Alberto Mengoni, Spokesperson for n_TOF at CERN, discusses the facility and how it utilises unique neutron beams to understand our Universe, advance nuclear technologies, and explore applications in fields such as nuclear medicine and material science.
n_TOF is a neutron time-of-flight facility established at CERN, inspired by the vision of Carlo Rubbia in the late 1990s and early 2000s. The facility, which has been operational for about 25 years, with the first beam produced in 2001, produces several neutron beams that are used in the study of neutron-induced nuclear reactions. A primary objective of the research activities at n_TOF is to understand the origins and formation of chemical elements in stars. There is a delicate balance between the formation and destruction of these elements during the various phases of stellar evolution, governed by their interactions with neutrons. The study of neutron-induced reactions is also a key to innovations in advanced nuclear technologies.
Our Collaboration consists of approximately 150 researchers, with around 30 PhD students participating each year, representing 40 research teams from various institutions. n_TOF is relatively large and active and stands out among the neutron research facilities worldwide. While most participants are based in Europe, we also have teams from Japan and the United States contributing to our efforts.
n_TOF’s objectives
n_TOF’s goals can be categorised into three main areas:
Nuclear astrophysics
Advanced nuclear technologies
Various applications, including nuclear medicine and material science
Our focus on nuclear astrophysics examines the processes involved in element formation in the cosmos and the nucleosynthesis of chemical elements in stars. Many elements are formed through interactions with neutrons within stars, and studying this process necessitates conducting experiments that focus on the interactions between neutrons and various isotopes.
The second focus is to provide insights for advanced nuclear technologies. This includes developing accelerators and devices that utilise nuclear processes, as well as exploring applications in energy production and nuclear medicine. Many innovations and advancements in these technologies depend on understanding neutron interactions, which are essential for the operation of nuclear reactors, where the neutrons generate nuclear fission and energy.
The types of accelerators employed to produce neutrons vary based on their intended purposes. Some are designed to produce neutron beams to study biological systems or materials science, while our focus is on nuclear physics. Consequently, the characteristics of our neutron beam produced by one of the CERN accelerators – in terms of energy resolution and neutron intensity – are truly unique.
Firstly, it offers an exceptional energy range. The facility can produce neutrons with kinetic energies across an impressive spectrum, from milli-electronvolts (meV) to giga-electronvolts (GeV), spanning 12 orders of magnitude with a single beam line. This capability is unmatched, with no other facility providing such a wide energy range.
Secondly, n_TOF utilises a specific type of accelerator known as the Proton Synchrotron (PS), and directs protons from the PS toward a target, where their interactions generate neutrons. Specifically, we use bunches of 20 GeV protons of extremely high intensity, separated by intervals of 1.2 seconds or more. Most standard neutron facilities use different types of accelerators that operate at lower energy levels with higher repetition frequencies. The latter can be a strong disadvantage in time-of-flight experiments.
Neutrons and their unique advantages
When we talk about fundamental physics today, we typically refer to particle physics. There are, however, aspects of neutron physics that are indeed fundamental as they relate to properties that concern fundamental physics. For example, the fundamental properties of the neutron, such as its decay or its electric dipole moment, represent fundamental physics. These topics are being explored through other methods in particle physics experiments and are not the focus of our work at n_TOF.
What we are doing with neutrons at n_TOF, ultimately, focuses on nuclear physics, specifically the study of nuclear reactions and nuclear structure.
The most significant advantage of neutrons is that they are neutral particles. They have no electric charge, therefore allowing us to explore the properties of nuclei without interference from electromagnetic interactions, particularly the Coulomb interaction, which complicates things significantly. Fast neutrons (neutrons with high kinetic energies) are essential for understanding nuclear properties because they can penetrate materials and directly interact with atomic nuclei. In this way, we study the properties of the nuclei that make up the materials rather than the macroscopic properties of the materials themselves.
Neutrons can also be utilised to examine the global properties of materials, as seen at facilities like the European Spallation Source, the American Spallation Neutron Source, or other facilities in Japan and China. These centres are more interested in studying biological systems or macroscopic material systems rather than focusing on nuclear physics and nuclei. In these cases, the neutrons are often very low-energy, typically cold or ultra-cold neutrons. When dealing with cold neutrons, their wavelengths become large enough to investigate the properties of the atoms and atomic arrangements within the material, rather than the properties of atomic nuclei.
