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

DANCE CINDORELA

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. 95Mo: Only 32/106 known res. have firm J.

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.

Smn

Experimental Technique Determining Resonance Spins in Odd-A Nuclides

• J=3- and 4- states in 148Sm formed at high Ex by s-wave neutron capture on 147Sm (I=7/2-).

• 148Sm decays to ground state by emitting M -rays.Simple dipole model predicts MJ=3=3 and MJ=4=4.Other multipolarities and decay statistics result in M distributions for J=3 and 4.

• Measurable differences in observables for different J’s.

148Sm

3-

2+

1-

0+

4-

3+

2-

1+

0+

cascades following 147Sm+n

Example: 147Sm(n,)

En from TOF, E from pulse height, M from counting coincidences.

Example 1: 147Sm(n,) at DANCE

Koehler et al., Phys. Rev. C 76, 025804 (2007).

• <M> 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).

Example 2: 95Mo(n,) at ORELA

• Coceva et al, Nucl. Phys. A117, 586 (1968).Larger J => higher multiplicity => more coincidences and softer singles spectrum.Measure: RJ = (”Hard” Singles)/Coincidences to identify J.

• 95Mo(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-.

ORELA Experiments

• 95Mo(n,): Two C6D6 -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 %).

• 95Mo total cross section.Measured via transmission on F.P.1.6Li-glass detector at 80 m.

The Oak Ridge Electron Linear Accelerator Facility

SAMMY Analysis

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-

Comparison to Previous Work

• Sheets et al., Phys. Rev. C 76, 064317.DANCE detector at the Lujan Center at LANSCE.

Sheets et al. DANCE

C6D6 at ORELA

Comparison of Recent 95Mo(n,) Experiments

• DANCE @ Lujan

• CINDORELA

Sheets et al., Phys. Rev. C 76, 064317

New Method for Determining Resonance J’s: Results

• Some combinations separate as well as J.

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.

147Sm(n,) results: Spins, Level Spacings, and Strength Functions

• 41 new J values for resonances with previously unknown (33) or tentative (8) J.

• Extracted D0’s and S0’s for each J (n

0 from Mizumoto).Results consistent with theory (spin cutoff parameter, statistical model S0 ratio).

• Very few resonances missed below 700 eV.Spacings agree with Wigner and 3.Two techniques used to correct for missed res.

147Sm(n,) results: Neutron Width Distributions

• Divided data into two regions: 0-350 eV and 350-700 eV.Previous 147Sm(n,) data revealed unexplained abrupt change in S ratio at 350 eV.

n0’s should follow a Porter-

Thomas (PT) distribution (=1).n’s Gaussian with 0 mean.n’s real (TRI).Single channel.

• En<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).

n0 Distribution, En<350 eV

147Sm(n,) results: Non-statistical Effect

• Neutron Width Data for 350<En<700 eV do not agree with Porter Thomas (=1).

• Data in good agreement with ≥2 (ML, =3.19±0.83).

• Similar effects reported for 232Th, 151Sm, 163Dy, 167Er, 175Lu, and 177Hf.

• No Known explanation.• TRIV implies = 2.

Rahn et al.

n0 Distribution, 350<En<700 eV

Wave Sound

Total-Radiation-Width Distributions for Pt Isotopes:Another “Non-Statistical” Effect?

• Measured neutron capture and total cross sections for 192,194,195,196Pt 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?

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 147Sm+n resonances below 700 eV.Similar data on 143Nd and 149Sm yet to be analyzed.

2) Spin Indices with C6D6 at ORELA.Should yield many, many new J’s for 95Mo+n resonances.Tested on previous 192,194,195,196Pt+n data (singles only) andappears to work well.

• Non-statistical effects.With new J’s, 147Sm+n data revealed deviation from expected Porter-Thomas distribution for n

0’s. distributions extracted from 192,194,195,196Pt+n data might be double peaked.New techniques for determining J values should allow better tests of previously reported non-statistical effects.

Collaborators

Klaus Guber and Jack Harvey, and Doro WiardaNuclear Science and Technology Division, ORNL

John Ullmann, Tod Bredeweg, John O’Donnell, Rene Reifarth, Bob Rundberg, Dave Vieira, and Jan

WoutersLANL

Frantisek Becvar and Milan KrtickaCharles University

Gencho Rusev TUNL

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.