Session NE: Nuclear Reactions: Hadrons/Light Ions

Surrogate Reaction Measurement of Angular Dependent $^{239}$Pu$(n,f)$ Probabilities

The surrogate method has previously been used to measure $(n,f)$ cross sections of difficult to produce actinide isotopes. These measurements have inaccuracies 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 experimental apparatus consists of charged particle detectors with 40 keV FWHM resolution at 13 angles up and downstream of the particle beam. A segmented array of photovoltaic (solar) cells is used to measure the angular distribution of fission fragments. 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 rates matrix obtained from this analysis determines fission probabilities of specific angular momentum states in the transition nucleus. Development of this scheme and first results will be discussed.   

Modified statistical model for alpha production in proton$+$Ag reactions at intermediate energies

Energy spectra measurements of alpha-particles emitted from an Ag target bombarded with 210, 300, and 480 MeV protons are analyzed by using different intra-nuclear cascade models implemented within Geant4 code plus the Statistical Model with Final State Interaction SMFSI. A key difference between SMFSI and other statistical evaporation models is that SMFSI allows fragments to be emitted in all possible states, ground states, excited states, as well as in the continuum, and hence fragments might be unstable while detected (similar to fission-like process). The SMFSI model reproduced the evaporative component of the energy spectra of alpha-particles reasonably well compared to the default evaporation models used in Geant4.   

Advances in Neutron Spectroscopy with Deuterated Organic Scintillators

Deuterated organic scintillators have shown promise as neutron detectors for nuclear science as well as applications in nuclear non-proliferation and safeguards. In particular, they can extract neutron spectra without the use of neutron time-of-flight measurement (n-ToF) utilizing spectrum unfolding techniques. This permits the measure of cross sections of bound and unbound states with high efficiency and angular coverage. In the case of measurements with radioactive ion beams where low beam intensities limit long path n-ToF, short path n-ToF can be used to discriminate neutrons of interest from room return and background neutrons. This presentation will provide recent advances with these types of detectors. Digital pulse-shape discrimination using fast waveform digitizers, spectrum unfolding methods for extraction of neutron spectra, and a new safer deuterated-xylene formulation EJ-301D will be discussed. In addition, experimental results from measurements of discrete and continuous neutron spectra which illustrate the advantage of these detectors for certain applications in nuclear physics research and nuclear security will be shown. This work is supported by NSF and DOE.   

Evaluation of Quantum Monte Carlo Overlaps via $^{6,7}$Li(\emph{d},\emph{p}) Reactions

A critical component in direct reaction theory is the treatment of the bound-state form factor which encapsulates all of the nuclear structure assumed in the reaction. In recent years, it has become possible to generate form factors for direct reactions using wave functions from modern ``ab initio'' nuclear models, such as the Quantum Monte Carlo (QMC) technique\footnote{I. Brida \emph{et al.}, Phys. Rev. \textbf{C}51 (2011) 024319}. To evaluate these overlap functions, a study of the $^6$Li(\emph{d},\emph{p})$^7$Li reaction ---where all the nuclei involved are well modeled by the QMC--- was performed at the Nuclear Structure Laboratory (NSL) at the University of Notre Dame. A 10-MeV deuteron beam was incident on 50-100 ug/cm$^2$ natural and $^6$Li-enriched LiF targets located in the 1.6-m diameter General Purpose Scattering Chamber (``R2D2''). Absolute cross sections were measured with an emphasis on reducing the systematic uncertainties. Preliminary results will be presented comparing conventional Woods-Saxon form factors and those derived from QMC overlap functions.   

Development of a Plastic Phoswich for Reaction Studies

In inverse kinematics, proton transfer reactions, such as (d,n), may be used to add a proton to a short-lived ion beam. By detecting the outgoing neutron, it is possible to extract spectroscopic information about the recoil nucleus. Plastic scintillators may be used for detecting these neutrons but are sensitive to gamma rays as well as neutrons, usually resulting in a large background. A clean tag on the recoil particle is often necessary for the removal of significant unwanted background from reactions with low cross sections. A plastic scintillator phoswich (phosphor-sandwich) was developed in order to separate the recoil nucleus from a radioactive ion beam cocktail. This phoswich is comprised of two layers of plastic scintillator, with two different pulse shape characteristics, fused together to produce a single assembly viewed by a PMT. Using pulse shape discrimination (PSD) on the resultant digitized light pulses allows for Z separation at rates of up to 1x10$^{6}$ pps. Since the recoil particle has one extra proton than the beam particle, it is only necessary to have separation in Z. This detector was successfully tested during a development experiment at the University of Notre Dame. An overview of the motivation, development, and analysis of this detector will be present   

