Electron scattering is a powerful tool for studying hadron and nuclear structure.  In particular, one‑photon (OPE) and two‑photon exchange (TPE) mechanisms play an important role in the scattering of electrons on nuclei. The OPE mechanism is the dominant interaction, while the TPE is a small but sometimes critical contribution. In terms of the state of the art, the OPE has been well measured and is well understood but the TPE is not.

A simple but extremely powerful tool that experimentalists can use to refine our knowledge of the TPE mechanism is “single-proton knockout” which, as the name implies, involves knocking one and only one proton out of a target nucleus without interacting with the rest of the nucleus or creating new particles. Single-proton knockout is used to study the momentum distributions of protons in the nucleus. To make precision measurements of these distributions, the elastic electron-proton interaction must be well understood. This means having a precise knowledge of the proton electromagnetic form factors which encode the spatial distribution of charge and magnetization of the proton and, ultimately, determine the e-p elastic scattering cross section [1].

With the completion of the 12-GeV upgrade at JLab, a new generation of precision nuclear structure measurements have been performed that are capable of exploring new kinematic regions. JLab E12-07-108 was one of the first experiments after the completion of the upgrade, and the experiment made an extensive set of precision elastic e-p scattering measurements for momentum transfers from 2 to 16 GeV². These data provide an improved understanding of the proton form factors and electromagnetic structure, the input needed for new proton knockout measurements, and strong experimental constraints on the impact of higher-order two-photon exchange (TPE) corrections in high-energy electron scattering. See Figure 1.

To effectively extract the elusive TPE contributions from other effects in the data, the team examined the impact of improved radiative corrections that go beyond the OPE. They found that these improved procedures reduced the discrepancy associated with TPE corrections in prior measurements by one-third but still require TPE contributions with a 4% angular dependence for Q² from 6 to 16 GeV² [2]. See Figure 2.

[1] “Review of two-photon exchange in electron scattering,” J. Arrington, P. G. Blunden, W. Melnitchouk, Prog. Part. Nucl. Phys. 66 (2011) 782

[2] “Form factors and two-photon exchange in high-energy elastic electron-proton scattering,” M. E. Christy, et al., Phys. Rev. Lett. 128 (2022) 102002.

A team from LBNL’s Applied Nuclear Physics program, in collaboration with scientists from Argonne National Laboratory, have been working to realize a city-scale multi-sensor testbed with which to explore new techniques in intelligent, networked sensing for radiological and nuclear detection. Figure 1 shows a prototype system installed at LBNL for evaluation. In June and July, four sensor systems will be deployed in Chicago. A full network of up to twenty systems is scheduled for completion by Spring 2023.

The detection, identification, and localization of illicit radiological and nuclear material continues to be a key component of non-proliferation and nuclear security efforts around the world. LBNL researchers have long been at the forefront of developing new detector systems and algorithms for these important applications. One place where advances in telecommunications, edge computing, and cloud computing technologies can have a significant impact on these efforts is the development of networked radiation detectors. Deploying multi-sensor systems at strategic locations in urban environments has the potential to provide persistent radiological/nuclear monitoring with a high probability of detecting mobile radiological sources or nuclear material. These concepts can be enhanced through the development of new networked detection capabilities including the integration of contextual sensors to aid in the interpretation of radiological measurements, algorithms that are able to adapt to changing environmental conditions, and the fusion of data from across the network of sensors.

The LBNL-led ‘Platforms and Algorithms for Networked Detection and Analysis’ project (PANDA) and the Argonne ‘Domain Aware Waggle Network’ project (DAWN) have developed unique sensor systems that combine large-volume gamma-ray detectors with a suite of contextual sensors including video, Lidar, meteorological sensors, edge computers, and network connectivity.

Figure 2 shows an engineering drawing of a PANDA-DAWN sensor system.

In addition to developing these new sensor systems, the LBNL team has also been developing and implementing advanced algorithms for radiological anomaly detection, contextual sensor processing, and data fusion. The data collected with the PANDA-DAWN network, along with advances in algorithms, system design, and network infrastructure developed through these projects, will pave the way for next-generation radiological detection in the field and serve as a model and springboard for other sensing modalities such as chemical, biological, and environmental.

