For over five years, a group of scientists from various universities around the world, known as the Majorana Collaboration, have been working together to study a very rare type of radioactive decay called neutrinoless double-beta decay. They used a special detector called the Majorana Demonstrator, which is located almost a mile underground in a laboratory called the Sanford Underground Research Facility in South Dakota. This unique location helps to shield the detector from background noise and interference, making it easier to study this rare phenomenon.
The large-scale experiment conducted by the Majorana Collaboration involved using 30 kilograms of very pure germanium crystals, which were kept in extremely cold conditions using special modules. These crystal modules were then protected by a layer of lead shielding. The final results of the experiment were published in a recent paper in PRL, and they provide new information about the behavior of neutrinoless double-beta decay (0νββ) of 76Ge. Essentially, the experiment helped to set new limits on how this rare form of radioactive decay works, which could help us better understand the fundamental nature of the universe.
“The primary objective of this 30-kg experiment was to establish the feasibility for a much larger experiment using germanium (Ge) detectors for neutrinoless double beta decay (0νββ),” Steve Elliott, one of the researchers who carried out the study, told Phys.org. “Previous work had demonstrated the viability of using Ge detectors for this research. This project was funded by the DOE and NSF after concepts were established using Laboratory Directed Research and Development (LDRD) funds from a number of national laboratories.”
Neutrinoless double beta decay (0νββ) is a rare type of radioactive decay that scientists predict could exist if certain characteristics of neutrinos are true. These characteristics include having a mass and being what’s called a Majorana particle, which means that they act as both a particle and its own antiparticle. While we already know that neutrinos have mass, observing this decay would be further evidence that they are Majorana particles. Essentially, this discovery would help us better understand the fundamental nature of the universe and the behavior of these mysterious subatomic particles.
“If the neutrino is a Majorana particle, it leads to theories that can motivate the preponderance of matter over anti-matter in the universe—a necessary condition for life as we know it to exist,” Elliott explained. “This decay, if it exists, would be extremely rare with its half-life being longer than 1026 years (1016 times longer than the age of the universe). Being so rare, any other radiation, from trace impurities in the shield or from cosmic rays, depositing energy in the Ge would mask the signal.”
The Majorana Demonstrator is the experimental set-up used by the Majorana Collaboration to search for 0νββ decay. This set-up is made up of a large collection of Ge crystals, which are located at the Sanford Underground Lab. These crystals are designed to act as radiation detectors, which means they can detect particles that are emitted during radioactive decay. Essentially, the Majorana Demonstrator is a complex machine that is used to study the behavior of subatomic particles in order to better understand the fundamental nature of the universe.
“The Ge material comprising these detectors is enriched in the isotope 76 to about 88%, whereas its natural abundance is about 7.75%,” Elliott said. “These detectors are contained within an ultra-low radioactivity shield and sited deep underground to reduce sources of energy deposit giving rise to background to the signal from 0νββ.”
The recent publication of the final results of the Majorana Demonstrator experiment in PRL brings us one step closer to answering important questions about the nature of the universe and the matter that makes it up. Although the experiment did not yet detect 0νββ decay, the efforts of the Majorana Collaboration demonstrate that their approach is feasible and effective. This means that they may be able to use a larger scale version of the experiment in the future to search for this rare form of radioactive decay. In other words, this experiment has paved the way for further research and discovery in the field of physics.
“The Majorana Demonstrator showed that a large array of Ge detectors with world leading energy resolution could be fabricated and operated in a radioactivity environment low enough to justify constructing a much larger project,” Elliott said. “Even this small experiment was able to compete with much larger 0νββ experiments due to its low background and excellent energy resolution.”
The Majorana Collaboration and the GERDA Collaboration, who both built similar detection experiments in different locations, have now joined forces to create the LEGEND-200 experiment. This is a collaborative effort to detect 0νββ decay and they recently started collecting data for this experiment in Italy, specifically at the Laboratori Nazionali del Gran Sasso. Essentially, the scientists from these two groups are working together to combine their expertise and resources in order to better study this rare form of radioactive decay. By pooling their efforts, they hope to make even greater progress in understanding the fundamental nature of the universe.
“We are proposing an experiment with 1,000 kg of Ge that is working through the DOE review process,” Elliott added.
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