Nuclear chemistry: an essential nuclear science

ABSTRACT

Nuclear chemistry is a small discipline that applies chemical methods to studies of the nucleus. Although there is substantial overlap with other fields (most notably nuclear physics, radiochemistry, and nuclear engineering), there are distinctive features of nuclear chemistry in either training, methods, or objectives that differentiate the field. Chemical methods often provide unique insights into fundamental nuclear science questions. Materials chemistry in particular plays an important role in studies of nuclear reactions and structure, but does not receive much recognition. Separations chemistry is also of great use in simplifying and separating complex nuclear reaction products. Despite this impact, nuclear chemistry is on an unsustainable path, and is facing existential challenges due to the scarcity of opportunities for study. This article discusses the essential role chemistry performs in nuclear science, providing definitions and historical context to a small but important field.

KEYWORDS:

• Academic disciplines
• nuclear chemistry
• nuclear physics
• nuclear science
• radiochemistry

1. Introduction

Chemistry may be the central science, but the role of chemistry in nuclear research is commonly underappreciated. As a discipline, nuclear chemistry bridges several technical areas focused on energy and structure of matter, similar to chemistry-at-large. Nuclear chemists provide experience and novel solutions that enable breakthrough nuclear research, by utilization of both nuclear and chemical aspects of the field. The role of nuclear chemistry is thus essential to nuclear research.

This article will discuss the field of nuclear chemistry and the challenges it faces. It will first define and distinguish nuclear chemistry from related sub-disciplines in science and engineering. Some of the impact that materials and separations chemistry have on fundamental and applied nuclear science will be briefly discussed. Finally, an assessment of the state of nuclear chemistry opportunities in the United States will be offered, with suggestions for addressing the challenges that the field faces.

2. Distinguishing nuclear chemistry

Most chemistry research does not feature much interaction with the atomic nucleus and focuses on electrons. Little is taught about the nucleus in most chemistry courses, other than the basics of its composition, and (perhaps) the nuclear forces that hold it together. The most common nuclear interactions that non-nuclear chemists concern themselves with are found in nuclear magnetic resonance spectroscopy or the kinetic isotope effect. It is therefore easy to assume that chemists have nothing to offer the study of the nucleus, but this assumption is in error.

Nuclear chemistry is traditionally a generalist area of inquiry, requiring skillsets and interests that transcend traditional scientific disciplines. Nuclear chemists must be fluent in chemistry and nuclear physics alike, as well as conversant with recent developments in nuclear engineering to ensure practical relevance. Researchers in the field often come from a traditional chemistry background (a BS or BA in Chemistry or similar) but they apply this knowledge to nuclear-specific research, particularly in energy regimes where the nucleus is still a coherent entity. Older definitions of the field defined nuclear chemistry by focusing on nuclear composition, changes, and structure, in parallel to other areas of chemistry.^[1] Indeed, early studies that probed the energy released in nuclear reactions and decays adopted language and methods of physical chemistry. Measurements of energy released in nuclear decays are still referred to as “Q-values,” as many of the earliest studies relied on calorimetry.^[2–4] The typical modern definition of nuclear chemistry is thus research that may include use of chemical methods or training to study the nucleus and its properties.^[5] While most nuclear chemists do not work exclusively to this end, it is the unifying feature of the field.

There are several areas of chemical and physical science that are commonly clustered together due to overlapping interests in chemistry and radiation, but which are nonetheless distinct areas of inquiry from nuclear chemistry. Some of these areas are described below, organized approximately by their proximity to nuclear chemistry. These areas are further summarized in Table 1.

• Nuclear physics is the field of physics concerned with nuclear phenomena. Its historic definition was defined by a focus on exact solutions to few-body systems, contrasted with the many-body systems studied statistically by nuclear chemistry. In the modern day, there often is very little difference between the two, other than training and chemical tools that are used infrequently by nuclear physicists. Together, the fields are referred to as nuclear science.^[6]

• Nuclear engineering is the discipline of engineering concerned with developing practical technologies from nuclear phenomena. It is distinguished from the other fields described by its objectives, training, and by being an engineering discipline. The work done by nuclear engineers is vital to the development of new nuclear knowledge, and nuclear science works closely with the field. These fields are sometimes collectively called “nuclear science and engineering.” Nuclear research is thus a triple point, where chemistry, physics, and engineering broadly overlap, and advances in complementary areas are necessary to advance the third.

