An abnormal production of events with almost equal-sized fragments was theoretically proposed as a signature of spinodal instabilities responsible for nuclear multifragmentation in the Fermi energy ...domain. On the other hand finite size effects are predicted to strongly reduce this abnormal production. High statistics quasifusion hot nuclei produced in central collisions between Xe and Sn isotopes at 32 and 45 A MeV incident energies have been used to definitively establish, through the experimental measurement of charge correlations, the presence of spinodal instabilities. N/Z influence was also studied.
Agata characterisation and pulse shape analysis Boston, A. J.; Crespi, F. C. L.; Duchêne, G. ...
The European physical journal. A, Hadrons and nuclei,
09/2023, Letnik:
59, Številka:
9
Journal Article
Recenzirano
Odprti dostop
The AGATA and GRETA spectrometers are large arrays of highly segmented HPGe detectors that use the technique of gamma ray tracking to reconstruct the scattering path of gamma rays interacting within ...the active material. A basic requirement is a precise reconstruction of the individual interaction locations within the detectors. This is possible through the use of pulse shape analysis which has to be conducted in real time due to the high data rates generated by the spectrometer. The methodologies that have been evaluated to perform this for AGATA are discussed along with the approaches used to calculate the pulse shape databases required by these algorithms. Finally, the performance and limitations of the existing approaches are reviewed.
Electric quadrupole matrix elements, Mp, for the Jπ=2+→0+, ΔT=0, T=1 transitions across the A=46 isobaric multiplet 46Cr-46V-46Ti have been measured at GSI with the FRS-LYCCA-AGATA setup. This allows ...direct insight into the isospin purity of the states of interest by testing the linearity of Mp with respect to Tz. Pairs of nuclei in the T=1 triplet were studied using identical reaction mechanisms in order to control systematic errors. The Mp values were obtained with two different methodologies: (i) a relativistic Coulomb excitation experiment was performed for 46Cr and 46Ti; (ii) a “stretched target” technique was adopted here, for the first time, for lifetime measurements in 46V and 46Ti. A constant value of Mp across the triplet has been observed. Shell-model calculations performed within the fp shell fail to reproduce this unexpected trend, pointing towards the need of a wider valence space. This result is confirmed by the good agreement with experimental data achieved with an interaction which allows excitations from the underlying sd shell. A test of the linearity rule for all published data on complete T=1 isospin triplets is presented.
.
For the first time, bases of signals delivered by highly segmented HPGe detectors, for identified hit locations, have been determined
in situ
, that is in the actual accelerator-target-detection ...system conditions corresponding to data acquisition during a physics experiment. As a consequence, these bases include all the genuine features and alterations of the signals induced by the experimental setup,
e.g.
diaphony, electronic response, specificity of individual crystals. The present pulse shape bases were constructed using calibration source data taken at the beginning of the AGATA campaign at GANIL. An experiment performed at GANIL using the AGATA
γ
-ray detector together with the VAMOS spectrometer was used to validate the bases. The performance of the bases when used for pulse-shape analysis has been compared to the performance of the standard bases, composed of pulse shapes generated by a computer simulation used for AGATA. This is done by comparing the Doppler correction capability. The so-called Jacobian method used to generate the
in situ
bases also produces correlations that can be applied to locate in a direct way (no search algorithm) the location where a
γ
-ray interacted given that only one segment is hit. As about 50% of all pulse-shape analysis is performed on crystals with only one segment hit this will allow for a large reduction in the needed computer power. Different ways to improve the results of this prospective work are discussed.
Background: Levels fulfilling the seniority scheme and relevant isomers are commonly observed features in semimagic nuclei; for example, in Sn isotopes (Z = 50). Seniority isomers in Sn, with ...dominantly pure neutron configurations, directly probe the underlying neutron-neutron (vv) interaction. Furthermore, an addition of a valence proton particle or hole, through neutron-proton (v pi) interaction, affects the neutron seniority as well as the angular momentum. Purpose: Benchmark the reproducibility of the experimental observables, like the excitation energies (E-x) and the reduced electric-quadrupole transition probabilities B(E2), with the results obtained from shell-model interactions for neutron-rich Sn and Sb isotopes with N < 82. Study the sensitivity of the aforementioned experimental observables to the model interaction components. Furthermore, explore from a microscopic point of view the structural similarity between the isomers in Sn and Sb, and thus the importance of the valence proton. Methods: The neutron-rich Sb122-131 isotopes were produced as fission fragments in the reaction Be-9(U-238, f) with 6.2 MeV/u beam energy. A unique setup, consisting of AGATA, VAMOS++, and EXOGAM detectors, was used which enabled the prompt-delayed gamma-ray spectroscopy of fission fragments in the time range of 100 ns to 200 mu s. Results: New isomers and prompt and delayed transitions were established in the even-A Sb122-131 isotopes. In the odd-A Sb122-131 isotopes, new prompt and delayed gamma-ray transitions were identified, in addition to the confirmation of the previously known isomers. The half-lives of the isomeric states and the B(E2) transition probabilities of the observed transitions depopulating these isomers were extracted. Conclusions: The experimental data was compared with the theoretical results obtained in the framework of large-scale shell-model (LSSM) calculations in a restricted model space. Modifications of several components of the shell-model interaction were introduced to obtain a consistent agreement with the excitation energies and the B(E2) transition probabilities in neutron-rich Sn and Sb isotopes. The isomeric configurations in Sn and Sb were found to be relatively pure. Furthermore, the calculations revealed that the presence of a single valence proton, mainly in the g(7/2) orbital in Sb isotopes, leads to significant mixing (due to the v pi interaction) of (i) the neutron seniorities (upsilon(v)) and (ii) the neutron angular momentum (I-v). The above features have a weak impact on the excitation energies, but have an important impact on the B(E2) transition probabilities. In addition, a constancy of the relative excitation energies irrespective of neutron seniority and neutron number in Sn and Sb was observed.
