Transcript Document

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.
The Surrogate Method
(Absolute probability variant)
b
A
a
D
B*
“Desired” reaction
PC 
“Surrogate”reaction
NC
 b C N b
C
comp reac
 A ( a ,x )C  Pc ( J ,  , E x ) a  A
( J,  , E x )
J ,
Weisskopf
 Ewing :  A ( a ,x )C  PC  a  A
comp reac
Central assumption: Both reactions form a compound nucleus

-2-
d
Surrogate Reaction “Flavors”
Surrogate Measurements
Absolute
BProbability
C
A
(Surrogate Method)
Relative Probability
(Ratio Method)
D
C
B
A
External
Ratio
External
Ratio
Same
channel
Same
channel
D
Different

Different
 CNCN
Internal
Ratio
Internal
Ratio
B
C
A
Different
Differentchannels
channels
Same
SameCN
CN
E

D
D
-3-
STARS+LiBerACE (Livermore-Berkeley
Array for Collaborative Experiments)
Target Chamber+6 “Clover” Ge
•
•
•
•
Interior w/S2 Si detectors
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
-5-
Benchmarking the external ratio method 234U(,’f)/236U(,’f) vs. 233U(n,f)/235U(n,f)
2.5
233U(n,f)/235U(n,f)
F is s io n R a t io
2
from ENDF-B7
234U(α,α'f)/236U(α,α'f)
Ratio from STARS
1.5
1
0.5
0
7
12
17
22
Excitation Energy (MeV)
Ratios work even when we are not in the Weissopf-Ewing limit
-6-
The External Ratio approach is predicted to work for (n,f) for
suitable spin distributions J. Escher & F.S. Dietrich, PRC 74 054601 (2006)
From STARS+LIBERACE data
-7-
Angular momentum differences in the entrance channel
are visible at low energy as a function of particle angle
238U(3He,f)
1
surrogate for 236U(n,f)
=39°-45°
= 57°-62°
Bin1: 39-45
0.9
Bin4: 57-62
236
U (n ,f ) C r o s s S e c tio n [ba r n s ]
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0.5
0.7
0.9
1.1
1.3
Equivalent Neutron Energy [MeV]
- 10 -
1.5
1.7
1.9
237U(n,destruction)
2.5
(n,f)+(n,2n)
3
C ross S ection (b a rn s)
C ross S ection (b )
4
cross sections measured
2
1
0
(n,f)
2
1.5
1
0.5
0
5
7.5
10
12.5
15
Neutron Energy (MeV)
17.5
0
20
2.5
5
7.5
10
12.5
Neutron Energy (MeV)
15
17.5
20
Direct Measurements would have required a 800+ Ci target!
1.8
(n,2n)
1.4
1.6
1.4
1.2
C ro ss S ectio n (b )
C ro ss S ectio n (b )
1.6
1
0.8
`
0.6
0.4
(n,)
1.2
1
0.8
0.6
0.4
0.2
0.2
0
STARS
LiBerACE
5
7.5
10
12.5
15
Neutron Energy (MeV)
17.5
20
0
0
PRC 73 054605 (2006)+submitted to PRC
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0.2
0.4
0.6
0.8
Neutron Energy (MeV)
1
1.2
1.4
We have also used the 238U(3He,t)238Np reaction to
get the 237Np(n,f) cross section (S. Basunia - LBNL)
