Lawrence Livermore National Laboratory



Nicholas Scielzo

Overview

We investigated the feasibility of establishing a state-of-the-art research program by bringing together the nuclear-science expertise of the Lawrence Livermore National Laboratory and the Soreq Nuclear Research Center in Israel. We concluded that there are three promising research topics that would benefit both laboratories and would take advantage of the capabilities available at the Soreq Applied Research Accelerator Facility, specifically state-of-the-art light-ion and neutron beams for research in low-energy nuclear physics. The three research topics are (1) precision beta-decay studies to test the Standard Model of particle physics and the solar neutrino flux, (2) direct and indirect determinations of (n,2n) reactions required as benchmarks for nuclear data needs, and (3) direct measurements of neutron-capture cross sections of importance to nuclear astrophysics. Our research was instrumental in enabling scientists of the two facilities to work together on these topics to determine feasible goals for future collaborations.

Background and Research Objectives

At the Soreq Nuclear Research Center (SNRC), the Soreq Applied Research Accelerator Facility (SARAF) provides light-ion and neutron beams for research in low-energy nuclear physics (Mardor et al. 2018). The first half of this facility, SARAF-I, was built primarily to test novel technologies for generating the highest-intensity beams and will run a campaign of science experiments until the end of 2019. The facility provides neutron beams with energies up to 20 MeV using a lithium fluoride thick target (Hirsh et al. 2012) and with an energy distribution similar to that of the 30-keV Maxwellian-averaged flux distribution, which is of interest for slow neutron-capture process (s-process) nucleosynthesis (whereby repeated neutron-capture reactions and beta decays build up elements over thousands of years, mostly in asymptotic giant branch stars [Tessler et al. 2015]). The facility is poised to deliver beams of radioactive noble gases (such as 6He) and a variety of unstable neon isotopes that can be delivered to experimental end-user stations (Marder et al. 2018). The second half of the facility, SARAF-II, is anticipated to come online in approximately five years and will serve as a user facility delivering neutron and light-ion beams, in some cases at intensities surpassing those of any other facility in the world.

At Lawrence Livermore National Laboratory, we are interested in a variety of experiments to study nuclear beta decay and measure nuclear-reaction cross sections that are achievable with these types of beams. A collaborative effort between Livermore and SNRC researchers could result in high-impact programs that use the capabilities of SARAF. During this study, we attempted to lay the groundwork for just such a collaboration by performing initial experimental investigations on three topics:

  1. Precision studies of nuclear beta decay to test the Standard Model (SM) of particle physics and solar neutrino predictions. Low-energy beta-decay measurements provide important experimental support for the SM description of the electroweak interaction. However, the SM can be tested more stringently by holding isotopes that beta decay in an atom trap or ion trap and studying the angular correlations between the emitted particles because these correlations are sensitive to the structure of the underlying electroweak interaction. Following beta decay, the low-energy nuclear recoil is available for study in coincidence with the beta particle, allowing the neutrino kinematics (and therefore beta-decay angular correlations) to be determined through conservation of energy and momentum. The large quantities of 6He, 18Ne, 19Ne, and 23Ne isotopes that can be produced at SARAF open up opportunities for important SM tests. In addition, neutrinos coming from the 8B beta decay produced in the Sun provide the dominant source of high-energy neutrinos and play a key role in the interpretation of the spectra in many experiments studying solar neutrino oscillations. The full decay kinematics can be reconstructed using trapped 8B ions, allowing the neutrino spectrum to be precisely determined and systematic uncertainties to be studied in detail.
  2. Measuring (n,2n) cross sections of importance to various applications. Neutron reactions on short-lived isotopes are extremely difficult to measure without an intense neutron source and the careful preparation of target materials. Surrogate nuclear reactions enable an indirect approach to determine nuclear cross sections (Escher et al. 2012). Multiple neutron-emission reactions, (n,2n) and (n,fxn), can be determined by using a charged-particle direct reaction to create the same desired nucleus as the neutron-induced reaction and then observe its decay probabilities (i.e., neutron emission in this case). As the surrogate method is developed, it is critical for the sake of comparison to have reliable benchmark data on stable nuclei that are within one to two nucleons of the surrogate reaction target nucleus. Recently a neutron detector array (NeutronSTARS) at Livermore was refurbished and commissioned to collect the data needed for surrogate (n,2n) cross sections and to obtain new fission neutron multiplicity data on any nucleus within one to two nucleons off of stability. There is currently a lack of high-quality data in this area, but this deficiency could potentially be resolved by directly measuring cross sections at SARAF.
  3. Measuring neutron-capture cross sections of importance to s-process nucleosynthesis. Nearly half of the elements (from iron to uranium) that are observed today were produced through the s-process. The cross sections for many of the long-lived radioactive nuclei (known as branch-point nuclei) along the nucleosynthesis pathway are important for interpreting the observed stable-isotope abundance pattern and understanding s-process dynamics. Direct measurements of branch-point nuclei require both large samples of the radioactive isotope of interest and an intense neutron flux with an energy distribution similar to that of the neutron-energy spectrum present during the s-process. Livermore has the radiochemistry expertise required to produce high-quality targets, nuclear experimentalists with experience performing cross-section measurements, and significant expertise in reaction theory. Such integral cross section measurements could be used to guide reaction-theory calculations to determine the neutron-energy dependence of the desired cross sections.

