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A monoenergetic positron (e § beam (A T/T< 5.10 -4) from the Stuttgart pelletron accelerator and a 4.6mg/cm 2 Be target has been employed to investigate e § e-scattering in the MeV region in a large solid angle with high statistics by means of a novel positionsensitive detector system. Superimposed on the Bhabha scattering, a structure has been found at an e § kinetic energy of 2263 keV (810keV excitation energy in the e+e -rest frame). From the energy-integrated resonance cross-section of ares' Fr;~ e-_____ 30 b. eV (c.m.) and the standard resonance cross-section we estimate partial resonance widths Fr~ e-~-72 meV or 24 meV for total angular momenta J = 0 or J = i, respectively. The structure, which has not been predicted within the framework of quantum electrodynamics, coincides with one of the sum energies of the correlated e § e-lines observed in heavy-ion collisions. 13.10.+q; 12.20.Fv; 14.80.Pb PACS: ExperimentThe detection of momentum-and energy-correlated e § e-pairs in heavy-ion collisions [-1, 2] has initiated a search for corresponding structures in elastic e § escattering [3][4][5][6][7][8]. Continuing our experiments on e § e-scattering at MeV energies [5], we have investigated the region around the structure observed at 810 keV excitation energy in heavy-ion collisions [2] with improved set-up and higher statistics.Positrons from a 26 mCi 22Na source, thermalized in a 5 gm tungsten moderator and accelerated in the Stuttgart pelletron [9], were deflected by 20 ~ in a large (if5 =40 cm) iron-free coil and focused onto a 4.6 mg/cm 2 polycrystalline beryllium target (thickness 25 ~m) or onto an aligned Be single crystal of 30 gm thickness (cf. Fig. 1). Position and divergence of the e + beam were monitored steadily by four active scintillator slits mounted 50 cm upstream of the target and leaving a 4 x 4 mm 2 aperture. Together with slits of 3.5 x 3.5 mm 2 aperture at the bending coil (4 m upstream) they defined size and divergence of the e § beam. By reversing the current of the coil, we could deflect the e § beam into the opposite direction and implant it into a window-less intrinsic Ge planar detector. This procedure enabled us to monitor the beam energy Tbe,m on-line within _+0.5keV. From the comparison with a 228Th source with its 2.615 MeV y line and the corresponding single and double escape photolines, we estimate that the quoted positron energies are accurate within _+ 4 keV on an absolute scale. The stability of the beam energy during a run of typically two hours was better than +_ 0.6 keV. The possibility to determine the beam energy at any time in a simple manner (without having to readjust the beam) allowed us to reproduce any beam energy for repeating the measurements.The detection system (cf.[5]) has been optimized

The paper reports on the recovery of irradiation‐induced defects in thin aluminium films as studied by measurements of the electrical 1/f noise. The defects are produced by irradiation with 1 MeV electrons at 10 K, which results in a strong increase of the noise. Subsequent isochronal annealing at progressively higher temperatures causes the 1/f noise measured at 10 K to recover in discrete steps which occur at the same temperatures as the well‐known recovery stages of the irradiation‐induced electrical resistivity increase. In the noise measurements at 40 K, however, an additional recovery step without analogue in the recovery spectrum of the electrical resistivity is observed at 70 K. The temperature dependence and annealing behaviour of the 1/f noise may be understood in terms of thermally activated motion of defects in a distorted lattice potential. A microscopic explanation of the present observation as well as of earlier measurements on Cu by Pelz and Clarke within the framework of the two‐interstitial model of radiation damage of metals is presented.

Independent investigations of positron creation in heavy-ion collisions at the UNILAC accelerator at GSI have reported unexpected narrow e+-lines [],2]. Recent e+e -coincidence measurements [3,4] hffve revealed even narrower e+e -sum lines at energies of about 620 keV and 8]0 keV with strong indication of back-to-back emission. Among the many suggested interpretations were the decay of a hitherto unobserved neutral particle [5,6,7] with a mass of about 1.8 MeV and the decay of a e+e -bound state with a radius of a few fro, formed by magnetic interaction [8,9]. This has prompted us to investigate the inverse process, viz. e+e -scattering corresponding to e+e -c.m. energies E* of &bout 620 keV and 810 keV. A monoene~getic beam of typically 1.3,104 e+s -t was obtained by thermalizing in a 3 #m thick W-moderator positrons from a 33 mCi 22Na source in the newly installed vertical electrostatic (pelletron) accelerator at the Max-Planck-Institut ffir Metallforschung. The beam (diameter ~ 5 ram) was focussed onto a low-Z target (3.2 mg/cm 2) of aluminized mylar (C~H402) mounted 40 cm downstream the collimator slits and was stopped 7 cm below the target in a pl&stic scintillator serving as veto counter. The beam energy was controlled using a rotating voltmeter calibrated by means of a Ge detector before and after the experiment (estimated uncertainty of its absolute value less than 4-20 keV, total energy spread < 5keV). The detector system, a conical plastic scintillator consisting of two optically separated parts (Fig.l), has been designed for optimal detection of e+e -scattering events dud permits a very effective background suppression through & complete determination of the scattering kinematics. Characteristic resolutions (FWt{M, measured with 1 MeV e +) are At = 8 * 10-1~ and AE/E = 7% in time and energy, A0 = 2.4 ~ and ACi = 8 ~ over the ranges 15 ~ < # _< 45 ~ and 0 ~ _< ~ < 360 ~ of the polar and azimuthal scattering angles # and q~. The e+e -scattering events are selected by means of the following constraints: l) Coincidence of two events ir~ the two detector parts, 2) sum of polar scattering angles 0t + 02 following relativistic e+e --scattering kinematics within A~ = +8 ~ 3) deviation of the azimuthM angles 0t and ~b2 from coplanarity by less than A0 = i15 ~ 4) energy sum E1 + E2 deviating from Eb~a,~ by less than +7%.target

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