Selective Population Of States Research Articles

Molecular resonance and highly deformed fission fragments in28Si+28Si

A high-resolution measurement of fragment-fragment-$\ensuremath{\gamma}$ triple coincidence events in the symmetric and near-symmetric mass exit channels from the ${}^{28}\mathrm{Si}{+}^{28}\mathrm{Si}$ reaction has been undertaken using the EUROGAM Phase II $\ensuremath{\gamma}$-ray spectrometer. The bombarding energy of ${E}_{\mathrm{lab}}{(}^{28}\mathrm{Si})=111.6 \mathrm{MeV}$ has been selected to populate the conjectured ${J}^{\ensuremath{\pi}}{=38}^{+}$ quasimolecular resonance in the ${}^{56}\mathrm{Ni}$ dinuclear system. In the ${}^{28}\mathrm{Si}{+}^{28}\mathrm{Si}$ symmetric mass exit channel, the resonance behavior is clearly verified at the chosen energy. The population of highly excited states in the ${}^{24}\mathrm{Mg},$ ${}^{28}\mathrm{Si},$ and ${}^{32}\mathrm{S}$ nuclei is discussed within a statistical fusion-fission model. Evidence is presented for selective population of states in the ${}^{28}\mathrm{Si}$ fragments arising from the symmetric fission of the ${}^{56}\mathrm{Ni}$ compound nucleus. The enhanced population of the ${K}^{\ensuremath{\pi}}{=3}_{1}^{\ensuremath{-}}$ band of the ${}^{28}\mathrm{Si}$ nucleus, indicative of an oblate deformed shape, suggests that the oblate configuration plays a significant role in the resonant process. Fragment angular distributions for the elastic and low-lying inelastic channels as well as $\ensuremath{\gamma}$-ray angular correlations for the mutual inelastic channel ${(2}^{+}{,2}^{+})$ indicate that the spin orientations of the outgoing fragments are perpendicular to the orbital angular momentum. This unexpected result, which is different from the alignment found for the resonance structures in the ${}^{24}\mathrm{Mg}{+}^{24}\mathrm{Mg}$ and ${}^{12}\mathrm{C}\mathrm{}{+}^{12}\mathrm{C}$ systems, suggests a situation where two oblate ${}^{28}\mathrm{Si}$ nuclei interact in an equator-to-equator stable molecular configuration. A discussion concerning the spin alignment and spin disalignment for different reactions such as ${}^{12}\mathrm{C}\mathrm{}{+}^{12}\mathrm{C},$ ${}^{24}\mathrm{Mg}{+}^{24}\mathrm{Mg},$ and ${}^{28}\mathrm{Si}{+}^{28}\mathrm{Si}$ is presented.

Selective population of states in fission fragments from the S32+24Mg reaction

Selective population of states in fission fragments from the S32+24Mg reaction

Cluster model theory of the Lambda 9Be hypernucleus.

The microscopic (\ensuremath{\alpha}+3N+N)+\ensuremath{\Lambda} cluster model is applied to achieve a unified understanding of the $_{\mathrm{\ensuremath{\Lambda}}}^{9}\mathrm{Be}$ hypernucleus in the energy region up to ${E}_{\ensuremath{\Lambda}}$\ensuremath{\simeq}20 MeV. The eigenstates of $_{\mathrm{\ensuremath{\Lambda}}}^{9}\mathrm{Be}$ consist of low-lying cluster states with $^{8}\mathrm{Be}$ (\ensuremath{\alpha}+\ensuremath{\alpha})+\ensuremath{\Lambda} structure and high-lying core-excited states with $^{8}\mathrm{*}$ (\ensuremath{\alpha}+\ensuremath{\alpha})+\ensuremath{\Lambda} structure (* denotes the intrinsic) excited state). With use of the $_{\mathrm{\ensuremath{\Lambda}}}^{9}\mathrm{Be}$ wave functions obtained, the production cross sections of $_{\mathrm{\ensuremath{\Lambda}}}^{9}\mathrm{Be}$ through the (${K}^{\mathrm{\ensuremath{-}}}$ in-flight,${\ensuremath{\pi}}^{\mathrm{\ensuremath{-}}}$), (${\ensuremath{\pi}}^{+}$,${K}^{+}$), and (${K}^{\mathrm{\ensuremath{-}}}$ stopped,${\ensuremath{\pi}}^{\mathrm{\ensuremath{-}}}$) reactions are evaluated. Each reaction process shows its own selective population of states and hence contributes to the comprehensive test of the theoretical predictions. A quantitative understanding is obtained for the three-peak structure observed in the (${K}^{\mathrm{\ensuremath{-}}}$ in-flight,${\ensuremath{\pi}}^{\mathrm{\ensuremath{-}}}$) reaction experiment. The predicted (${\ensuremath{\pi}}^{+}$,${K}^{+}$) excitation function is in good agreement with recent experimental data, and for the first time genuine hypernuclear states came into observation.

