Coherent and incoherent π0 photoproduction from the deuteron

Differential cross-sections for the reactions d (γ,π⁰)d and d (γ,π⁰)pn have been measured at MAMI with the TAPS detector setup in the energy range 140 MeV < E _γ < 306 MeV. By use of the Glasgow tagging spectrometer an 0.8 MeV energy resolution for photons incident on the target was achieved. The π⁰ missing energy resolution was sufficient for a reliable separation of coherent and incoherent channels. The data for the break-up channel exhibit very strong final state interaction effects, whereas the observed angular dependence of the inclusive process d (γ,π⁰)X is in quantitative agreement with predictions for a quasi-free process. The observed absolute d (γ,π⁰)X cross-sections, on the other hand, are significantly smaller than predicted by the quasi-free process for E _γ >∼ 250 MeV. Associating this failure with the π⁰ photoproduction on the neutron would suggest that its cross-section is up to 25% below the presently believed value. Received: 13 February 2001 / Accepted: 13 April 2001     

Molecular relaxation effects in hydrogen chloride photoacoustic detection

A photoacoustic (PA) sensor has been developed to monitor hydrogen chloride at sub-ppm level in the 1740-nm region. The system was designed to control the process in the novel low-water-peak optical fiber manufacturing process. Relaxation effects in hydrogen chloride PA detection in oxygen–helium and nitrogen–helium gas mixtures are presented, showing that the generation of the PA signal is strongly affected by the ratio of these substances. In addition, the role of water vapor in the PA signal is investigated.     

Recombination of lithium-like bismuth and uranium ions with free electrons at the ESR

In two separate experiments at the ESR recombination of 230 MeV/u Bi⁸⁰⁺ and 170 MeV/u U⁸⁹⁺ ions was studied. The analysis of these experiments has been carried on in order to extract information about radiative recombination (RR) of very highly charged ions exposed to electrons with a density of few 10⁶ cm^-3. In the light of the recombination rate enhancement seen in almost all recent experiments at a relative energy E_rel = 0 eV, a surprising result is found for the very highly charged ions, which could add some confusion to an already puzzling picture: in spite of the high charge state of the investigated ions there is no evidence of enhanced recombination beyond the expected RR rate. This observation appears to be supported by other experimental findings at the ESR as well. We suspect, however, that this is a consequence of excessive transverse magnetic field components in the ESR cooler (which have meanwhile been corrected for) causing variable degrees of angular mismatch between the ion beam and the electron beam occurring when the ring settings are tuned for optimum cooling. This revised version was published online in August 2006 with corrections to the Cover Date.     

High‐pressure μSR studies of ferro‐ and antiferromagnetic metals

High‐pressure μSR experiments on ferromagnetic nickel and \alpha‐iron and antiferromagnetic chromium are reported. In Ni above 260 K B_Fermi was found to be proportional to the saturation magnetization, whereas at lower temperatures it is temperature independent apart from a small anomaly below 30 K which is presumably caused by a magnetoelastic interaction. There was no evidence for an occupation of metastable sites by the μ⁺ below the Curie temperature. By contrast, in \alpha‐Fe the temperature dependence of \curpartialB_μ/\curpartialp shows a structure which might be attributed to the occupation of excited muon states at elevated temperatures. High‐pressure zero‐field experiments on Cr performed in the temperature regime between 4.5 K and 8 K revealed a pressure dependence of B_μ as large as \curpartialB_μ/\curpartialp=-(89.15\pm 0.06)\times 10^-12 T/Pa. In terms of volume dependence a very large negative Grüneisen parameter \gamma =-27 was obtained. This revised version was published online in July 2006 with corrections to the Cover Date.     

P-N junctions in semiconducting TiO2 studied through computer modelling

The dependence of the formation of a p-n or p-p⁺ junction in semiconducting TiO₂ as a function of various parameters is investigated through a computer model. The effect of the polarizing conditions on the I–V characteristics is also discussed and related to experimental results. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Sept. 15–22, 1996     

Probing magnetic order in heavily doped La2-xSrxNiO4+\delta with μSR

We report μSR measurements on a series of compounds with composition La_2-xSr_xNiO_4+\delta where the net hole concentration x+2\delta is greater than 0.4. A magnetic transition is found in all the samples studied which occurs at a composition‐dependent temperature T_M. Below T_M, clear precession signals are observed in zero applied magnetic field. The possible muon sites are discussed within the context of dipole‐field calculations. This revised version was published online in July 2006 with corrections to the Cover Date.     

Generation of very slow polarized muons by moderation

At the Paul Scherrer Institute very slow, nearly 100% polarized, positive muons with an energy of \sim\mbox10 eV are produced by moderating a secondary beam of surface muons in a thin film of an appropriate condensed gases. These epithermal muons can be used as a source of a tertiary beam of tunable energy between \sim\mbox10 eV and \sim\mbox20 keV. Such a beam allows the μSR technique to be extended to the study of thin films and surfaces. In order to be able to perform time differential μSR experiments we have developed an ultra‐thin detector that registers the passage of keV muons and permits to trigger the experiment. The results achieved so far demonstrate that first investigations of thin film samples can be performed with the present set‐up. This revised version was published online in August 2006 with corrections to the Cover Date.     

On nearly parallel G2-structures

A nearly parallel G₂-structure on a seven-dimensional Riemannian manifold is equivalent to a spin structure with a Killing spinor. We prove general results about the automorphism group of such structures and we construct new examples. We classify all nearly parallel G₂-manifolds with large symmetry group and in particular all homogeneous nearly parallel G₂-structures.     

The structure of 32S I. Spectroscopy of highly-excited states

Assignments of I, π, T are made to 30 levels in ³²S between 7.35 and 11.76 MeV excitation energy, making the spectroscopy of the T= 0 states rather complete up to 10 MeV and that of the T = 1 states up to 12 MeV. A reassessment of existing data in the light of the new results clarifies the spectrum of I ^π = 1⁺, T = 1 states up to 15 MeV excitation energy. High-spin states (I = 52 - 7) below 10 MeV excitation energy have been investigated by n t γ angular-correlation measurements with the ²⁹Si(α, nγ) reaction at E _α 14.4 MeV. Five g-wave resonances of the ³¹P(p, γ) reaction, leading to the formation of I _π + 4⁺, 5⁺ states in ³²S, have been identified between 10 and 12 MeV excitation energy. The spectrum of T = 1 states between 10.7 and 12 MeV, has been investigated by measurements of γ-ray angular distributions on resonances of the ³¹P(p, γ) reaction and by measurements of resonance strengths. Several ³²S levels between 7.35 and 8.75 MeV excitation energy were studied as final states in resonance decays. Finally a search was performed for I ^π = 0⁺ resonances of the ²⁸Si(α, γ) reaction.     

Structure and polarity of 8CB films evaporated onto solid substrates

Brewster-angle reflection ellipsometry and surface optical second harmonic generation were used to study the growth of 4'-n-octyl-4-cyanobiphenyl (8CB) films evaporated in air onto polymeric and quartz glass substrates. The layer-by-layer growth of the films terminates after formation of two distinctive interfacial layers. Both of these two layers are polar and tilted. In the first layer the molecules lie nearly flat on the surface, while in the second layer they point on average about 50° toward the surface normal. The dipole moment of the second layer has a lower magnitude and an opposite direction with respect to the dipole moment of the first layer.