Thursday, May 19, 2022

Physicists design an experiment to pin down the origin of the elements

With help from next-generation particle accelerators, the approach may nail down the rate of oxygen production in the universe.

All the heavier chemical elements — such as oxygen, nitrogen, carbon, and iron, which are essential to life — were forged in the bellies of stars like our sun. Once these stars contract and burn, they set off thermonuclear reactions inside their centers, where cores of carbon and helium can impact and fuse in a rare through an essential nuclear reaction that produces a significant part of the oxygen known to the universe.

However, pinpointing the rate of oxygen-generating reaction is quite tricky. Be that as it may, if specialists can get an adequate gauge of what’s known as the “radiative capture reaction rate,” they can start to work out the answers to crucial inquiries, for example, the ratio of carbon to oxygen. An exact rate may help them to determine if an exploding star will sink into the type of a black hole or a neutron star.

Recently physicists at MIT’s Laboratory for Nuclear Science (LNS) have concocted an experimental design that could pinpoint the rate of this oxygen-generating reaction. The methodology requires a kind of particle accelerator that is still under construction, in a few areas around the globe.

Once fully operational, such “multimegawatt” linear accelerators may give only the right conditions to run the oxygen-generating reaction in reverse, as though getting back to better days of star formation.

According to scientists, such an inverse reaction should give them an estimate of the reaction rate that occurs in stars, with higher accuracy than has previously been achieved.

Richard Milner, professor of physics at MIT said, “The job description of a physicist is to understand the world, and right now, we don’t quite understand where the oxygen in the universe comes from, and, how oxygen and carbon are made. If we’re right, this measurement will help us answer some of these important questions in nuclear physics regarding the origin of the elements.”

The radiative capture reaction rate alludes to the response between a carbon-12 nucleus and a helium nucleus, otherwise called an alpha particle that happens inside a star. At the point when these two nuclei collide, the carbon nucleus adequately captures the alpha molecule, and all the while is energized and emanates energy as a photon. What’s deserted is an oxygen-16 nucleus, which eventually rots to a steady type of oxygen that exists in our climate.

However, the odds of this reaction usually happening in a star are incredibly slim, because of the way that both an alpha particle and a carbon-12 nucleus are exceptionally positively charged. If they do come in close contact, they are generally disposed to repulse, in what’s known as a Coulomb’s force.

To fuse to form oxygen, the pair would need to crash at adequately high energies to conquer Coulomb’s force. Such an exceedingly low reaction rate would be difficult to identify at the vitality levels that exist inside stars.

During the study, scientists resurrect a previous notion, to produce the inverse of the oxygen-generating reaction. The aim, necessarily, is to start from oxygen gas and split its nucleus into its starting ingredients: an alpha particle and a carbon-12 nucleus.

The team reasoned that the probability of the reaction happening in reverse should be higher, and therefore more easily measured, than the same result run forward. The inverse response should also be possible at energies nearer to the energy range within actual stars.

The group proposed an investigation to create the opposite reaction by shooting an electron beam at a cold, ultradense cloud of oxygen. On the off chance that an electron effectively crashed into and split an oxygen atom, it should dissipate away with a specific measure of energy, which physicists have recently anticipated. The scientists would segregate the impacts, including electrons inside this given energy range, and from these, they would separate the alpha particles produced in the aftermath.

Alpha particles are produced when O-16 atoms split. The splitting of other oxygen isotopes can also result in alpha particles, but these would scatter away slightly faster — about ten nanoseconds faster — than alpha particles produced from the splitting of O-16 atoms. So, the team reasoned they would isolate those alpha particles that were slightly slower, with a somewhat shorter time of flight.

Milner said, “We’re essentially doing the time-reversed reaction. If you measure that at the precision we’re talking about, you should be able to directly extract the reaction rate, by factors of up to 20 beyond what anybody has done in this region.”

Scientists are now collaborating with physicists there to design the experiment, in hopes that, once up and running, they can put their experiment into action to truly pin down the rate at which stars churn oxygen out into the universe.

Milner said, “If we’re right, and we make this measurement, it will allow us to answer how much carbon and oxygen is formed in stars, which is the largest uncertainty that we have in our understanding of how stars evolve.”

Milner is a co-author of a paper appearing today in the journal Physical Review C, along with lead author and MIT-LNS postdoc Ivica Friščić and MIT Center for Theoretical Physics Senior Research Scientist T. William Donnelly.



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