Study of the photon strength function and Nuclear Level

Density of 152Sm.

S. Marrone, M. Krtička, N. Colonna and F. Gunsing

n_TOF meeting 28-30 November 2007, BARI.

• Scientific motivations

• The method: Experiment and Simulations.

• Preliminary Results on 152Sm

Outline

152Sm is a Very Interesting Isotope :•Transition region from spherical vibrator to axial rotor (=0.243)•Critical point of phase transition.•The variation of the nuclear properties affect both the PSF and the NLD. Trend already observed in rare earth nuclei (Nd, Sm, Gd, Dy), in particular in Sm (several stable isotopes from 144 to 154).•Possible presence of scissor mode (M1 strength proportional to square of deformation?)

Scientific Motivation Nuclear Structure

Pietralla et al. PRC 58 (1998).

Richter, PNPP 34 (1995)Richter, PNPP 34 (1995)

151Sm(n,) is a branching-point isotope in s process but not only…

Scientific Motivations Nuclear Astrophysics

S. Goriely PLB 436, 110 (1998)

…strong implications in r process PATH!

Disadvantages:• poor -ray resolution of C6D6.

• statistics at high energy is limited

Proposed solution: filter model predictions through detector’s response

The analysis

Advantages:• very good signal-to-background

ratio

• high resolution allows the selection of different resonances

• accurate study of the detector response (MC simulations and data)

151Sm J = 5/2+

Capture resonances J = 2+ or 3+

Selected different resonances between 1 and 400 eVAll s-wave (but impossible to tell J)study with different l, S, J, p.low-energy -rays up to Bn=8.258 MeV. Nuclei difficult to measure otherwise

Background:• small ambient background

(measured with Ti-can)• negligible from radioactivity • n/ discrimination to suppress

neutrons• threshold 200 keV to further

minimize background

Calibrations• -spectrum accurately

calibrated with 137Cs, 60Co, Pu/C

• checked stability over all runs

• verified that coincidence probability is small

-ray spectrum

The comparison method

The low resolution of the experimental -ray spectrum (with C6D6) makes difficult to obtain direct information on PSF.

Proposed solution (indirect method):• generate decay spectra with models (different combinations of PSF and NLD

assumptions, parameters, etc…) using DICEBOX MC.

• filter the predicted spectra through experimental apparatus with MC tracking codes: GEANT-3, GEANT-4 and MCNP.

• compare filtered theoretical spectra with experimental one (calculate the 2)

• draw some conclusions.

E1 M1 NLDBrink-Axel model Single particle Constant temperature

Kadmenskij-Markushev-Furman Scissors Resonance+Spin-Flip

Back-shifted Fermi Gas

... …. ….

The models for -decay

E1 photon strength functions

•Brink-Axel model (BA) …………………………………. check validity below Bn (8.26 MeV)

•Kadmenskij-Markushev-Furman (KMF) ……… works well on 148,150Sm but may not be appropriate for deformed nuclei

•Enhanced Generalized Lorentzian (EGLO) … spherical and deformed nuclei

•KMF at low E + BA at high E (K) …………….. linear combination in between 4-8MeV

Decay spectra of 152Sm simulated with the DICEBOX algorithm. Extreme statistical model embodying:

• Bohr’s idea of compound nucleus

• Fragmentation of photon strength

• Brink hypothesis

Decay of highly excited nuclear states described in terms of:

• Photon Strength Functions for various types of multipolarities of emitted -rays, fXL (X=E/M, L=multipolarity)

• Nuclear Level Density (function of excitation energy and spin)

The models for -decay

M1 Photon Strength function

•Single Particle (SP) …………………….. Energy independent

•Spin Flip (SF) …………………………… Lorentzian shape with suitable parameters

•Scissor Resonance (SR) …………... In transitional and deformed nuclei

The SR is assumed to occurr around 3 MeV, with strength proportional to deformation

Il could play an important role in 152Sm, since this is a deformed nucleus

E2 Photon Strength Function

Single particle (SP) …………………………… constant value (=10-10 MeV-5)Nuclear level density

•Constant temperature formula (CTF)

•Back-shifted Fermi Gas (BSFG)

Models of Photon Strength Function

E G

12.38 MeV 2.97 MeV 176 mb

15.74 MeV 5.22 MeV 234 mb

Brink-Axel model

Models and parameters

E G

12.38 MeV 2.97 MeV 176 mb

15.74 MeV 5.22 MeV 234 mb

E EL

8 MeV 4 MeV

Kadmenskij-Markushev-Furman

Combination of BA and KMF

k0

3 4.5 MeV

Enhanced Generalized Lorentzian

Nuclear Level Density: Models and parameters

Nuclear level Density

CTF

BSFG

E0 T 0.37 MeV-1 0.559 MeV

E1 a T 0.37 Mev 18.57 MeV-1 0.559 MeV

n_TOF Experimental Point at BnJ = 2

To simulate the detector response, used three different Monte Carlo codes:

• MCNP-X

• GEANT 3.21

• GEANT 4

Accurate implementation of the materials and detailed geometry of experimental apparatus

Monte Carlo Simulations

-rays are generated uniformily in the sample Used same cuts as in the experiment (threshold of 200 keV) Energy resolution of the detectors (measured with sources) included in

the simulations

Problems

Some disagreement between different simulations is observed below 1 MeV

Probably due to details on the experimental apparatus

Still investigating the origin but there strong indications that is the material definition.

GEANT 4 in between MCNP and Geant 3. For all comparison, used GEANT 4

The region between 200 and 800 keV is important for comparison with models: need to understand the problem before a final comparison

GEANT-3

GEANT-4

MCNP

Angular momentum

In data:

• not possible to distinguish between J=2 and J=3

• All resonances summed together

In models:

• Little difference between 2 and 3

• Mixed together according to spin probability distribution function (c spin cut-off factor = 0.98A0.29):

Sensitivity of results on the nuclear realization (level structure and decay scheme): NONE

A few checks

A bad caseDICEBOX choice:

PSF E1 KMF

PSF M1 SR (0.5) +SF

NLD BSFG

Normalization done for the same number of cascades.In general, the use of the BA or the KMF model alone results in a poor agreement. Also important the strength of the SR.The predicted radiation width is too low 73(2), relative to the experimental value of 108(15).

Reasonable agreement for -ray energy above 2 MeV.

The most sensitive part is below 2 MeV.

Not very good agreement in this case

Filtered DICEBOX

n_TOF data

Best case

DICEBOX choice:

E1 PSF BA (8) + KMF (4)

M1 PSF SR (0.4) + SF + SP

NLD BSFG

The best agreement is obtained by combining BA+KMF, and assuming a Scissor resonance for M1. Need to consider also a constant SP background in M1.

Filtered DICEBOX

n_TOF data

A more accurate comparison (and conclusion) requires fixing some uncertainty in the MC filtering code.

Conclusions

WORK IN PROGRESS

• Possibility to study Photon Strength Function in neutron capture reactions at n_TOF

• Data on many interesting isotopes.

• Some data taken with C6D6: low-sensitivity, low-background, but also … low-resolution.

• Indirect method: filter model predictions through the detector’s response with MC simulations.

• For 152Sm preliminary results indicate that a good reproduction of the data can be obtained with BA+KMF for E1, a SR of strength 0.4+SF+SP for M1, and BSFG.

• Need still to check the reliability of the comparison (in particular, the filtering MC codes).

• A method is here proposed, which could be applied to a wealth of n_TOF data.

• An even more reliable comparison can be performed with the TAC data.