Key projects and experiments
In nuclear physics, experiments tend to be on a smaller scale compared to those in particle physics. Particle physics experiments such as the ATLAS and CMS collaborations are extensive, each yielding many aspects, activities, and outcomes from a single experiment. On the other hand, we conduct many smaller experiments, each with its own distinct purpose and characteristics. At n_TOF, we have performed over 150 individual experiments over the past 20 years.
One ongoing experiment focuses on understanding the neutron interaction with calcium-41, which is a radioactive isotope of calcium with a half-life of 100,000 years. There is evidence that calcium-41 existed at the formation of the solar system. Although calcium-41 is no longer detectable today due to its radioactivity, understanding both its formation and destruction, in addition to decay, is crucial for comprehending the origins of our solar system. We can infer its presence by examining the abundance of its decay product, potassium-41. This field of study is known as nucleosynthesis, which aims to explain how the chemical elements we observe now were formed and originated. In the specific experiments we conduct, we analyse neutron interactions with calcium-41, which have led to its destruction in stellar environments.
We also recently investigated potassium-40, which, while nearly stable, has a very long half-life of 1.3 billion years. Potassium-40 is significant because, along with uranium and thorium, potassium is one of the elements contributing to the Earth’s internal heat generation – not only from solar radiation but also through its radioactive decay. Initially, during the formation of the solar system, the decay of potassium was the primary source of this internal heat. Understanding potassium’s formation and decay is essential for discerning how planets form – not just Earth, but also exoplanets, as the internal heat of these planets can be largely attributed to potassium.
Real-world applications: Advancing energy and medical research
n_TOF places a considerable focus on developing nuclear technologies, conducting numerous experiments that provide critical nuclear physics information and nuclear data.
For example, we have performed experiments on neutron interactions with plutonium—specifically, plutonium-239. Plutonium-239 is known for its use in nuclear fission; it serves as fuel for nuclear reactors. While the interaction of neutrons with plutonium is well understood, it still requires more precise measurements for the safe operation of nuclear reactors. This is part of our ongoing research, which also includes experiments involving uranium, thorium, americium, and other actinide nuclei. Our research is entirely public; the results are regularly published, with approximately 95% of our recent publications available open-access (with no charged-subscription to journals where it is published). We are committed to advancing nuclear technologies exclusively for peaceful purposes, emphasising their proper use in energy generation and scientific inquiry.
Shifting to applications in the medical field, there are several nuclear technologies utilised in medical science. One notable example is boron neutron capture therapy (BNCT), which is related to cancer treatment using neutrons. The basic concept of BNCT is that cancer cells can be targeted with radiation. Traditional therapies often use X-rays to bombard and destroy cells, but in BNCT, a specific drug containing boron is injected into the cancer cells. When exposed to neutrons, the boron interacts and produces radiation that effectively kills the cancer cells. While the concept of injecting a target into the cancer cells to facilitate their destruction is straightforward, it is essential to understand how neutrons interact not only with the drug but also with the surrounding biological materials, such as nitrogen and chlorine, that exist in our bodies.
To evaluate potential collateral damage from neutron treatments, we have to understand neutron-induced nuclear interactions with various materials. This is a critical aspect of our research at n_TOF, as we provide accurate nuclear data that supports the development of effective therapies using nuclear radiation. We have conducted experiments on materials such as nitrogen-14, chlorine-35, sulphur, and carbon, focusing on dosimetry studies and the sensitivity of healthy tissues to neutron radiation.
Applications in astrophysics
An interesting example is the nucleosynthesis of the Big Bang. We know that our Universe began with the Big Bang, and the early moments of the Universe, specifically during the first extremely small fractions of a second, are particularly fascinating. At this stage, significant fundamental physics events occurred (the domain of astro-particle physics. After a few seconds to a few minutes after the Big Bang, nuclear physics took over as the first nuclei began to form.
Initially, we had protons and neutrons, which interacted to create chemical elements. Interestingly, the conditions present during the Big Bang allowed for the creation of only three elements: hydrogen, helium, and a very limited quantity of lithium. However, the expected amount of lithium does not align with our observations, giving rise to what is known as the cosmological lithium problem.
One proposed explanation is that there might be a mechanism involving neutrons that destroys lithium, or more specifically, beryllium-7, which is the precursor to lithium. Therefore, we initiated research on neutron interactions with beryllium-7 through an experiment known as the neutron and alpha reaction of beryllium-7.
