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New Research: Photon emission of exotic nuclei in stars

Experimental data (blue) showing the enhancement in the low-energy photon emission, compared to theoretical predictions without this enhancement (pink). Background image: artist’s impression of a neutron-star merger event by Erin O’Donnell.

Atomic nuclei are complex little beasts. If, however, you are curious enough to look closely, you’ll see that these tiny constituents of matter are quite fascinating, and hide a lot of surprises. Together with being curious about the nuclei themselves, I personally am even more fascinated by how much the secret lives of atomic nuclei drive astrophysical phenomena. It is through nuclear processes that the elements we find in the Universe are created.

Together with a group of collaborators we recently found a new property of nuclei that we all got excited about. I’ll try to explain here what we found, in the hopes that maybe you’ll find it at the very least interesting, or maybe even exciting, like we did.

For the purpose of this discussion, we don’t really need to dive into the full complex arrangements of protons and neutrons in a nucleus. Let’s look at it as a large blob of matter. If we somehow give energy to this blob of matter it can do many different things: vibrate, rotate, wobble, and more. It becomes “excited” as we call it. It reminds me of a situation where a group of toddlers just ate birthday cake at a party, and you can watch them do all the random things they can think of, like jump up and down, run around, shake, dance, turn, and spin; until they run out of energy. In the case of nuclei, they will also eventually run out of energy, usually by emitting photons, i.e. electromagnetic radiation.

The photons that nuclei emit in the process of “calming down” can carry different amounts of energy. There is a higher chance to emit photons with high energy, and the emission probability gets smaller for lower energies. It follows more or less the pink line in the figure. Here’s where our new research comes into play. We found that when you go to very low energies, this probability takes a surprising turn upward.

This kind of “upbend” as it was called, has been seen in stable “normal” nuclei before, but no one really knows exactly why it shows up, which nuclei should have it, what causes it, and what is its nature. Looking for answers, we did an experiment at the NSCL, and saw this behavior, for the first time, in an exotic nucleus. This was the nucleus nickel-70, which is exotic because it has way more neutrons than protons (42 neutrons vs 28 protons) and only lives for 6 seconds.

Looking at the new experimental results (blue shaded area in the figure), our group, led by the nuclear physics group at the University of Oslo, also tried to approach the same nucleus from a theoretical standpoint. In the end, we did find a model that fits the experimental data, which is another reason we are all excited about this new research; it means we are a step closer to understanding the cause of this phenomenon, and not just observe it experimentally.

Exotic nuclei, like nickel-70, are the ones driving a lot of the astrophysical processes in the hearts of stars, and also during stellar explosions. So we didn’t just learn something new about nuclei themselves, but we also got a step closer to understanding the astrophysical processes that create our whole Universe.

I don't know about you but in my mind this kind of result counts as a success in research...

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