Where do Heavy Elements Come From?

In terms of nuclear structure, iron is a pretty stable element. Smaller elements fuse together up to iron, releasing lots of energy in the process and feeding stars. Larger elements tend to be unstable and will undergo fission or beta decay to get down to a stable element, releasing energy in the process. Iron seems to have that ying yang balance!

But how do we get larger elements in the first place?

Before we start, Do You Know About Beta Decay?

At school, we learn that there are 3 types of radioactive decay: alpha, beta, and gamma. Beta tends to get neglected, like the middle sibling. However beta decay is arguably the most useful of the 3.

Beta decay occurs in a nucleus when there are too many of one nucleon (a proton or neutron), and what happens is that these excess nucleons simply turn into the other. Not all the excess particles will change.

For example, carbon-14 has 2 too many neutrons, so it will convert 1 (not both!) into a proton. This makes Nitrogen-14, a nice and stable element. To balance out charges, an electron (plus an antineutrino) will also be released. In the case of turning protons to neutrons, a positron (and a neutrino) will be released.

Introducing Neutron Capture!

Neutrons with high enough energy can be captured by a nucleus, an occurrence that happens from the biggest stars to our tiny nuclear reactors on Earth. Protons can also be captured, but this is far rarer since they are positively charged and experience electrostatic repulsion with the nucleus.

There are 2 main catergories we need to consider: Slow s-process neutron capture, and rapid r-process neutron capture.

S-Process Neutron Capture is most common

In this case, neutrons are acquired one at a time until the nucleus becomes unstable. To stabilise itself, one or a few neutrons will undergo beta decay and turn into protons (plus some electrons and antineutrinos to balance things out).

It is the proton number that determines what the element is, so since the amount of protons has increased we end up with an entirely different element!

It’s not easy to visually represent the transition, but this graph by Rursus Siderespector shows squares moving to the right representing the addition of a neutron, and then the diagonal lines show the nucleus converting a neutron into a proton and becoming a new element. The process starts again with more neutrons being captured.

Credit: Rursus Siderespector

There are some more translucent boxes which we don’t attain, like in the cadmium row. This is where we need to increase the neutron number much more quickly than what the s-process can offer. Hence, we need rapid process capture!

R-Process is rare, but extremely vital!

The Rapid R-Process is not seen often, but we know it must occur in order for us to obtain some of the elements.

In this process, the nucleus is bombarded with neutrons. These are captured far more quickly than they can decay, so that when decay does occur it is into completely different elements. How cool is that?

Astronomers suspect r-process occurs in places such as colliding neutron stars, which the thought of it just amazes me!

Inside the Crab Nebula is a beautiful pulsar, a rapidly spinning neutron star!

That’s it for this week! Next week we will say goodbye to stars and have a look at something a little more down to Earth!

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