When a heavy atomic nucleus splits (fission), the resulting fragments are observed to emerge spinning
; this phenomenon has been a mystery in nuclear physics for over 40 years
. The internal ...generation of typically six or seven units of angular momentum in each fragment is particularly puzzling for systems that start with zero, or almost zero, spin. There are currently no experimental observations that enable decisive discrimination between the many competing theories for the mechanism that generates the angular momentum
. Nevertheless, the consensus is that excitation of collective vibrational modes generates the intrinsic spin before the nucleus splits (pre-scission). Here we show that there is no significant correlation between the spins of the fragment partners, which leads us to conclude that angular momentum in fission is actually generated after the nucleus splits (post-scission). We present comprehensive data showing that the average spin is strongly mass-dependent, varying in saw-tooth distributions. We observe no notable dependence of fragment spin on the mass or charge of the partner nucleus, confirming the uncorrelated post-scission nature of the spin mechanism. To explain these observations, we propose that the collective motion of nucleons in the ruptured neck of the fissioning system generates two independent torques, analogous to the snapping of an elastic band. A parameterization based on occupation of angular momentum states according to statistical theory describes the full range of experimental data well. This insight into the role of spin in nuclear fission is not only important for the fundamental understanding and theoretical description of fission, but also has consequences for the γ-ray heating problem in nuclear reactors
, for the study of the structure of neutron-rich isotopes
, and for the synthesis and stability of super-heavy elements
.
Excited states in the Tz=0,−1 nuclei 62Ga and 62Ge were populated in direct reactions of relativistic radioactive ion beams at the RIBF. Coincident γ rays were measured with the DALI2+ array and ...uniquely assigned to the A=62 isobars. In addition, 62Ge was also studied independently at JYFL-ACCLAB using the 24Mg(40Ca,2n)62Ge fusion-evaporation reaction. The first excited T=1,Jπ=2+ states in 62Ga and 62Ge were identified at 979(1) and 965(1) keV, respectively, resolving discrepant interpretations in the literature. States beyond the first 2+ state in 62Ge were also identified for the first time in the present work. The results are compared with shell-model calculations in the fp model space. Mirror and triplet energy differences are analyzed in terms of individual charge-symmetry and charge-independence breaking contributions. The MED results confirm the shrinkage of the p-orbits' radii when they are occupied by at least one nucleon on average.
A new β-decay station, COnversion electrons Chasing at Orsay (COeCO), has been developed at ALTO to perform conversion electron spectroscopy studies of neutron-rich nuclei produced by photo-fission ...of a uranium carbide target. It is based on the collection of a low-energy ISOL beam on a mylar tape, and the transportation of the electrons emitted by the produced radioactive source through a magnetic field induced by two copper coils, towards a cooled Si(Li) detector. In this article, a detailed description of the new decay station and its components is given. The magnetic field induced by the coils was measured and compared to simulations performed with the COMSOL® software. The efficiency of the detection setup was estimated using a 207Bi and a 152Eu source as an off-line commissioning. Finally, the results obtained with a 96Rb radioactive beam for the on-line commissioning are presented.
The β-decay half-lives of 55 neutron-rich nuclei 134−139Sn, 134−142Sb, 137−144Te, 140−146I, 142−148Xe, 145−151Cs, 148−153Ba, and 151−155La were measured at the Radioactive Isotope Beam Factory ...employing the projectile fission fragments of 238U. The nuclear level structure, which relates to deformation, has a large effect on the half-lives. The impact of newly measured half-lives on modeling the astrophysical origin of the heavy elements is studied in the context of r process nucleosynthesis. For a wide variety of astrophysical conditions, including those in which fission recycling occurs, the half-lives have an important local impact on the second (A ≈ 130) peak.
The population of isomeric states in the prompt decay of fission fragments-so-called isomeric yield ratios (IYRs)-is known to be sensitive to the angular momentum J that the fragment emerged with, ...and may therefore contain valuable information on the mechanism behind the fission process. In this work, we investigate how changes in the fissioning system impact the measured IYRs of fission fragments to learn more about what parameters affect angular momentum generation. To enable this, a new technique for measuring IYRs is first demonstrated. It is based on the time of arrival of discrete gamma rays, and has the advantage that it enables the study of the IYR as a function of properties of the partner nucleus. This technique is used to extract the IYR of 134Te, strongly populated in actinide fission, from the three different fissioning systems: 232Th(n, f), 238U(n, f), at two different neutron energies, as well as 252Cf(sf). The impacts of changing the fissioning system, the compound nuclear excitation energy, the minimum J of the binary partner, and the number of neutrons emitted on the IYR of 134Te are determined. The decay code TALYS is used in combination with the fission simulation code FREYA to calculate the primary fragment angular momentum from the IYR. We find that the IYR of 134Te has a slope of 0.004 +/- 0.002 with increase in compound nucleus (CN) mass. When investigating the impact on the IYR of increased CN excitation energy, we find no change with an energy increase similar to the difference between thermal and fast fission. By varying the mass of the partner fragment emerging with 134Te, it is revealed that the IYR of 134Te is independent of the total amount of prompt neutrons emitted from the fragment pair. This indicates that neutrons carry minimal angular momentum away from the fission fragments. Comparisons with the FREYA+TALYS simulations reveal that the average angular momentum in 134Te following 238U(n, f) is 6.0 h over bar . This is not consistent with the value deduced from recent CGMF calculations. Finally, the IYR sensitivity to the angular momentum of the primary fragment is discussed. These results are not only important to help understanding the underlying mechanism in nuclear fission, but can also be used to constrain and benchmark fission models, and are relevant to the gamma -ray heating problem of reactors.
