Fusion06 – Intl. Conf. on Reaction Mechanisms
and Nuclear Structure at the Coulomb Barrier
S. Servolo, Venezia,
March 19-23, 2006
Fusion-fission of 16O+197Au
at sub-barrier energies
Birger Back
Argonne National Laboratory
Argonne National Laboratory is managed by
The University of Chicago for the U.S. Department of
Energy
Outline
•
Motivation: Does sub-barrier hindrance apply to fusionfission processes?
•
Experiment –
•
Cross section results
•
S-factor representation
•
What’s next
16O+197Au
fission at sub-barrier energies
2
Pioneering
Science and
Technology
Fusion06 – Intl. Conf. on Reaction Mechanisms and Nuclear Structure at the Coulomb Barrier
S. Servolo, Venezia , March 19-23, 2006
B.B.Back, Argonne National Laboratory
Sub-barrier fusion hindrance
•
Deviation from standard fusion models at sub µb level
-
•
Jiang et al. PRL 89, 52701 (2002)
Jiang et al. PRC 69, 14604 (2004)
Jiang et al. PRL 93, 12701 (2004)
Jiang et al. PRC 71, 44613 (2005)
Jiang et al. PRC 73, 014613(2006)
Onset of fusion hindrance
Most cases evaporation residues
16
O+208 Pb
3
Pioneering
Science and
Technology
Fusion06 – Intl. Conf. on Reaction Mechanisms and Nuclear Structure at the Coulomb Barrier
S. Servolo, Venezia , March 19-23, 2006
B.B.Back, Argonne National Laboratory
Existing data
16O+197Au
cross sections:
Fission:
T.Sikkeland et al., Phys. Rev. 135, 669 (1964)
D.J.Hinde et al., Nucl. Phys. A452, 550 (1986)
Evaporation Residues:
K.-T.Brinkmann et al, PRC 50, 309 (1994)
D.J.Hinde et al.
Need data down to µb level to study
sub-barrier hindrance
Figure from: L.Corradi et al. Phys. Rev. C71, 14609 (2005)
4
Pioneering
Science and
Technology
Fusion06 – Intl. Conf. on Reaction Mechanisms and Nuclear Structure at the Coulomb Barrier
S. Servolo, Venezia , March 19-23, 2006
B.B.Back, Argonne National Laboratory
Experimental arrangement
Mon
n Measure fission fragments emitted into
backwards angles to minimize elastic
scattering count-rate.
n Large solid angle – four quadrants
5
Pioneering
Science and
Technology
Fusion06 – Intl. Conf. on Reaction Mechanisms and Nuclear Structure at the Coulomb Barrier
S. Servolo, Venezia , March 19-23, 2006
B.B.Back, Argonne National Laboratory
Quad PPAC detector – posisition sensitive
y (inches)
A
B
D
C
x (inches)
6
Pioneering
Science and
Technology
Fusion06 – Intl. Conf. on Reaction Mechanisms and Nuclear Structure at the Coulomb Barrier
S. Servolo, Venezia , March 19-23, 2006
B.B.Back, Argonne National Laboratory
Identification of fission fragments
Wedge B – 76.9 MeV σfis ~2 mb 16O+197Au
Time-of-flight (channels)
fixed ∆E ratio
Analysis method:
nDetermine location and shape of fission fragment
distributions at above barrier energies
Fission
n Fixed TOF difference between fission and elastics
n Fixed ratio of fission and elastics energy loss
n Determine fraction of fission fragments falling
within five shrinking elliptical cuts of 100%, 90%,
80%,70%, and 60% of original cut (linear)
fixed ∆T
16O
elastic
n Use same cut to count fission fragments at low
beam energies
Energy Loss (channels)
7
Pioneering
Science and
Technology
Fusion06 – Intl. Conf. on Reaction Mechanisms and Nuclear Structure at the Coulomb Barrier
S. Servolo, Venezia , March 19-23, 2006
B.B.Back, Argonne National Laboratory
Correction for size of ellipse
As ellipse shrinks – fewer fission fragments
are included
Average correction factor computed for
11 measurements above 74.8 MeV
70%
100%
90%
60%
80%
Final cross section is independent of
which cut is used
Smaller cut will improve signal/background
ratio
Decided to use 80% cut for cross section
calculation
8
Pioneering
Science and
Technology
Fusion06 – Intl. Conf. on Reaction Mechanisms and Nuclear Structure at the Coulomb Barrier
S. Servolo, Venezia , March 19-23, 2006
B.B.Back, Argonne National Laboratory
Background subtraction at lowest energies
Wedge B – 72.0 MeV σfis ~6 µb 16 O+197Au
Time-of-flight (channels)
fixed ∆E ratio
Background: 2
Fission:14
fixed ∆T
16O
elastic
Energy Loss (channels)
9
Pioneering
Science and
Technology
Fusion06 – Intl. Conf. on Reaction Mechanisms and Nuclear Structure at the Coulomb Barrier
S. Servolo, Venezia , March 19-23, 2006
B.B.Back, Argonne National Laboratory
Correction for anisotropy - PRELIMINARY
14N+197Au
V.E.Viola et al, Phys. Rev. 129, 2710 (1963)
Lab range: 2.2 – 23.6 degrees
CM range: 2 – 22 degrees
10
Pioneering
Science and
Technology
Fusion06 – Intl. Conf. on Reaction Mechanisms and Nuclear Structure at the Coulomb Barrier
S. Servolo, Venezia , March 19-23, 2006
B.B.Back, Argonne National Laboratory
Fission cross section
n Fission cross section extended by
three orders of magnitude
n Good agreement with Sikkeland data
(except for lowest point ?)
PRELI
MINAR
Y
n Discrepancy with Hinde data ???
11
Pioneering
Science and
Technology
Fusion06 – Intl. Conf. on Reaction Mechanisms and Nuclear Structure at the Coulomb Barrier
S. Servolo, Venezia , March 19-23, 2006
B.B.Back, Argonne National Laboratory
Fission cross sections & S-factor maximum
Do we see a maximum in the S-factor?
Fit to logarithmic slope excluding lowest
points (Cheng-Lie Jiang)
Maybe?
12
Pioneering
Science and
Technology
Fusion06 – Intl. Conf. on Reaction Mechanisms and Nuclear Structure at the Coulomb Barrier
S. Servolo, Venezia , March 19-23, 2006
B.B.Back, Argonne National Laboratory
Comparison to systematics
13
Pioneering
Science and
Technology
Fusion06 – Intl. Conf. on Reaction Mechanisms and Nuclear Structure at the Coulomb Barrier
S. Servolo, Venezia , March 19-23, 2006
B.B.Back, Argonne National Laboratory
Future plans – measure σER to 10 µb level
ATLAS approved experiment
to measure evaporation residues
via delayed α decays
14
Pioneering
Science and
Technology
Fusion06 – Intl. Conf. on Reaction Mechanisms and Nuclear Structure at the Coulomb Barrier
S. Servolo, Venezia , March 19-23, 2006
B.B.Back, Argonne National Laboratory
Summary
fission cross section to µb level
•
16O+197Au
•
Maybe observe sub-barrier hindrance in fission channel
•
Future: Measure evaporation residues to same or lower energies
•
Compare to Misicu/Esbensen model
15
Pioneering
Science and
Technology
Fusion06 – Intl. Conf. on Reaction Mechanisms and Nuclear Structure at the Coulomb Barrier
S. Servolo, Venezia , March 19-23, 2006
B.B.Back, Argonne National Laboratory