Lawrence Livermore National Laboratory Nicholas Scielzo Physics Division, Physical and Life Sciences LLNL-PRES Lawrence Livermore National Laboratory, P. O. Box 808, Livermore, CA This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 Using surrogate nuclear reactions to determine (n,f) and (n,  ) cross sections August 8, 2009

2 Lawrence Livermore National Laboratory Surrogate Nuclear Reactions Approach The Surrogate Nuclear Reactions approach is an indirect method for determining cross sections of compound-nuclear reactions Used when direct measurements are not possible because of beam and/or target limitations – create compound nucleus through reaction of light-ion beam on a (more) stable isotope Can be used in regular or inverse kinematics

3 Lawrence Livermore National Laboratory Surrogate nuclear reaction method using inelastic scattering “Desired” reaction  n 153 Gd 154 Gd “Surrogate” reaction  p 154 Gd p Hauser-Feshbach theory describes the “desired” reaction as a product of entrance channels (   CN – can be calculated reliably) and exit-channel branching ratios (G  CN – can’t be calculated reliably) Alternative (“surrogate”) reaction forms the same compound-nucleus and determines G  CN We measure this ratio t 1/2 =240 days stable

4 Lawrence Livermore National Laboratory Approximation simplifies technique above ~MeV “Desired” reaction  n 153 Gd 154 Gd “Surrogate” reaction  p 154 Gd p Hauser-Feshbach theory describes the “desired” reaction as a product of entrance channels (   CN – can be calculated reliably) and exit-channel branching ratios (G  CN – can’t be calculated reliably) Alternative (“surrogate”) reaction forms the same compound-nucleus and determines G  CN t 1/2 =240 days stable Weisskopf-Ewing Approximation: branching ratios G  CN are independent of spin and parity when many decay channels are open

5 Lawrence Livermore National Laboratory Gamma Ray Detectors Up to 4×1000 µm E detectors  -electron & fission fragment shield p, d,  He, , 18 O beam Scattered particle 140 µm or 500 µm  E detector Fission Fragments   140 µm fission detector Silicon Telescope Array for Reaction Studies (STARS) Livermore Berkeley Array for Collaborative Experiments (LIBERACE) Particle solid angle: 20%  -ray 1 MeV: 1% Fission fragment solid angle: 2 × 20% E n determined from scattered particle energy:

6 Lawrence Livermore National Laboratory Surrogate (n,f) measurements Surrogate reactions approach has successfully determined (n,f) cross sections in actinides 237 U(n,f)/ 235 U(n,f) from 238 U( ,  f)/ 236 U( ,  f) 233 U(n,f)/ 235 U(n,f) from 234 U( ,  f)/ 236 U( ,  f) 237 Np(n,f) from 238 U( 3 He,tf) S.R. Lesher et al., Phys. Rev. C 79, (2009). J.T. Burke et al., Phys. Rev. C 79, (2006). M.S. Basunia et al., Nucl. Instrum. Meth. B, in press (2009).

7 Lawrence Livermore National Laboratory Surrogate (n,  ) measurements Extract most-likely J  distribution from comparison of data and calculations… …and use this information to move beyond Weisskopf- Ewing approximation to extract reliable (n,  ) results The measured  -ray yields compared to calculated yields for different spin distributions (error bars not shown). Compound-nuclear J  distribution is important… SnSn Probability of  -ray emission for 156 Gd(p,p’)

8 Lawrence Livermore National Laboratory Requirements Experiments benefit from:  up to nano-Amp beams (regular or inverse kinematics)  light-ion reactions  efficient particle detectors with excellent PID and energy resolution  high-efficiency  -ray detector arrays

9 Lawrence Livermore National Laboratory Collaborators Lawrence Livermore National Laboratory L.A. Bernstein, D.L. Bleuel, J.T. Burke, F. Dietrich, J. Escher, S.R. Lesher, E.B. Norman, N.D. Scielzo, S. Sheets, I. Thompson, M. Wiedeking U.C. Berkeley and Lawrence Berkeley National Laboratory M.S. Basunia, R.M. Clark, P. Fallon, J. Gibelin, R. Hatarik, B. Lyles, M.A. McMahan, L. Moretto, E.B. Norman, L. Phair, S.G. Prussin, E. Rodriguez-Vieitez University of Richmond J.M. Allmond, C. Beausang Rutgers University J.A. Cizewski, R. Hatarik, P.D. O’Malley and T. Swan