• Radiochemistry is commonly conflated with nuclear chemistry, but this is an oversimplification that confuses the fields. The typical description of the field is a near inverse of the definition of nuclear chemistry, as radiochemists use radiation and physics techniques to study chemical processes, often for nuclear technologies or medicines.^[5] The tools developed by radiochemists are aptly called “radiochemical methods,” although they are broadly applied in other areas, including nuclear chemistry.

• Radiation chemistry is a related field, but that again has quite different goals and is distinct from radiochemistry. Radiation chemists apply radiation (nuclear or otherwise) to change the studied chemical environment. While radiation-induced chemical degradation commonly occurs in radiochemistry, it is rarely the intent, and is often considered a confounding factor. Radiation-induced radical formation can produce unusual or exceptional chemistry and change redox potentials,^[7] or create transient chemical species that play important roles in bulk corrosion. Such understanding has broad relevance for nuclear technologies and nuclear engineering.^[8]

• Actinide chemistry is a specialized form of inorganic chemistry with vital implications for nuclear research, but it is distinct from nuclear chemistry.^[9] All known isotopes of the actinide family are radioactive, but some occur in nature due to long half-lives. Actinide chemists use many of the same tools as nuclear and radiochemists, but are differentiated by objectives. The nuclear properties of the actinides is only incidentally related to the goals of the research in actinide chemistry, rather than the focus. The most common definition of actinide chemistry is not the study of radiation or nuclear properties of these elements, but rather the study of fundamental or applied chemistry of these elements.

Table 1. Discussed disciplines summarized per definitions given in the text.

Area of Inquiry	Primary Phenomena of Study	Methods	Typical Training
Nuclear Chemistry	Nuclear Phenomena	Chemical and Physical	Chemical
Nuclear Physics	Nuclear Phenomena	Physical	Physical
Nuclear Engineering	Nuclear Applications	Engineering	Engineering
Radiochemistry	Chemical Phenomena	Chemical and Physical	Chemical
Radiation Chemistry	Chemical Phenomena	Chemical	Chemical
Actinide Chemistry	Chemical Phenomena	Chemical	Chemical

While these fields often use similar tools, their principal objectives and training ensure they are nonetheless distinct. Nuclear chemistry is particularly of value when examining novel nuclear phenomena, where chemical intuition and understanding can enhance observations. As such, nuclear chemists are often found in nuclear physics or nuclear engineering departments, or at facilities that generate nuclear reactions such as nuclear reactors. Radiochemists and actinide chemists have a larger population in chemistry departments than nuclear chemists, but they are commonly found in engineering departments as well. Over the last few decades, it has been increasingly rare to find nuclear chemists in chemistry departments, but chemistry skills remain vital for further inquiry into nuclear science.

3. Chemistry that enhances nuclear science

Nuclear science is popularly misunderstood to be a physics discipline, and is often contrasted with other areas of chemistry that focus on electron behavior. This oversimplification is perhaps understandable, as nuclear physics is a much larger field with far more public awareness. The historic and modern role of chemistry in advancing fundamental nuclear science is therefore somewhat lost without explicit acknowledgment.

Nuclear science has addressed, although incompletely thus far, many questions about the nature of the universe’s composition, one of the most fundamental questions of science. These open areas of inquiry aim to reduce uncertainties that feed into models of diverse natural phenomena. Outstanding uncertainties impact the incredibly vast synthesis of the chemical elements in astrophysical phenomena such as neutron stars, to the atomic scale individual atoms of rare isotopes produced in particle accelerators. Our finite understanding from repeated measurements is in many experiments not limited by the physics or apparatus, but rather by the influence chemistry has on the results. Two areas of chemistry – materials chemistry and separations chemistry – are particularly important for nuclear science, and their influences are briefly presented below.