However, the results of our experiment indicated that this was not the case. It turned out that neutrons cannot destroy enough beryllium-7 during the Big Bang nucleosynthesis to account for the observed lithium deficiency. Thus, the proposed mechanism was ruled out, as our findings showed that the neutron interactions with beryllium-7 were insufficient to cause significant destruction of beryllium-7.
This is one example of fundamental science that we have achieved at a high level. Another significant example comes from astrophysics, specifically regarding the age of the Universe. Current estimates suggest that our universe is approximately 13.8 billion years old. But how do we know this?
There are three different methods to estimate the age of the Universe. The first method is the cosmological approach. By examining the rate of expansion and tracing it back to the origin, we can estimate how long the Universe has existed. This method, based on observations of the cosmic microwave background radiation and other aspects of modern cosmology, is highly accurate and gives us an estimate of 13.8 billion years.
However, if our understanding of cosmology changes, the validity of this age determination could be affected. Throughout history, we’ve observed various cosmological theories, so the accuracy of this approach hinges on the correctness of the current model.
The second method to estimate the Universe’s age involves determining the age of stars or groups of stars using globular clusters to assess the age of our galaxy. It is generally accepted that stars and galaxies began to form relatively shortly after the Big Bang, possibly just a few hundred million years later. By knowing the age of these stars or globular clusters, we can approximate the Universe’s age. This method has also produced results consistent with those from cosmological models.
The third method for determining the age of the Universe is rooted in nuclear physics. This approach utilises what are known as ‘cosmic clocks,’ which are based on certain atomic nuclei that have long half-lives, often spanning billions of years. At n_TOF, we conducted an experiment using one of these atomic clocks, known as the Rhenium-Osmium clock. By analysing the abundance of these nuclei in the Universe, scientists can estimate how long they have been present. The advantage is that this method provides an independent estimate of the age of the Universe that does not rely on cosmological models or on astronomical observations of stars.
Identifying the challenges in nuclear physics research
One of the most significant challenges in nuclear physics is the understanding of unstable nuclei. While we can study stable or near-stable nuclei, such as certain isotopes of gold, many isotopes are unstable and exist for only short periods – sometimes only hours, minutes, seconds, or even milliseconds. Studying the properties of these short-lived nuclei is complicated because they do not last long enough for easy experimentation in the laboratory. The Isotope mass Separator On-Line facility, ISOLDE, is focusing on experimental studies of exotic nuclei, which are short-lived and therefore difficult to analyse, and we also contribute to this work to some extent.
An aspect to consider in our actions is research policies and their impact on nuclear physics activities. Several laboratories in Europe, including CERN, are involved in nuclear physics research. The future of nuclear physics at CERN is a topic up for discussion in the upcoming years, especially regarding whether CERN will host the Future Circular Collider (FCC) or other particle physics machines. This decision is interconnected with nuclear physics because nuclear research at CERN relies on specific machines of the accelerator complex: the injectors. If there is a complete overhaul of the chain of accelerators at CERN, including changes to the injectors, this will inevitably affect nuclear physics research. n_TOF currently has a scientific programme and facility that will certainly operate until LS4. What follows is linked to CERN’s broader vision for the future.
The evolving role of neutrons in science
The challenges we face in nuclear physics are closely tied to nuclear astrophysics and advanced nuclear technologies. Understanding the nucleosynthesis of chemical elements in explosive scenarios is vital, not only during the stable lifetimes of stars but particularly at the end of their lives, such as during supernova explosions or neutron star/black hole mergers. These events are significant in astrophysics and have profound implications for nuclear astrophysics, as the nucleosynthesis occurring, e.g. during neutron star mergers, resembles that of supernovae.
To comprehend these phenomena, we need to analyse data from astronomical observations, which is rapidly expanding. This influx of data provides insights into stellar operations throughout their lifetimes and requires an understanding of the relevant nuclear physics, specifically the role of neutrons.
There is considerable work to be done in these fields, with many new experiments and ideas emerging. It is crucial that the facility continues its operations, and we are preparing for developments beyond LS4. n_TOF relies on a lead target, which has a lifespan of about ten years. Currently, we are using our third-generation target, having already utilised the first and second generations. The lifespan of this third-generation target coincides with the end of LS4. We have begun implementing the technological aspects for the next target, which will be similar to the current one, as the existing target has been performing very well.
Please note, this article will also appear in the 23rd edition of our quarterly publication.
Source link
