Strong gamma-ray emission from neutron unbound states populated in beta-decay: Impact on (n,gamma) cross-section estimates

Total absorption gamma-ray spectroscopy is used to measure accurately the intensity of gamma emission from neutron-unbound states populated in the β-decay of delayed-neutron emitters. From the comparison of this intensity with the intensity of neutron emission one can deduce information on the (n,γ) cross section for unstable neutron-rich nuclei of interest in r process abundance calculations. A surprisingly large gamma branching was observed for a number of isotopes. The results are compared with Hauser-Feshbach calculations and discussed

Measurement of very low (alpha,n) cross sections of astrophysical interest

The reactions C-13(alpha,n)O-16 and Ne-22(alpha,n)Mg-25 are the primary sources of neutrons for the astrophysical s-process. The feasibility of cross section measurements within the respective Gamow windows is discussed in quantitative terms for a 4 pi neutron counter, based on He-3 tubes and a neutron moderator, placed in an underground lab

Shapes of Pb-192,Pb-190 ground states from beta-decay studies using the total-absorption technique

The beta decay of Pb-192,Pb-190 has been studied using the total absorption technique at the ISOLDE (CERN) facility. The beta-decay strength deduced from the measurements, combined with QRPA theoretical calculations, allow us to infer that the ground states of the Pb-192,Pb-190 isotopes are spherical. These results represent the first application of the shape determination method using the total absorption technique for heavy nuclei and in a region where there is considerable interest in nuclear shapes and shape effects

Experimental study of the O-15(2p, gamma) Ne-17 cross section by Coulomb Dissociation for the rp process

The time-reversed reaction O-15(2p, gamma) Ne-17 has been studied by the Coulomb dissociation technique. Secondary 17Ne ion beams at 500 AMeV have been produced by fragmentation reactions of Ne-20 in a beryllium production target and dissociated on a secondary Pb target. The incoming beam and the reaction products have been identified with the kinematically complete LAND-(RB)-B-3 experimental setup at GSI. The excitation energy prior to decay has been reconstructed by using the invariant-mass method. The preliminary differential and integral Coulomb Dissociation cross sections (sigma(Coul)) have been calculated, which provide a photoabsorption (sigma(photo)) and a radiative capture cross section (sigma(cap)). Additionally, important information about the nuclear structure of the Ne-17 nucleus will be obtained. The analysis is in progress

Properties of low-lying intruder states in Al-34 and Si-34 populated in the beta-decay of Mg-34

The results of the IS530 experiment at ISOLDE revealed new information concerning several nuclei close to the N approximate to 20 'Island of Inversion' - Mg-34, Al-34, Si-34. The half-life of Mg-34 was found to be three times larger than the adopted value (63(1) ms instead of 20(10) ms). The beta-gamma spectroscopy of Mg-34 performed for the first time in this experiment, led to the first experimental level scheme for Al-34, also showing that the full beta strength goes through the predicted 1(+) isomer in Al-34 [1] and/or excited states that deexcite to it. The subsequent beta-decay of the 1(+) isomer in Al-34 allowed the observation of new gamma lines in Si-34, (tentatively) associated with low-spin high-energy excited states previously unobserved

Direct experimental evidence for a multiparticle-hole ground state configuration of deformed Mg-33

©2016 American Physical Society. Articulo firmado por más de 10 autores. The authors wish to thank the accelerator staff of GSI for their active support during the experiment. Ushasi Datta acknowledges the Alexander von Humboldt Foundation and SEND project (PIN: 11-R&D-SIN-5.11-0400), Govt. of India, for their support of the experimental investigation and is also grateful to Prof. B. M. Sherrill, NSCL, and Prof. Larry Zamick, Rutgers University, for many valuable suggestions and discussion.The first direct experimental evidence of a multiparticle-hole ground state configuration of the neutron-rich Mg-33 isotope has been obtained via intermediate energy (400 A MeV) Coulomb dissociation measurement. The major part similar to(70 +/- 13)% of the cross section is observed to populate the excited states of Mg-32 after the Coulomb breakup of Mg-33. The shapes of the differential Coulomb dissociation cross sections in coincidence with different core excited states favor that the valence neutron occupies both the s(1/2) and p(3/2) orbitals. These experimental findings suggest a significant reduction and merging of sd-pf shell gaps at N similar to 20 and 28. The ground state configuration of Mg-33 is predominantly a combination of Mg-32(3.0,3.5MeV; 2(-), 1(-)) circle times nu(s1/2), Mg-32(2.5MeV; 2(+)) circle times nu(p3/2), and Mg-32(0; 0(+)) circle times nu(p3/2). The experimentally obtained quantitative spectroscopic information for the valence neutron occupation of the s and p orbitals, coupled with different core states, is in agreement with Monte Carlo shell model (MCSM) calculation using 3 MeV as the shell gap at N = 20.Alexander von Humboldt FoundationSEND project, Govt. of IndiaDepto. de Estructura de la Materia, Física Térmica y ElectrónicaFac. de Ciencias FísicasTRUEpu