3.1. Materials chemistry and nuclear science

Low-energy nuclear reactions typically have two principal components: a static target and a source that irradiates the target with a projectile. With modern irradiation facilities, radiation detectors, and detector electronics all reaching exceptional precision, the quality of a nuclear physics study often depends primarily on the quality of the target. Targets must be of clearly defined chemistry, geometry, and physical homogeneity. Inhomogeneities (physical or chemical) in a nuclear reaction target often limits the uncertainty of the obtained results to that same order of magnitude or worse. These targets are largely a “black box” for chemists and physicists alike, with only rudimentary compositional understanding. It is perhaps understandable why this is the case. The development of a new method to make a uniform nanometers-thin film of a rare and often radioactive substance is somewhat of a niche interest to most chemists. Unless the research is applicable to the semiconductor industry, these questions are unusual for most chemists.

Thin deposits of highly reactive metals (such as the actinides) have typically relied on decades-old methods that deposit matter with an unknown composition and excess “crud” that adheres to the desired material. Alternative deposition methods, like physical vapor deposition, can provide high uniformity without crud, but at the cost of very inefficient material use, making some experiments simply infeasible due to material limitations.^[10,11] Exceptional efforts are being made among the small nuclear targetry community to use inkjet printing^[12] or other technologies to produce targets with high efficiency and known composition, but these are not yet general methods. It is far more common to have an empirical recipe used by a particular laboratory, that, having been developed long ago, is adhered to because it works. These recipes have compromises, uncertainties, and present a major problem for nuclear science.

If a nuclear physics lab produces targets, it is most often produced by a skilled technician who works by proven methods to provide a target. It takes substantial time and effort to make a new target even by established methods. The risks and costs associated with developing a new method provide little incentive for individual physics labs to develop new methods unless absolutely required for the experiment to succeed. Such cookbook syntheses are known to work, but often lack the detail needed for high resolution studies. As the nuclear data community seeks higher resolution studies into diverse phenomena, the limiting factor will often be the materials chemistry of the target, yet relatively little research is being done to address these uncertainties.

For example, if a physicist wishes to do a knockout experiment (a projectile knocks out a part of the target’s nucleus, a method that informs understanding of the nucleus’s structure), chemical composition of the target plays a dominating role in the data collection and analysis. Every element has a different ability to slow down the particle beam, a different “stopping power,” which varies with the energy and element of the beam. If the target composition and associated stopping power is known for each component of the target, the energy loss of the beam and outgoing charged projectile in the target can be accounted for and the data corrected. Inhomogeneities in the distribution of the material will obscure the scattering angles and blur the energy losses, while crud or other contaminants will systematically skew the data by an unknown amount.^[11] As we seek refined knowledge of nuclear structure, particularly in the material-limited actinides, target chemistry will become the limiting factor in the data that can be collected.

The last comprehensive measurements of stopping power for a wide variety of target-beams combinations were collected in the 1970s.^[13] The 2023 Second Report of the Nuclear Data Charge Subcommittee of the Nuclear Science Advisory Committee identified stopping power measurements as a key experimental gap that requires addressing. Outside of a single study with uranium, no stopping power measurements have been made with actinide targets and proton beams, while heavier projectiles have no points of comparison.^[14] A common approach for scientists working with actinide targets is to approximate the target composition using uranium as a replacement for other actinides, a useful but crude adjustment. Without improved chemistry that produces highly-homogenous target with simple chemistry and efficient use of material, high resolution stopping power measurements with transuranium isotopes are a near-impossibility.

It is not easy to make a high-quality target of any kind, by any method, and developing superior methods is a true challenge. Nuclear chemists have the skillset and inclination to address this challenge, bringing chemistry skills to a problem that benefits fundamental nuclear science.

