6.95 have been measured in the superconducting and normal states. No significant differences were found between the two spectra when comparing the intensities of 27 lines.Dedicated to Prof. Dr. H. J. Mang on the occasion of his 60th birthday     

The interplay of theK + K − atom and thef 0(975) resonance

We predict that production of theK ⁺K^– atom inpd ³ He X and similar reactions exhibits a drastic missing mass spectrum due to the interplay withf ₀(975) resonance. We point out that high precision studies of theK ⁺K^– atom may shed a new light on the nature off ₀(975).Communicated by: B. Povh     

A possible way to promote antihydrogen formation via metastable antiprotonic helium atoms

A new way to promote antihydrogen formation via the recently discovered long-lived metastable states of antiprotonic helium atoms is discussed. Recombination processes such ase ^– ¯pHe⁺⁺ +e ⁺ e ^– e ⁺ ¯p + He⁰ are possible in this respect.Dedicated to Prof. Dr. P. Kienle on the occasion of his 60th birthday     

Positron mobility in polyethylene in the 60–400 K temperature range

We have determined the positron mobility (₊) in polyethylene samples (67.2% crystalline, glass transition temperatureT _g=151 K) in the 64–400 K temperature range by Doppler shift measurements. A method based on the simulataneous observation of two lines from¹³³Ba and¹³⁷Cs radioactive sources together with the positron annihilation line, was employed to measure the Doppler shift of the 511 keV line as a function of the electric field applied to the samples. With this method we were able to measure at the same time the drift velocity of positrons and theS parameter. This parameter is very important in the interpretation of the mobility trend in samples where the positron states change with temperature. The positron mobility was corrected for positronium formation. ₊ at 64 K is 31.7±0.8 cm² V^–1 s^–1 then decreases up to 123 K, increases at 148 K and decreases again up to 170 K (₊=26.9±0.8 cm² V^–s^–). This sharp change in mobility is centred around the glass transition temperature of our samples. Then the mobility remains almost constant up to 230 K. From 250 K to 377 K, ₊ increases and reaches the value of 38.4±1.0 cm² V^–1s^–1. The corrected experimental data were well fitted by a simple model taking into account scattering and a thermally activated process (hopping mechanism).     

Monte Carlo simulations of the <Emphasis Type="Italic">μ</Emphasis>CF processes kinetics in deuterium gas

The kinetics of muon-catalyzed-fusion processes (μCF) in pure D₂ gas have been studied by means of Monte Carlo simulations for various target temperatures and densities. In particular, the role of resonant and non-resonant ddμ formation in μCF has been investigated. It has been shown that non-resonant formation can be directly observed at very short times in the neutron time spectra from μCF for low-density D₂ targets. The time spectra of neutrons from the low-temperature ortho-D₂ and para-D₂ gas targets have been calculated. These spectra display a strong ortho-para effect, which agrees with experimental results for the dilute-gas D₂ targets.     

Precision measurement of antiprotonic hydrogen and deuterium X-rays

X-rays from antiprotonic hydrogen and deuterium have been measured at low pressures. Using the cyclotron trap, a 105 MeV/c antiproton beam from LEAR was stopped with an efficiency of 86% in 30 mbar hydrogen gas in a volume of only 100 cm³. The X-rays were measured with Si(Li) detectors and a Xe-CH₄ drift chamber. The strong interaction shift and broadening of the Lyman transition and the spin-averaged 2p width in antiprotonic hydrogen was measured with unprecedented accuracy. The triplet component of the ground state in antiprotonic hydrogen was determined for the first time.The authors would like to thank the LEAR staff for their efforts in providing the antiproton beam. The help of P. Gauss from the CERN cryogenic group and of P. Zettwoch from the PS workshop is gratefully acknowledged. We wish to thank K.-P. Wieder for his help with the drawings. This work is part of the Ph.D. thesis of one of us (K.H., University of Karlsruhe, 1990).     

Semiclassical approach to the three-body muon-transfer collisions

Muon-transfer rates in collisions of hydrogen-like atoms or with light nuclei t, ³He, ⁴He, ⁶Li or ⁷Li, are calculated in a semiclassical approximation to the Faddeev-Hahn equations. The two nuclei involved are treated classically, while the motion of the muon in their Coulomb field is considered from the quantum mechanical point of view. The experimentally observed strong dependence on the charge of the nuclei is reproduced. Received: 1st November 1997 / Revised: 26 March 1998 / Accepted: 18 August 1998     

The decay of the isotopes179Ir and180Ir

The probability for non-radiative (n.r.) excitations in muonic²⁰⁹Bi was determined from a ( ^–,)-measurement by comparing the intensities of muonic X-ray transitions in single and coincidence spectra. The values of P_n.r(3p1s)=(17.9±2.0)% and P_n.r.(3d1s)=(3.0±2.2)% were measured for the first time. The strength of the n.r. decay of the 2p-level was found to be (4.2±2.2)%. The n.r. transition probabilities of two subcomplexes of the (2p1s)-transition leading to different mean excitation energies are (3.2±1.8)% and (5.0±2.0)%, respectively.We are indebted to the following institutes or organizations for financial support: Bundesministerium ffir Forschung und Technologic der Bundesrepubfik 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).     

