Presentation on theme: "n_TOF meeting November 2007, BARI."— Presentation transcript:
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. Gunsingn_TOF meeting November 2007, BARI.OutlineScientific motivationsThe 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 eVAll 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 otherwiseAdvantages:very good signal-to-background ratiohigh resolution allows the selection of different resonancesaccurate study of the detector response (MC simulations and data)Disadvantages:poor g-ray resolution of C6D6.statistics at high energy is limitedProposed solution: filter model predictions through detector’s response
5 g-ray spectrum Calibrations Background: g-spectrum accurately calibrated with 137Cs, 60Co, Pu/Cchecked stability over all runsverified that coincidence probability is smallBackground:small ambient background (measured with Ti-can)negligible from radioactivityn/g discrimination to suppress neutronsthreshold 200 keV to further minimize background
6 The comparison methodThe 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.E1M1NLDBrink-Axel modelSingle particleConstant temperatureKadmenskij-Markushev-FurmanScissors Resonance+Spin-FlipBack-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 nucleusFragmentation of photon strengthBrink hypothesisDecay 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 functionsBrink-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 nucleiEnhanced Generalized Lorentzian (EGLO) … spherical and deformed nucleiKMF 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 independentSpin Flip (SF) …………………………… Lorentzian shape with suitable parametersScissor Resonance (SR) …………... In transitional and deformed nucleiThe SR is assumed to occurr around 3 MeV, with strength proportional to deformationIl could play an important role in 152Sm, since this is a deformed nucleusE2 Photon Strength FunctionSingle particle (SP) …………………………… constant value (=10-10 MeV-5)Nuclear level densityConstant temperature formula (CTF)Back-shifted Fermi Gas (BSFG)
9 Models of Photon Strength Function Brink-Axel modelEg Gg sG12.38 MeV MeV 176 mb15.74 MeV MeV 234 mb
10 Models and parameters Kadmenskij-Markushev-Furman Eg Gg sG12.38 MeV MeV 176 mb15.74 MeV MeV 234 mbCombination of BA and KMFEH EL8 MeV MeVEnhanced Generalized Lorentziank GgMeV
11 Nuclear Level Density: Models and parameters J = 2n_TOF Experimental Point at BnNuclear level DensityCTFE T0.37 MeV MeVBSFGE a T0.37 Mev MeV MeV
12 Monte Carlo Simulations To simulate the detector response, used three different Monte Carlo codes:MCNP-XGEANT 3.21GEANT 4Accurate implementation of the materials and detailed geometry of experimental apparatusg-rays are generated uniformily in the sampleUsed same cuts as in the experiment (threshold of 200 keV)Energy resolution of the detectors (measured with sources) included in the simulations
13 ProblemsSome disagreement between different simulations is observed below 1 MeVProbably due to details on the experimental apparatusStill investigating the origin but there strong indications that is the material definition.GEANT-3GEANT-4MCNPGEANT 4 in between MCNP and Geant 3. For all comparison, used GEANT 4The 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=3All resonances summed togetherIn models:Little difference between 2 and 3Mixed 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 caseFiltered DICEBOXn_TOF dataNormalization 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 + SPNLD BSFGFiltered DICEBOXThe 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 dataA 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_TOFData 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.