3.2. Separations chemistry and nuclear science

It is more than mere trivia that chemists were the first to identify and describe nuclear fission. In the 1930s, there was no viable physics mechanism to explain the division of a large nucleus, like is found in uranium. Indeed, when famed physicist and Nobel Laureate Enrico Fermi first split the atom, he claimed the production of a transuranium element based on a plausible nuclear reaction and an unknown radiation that was isolated. This was the conventional explanation of his data, and he had compelling physical evidence for such a transformation. Nuclear chemist Ida Noddack (co-discoverer of rhenium) suggested that the chemical evidence for a new element was scant, and that lighter elements may have been produced instead, as Fermi had not tested elements lighter than lead, as such an outcome was considered implausible.^[15] Studies by the nuclear chemists Otto Hahn and Fritz Strassmann attempted to study the chemistry of this “new element,” but instead confirmed the production of the much lighter element barium using separations chemistry. This indicated a splitting of the uranium nucleus, as 40% of the nucleus was carved off by this reaction. The evidence provided by chemical analysis of the appearance of barium and other lighter elements was indisputable, and nuclear physics had to adapt. The earliest model was provided by Hahn’s nuclear physicist collaborator Lise Meitner, shortly after fleeing Nazi Germany.^[16] Separations chemistry continues to play a key role in modern nuclear science, and it is sometimes the only or best way to obtain meaningful data.

There are ongoing and extraordinary efforts by nuclear chemists to use radiochemical methods in the service of understanding nuclear fission. Overlapping physics observables from the many hundreds of isotopes produced in fission obscure analyses that use radiation detection alone. By separations chemistry, the individual components of the nearly 50 elements (and over 500 isotopes) produced in fission can be isolated and then measured. This herculean task is necessary for understanding a wide range of nuclear phenomena, including models of nucleosynthesis in neutron star mergers that produce fissioning isotopes. Back on Earth, new reactor designs are being developed that rely on different fission mechanisms, with very different fission fragment distributions, and thus different engineering and nuclear waste considerations. Without further resolution of fission fragment production and covariant observations, a generalizable model of fission remains elusive and nuclear engineering models cannot be updated.^[17] Such separations studies are also of increasing importance as nuclear science seeks to design consistent, predictive, and useful theoretical and computational tools.

Rapid chemical separations can also provide increased understanding of new nuclear products, such as in the synthesis of super-heavy elements (those with more than 100 protons, which cannot be produced on earth except in a particle accelerator). The discovery of new chemical elements historically required the separation of the new candidate element from all other elements, as Fermi’s new element error failed to do. In the modern day, where individual atoms of the rarest elements are only produced after years-long experiments, unique chemistry is no longer required to identify a new element.^[18] Instead, decades of statistical “nuclear genetics” correlated radioactive decay lifetimes of and energies of parent and daughter isotopes. Nuclear scientists pieced together data about these short-lived species into a coherent picture, but such models were only tentative correlations and inferences. Recent work led by nuclear chemists employed mass spectroscopy to determine the masses of two super-heavy isotopes, moscovium-288 and its alpha-decay product nihonium-284.^[19] In this single experiment, dozens of experiments representing decades of effort were confirmed thanks to chemistry methods. Other experiments have enabled the chemistry of these elements to be studied in the gas phase, both by looking at charge-exchange reactions, and by inclusion of reactive gases.^[19–21] This could enable further separations of isobars by differential reaction chemistry, increasing sensitivity for nuclear reaction studies.

Synthesizing such short-lived isotopes requires months of irradiation with billions of projectile ions sent through targets every second. Most ions do nothing other than lose energy and heat the target. In a rare (roughly one per billion ions) collision, the projectile will partially fuse to a target nucleus but then break apart faster than necessary to form an electron cloud (roughly 10⁻¹⁵ seconds). The real objective of such experiments is much less probable still, as roughly one per billion of these collisions will result in a surviving (not breaking apart) compound nucleus. In the search for new super-heavy elements, these experiments have become more time-consuming and expensive, as 10^[17,18] ions were required to produce even a singular super-heavy atom of moscovium for mass spectroscopy.^[19] This is worthwhile science, however, as such efforts are remarkably useful tests of our physical and chemical understanding, providing increasingly extreme environments to delineate differences that are subtle in other elements. Unfortunately, the physics underlying these experiments increase in improbability by roughly an order of magnitude for each additional proton. It thus becomes dramatically more expensive to expand the periodic table, much less to synthesize sufficient sample sizes to enable discriminating chemistry.