The PANDA project is led by Ren Cooper. Nico Abgrall, Mark Bandstra, Dan Hellfeld, Tenzing Joshi, Victor Negut, Dan Parker, Brian Quiter, Emil Rofors, and Marco Salathe have all made significant contributions. The project is supported by the Department of Energy, National Nuclear Security Administration, Office of Defense Nuclear Nonproliferation Research and Development.

The Facility for Rare Isotope Beams (FRIB) officially began operations and started its user program in May. A ribbon-cutting ceremony was held May 2, 2022 to officially mark the “start of FRIB’s scientific mission” with U. S. Secretary of Energy Jennifer Granholm and the president of Michigan State University Samuel Stanley in attendance. The first FRIB experiment, E21062, ran from May 10-18, 2022 and studied the decays of rare isotopes produced following the fragmentation of a high-intensity (1 kW) ⁴⁸Ca primary beam. The exotic nuclei produced in these reactions were delivered to the center of the FRIB Decay Station Initiator (FDSi), which is a detector system designed to provide discovery potential for the decay of isotopes reaching towards the limit of stability at the neutron dripline. The FDSi detector system for the first FRIB measurement included charged particle, gamma-ray, and neutron detection systems.

The first FRIB experiment was led by spokesperson Heather Crawford (LBNL) and co-spokespersons Mitch Allmond (ORNL), Ben Crider (Mississippi State University), Robert Grzywacz (University of Tennessee Knoxville) and Vandana Tripathi (Florida State University). The team included over 50 participants from ten universities and national laboratories. While the initial beam intensity for this measurement was just 10% of that planned for the ultimate experiment, the preliminary results nonetheless show a number of isotopes with newly measured half-lives. See Figure 1. The  red line in the figure separates nuclei with known half-lives – to the left – from those with no literature value – on the right.

Analysis of the data is now underway, with involvement from across the collaboration. Stay tuned for the first set of publications!

The building blocks of matter, protons, and neutrons, are made of quarks and held together by gluons. (See Figure 1.)  Each of these objects – quarks, gluons, protons, and neutrons – has spin.  Quarks and gluons have an internal spin, with values of 1/2 and 1 in particle physics units.  They can also orbit around each other –  this type of spin is called orbital angular momentum and has integer values – 1, 2, etc.  Protons and neutrons are very complicated objects in one sense – they are made of 3 “valence” quarks – quarks that set the quantum identifiers of the nucleon – as well as a sea of quark-antiquark pairs and gluons.  All of it spinning in a variety of ways.  From the outside, however, protons and neutrons are very simple spin-1/2 objects and so a  fundamental question is how do they get their spin?  How do they end up with spin-1/2 when they contain within themselves any number of spin-1 and spin-1/2 objects, which may also be spinning around each other?  This is a question for theorists and experimentalists at the future Electron-Ion Collider (EIC) accelerator.

Recently, a team from the Berkeley Lab, Jefferson Lab, Brookhaven, MIT, and industry laid out a roadmap [1] for discovering the spin structure of the neutron by probing it with an electron beam.  Oddly enough, the key to this work is making sure we know that the neutron is the target of the probe. Why is it so hard to know that we probed a neutron with the electron beam?  Because building a pure neutron target is not easy.  Free neutrons have a half-life of about 15 minutes so any bottle of neutrons left alone in the laboratory will be gone quickly.  We need to stabilize the neutrons in order to keep them longer.  This is done in Nature by combining neutrons with protons – i.e., by binding them in  a nucleus such as helium-3. 

The team’s roadmap proposes to do an EIC experiment using helium-3 as the target because helium-3 contains two protons and one neutron and, most importantly, the neutron is accessible and stable. By putting silicon detectors (nicknamed “Roman Pots”) very close to the axis of the beam, the team will be able to detect the protons kicked out of the helium nucleus. (See Figure 2).  So, when two simultaneous protons are detected coming from the target  (called “double-spectator tagging”),  the team will know that the neutron was hit.  It is a very simple and effective technique.  From this knowledge, a set of spin observables can be measured to obtain the valence quark contribution to the neutron’s spin that is 3 to 10 times more precise than our best experiments to date.  If the experiment can be successfully carried out, then we will be taking a mighty leap forward in understanding how the neutron gets its spin.

Jennifer Rittenhouse West from the Nuclear Theory group in NSD was the principal theorist on this work.

[1]  I. Friscic et al., Physics Letters B, Volume 823, 10 December 2021, 136726