237
Np(n, f) using 238U(3 He, t )238Np
2.5
Cross Section (barn)
2.4
2.3
2.2
2.1
This Expt.
EXFOR/CSISOR-Ref 12
ENDF/B-VII.0
JENDL 3.3
2.0
1.9
1.8
10
12
14
16
18
Equivalent neutron energy (M eV)
Only statistical uncertainty
Ref. Interaction of Neutrons with Nuclei, Dubna 2001 p.288
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20
A recent 237Np(n,f)/235U(n,f) ratio measurement allows
a comparison between our result and the “real deal”
237
Np/235U cross section ratio
1.5
This work
1.4
LANL-2007
Ratio
1.3
1.2
1.1
1.0
0.9
0.8
10
12
14
16
18
Neutron Energy (MeV)
Where has all the pre-equilibrium gone?
- 14 -
20
Surrogates for nuclear astrophysics:
The s-process (slow neutron capture)
• s-process: slow neutron capture moves along
valley of stability with branch points where
-decay competes with capture.
s-process branching near Sm-Eu-Gd
s- vs. r-process abundances
ln()
158Gd
154,156,158Gd(p,p’)
scheduled
for next week (5/30-6/4)
- 15 -
Z
The surrogate ratio method can also be applied to
other areas: Generation-IV reactor design
Target Surrogate Reactions
Ratio Reactions
Reactor Type*
238Pu
239Pu(,’)
235U(,’)
LFR,SFR
239Pu
239Pu(d,p), 240Pu(,’)
235U(d,p), 236U(,’)
GFR,LFR,SFR,EFR
240Pu
240Pu(d,p), 242Pu(3He,)
236U(d,p), 236,238U(3He,)
GFR,LFR,SFR,EFR
241Pu
242Pu(,’)
236U(,’)
GFR,LFR,SFR,EFR
241Am
243Am(3He,), 240Pu(3He,d)
235U(3He,),
242mAm
242Pu(3He,t), 243Am(,’)
238U(3He,t), 235U(,’)
LFR,SFR
243Am
243Am(d,p), 242Pu(3He,d)
239Pu(d,p),
SFR
242Cm
243Am(3He,t)
238U(3He,t)
EFR
243Cm
245Cm(3He,), 243Am(3He,t)
235U(3He,), 238U(3He,t)
GFR,EFR
244Cm
245Cm(,’), 243Am(3He,d)
235U(,’),
GFR,LFR,SFR,EFR
245Cm
247Cm(3He,), 245Cm(d,p)
235U(3He,), 235U(d,p)
*from Aliberti
et al.,
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none yet
none yet
none yet
GFR,LFR,SFR
LFR,SFR,EFR
- 17 -
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
Indirect drive: X-rays drive implosion
Hohlraum ~ 10 mm long
Target ~ 1 mm radius
Optical pulse ~ few ns
Burn ~ few ps
Ablator
Can insert
≤ 1015 nuclei
rinitial =1 mm
rfinal =30 µm
DT
Ice
DT
Gas
Up to 300 shots/year with ≈15% dedicated for basic science (Ride-along also possible)
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NIF provides two unique environments for
Nuclear Physics studies
Reactions on short-lived states
Stellar-like conditions
1040
{
He-Burning
Ignition
Flux (n/s/cm2)
1030
HBurning
Non-Ignition
1020
Density (atoms/cm3)
1033-35

