Transcript Fission fragment angular distribution of U

Fission fragment angular distribution
of U-234(n,f) and U-236(n,f)
Durán, C. Paradela, D. Tarrío
Universidad Santiago de Compostela
L. Audouin, L.S. Leong, L. Tassan-Got
Institute de Physique Nucleáire d´Orsay
Motivation
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FFAD are scarcely known above several MeV for most of the isotopes,
including second and third fission chance of uranium isotopes.
U-234 exhibits a vibrational resonance in the threshold of the fission
chance. Large fluctuations [1] in the FF properties have been observed
around these type of resonances. Better candidate than Th-232 because of
the higher cross section.
U-236 cross section has been measured with FIC and a PPAC
measurement can confirm it and extend the energy range beyond 2 MeV.
Even stronger anisotropies have been measured, but less information
available in databases.
Good knowledge of FFAD would improve significantly the uncertainty of
PPAC measured cross section at threshold regions (strong anisotropies)
and provides additional information (sensitive to the quantum numbers of
the deformed fissioning nucleus [2]).
Information on angular momentum deposition at intermediate energies.
Preliminary data from Th are in agreement with previous data, but
discrepant with respect to proton data.
[1] A. Goverdovsky et al., Sov. J. Nucl. Phys. 44, 179 (1986)
[2] N. H. Behkami et al., Phys. Rev 171, 1267 (1968)
U-234 case
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Three targets already available at IPN.
Past PPAC measurements are short in
statistics (two targets during less than
two weeks measurement) to perform a
detailed studied on the vibrational
resonance region.
With PPAC Phase-I setup, poor angular
distribution was obtained because the
limited angular acceptance and high
detection thresholds. Lower efficiency for
the strongly anisotropic energies of
interest.
Old and recent measurements in FFAD
[1,2] present limitations in the studied
energies. Low sensitivity to vibrational
resonance fluctuations, if they exist. At
n_TOF high-resolution,
[1] Leachman, Phys. Rev 137, B814
[2] J. Hambsch, ND2010 proceedings
U-234 case: fission threshold landscape
U-236 case
• Cross section similar to U-234, even larger
anisotropies at fission threshold and less
experimental data.
• Completing systematics angular
distributions for uranium isotopes at the
second [*] and third chance energies.
• No targets yet, material maybe available at
Munich (?), possible target fabrication at
CACAO.
[*] Simmons et al.,Phys. Rev. 120 198 (1960)
Intermediate energy situation
• Preliminary results on Th-232
measurements look in
agreement with Tutin data.
• High anisotropy is still
remaining at intermediate
energies for neutron data.
• Study of new isotopes will
confirm such trend.
Proton request
• Some numbers for U-234 and 3x1018 protons request taking 2010
Thorium data as reference:
– 5000 events per target and per bin around 800 keV (100 log bins
per decade)=> 50 cos (theta) bins with 100 counts per bin. To be
multiplied by the number of targets.
– Factor 3 less events at 500 keV and factor 10 at 300 keV.
– Even more dense binning can be performed for studying the cross
section: 1000 bins per decade=> 50 counts per bin and target in
the 300 keV region
– At second and fission chances, similar counting rates to those
expected at 500 keV.
– Slightly less counts are expected for U-236 around the threshold.
• 9 target slots: 2-3 U-234 + 1 or 2 U-236 + 1 U-235+ 2-4 Th-232
Expected angular distributions
7/2 spin for compound 235U*
Another FF properties with PPAC
• Separation of fission
modes by time difference
measurement.
• At least symmetric from
asymetric modes seems
possible.
Proton request
•
Some numbers for U-234 and 3x1018
protons request taking 2010 Thorium data
as reference:
–
–
–
–
–
5000 events per target and per bin around
800 keV (100 log bins per decade)=> 50 cos
(theta) bins with 100 counts per bin. To
multiply by the number of targets.
Factor 3 less events at 500 keV and factor 10
at 300 keV.
Even more dense binning can be performed
for studying the cross section: 1000 bins per
decade=> 50 counts per bin and target in the
300 keV region
At second and fission chances, similar
counting rates to those expected at 500 keV.
Slightly less counts are expected for U-236
around the threshold.
Figure 44 :: Number
Number of
of counts
counts in
in each
each energy
energy bin
bin
Figure
234
234
(size of
of 0.01)
0.01) for
for the
the 22 targets
targets of
of U
U and
and for
for 33
(size
18
18
10 incident
incident protons
protons
10