Trace anomaly and chiral symmetry breaking scale in the Nambu and Jona-Lasinio model

Bound state pion production in⁴⁰Ca and¹²⁰Sn nuclei in (p,2p) and (n,d) reactions is investigated. The differential and total cross sections have been obtained. It is shown that the cross sections are determined by the overlap function of the bound states of the nucleon and pion, and as consequence,s-wave part of the pion-nucleus optical potential. The contributions from the nucleons and deuteron distortions are evaluated.The authors would like to express their gratitude to R.A. Eramzhyan, S.N. Gninenko, V.B. Kopeliovich, V.M. Lobashev, B.B. Matveev and E. Oset for very useful and stimulating discussions.     

The influence of Debye plasma on the ground state energies of exotic systems

The effect of plasma environment on the ground state energies of exotic systems ppμ, ddμ and ttμ has been analyzed within a generalized three-body formalism using multi-term correlated basis sets. The Debye screening model of the plasma has been adopted for such a study. The binding energies of p with pμ, d with dμ and t with tμ have been estimated for a range of values of the Debye screening parameters. The systems tend toward instability for increased screening. The effect of particle correlation has been investigated in detail and is found to play an important role for the stability in these systems.     

Σatomic states and the nucleon distribution in Pb

The analyses of (K^-,π) and (π^-, K⁺) reactions indicate that the nuclear potential of the Σ-hyperon is repulsive inside the nucleus, in agreement with the prediction of model F of the Nijmegen baryon-baryon interaction. This is consistent with the recent calculation of the strong-interaction shifts and widths of the observed levels of Σ^- atoms, including the precise data on the Σ^-Pb atom. In this paper, the sensitivity of this calculation to the neutron and proton density distributions is used to determine these densities in ²⁰⁸Pb.     

Formation and escaping of positronium in porous SiO2 films at low temperature

Positronium formation and escaping has been studied in porous silica thin films at temperature ranging from 13 to 300 K by 2-3 gamma ratio of positronium (3γ-PAS) measurements. Nanoporous silica thin films were deposited by spin coating on p-type (1 0 0) Si substrates and thermal treated in air at temperatures of 600 °C. Two different molar ratios of porogen (polyvinylpyrrolidone) were used in the TEOS-ethanol mixture to obtain samples with close porosity and connected porosity with the surfaces. In both types of sample a reduction of the 2-3 gamma ratio of positronium was observed by decreasing the temperature. This finding, in disagreement with the theoretical expectation, is discussed on the basis of the possible quenching mechanisms.     

Kaon absorption from kaonic atoms and formation spectra of kaonic nuclei

We considered the kaon absorption from atomic states into the nucleus. We found that the nuclear density probed by the atomic kaon significantly depends on the kaon orbit. Then, we re-examined the meanings of the observed strengths of one-body and two-body kaon absorption, and investigated the effects to the formation spectra of kaon bound states by in-flight (K ^-, p) reactions. As a natural consequence, if the atomic kaon probes a smaller nuclear density, the ratio of the two-body absorption at nuclear center is larger than the observed value in kaonic atoms, and the depth of the imaginary potential is deeper even at smaller kaon energies as in kaonic nuclear states because of the large phase space for the two-body processes. This deeper imaginary potential makes the signals of kaonic nucleus formation more unclear in the (K ^-, p) spectra.     

Determination of the central density on the basis of its moments

The value of the central density is of key importance for annihilation processes. For the ground state we discuss its determination from the moments of the ground state density. We first review the way of reaching the moments from the spectrum. In particular we show how to get the lowest moments in D = 3, namely 〈r⁻²〉 and 〈r⁻¹〉 from the series expansion of the Laplace transform of the density. We then recall a method to obtain the central density based on the Stieltjes moment problem. If the number of known moments is finite, this technique yields a lower bound. We investigate the possibilities to estimate the accuracy of the bound and the corresponding asymptotic value. An application to the muonic ²⁰⁸Pb atom is presented.