Graphical Methods for Separating Beam and Target Fragmentation Regions

For reactions involving three or more final-state particles, graphical methods can help to elucidate the dominant reaction mechanism. Van Hove [1] introduced a longitudinal phase space plot, which categorizes reaction products in terms of their longitudinal moments. We review the construction of such plots, and show how they are useful in separating beam and target fragmentation regimes. We summarize the information that can be obtained from Van Hove plots, and use these plots to analyze reactions with three or four strongly-interacting particles in the final state. As an example, we apply these methods to simulated data for the reaction $\pi^{-} + p \rightarrow \pi^{-} + \eta \, (\eta^{'}) + p$. We show how cuts in the Van Hove plot can be utilized to isolate various two-body processes that contribute to this reaction. We also show how the dominant reaction processes change with the beam energy.\\[4pt] [1] L. Van Hove, Phys. Lett. B28, 429 (1969).   

Cross section measurements for $\gamma$-rays emitted in $^{109}$Ag($n, x n y p \gamma$) reactions

Absolute partial cross sections for production of discrete $\gamma$-rays using $^{109}$Ag($n, x n y p \gamma$) reactions with $x \leq 7$ and $y \leq 1$ in a total of 12 reaction channels were measured. The data were taken using the GEANIE spectrometer comprised of 20 high-purity Ge detectors with BGO escape-suppression shields. The broad-spectrum pulsed neutron beam of the Los Alamos Neutron Science Center's (LANSCE) WNR facility provided neutrons in the energy range from 1 to 300~MeV. The time-of-flight technique was used to determine the incident neutron energies. Partial $\gamma$-ray cross sections for a total of 109 transitions and for neutron energies 1~MeV$< E_{n} <$300~MeV were obtained. Five previously unknown transitions were identified, were assigned to $^{109}$Ag, and were placed in the level scheme. An estimate of the population of isomers in the ($n,n^{\prime}$), ($n,2n$) and ($n,3n$) channels was made. Theoretical calculations up to $E_{n} = 100$~MeV from the CoH$_{3}$ reaction model are compared to the experimental results.   

Cross-Section Measurement of the $^{169}$Tm(n,3n)$^{167}$Tm Reaction and Constraining the Branching Ratio of $^{167}$Tm

The cross section of the $^{169}$Tm(n,3n)$^{167}$Tm reaction has been measured from 17.5 to 21.5 MeV using activation technique. This energy region was chosen to resolve the two different trends of the previous (n,3n) cross section measurements on $^{169}$Tm. In addition, the branching ratio of the 207.8 keV $\gamma$-ray line stemming from electron capture of $^{167}$Tm was measured to be 0.419(16). The result of these measurements provide more accurate diagnostic estimation of the so called reaction-in-flight neutrons produced via the internal confinement fusion plasma in deuterium-tritium capsules at the National Ignition Facility.   

Photon-induced Fission Product Yield Measurements on $^{235}$U, $^{238}$U, and $^{239}$Pu

During the past three years, a TUNL-LANL-LLNL collaboration has provided data on the fission product yields (FPYs) from quasi-monoenergetic neutron-induced fission of $^{235}$U, $^{238}$U, and $^{239}$Pu at TUNL in the 0.5 to 15 MeV energy range. Recently, we have extended these experiments to photo-fission. We measured the yields of fission fragments ranging from $^{85}$Kr to $^{147}$Nd from the photo-fission of $^{235}$U, $^{238}$U, and $^{239}$Pu using 13-MeV mono-energetic photon beams at the HIGS facility at TUNL. First of its kind, this measurement will provide a unique platform to explore the effect of the incoming probe on the FPYs, i.e., photons vs. neutrons. A dual-fission ionization chamber was used to determine the number of fissions in the targets and these samples (along with Au monitor foils) were gamma-ray counted in the low-background counting facility at TUNL. Details of the experimental set-up and results will be presented and compared to the FPYs obtained from neutron-induced fission at the same excitation energy of the compound nucleus.