Session X4: Nuclear Reactions: Light and Heavy Ions

Transverse Isotropy: Disappearance of Mott oscillations in sub-barrier elastic scattering of identical heavy ions and the nuclear ineraction

It is found that at a certain critical value of the Sommerfeld parameter the Mott oscillations usually present in the scattering of identical heavy ions, disappear and the cross section becomes quite flat. We call this effect Transverse Isotropy (TI) (L. F. Canto, R. Donangelo and M. S. Hussein, Mod. Phys. Lett. A, \textbf{16}), 1027 (2001).. The critical value of the Sommerfeld parameter at which TI sets in is found to be $\eta_{c} = \sqrt{3s +2}$, where $s$ is the spin of the nuclei participating in the scattering. No TI is found in the Mott scattering of identical Fermionic nuclei. The critical center of mass energy corresponding to $\eta_c$ is found to be $E_c$ = 0.40 MeV for $\alpha + \alpha$ (s = 0), and 1.2 MeV for $^{6}$Li + $^{6}$LI (s = 1). We further found that the inclusion of the nuclear interaction induces a significant modification in the TI (L. F. Canto, M. S. Hussein and W. Mittig, Phys. Rev. C, \textbf{89} 024610 (2014)). This can be verified by calculating the second derivative of the cross section at $\theta = 90^{\circ}$. We suggest measurements at these sub-barrier energies for the purpose of extracting useful information about the nuclear interaction between light heavy ions.   

The Breakup Cross Section of the D+D Reaction at 6.94 MeV

The D+D reactions are well known and widely used for a variety of purposes, mainly due to the mono-energetic neutrons from the D$(d,n)^3$He reaction. The least studied of the D+D reactions is the D$(d,np)$D reaction known as the deuteron breakup reaction. The D$(d,np)$D reaction produces a continuum of neutrons at energies lower than that of the mono-energetic peak. In this work, the D\emph{(d,np)}D reaction has been studied for the purpose of use as a neutron source for the active interrogation of hidden fissile materials. The neutron energy distribution as a function of angle for the cross section, $\frac{d^{2}\sigma}{d\Omega dE}$, of the D\emph{(d,np)}D reaction has been measured at the Edwards Accelerator Laboratory of Ohio University, using a 6.94-MeV pulsed deuteron beam incident upon a D$_2$ gas target. The time-of-flight technique was used to determine the energy of the neutrons detected in the array of two lithium glass scintillators and one NE-213 scintillator. The breakup cross section was determined as low as 225-keV neutron energy in the lithium glass detectors.   

Measuring the Fusion Cross-Section of $^{18,19}$O + $^{12}$C with Low-Intensity Beams at Energies Near and Below the Coulomb Barrier

Fusion of neutron-rich light nuclei has been proposed as a heat source that triggers an X-ray superburst in the crust of an accreting neutron star. To investigate this hypothesis the total fusion cross-section for beams of low-intensity, neutron-rich nuclei ($<10^5$ ions/s) on light targets has been measured at energies near and below the Coulomb barrier. Evaporation residues, resulting from the fusion of oxygen and $^{12}$C nuclei, were identified by their energy and Time-of-flight. Using this technique, the fusion excitation function was measured in the sub-barrier domain down to the 2 mb level. Comparison of the measured fusion excitation function with the predictions of a density constrained TDHF model reveals that the experimental data exhibit a smaller decrease in cross-section with decreasing energy than is theoretically predicted. This difference can be interpreted as a larger tunneling probability for the experimental data as compared to the theoretical predictions. To determine if this difference increases in magnitude with decreasing incident energy improvements have been implemented to enable measurement of the fusion cross-section to an even lower level.   

Alpha Emission in the De-excitation of $^{30}$Si Nuclei at E$^{*}$ = 30 to 38 MeV

Compound nuclei produced in low-energy fusion reactions de-excite via emission of neutrons, protons, and alpha particles. Although the statistical model has been successful in describing this de-excitation for heavy nuclei, its applicability for light nuclei, particularly at low excitation is questionable. Understanding the de-excitation modes of such light nuclei is of significant importance as they play a role in stellar nucleosynthesis. To investigate this topic we have measured the alpha particles emitted in the de-excitation of the $^{30}$Si nucleus produced by fusion of $^{18}$O ions with $^{12}$C target nuclei. Both the alpha particles and the coincident evaporation residues were identified by utilizing the energy versus time-of-flight method. The energy spectra and angular distributions of the residues reveal that alpha emission plays a significant role in the de-excitation of the compound nucleus. Comparison of the residue angular distributions and alpha particle yields and energy spectra with a statistical model shows that alpha emission is significantly under-predicted by the model. The details of this comparison and its possible implications will be presented.   

