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Register a new userEvidence for the reduction of nuclear level density away from the $beta $-stability line
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The isospin dependence of nuclear level density has been investigated by analyzing the spectra of evaporated neutrons from excited $^{116}$Sn and $^{116}$Te nuclei. These nuclei are populated via $p$ + $^{115}$In and $^{4}$He + $^{112}$Sn reactions in the excitation energy range of 18 - 26 MeV. Because of low excitation energy, the neutron spectra are predominantly contributed by the first-chance decay leading to the $beta$-stable $^{115}$Sn and neutron-deficient $^{115}$Te as residues for the two cases. Theoretical analysis of the experimental spectra have been performed within the Hauser-Feshbach formalism by employing different models of the level density parameter. It is observed that the data could only be explained by the level density parameter that decreases monotonically when the proton number deviates from the $beta$-stable value. This is also confirmed by performing a microscopic shell-model calculation with the Wood-Saxon mean field. The results have strong implication on the estimation of the level density of unstable nuclei, and calculation of astrophysical reaction rates relevant to $r$- and $rp$-processes.
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The general phenomenon of shell structure in atomic nuclei has been understood since the pioneering work of Goeppert-Mayer, Haxel, Jensen and Suess.They realized that the experimental evidence for nuclear magic numbers could be explained by introducing a strong spin-orbit interaction in the nuclear shell model potential. However, our detailed knowledge of nuclear forces and the mechanisms governing the structure of nuclei, in particular far from stability, is still incomplete. In nuclei with equal neutron and proton numbers ($N = Z$), the unique nature of the atomic nucleus as an object composed of two distinct types of fermions can be expressed as enhanced correlations arising between neutrons and protons occupying orbitals with the same quantum numbers. Such correlations have been predicted to favor a new type of nuclear superfluidity; isoscalar neutron-proton pairing, in addition to normal isovector pairing (see Fig. 1). Despite many experimental efforts these predictions have not been confirmed. Here, we report on the first observation of excited states in $N = Z = 46$ nucleus $^{92}$Pd. Gamma rays emitted following the $^{58}$Ni($^{36}$Ar,2$n$)$^{92}$Pd fusion-evaporation reaction were identified using a combination of state-of-the-art high-resolution {gamma}-ray, charged-particle and neutron detector systems. Our results reveal evidence for a spin-aligned, isoscalar neutron-proton coupling scheme, different from the previous prediction. We suggest that this coupling scheme replaces normal superfluidity (characterized by seniority coupling) in the ground and low-lying excited states of the heaviest N = Z nuclei. The strong isoscalar neutron- proton correlations in these $N = Z$ nuclei are predicted to have a considerable impact on their level structures, and to influence the dynamics of the stellar rapid proton capture nucleosynthesis process.
The low-lying excited states in the neutron-deficient $N=Z+1$ nucleus $^{87}_{43}$Tc$^{ }_{44}$ have been studied via the fusion-evaporation reaction $^{54}$Fe($^{36}$Ar, $2n1p$)$^{87}$Tc at the Grand Accelerateur National dIons Lourds (GANIL), France. The AGATA spectrometer was used in conjunction with the auxiliary NEDA, Neutron Wall, and DIAMANT detector arrays to measure coincident prompt $gamma$-rays, neutrons, and charged particles emitted in the reaction. A level scheme of $^{87}$Tc from the (9/2$^{+}_{g.s.}$) state to the (33/2$^{+}_{1}$) state was established based on 6 mutually coincident $gamma$-ray transitions. The constructed level structure exhibits a rotational behavior with a sharp backbending at $hbaromegaapprox 0.50$ 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.
The last decades brought an impressive progress in synthesizing and studying properties of nuclides located very far from the beta stability line. Among the most fundamental properties of such exotic nuclides, usually established first, is the half-life, possible radioactive decay modes, and their relative probabilities. When approaching limits of nuclear stability, new decay modes set in. First, beta decays become accompanied by emission of nucleons from highly excited states of daughter nuclei. Second, when the nucleon separation energy becomes negative, nucleons start to be emitted from the ground state. Here, we present a review of the decay modes occurring close to the limits of stability. The experimental methods used to produce, identify and detect new species and their radiation are discussed. The current theoretical understanding of these decay processes is overviewed. The theoretical description of the most recently discovered and most complex radioactive process - the two-proton radioactivity - is discussed in more detail.
Constraining excitation energy at which nuclear shell effect washes out has important implications on the production of super heavy elements and many other fields of nuclear physics research. We report the fission fragment mass distribution in alpha induced reaction on an actinide target for wide excitation range in close energy interval and show direct evidence that nuclear shell effect washes out at excitation energy ~40 MeV. Calculation shows that second peak of the fission barrier also vanishes around similar excitation energy.
We present an account of the current status of the theoretical treatment of inclusive $(d,p)$ reactions in the breakup-fusion formalism, pointing to some applications and making the connection with current experimental capabilities. Three independent implementations of the reaction formalism have been recently developed, making use of different numerical strategies. The codes also originally relied on two different but equivalent representations, namely the prior (Udagawa-Tamura, UT) and the post (Ichimura-Austern-Vincent, IAV) representations. The different implementations have been benchmarked, and then applied to the Ca isotopic chain. The neutron-Ca propagator is described in the Dispersive Optical Model (DOM) framework, and the interplay between elastic breakup (EB) and non-elastic breakup (NEB) is studied for three Ca isotopes at two different bombarding energies. The accuracy of the description of different reaction observables is assessed by comparing with experimental data of $(d,p)$ on $^{40,48}$Ca. We discuss the predictions of the model for the extreme case of an isotope ($^{60}$Ca) currently unavailable experimentally, though possibly available in future facilities (nominally within production reach at FRIB). We explore the use of $(d,p)$ reactions as surrogates for $(n,gamma)$ processes, by using the formalism to describe the compound nucleus formation in a $(d,pgamma)$ reaction as a function of excitation energy, spin, and parity. The subsequent decay is then computed within a Hauser-Feshbach formalism. Comparisons between the $(d,pgamma)$ and $(n,gamma)$ induced gamma decay spectra are discussed to inform efforts to infer neutron captures from $(d,pgamma)$ reactions. Finally, we identify areas of opportunity for future developments, and discuss a possible path toward a predictive reaction theory.
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