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Study of the photon strength function and Nuclear Level Density of 152 Sm.

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### 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.

Outline

- Scientific motivations
- The method: Experiment and Simulations.
- Preliminary Results on 152Sm

Scientific Motivation Nuclear Structure

Richter, PNPP 34 (1995)

Pietralla et al. PRC 58 (1998).

152Sm is a Very Interesting Isotope :

- Transition region from spherical vibrator to axial rotor (b=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 Motivations Nuclear Astrophysics

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

S. Goriely PLB 436, 110 (1998)

…strong implications in r process PATH!

151Sm Jp = 5/2+

Capture resonances J = 2+ or 3+

Selected different resonances between 1 and 400 eV

All s-wave (but impossible to tell J)

study with different l, S, J, p.

low-energy g-rays up to Bn=8.258 MeV.

Nuclei difficult to measure otherwise

- 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)

- Disadvantages:
- poor g-ray resolution of C6D6.
- statistics at high energy is limited
- Proposed solution: filter model predictions through detector’s response

- Calibrations
- g-spectrum accurately calibrated with 137Cs, 60Co, Pu/C
- checked stability over all runs
- verified that coincidence probability is small

- Background:
- small ambient background (measured with Ti-can)
- negligible from radioactivity
- n/g discrimination to suppress neutrons
- threshold 200 keV to further minimize background

- The low resolution of the experimental g-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.

- 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 g-rays, fXL (X=E/M, L=multipolarity)
- Nuclear Level Density (function of excitation energy and spin)

- 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 Eg + BA at high Eg (K) …………….. linear combination in between 4-8MeV

- 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)

- Nuclearlevel density
- Constant temperature formula (CTF)
- Back-shifted Fermi Gas (BSFG)

Models of Photon Strength Function

Brink-Axel model

EgGgsG

12.38 MeV 2.97 MeV 176 mb

15.74 MeV 5.22 MeV 234 mb

Kadmenskij-Markushev-Furman

EgGgsG

12.38 MeV 2.97 MeV 176 mb

15.74 MeV 5.22 MeV 234 mb

Combination of BA and KMF

EH EL

8 MeV 4 MeV

Enhanced Generalized Lorentzian

k0Gg

3 4.5 MeV

Nuclear Level Density: Models and parameters

J = 2

n_TOF Experimental Point at Bn

Nuclear level Density

CTF

E0 T

0.37 MeV-1 0.559 MeV

BSFG

E1 a T

0.37 Mev 18.57 MeV-1 0.559 MeV

- To simulate the detector response, usedthreedifferent Monte Carlo codes:
- MCNP-X
- GEANT 3.21
- GEANT 4
- Accurate implementationof the materials and detailedgeometryofexperimentalapparatus

- g-rays are generateduniformily in the sample
- Usedsamecutsas in the experiment (thresholdof 200 keV)
- Energy resolutionof the detectors (measuredwithsources) included in the simulations

- 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-3

GEANT-4

MCNP

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

- Angularmomentum
- In data:
- notpossibletodistinguishbetween J=2 and J=3
- Allresonancessummedtogether
- In models:
- Little differencebetween 2 and 3
- Mixedtogetheraccordingtospinprobabilitydistributionfunction (scspin cut-off factor = 0.98A0.29):

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

DICEBOX choice:

PSF E1 KMF

PSF M1 SR (0.5) +SF

NLD BSFG

Reasonable agreement for g-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

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).

DICEBOX choice:

E1 PSF BA (8) + KMF (4)

M1 PSF SR (0.4) + SF + SP

NLD BSFG

Filtered DICEBOX

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.

n_TOF data

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

- 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.

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