Session C11: Nuclear Structure and Reactions I

Hypernuclei production experiment E05-115 at Jefferson Laboratory HallC by (e, e'K$^{+}$) reaction

The third-generation (\emph{e,e'K$^{+}$}) hypernuclear experiment E05115 at JLAB HallC was carried out in 2009. The goal of the experiment is to obtain high precision spectroscopy for medium-heavy hypernuclei $^{52}_\Lambda$Cr as well as light p shell hypernuclei $^{12}_\Lambda$B, $^{7}_\Lambda$He, $^{9}_\Lambda$B and $^{10}_\Lambda$Be. The application of brand new electron spectrometer and higher energy beam makes the ``tilt method'' more effective than last JLAB HallC hypernuclear experiment which was taken in 2005. The system optical calibration is being studied in detail, since there is a fringe field cross talk between the magnets of the spectrometers. The updated optics status and preliminary spectrum will be presented in this talk.   

The direct measurement of the lifetime of the heavy $\Lambda$ hypernuclei

The experiment E02-017 for direct measurement of the lifetime of heavy $\Lambda$ hypernuclei was carried out at Jefferson Lab Hall C in 2009. Fe, Cu, Ag, Au, Bi and U were employed as the target materials.The delayed fission from heavy hypernuclei produced by photon-nuclei reaction has been measured in this experiment.The only and recent COSY-13 result on the lifetime of heavy hypernuclei in the A range of 180-230 claimed to have the best accuracy but appeared quite controversial to the conclusion obtained from the studies of light hypernuclei. This work will provide observable lifetime in the high A range as an independant measurement to help further understanding on this issue. We have extracted the timing spectrum, the fitting strategy is under investigation. The data analysis status will be presented and the prelimilary result is expected in the near future which will either confirm or challenge the COSY-13 result, either way has significant impact in helping to fully understand the $\Lambda$N$\rightarrow$NN weak interaction and non-mesonic decay.   

Calculations of reaction and interaction cross sections for $^{11}$Li scattering on carbon target

In the high energy light ion scattering experiments interaction (not reaction) cross-sections are measured. The difference between interaction and reaction cross-sections is that the reaction cross-sections include the cross-sections of all processes except of the elastic scattering, whereas the interaction cross-sections do not include the elastic scattering as well as the processes with a target nuclei excitation or disintegration. Usually this difference is neglected in the analyses of experimental data. We present the results of calculation of the difference between interaction and reaction cross sections for scattering stable isotopes with atomic weight $A<40$ and unstable $^{11}$Li isotope on carbon target. Calculations were done in optical approximation and in the Glauber Theory framework, in which all processes were included. We show that this difference is significant and cannot be neglected in further analysis.   

Studies of the $\mathrm{^{19}O(d,p)^{20}O}$ reaction at {\sc resolut}

The energetic location of the d$_{3/2}$--orbital in neutron-rich nuclei is of particular interest as it determines the location of the drip-line in the oxygen isotopes. Its behaviour has recently been discussed as a consequence of three-body forces\footnote{T. Otsuka et al., Phys. Rev. Lett 105, 032501 (2010)}. Manifestations of such forces are traced through the location of the d$_{3/2}$ orbital, which closer to stability leads to highly excited states. In order to study the location and fragmentation of this orbital in $^{20}$O, we performed an experiment at the {\sc resolut} radioactive beam facility of the Florida State University accelerator laboratory. We produced a beam of the short-lived $^{19}$O isotope with an intensity of $1 \times 10^5$ pps, 65$\%$ purity and 4.11 MeV/u. This beam was used to study the spectroscopic factors of bound and unbound states of $^{20}$O using the $(d,p)$ reaction in inverse kinematics and components of the new {\sc anasen} detector array. We will present the methods used and the first results of this experiment regarding the spectroscopic factors of unbound d$_{3/2}$ resonances in $^{20}$O.   

