Session DB: Mini-Symposium on Direct Reactions as a Probe of Nuclear Structure and Nuclear Astrophysics-New Techniques and Instrumentation II

Test of the combined method for extracting spectroscopic factors in N$=$50 nuclei

The single-particle properties of nuclei near shell closures and r-process waiting points can be observed using single-nucleon transfer reactions with beams of rare isotopes. However, approximations have to be made about the final bound state to extract spectroscopic information. An approach to constrain the bound state potential has been proposed by Mukhamedzhanov and Nunes [1]. At peripheral reaction energies (\textasciitilde 5 MeV/u), the ANC for the nucleus can be extracted, and is combined with the same reaction at higher energies (\textasciitilde 40 MeV/u). These combined measurements can constrain the shape of the bound state potential, and the spectroscopic factor can be reliably extracted. To test this method, the $^{\mathrm{86}}$Kr($d,p)$ reaction was performed in inverse kinematics with a 35 MeV/u beam at the National Superconducting Cyclotron Laboratory (NSCL) with the ORRUBA and SIDAR arrays of silicon strip detectors coupled to the S800 spectrometer. Successful results supported the measurement of a radioactive ion beam of $^{\mathrm{84}}$Se at 45 MeV/u at the NSCL to be measured at the end of 2017. Results from the $^{\mathrm{86}}$Kr($d,p)$ measurement will be presented as well as preparations for the upcoming $^{\mathrm{84}}$Se($d,p)$ measurement. [1] A.M. Mukhamedzhanov and F.M. Nunes, Phys. Rev. C 72, 017602 (2005)   

Elastic and inelastic scattering of $^{134}$Xe beams on C$_{2}$D$_{4}$ targets measured with GODDESS

The GODDESS (Gammasphere-ORRUBA: Dual Detector for Experimental Structure Studies) coupling of the ORRUBA charged-particle array with Gammasphere is designed to enable high-resolution particle-gamma measurements in inverse kinematics with radioactive beams. The high resolution and coverage of GODDESS allows for multiple reaction channels to be studied simultaneously. For the stable-beam commissioning of GODDESS, the $^{134}$Xe(d,p$\gamma$)$^{135}$Xe reaction was measured using a beam of $^{134}$Xe at 8 MeV/A, delivered by the ATLAS facility at Argonne National Laboratory. The beam impinged on an 800 $\mu$g/cm$^{2}$ C$_{2}$D$_{4}$ target, and charged particles were detected in the GODDESS silicon array between 15 and 165 degrees. Coincident gamma rays were measured with Gammasphere, with 10$\%$ efficiency at 1.3 MeV. In the detectors downstream of the target, elastically- and inelastically-scattered target ions (deuterium and carbon) were detected, populating the ground and low-lying excited states in $^{134}$Xe. An overview of GODDESS will be presented, along with the analysis of the downstream data, including the differential scattering cross sections and population of collective states in $^{134}$Xe.   

Developing the (d,p$\gamma )$ reaction as a surrogate for (n,$\gamma )$ in inverse kinematics

The r-process that proceeds via (n,$\gamma )$ reactions on neutron-rich nuclei is responsible for the synthesis of about half of the elements heavier than iron. Because (n,$\gamma )$ measurements on short-lived isotopes are not possible, the (d,p$\gamma )$ reaction is being investigated as a surrogate for (n,$\gamma )$. The experimental setup GODDESS (Gammasphere ORRUBA: Dual Detectors for Experimental Structure Studies) has been developed especially for this purpose. The Oak Ridge Rutgers University Barrel Array (ORRUBA) of position-sensitive silicon strip detectors was augmented with annular arrays of segmented strip detectors at backward and forward angles, resulting in a high-angular coverage for light ejectiles (20 to 160 degrees in the laboratory frame). The $^{\mathrm{134}}$Xe(d,p$\gamma )$ reaction, used to commission the setup, was measured in inverse kinematics with stable beams from ATLAS impinged on C$_{\mathrm{2}}$D$_{\mathrm{4}}$ targets. Reaction protons were measured (ORRUBA) in coincidence with gamma rays (Gammasphere). An overview of GODDESS and preliminary results from the $^{\mathrm{134}}$Xe(d,p$\gamma )$ study will be presented.   

Low-energy nuclear astrophysics studies at the Multicharged Ion Research Facility

As low-energy nuclear astrophysics progresses toward measuring reaction cross sections in the stellar burning regimes, a worldwide effort is underway to continue these measurements at underground laboratories to achieve the requisite ultra-low-background environment.~ These facilities are crucial for providing the required low-background environments to perform such measurements of astrophysical importance.~ While advances have been made in the use of accelerators underground, of equal importance is the detectors, high-current targets, and techniques required to perform such measurements.~ With these goals in mind, a newly established astrophysics beamline has been built at the Multicharged Ion Research Facility (MIRF) located at Oak Ridge National Laboratory. The unique capabilities of MIRF will be demonstrated through two recent low-energy above-ground measurements of the~dominant~s-process neutron source~$^{\mathrm{13}}$C($\alpha $,n)$^{\mathrm{16}}$O and associated beam-induced background source~$^{\mathrm{13}}$C(d,n)$^{\mathrm{14}}$N.~ This material is based upon work supported by the U.S. DOE, Office of Science, Office of Nuclear Physics. Research sponsored by the LDRD Program of ORNL, managed by UT-Battelle, LLC, for the U.S. DOE.   

