IDENTIFICATION OF EXCITED 10BE CLUSTERS BORN IN
TERNARY FISSION OF 252CF
A. V. DANIEL, G. M. TER-AKOPIAN, G. S. POPEKO, A. S. FOMICHEV,
A. M. RODIN, YU. TS. OGANESSIAN
Flerov Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research,
Dubna 141980, Russia
J. H. HAMILTON, A. V. RAMAYYA, J. KORMICKI, J. K. HWANG,
D. FONG, P. GORE
Department of Physics, Vanderbilt University,
Nashville, TN 37235, USA
J. D. COLE
Idaho National Engineering and Environmental Laboratory,
Idaho Falls, ID 83415, USA
M. JANDEL, L. KRUPA AND J. KLIMAN
Institute of Physics, Slovak Academy of Sciences,
Dubravska cesta 6, Bratislava, Slovak Republic and FLNR, JINR, Dubna, Russia
J. O. RASMUSSEN, A. O. MACCHIAVELLI, I. Y. LEE AND S. -C. WU
Lawrence Berkeley National Laboratory,
Berkeley, CA 94720, USA
M. A. STOYER
Lawrence Livermore National Laboratory,
Livermore, CA 94550, USA
R. DONANGELO
Instituto de Fisica Universidade,
Federal do Rio de Janeiro, Rio de Janeiro, 21945-970, Brazil
Ternary fission of 252Cf was studied at Gammasphere with using eight E×E particle
telescopes. The 3368 keV gamma transition from the first excited state in 10Be was
undoubtedly found. The ratio of the population probabilities for these two levels was
estimated as 0.160±0.025. The nuclear temperature of the neck region near the scission
point was estimated as 1.0±0.2 MeV. No evidence was found for 3368 keV γ rays
emitted from a triple molecular state.
1
2
1.
Introduction
The interest to nuclear fission accompanied by light charged particle (LCP)
emission is connected with the possibility to obtain additional data about the
fission mechanism. In spite of intensive investigations of these phenomena (see
reviews1, 2 and papers cited there a good quantitative explanation of the observed
kinematical characteristics and yield of LCPs has not been achieved. Mainly,
theoretical models concentrated only on the explanation of α-particle emission in
the fission for which the main experimental data have been obtained. More
recently, information about the kinematical characteristics of heavy LCPs and
their yields were improved3, 4 for the 252Cf spontaneous fission. The problem still
remained is a relatively high energy limit above which heavy LCPs were
measured.
A more direct way allowing the exploring of the fission nucleus
characteristics may be based on the assumption5 that the LCPs could be emitted
in their excited states. If thermal equilibrium is maintained near the scission
point then population of the LCPs in excited state can be connected with the
nuclear temperature. Reports of the observation of the 3368 keV γ ray
corresponding to the 10Be 2+→0+ transition in a ternary fission experiment with
252
Cf have been made6, 7. There was some evidence that the γ peak of 10Be was
seen without Doppler broadening. Taking into consideration the lifetime 125 fs
of 2+ level in 10Be it means6, 7 that the 10Be stays at rest for an unusual long time
or γ rays are emitted predominantly in the orthogonal direction to the 10Be
momentum. It was not the undoubted result because of the poor energy
resolution of NaI detectors used in the experiment. This idea has been tested by
using data of γ-γ-γ coincidences obtained at the experiment made on the
Gammashpere8 with high energy resolution. The data gave support for that
result, but with limited statistics and no direct LCPs identification. The
possibility that the 10Be nucleus may stay between two fission fragments for a
long time ~10-13s to create a so-called triple nuclear molecule opens up exciting
possibility discussed in9.
2.
Experiment
The experiment has been carried out at the Lawrence Berkeley National
Laboratory by using Gammasphere and eight light charged particle detectors.
Gammasphere was set to record γ rays with energy less then ~5.4 MeV. The
efficiency varied from a maximum value ~17% to ~4.6% at the γ energy
3368 keV.
A sample of 252Cf giving ~4×106 spontaneous fissions per second was
installed in the center of the reaction chamber, which was placed in a hollow
3
sphere inside Gammasphere. The source was prepared from a 252Cf specimen
that was deposited in a 5 mm spot on a 1.8 micron titanium foil and was tightly
covered on both sides by gold foils to exclude the coming out of fission
fragments from source.
Eight similar E×E Si detector telescopes were used to measure LCPs
emitted in the ternary fission. They were arranged in the reaction chamber with
four telescopes centered at the polar angle θ=30o (azimuth angles φ = 45o, 135o,
225o and 315o, and four at θ =150o (azimuth angles were the same). Each E
detector had an area 10×10 mm2 and thickness slightly varying between 9 μ and
10.5 μ. Each E detector was 400 μ thick and was 20×20 mm2 in area. The
distance from the source to E detector was 27 mm and from E detector to E
detector was 13 mm in all telescopes.
3.
Results and Discussion
The resolution of the E×E telescopes allowed us to well identify helium,
beryllium, boron and carbon nuclei, when energy deposition in the E detector
was greater then 5 MeV. From this value 5 MeV, we calculated the lower
primary energy of the LCPs registered in the experiment, as 9, 20, 24, 32 MeV
for He, Be, B, and C LCPs, respectively. The lithium region was shadowed by
the random coincidence of the ternary helium LCPs with the 252Cf α-decay
particles.
