PHYSICAL REVIEW C 74, 017309 (2006)
New results for the intensity of bimodal fission in barium channels of the
spontaneous fission of
252
Cf
C. Goodin,
1
D. Fong,
1
J. K. Hwang,
1
A. V. Ramayya,
1
J. H. Hamilton,
1
K. Li,
1
Y. X. Luo,
1,2,3
J. O. Rasmussen,
3
S. C. Wu,
3
M. A. Stoyer,
4
T. N. Ginter,
5
S. J. Zhu,
1,2,6
R. Donangelo,
7
G. M. Ter-Akopian,
8
A. V. Daniel,
8
G. S. Popeko,
8
A. M. Rodin,
8
and A. S. Fomichev
8
1
Physics Department, Vanderbilt University, Nashville,Tennessee 37235, USA
2
Joint Institute for Heavy Ion Research, Oak Ridge, Tennessee 37830, USA
3
Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
4
Lawrence Livermore National Laboratory, Livermore, California 94550, USA
5
National Superconducting Cyclotron Laboratory, Michigan State University, East Lansing, Michigan 48824, USA
6
Department of Physics, Tsinghua University, Beijing 100084, People’s Republic of China
7
Universidade Federal do Rio de Janeiro, CP 68528, RG Brazil
8
Flerov Laboratory of Nuclear Reactions, JINR, Dubna, Russia
(Received 5 May 2006; published 21 July 2006)
Triple coincidence data from the fission of
252
Cf were used to deduce the intensity of the proposed “hot” mode
in barium channels. γ -γ -γ and α-γ -γ fission data were analyzed to find the neutron multiplicity distribution for
several binary and ternary charge splits. The binary channels Xe-Ru and Ba-Mo were analyzed, as well as the
Ba-α-Zr, Mo-α-Xe, and Te-α-Ru ternary channels. An improved method of analysis was used to avoid many of
the complexities associated with fission spectra. With this method, we were unable to confirm the second mode
in the either the Ba-Mo or Ba-α-Zr splits.
DOI: 10.1103/PhysRevC.74.017309 PACS number(s): 25.85.Ca, 27.90.+b
Observations of prompt γ rays produced in the spontaneous
fission of
252
Cf have shown evidence for a “hot” fission mode in
the Ba-Mo channel. The evidence for this mode is observed as
a higher relative intensity for the 7–10 neutron channels [1–3].
Later analysis [4–6] did not confirm the second mode, but
Refs. [5,6] did show an “irregularity” around the eight-neutron
channel. In recent years, more complete data on the levels and
relative intensities of transitions in barium and molybdenum
isotopes has become available. There is also recent evidence
that the second mode might be seen in ternary fission [7].
Because fission spectra are often complex and the events of
interest are rare compared with other channels, this type of
analysis is difficult and prone to errors caused by random co-
incidences. Therefore an improved method that avoids many of
these complexities was developed to determine the relative in-
tensity of the second mode in both the binary and ternary cases.
The data for this analysis come from two experiments using
the Gammasphere detector array located at Lawrence Berkeley
National Laboratory. The binary data were taken in November
2000. A 62-µCi source was placed between two iron foils to
stop fission fragments. This arrangement was then put in a
7.62-cm polyethylene ball and placed in Gammasphere. The
5.7 × 10
11
γ -γ -γ events were recorded. A coincidence cube
was then constructed using the RADWARE software package [8].
The ternary data were taken in December 2001. A 35-µCi
252
Cf
source was deposited as a 5-mm spot on a 1.8-µ Ti foil covered
on both sides by gold foils. Eight E-E detectors were placed
around the source to detect light-charged particles (LCPs).
The 9.0 × 10
5
LCP-γ -γ events were recorded [9] and a γ -γ
matrix was constructed. The ternary data were analyzed using
the γ -γ matrix peak fitting software written by Andrei Daniel.
Earlier versions of this software were used by Refs. [1–3].
The number of prompt neutrons emitted in a fission event
can be determined by finding the mass number of the fragments
produced in the event. For example, if the fission fragments
of
252
Cf are determined by some method to be
144
Ba and
103
Mo, then five neutrons must have been emitted. The relative
intensity of a particular neutron channel can be found from
triple coincidence data by double gating on a pair of transitions
in the heavy fragment, then measuring the intensity of its
partners. This must be done for each isotope of the heavy
partner. The yield as a function of neutrons emitted can then
be determined by summing the contributions of all possible
pairs.
In practice, the yield is found by fitting peaks in double-
gated spectra to find the area. For example, if a double
gate is taken on
142
Ba, then the resulting spectra will show
the 171.5-keV peak from
106
Mo, as well as the unresolved
peaks of
104,108
Mo at 192.0 and 192.7 keV, respectively. The
number of counts in the
106
Mo peak gives the relative intensity
of the
142
Ba-
106
Mo part of the four neutron channel. This
number must be corrected for detector efficiencies and internal
conversion coefficients. In principle this must be done for all
possible sets of transitions to the ground state in each barium
and molybdenum isotope to count all events. However, for
this analysis only the strongest transitions in each barium
and molybdenum isotope were measured. Because the relative
intensities of other transitions are now known in the isotopes
of interest, these values were used instead of attempting to
measure each peak. Tables I and II show the yield matrices for
the Ba-Mo and Ba-α-Zr splits calculated in this way. Note that
the values given in the table are scaled and do not match the
values in the following figures, which are normalized so that
the total area of the Gaussian fit is equal to 1. Summing along
0556-2813/2006/74(1)/017309(4) 017309-1 ©2006 The American Physical Society