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Register a new userProbing the width of compound states with rotational gamma rays
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The intrinsic width of (multiparticle-multihole) compound states is an elusive quantity, of difficult direct access, as it is masked by damping mechanisms which control the collective response of nuclei. Through microscopic cranked shell model calculations, it is found that the strength function associated with two-dimensional gamma-coincidence spectra arising from rotational transitions between states lying at energies >1 MeV above the yrast line, exhibits a two-component structure controlled by the rotational (wide component) and compound (narrow component) damping width. This last component is found to be directly related to the width of the multiparticle-multihole autocorrelation function.
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Angular distribution of individual $gamma$-rays, emitted from a neutron-induced compound nuclear state via radiative capture reaction of ${}^{139}$La(n,$gamma$) has been studied as a function of incident neutron energy in the epithermal region by using germanium detectors. An asymmetry $A_{mathrm{LH}}$ was defined as $(N_{mathrm L}-N_{mathrm H})/(N_{mathrm L}+N_{mathrm H})$, where $N_{mathrm L}$ and $N_{mathrm H}$ are integrals of low and high energy region of a neutron resonance respectively, and we found that $A_{mathrm{LH}}$ has the angular dependence of $(Acostheta_gamma+B)$, where $theta_gamma$ is emitted angle of $gamma$-rays, with $A= -0.3881pm0.0236$ and $B=-0.0747pm0.0105$ in 0.74 eV p-wave resonance. This angular distribution was analyzed within the framework of interference between s- and p-wave amplitudes in the entrance channel to the compound nuclear state, and it is interpreted as the value of the partial p-wave neutron width corresponding to the total angular momentum of the incident neutron combined with the weak matrix element, in the context of the mechanism of enhanced parity-violating effects. Additionally we used the result to quantify the possible enhancement of the breaking of the time-reversal invariance in the vicinity of the p-wave resonance.
A correlation term ${{ sigma}_{n} }cdot ({ k_{n}times k_gamma}) $ in the ${}^{139}{rm La}(vec{n},gamma)$ reaction has been studied utilizing epithermal polarized neutrons and germanium detectors. The transverse asymmetry for single $gamma$-ray transition was measured to be $0.60pm0.19$ in the $p$-wave resonance.
A method has been recently proposed to establish the geometry of the alpha-cluster arrangement in $^{12}$C making use of polarized gamma-rays. The ratio of intensities of scattered radiation at 90 degrees along and perpendicular to the initial direction of the electric field vector, called depolarization ratio, is a key quantity that allows to underpin the nature of totally symmetric modes of vibrations. This allows to connect with the underlying point-group structure and therefore to the geometric shape of the nuclear molecule. This method is reviewed for $^{12}$C and extended to other configurations, such as three unequal clusters and four identical clusters (e.g. $^{16}$O).
While Josephson-like junctions, transiently established in heavy ion collisions ($tau_{coll}approx10^{-21}$ s) between superfluid nuclei --through which Cooper pair tunneling ($Q$-value $Q_{2n}$) proceeds mainly in terms of successive transfer of entangled nucleons-- is deprived from the macroscopic aspects of a supercurrent, it displays many of the special effects associated with spontaneous symmetry breaking in gauge space (BCS condensation), which can be studied in terms of individual quantum states and of tunneling of single Cooper pairs. From the results of studies of one- and two- neutron transfer reactions carried out at energies below the Coulomb barrier we estimate the value of the mean square radius (correlation length) of the nuclear Cooper pair. A quantity related to the largest distance of closest approach for which the absolute two-nucleon tunneling cross section is of the order of the single-particle one. Furthermore, emission of $gamma$-rays of (Josephson) frequency $ u_J=Q_{2n}/h$ distributed over an energy range $hbar/tau_{coll}$ is predicted.
The emission of prompt fission $gamma$ rays within a few nanoseconds to a few microseconds following the scission point is studied in the Hauser-Feshbach formalism applied to the deexcitation of primary excited fission fragments. Neutron and $gamma$-ray evaporations from fully accelerated fission fragments are calculated in competition at each stage of the decay, and the role of isomers in the fission products, before $beta$-decay, is analyzed. The time evolution of the average total $gamma$-ray energy, average total $gamma$-ray multiplicity, and fragment-specific $gamma$-ray spectra, is presented in the case of neutron-induced fission reactions of $^{235}$U and $^{239}$Pu, as well as spontaneous fission of $^{252}$Cf. The production of specific isomeric states is calculated and compared to available experimental data. About 7% of all prompt fission $gamma$ rays are predicted to be emitted between 10 nsec and 5 $mu$sec following fission, in the case of $^{235}$U and $^{239}$Pu $(n_{rm th},f)$ reactions, and up to 3% in the case of $^{252}$Cf spontaneous fission. The cumulative average total $gamma$-ray energy increases by 2 to 5% in the same time interval. Finally, those results are shown to be robust against significant changes in the model input parameters.
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