3He cluster structure of 12C.

The reaction $^{9}\mathrm{Be}$${(}^{6}$Li,t${)}^{12}$C was studied at 32 MeV. Selective population of states was observed up to the 14.1 MeV state in $^{12}\mathrm{C}$. The high probability for three or more body processes resulted in a large continuum yield which prevented the observation of the population of states in the 17--22 MeV excitation region of $^{12}\mathrm{C}$. Hauser-Feshbach calculations show that the statistical compound nucleus contribution to the cross section is small. The experimental angular distributions are well reproduced by distorted-wave Born approximation calculations. The extracted relative spectroscopic strengths are in good agreement with theoretical values. The T=1 15.1 and 16.1 MeV states were not observed. A measurement of the $^{9}\mathrm{Be}$${(}^{6}$Li${,}^{3}$He${)}^{12}$B cross sections to their analog states shows that these states would have a peak height of only 10% of the continuum yield, making it difficult to determine if these states are populated in the present reaction.

Charge exchange reactions (13C,13N) and (13C,13B) on12C

The properties of charge exchange reactions induced by13C projectiles of 30 MeV/u are discussed. Spectra on12C as target have been measured and are used to illustrate the properties of the (13C,13N) and (13C,13B) reactions. Both reactions show selective population of states, the (13C,13B) reactions favours the spin-flip (Δs=1) transitions.The Giant Dipole Resonance is strongly excited in both reactions.

Transfer reaction studies to states in 14N and 14C

States in 14N and 14C have been populated by the Li-induced reactions 10B( 6Li, d), 11B( 6Li, t/ 3He), 11B( 7Li, α), 12C( 6Li, α), and 13C( 7Li, 6He/ 6Li) at E Li = 32 and 34 MeV. Selective population of states is observed with these reactions. Angular distributions were measured for the 11B( 6Li t/ 3He) analog reactions. Experimental criteria are developed to allow the identification of analog states up to 18.40 and 16.45 MeV excitation in 14N and 14C, respectively.

Selective population of states inAr36by theC12(Si28, α)Ar36reaction

Energy spectra of $\ensuremath{\alpha}$-particles were measured at 0\ifmmode^\circ\else\textdegree\fi{} for the systems $^{28}\mathrm{Si}$+$^{12,13}\mathrm{C}$ in the range ${E}_{\mathrm{lab}}(^{28}\mathrm{Si})=75\ensuremath{-}102$ MeV in 0.750 MeV steps. Transitions to discrete states in $^{36}\mathrm{Ar}$ up to \ensuremath{\sim}20 MeV excitation energy via the $^{12}\mathrm{C}(^{28}\mathrm{Si}, \ensuremath{\alpha})^{36}\mathrm{Ar}$ reaction are observed. The excitation functions measured for some of these transitions show structures \ensuremath{\sim}1 MeV wide. No strong ($^{28}\mathrm{Si}$, $\ensuremath{\alpha}$) transitions to discrete states are observed for the $^{28}\mathrm{Si}$+$^{13}\mathrm{C}$ system.NUCLEAR REACTIONS $^{12,13}\mathrm{C}(^{28}\mathrm{Si}, \ensuremath{\alpha})$, $E=75\ensuremath{-}102$ MeV, measured $\ensuremath{\sigma}(E)$, $^{36}\mathrm{Ar}$ deduced levels.

The 12C( 9Be, n) 20Ne reaction

A study of the 12C( 9Be, n) 20Ne reaction has been carried out at bombarding energies of 16 and 24 MeV. The spectra at both incident energies are dominated by a consistent set of levels between an excitation energy of 7.3 ± 0.4 and 15.7 ± 0.3 MeV. The rotational band based on the K π = 0 3 + state appears to be strongly populated. Based on this selectivity, additional evidence is provided in favor of identification of the 8 + state at 15.9 MeV with this 0 3 + band. Angular distributions measured at the higher bombarding energy are compared with statistical compound-nuclear calculations. It appears that a non-statistical mechanism is responsible for the reaction's selective population of states with 8p-4h configuration. Such a mechanism, involving the preferential breakup of the 9Be into 8Be plus a neutron, is discussed.

Selective population of three-particle states at high excitation energies in 15N

Selective population of states in 15N has been observed up to 24.5 MeV in excitation in 15N with the 12C( 7Li, α) reaction at E( 7 Li) = 48 MeV . Strong states are observed at 17.95, 18.70, 19.68, 20.93 and 24.75 MeV. The width of the 18.70 MeV state is 100 keV or less even though it is 4 MeV above the 12C + t breakup threshold. Comparison between the 12C( 7Li, α) and previously reported 12C(α, p) data shows only the 17.95 MeV state to be strongly excited in (α, p).