Photon scattering cross sections below the photoneutron threshold for Pb206, Pb208, and Bi209
Stein, Nelson
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https://hdl.handle.net/2142/23960
Description
Title
Photon scattering cross sections below the photoneutron threshold for Pb206, Pb208, and Bi209
Author(s)
Stein, Nelson
Issue Date
1964
Doctoral Committee Chair(s)
Axel, P.
Department of Study
Physics
Discipline
Physics
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
photon scattering
photoneutron threshold
Bremsstrahlung monochromator
monoenergetic photons
Language
en
Abstract
The elastic scattering of photons has been measured as a function
of energy below the (r,n) threshold. A bremsstrahlung monochromator
was used to provide nearly monoenergetic photons of energy resolution
between 1 and 2%. Cross sections were obtained for Pb 206 between 5.9
and 8.2 MeV, for Pb 208 between 6.6 and 7.5 MeV, and for B~0 209 between
5.1 and 7.7 MeV. In all three cases, structure and sharp fluctuations
with energy were observed.
The Pb208 cross section contains three strong peaks centered
at 6.72, 7.03~ and 7.29 MeV. Each peak is consistent with the experimental
resolution and is most likely dominated by a simple strong level whose
width is much less than the resolution. The quantity r 2/r (r and r o 0
are the total and ground state radiative widths) for each peak is
14 ~ 7 eV at 6.72 MeV, 30 t 13 eVat 7.03 MeV, and 41 ~ 10 eV at 7.29 MeV.
Additional resonant absorption measurements indicate that r o/ r is very
close to 1.
The widths associated with the structure in Pb206 and Bi209~ere
generally greater than the resolution. In Pb 206 a well defined
resonance about 400 kev wide is centered at 7.85 MeV with a peak cross
section of 20 mb. Another maximum occurs at 7.45 MeV, and further
structure superimposed on a generally decreasing cross section is
suggested as the energy decreases. In ii2 09 the scattering strength
is concentrated into two large peaks with maximum cross sections of 21
and 27 mb at 5.5 and 7.3 MeV, respectively. Separating the peaks is a
broad valley in which the average cross section is about 1/7 of the
observed maxima.
o 208 . 206
In contrast w~th Pb ,the structure observed in the Pb and
ii2 09 cross sections represents the average contribution from many unresolved
narrow levels included within the resolution. Resonant
absorption measurements indicate that at 7.3 MeV in ii209 the average
spacing, D, between levels having the ground state spin is about 1.5 kev,
and the average radiative width, r-o, to the ground state is about 0.2 eVe
Estimates at 7 MeV for Pb206 indicate approximately similar values.
Since ~(1,1) averaged over the resolution is proportional to
(r 2/r )/D, it can be shown that similar structure is very likely present o
in the ground state photon transition strength function» ro/D. The widths
of only several hundred kev that are involved in this structure may be
important for the general question of the distribution of nuclear
transition strength over available energy levels. Accepted values of
the imaginary part, W, of the optical potential would imply strength
function widths much larger than those observed in this experiment.
Fluctuations in rolD, rather than the monotonic energy dependence
usually expected, may require a more cautious interpretation of measurements
of gamma ray strength than has appeared necessary up to now.
Where structure is present, results obtained either in isolated narrow regions or over broad regions with poor resolution may not be reliable
for determining the general behavior of r-o/ D.
The results provide lower limits for the photon absorption
cross sections below the (r,n) threshold; for Pb 208 ,upper limits are
also given. Similarities among the three nuclei, both in scattering
and in the photoneutron cross sections just above the threshold,
suggest the likelihood that the observed photon strength involves
primarily the Pb208 core. The magnitude of the total strength observed
throughout the threshold region is consistent with the two estimates of
electric dipole strength that are closely related to the giant dipole
resonance.
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