When a heavy atomic nucleus splits (fission), the resulting fragments are observed to emerge spinning 1 ; this phenomenon has been a mystery in nuclear physics for over 40 years 2,3 . The internal generation of 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 [4][5][6][7][8][9][10][11][12] . 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 13,14 , for the study of the structure of neutron-rich isotopes 15,16 , and for the synthesis and stability of super-heavy elements 17,18 .

A measurement of the reduced transition probability for the excitation of the ground state to the first 2 þ state in 104 Sn has been performed using relativistic Coulomb excitation at GSI. 104 Sn is the lightest isotope in the Sn chain for which this quantity has been measured. The result is a key point in the discussion of the evolution of nuclear structure in the proximity of the doubly magic nucleus 100 Sn. The properties of many composite quantum objects that represent building blocks of matter, such as hadrons, atomic nuclei, atoms, and molecules are governed by energy gaps between quantum states which originate in the forces between their fermionic constituents. In the case of atomic nuclei, the energy gaps manifest themselves by the existence of specific stable isotopes. These include, e.g., the double shell-closure nuclei 4 He, 16 O,40;48 Ca, and 208 Pb, which are particularly robust against particle separation and intrinsic excitation. The -unstable isotopes 56 Ni, 78 Ni, and 100;132 Sn are also expected to correspond to double shell closures. However, data for 78 Ni and 100 Sn are scarce due to their exotic neutron-to-proton ratios. Therefore, there is considerable interest in finding more proof for the magicity of these isotopes. In addition, the single particle energies relative to 100 Sn are largely unknown experimentally. Data are limited to the energy splitting between the two lowest-energy orbitals [1,2] while extrapolations from nearby nuclei are available with a typical uncertainty of a few hundred keV for the orbitals of higher energy [3]. Since 100 Sn is predicted to be a doubly magic nucleus, it would provide an approximately inert core on top of which simple excitations can be formed by adding few particles or holes. For this reason, it presents an ideal testing ground for fundamental nuclear models. Another cause for increased interest in nuclear structure in this region comes from the rp process of nuclear synthesis [4]. It has been concluded recently that this reaction sequence comes to an end near 100 Sn [4]. In addition, 100 Sn itself is expected to be the heaviest self-conjugate PRL 110,

Prompt γ-ray spectroscopy of the neutron-rich ^{96}Kr, produced in transfer- and fusion-induced fission reactions, has been performed using the combination of the Advanced Gamma Tracking Array and the VAMOS++ spectrometer. A second excited state, assigned to J^{π}=4^{+}, is observed for the first time, and a previously reported level energy of the first 2^{+} excited state is confirmed. The measured energy ratio R_{4/2}=E(4^{+})/E(2^{+})=2.12(1) indicates that this nucleus does not show a well-developed collectivity contrary to that seen in heavier N=60 isotones. This new measurement highlights an abrupt transition of the degree of collectivity as a function of the proton number at Z=36, of similar amplitude to that observed at N=60 at higher Z values. A possible reason for this abrupt transition could be related to the insufficient proton excitations in the g_{9/2}, d_{5/2}, and s_{1/2} orbitals to generate strong quadrupole correlations or to the coexistence of competing different shapes. An unexpected continuous decrease of R_{4/2} as a function of the neutron number up to N=60 is also evidenced. This measurement establishes the Kr isotopic chain as the low-Z boundary of the island of deformation for N=60 isotones. A comparison with available theoretical predictions using different beyond mean-field approaches shows that these models fail to reproduce the abrupt transitions at N=60 and Z=36.

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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 (νν) interaction. Furthermore, an addition of a valence proton particle or hole, through neutron-proton (νπ) interaction, affects the neutron seniority as well as the angular momentum.

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Background: In the neutron-rich A ≈ 100 mass region, rapid shape changes as a function of nucleon number as well as coexistence of prolate, oblate, and triaxial shapes are predicted by various theoretical models. Lifetime measurements of excited levels in the molybdenum isotopes allow the determination of transitional quadrupole moments, which in turn provides structural information regarding the predicted shape change. Purpose: The present paper reports on the experimental setup, the method that allowed one to measure the lifetimes of excited states in even-even molybdenum isotopes from mass A = 100 up to mass A = 108, and the results that were obtained. Method:The isotopes of interest were populated by secondary knock-out reaction of neutron-rich nuclei separated and identified by the GSI fragment separator at relativistic beam energies and detected by the sensitive PreSPEC-AGATA experimental setup. The latter included the Lund-York-Cologne calorimeter for identification, tracking, and velocity measurement of ejectiles, and AGATA, an array of position sensitive segmented HPGe detectors, used to determine the interaction positions of the γ ray enabling a precise Doppler correction. The lifetimes were determined with a relativistic version of the Doppler-shift-attenuation method using the systematic shift of the energy after Doppler correction of a γ -ray transition with a known energy. This relativistic Doppler-shiftattenuation method allowed the determination of mean lifetimes from 2 to 250 ps. Results: Even-even molybdenum isotopes from mass A = 100 to A = 108 were studied. The decays of the low-lying states in the ground-state band were observed. In particular, two mean lifetimes were measured for the first time: τ = 29.7 Conclusions:The reduced transition strengths B(E2), calculated from lifetimes measured in this experiment, compared to beyond-mean-field calculations, indicate a gradual shape transition in the chain of molybdenum isotopes when going from A = 100 to A = 108 with a maximum reached at N = 64. The transition probabilities decrease for 108 Mo which may be related to its well-pronounced triaxial shape indicated by the calculations.