1020
Supernovae
≈ 10-12 s
100
10-1
100
101
Temperature (keV)
LANSCE/WNR
102
Reactor
SNS
Consider the following possible programs
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NIF
Stellar reaction cross section measurements at NIF are
enhanced by 2 compared to accelerator-based experiments
Consider
Assumptions
–
A ( B, X )Y
N Y   ( b ,x ) N A N B area
1 mm diameter initial pellet size with
density≈0.1 g/cm3 Compression to 30 µm
diameter
No fuel loaded. 50/50 mix of A and B
for  ( b ,x )  0.01 pb (10
N Y  10
Accelerator-Based Experiments
S-Factor (keV.barn)
–
the reaction
38
cm
2
10
20
 10
20
38

4  ( 30  m )
NIF-Based Experiments

0.6
 (E ) 
S(E )
E
0.4


exp  

E turn
Ablator

E 

50/50 mix
of A, B
Resonance
Gamow window
0.2
0
400
800
√
E (keV)
Mono-energetic
√
High Count rate (3x105 atoms/shot)

Low event rate (few events/month)
√
Small, manageable screening

Significant screening corrections needed
√
Energy window is better

Not performed at relevant energies

Integral experiment

7Be
- 20 -
2
cm )
background
2
  10
6

CNO Cycle cross section measurements
possible at NIF
•
•
First proposed by Bethe in 1938
Important Hydrogen-burning mechanism
in massive stars
–
–
16O
•
17F
Measured down to kBT≈8 keV
–
•
17O
Makes ≈1.7% of all Helium in low-mass
stars like the sun
Very massive stars have two other minor
CNO cycles
“Gamow” window near 2 keV
Reactions that lead to radioactive
products are best for NIF
Products formed at kBT≈6 keV
The only radioactivity after
a C6H6 capsule shot would be
13N (all other have larger E
)
coul
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Reaction (cycle #)
12C(p,)13N (I)
14N(p,)15O (I)
16O(p,)17F (II)
17O(p,)18F (III)
Products/shot
≈1 x 107
≈2 x 105
≈7 x 103
≈7 x 103
NIF may allow for the first direct observation
of a 3-body nuclear reaction
  10
For every 10
22
s : P3 body  N
P3 body  10 10 10
8
Total
6
22
8
 27
5
1:105
s
N n  capture / 4  r
He
/10
6
5
He per shot : 10 10
 10
11
2
6
 10
5
assumes  capture  1 mb
 +  + n  9Be also possible
- 22 -
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 (t≈50-400 fs).
Assume 14 MeV neutrons induce (n,3n) rather than (n,2n) on all nuclei still at
Ex≈Sn 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 (En>14 MeV) neutrons (103-5 fewer than at 14 MeV) do (n,3n) on ground
states
A-4
A-3
A-2
A-1
A
This type of analysis is quantitatively understood at LLNL
- 24 -
The model show that almost all higher-order reaction
products are from reactions on excited states with ≥20 fs
1.00E-01
20 fs excited states
“Rule of thumb”
≥ burnburn
produces a
clear signal
<2 fs excited states
1.00E-02
1.00E-03
1.00E-04
1.00E-05
A-1/A
A-2/A-1
A-3/A-2
A-4/A-3
Successfully reproduces the results of more sophisticated modeling
- 25 -
How do these lifetimes  compare to lifetimes of
states with Ex≤ Sn?
% of states at the neutron separation
energy remaining after 20 fs
Statistical Cascade
Lifetimes
100
1.E-10
Asumptions
1.E-11
:
T(E )  E
3
L ife tim e (s )
P e rce n ta g e
10
1
Fermi  gas  ( E x )
1.E-12
1.E-13

0.1
1.E-14
1.E-15
0.01
0
100
200
300
\
0
2.5
5
7.5
U (MeV)
Mass
Product yields are very sensitive to quasi-continuum lifetimes
- 27 -
10
Proposed “Radchem” Gas Collection System using
the existing NIF Chamber Vacuum System
NIF Chamber
Existing NIF Chamber
Vacuum System
4xCryo
Pumps
(3000l/s)
(One of four cryo/turbo
Pump systems)
Turbo
Pump
2000l/s
Turbo
Pump
500l/s
Cryogenic Collection
and Detector System
1. Primary
Cryo (T1 K)
Collector
Hot He Gas
Turbo
Pump
Detector
2. Prim
Cryo (T2 K)
Collector
Roughing
Pump
Turbo
Pump
Shielding
RGA
RGA
- 30 -
Detector (4 Ge det.)
(event mode)
Hot He Gas
Second Cryo Collector (4K)
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 @ NIF planned for August 28-31, 2007
LLNL is advertising for a Special Nuclear Chemistry Post-doctoral position
– Non-U.S. applicants welcome!
- 32 -
Collaborators
(students in red post-docs underlined)
STARS
“NIFflers”
Surrogate Physicists
R.D. Hoffman, M.A. Stoyer, C. Cerjan,
K. Moody D.H.G. Schneider, R. Boyd
LLNL
LiBerACE
L.A. Bernstein, J.T. Burke, E.B. Norman,
L. Ahle, K. Moody, B. F. Lyles1 LLNL
Yale
L.G. Moretto, L.W. Phair, I.Y. Lee, D.L. H. Ai, C.W. Beausang, S. Lesher
Bleuel, M.A. McMahan
University / U. of Richmond
LBNL
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
U. Greife
Colorado School of Mines
S. Grimes
Ohio University
LBNL
1U.C.
Berkeley Dept. of Nucl. Eng.
- 33 -