Nuclear Physics

Mission Statement

The primary mission for the Nuclear Team is to study thermonuclear processes from inertial confinement fusion (ICF) experiments at the Omega Laser Facility relevant to ICF, the Stockpile Stewardship Program (SSP), stellar nucleosynthesis (SN), and the big-bang nucleosynthesis (BBN). Additional efforts include experimental operations on the OMEGA EP and Multi-Terawatt (MTW) Laser Systems.  These ongoing experiments are valuable hands-on experiences that involve developing state-of-the-art nuclear diagnostics and advanced analysis techniques.

Thermonuclear Fusion​

“Harnessing the Energy that Powers the Sun”​

Thermonuclear fusion is the process in which two or more atomic nuclei fuse together and form one or more different atomic nuclei and subatomic particles (neutrons or protons).  The difference in mass between the reactants and products is manifested as either the release or the absorption of energy.  This difference in mass arises due to the difference in atomic binding energy between the nuclei before and after the reaction.  In addition, fusion is the process that powers active or main sequence stars and other high-magnitude stars, where large amounts of energy are released.
One of the main objectives for the Laboratory for Laser Energetics (LLE) is to perform cryogenic direct-drive inertial confinement fusion (ICF) experiments with cryogenic deuterium–tritium target shells. In the ICF process, a target is filled with a mixture of two isotopes of hydrogen [deuterium (D) and tritium (T)], which then form a crystal layer at cryogenic temperatures. The target is subjected to a sudden laser irradiation resulting in intense pressures and high temperatures sufficient for thermonuclear fusion reactions to be initiated.
Thermonuclear fusion illustration.

HDC: high-density carbon

S-Factor Measurements1

Theoretical and computational models require accurate nuclear reaction rate or cross-section measurements to understand astrophysical objects and to predict how they will evolve. Within this context, the S factor is often used as a representation of the cross section that removes Coulomb effects so as to isolate the nuclear component of the interaction.
Accelerator-based experiments for these measurements are particularly challenging at the low center-of-mass energies relevant to stellar and big-bang nucleosynthesis since the reaction cross sections fall rapidly with decreasing energy and bound electron screening corrections may become significant. In contrast, ICF experiments offer a unique opportunity to study these reactions using a population of high-temperature ions in which low center-of-mass (CM) energies are readily accessible. The target and laser parameters of an implosion can be varied to study these reactions across a range of CM energies. Warm implosions on OMEGA have recently been used to study S factors for the gamma-branch fusion reactions D(T,5He)γ, H(D,3He)γ, and H(T,4He)γ using the D(T,4He)γ and/or D(D,3He)γ reactions as references.​
S-factor and branching ratio measurements.

Nuclear Structure and Nucleon–Nucleon Interactions2

Few-nucleon systems and reactions between light nuclei continue to attract interest since they provide testing grounds for modern microscopic nuclear theory. Numerous experiments conducted over the past several decades have searched for evidence of a three-nucleon force (3NF) in addition to the nucleon–nucleon (NN) interactions. A novel approach to measure the neutron-induced scattering reactions between light-Z nuclei has been developed at the Omega Laser Facility is shown in in the figure below.

Experimental setup.

Experimental setup with a high-yield neutron source incident on a nuclear reaction vessel positioned 9 cm from target chamber center. The vessel contains nondeuterated or corresponding deuterated target compounds. Because of the geometry of the reaction volume with respect to the diagnostic instrument, this measurement will cover an emission angle of the breakup neutrons from the reaction vessel.

nTOF: neutron time of flight

A high-intensity pulse of 14-MeV neutrons produced from direct-drive ICF implosions on the OMEGA Laser System with a luminosity of L = 1024 1/s at the University of Rochester’s Laboratory for Laser Energetics is used to irradiate axially symmetric vessels that contains a light-Z element. Both elastic scattering and inelastic reactions are measured through the detection of neutrons using a highly collimated spectrometer in line with the reaction vessel.

This platform achieved the first measurement of the deuteron breakup using a new laser-based ICF platform developed on the Omega Laser System and was compared to a recent ab initio theoretical prediction2.
Plots for Double-differential cross section (d2σ/ddE) for neutron-induced deuteron breakup.

Double-differential cross section (d2σ/ddE) for neutron-induced deuteron breakup at 14 MeV using two different reaction vessels with (a) deuterated water, (b) deuterated benzene, and (c) the averaged values from the separate measurements  for a near-zero forward angle. The solid circles are the data reported in this paper, the squares are the data of M. Brüllmann et al., Phys. Lett. B 25, 269 (1967), and the diamonds are the neutron-induced breakup data of J. Kecskeméti, T. Czibók, and B. Zeitnitz, Nucl. Phys. A 254, 110 (1975). The solid lines represent Faddeev-type calculation of the neutron-induced deuteron breakup using the realistic CD Bonn +  + U1 model, averaged over forward neutron emission angles at d2σ/dEndn0◦<θ<7.4◦.

Ion Acceleration Using the TNSA Process3

The past few decades have seen increased interest by the nuclear science community in the structure of light nuclei and their interactions at energies approaching a MeV. This shift in research is largely attributable to challenges persisting in nuclear astrophysics and the progress in sophisticated quantitative nuclear structure modeling. As an example, astrophysical nuclear reaction mechanisms, proposed to bridge the A = 5 and A = 8 gaps in the Segré chart, and various “Li puzzles” still challenge models of primordial (big bang) and contemporary, cosmic modes of nucleosynthesis.
TNSA process illustration.

Target normal sheath acceleration (TNSA) process: Laser beams from left force sheath of electrons to separate from the backside of a metallic converter foil. Resulting Coulomb field accelerates ions (here tritons) in the near-surface domain of foil. A beam of ions is accelerated toward the physics target to be studied.

The Tritium Laser-Ion Acceleration for Nuclear Science (TLIANS) project at the Omega Laser Facility is developing a controllable triton (3H) beam and experimental platform to study triton-induced reaction on light nuclei. It is based on the principle of the laser-induced ionization process known as TNSA, for “target normal (electron) sheath acceleration.3” An illustration of this process is shown is shown in the figure above in which enormous Coulomb fields (~ TV/m) are generated suddenly (within picoseconds) on the back side of a metallic foil irradiated with high-energy, short-pulse laser beams.

1 Z. L. Mohamed et al., Bull. Am. Phys. Soc., ZO04.00011 (2021).
2 C. J. Forrest et al., Phys. Rev. C 100, 034001 (2019).
3 E. L. Clark et al., Phys. Rev. Lett. 84, 670 (2000); R. A. Snavely et al., Phys. Rev. Lett. 85, 2945 (2000); A. Maksimchuk et al., Phys. Rev. Lett. 84, 4108 (2000).
4 A. K. Schwemmlein et al., Nucl. Instrum. Methods Phys. Res. B 522, 27 (2022).