Measurement of picosecond lifetimes in neutron-rich Xe isotopes

Abstract

Background: Lifetimes of nuclear excited states in fission fragments have been studied in the past following isotope separation, thus giving access mainly to the fragments' daughters and only to long-lived isomeric states in the primary fragments. For the first time now, short-lived excited states in the primary fragments, produced in neutron-induced prompt fission of 235 U and 241 Pu, were studied within the EXILL&FATIMA campaign at the intense neutron-beam facility of the Institute Laue-Langevin in Grenoble. Purpose: We aim to investigate the quadrupole collective properties of neutron-rich even-even 138,140,142 Xe isotopes lying between the double shell closure N = 82 and Z = 50 and a deformed region with octupole collectivity. Method: The γ rays emitted from the excited fragments were detected with a mixed array consisting of 8 HPGe EXOGAM Clover detectors (EXILL) and 16 LaBr 3 (Ce) fast scintillators (FATIMA). The detector system has the unique ability to select the interesting fragment making use of the high resolution of the HPGe detectors and determine subnanosecond lifetimes using the fast scintillators. For the analysis the generalized centroid difference method was used. Results: We show that quadrupole collectivity increases smoothly with increasing neutron number above the closed N = 82 neutron shell. Our measurements are complemented by state-of-the-art theory calculations based on shell-model descriptions. Conclusions:The observed smooth increase in quadrupole collectivity is similar to the evolution seen in the measured masses of the xenon isotopic chain and is well reproduced by theory. This behavior is in contrast to higher Z even-even nuclei where abrupt change in deformation occurs around N = 90.

Low- Z boundary of the N=88 –90 shape phase transition: Ce148 near the critical point

“…For optimum efficiency the FATIMA detectors were placed as close as possible to the target, at 8.5 cm. Several results for lifetimes in the picosecond and nanosecond region have already been published from the EXILL&FATIMA campaign [22][23][24][25][26]. A more detailed description of the experimental setup can be found in Ref.…”

Coulomb Excitation of $^{142}$Xe

“…At this energy, based on prompt fission fragment spectroscopy following the spontaneous fission of 248 Cm [7], a candidate transition was proposed to be the decay of the first 3 − state to the first 2 + state. However, the respective coincidences have not been observed following neutron-induced fission of 235 U [8] nor in the spontaneous fission of 252 Cf [9]. It should be noted though that there are three peaks visible, which have not been observed before, and they lie in the same energy range where the missing 3 − → 2 + transition is expected.…”

Nuclear structure of Te isotopes beyond neutron magic number N=82

“…On the other hand, the recent experimental result of Ref. [13] beyond the neutron magic number N = 82 turn out to show a different nature in the structure evolution as compared to the neighboring Sn [14] and Xe [52] nuclides.…”