x-ray superburst ~10 - indiana university...
TRANSCRIPT
Neutron stars, remnant cores following supernova explosions, are highly interesting astrophysical environments
In particular, accreting neutron stars presents a unique environment for nuclear reactions
Identified as the origin of energetic X-ray superbursts (1,2)
X-ray superburst ~1042 ergs
Annual solar output ~1041 ergs
X-ray superbursts thought to be fueled by 12C + 12C fusion in the outer crust (3)
However, the temperature of the outer crust is too low (~ 3x106 K) for 12C fusion (4)
(1) Cumming et al., Astrophys. J. Lett. 559, L127 (2001) (2) Strohmayer et al., Astrophys. J. 566, 1045 (2002)
(3) Horowitz et al., Phys. Rev. C 77, 045807 (2008) (4) Haensel et al., Neutron Stars 1, 2007
One potential heat source, proposed to heat the crust of neutron stars and allow 12C fusion, is the fusion of neutron-rich light nuclei (ex. 24O + 24O) (1)
If valence neutrons are loosely coupled to the core, then polarization can result and fusion enhancement will occur
16O
Core
16O
Core
valence
neutrons
(1) Horowitz et al., Phys. Rev. C 77, 045807 (2008)
2
State of the art TDHF calculations, which follow the collision dynamics, predict a fusion enhancement for neutron-rich system
Experimental measurements of the fusion cross-section provide a test of fusion models
24O is currently inaccessible for reaction studies – instead study other neutron-rich isotopes of oxygen on 12C
Umar et al., Phys. Rev. C 85, 055801 (2012)
Exited compound nuclei decay by emission of p, n, and α particles – the resulting heavy nuclei are called evaporation residues.
Emission of these evaporated particles kick evaporation residues off of zero degrees, thus allowing direct measurement of evaporation residues
The fusion cross-section can be measured by measuring the number of evaporation residues relative to incident oxygen nuclei
Evaporation residues Evaporated particles
3
Stop time
Energy
Start time
Beam Residue
To distinguish fusion residues from beam particles, one needs to measure:
Energy of the particle (ΔE/E ~ 2%)
Time-of-flight of the particle (Δt/t ~ 7%)
18O beam was provided by the Tandem van de Graaff accelerator at Florida State University
18O @ Elab = 13.75 – 36 MeV IBeam ~ 1 - 4.5x105 p/s
John D. Fox Accelerator Laboratory
4
Time-of-flight of beam measured between US and TGT MCP detector
Elastically scattered beam particles and evaporation residues:
Time-of-flight measured between TGT MCP and Si detectors
Energy measured in annular Si detectors (T2, T3)
DS MCP detector allows random rejection
18O BEAM
US MCP
Detector
TGT MCP
Detector T2
~ 130 cm ~ 16 cm
DS MCP
Detector
T3 LCP Det.
Array
Bowman et al., Nucl. Inst. and Meth. 148, 503 (1978)
Steinbach et al., Nucl. Inst. and Meth. A 743, 5 (2014)
5
TOF (ns)5 10 15 20 25 30 35 40
E (
Me
V)
0
5
10
15
20
25
30
35C
12O +
18
= 14.4 MeVc.m.E
Elastic scattering peak
Slit Scattering
Fusion residues
18O + 12C @ ELab = 36 MeV
σ = fusion cross section
t = target thickness
I = number of incident beam particles
N = number of reactions
Steinbach et al., PRC 90, 041603(R) (2014)
6
(MeV)c.m.E6 7 8 9 10 11 12 13 14
(m
b)
s
10
210
310
Eyal Low Cross-Section
C12O + 18
, 1527 (1976) 13 PRCet al.Eyal
Present Work
Fit of Exp Values
Wong, PRL 31, 766 (1973)
Steinbach et al., PRC 90, 041603(R) (2014)
Measured the cross section for ECM ~ 6-14 MeV
Measured down to the 2.8 mb level, well below prior measurement of 25 mb
Fit data to functional form that describes the penetration of an inverted parabolic barrier:
Best fit to experimental data has values:
Rc = radius of the fusion barrier
V = height of the fusion barrier
ħω = curvature of the fusion barrier
Rc = 7.24 ± 0.16 fm V = 7.62 ± 0.14 MeV ħω = 2.78 ± 0.29 MeV
7
Steinbach et al., PRC 90, 041603(R) (2014)
(MeV)cmE6 7 8 9 10 11 12 13 14
(m
b)
s
1
10
210
310
C12
O + 18
Present Work
Fit of Exp Values
DC-TDHF - No Pairing
DC-TDHF - Pairing
Coupled Channels
(MeV)c.m.E6 7 8 9 10 11 12 13 14
DC
-TD
HF
s /
E
xp
eri
me
nt
s 0.5
1
1.5
Trend in ratio emphasizes that the experimental and theoretical excitation functions have different shapes
Increase in the ratio can be interpreted as a larger tunneling probability
This enhanced tunneling probability can be associated with a narrower barrier
Dramatic increase in experimental cross-section relative to DC-TDHF occurs at energies below 7 MeV
Demonstrates the importance of measuring the sub-barrier fusion cross-section
8
Measured the fusion cross-section of 18O + 12C down to the ~ 650 μb level!
New data is consistent with previous measurement
Trend of experimental data being above TDHF predictions continues as Ecm decreases
(MeV)cmE5 6 7 8 9 10 11 12 13 14
(m
b)
s
1
10
210
310
Eyal Low Cross-Section
C12
O + 18
2014 Measurement
Fit of 2014 Exp
2015 Measurement
DC-TDHF - Pairing
9
Extraction of fusion cross-section in sub-barrier domain can be accomplished by direct measurement of evaporation residues using low intensity beams
Measured 18O+12C fusion cross-section 40 times lower than previous measurements (~ 650 μb level)
Comparison of experimental cross-section with DC-TDHF prediction reveals a difference in the shape of the fusion excitation function (i.e. different barrier)
Rapid increase in the ratio of the experimental to theoretical cross-section demonstrates the importance of measuring the fusion cross-section below the barrier
Ready to measure 19O + 12C fusion excitation function (May 2015)
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Indiana University Nuclear Chemistry: R.T. deSouza, S. Hudan, V. Singh, J. Vadas,
B.B. Wiggins, J. Schmidt
Florida State University: I. Wiedenhover, L. Baby, S. Kuvin
Vanderbilt University:
A.S. Umar, V.E. Oberacker
DOE under Grant No. DEFG02-88ER-40404