Constraint of the $^{13}$C($\alpha$,n) Cross Section Toward Astrophysical Energies for the Main s-Process

The slow neutron capture process (s-process) typically occurs in relatively low neutron flux environments, such as AGB stars, and is a key mechanism in heavy-element synthesis. The dominant source of neutrons for the main s-process is the $^{13}$C($\alpha$,n) reaction, which proceeds at stellar temperatures ($\sim 0.1$GK, 200 keV), via reactions well below the Coulomb barrier. Direct measurements of the reaction rate in the Gamow window ($\sim 140-230$ keV) is difficult, complicated by the low yields and high beam currents required. Current measurements have constrained the cross section down to approximately 320 keV - still well above stellar conditions- with significant statistical uncertainties. These uncertainties, and the influence of a near-threshold $1/2^+$ state at 6.4 MeV, means that extrapolation of the data into the Gamow window is unreliable. These measurements typically use high-efficiency moderated neutron counter detectors, meaning energy information of the incident neutrons is lost. A quasi-spectroscopic approach has been used to measure the $^{13}$C($\alpha$,n) reaction rate at energies between 300-350 keV with the aim of reducing uncertainties in current measurements.   

Astrophysics Unearthed: Measuring the Beam-Induced $^{\mathrm{13}}$C(d,n) Background in Underground Nuclear Astrophysics Experiments

The slow neutron capture process (s-process) is a series of nuclear reactions responsible for the synthesis of approximately half the atomic nuclei heavier than iron. The s-process proceeds via a series of neutron capture and beta decay reactions in the low neutron flux environment of AGB stars. The primary source of neutrons for the s-process is the $^{\mathrm{13}}$C($\alpha $,n)$^{\mathrm{16}}$O reaction. Extrapolation of the $^{\mathrm{13}}$C($\alpha $,n) S-factor into the Gamow window is complicated by the unknown influence of a $\frac{1}{2}^{+}$ resonance in $^{\mathrm{17}}$O near the alpha capture threshold, prompting an international effort to directly measure the cross section at low energies. Measurement of the $^{\mathrm{13}}$C($\alpha $,n) cross section is made difficult by beam-induced background, such as deuterium contamination in the alpha particle beams of most accelerators. At astrophysically relevant energies, the $^{\mathrm{13}}$C(d,n) cross section is many orders of magnitude higher than that of $^{\mathrm{13}}$C($\alpha $,n). Accordingly, the $^{\mathrm{13}}$C(d,n) cross section was measured at laboratory energies below 250 keV (corresponding to alpha beam energies of 500 keV and below) at Oak Ridge National Laboratory's Multicharged Ion Research Facility. Preliminary results and the implications of this work are discussed.   

Understanding the Limits of the Statistical Model: Indirect Probes of 96Zr(n,gamma)

A major barrier in the study of neutron-induced nuclear reactions is the impossibility of direct measurements with short-lived radioactive isotopes. For these exotic nuclei, theoretical inputs such as the Photon Strength Function (PSF) are poorly constrained. Recently, a program to investigate the PSF for medium-mass nuclei has begun as a collaboration between Los Alamos National Laboratory (LANL) and Argonne National Laboratory (ANL). At LANL, The Detector for Advanced Neutron Capture Experiments (DANCE) provides direct measurements of gamma ray cascades following neutron capture reactions on stable or long-lived radioactive nuclei. At ANL, single neutron transfer reactions in inverse kinematics provides complementary data on short-lived radioactive nuclei. As a test case for this research program, the 96Zr(n,$\gamma )$ reaction was measured using DANCE and the 96Zr(d,p) reaction was measured using HELIOS$+$APOLLO. Results from the 96Zr(d,p) and 96Zr(n,$\gamma )$ measurements will presented.   

Heavy-ION Induced Transfer Reactions On 130SN

Nuclear data in the region of the doubly-magic nucleus 132SN is of particular interest for R-process nucleosynthesis as well as benchmarking nuclear structure models. The j-dependence and selectivity of heavy-ion induced transfer reactions to the bound-state~wave function~of a target nucleus can be used to deduce the spin and parity of the directly populated states. Results from 130SN(9BE, 8BE \textsc{}$\gamma $\textsc{) and 130SN(13C, 12C}\textsc{}$\gamma $) single-neutron transfer reactions will be presented. By comparing the relative cross sections of the populated states from two different targets, spin-parity assignments were confirmed. In addition, limits on the lifetimes of some of the populated states have been inferred through the doppler shift.