Nuclear Physics in the Continuum:

1
Nuclear Physics in the Continuum Surrogate
reactions and Nuclear Physics using the National
Ignition Facility
L.A. Bernstein LLNL
Workshop on Level Density and Gamma Strength in
the Continuum 24 May, 2007 University of
Oslo Oslo, Norway
Two talks in one!
This work was performed under the auspices of the
U.S. Department of Energy by the University of
California, Lawrence Livermore National Laboratory
under Contract No. W-7405-Eng-48.
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Many indirect methods exist for determining cross
sections
ANC method for low-energy radiative-capture
reactions A(a,g)B. Determines the asymptotic
normalization (ANC) of the radial overlap IBAa
for B?Aa Coulomb dissociation for
low-energy radiative-capture reactions b(c,g)a.
Studies the time-reversed reaction a g ? b c
and uses detailed balance.
A
BaA
a
b
dab
J. Escher, LLNL, N Div,
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The Surrogate Method (Absolute probability
variant)
C
Central assumption Both reactions form a
compound nucleus
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Surrogate Reaction Flavors
Surrogate Measurements
Absolute Probability (Surrogate Method)
Relative Probability (Ratio Method)
External Ratio Same channel Different CN
Internal Ratio Different channels Same CN
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Surrogate Reaction Flavors
Surrogate Measurements
Absolute Probability (Surrogate Method)
Relative Probability (Ratio Method)
External Ratio Same channel Different CN
Internal Ratio Different channels Same CN
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STARSLiBerACE (Livermore-Berkeley Array for
Collaborative Experiments)
Interior w/S2 Si detectors
Target Chamber6 Clover Ge

• Initiated in 12/04
• Up to 128 Si channels (S1, S2 W1 StripES
  detectors)
• 39 experiments covering a wide range of
  low-energy nuclear topics
• A small sample of surrogate data taken from
  12/04-5/06 shown here

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Benchmarking the external ratio method -
234U(?,?f)/236U(?,?f) vs. 233U(n,f)/235U(n,f)
Ratios work even when we are not in the
Weissopf-Ewing limit
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The External Ratio approach is predicted to work
for (n,f) for suitable spin distributions J.
Escher F.S. Dietrich, PRC 74 054601 (2006)
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The surrogate method in the Weisskopf-Ewing limit
might work for (n,?) as well(J. Escher F.S.
Dietrich UCRL-TR-212509)
Absolute Probability (Weisskopf-Ewing limit)
External Ratio (Weisskopf-Ewing limit)
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Weve also examined the effects ground state Jp
differences (0 vs. 7/2-) 236U(n,f) from
238U(3He,?f)/235U(3He,?f) ratio and absolute
Ph.D. thesis project Bethany Lyles
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Angular momentum differences in the entrance
channel are visible at low energy as a function
of particle angle
238U(3He,?f) surrogate for 236U(n,f)
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The surrogate ratio method has been applied the
237U(n,x) cross sections
6.75 d
23.4 My
stable
23.4 min
Uranium Reaction Network

• Early attempts to measure 237U(n,f) have used
  unconventional neutron sources (bombs)
• Activity associated with a 10 mg target 810
  Curies!
• Lets just say that (n,?) would be very hard

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237U(n,destruction) cross sections measured
Direct Measurements would have required a 800 Ci
target!
PRC 73 054605 (2006)submitted to PRC
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We have also used the 238U(3He,t)238Np reaction
to get the 237Np(n,f) cross section (S. Basunia
- LBNL)
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A recent 237Np(n,f)/235U(n,f) ratio measurement
allows a comparison between our result and the
real deal
Where has all the pre-equilibrium gone?
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Surrogates for nuclear astrophysicsThe
s-process (slow neutron capture)

• s-process slow neutron capture moves along
  valley of stability with branch points where
  ?-decay competes with capture.

158Gd
154,156,158Gd(p,p) scheduled for next week
(5/30-6/4)
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The surrogate ratio method can also be applied to
other areas Generation-IV reactor design
from Aliberti et al.,
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(No Transcript)
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The National Ignition Facility (NIF) A new kind
of nuclear laboratory

• NIF is designed to implode D-T (or other) pellets
  to achieve thermonuclear fusion
• Standard ignition configuration 192 beams, 1.8MJ
  in 3? light

Up to 300 shots/year with 15 dedicated for
basic science (Ride-along also possible)
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NIF provides two unique environments forNuclear
Physics studies
Stellar-like conditions
? 10-12 s
He-Burning
H-Burning
Supernovae
1030
Density (atoms/cm3)
Ignition
Non-Ignition
1020
10-1 100 101
102
Temperature (keV)
Consider the following possible programs
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Stellar reaction cross section measurements at
NIF are enhanced by ?2 compared to
accelerator-based experiments

• Assumptions
• 1 mm diameter initial pellet size with
  density0.1 g/cm3 Compression to 30 µm diameter
• No fuel loaded. 50/50 mix of A and B

NIF-Based Experiments
Accelerator-Based Experiments
Ablator
0.6
50/50 mix of A, B
S-Factor (keV.barn)
0.4
Resonance
0.2
0 400
800
E (keV)

• High Count rate (3x105 atoms/shot)
• Small, manageable screening
• Energy window is better
• Integral experiment
• 7Be background

• Mono-energetic
• Low event rate (few events/month)
• Significant screening corrections needed
• Not performed at relevant energies

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CNO Cycle cross section measurements possible at
NIF

• First proposed by Bethe in 1938
• Important Hydrogen-burning mechanism in massive
  stars
• Makes 1.7 of all Helium in low-mass stars like
  the sun
• Very massive stars have two other minor CNO
  cycles
• Measured down to kBT8 keV
• Gamow window near 2 keV
• Reactions that lead to radioactive products are
  best for NIF