Excited states in 56Zn were populated following one-neutron removal from a 57Zn beam impinging on a Be target at intermediate energies in an experiment conducted at the Radioactive Isotope Beam ...Factory at RIKEN. Three γ rays were observed and tentatively assigned to the 6+→4+→2+→0+ yrast sequence. This turns 56Zn into the heaviest Tz=−2 nucleus in which excited states are known. The excitation-energy differences between these levels and the isobaric analogue states in the Tz=+2 mirror partner, 56Fe, are compared with large-scale shell-model calculations considering the full pf valence space and various isospin-breaking contributions. This comparison, together with an analysis of the mirror energy differences in the A=58, Tz=±1 pair 58Zn and 58Ni, provides valuable information with respect to the size of the monopole radial and the isovector multipole isospin-breaking terms in the region above doubly-magic 56Ni.
Response of the FAst TIMing Array (FATIMA) for DESPEC at FAIR Phase-0 Chishti, M.M.R.; Jazrawi, S.; Shearman, R. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
11/2023, Letnik:
1056
Journal Article
Recenzirano
The Monte-Carlo simulated response for γ-ray detection of the FAst TIMing Array (FATIMA) for exploitation within the DEcay SPECtroscopy (DESPEC) experimental system at the FAIR Phase-0 facility at ...Darmstadt, Germany is presented. In this configuration, FATIMA consisted of 36 LaBr3(Ce) detectors surrounding the AIDA, position-sensitive charged-particle active stopper. The decay of the Iπ=8+ isomer-fed decay cascade in 96Pd, measured in the first DESPEC experiment at the FAIR-0 facility was used to validate the simulations. The experimental data yielded in-situ full-energy peak efficiency values for FATIMA of 11.2(11)%, 6.8(7)%, 3.8(4)% and 2.1(4)% at 106, 325, 684 and 1415 keV respectively, consistent with the values derived from the simulated response.
Structure of the neutron-rich N=86 isotope 140Xe, located northeast of a doubly-magic nucleus 132Sn, is investigated by β−γ spectroscopy. Two β-decay isomers in 140I are newly found in the study of ...two different β decays of 140I which were produced by two reactions (i) direct in-flight fission at a primary target and (ii) β decay of 140Te at an active stopper. Half-lives of the β decays of the ground state, the low-spin isomer, and the high-spin isomer are determined to be 0.38(2), 0.91(5), and 0.47(4) s, respectively. Decay schemes of the β decay of the high-spin isomer and of the mixed β decays of the ground state and the low-spin isomer in 140I to 140Xe are constructed using the information on γ-ray coincidence relation and γ-ray intensity. Nuclear structures of the low-lying states in 140Xe and 140I are discussed by comparing the experimental results to two theoretical calculations based on a large-scale shell model and the deformed Skyrme Hartree-Fock-Bogoliubov plus deformed quasiparticle-random-phase approximation. Possible candidates for (quasi-)γ-band members of 2+ and 4+ states and the octupole collective 1− state are proposed in 140Xe. Increase of quadrupole, triaxial, and octupole collectivities is discussed with the increase of neutron and proton numbers.
Newly observed decay schemes of the nuclei 137Sb and 138Sb are reported. The neutron-rich Sb isotopes were produced by the in-flight fragmentation of a 238U primary beam with an energy of 345 ...MeV/nucleon. Several new excited states of 137Te with tentatively assigned spin-parities of (5/2−), (9/2−), and (7/2) have been established which play an important role in the evolution of neutron levels beyond N=82. The study of the β decay of 138Sb led to a considerable extension of the level scheme of 138Te including the identification of several nonyrast states. The structure of 137Te and 138Te is discussed on the basis of large-scale shell-model calculations performed using two different effective interactions.
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