Nuclear Science Division Newsletter, January 2023

In a January 2023 publication by nuclear theory postdoctoral fellow Jennifer Rittenhouse West, a quantum chromodynamics (QCD) bond between quarks from different nucleons is proposed as the cause of short-range correlated pairs in nuclei – a fundamental particle physics explanation for roughly 20% of nuclear structure.

Nuclei were found to have their own internal structure composed of neutrons and protons arranged in shells of quantized energy by Nobel Laureate Maria Goeppert Mayer in 1948.  Decades later it was shown, surprisingly, that not all nucleons are contained within shells. Nearly 20% of nucleons are paired into highly overlapped states called short-range correlations (SRC) and the cause of this pairing is unknown.

The uncertainty principle of quantum physics allows nucleons to suddenly come very close together in the nucleus – forcing their wavefunctions to strongly overlap – but the overlap lasts only momentarily before the nucleons must fly apart again.  How the nucleon pairs in SRC stick together is a longstanding mystery.  This is where the new work by Rittenhouse West comes into play.

At a fundamental particle physics level, QCD describes interactions between quarks, antiquarks and gluons, all of which have “color charge,” in analogy to the electric charge of quantum electrodynamics (QED).  Two color-charged quarks can bind together into a diquark if they come into very close range, a process that typically only occurs within the close quarters of a single nucleon; for example, an up quark and a down quark inside a proton forming a diquark. However, the large nucleon-nucleon overlap due to the quantum uncertainty principle allows quarks from different nucleons to come close enough to form a diquark (see Fig.1 for a plot of the large wavefunction overlap between SRC nucleons in the nucleus).  It is this attractive short-range diquark bond across a nucleon pair that is proposed to cause the correlation between nucleons.  Rittenhouse West’s calculation of the diquark binding energy shows that certain diquarks – those made of up and down quarks whose spins cancel out – are energetically favored to form in the nuclear environment.  In her model, the energy “borrowed” by the quantum uncertainty principle in order to force nucleons very close together is, at least temporarily, paid back in the form of significant diquark binding energy (see Fig.2 for a schematic of the diquark bond process).

Diquark formation between nucleons is poised to be a major leap forward in understanding the structure of nuclei in terms of the underlying particle theory of QCD if proven to be correct.  Recent experimental results acquired at Jefferson Lab by a collaboration led by NSD scientists including Shujie Li and John Arrington are favorable to Rittenhouse West’s model. 

[1] Jennifer Rittenhouse West, Nuclear Physics A 1029 (2023)

Diquark induced short-range nucleon-nucleon correlations & the EMC effect

[2] S. Li, R. Cruz-Torres, et al. with J. Arrington, Nature 609 (2022)

Revealing the short-range structure of the mirror nuclei 3H and 3He

Figure 2: A diquark bond formed across a nucleon pair. Quarks are represented by “color-charged” circles in red, green and blue. Gluons, the mediators of the strong force, are represented by black spirals. The panels evolve from left to right as the nucleons begin as neighbors, undergo a quantum fluctuation into an overlapped pair, followed by gluon exchange between a blue quark from the proton and a red quark from the neutron, ending with the diquark bond formed as represented by the lime green oval in the final panel. The mathematical transformation of 2 quarks into a diquark changes the color-charge of the final state. The diquark is no longer a simple red quark-blue quark object, but an anti-green object, by the mathematical rules of the SU(3) group theory that QCD is based on. (Lime green was chosen to represent the anti-color charge called anti-green.)

The LBNL Scintillator Research Group, formerly part of the Material Sciences Division, has joined the Nuclear Science Division. This move creates new opportunities for the development of novel radiation detection systems for a range of applications from basic science to nuclear security and medical imaging.

The detection of ionizing radiation underpins many applications that cover a large number of fields that span from the medical, to security inspection and non-destructive testing, to nuclear non-proliferation and emergency response, and to High Energy Physics. Many of these technologies rely on the capability of a group of materials, called scintillators, to efficiently convert the ionizing radiation into UV or visible light that can be more easily detected and analyzed. No single scintillator is, however, able to efficiently interact with all types of ionizing radiation, leading to the development of a large number of different materials tailored for each specific application. The discovery and the optimization of scintillators is still a thriving research field, as new needs arise.

The Scintillator Research Group at LBL has been active in inorganic scintillator research for over thirty years, building an almost unique laboratory in which all aspects of scintillator research can be tackled: from materials synthesis and growth into single crystals, to the characterization of their luminescence and scintillation properties. Over the past 15 years the Group discovered over 22 new scintillating materials. The Group is able to grow single crystals, Fig. 1, through a variety of methods (from Czochralski to Bridgman-Stokbarger and micro-Pulling Down) on both oxide- and halide-based scintillators. Optical, luminescence and scintillation properties of the obtained materials are investigated with a variety of methods (from photo- and radioluminescence both in steady state and time resolved (Fig. 2) manner, to pulse height spectra with gamma ray sources and thermally stimulated luminescence) in order to qualify these materials. 

Primary interests of the Group include the discovery of new scintillators, the optimization of scintillator properties through suitable chemical addition/modification of their composition, fundamental physic phenomena underlying the scintillation process itself including the role of defects, and synthesis strategies. Although the Group focus has been on inorganic scintillators, it recently expanded also to water-based liquid scintillators as well as organic/inorganic composites. 

The addition of the Scintillator Research Group strengthens NSD’s existing capabilities in radiation detector development and compliments the longstanding expertise in semiconductor development.

Federico Moretti and Weronika Wolszczak contributed to this article. Dr. Moretti is the PI of the Scintillator Research Group and Dr. Wolszczak is a Postdoctoral Researcher.

Figure 1: a polished piece of Cs₂LiLaBr₆:Ce (CLLB) single crystal grown by Bridgman Stockbarger.

Figure 2: Scintillation decay profile as a function of the temperature of a newly developed Tl₂La_0.95Ce_0.05Cl₅ single crystal measured with a table-top light-excited x-ray tube working at 40 kV_p. The system has an instrument response function of about 100 ps. Measurements were performed in the 15-310 K. These scintillation decays allow to clarify how much charge carrier transport toward the luminescence centers is affected by the temperature. The curves have been shifted along the ordinate axis for clarity.