The detection of prompt γ rays coinciding with the Be LCPs permits to
search γ rays corresponding to the transition from the first excited state in 10Be.
The energy resolution of the E detectors does not allow the separation of
beryllium isotopes in the E×E plot, but experimental results presented in10 show
that the yield of beryllium LCPs emitting in the 252Cf spontaneous fission consist
of 10Be on ~80%.
In Fig. 1 the dotted line shows the spectrum of γ rays in coincidence with the
beryllium accompanied 252Cf spontaneous fission. The solid line on this figure
demonstrates the same spectrum after applying a Doppler correction. The
distinct peak in the last spectrum at the energy 3368 keV corresponds to the
2+→0+ transition in 10Be. The width FWHM=65.7±4.5 keV was calculated for
the peak by using a Gaussian fitting. This width is compatible with the value
FWHM=62.1 keV obtained by the Monte Carlo simulation of the procedure
introducing a Doppler correction for the γ rays emitting from the moving 10Be
nucleus. The simulation has been done taking into consideration the
experimental kinetic energy distribution of 10Be, the Gammasphere energy
resolution and the real 3d-position of all gamma and particle detectors. For
comparison, the dotted line in the Fig. 1 shows the spectrum of γ rays coinciding
4
with the emission of helium nuclei. It reflects, to some extent, the background
from the γ rays emitted from fission fragments.
One infers from Fig. 1 that the 3368 keV peak restored after Doppler
correction (see the solid line histogram) is smeared in the raw spectrum (dashed
line histogram) over an energy range extending from about 3000 keV to 3700
keV. Comparison made in Fig. 2 between the spectrum of γ rays coinciding with
Be LCPs and the spectrum obtained by Monte Carlo simulation gives evidence
that the 3368 keV γ rays are mainly emitted by moving 10Be nuclei.
Figure 1: The solid and dashed lines show the spectra of γ rays coinciding with Be LCPs
after and before the Doppler shift correction, respectively. The dotted line shows the
spectrum of γ rays coinciding with He LCPs. The last spectrum was normalized to the total
number of counts obtained in the whole spectrum associated with the Be LCPs
E  90  5000 keV .
In the raw spectrum of γ rays recorded in coincidence with Be LCPs we do
not see any distinct peak near 3368 keV, which could be associated with γ
emission from stationary 10Be nuclei (see Figs. 1 and 2). Obviously, γ rays
emitted from the moving 10Be in directions almost orthogonal to the 10Be
momentum result in a broad bump centered on the maximum of the 3368 keV
peak. To get rid of this bump we picked out only such events where the angles
between the trajectories of γ rays and 10Be nuclei were less than 45o or greater
than 135o. The resulting spectrum obtained after the background subtraction is
shown in Fig. 3. The energy region around 3368 keV now has a clear zero. In
other words, there is a lack of any γ line which could be attributed to the
emission from 10Be standing in a triple nuclear molecule.
5
Figure 2: Histogram drawn with a dotted line shows the spectrum of γ rays coinciding with Be
LCPs. It is obtained from the dashed line histogram of Fig. 1 after the subtraction of the
background shown by the dotted line in Fig. 1. The solid and dashed line histograms
demonstrate the spectra simulated using experimental angular distribution of 3368 keV γ and
under the assumption of isotropic γ emission from 10Be, correspondingly. Both spectra were
normalized to the number of counts obtained in the first one in the energy range
3000-3800 keV.
Figure 3: Histogram shows the spectrum of γ rays detected at angles <45o or >135o to the Be
momentum.
Using results on Fig. 1 we estimated the population ratio of the excited 2+ to
the ground-state 0+ levels in 10Be as 0.160±0.025. Having spin and energy of
these two states and assuming thermal equilibrium near the scission point, this
value allows one to estimate the temperature parameter as 1.0±0.2 MeV using
Boltzmann distribution. Most likely it is the estimation of the nuclear
temperature of the neck region at the scission point.
6
The upper limit for the probability that 10Be emits its γ rays being in rest
makes only 2% of this ratio. This result is valid for 10Be LCPs having at infinity
the kinetic energy more than 20 MeV. One can not exclude that the decay
channel of the hypothetical triple molecule is characterized by the low kinetic
energy of the 10Be LCPs which was unreachable in our experiment. The
Gammasphere experiment13 relied solely on the observation of γ-γ-γ coincidence
events, a possibility still remains open that a narrow γ peak characteristic to the
motionless 10Be can be found if the low energy part of the energy distribution of
10
Be LCPs is detected. The present result excludes any possibility that an effect
of triple quasi molecular state, involving beryllium LCPs, could be observed
in6, 7 where the energy cut off was 26 MeV for these clusters.
Acknowledgments
Work at Joint Institute for Nuclear Research was supported in part by the US
Department of Energy contract \#DE-AC011- 00NN4125, BBW1 Agreement
No.~3498 (CRDF grant RPO-10301-INEEL) and by the joint RFBR-DFG grant
(RFBR No. 02-02-04004, DFG No. 436RUS 113/673/0-1(R)). Work at
Vanderbilt University, LBNL, LLNL and INEEL are supported by U.S.
Department of Energy under Grant No. DE-FG05-88ER40407 and Contract
Nos. W-7405-ENG48, DE- AC03-76SF00098 and DE-AC07-76ID01570.
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