A different physics mechanism for producing many new isotopes at once, multi-nucleon transfer reactions (MNTR), may provide an alternative approach. Where current fusion reactions produce only a few isotopes of a single element with a very low probability, MNTR has a very high probability, but produces potentially dozens of isotopes for each of tens of elements simultaneously, including more massive isotopes.^[22] The radioactive signals produced by such reactions are overwhelming and overlapping, potentially paralyzing analysis, although a few studies have attempted to clarify these products using physics methods alone.^[23] Use of chemistry, as in past decades,^[24–27] to quickly sift through MNTR products may both improve the models underpinning MNTR and identify new elements and isotopes, enabling new investigation of extreme nuclear and atomic matter.

Separations chemistry can also improve understanding of more applied nuclear reactions. Medical procedures that benefit from radioactivity are now conventional techniques, with even small and midsized city hospitals often featuring entire nuclear medicine departments. Most nuclear medicine procedures in the United States use only one of a few different isotopes (predominantly fluorine-18 and technetium-99 m for diagnostics),^[28] but nuclear chemistry is being employed to develop new prospective diagnostic and therapeutic isotopes. The nuclear reactions that produce these isotopes often result in several simultaneous products, and clever target and separations chemistry are required to evaluate the products. The skillset required for these studies (and others) is of great value, but the field is not growing at a rate that approaches the demand.

4. Nuclear chemistry’s declining numbers

In these examples, the essential role of chemistry for the advancement of nuclear science and nuclear technology has been described, but nuclear chemistry is a field struggling to survive. Much of science is, and must always be, composed of specialists. Individuals who focus on a narrow set of problems and become expert with a particular method are of course common in science and play key roles in advancing our understanding of nature. There remains, however, a vital need for generalists who bring diverse skillsets and who connect the different regions of science. Nuclear chemists are very often generalists, bringing chemical intuition and methods to problems most often posed by physics. In this role, they can perform an integrative function. This virtue is confusing to some, and nuclear chemists are thus often caught between fields, without a clear home among the sciences.

As a junior nuclear chemist working in a College of Engineering, when I discuss the breadth of my research, chemists wonder why I am not in physics, and physicists query why I am not in chemistry. The novel work done in nuclear chemistry labs around the world is in a blind spot for chemistry and physics departments alike. Chemists see such work as for the benefit of physics rather than chemistry, while physicists see the approaches as chemistry and foreign (or at least unorthodox). As we push the limits of our current technology and ask more detailed scientific questions, there are more nuclear challenges awaiting that will benefit from chemistry. Nuclear science will require further graduates with this expertise to move forward, but opportunities for such training are perilously limited.

There are laudable efforts in the United States to train undergraduate students in nuclear chemistry. I particularly acknowledge the annual summer school sponsored by the Department of Energy through the Division of Nuclear Chemistry and Technology of the American Chemical Society (NUCL-ACS). This program, providing both college credit and a study stipend, instructs two dozen students per year. Students gain a basic knowledge of both nuclear chemistry and radiochemistry, with laboratory experiences, field trips, and opportunities to meet with faculty in the field.^[29] Programs into related specialties (for example: radiochemistry, the nuclear fuel cycle, and nuclear forensics) have lapsed in the past decade, and it is fortunate that the nuclear chemistry summer school has survived for nearly four decades. These students are more important than ever, as the number of nuclear chemists has decreased.

A recent workforce analysis of nuclear chemistry and radiochemistry (combined) found that, from 2011-2022, 55 PhDs were awarded in the United States, an increase over previous periods.^[5] Enthusiasm must be tempered, however, as most years see over 2000 PhDs in Chemistry and over 1500 PhDs in Physics awarded, suggesting that the combined disciplines of nuclear chemistry and radiochemistry typically graduate less than 0.25% of all Chemistry PhDs.^[30] The field remains, “small and fragile,”^[5,31] with fewer than forty institutions of higher education offering formal training in nuclear or radiochemistry.