The properties of pygmy dipole states in Pb-208 were investigated using the Pb-208(O-17, O-17'gamma) reaction at 340 MeV and measuring the gamma decay with high resolution with the AGATA demonstrator ...array. Cross sections and angular distributions of the emitted gamma rays and of the scattered particles were measured. The results are compared with (gamma, gamma') and (p, p') data. The data analysis with the distorted wave Born approximation approach gives a good description of the elastic scattering and of the inelastic excitation of the 2(+) and 3(-) states. For the dipole transitions a form factor obtained by folding a microscopically calculated transition density was used for the first time. This has allowed us to extract the isoscalar component of the 1(-) excited states from 4 to 8 MeV.
The low-lying excited states in the neutron-deficient N=Z+1 nucleus 8743Tc44 have been studied via the fusion-evaporation reaction 54Fe(36Ar, 2n1p)87Tc at the Grand Accélérateur National d'Ions ...Lourds (GANIL), France. The AGATA spectrometer was used in conjunction with the auxiliary NEDA, Neutron Wall, and DIAMANT detector arrays to measure coincident prompt γ rays, neutrons, and charged particles emitted in the reaction. A level scheme of 87Tc from the (9/2+g.s.) state to the (33/2+1) state was established based on six mutually coincident γ-ray transitions. The constructed level structure exhibits a rotational behavior with a sharp backbending at ℏω≈0.50 MeV. A decrease in alignment frequency and increase in alignment sharpness in the odd-mass isotonic chains around N=44 is proposed as an effect of the enhanced isoscalar neutron-proton interactions in odd-mass nuclei when approaching the N=Z line.
We performed a gamma-particle coincidence experiment for the Ni-60 + Sn-116 system to investigate whether the population of the two-neutron pickup channel leading to Ni-62 is mainly concentrated in ...the ground-state transition, as has been found in a previous work D. Montanari et al., Phys. Rev. Lett. 113, 052501 (2014). The experiment has been performed by employing the PRISMA magnetic spectrometer coupled to the Advanced Gamma Tracking Array (AGATA) demonstrator. The strength distribution of excited states corresponding to the inelastic, one-and two-neutron transfer channels has been extracted. We found that in the two-neutron transfer channel the strength to excited states corresponds to a fraction (less than 24%) of the total, consistent with the previously obtained results that the 2n channel is dominated by the ground-state to ground-state transition.
A first performance test of the Coulomb excitation multipolarimetry (
Coulex-multipolarimetry
) method is presented. It is based on a
85
Br
π
p
3
/
2
→
π
p
1
/
2
spin-flip experiment performed as ...part of the PreSPEC-AGATA campaign at the GSI Helmholtzzentrum für Schwerionenforschung (GSI). Via determination of background levels around the expected
85
Br
excitations as well as measured
197
Au
excitations, an upper limit for the
M
1 transition strength of the
1
/
2
1
-
→
3
/
2
g.s.
-
transition in
85
Br
and a lower beam time limit for upcoming experimental campaigns utilizing
Coulex-multipolarimetry
have been inferred. The impact of the use of AGATA in its anticipated
1
π
configuration on these estimates is deduced via
Geant4
simulations.
The isospin mixing was deduced in the compound nucleus Zr-80 at an excitation energy of E* = 54 MeV from the gamma decay of the giant dipole resonance. The reaction Ca-40 + Ca-40 at E-beam = 136 MeV ...was used to form the compound nucleus in the isospin I = 0 channel, while the reaction Cl-37 + Ca-44 at E-beam = 95 MeV was used as the reference reaction. The. rays were detected with the AGATA demonstrator array coupled with LaBr3:Ce detectors. The temperature dependence of the isospin mixing was obtained and the zero-temperature value deduced. The isospin-symmetry-breaking correction delta(C) used for the Fermi superallowed transitions was extracted and found to be consistent with beta-decay data.
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