Coherent pi0 photoproduction on nuclei
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Date
08/2007Author
Tarbert, Claire M.
Metadata
Abstract
The coherent nuclear ¼0 photoproduction reaction is sensitive to the nuclear
matter form factor i.e. the distribution of matter within the nucleus. Accurate
measurements of the nuclear matter distribution as charaterised by
the r.m.s. radius are important for applications to nuclear theories, neutron
stars, atomic parity non-conservation and heavy ion collisions.
This thesis presents the results from a new experiment to measure coherent
¼0 photoproduction on 208Pb, 40Ca, 16O and 12C. The ultimate goal of the
research programme is to make a high precision measurement of the nature
of the neutron skin of 208Pb. The first major step in this direction is to
achieve an accurate data set of coherent pion photoproduction which is
presented in this thesis.
The experiment was performed in the A2 hall of the MAMI electron accelerator
facility at the InstitÄut fur Kernphysik, Mainz, Germany. An 883 MeV
beam of electrons was directed on to a 10¹m nickel radiator producing a
Bremsstrahlung photon beam which was then 'tagged' with a resolution
of 2 MeV using the Glasgow Photon Tagging Spectrometer. The photon
beam was incident on one of the 4 experimental targets inducing the reaction
A(°,¼0)A. The neutral ¼0s were then detected via their two photon
decay in the newly installed 4¼ Crystal Ball and TAPS detector systems.
The results from all four targets are presented as differential and total
cross sections covering the energy range E°=(135-300) MeV and covering
the full 180± of the pion polar angle. Comparisons have been made with
previous data and with the latest theoretical calculations of Dreschel et. al.
which employ detailed pion optical potentials in describing the pion-nucleus
final state interaction. It is concluded that the 208Pb cross sections show
good agreement with the calculations indicating that the pion-nucleus FSI
distortions are well accounted for by the model. A pleasing reduction in
statistical and systematic uncertainties from previous measurements is also
observed. While the 208Pb cross sections are finalised it is suggested that
the 12C and 16O data would benefit from a further analysis utilising the
coincident detection of nuclear decay photons to isolate incoherent events
and allow a more detailed comparison with theory.
A first comparison of the theoretical model with the new high quality data
gives first indications of a neutron skin on 208Pb. A future scheme for the
full detailed extraction of the matter distribution from the new data set is
also suggested.