Ground-state configuration of neutron-rich ³⁵Al via Coulomb breakup

The ground-state configuration of ³⁵Al has been studied via Coulomb dissociation (CD) using the LAND-FRS setup (GSI, Darmstadt) at a relativistic energy of ~ 403 MeV/nucleon. The measured inclusive differential CD cross section for ³⁵Al, integrated up to 5.0 MeV relative energy between the ³³Al core and the neutron using a Pb target, is 78(13) mb. The exclusive measured CD cross section that populates various excited states of ³³Al is 29(7) mb. The differential CD cross section of ³⁵Al -> ³⁴Al + n has been interpreted in the light of a direct breakup model, and it suggests that the possible ground-state spin and parity of ³⁵Al could be, tentatively, 1/2⁺ or 3/2⁺ or 5/2⁺. The valence neutrons, in the ground state of ³⁵Al, may occupy a combination of either l = 3,0 or l = 1,2 orbitals coupled with the ³⁴Al core in the ground and isomeric state(s), respectively. This hints of a particle-hole configuration of the neutron across the magic shell gaps at N = 20,28 which suggests narrowing the magic shell gap. If the 5/2⁺ is the ground-state spin-parity of ³⁵Al as suggested in the literature, then the major ground-state configuration of ³⁵Al is a combination of ³⁴Al (g. s.; 4⁻) circle times ν_(p_(3/2)) and ³⁴Al (isomer; 1⁺) circle times ν _(d_(3/2)) states. The result from this experiment has been compared with that from a previous knockout measurement and a calculation using the SDPF-M interaction

The Generalized Centroid Difference method for lifetime measurements via gamma-gamma coincidences using large fast-timing arrays.

A novel method for direct electronic "fast-timing" lifetime measurements of nuclear excited states via gamma-gamma coincidences using an array equipped with N very fast high-resolution LaBr3(Ce) scintillator detectors is presented. The generalized centroid difference method provides two independent "start" and "stop" time spectra obtained without any correction by a superposition of the N(N - 1)/2 calibrated gamma-gamma time difference spectra of the N detector fast-timing system. The two fast-timing array time spectra correspond to a forward and reverse gating of a specific gamma-gamma cascade and the centroid difference as the time shift between the centroids of the two time spectra provides a picosecond-sensitive mirror-symmetric observable of the set-up. The energy-dependent mean prompt response difference between the start and stop events is calibrated and used as a single correction for lifetime determination. These combined fast-timing array mean gamma-gamma zero-time responses can be determined for 40 keV < E-gamma < 1.4 MeV with a precision better than 10 ps using a Eu-152 gamma-ray source. The new method is described with examples of (n,gamma) and (n,f,gamma) experiments performed at the intense cold-neutron beam facility PF1B of the Institut Laue-Langevin in Grenoble, France, using 16 LaBr3(Ce) detectors within the EXILL&FATIMA campaign in 2013. The results are discussed with respect to possible systematic errors induced by background contributions

Low-Z boundary of the N=88-90 shape phase transition: Ce-148 near the critical point

Excited states of the neutron-rich nucleus Ce-148 have been populated by neutron-induced fission of a U-235 target. Its electromagnetic decays were studied by means of gamma-ray coincidence spectroscopy with fast-timing capabilities. Lifetimes of the 2(1)(+) and 4(1)(+) states of Ce-148 were obtained and their E2 decay rates deduced. The B-4/2 = B(E2; 4(1)(+) -> 2(1)(+))/B(E2; 2(1)(+) -> 0(1)(+)) ratio indicates that Ce-148 is a transitional nucleus while the N = 88/90 shape phase transition evolves into a gradual change of nuclear deformation for proton numbers Z < 60