Photoproduction of neutral pions in helium by 335 MEV Bremsstrahlung
Staples, John William
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https://hdl.handle.net/2142/25778
Description
Title
Photoproduction of neutral pions in helium by 335 MEV Bremsstrahlung
Author(s)
Staples, John William
Issue Date
1969
Doctoral Committee Chair(s)
Hanson, A.O.
Department of Study
Physics
Discipline
Physics
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
neutral pions
helium
Bremsstrahlung
recoil nucleus
semiconductor telescope
Language
en
Abstract
The differential cross section was measured for the, coherent production of neutral pions from He4 by photons to
an energy of 335 MeV. Bremsstrahlung from the University of
Illinois 340 MeV betatron impinged upon a gaseous He4 target
cooled to 20.4oK. The recoil nucleus was detected with a semiconductor
telescope in coincidence with at least one gamma
resulting from the decay of the neutral pion. The particle
events were recorded on photographic film in both analogue
and digital form. The resolution of the recoil telescope 4
was sufficient to permit separation of the He recoils from
the background of He3 recoilS and a11owed'a better identification
of the coherent He4 events that had been achieved in
earlier experiments.
The laboratory cross sections were found to vary
from 40 ub/sr at 200 and 230 MeV photon energy decreasing with
energy to 55 ub/sr for the single measurement at 500 for a
photon energy of 320 MeV. These cross sections are, moderate1y
smaller than those given by Palit and Bellamy and are much
smaller than earlier measurements by DeSaussure and Osborne.
The experimental cross sections are compared with
those calculated by means of an impulse approximation, using
the amplitudes for the photoproduction of neutral pions from
nucleons.
The laboratory energy and angle of the He4 recoil
define the kinematic variables in the He4 system but do not
explicitly define the variables in the nucleon system, which
are required to evaluate the nucleon photoproduction amplitude.
The correct evaluation of these variables is a difficult problem
involving appropriate averages of the effects of nucleon
motion within the nucleus. Three simplified ways of specifying
the variables in the nucleon frame are presented and
compared to each other with respect to their usefulness of
representing the experimental results. Calculations done
elsewhere show that corrections to the impulse approximation
for interactions of the pion in the final state reduce the
predicted cross section by about a factor of two in the
resonance region. Using this correction and specifying the
center of mass pion momentum and gamma energy, variables equal
in the nucleon and helium frames gives reasonable agreement
with the experimental cross sections.
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