S. Micheletti

INFN - Istituto Nazionale di Fisica Nucleare, Frascati, Latium, Italy

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Publications (85)151.07 Total impact

  • 12/2007: pages 211-225;
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    ABSTRACT: The inclusive and exclusive reactions have been studied with a beam energy of 171 MeV and scattering angles for the reaction θ=0° and 3°. The studies focused on the separation of the isovector spin-dipole resonance (IVSGDR) into its components by measuring tensor-analysing powers and observing the direct neutron decay to the low-lying proton-hole states in 11B. Merging the information obtained from both measurements resulted in the first-time verification of model-independent predictions of tensor-analysing powers at extreme forward angles and the experimental decomposition of the IVSGDR into its Jπ components. The experimental results are in reasonable agreement with theoretical estimates based on shell-model calculations.
    Physics Letters B 01/2007; · 4.57 Impact Factor
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    ABSTRACT: Without Abstract
    04/2006: pages 311-316;
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    ABSTRACT: Neutron inner hole responses in 115Sn and 119Sn nuclei have been studied via the (d⃗,t) reaction at Ed=200MeV using a polarized beam with both vector and tensor components. One-step pickup observables corresponding to the overlapping 1g9/2, 1f5/2, and 1f7/2 responses were analyzed between 3° and 15° via a least square fit procedure up to EX=21.5MeV and 20 MeV in 115Sn and 119Sn, respectively. The relative enhancement of transitions with high total angular momentum j and the strongly characteristic angular distributions of j-=l-1/2 versus j+=l+1/2 vector and tensor analyzing powers allow the extraction for the first time of 1f5/2 and 1f7/2 strength distributions and of new 1g9/2 strength in addition to its previously known main component. The standard DWBA analysis is complemented by a refined analysis taking into account the form factor dependence on excitation energy due to the hole coupling with surface vibrations calculated in the framework of the quasiparticle-phonon coupling model (QPMFF). Residual nucleus spectra were measured, mainly in 115Sn, up to EX∼45MeV where the underlying background of multistep reactions is dominant and can be calibrated. The background of multistep pickup cross section is calculated for the first time in the forward angle region, assuming a dominant role of collective excitations in inelastic steps. Coupled channel calculations for two-step pickup observables involving low multipole collective excitations are performed for comparison. Integrated strengths deduced in the present work are compared with previous data on the 1g9/2 structure and tail, and with the very limited previous information existing on the 1f strengths. The 1g9/2, 1f5/2, and 1f7/2 QPMFF and standard strength distributions are compared with available theoretical predictions. In particular, we find that the 1f5/2 and 1f7/2 strength distributions are not well reproduced by the QPM model. The QPMFF spreading widths are found much narrower than the standard ones, but nevertheless much wider than predicted by theoretical calculations.
    Physical Review C 11/2002; 66(5). · 3.72 Impact Factor
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    ABSTRACT: The study of the 208Pb(d⃗,t)207Pb reaction at Ed=200MeV has been extended up to typically Ex=40MeV in 207Pb using a polarized beam with both vector and tensor components. Two-step pickup reactions involving low multipolarity collective transitions have been evaluated for the first time via systematic coupled channel calculations, allowing a new approach of the background determination. The (d⃗,t) observables corresponding to the overlapping 1h11/2, 1g7/2, and 1g9/2 inner hole responses have been analyzed up to Ex=25MeV via a least squares fit procedure. Necessary input values were deduced for hole states of interest from finite range distorted wave (DWBA) calculations. The optical parameters and the range function were those successfully used in a previous survey of valence state observables. The highest j transitions are enhanced in the reaction and analyzing powers exhibit strongly characteristic features for j-=l-1/2 versus j+=l+1/2 states. We have calculated for the first time the separation energy dependence of nlj transition observables, taking into account the form factor modifications induced by the hole coupling with surface vibrations. The calculations have been performed in the framework of the quasiparticle-phonon model (QPM). This analysis (QPMFF) predicts a large variation of differential cross sections with excitation energy of the hole fragments, while angular distribution shapes remain quite stable. The strength distributions resulting of the QPMFF analysis and of a standard analysis using DWBA observables calculated at the centroid energies are systematically compared. As a general rule, the QPMFF analysis increases the strength concentration toward lower excitation energy. The corresponding 1h11/2, 1g7/2, and the tentative 1g9/2 strength distributions are compared and discussed with the available theoretical calculations. In particular, the narrower spreading widths deduced via the QPMFF analysis are quite well predicted by the calculation of spectral functions in a modified mean field. The 1i13/2 and 1h9/2 valence strength distributions are revisited along this new approach and found to be in fair agreement with the fragmentation predicted by the QPM, which is not the case of inner hole strength distributions.