It is becoming less and less common to see nuclear chemists in chemistry departments, as more faculty retire and universities do not replace them. This trend can be seen by a brief survey of the self-selected faculty listed on the NUCL-ACS graduate faculty website. While this list is incomplete, it is illustrative of the broader trends among faculty chemists who work with nuclear radiation.^[32] Faculty were categorized into three fields: actinide chemistry (23 faculty), radiochemistry (17), and nuclear chemistry (17), based on their publications and the definitions of the fields provided earlier in this work. (Two faculty were excluded, as they could not be readily categorized by this method.) Figure 1 shows the trends in the areas of research among these faculty, organized by the year in which they received their PhD. The 57 categorized faculty are affiliated with 31 universities in the United States and Canada, with 15 universities sponsoring actinide chemistry, 13 supporting radiochemistry, 11 supporting nuclear chemistry, and 4 supporting two or more categories. 19 universities have hired faculty who received their PhD in the last 20 years (since 2004), with three institutions hiring such nuclear chemists in this timeframe. Only two universities (three hires at Michigan State University, and one at Texas A&M University) have these more junior nuclear chemistry faculty in their Chemistry Departments. The opportunities for students to study nuclear science in a chemistry department have therefore dwindled substantially, a development with no clear reversal in sight.

Figure 1. A comparison of the years that faculty listed on the NUCL-ACS webpage were awarded their PhDs, by sub-field of chemistry as defined in this work. Data were collected by surveying the faculty webpages and/or by the publication date of their dissertation. Data are binned on a half-decadal basis. The bar graphs are on the scale to the left y-axis, while the cumulative timeline is scaled to the right.

4.1. Investing in nuclear chemistry

Recent efforts at solving the nuclear and radiochemistry workforce problem (such as the 2011-2021 SUCCESS PIPELINE^[31]) have focused on directing graduates of these programs into the national laboratory system. There is an ongoing and rapidly increasing demand for national laboratory PhD hires, both to replace retiring experts and to fill new positions, but graduates are not being produced at the rate that is necessary to meet either demand. It seems reasonable, considering the faculty hiring patterns in Figure 1, to suggest that the bottleneck is not interest in these positions, but is rather training capacity. As more faculty are retiring and fewer are being hired, remaining faculty are being relied upon to train more students. Programs to incentivize universities with graduate programs in chemistry or nuclear science to hire and train new nuclear chemists would address this bottleneck and provide more candidates for growing workforces.

With so few new hires in nuclear chemistry, PhDs seeking faculty positions face a hard job market. Funding programs aimed at new graduates to “write their own ticket” into postdoctoral research positions in nuclear chemistry should be developed, both to provide further training, and to enhance competitiveness for future faculty job applications. Such a program could also match chemistry graduates with nuclear physics faculty, and nuclear physics graduates with chemistry faculty to supplement the small number of available nuclear chemist mentors. This would aid the transition from graduate school, but positions would still need to be made available to then make use of this greater capacity for training.

Programs should also be established to encourage chemistry departments to hire nuclear chemistry faculty, perhaps by supplementing startup funds and guaranteeing student fellowships for these hires over a set number of years. Funding for courses in nuclear chemistry and related fields would be appropriate as well, enabling more undergraduate students to experience the field than the few dozen per year that currently receive such instruction. This faculty-forward approach would provide opportunities for invention and novelty, all while increasing new scholarship. With increased faculty numbers, more students will be graduated per year, addressing the workforce deficit, and providing for sustainability in the field. Without intervention, there is a real risk that nuclear chemistry will become a defunct discipline over the next generation.

5. Conclusions

Nuclear research is inherently interdisciplinary, with important contributions from all areas of science and engineering. As nuclear science seeks to refine understanding of the atomic nucleus, nuclear chemistry must play a vital role. The impact of materials chemistry and separations chemistry on nuclear science were briefly discussed, both in the historical context and the modern day. A brief discussion of the hiring patterns of chemists who belong to the Nuclear Chemistry and Technology Division of the American Chemical Society was produced, highlighting current challenges to the field. The development of new post-graduate training and faculty job opportunities was suggested, with a brief justification of their potential value on the nuclear workforce more generally.

Acknowledgments

The author acknowledges a startup package from Oregon State University for this work, and thanks the many colleagues with whom he discussed the ideas presented here.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by a startup package by the Oregon State University College of Engineering.