In this issue:
- Director’s Corner
- Diquarks in Nuclei: A Quantum Chromodynamics Basis for nearly 20% of Nuclear Structure
- Scintillator Research Group moves to NSD
- Inclusion, Diversity, Equity, and Accountability Moments
2022 has been an eventful year for the Nuclear Science Division with many successes in science, funding, awards, and advancements of major projects. I feel privileged to have joined Berkeley Lab and the NSD in mid-2022.
2023 promises to be an exciting year. The U.S. nuclear science community, with strong involvement by division members, is developing its new NSAC Long Range Plan. Major projects of the division are advancing with GRETA moving into the integration phase, EIC R&D and design efforts ramping up, infrastructure renewals at the 88-Inch cyclotron progressing, the MARS ECR magnet advancing, and CUPID preparing for its CD-1 review later this year. Members of the NSD Theory group are involved in 5 newly funded topical collaborations and are welcoming Raul Briceno to Berkeley, joining LBNL and the UC Berkeley Physics Department on a joint appointment.
This issue of the NSD Newsletter touches on color in two very different contexts: (i) the theory of quantum chromodynamics revealing how short range correlations of nucleons might be facilitated by di-quark correlations; and (ii) considerations when choosing colors for graphics and signs in order to be inclusive of people that are color blind. This issue also includes a brief portrait of the LBNL Scintillator Research Group, which recently joined the Nuclear Science Division, further bolstering the division’s capacity and expertise for advanced research on radiation detectors and their applications.
I am wishing everyone a healthy, safe, and successful year 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.
 Jennifer Rittenhouse West, Nuclear Physics A 1029 (2023)
 S. Li, R. Cruz-Torres, et al. with J. Arrington, Nature 609 (2022)
Figure 1: The overlap of two nucleons in a SRC. The relative momentum between them is 400 MeV/c (taken from neutron-proton SRC measurements in carbon-12), corresponding to a separation distance of ~0.5 femtometers – a very small separation considering the rms radius of a nucleon is 0.84 femtometers. Nearly 60% of the volume of each nucleon is within the overlap region. Fundamental QCD effects – like attractive potentials between quarks – are nearly certain to have an effect with such large wavefunction overlap.
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 Tl2La0.95Ce0.05Cl5 single crystal measured with a table-top light-excited x-ray tube working at 40 kVp. 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.
Colorblindness Inclusion in NSD
In September 2022, Jayson Vavrek, a postdoctoral researcher within the Applied Nuclear Physics program, gave a presentation at the NSD staff meeting about the need to consider the choice of colors when creating figures and other graphics so that we can better include those who are colorblind or are affected by another form of color vision deficiency (CVD). Measurements vary, but approximately 2–10% of men and 0.1–3% of women have some form of CVD, so a substantial proportion of your audience (roughly 5%) may have some difficulty perceiving color.
Jayson pointed out that although many popular scientific plotting packages have switched to colorblind-friendly default palettes in recent years, it is still important to consider how one’s plots and other graphics will be perceived by those with different abilities to see color. For example, here is a plot using matplotlib’s default color scheme, with the version on the right being adjusted to what a person with deuteranopia (or red-green colorblindness, the most common form of CVD) would see:
Instead of relying on color alone to convey meaning, one can also use text, symbols, and/or texture to signal the difference. In the above example, different line dashing styles would improve the differentiation between the lines. Another key takeaway is to use perceptually uniform colormaps instead of rainbow (or “jet”) colormaps, since they are more inclusive for viewers with CVD (and they can be misleading even for those without CVD!).
These days there are many technological solutions to help those without CVD to be more inclusive of those with CVD. There are specially designed color palettes (e.g., Color Brewer), and both desktop simulation tools (e.g., Color Oracle) and augmented reality mobile apps (e.g., Sim Daltonism) to allow one to see how a given plot or graphic will look under different types of CVD.
Coincidentally, on the very same day that Jayson gave his presentation, one of our colleagues within NSD had just alerted the IDEA Council that several signs and markings within the Laboratory make work harder for colleagues with CVD. In response, the IDEA Council initiated contact with Creative Services, Strategic Communications, and other offices and groups within LBL and has convened a working group to understand the existing policies for colorblindness inclusion at the lab, and to potentially craft new lab-wide recommendations for all staff to be aware of. The council will be informing the division of the progress on these recommendations so that we can continue to build a more safe and inclusive workplace at the lab.
Recent DEI topics @ NSD Staff Meetings
October 11, 2022 – Disability Awareness Month
October 25, 2022 – DNP 2022 and Code of Conduct for APS meetings
November 22, 2022 – PIER plans for DOE proposals
December 20, 2022 – Winter Break 2022
To recognize their efforts in the area(s) of Inclusion, Diversity, Equity, and Accountability, the following people received a Luminary Card:
Alan Poon and multiple anonymous recipients
Staff and students from the Applied Nuclear Physics (ANP) program attended the IEEE Nuclear Science Symposium and Medical Imaging Conference in Milan in November. The meeting is the largest annual radiation instrumentation conference. Members of ANP and affiliated graduate students delivered a total of 19 presentations.
Nu Xu, from NSD’s Relativistic Nuclear Collisions Program has been awarded a prestigious Research Award from the Humboldt Foundation in Germany. Humboldt Research Awards are awarded to internationally leading researchers of all disciplines in recognition of their academic accomplishments.
Agnieszka Sorensen, who carried out a substantial amount of her research at LBNL under the supervision of NSD’s Volker Koch, has received the American Physical Society’s 2023 Dissertation Award in Nuclear Physics for her work using an innovative approach to study the speed of sound in dense nuclear matter.
Raul Briceno has joined the UC Berkeley Physics Department as an Assistant Professor with a joint affiliation with the NSD Nuclear Theory Program. Raul’s research focuses on understanding the emergence of low-energy nuclear physics directly from quantum chromodynamics (QCD).
Nicolas Abgrall has reached the milestone of 10 years of service.
Andre Walker-Loud has reached the milestone of 10 years of service.
Scott Matthew Small has reached the milestone of 10 years of service.
Feng Yuan has reached the milestone of 15 years of service.
Paul Fallon has reached the milestone of 30 years of service.
Grazyna Odyniec has reached the milestone of 40 years of service.
Congratulations to all of you for your achievements!