Session K7: Mini-Symposium on Electromagnetic Reactions Involving Light Nuclei II

An overview of the research program at the High Intensity Gamma-Ray Source (HIGS) to study light nuclei

A program is underway at the HIGS facility to study the response of nucleons and light nuclei, namely the deuteron and 3He, to gamma rays having energies between photodisintegration threshold and 100 MeV. Major components of this program are: 1) the spin response of polarized deuterium and polarized 3He to circularly polarized gamma rays to study the Gerasimov-Drell-Hearn (GDH) sum rule; 2) Compton scattering from protons and deuterons to extract the static electromagnetic polarizabilities of the nucleons; 3) A first measurement of the proton spin-polarizabilities; and 4) measurement of total and differential cross sections of the deuteron and 3He at energies relevant to Big-Bang Nucleosynthesis (BBN). An overview of these programs and initial results will be presented.   

The Near-Threshold Pion Photoproduction Program at MAX-lab

One of the important questions in nuclear science is to describe the properties of the nucleon in terms of the framework provided by QCD. In the low-energy nuclear region the QCD calculations are impossible to do using standard techniques, so it is necessary to use alternative approaches to solve them. One process where these other techniques are both valid and useful is pion photoproduction near threshold. Since this process involves a rearrangement of the quarks in the nucleon it is directly accessing the underlying quark structure. Measurements of this fundamental process can be used to test the predictions from chiral effective-field theories, dispersion theory approaches and other quark-based models of the nucleon. The new Photon Tagging Facility at MAX-lab in Lund, Sweden is uniquely suited to perform measurements of pion photoproduction at energies between threshold and the $\Delta$-resonance. A comprehensive program to investigate pion photoproduction is currently underway at MAX-lab. These measurements will provide important new information on the $p$-wave contributions to charged pion photoproduction, resolve questions regarding the threshold $E_{0^{+}}\left(\pi^{-}n\right)$ amplitude, and will be the first measurement of neutral pion production from the neutron. A brief overview of the pion program at MAX-lab, together with preliminary results from measurements already performed will be presented.   

Program of Compton Scattering Studies on the Deuteron at HIGS

The High Intensity Gamma Source (HIGS) at Duke University will deliver intense mono-energetic photon beams with high degrees of linear or circular polarization by backscattering of free-electron laser (FEL) photons. To exploit the unique capabilities of this facility, a program of Compton scattering studies on light nuclei ($p$, $d$, $^{3}$He) is planned. Experiments using deuterium targets will elucidate the electromagnetic polarizabilities of the neutron ($\alpha _{n}$ and $\beta _{n})$ and provide high precision data at low energies ($E_{\gamma }$ = 30-50 MeV) for comparison with chiral Effective Field Theory (EFT) calculations. Absolute cross sections for elastic Compton scattering on deuterium will be measured for the first time in this energy region, and with the advent of polarized deuterium targets at HIGS, additional constraints on $\alpha _{n}$ and $\beta _{n}$ can be imposed. Furthermore, the prospect of measuring double polarization observables with circularly polarized photons in Compton scattering at higher energies ($E_{\gamma }$ = 100-120 MeV) will enable the spin polarizabilities ($\gamma _{1}$, $\gamma _{2}$, $\gamma _{3}$, $\gamma _{4})$ of the neutron to be determined for the first time and will complement parallel studies on the proton. The plans for Compton scattering experiments on deuterium will be outlined and the impact on the neutron electromagnetic and spin polarizabilities will be discussed.   

Interaction current in $p p \to p p \gamma$

The nucleon-nucleon bremsstrahlung reaction is investigated based on a fully gauge-invariant relativistic meson-exchange model approach. In order to account consistently for the complicated part of the interaction current (which at present is too demanding to be calculated explicitly), a generalized contact current is introduced following the approach of Haberzettl, Nakayama, and Krewald [PRC 74, 045202 (2006)]. The contact interaction current is constructed phenomenologically such that the resulting full bremsstrahlung amplitude satisfies the generalized Ward-Takahashi identity. The formalism is applied to describe the high-precision proton-proton bremsstrahlung data at 190 MeV obtained at KVI [PRC 65, 031001 (R) (2002)]. The present results show good agreement with the data, thus removing the longstanding discrepancy between the theoretical predictions and experimental data. The present investigation, therefore, points to the importance of properly taking into account the interaction current for this reaction.   

First Study of Three-body Photo-disintegration of $^3$He $\overrightarrow{^3He}(\vec{\gamma},n)pp$ with Double Polarizations

The study of the three-nucleon system has long been of fundamental importance to nuclear physics. We report on a first study of three-body photo-disintegration of polarized $^3$He with a circularly polarized $\gamma$-ray beam at an incident energy of 11.4 MeV. This experiment was carried out at the High Intensity $\gamma$-Ray Source (HI$\gamma$S) facility located at the Duke University Free Electron Laboratory. A high-pressure polarized $^3$He target based on spin-exchange optical pumping of hybrid alkali was used in the experiment. Both differential cross sections and asymmetries were extracted from the experiment. The results are compared to the three-body calculations using both CD Bonn and AV18 potentials and are in agreement within experimental uncertainties.   

Photodisintegration of Lithium Isotopes

It is clear that new experimental data is needed to compare with recent Lorentz Integral Transform calculations of the photodisintegration cross sections of the lithium isotopes. We describe a recent measurement on these isotopes performed with the monochromatic, polarized photon beam at the High Intensity Gamma-Ray Source (HIGS) at Duke University in Durham, NC, USA. The Blowfish Neutron Detector Array, a segmented neutron detector array with good angular resolution that covers $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 4$} $ of 4$\pi $ steradians, was used to detect photoneutrons. Clear separation of various reaction channels is possible which allows detector efficiencies to be accurately modeled using a GEANT4 simulation. Several methods for obtaining the incident photon flux are available so precision cross sections between 8 and 35 MeV can be obtained.   

A New Measurement of the Total Cross Section for the Photodisintegration of $^{9}$Be Near Threshold

The hot and neutron rich dense matter in Type II supernovae is a plausible environment where r-process nuclei are synthesized. The $\alpha(\alpha n,\gamma)^{9}Be(\alpha,n)^{12}C$ sequence is the favored reaction chain for synthesizing carbon in this explosive environment. Nucleosynthesis network models indicate that the ratio of neutrons to seed nuclei at the onset of the r-process is highly sensitive to the rate of the $\alpha(\alpha n,\gamma)^{9}Be(\alpha,n)^{12}C$ chain relative to the $\alpha(\alpha\alpha,\gamma)^{12}C$ reaction. The rate of the $\alpha(\alpha n,\gamma)^{9}Be$ reaction is derived from the measured cross section for photodisintegration of $^{9}Be$. New cross section measurements for this reaction have been made from threshold to 5 MeV using the High Intensity Gamma-ray Source (HI$\gamma$S) at TUNL. The low energy spread (as low as 1\%) of the beam at HI$\gamma$S enabled high precision measurements at the reaction threshold energy and of the narrow resonance at 2.43 MeV. Experimental techniques and results will be presented and astrophysical consequences will be discussed.