Impact on Mission

Our research began the process of establishing strong partnerships between nuclear science researchers at Livermore and SNRC. We envision these partnerships opening up new opportunities to perform measurements of interest to Livermore’s future nuclear science missions. A strong collaboration with researchers at SNRC and Israeli universities can provide access to the state-of-the-art light-ion and neutron beams available at SARAF that are currently of limited availability or are not available in the U.S. Livermore researchers can also benefit from the expertise developed in Israel for carrying out low-energy nuclear science experiments.

Conclusion

The SARAF facility at SNRC in Israel has begun delivering high-intensity neutron and light-ion beams that would benefit a variety of nuclear-science programs, ranging from studies of nuclear beta decay to the determination of neutron-induced reaction cross sections. We worked with SNRC to determine the feasibility of performing these types of measurements at SARAF. These efforts led to the planning and design of an experiment to measure the 23Ne beta-decay transition intensities, a novel method to calibrate silicon detectors (which resulted in a joint LLNL-SNRC publication), as well as the initiation of a Livermore-SNRC collaborative effort to investigate neutron-induced reactions using surrogate-reach approaches at Texas A&M University and direct-measurement approaches at SARAF. We hope to obtain support to carry out experiments at SARAF to measure the 23Ne beta-decay transition intensities and to utilize the high-intensity neutron beams to directly measure neutron-induced reaction cross sections. We envision that there will be a great benefit to continuing to foster this collaboration so that Livermore will be well-positioned to use the unique capabilities that will become available when SARAF-II comes online in the coming years.

References

Akindele, O. A., et al. 2017. "NeutronSTARS: A Segmented Neutron and Charged Particle Detector for Low-Energy Reaction Studies." Nuclear Instruments and Methods in Physics Research A 872, 112–118. doi: 10.1016/j.nima.2017.07.069.

Carlson, T. A. 1963. "Recoil Energy Spectrum of Sodium Ions Following the Beta Decay of 23Ne." Physical Review 132, 2239–2242. doi: 10.1103/PhysRev.132.2239.

Hirsh, T. Y., et al. 2012. "Towards an Intense Radioactive 8Li Beam at SARAF Phase I." Journal of Physics Conference Series 337, 012010. doi: 10.1088/1742-6596/337/1/012010.

——— . 2018. "The Use of Cosmic-Ray Muons in the Energy Calibration of the Beta-decay Paul Trap Silicon-Detector Array." Nuclear Instruments and Methods in Physics Research A 887, 122–127. oi: 10.1016/j.nima.2018.01.021.

Mardor, I., et al. 2018. "The Soreq Applied Research Accelerator Facility (SARAF): Overview, Research Programs and Future Plans." European Physics Journal A 54, 1–32.

Tessler, M., et al. 2015. "Stellar 30-keV Neutron Capture in 94,96Zr and the 90Zr(γ,η)89Zr Photonuclear Reaction with a High-Power Liquid-Lithium Target." Physics Letters B 751, 418–422. doi: 10.1016/j.physletb.2015.10.058.

Publications and Presentations

Hirsh, T. Y., et al. 2018. "The Use of Cosmic-Ray Muons in the Energy Calibration of the Beta-Decay Paul Trap Silicon-Detector Array." Nuclear Instruments and Methods in Physics Research A 887, 122–127. LLNL-JRNL-732612.