    Physical Review C 10/1998; 58(4). · 3.72 Impact Factor
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    ABSTRACT: Low-lying states at Ex=4.4-8.6 MeV of excitation energy in 56Fe have been studied by means of inelastic deuteron scattering at Ed=56 MeV. The spin, parity, and isoscalar matrix element of detected levels have been deduced by comparing the measured cross sections with coupled-channel calculations using collective form factors. The present data for higher excitation energies have been analyzed by combining with previous data at lower excitation energies. The low-energy octupole strength in 56Fe is found to be mostly located in the excitation energy range of Ex=4.4-8.6 MeV. The strength is weakly fragmented, and the 3-1 level exhausts the major part of the octupole energy-weighted-sum-rule value. A similar behavior is observed for the quadrupole strength in 56Fe, which is mostly located below Ex=4.4 MeV. This is in contrast with the behavior of the 3- and 2+ states in 54Fe, which are heavily fragmented despite the weaker quadrupole deformation. The hexadecapole strength is strongly fragmented both in 56Fe and in 54Fe. The strength distributions for the transitions with multipolarities λ=2, 3, and 4 in 54,56Fe have been compared with random phase approximation calculations. The theoretical calculations show that the closure of the νf7/2 shell is responsible for the observed fragmentation of the 2+ and 3- strengths in 54Fe, and that the collectivity of the 2+1 and 3-1 levels of 56Fe is restored by the presence of two neutrons outside the νf7/2 shell.
    Physical Review C 04/1998; 57(4):1604-1616. · 3.72 Impact Factor
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    ABSTRACT: The 154Sm(d-->,t) reaction at high energy resolution (n,gamma), average resonance capture (ARC), and coincidence measurements were performed to study the deformed nucleus 153Sm. Strength distributions from (d-->,t) and completeness for Ipi=12- and 32- states up to 1500 keV from ARC provide one of the first detailed tests of the interacting boson fermion model (IBFM) in a deformed nucleus in a multiorbit environment. For negative parity states the model accounts for the large number of low spin (12-, 32-) states much better than the Nilsson model since the even-even core in the IBFM calculations automatically includes excited vibrational states. The IBFM calculations also predict (d,t) spectroscopic factors better than the Nilsson model with pairing and Coriolis mixing. Neither the IBFM nor the Nilsson approach can explain the low lying positive parity states. The IBFM calculations show that for certain combinations of parameters, the monopole term in the boson-fermion Hamiltonian has more than a scaling effect: it can attenuate the Coriolis mixing (energy staggering). Finally suggested improvements in the treatment of pairing in the IBFM are made.
    Physical Review C 01/1998; 57(6). · 3.72 Impact Factor
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    ABSTRACT: Excited states in the 153Sm nucleus have been investigated via the (p,d) reaction performed with a good resolution. About 170 levels were observed below 2.2 MeV excitation energy; for most of them the ℓ-transfer value could be assigned. Distorted wave calculations have been performed and discussed. The 153Sm is already rather deformed, and can be described in terms of the Nilsson model. Levels below 0.5 MeV were identified as members of rotational bands based on the Nilsson orbitals. The strength distributions have been described within the Quasiparticle Phonon Model.
    Nuclear Physics A 01/1997; 624(3):433-448. · 2.50 Impact Factor
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    ABSTRACT: Low-lying states in 56Fe, up to an excitation energy of about 4 MeV, have been investigated by means of inelastic proton and deuteron scattering experiments at Ep=65 and 400 MeV and at Ed=56 MeV, respectively. Measured cross sections and analyzing powers have been compared with coupled-channels calculations using collective form factors; calculations in both the Schrödinger and Dirac formalisms have been carried out for the proton data. For each probe, the matrix elements have been deduced for transitions from the ground state and from the 2+1 state to six quadrupole (2+) states to one octupole (3-1) and two hexadecapole (4+1 and 4+2) states. The obtained matrix elements and the previous values from gamma decay or electron inelastic scattering have been used to evaluate the isospin character of the transitions. To discuss the quadrupole mixed-symmetry states in 56Fe, the deduced neutron (Mn) and proton (Mp) components of the matrix elements, or equivalently the isoscalar (Ms) and isovector (Mv) parts, have been compared with theoretical calculations based on the neutron-proton interacting boson model and on the shell model evaluated in a full f-p configuration space.
