1. Two New Techniques for Determining J  ’s of Neutron Resonances, and the Search for Non-Statistical Effects in Neutron Capture Paul Koehler Physics Division, Oak Ridge National Laboratory DANCECINDORELA

2. Difficulties Determining J  Values Zero-spin targets. Often very difficult or impossible to distinguish small s-wave from large p-wave resonances. Odd-A targets. Typically twice as many possible J  values as zero- spin targets. Two spin possibilities for s waves. Three to four possibilities for p waves. Ex. 95 Mo: Only 32/106 known res. have firm J .

3. Reasons for Determining J  Values Improve Nuclear Models. Level densities, neutron and gamma strength functions for each J . Improve Astrophysical Reaction Rates. (n,  ) rates for s-process branchings and r-process freeze-out. Better ( ,  ), etc. rates for explosive nucleosynthesis via (n,  ) measurements. Test model assumptions. Can a simple strength-function model be used for alphas? Do neutron widths follow a Porter-Thomas distribution? Are gamma width distributions Gaussian? Not much data for odd-A Nuclides.  Sm  n 

4. Experimental Technique Determining Resonance Spins in Odd-A Nuclides J  =3 - and 4 - states in 148 Sm formed at high E x by s-wave neutron capture on 147 Sm (I  =7/2 - ). 148 Sm decays to ground state by emitting M  -rays. Simple dipole model predicts M J=3 =3 and M J=4 =4. Other multipolarities and decay statistics result in M distributions for J=3 and 4. Measurable differences in observables for different J’s. 148 Sm 3-3- 2+2+ 1-1- 0+0+ 4-4- 3+3+ 2-2- 1+1+ 0+0+  cascades following 147 Sm+n Example: 147 Sm(n,  ) E n from TOF, E  from pulse height, M from counting coincidences.

5. Example 1: 147 Sm(n,  ) at DANCE Koehler et al., Phys. Rev. C 76, 025804 (2007). Problematical when resonances not well resolved. Noisy. Used “Judicious Linear Combinations” of M’s to determine J’s. JLC’s measure difference between data and prototypical J=3 and 4 M distributions. Revealed 6 previously unknown doublets. 41 new J  values (out of 110 below 700 eV).

6. Example 2: 95 Mo(n,  ) at ORELA Coceva et al, Nucl. Phys. A117, 586 (1968). Larger J => higher multiplicity => more coincidences and softer singles spectrum. Measure: R J = (”Hard” Singles)/Coincidences to identify J. 95 Mo(n,  ): Near peak of p- and valley of s-wave strength functions. I  = 5/2 + : l = 0 => J  = 2 + or 3 +. 1 => J  = 1 -, 2 -, 3 -, or 4 -.

7. ORELA Experiments 95 Mo(n,  ): Two C 6 D 6  -ray detectors. Used to measure many (n,  ) astrophysical rates to high accuracy. New apparatus on F.P.6 in the 40-m station. Small change made to keep track of coincidences (~few %). 95 Mo total cross section. Measured via transmission on F.P.1. 6 Li-glass detector at 80 m. The Oak Ridge Electron Linear Accelerator Facility SAMMY Analysis

8. New Method for Determining J and  Take data in event mode. Use several “off-line” singles and coincidence gates to construct ratios maximizing J and  differences. Pulse-height spectra: 2 + vs. 3 + Pulse-height spectra: 3 + vs. 3 -

9. Comparison to Previous Work Sheets et al., Phys. Rev. C 76, 064317. DANCE detector at the Lujan Center at LANSCE. Sheets et al. DANCEC 6 D 6 at ORELA

10. Comparison of Recent 95 Mo(n,  ) Experiments DANCE @ Lujan CINDORELA Sheets et al., Phys. Rev. C 76, 064317

11. New Method for Determining Resonance J  ’s: Results Some combinations separate  as well as J.

12. New Method for Determining Resonance J  ’s: To Do Simulate experiment using DICEBOX and GEANT. F. Becvar, M. Krticka, and G. Rusev. Compare to theory. Better J  indices? Complete resonance analysis and compare to theory. Neutron width distributions.  -width distributions. Spacing distributions. Apply to other nuclides. Improved checks of reported non-statistical effects. Apply retroactively to old ORELA data.

13. 147 Sm(n,  ) results: Spins, Level Spacings, and Strength Functions 41 new J values for resonances with previously unknown (33) or tentative (8) J. Extracted D 0 ’s and S 0 ’s for each J (  n 0 from Mizumoto). Results consistent with theory (spin cutoff parameter, statistical model S 0 ratio). Very few resonances missed below 700 eV. Spacings agree with Wigner and  3. Two techniques used to correct for missed res.

14. 147 Sm(n,  ) results: Neutron Width Distributions Divided data into two regions: 0-350 eV and 350-700 eV. Previous 147 Sm(n,  ) data revealed unexplained abrupt change in S  ratio at 350 eV.  n 0 ’s should follow a Porter- Thomas (PT) distribution ( =1).  n ’s Gaussian with 0 mean.  n ’s real (TRI). Single channel. E n <350 eV data agree with PT. Used standard technique (Fuketa and Harvey) to calculate that only 3 small resonances missed by 350 eV (8 by 700 eV).  n 0 Distribution, E n <350 eV

15. 147 Sm(n,  ) results: Non-statistical Effect Neutron Width Data for 350<E n <700 eV do not agree with Porter Thomas ( =1). Data in good agreement with ≥2 (ML, =3.19±0.83). Similar effects reported for 232 Th, 151 Sm, 163 Dy, 167 Er, 175 Lu, and 177 Hf. No Known explanation. TRIV implies = 2. Rahn et al.  n 0 Distribution, 350<En<700 eV

16. Total-Radiation-Width Distributions for Pt Isotopes: Another “Non-Statistical” Effect? Measured neutron capture and total cross sections for 192,194,195,196 Pt at ORELA. Used SAMMY to extract resonance parameters. 1262 resonances. Data in good agreement with Porter-Thomas and Wigner. Total-radiation-width distributions look double peaked. Statistically significant?

17. Summary and Conclusions Two new methods for determining J  ’s of neutron resonances. 1) JLC’s at DANCE at LANSCE. J  ’s for almost all know 147 Sm+n resonances below 700 eV. Similar data on 143 Nd and 149 Sm yet to be analyzed. 2) Spin Indices with C 6 D 6 at ORELA. Should yield many, many new J  ’s for 95 Mo+n resonances. Tested on previous 192,194,195,196 Pt+n data (singles only) and appears to work well. Non-statistical effects. With new J  ’s, 147 Sm+n data revealed deviation from expected Porter-Thomas distribution for  n 0 ’s.   distributions extracted from 192,194,195,196 Pt+n data might be double peaked. New techniques for determining J  values should allow better tests of previously reported non-statistical effects.

18. Collaborators Klaus Guber and Jack Harvey, and Doro Wiarda Nuclear Science and Technology Division, ORNL John Ullmann, Tod Bredeweg, John O’Donnell, Rene Reifarth, Bob Rundberg, Dave Vieira, and Jan Wouters LANL Frantisek Becvar and Milan Krticka Charles University Gencho Rusev TUNL

19. New Method for Determining Resonance J  ’s: Results Constructed “Spin Indices” by scaling ratios of singles and coincidences in different pulse-height gates. Singles data alone works about as well as Coceva method. Combination of singles- and coincidence-data ratios results in even better J separation.