Session B13: Nuclear Reactions: Heavy Ions/Rare Isotope Beams

Study of light tin isotopes via single-nucleon knockout reactions

Spectroscopic studies have been performed close to $^{100}$Sn, utilizing the S800 and CAESAR at the NSCL. These studies make use of a single neutron knockout reaction on $^{108}$Sn and $^{106}$Sn beams. The momentum distributions of the resulting residues indicate the $\ell$-value of the removed neutron and the spectroscopic factors for the even-mass nuclei. Additionally, $\gamma$-rays were measured in coincidence with the momentum distributions allowing the separation of the knockout channel where the residue is left in an excited state from the channel to the ground state. The odd-mass residue can then be characterized in terms of a hole in the d- or g- orbital with reference to the even-mass nucleus. The current status of the analysis will be presented.   

First Lifetime Results from a Systematic Study of Odd-Mass Neutron-Rich Nuclides Near A$\sim$110

Our understanding of the astrophysical r-process nucleosynthesis path relies strongly on input from nuclear models. The mass region surrounding A$\sim$110 is known to exhibit rapid structural evolution, however experimental data is lacking for nuclides away from stability making the prediction of important nuclear properties in the r-process region difficult. In this talk we report new pico-second lifetimes of excited states populated via beta decay of fission fragments produced at IGISOL in Jyv\"{a}skyl\"{a}, Finland. The measurements utilized triple coincidences ($\beta\gamma\gamma$) where two fast scinitillators (plastic for $\beta$ and LaBr$_3$ for $\gamma$) were used in conjunction with a HPGe detector. Details of the experimental technique will be highlighted along with the presentation of eight new lifetimes in $^{109}$Pd, which range between 10-1500 ps, representing the first results from a broad experimental campaign.   

Production of Nuclides Near the $N =$ 126 Shell Using Projectiles with $Z$ \textgreater\ 20

Recent experiments at Texas A{\&}M University have shown that the cross sections for fusion-evaporation products in the reactions of $^{48}$Ca, $^{45}$Sc, $^{50}$Ti, and $^{54}$Cr with lanthanide targets are heavily influenced by the atomic number of the projectile. In particular, the cross sections for the 4n evaporation channel in the $^{48}$Ca-induced reactions are substantially larger than for the other projectiles. The compound nuclei in these reactions are near the $N =$ 126 shell, and the sharp decrease in cross section is attributed to an enhancement of the fission level density, along with a smaller change in the probability of compound nucleus formation. Through careful correction for the neutron binding energy, fission barrier, and excitation energy available for neutron emission, it may be possible to use these results to make inferences on the likelihood of future superheavy element synthesis. This talk will discuss recent results and the difficulty of synthesizing evaporation residues using projectiles with $Z$ \textgreater\ 20.   

A Signature Distinguishing Fissile From Non-Fissile Materials Using Linearly Polarized Gamma Rays

Photofission of $^{233,235,238}$U, $^{239,240}$Pu, and $^{232}$Th was induced by nearly 100\% linearly polarized, high intensity ($\sim$10$^{7}$ $\gamma$s per second), and nearly-monoenergetic $\gamma$-ray beams of energies between 5.6 and 7.3 MeV at the High Intensity $\gamma$-ray Source (HI$\gamma$S). An array of 18 liquid scintillating detectors was used to measure prompt fission neutron angular distributions. The ratio of prompt fission neutron yields parallel to the plane of beam polarization to the yields perpendicular to this plane was measured as a function of beam and neutron energy, as described in a recent publication showing results from $^{235,238}$U, $^{239}$Pu, and $^{232}$Th [1]. A ratio near unity was found for $^{233,235}$U and $^{239}$Pu while a significant ratio ($\sim$1.5-3) was found for $^{238}$U, $^{240}$Pu, and $^{232}$Th. This large difference could be used to distinguish fissile isotopes (such as $^{233,235}$U and $^{239}$Pu) from non-fissile isotopes (such as $^{238}$U, $^{240}$Pu, and $^{232}$Th). Polarization ratios as a function of the relative abundance of fissile to non-fissile isotopes will be presented. \\[4pt] [1] J. M. Mueller $\emph{et al.}$, Phys. Rev. C 85, 014605 (2012).   

A Measurement of Cross Sections for Neutron-Induced Transitions in Germanium and $^{136}$Xe

We report on a measurement of cross sections for neutron-induced transitions on germanium, using one target with natural isotopic abundances and another enriched in $^{76}$Ge, and $^{136}$Xe. These reactions were investigated to determine neutron-induced contributions to background contamination in the regions of interest for searches of neutrinoless double beta decay of $^{76}$Ge and $^{136}$Xe; an accurate understanding of this background is a requisite for any compelling result. These measurements were undertaken at Triangle Universities Nuclear Laboratory (TUNL) using a neutron beam with 8 MeV energy generated with the $^{2}$H(d,n)$^{3}$He source reaction.   

Neutron-capture cross-section measurements of $^{40}$Ar and $^{136}$Xe in the energy region 0.7-14.8 MeV

Cross-section measurements for the reaction $^{40}$Ar(n,$\gamma $)$^{41}$Ar have been carried out with the activation method in the neutron energy region 0.7-14.8 MeV. These results are important to identify backgrounds in liquid argon based neutrino detectors and in the neutrinoless double-$\beta $ decay experiment GERDA, which uses argon as cooling and shielding medium. A high-pressure gas cell of $^{40}$Ar was irradiated with monoenergetic neutrons produced either by the $^{3}$H(p,n)$^3$He, $^2$H(d,n)$^3$He or $^3$H(d,n)$^4$He reactions. Indium and Au monitor foils were irradiated simultaneously to determine the incident neutron flux. The activities induced by the reaction products were measured using high-resolution $\gamma $-ray spctroscopy. The data are compared with the available data evaluations. Measurements of the $^{136}$Xe(n,$\gamma $)$^{137}$Xe cross section are underway and preliminary results will be presented as well. The latter data are important for the EXO and KamLAND-Zen neutrinoless double-$\beta $ decay searches.   

Double-$\beta$ decay nuclear structure via electron capture on $^{116}$In

The small electron-capture decay branch of $^{116}$In has been measured using Penning trap assisted decay spectroscopy. The deduced Gamow-Teller transition strength helps to resolve longstanding differences between scattered charge-exchange reaction values and a previous electron-capture decay value that was less statistically significant than the present one. We argue that this transition can now be used as a reliable benchmark for nuclear-structure calculations of the matrix element for the neutrinoless double-$\beta$ decay of $^{116}$Cd and other nuclides.