1. n_TOF meeting 28-30 November 2007, BARI.
Study of the photon strength function and Nuclear Level Density of 152Sm.
S. Marrone, M. Krtička, N. Colonna and F. Gunsing
n_TOF meeting November 2007, BARI.
Outline
Scientific motivations
The method: Experiment and Simulations.
Preliminary Results on 152Sm

2. Scientific Motivation Nuclear Structure
Richter, PNPP 34 (1995)
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?)
Pietralla et al. PRC 58 (1998). 2

3. 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!

4. The analysis Advantages: Disadvantages: 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

5. g-ray spectrum Calibrations Background:
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

6. The comparison method
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. E1 M1
NLD
Brink-Axel model
Single particle
Constant temperature
Kadmenskij-Markushev-Furman
Scissors Resonance+Spin-Flip
Back-shifted Fermi Gas
... …. 6

7. The models for g-decay E1 photon strength functions
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

8. The models for g-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)

9. Models of Photon Strength Function
Brink-Axel model
Eg Gg sG
12.38 MeV MeV 176 mb
15.74 MeV MeV 234 mb

10. Models and parameters Kadmenskij-Markushev-Furman
Eg Gg sG
12.38 MeV MeV 176 mb
15.74 MeV MeV 234 mb
Combination of BA and KMF
EH EL
8 MeV MeV
Enhanced Generalized Lorentzian
k Gg
MeV

11. Nuclear Level Density: Models and parameters
J = 2
n_TOF Experimental Point at Bn
Nuclear level Density
CTF
E T
0.37 MeV MeV
BSFG
E a T
0.37 Mev MeV MeV

12. Monte Carlo Simulations
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
g-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

13. 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-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

14. A few checks 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 (sc spin cut-off factor = 0.98A0.29):
Sensitivity of results on the nuclear realization (level structure and decay scheme): NONE

15. A bad case 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).

16. Best case 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.

17. 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.