Photoneutron spectra from monoenergetic photons on Pb208 in the giant resonance
Calarco, John Richard
This item is only available for download by members of the University of Illinois community. Students, faculty, and staff at the U of I may log in with your NetID and password to view the item. If you are trying to access an Illinois-restricted dissertation or thesis, you can request a copy through your library's Inter-Library Loan office or purchase a copy directly from ProQuest.
Permalink
https://hdl.handle.net/2142/25789
Description
Title
Photoneutron spectra from monoenergetic photons on Pb208 in the giant resonance
Author(s)
Calarco, John Richard
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)
photoneutron spectra
monoenergetic photons
giant resonance
time-of-flight of photoneutrons
Language
en
Abstract
The time-of-flight of photoneutrons from Pb208 was
observed at 1150 for incident gamma ray energies of 9.90 MeV
to 16.87 MeV. The University of Illinois 25 MeV betatron and
bremsstrahlung monochromator facility were used to produce the
gammas and determine their energies within 1.7% (full width).
Neutrons above 200 keV were detected with a 5 inch
diameter by 2 inch deep glass cell filled with NE213 liquid
sCintillator, viewed by three RCA 8575 photomultipliers.
Coincidences between the photomultipliers were used to reject
random noise pulses', and neutrons were identified against the
high gamma background by pulse-shape discrimination.
The neutron energy resolution at 1.46 m was +3.5%
at 6 MeV and +2.5% for en~rgies less than about 2.5 MeV.
The total cross section was extracted as well as
partial cross sections fur those final states which could be
resolved. The gamma absorption cross section reached a maximum
of 650 mb at 13.25 MeV, with a width of about 4 MeV. The
shape was generally Lorentzian, but structure was observed at
10 MeV, 11 MeV, and 12 MeV. The peak at 10 MeV was found to
be due to neutrons leaving Pb207 in the (P3/2)- state.
The spectra were analyzed by assuming that the gamma
ray is absorbed by exciting a single, well defined dipole state,
which is a linear combination of elementary particle-hole
excitations. The decay of this state was assumed to be either
by direct neutron emission from a particle-hole state or by
thermalization and subsequent neutron evaporation.
At low gamma energies, individual final states were
resolved, but the partial cross sections were not accounted
for in detail by the model. At higher energies, between 12 MeV
and 17 MeV, the majority of emitted neutrons were distributed
as would be expected from neutron evaporation characterized by
a nuclear temperature of 1.0 MeV. An excess of fast neutrons
was observed in all the speotra which increased from about 10%
at 12 MeV to about 15% at 14.5 MeV where it leveled off. This
excess was found to be consistent with that expected from
direct emission using available particle-hole amplitudes, it
was dominated by emission to the (f5
/ 2 )- and (f7/ 2 )- states in
Pb207 •
The fast excess did not increase as rapidly as
expected. This indicated that the total width of the dipole
state increased by about a factor of 3 between 12 MeV and 17 MeV.
A crude estimate of the absolute magnitude of this total width,
based on the observed 15% fast exoess, was smaller than the
observed width of the giant resonance by about a factor of 2.
This discrepancy and the observed Variation in the direct excess
indicated either a breakdown of the assumption of a single
dipole state or structure in the statistical width.
Use this login method if you
don't
have an
@illinois.edu
email address.
(Oops, I do have one)
IDEALS migrated to a new platform on June 23, 2022. If you created
your account prior to this date, you will have to reset your password
using the forgot-password link below.