Gamma ray spectroscopy and shell model calculations reveal the structure of $^{25}$Na

$^{25}$Na was produced in a $^{9}$Be ($^{18}$O, pn) reaction at a beam energy of 35 MeV. Gammas were detected using the FSU compton-supressed germanium array in coincidence with protons from the reaction. New states and transitions were observed. The level scheme has been compared with shell model calculations using the COSMO code with the USDA and the WBP with cross-shell interactions.   

Gamma ray spectroscopy of $^{31}$Si

$^{31}$Si was produced through the $^{18}$O ($^{18}$O, $\alpha$n) reaction at the beam energy of 24 MeV, which preferentially populates the high spin states. The $\alpha$ particles were detected in Microball and the multiple $\gamma$-ray coincidences were detected by Gammashpere. There are 11 newly observed states and 22 new discovered $\gamma$ transitions. A strong competition is seen between negative-parity {}``intruder'' states and positive-parity pure s-d states. Shell model calculations agree relatively well with both groups of states.   

Core-coupled protons, $f_{7/2}$ intruder states, and competing $g_{9/2}$ proton and neutron structures in $^{65,67}$Cu

The nuclei $^{65,67}$Cu were studied in reactions between a 430-MeV $^{64}$Ni beam and a thick $^{238}$U target with the Gammasphere array. Decay schemes for both nuclei have been extended, with spin and parity assignments constrained by gamma-ray angular distributions and correlations. Positive-parity structures, based on $p_{3/2}$ protons coupled to negative-parity Ni core states, have been identified above the known $9/2^+$ states. In $^{67}$Cu, a negative-parity dipole band built upon a $\pi f_{7/2}^{-1}$ state has been observed, as were two shorter negative-parity sequences. A qualitative description of the level structures has been obtained through comparison with odd-$A$ Cu systematics and neighboring even-even Ni and Zn cores. Shell-model calculations using JUN45 and jj44b effective interactions were performed for $^{65,67}$Cu; the successes and limitations of the calculations will be discussed.   

Negative $g$ factor for the 4$^+_1$ state in $^{86}$Sr: a test of the shell model near $N$ = 50

Lifetimes and $g$ factors of several low-lying states have been measured by Coulomb excitation, in inverse kinematics. The $_{38}^{86}$Sr$_{48}$ nucleus lies near the $N$ = 50 shell closure. $^{86}$Sr is a particularly interesting case because $^{88}$Sr is often taken as the core nucleus in shell model calculations in this region. Noteworthy results of the experiment include the negative $g$ factor for the 4$^+_1$ state of $^{86}$Sr, in contrast to the positive $g$ factor of the 2$^+_1$ state, and similarities between the two isotones $^{86}$Sr and $^{88}$Zr. Large scale shell model calculations using the JJ4B and JUN45 interactions have been carried out. The results will be presented and compared to the data from the $^{84-102}$Zr and $^{84,88}$Sr isotopes.   

Induced depletion of $^{108m}$Ag with 6 MeV bremsstrahlung

The nuclide $^{108}$Ag possesses an interesting combination of a long-lived isomer (T$_{1/2 }$= 418 years, I$^{\pi }$ = 6$^{+}$, E = 109 keV) and a short-lived ground state (T$_{1/2}$ = 2.37 minutes, I$^{\pi }$ = 1$^{+})$. The ground state decays primarily by $\beta ^{-}$ emission with Q$_{\beta -}$ = 1,649 keV. A search of the available nuclear data (e. g., ENSDF and Phys. Rev. C 52, 104 (1995)) suggests two possible transitions at energies below 500 keV from the isomer to higher-lying levels, whose subsequent decay can branch to the ground state. This process would lead to a partial depletion of any population trapped within the isomeric state, $^{108m}$Ag. Currently, the cross section for induced isomer depletion via these transitions cannot be accurately deduced due to unknown branching ratios, and level widths and spins. Other ``depletion'' levels requiring excitation $>$ 500 keV are also likely. An experimental test of $^{108m}$Ag depletion has been performed using 6 MeV bremsstrahlung at the US Army Research Laboratory, with isomeric targets and a computer-controlled repetitive measurement system. The design of the system and experimental results will be discussed.