    Physical Review C 07/1996; 53(6):2718-2731. · 3.72 Impact Factor
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    ABSTRACT: Excited states in 140, 142, 144, 146Nd nuclei, up to an excitation energy of about 5 MeV, were investigated by (p,t) experiments performed with a good energy resolution. These data, together with proton and deuteron scattering data from a previous experiment, are compared with Quasi-Particle Phonon Model evaluations, in which the competition between particle-hole and particle-particle residual interactions is considered. The B(Eλ) distributions are satisfactorily reproduced. The 146, 148Nd(p,t) reaction data are well accounted for, while difficulties are found in reproducing those for 142, 144Nd(p,t). Limitations and improvements of the model are discussed.
    Nuclear Physics A 01/1996; 601(1):1-40. · 2.50 Impact Factor
  • Physical Review C 01/1995; 50(6):2935-2946. · 3.72 Impact Factor
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    ABSTRACT: A cross section enhancement, persisting to high momentum transfer, has been observed around 1ħω in excitation energy in spectra from inelastic proton-, alpha- and electron scattering on 90Zr, 116Sn, 196Pt and 208Pb. Data are presented for (p,p') at 201 MeV, (α, α') at 120 MeV and analyzed together with existing data obtained with 133.8 MeV polarized (,p') on 116Sn and electron-scattering data on 116Sn and 196Pt. Two different interpretations of the observed enhancement are discussed: that of the incoherent sum of all 1ħω cross section and alternatively the sum over the isoscalar normal modes of all multipolarities.
    Nuclear Physics A 01/1995; · 2.50 Impact Factor
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    ABSTRACT: High-resolution inelastic-scattering measurements on 112Cd with polarized deuterons (20 MeV) and polarized protons (16 and 65 MeV) provide complete spectroscopic information. Transition moments are obtained from the angular distributions of the cross section and analyzing power in a coupled-channel reaction analysis. The results are compared with interacting boson model predictions aiming to describe intruder states, quadrupole-hexadecapole and quadrupole-octupole excitations.
    Nuclear Physics A 07/1994; 574(3):414-452. · 2.50 Impact Factor
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    ABSTRACT: Differential cross sections, vector and tensor analyzing powers of the (d→,t) reaction on 120Sn, 116Sn, 90Zr, 58Ni, 16O, and 12C have been measured at 200 MeV bombarding energy. Deuteron elastic scattering measurements have been performed on 116Sn and 208Pb at the same energy. These data have been analyzed together with previous ones on 58Ni and 16O to get best fit optical parameters describing deuteron elastic scattering. The (d→,t) experimental survey bears on 28 transitions populating well known valence levels, including previous data in 207Pb and 27Si. The vector and tensor analyzing powers exhibit striking similarities for transitions measured in different nuclei. The angular distributions are found to strongly depend on the number of nodes in the neutron form factor and on the coupling of spin and angular momentum j-=l-1/2 versus j+=l+1/2. The j effect is especially pronounced, for both analyzing powers for n=1 transitions. The slopes of the differential cross sections in different nuclei depend mainly on the number of nodes. Exact finite range calculations including S and D components have been performed, using two sets of deuteron parameters together with a deep triton potential. Both analyses reproduce rather well the differential cross sections and currently adopted spectroscopic factors. The conventional analyses with deuteron parameters fitting elastic scattering data reproduce rather well analyzing powers of n>1 transitions (with l=0,1,2), but disagree with the data for n=1 transitions (except for j+Ayy values). Good or qualitative agreement is achieved for all transitions with the second deuteron potential, characterized by larger spin orbit terms and an additional imaginary tensor term. This allows using the reaction as a spectroscopic tool.
    Physical Review C 01/1994; 50(6). · 3.72 Impact Factor
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    ABSTRACT: Low-lying hexadecapole states in stable even-even neodymium isotopes have been investigated by means of inelastic electron scattering. Transition charge densities were extracted in a Fourier-Bessel analysis of the form factors. The analysis of the experimental results within the interacting sdg-boson model with only one g boson allowed the extraction of the radial shapes of the hexadecapole structure functions of three of the different boson-pair configurations (i.e., dd, sg, and dg) involved in the hexadecapole excitations.
    Physical Review C 05/1993; 47(4):R1347-R1350. · 3.72 Impact Factor
  • Physical Review C 05/1993; 47(4):1571-1585. · 3.72 Impact Factor
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    ABSTRACT: The low-lying states of 144Nd have been investigated up to an excitation energy of 3.1 MeV by means of high-resolution inelastic electron scattering. Transition charge densities have been extracted for natural-parity states. The experimental data have been compared with the predictions of the quasiparticle-phonon model. The calculations show that both collective and single-particle degrees of freedom are important for describing the low-lying states of 144Nd. A comparison of the present data with data for 142Nd and 142Ce emphasizes the role played by the two valence neutrons outside the N = 82 closed shell.
