Physicists design an experiment to pinpoint the origin of the elements

Almost all the oxygen in our universe is wrought in the belly of massive stars like our sun. When these stars contract and burn, they trigger thermonuclear reactions in their nuclei, where carbon and helium nuclei can collide and merge into a rare but essential nuclear reaction that generates much of the energy. oxygen of the universe.

The rate of this oxygen – generating reaction has been incredibly difficult to pin down. But if researchers can get a reasonably accurate estimate of what is called the "radiative capture rate," they can begin to develop answers to fundamental questions, such as the ratio of carbon to oxygen in the universe. A precise rate could also help them determine whether an exploding star will settle in the form of a black hole or a neutron star.

Physicists at the MIT Nuclear Science Laboratory (LNS) have developed an experimental plan that could help determine the rate of this oxygen – generating reaction. The approach requires a type of particle accelerator still under construction, at several locations around the world. Once in service, these "multi-megawatt" linear accelerators can provide the proper conditions to perform the reaction generating an oxide in the opposite direction, as if going back up the time of star formation.

Researchers say that such a "reverse reaction" should give them an estimate of the reaction rate actually present in stars, with greater precision than previously obtained.

"The job description of a physicist is to understand the world and, at the moment, we do not understand very well where the oxygen from the universe comes from and how is made of it. Oxygen and carbon, "says Richard Milner, professor of physics at MIT. "If we are right, this measure will help us answer some of these important questions of nuclear physics regarding the origin of the elements."

Milner is co-author of an article published today in the journal Physical Review C, alongside MIT-LNS lead author and postdoc, Ivica Friščić, and lead researcher T. William Donnelly from the Center for Theoretical Physics at MIT.

A precipitous drop

The radiative capture reaction rate refers to the reaction between a carbon nucleus 12 and a helium nucleus, also called an alpha particle, which takes place in a star. When these two nuclei collide, the carbon core effectively "captures" the alpha particle and, in doing so, is excited and emits energy in the form of a photon. What remains is an oxygen-16 nucleus, which eventually disintegrates into a stable form of oxygen existing in our atmosphere.

But the chances of this reaction occurring naturally in a star are incredibly slim, since an alpha particle and a carbon 12 nucleus have a very positive charge. If they come in close contact, they are naturally inclined to repel each other under the name of Coulomb's force. To merge to form oxygen, the couple should collide at energies high enough to overcome the Coulomb force – a rare event. Such a low reaction rate would be impossible to detect at the energy levels in the stars.

Over the last five decades, scientists have attempted to simulate the rate of radiative capture reaction in small but potent particle accelerators. They do this by colliding beams of helium and carbon in the hope of fusing the nuclei of the two beams to produce oxygen. They were able to measure these reactions and calculate the associated reaction rates. However, the energies at which such accelerators collide are much higher than what happens in a star, so much so that current estimates of the rate of reaction generating oxygen are difficult to extrapolate to what is happening. actually goes into the stars.

"This reaction is quite well known at higher energies, but it decreases precipitously down energy, towards the interesting astrophysical region," Friščić explains.

Time in the opposite direction

In the new study, the team decided to resurrect a previous notion, in order to produce the reverse of the generating oxygen response. The goal is essentially to start gaseous oxygen and divide its nucleus into its starting ingredients: an alpha particle and a carbon core 12. The team felt that the likelihood that the reaction would occur in the sense inverse should be larger, and therefore more easily measurable, than the same reaction. The opposite reaction should also be possible at energies closer to the energy range in real stars.

To separate the oxygen, they would need a high intensity beam, with an extremely high concentration of electrons. (The larger the number of electrons that bombard a cloud of oxygen atoms is important, the greater the chance that an electron on billions has the energy and the the pulse needed to collide with an oxygen core and split it.)

The idea was launched by MIT researcher Genya Tsentalovich, who led a proposed experiment in the electron storage ring of MIT-Bates South Hall in 2000. Although the experiment was never conducted at Bates accelerator, which ceased operations in 2005, Donnelly and Milner felt that the idea deserved to be studied in detail. With the launch of the construction of next-generation linear accelerators in Germany and Cornell University, capable of producing electron beams of sufficient intensity or current to eventually trigger the opposite reaction, and Friščić arrived at MIT in 2016, the study began.

"The possibility of these new high-intensity electron machines, with tens of milliamperes of current, has awakened our interest in this technology. [inverse reaction] idea, "says Milner.

The team proposed an experiment aimed at producing the opposite reaction by projecting an electron beam on a cloud of ultra-cold and cold oxygen. If an electron collides with an oxygen atom and separates it, it should disperse with a certain amount of energy, which physicists had previously predicted. Researchers would isolate collisions involving electrons in this given energy range and, from them, would isolate the alpha particles produced as a result.

Alpha particles are produced by splitting from 0 to 16 atoms. The splitting of other isotopes of oxygen can also produce alpha particles, but these would disperse a little faster (about 10 nanoseconds faster) than the alpha particles produced by the splitting of atoms. from O-16. The team decided to isolate the alpha particles, which were slightly slower and whose "flight time" was slightly shorter.

The researchers could then calculate the rate of reverse reaction, given the frequency with which the slower alpha particles – and, by proxy, the O-16 atom division – occur. They then developed a model to relate the reverse reaction to the direct direct reaction of oxygen production that occurs naturally in the stars.

"We are essentially doing the opposite reaction of time," says Milner. "If you measure that with the precision we're talking about, you should be able to extract the reaction rate directly, by factors up to 20 times more than anyone else in this region."

At present, a multi-wavelength linear accelerator, MESA, is under construction in Germany. Friščić and Milner collaborate with physicists for the design of the experiment, in the hope that once launched, they will be able to concretize their experience in order to accurately determine the speed at which the stars are injecting. Oxygen in the universe.

"If we are right and we do this, it will allow us to determine the amount of carbon and oxygen that is formed in the stars, which is the biggest uncertainty about how stars evolve" says Milner.

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