Nuclear Physics in the Continuum: Surrogate reactions and Nuclear Physics using the National Ignition Facility

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.

2. b A a D d “Desired” reaction B* “Surrogate”reaction The Surrogate Method (Absolute probability variant) C Central assumption: Both reactions form a compound nucleus

3. B B  C C  A A  D D D External Ratio Same channel Different CN Internal Ratio Different channels Same CN B C A E D 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

4. STARS+LiBerACE (Livermore-Berkeley Array for Collaborative Experiments) Interior w/S2 Si detectors Target Chamber+6 “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

5. 233U(n,f)/235U(n,f) from ENDF-B7 234U(α,α'f)/236U(α,α'f) Ratio from STARS 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

6. From STARS+LIBERACE data The External Ratio approach is predicted to work for (n,f) for suitable spin distributions J. Escher & F.S. Dietrich,PRC 74 054601 (2006)

7. =39°-45° = 57°-62° 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)

8. (n,f)+(n,2n) (n,f) (n,2n) (n,) STARS LiBerACE 237U(n,destruction) cross sections measured Direct Measurements would have required a 800+ Ci target! PRC 73 054605 (2006)+submitted to PRC

9. We have also used the 238U(3He,t)238Np reaction to getthe 237Np(n,f) cross section (S. Basunia - LBNL)

10. 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?

11. Surrogates for nuclear astrophysics:The s-process (slow neutron capture) s-process branching near Sm-Eu-Gd s- vs. r-process abundances ln() Z • 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)

12. The surrogate ratio method can also be applied to other areas: Generation-IV reactor design *from Aliberti et al.,

14. Indirect drive: X-rays drive implosion Hohlraum ~ 10 mm long Target ~ 1 mm radius Optical pulse ~ few ns Burn ~ few ps Ablator rinitial =1 mm rfinal =30 µm Can insert ≤ 1015 nuclei DT Ice DT Gas 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)

15. Reactions on short-lived states 1040 1033-35{ Flux (n/s/cm2) 1020 100 LANSCE/WNR Reactor SNS NIF NIF provides two unique environments forNuclear Physics studies Stellar-like conditions  ≈ 10-12s 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

16. Gamow window Stellar reaction cross section measurements at NIF are enhanced by 2 compared to accelerator-based experiments Assumptions • 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 Accelerator-Based Experiments NIF-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

17. 16O 17F 17O 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 kBT≈8 keV • “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 Ecoul)

18. 3.5 MeV 99.999% 14 MeV 1:105 NIF may allow for the first direct observation of a 3-body nuclear reaction  +  + n  9Be also possible

19. 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

20. “Rule of thumb” ≥ burnburn produces a clear signal 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

21. How do these lifetimes compare to lifetimes of states with Ex≤ Sn? Product yields are very sensitive to quasi-continuum lifetimes

22. 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)

23. 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!

24. STARS LiBerACE Collaborators (students in red post-docs underlined) L.A. Bernstein, J.T. Burke, E.B. Norman, L. Ahle,K. Moody, B. F. Lyles1LLNL H. Ai, C.W. Beausang, S. LesherYale 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 “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 1U.C. Berkeley Dept. of Nucl. Eng.