    Nuclear Physics A 01/1993; · 2.50 Impact Factor
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    ABSTRACT: High resolution inelastic scattering measurements with 20 MeV polarized deuterons and results of recent Coulomb excitation studies of Hasselgren et al. on 110Pd are compared with predictions from new IBA calculations, where configuration mixing or QQ interaction between like bosons was included to describe intruder states and triaxial features, respectively. The spectroscopic properties of the four low-energy collective bands are well described.
    Nuclear Physics A 01/1993; 562(2):157-179. · 2.50 Impact Factor
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    ABSTRACT: Highly excited neutron hole states in 207Pb have been studied via the (d→,t) reaction at Ed=200 MeV using for the first time a polarized beam, with both vector and tensor components. The determination of overlapping neutron hole response functions takes advantage of the strong characteristic features of j-=l-1/2 versus j+=l+1/2 level analyzing powers and of the good distorted wave Born approximation (DWBA) description of the reaction. A least-squares fit analysis of the σ, Ay, and Ayy angular distributions has allowed a determination of the high j transition spectroscopic factors contributing to the excitation energy bins up to Ex=14.5 MeV. The results are compared in details with those relying only on l identification, previously obtained in the 1i13/2 and 1h9/2 valence state fragmentation up to Ex=6.7 MeV and on the 1h11/2 strength up to ∼10 MeV. Several previous conclusions are unambiguously confirmed, in particular, the attribution of the bump around Ex=8.2 MeV to the 1h11/2 strength. In addition, the present experiment settles the attributions of several j- versus j+ valence groups and indicates that the 1h9/2 strength is spread up to ∼10.8 MeV.The analysis performed up to Ex=14.5 MeV gives the first determination of the 1h11/2 strength beyond the bump, and a first reliable evidence for the 1g7/2 strength, with the maximum around Ex=11 MeV. ∼85% of these inner hole state sum rules are exhausted in the studied excitation energy range. The spin-orbit splitting of 1h orbitals in 208Pb deduced from the strength centroids is 5.2 MeV. The experimental strength distributions and the integral characteristics of the 1i13/2, 1h9/2, 1h11/2, and 1g7/2 hole states are compared with the results from different theoretical approaches, i.e., microscopic calculations of the fragmentation and phenomenological calculations of spectral functions in a modified mean field. The comparison reports on the sharing of the valence strengths between each quasihole level and the other fragments, characterized by their centroids and widths, and on the shape of the inner hole strength distributions, the centroids and spreading widths.
    Physical Review C 01/1993; 47(4). · 3.72 Impact Factor
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    ABSTRACT: Excited states in even Nd isotopes, up to excitation energies of 3–4 MeV, were investigated in proton- and deuteron-scattering experiments performed with high-energy resolution. More than 300 transitions were studied. For several new excited states spin and parity assignments have been suggested. Reduced transition probabilities were extracted for natural-parity states from O+ up to 6+. The experimental strength distributions have been compared with the predictions of the interacting boson model (IBM) and of the quasi-particle-phonon model (QPM). The octupole transition probabilities are well described in both models as produced by the fragmentation of the f-boson or of E3 phonons. IBM-sdf calculations seem to account also for the transitions to the low-lying 1− states. Quadrupole and hexadecapole distributions are well described in the QPM. The leading configurations are due to 6–8 low-lying one-phonon states. The two- and three-phonon states play an important role especially in 146Nd. The failure of IBM quadrupole and hexadecapole calculations clearly points out the need of introducing additional bosons lying at high excitation energies. QPM evaluations account also for other features of the experimental data, as the E5 and E6 strength distributions and the isovector components. The limits of the two models are discussed.
    Nuclear Physics A 01/1993; · 2.50 Impact Factor

Publication Stats

594 Citations
151.07 Total Impact Points

Institutions

  • 1957–2007
    • INFN - Istituto Nazionale di Fisica Nucleare
      Frascati, Latium, Italy
  • 1963–2002
    • University of Milan
      • • Department of Physics
      • • Istituto di Scienze Dermatologiche
      Milano, Lombardy, Italy
  • 1988
    • VU University Amsterdam
      • Department of Physics and Astronomy
      Amsterdamo, North Holland, Netherlands
  • 1983
    • University of Groningen
      • Kernfysisch Versneller Instituut (KVI)
      Groningen, Province of Groningen, Netherlands
  • 1981
    • Università degli Studi di Bari Aldo Moro
      Bari, Apulia, Italy