The $^{136}$Xe + $^{208}$Pb reaction: A test of models of multi-nucleon transfer reactions

The yields of over 200 projectile-like and target-like fragments from the interaction of $^{136}$Xe (E$_{c.m.}$=450 MeV) with a thick target of $^{208}$Pb were measured using Gammasphere and off-line $\gamma$-ray spectroscopy, giving a comprehensive picture of the production cross sections in this reaction. The measured yields were compared to predictions of the GRAZING model (with fission competition) and those of Zagrebaev and Greiner. There is good agreement between the measurements and the predictions of Zagrebaev and Greiner for nuclei near or below the target (Z = 74, 76, 78, 80, 82). However, the measured cross sections exceed the predicted values by up to an order of magnitude for neutron-rich trans-target nuclei (Z = 84, 86, 88). The GRAZING model predictions are adequate for nuclei near the target (Z = 81-83) but grossly underestimate the yields of all other products.   

$^{89}$Zr(n,$\gamma )^{90}$Zr from a surrogate reaction approach

While recent studies have demonstrated the validity of the surrogate reaction approach for studying fission cross sections of short-lived actinides, its applicability for (n,$\gamma )$ is still under investigation. We studied the $\gamma $-decay of $^{90}$Zr produced by $^{91}$Zr(p,d) and $^{92}$Zr(p,t) in order to infer the $^{89}$Zr(n,$\gamma )$ cross sections. The experiments were carried out at the K150 Cyclotron facility at Texas A{\&}M University with a 28.5-MeV proton beam. The reaction deuterons and tritons were measured at forward angles of 30-60$^{\circ}$ with the STARS (Silicon Telescope Array for Reaction Studies) array of three segmented Micron S2 silicon detectors. Compound nuclei with energies up to a few MeV above the neutron separation thresholds were populated. The coincident $\gamma $-rays were measured with the LiTeR (Livermore Texas Richmond) array of five Compton-suppressed HPGe clovers. We will present results of $\gamma $-emission probabilities of $^{89}$Zr(n,$\gamma )$ and some theoretical discussions.   

Measurement of the Total Kinetic Energy Release (TKE) in $^{232}$Th(n,f) with E$_{n}$ = 2.59 - 87.31 MeV

Experimental results for the Total Kinetic Energy Release (TKE) of $^{232}$Th(n,f) with E$_{n}$ = 2.59 - 87.31 MeV will be presented. The experiment was performed at the 15R beamline at the Weapons Neutron Research(WNR) facility at LANL-LANSCE. WNR provides a white spectrum of neutrons peaking at 2 MeV and reaching up to 800 MeV, with neutron energies being deduced from measurements of the neutron time of flight (TOF). A thin-backed $^{232}$ThF$_{4}$ target of 2 cm diameter with a thorium areal density of 178.9 $\mu$g/cm$^{2}$ was placed between two arrays of Hammamatsu PIN diodes (active area 4 cm$^{2}$ each). The beam was collimated to 1 cm diameter. The target was placed 45 degrees off of the beam axis, with the detectors at 60 degrees and 120 degrees from the beam axis. Over 25,000 fission fragment coincidence events were recorded, allowing for sixteen energy bins between 2.59 and 87.31 MeV. We believe that this will be the most comprehensive published measurement of the TKE for $^{232}$Th(n,f) with E$_{n}$ = 2.59 - 87.31 MeV.\\[4pt] This work was supported in part by the Director, Office of Energy Research, Division of Nuclear Physics of the Office of High Energy and Nuclear Physics of the USDoE under Grant DE-FG06-97ER41026. This work has benefited from the use of the Los Alamos Neutron Science Center at the Los Alamos National Laboratory. This facility is funded by the USDoE under DOE Contract No. DE-AC52-06NA25396.   

Measurement of Neutron-Induced, Angular-Momentum-Dependent Fission Probabilities Direct Reactions

The surrogate method has previously been used to successfully measure $(n,f)$ cross sections of a variety of difficult to produce actinide isotopes. These measurements are inaccurate at excitation energies below 1.5 MeV where the distribution of angular momentum states populated in the compound nucleus created by neutron absorption significantly differs from that arising from direct reactions. A method to measure the fission probability of individual angular momentum states arising from $^{239}$Pu$(d,pf)$ and $^{239}$Pu$(\alpha,\alpha'f)$ reactions has been developed. This method consists on charged particle detectors with 40 keV FWHM resolution at 13 angles up and downstream of the beam. An array of photovoltaic (solar) cells is used to measure the angular distribution of fission fragments with high angular resolution. This distribution uniquely identifies the populated angular momentum states. These are fit to expected distributions to determine the contribution of each state. The charged particle and fission matrix obtained from these measurements determines fission probabilities of specific angular momentum states in the transition nucleus. Development of this scheme and first results will be discussed.