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Register a new userTwo-neutron transfer reaction mechanisms in $^{12}$C($^6$He,$^{4}$He)$^{14}$C using a realistic three-body $^{6}$He model
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The reaction mechanisms of the two-neutron transfer reaction $^{12}$C($^6$He,$^4$He) have been studied at 30 MeV at the TRIUMF ISAC-II facility using the SHARC charged-particle detector array. Optical potential parameters have been extracted from the analysis of the elastic scattering angular distribution. The new potential has been applied to the study of the transfer angular distribution to the 2$^+_2$ 8.32 MeV state in $^{14}$C, using a realistic 3-body $^6$He model and advanced shell model calculations for the carbon structure, allowing to calculate the relative contributions of the simultaneous and sequential two-neutron transfer. The reaction model provides a good description of the 30 MeV data set and shows that the simultaneous process is the dominant transfer mechanism. Sensitivity tests of optical potential parameters show that the final results can be considerably affected by the choice of optical potentials. A reanalysis of data measured previously at 18 MeV however, is not as well described by the same reaction model, suggesting that one needs to include higher order effects in the reaction mechanism.
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Measurements of the t-t and p-t coincidence events in the $^3$H ($alpha$, ttp) reaction have been obtained at $E_alpha$ incident energy of 67.2 MeV. Various appropriate angular configurations of detectors were chosen in order to observe the population of the $^6$He$^*$ state at around 18 MeV. Its contribution appears at the $E_{rm tt}$ relative energy of 6.0 MeV by the analysis of bidimensional spectra. We found the formation of the $^6$He excited state at $E^* = 18.3 pm 0.2$ MeV (with a $Gamma$ width of 1.1 $pm$ 0.3 MeV) by the decay into the t+t binary channel, since the threshold energy of the t+t channel is 12.31 MeV. In each analyzed bidimensional energy spectrum of ($E_{rm t}$, $E_{rm t}$) and ($E_{rm p}$, $E_{rm t}$) coincidence events resonance structures are present due to the formation of both $^6$He$^*$ and $^4$He$^*$ excited states. Our results on the $E^*$ and $Gamma$ values regarding the $^6$He$^*$ level of about 18 MeV are compared with the results obtained by other reactions. Moreover, we also found new $Gamma$ width values of 0.7 $pm$ 0.3 and 0.8 $pm$ 0.4 MeV for the 14.0 $pm$ 0.4 and 16.1 $pm$ 0.4 MeV $^6$He levels, respectively.
The low-energy behavior of the strength function for the $1^-$ soft dipole excitation in $^{6}$He is studied theoretically. Use of very large basis sizes and well-grounded extrapolation procedures allows to move to energies as small as 1 keV, at which the low-energy asymptotic behavior of the E1 strength function seems to be achieved. It is found that the low-energy behavior of the strength function is well described in the effective three-body dynamical dineutron model. The astrophysical rate for the $alpha$+$n$+$n rightarrow ^6$He+$gamma$ is calculated. Comparison with the previous calculations is performed.
$^{6}$He+$t$ cluster states of exited $^{9}$Li have been measured by 32.7 MeV/nucleon $^{9}$Li beams bombarding on $^{208}$Pb target. Two resonant states are clearly observed with the excitation energies at 9.8 MeV and 12.6 MeV and spin-parity of 3/2$^{-}$ and 7/2$^{-}$ respectively. These two states are considered to be members of K$^{pi}$=1/2$^{-}$ band. The spin-parity of them are identified by the method of angular correlation analysis and verified by the continuum discretized coupled channels (CDCC) calculation, which agrees with the prediction of the generator coordinate method (GCM). A monopole matrix element about 4 fm$^{2}$ for the 3/2$^{-}$ state is extracted from the distorted wave Born approximation (DWBA) calculation. These results strongly support the feature of clustering structure of two neutron-rich clusters in the neutron-rich nucleus $^{9}$Li for the first time.
Two new low-lying $^6$He levels at excitation energies of about 2.4 and 2.9 MeV were observed in the experimental investigation of the p-$alpha$ coincidence spectra obtained by the $^3$H($^4$He,p$alpha$)2n four-body reaction at $E_{rm ,^4He}$ beam energy of 27.2 MeV. The relevant $E^*$ peak energy and $Gamma$ energy width spectroscopic parameters for such $^6$He$^*$ excited states decaying into the $alpha$+n+n channel were obtained by analyzing the bidimensional ($E_{rm p}$, $E_{rm alpha}$) energy spectra. The present new result of two low-lying $^6$He$^*$ excited states above the $^4$He+2n threshold energy of 0.974 MeV is important for the investigation of the nuclear structure of neutron rich light nuclei and also as a basic test for theoretical models in the study of the three-cluster resonance feature of $^6$He.
The space and time configurations of the dissociation of $^8$He into $^6$He+$n$+$n$, on C and Pb targets, have been explored simultaneously for the first time. The final-state interactions in the $n$-$n$ and $^6$He-$n$ channels are successfully described within a model that considers independent emission of neutrons from a Gaussian volume with a given lifetime. The dissociation on C target exhibits a dominant sequential decay through the ground state of $^7$He, consistent with neutrons being emitted from a Gaussian volume of $r_{nn}^{rm{rms}}=7.3pm0.6$~fm with a $n$-$n$ delay in the sequential channel of $1400pm400$~fm/$c$, in agreement with the lifetime of $^7$He. The lower-statistics data on Pb target correspond mainly to direct breakup, and are well described using the $n$-$n$ volume measured, without any $n$-$n$ delay. The validity of the phenomenological model used is discussed.
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