2,149 research outputs found
First Energy and Angle differential Measurements of e^+e^- -pairs emitted by Internal Pair Conversion of excited Heavy Nuclei
We present the first energy and angle resolved measurements of e+e- pairs
emitted from heavy nuclei (Z>=40) at rest by internal pair conversion (IPC) of
transitions with energies of less than 2MeV as well as recent theoretical
results using the DWBA method, which takes full account of relativistic
effects, magnetic substates and finite size of the nucleus. The 1.76MeV E0
transition in Zr90 (Sr source) and the 1.77MeV M1 transition in Pb207 (Bi
source) have been investigated experimentally using the essentially improved
set-up at the double-ORANGE beta-spectrometer of GSI. The measurements prove
the capability of the setup to cleanly identify the IPC pairs in the presence
of five orders of magnitude higher beta- and gamma background from the same
source and to yield essentially background-free sum spectra despite the large
background. Using the ability of the ORANGE setup to directly determine the
opening angle of the e+e- pairs, the angular correlation of the emitted pairs
was measured. In the Zr90 case the correlation could be deduced for a wide
range of energy differences of the pairs. The Zr90 results are in good
agreement with recent theory. The angular correlation deduced for the M1
transition in Pb207 is in strong disagreement with theoretical predictions
derived within the Born approximation and shows almost isotropic character.
This is again in agreement with the new theoretical results.Comment: LaTeX, 28 pages incl. 10 PS figures; Accepted by Z.Phys.
Performance of the PADME calorimeter prototype at the DANE BTF
The PADME experiment at the DANE Beam-Test Facility (BTF) aims at
searching for invisible decays of the dark photon by measuring the final state
missing mass in the process , with undetected. The
measurement requires the determination of the 4-momentum of the recoil photon,
performed using a homogeneous, highly segmented BGO crystals calorimeter. We
report the results of the test of a 55 crystals prototype performed
with an electron beam at the BTF in July 2016
Positron spectra from internal pair conversion observed in {238}U + {181}Ta collisions
We present new results from measurements and simulations of positron spectra,
originating from 238U + 181Ta collisions at beam energies close to the Coulomb
barrier. The measurements were performed using an improved experimental setup
at the double-Orange spectrometer of GSI. Particular emphasis is put on the
signature of positrons from Internal-Pair-Conversion (IPC) processes in the
measured e+ energy spectra, following the de-excitation of electromagnetic
transitions in the moving Ta-like nucleus. It is shown by Monte Carlo
simulations that, for the chosen current sweeping procedure used in the present
experiments, positron emission from discrete IPC transitions can lead to rather
narrow line structures in the measured energy spectra. The measured positron
spectra do not show evidence for line structures within the statistical
accuracy achieved, although expected from the intensities of the observed
transitions (E keV) and theoretical conversion
coefficients. This is due to the reduced detection efficiency for IPC
positrons, caused by the limited spatial and momentum acceptance of the
spectrometer. A comparison with previous results, in which lines have been
observed, is presented and the implications are discussed.Comment: LaTeX, 20 pages including 5 EPS figures; Accepted by Eur. Phys.Jour.
Characterization and Performance of PADME's Cherenkov-Based Small-Angle Calorimeter
The PADME experiment, at the Laboratori Nazionali di Frascati (LNF), in
Italy, will search for invisible decays of the hypothetical dark photon via the
process , where the escapes detection. The
dark photon mass range sensitivity in a first phase will be 1 to 24 MeV. We
report here on measurement and simulation studies of the performance of the
Small-Angle Calorimeter, a component of PADME's detector dedicated to rejecting
2- and 3-gamma backgrounds. The crucial requirement is a timing resolution of
less than 200 ps, which is satisfied by the choice of PbF crystals and the
newly released Hamamatsu R13478UV photomultiplier tubes (PMTs). We find a
timing resolution of 81 ps (with double-peak separation resolution of 1.8 ns)
and a single-crystal energy resolution of 5.7%/ with light yield of
2.07 photo-electrons per MeV, using 100 to 400 MeV electrons at the Beam Test
Facility of LNF. We also propose the investigation of a two-PMT solution
coupled to a single PbF crystal for higher-energy applications, which has
potentially attractive features.Comment: 12 pages, 19 figures. v2: added section on radiation damage studie
HITRAP: A facility at GSI for highly charged ions
An overview and status report of the new trapping facility for highly charged
ions at the Gesellschaft fuer Schwerionenforschung is presented. The
construction of this facility started in 2005 and is expected to be completed
in 2008. Once operational, highly charged ions will be loaded from the
experimental storage ring ESR into the HITRAP facility, where they are
decelerated and cooled. The kinetic energy of the initially fast ions is
reduced by more than fourteen orders of magnitude and their thermal energy is
cooled to cryogenic temperatures. The cold ions are then delivered to a broad
range of atomic physics experiments.Comment: 8 pages, 11 figure
Isotope shift in the dielectronic recombination of three-electron ^{A}Nd^{57+}
Isotope shifts in dielectronic recombination spectra were studied for Li-like
^{A}Nd^{57+} ions with A=142 and A=150. From the displacement of resonance
positions energy shifts \delta E^{142,150}(2s-2p_1/2)= 40.2(3)(6) meV
(stat)(sys)) and \delta E^{142,150}(2s-2p_3/2) = 42.3(12)(20) meV of 2s-2p_j
transitions were deduced. An evaluation of these values within a full QED
treatment yields a change in the mean-square charge radius of ^{142,150}\delta
= -1.36(1)(3) fm^2. The approach is conceptually new and combines the
advantage of a simple atomic structure with high sensitivity to nuclear size.Comment: 10 pages, 3 figures, accepted for publication in Physical Review
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