Products formed at kBT6 keV
The only radioactivity after a C6H6 capsule shot
would be 13N (all other have larger Ecoul)
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NIF may allow for the first direct observation
of a 3-body nuclear reaction
? ? n ? 9Be also possible
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The short time scale of a NIF burn matches the
lifetime of quasi-continuum states
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A simple toy model can be used to determine the
effects of excited state lifetimes on what
reaction products are formed

• Divide NIF burn time into 100 equal-flux time
  bins (?t50-400 fs).
• Assume 14 MeV neutrons induce (n,3n) rather than
  (n,2n) on all nuclei still at ExSn after 1 bin
  and that these nuclei
• Include two neutron energy bins
• 14 MeV can do (n,n) (n,2n) on ground and
  (n,3n) on excited states
• Tertiary (Engt14 MeV) neutrons (103-5 fewer than
  at 14 MeV) do (n,3n) on ground states

A-3
A-2
A-1
A
A-4
This type of analysis is quantitatively
understood at LLNL
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The model show that almost all higher-order
reaction products are from reactions on excited
states with ?20 fs
Successfully reproduces the results of more
sophisticated modeling
27
The toy model is very successful at reproducing
the results of more sophisticated simulations

• 3 x 1013 atoms of Iridium doped into the
  ice-ablator interface.

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How do these lifetimes ? compare to lifetimes of
states with Ex Sn?
Product yields are very sensitive to
quasi-continuum lifetimes
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The NIF time scale is ideal for populating
excited states near Sn
Up to 30 of all nuclear states near Sn have
not had time to emit even a single ?-ray in 20 fs
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NIF will use these types of experiments
diagnostics to address issues like mix and drive
asymmetry

• Implanting a noble-gas tracer in the inner part
  of the ablator to explore drive asymmetry via the
  (n,2n)/(n,?) ratio
• Implanting a halogen tracer on the ice-ablator
  interface to explore fuel-ablator mix via the
  (d,2n)/(n,2n) ratio

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Proposed Radchem Gas Collection System using
the existing NIF Chamber Vacuum System
Target Chamber Wall
NIF Chamber
Existing NIF Chamber Vacuum System (One of four
cryo/turbo Pump systems)
4xCryo Pumps (3000l/s)
Cryogenic Collection and Detector System
1. Primary Cryo (T1 K) Collector
Turbo Pump 2000l/s
Turbo Pump 500l/s
Hot He Gas
2. Prim Cryo (T2 K) Collector
Turbo Pump
Detector
Roughing Pump
Turbo Pump
RGA
Hot He Gas
Shielding
RGA
Second Cryo Collector (4K)
Detector (4 Ge det.) (event mode)
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Collaborations are welcome

• Scientific collaborations are welcome
• Nuclear Astrophysics measurements
• Nuclear Structure/Reactions measurements
• Nuclear Chemistry
• Gas and Plasma phase radiochemistry is
  particularly important for collection of
  non-gaseous products.
• Workshop on Nuclear Astrophysics _at_ NIF planned
  for August 28-31, 2007
• Joint with LBNL
• LLNL is advertising for a Roger Batzel Nuclear
  Chemistry Post-doctoral position
• Non-U.S. applicants welcome!

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Conclusions
Surrogate Reactions

• Surrogate methods are highly successful in
  reproducing Actinide fission exit channel cross
  sections
• Surrogate direct reaction do indeed produce a
  compound nucleus.
• Future plans include
• Surrogate measurements for Astrophysics and
  Nuclear Energy
• Further experiments to explore the limits of the
  technique

Nuclear Physics using NIF

• Integral cross section measurements for stellar
  energy production
• pp-chain, CNO cycle
• Reactions on excited states
• Three-body nuclear reactions
• Scattering off of weakly bound excited states
• Workshop on Nuclear Astrophysics _at_ NIF planned
  for August 28-31, 2007
• LLNL is advertising for a Special Nuclear
  Chemistry Post-doctoral position
• Non-U.S. applicants welcome!

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Collaborators (students in red post-docs
underlined)
NIFflers
Surrogate Physicists
R.D. Hoffman, M.A. Stoyer, C. Cerjan, K. Moody
D.H.G. Schneider, R. Boyd LLNL L.G. Moretto,
L.W. Phair, I.Y. Lee, D.L. Bleuel, M.A.
McMahan LBNL U. Greife Colorado School of
Mines S. Grimes Ohio University

• L.A. Bernstein, J.T. Burke, E.B. Norman, L. Ahle,
  K. Moody, B. F. Lyles1 LLNL
• H. Ai, C.W. Beausang, S. Lesher Yale
  University / U. of Richmond
• L.W. Phair, S. Basunia, D.L. Bleuel, P.Fallon,
  R.M. Clark, M.A.
  Delaplanque-Stephens, I.Y. Lee, A.O.
  Macchiavelli, M.A. McMahan, E. Rodriguez-Vieitez,
  F.S. Stephens, M.Wiedeking, J.D. Gibelin
  LBNL

1U.C. Berkeley Dept. of Nucl. Eng.
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After 70 years we still dont completely
understand Vnn in complex nuclei

• Nuclei exist in a three-dimensional phase space
• Vnn(J,T) have been explored actively
• Gamma-ray spectroscopy using direct and heav-ion
  reactions for J
• Rare isotope beams (RIA etc.) which probes N/Z
  far off stability for T
• Ex has only been explored passively (decay of
  excited states)

?
Ex
Isospin (T)
J