Signs of an exotic atom-like system made up of a neutral meson bound to an atomic nucleus via the strong interaction have emerged in experimental data from two international collaborations. If confirmed, this hitherto unobserved system could shed light on the origins of hadron masses and provide new insights into the fundamental symmetries of quantum chromodynamics in nuclear matter.

The strong interaction is one of the four fundamental forces of nature, alongside gravity, electromagnetism and the weak interaction. It is responsible for binding quarks into hadrons, which are three-quark particles such as protons and neutrons, and for holding protons and neutrons together within atomic nuclei. Electrically neutral mesons – short-lived particles made up of a quark and an antiquark – are likewise subject to the strong interaction, which can bind them to atomic nuclei in a way that is conceptually similar to an electron bound to a nucleus by the electromagnetic force.

Studying these meson-based nuclear systems is important because it helps us better understand the properties of the strong interaction, says study co-leader Yoshiki Tanaka of RIKEN in Japan. The eta prime meson, η′, is particularly interesting, Tanaka adds, because its relatively large mass cannot be explained by a simple quark model. “This U(1) problem, as it known, was raised as long ago as the 1970s by the physicist Steven Weinberg,” he notes.

Direct experimental access to the 𝜂′-meson mass in nuclei

Modern theories attribute the η′ meson’s large mass to the presence of chiral symmetry breaking in quantum chromodynamics, which is the fundamental theory of the strong force. These theories predict that this mass should be reduced in a nuclear system, and this is what Tanaka and colleagues set out to test.

“Spectroscopy studies of 𝜂′-mesic nuclei provide direct experimental access to the 𝜂′-meson mass in nuclei and offer a unique opportunity to investigate the underlying mechanisms of how the mass of hadrons comes about,” he explains.

In the team’s study, a beam of protons strikes a ¹²C atomic nucleus at near-relativistic speeds and removes a neutron from it. This neutron, together with a proton, forms a deuteron that propagates away in a forward direction, leaving behind a nucleus of ¹¹C in a highly energetic state. It is this excess energy that gives rise to an 𝜂′-meson.

In rare cases, the researchers explain, the meson then binds to the ¹¹C nucleus, forming an 𝜂′-mesic nuclear system. But because these events are so rare, they are hard to find. “One of the major challenges we encountered in the work was the very large amount of background events we registered during our measurements,” Tanaka recalls. “These were about 100 to 1000 times higher than the signal events.”

The researchers overcame this problem by developing a new experiment that allows them to efficiently select signal events associated with the formation of 𝜂′-mesic nuclei by “tagging” the particles they decay into. This enabled them to measure not only the forward-travelling deuteron, but also the decay products of the short-lived 𝜂′-mesic nuclear state.

The researchers say that their results, which they describe in Physical Review Letters, indicate that the 𝜂′-meson mass drops by about 60 MeV in nuclear matter. “This result qualitatively supports the theoretical scenario [that attributes] the origin of the 𝜂′-meson mass to chiral symmetry breaking together with the dynamics of gluons (massless particles that mediate the strong nuclear force) in general,” Tanaka says.

Members of the team, which also includes researchers from the η-PRiME Collaboration and the Super Fragment Separator Experiment Collaboration, together with physicists from Justus Liebig University Giessen in Germany with their working groups GSI/FAIR, say they are now planning follow-up experiments to confirm that they have indeed observed 𝜂′-mesic nuclei. “We also aim to increase the significance to the 5σ level, which is required to firmly establish the discovery on new quantum states in particle and nuclear physics,” Tanaka says.

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The discovery of X-rays and radioactivity in the late 19th century gave rise to a surge of interest from the scientific community, shortly followed by the realization of the adverse effects of ionizing radiations on health. By about 1910 the dangers were widely recognised and some basic protection measures were being adopted. It was not until 1934, however, that the first quantitative standards of radiological protection were published.

Of course, protection against the adverse effects of ionizing radiation is as important today as ever, particularly for those working within nuclear and defence-related industries, medicine and R&D, as well as hospital patients undergoing radiation-based procedures and members of the general public. As such, the last century has seen the development of a complex international regulatory system, with recommendations on occupational and public exposures to radiation – from organizations such as the International Commission on Radiological Protection (ICRP) and others – continually revised and updated.

A new book, Principles and Techniques of Radiological Protection, provides a comprehensive overview of the current regulatory context for radiological protection. The text also provides an overview of the scientific issues relating to radiological protection and the current state-of-the-art tools used to comply with the relevant legislation and guidance.

Targeted at postgraduate students and new entrants to the field, the textbook is designed to cover a wide range of topics that an early-career radiation protection professional might need, or want, to know about. It also serves as a day-to-day reference work for specialists such as radiation protection advisors (RPAs) to identify appropriate techniques to address radiological protection issues as they arise.

“I aimed to produce a book that I would have liked to have had available when I started work in radiological protection just over 50 years ago,” explains the book’s editor Michael Thorne. “As I come towards the end of my career in the field, I aimed to include information, tools and techniques that I would have liked to have had readily accessible in a single volume.”

History, theory and practical applications

Thorne begins the book with a brief history of radiological protection and how historical developments continue to influence the discipline today. The next chapters examine the physical aspects of radiological protection, including an overview of basic nuclear physics and the sources of radiation, radiation transport through and interactions with matter, and the instruments used to detect and monitor radiation. Later chapters cover the principles of internal dosimetry, phantoms and biokinetic models, and mathematical modelling of radionuclide transport.

“I have also given a detailed account of natural background radiation and modelling the transport of radionuclides in the environment; and I have included a chapter on the effects of radiation on the environment, with specific emphasis on non-human biota,” says Thorne. “Throughout, I have recruited co-authors with decades of relevant experience to capture their expertise in each of the specialized areas.”

The book also provides examples of how this information is employed practically within various fields, including the nuclear industry and industries handling naturally occurring radioactive materials. Several chapters and themes are of particular relevance to those working within medical physics.

“There are two chapters specifically on radiology and nuclear medicine, written by Colin Martin, who is well known internationally for his work in this area,” Thorne tells Physics World. “There are also specialized chapters on biokinetic modelling, the nature and use of both mathematical and physical phantoms in radiation dosimetry, and on the use and abuse of instruments for radiation monitoring.”

The book rounds off with a look at the some of the major and minor accidents that led to exposure of members of the public and workers using radioactive sources. The final chapter addresses emergency planning and response for such incidents, including suggested protective actions and the roles and responsibilities of various organizations.

“Throughout, the emphasis is on broad principles and widely applicable techniques,” says Thorne. “It is considered that an individual who gains a clear understanding of these principles and techniques will be readily able to apply that understanding to the diverse and changing set of challenges that arise.”

• Individual copies of Principles and Techniques of Radiological Protection can be purchased at the IOP Publishing Bookstore.

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