Medieval alchemists dreamed of turning lead into gold.
Today, we know that lead and gold are different substances, and no chemistry can convert one into the other.
But our modern knowledge tells us the fundamental difference between a lead atom and a gold atom: a lead atom has exactly three more protons. So can we make a gold atom by removing three protons from a lead atom?
As it turns out, we can. But it is not easy.
While smashing some lead atoms together at super-fast speeds in an attempt to simulate the state of the universe just after the Big Bang, physicists working on the ALICE experiment at the Large Hadron Collider in Switzerland produced a tiny bit of gold.
The amount is very small, in fact: a total of 29 trillionths of a gram.
How to steal a proton
Protons are found in the nucleus of an atom. How can they be removed?
No, protons have an electric charge, which means that an electric field can pull or push them around. Placing the nucleus of an atom in an electric field can do it.
However, the nuclei are held together by a very strong force with a very short range, which is theoretically known as the nuclear force. This means that a very strong electric field is needed to eject the protons – about a million times stronger than the electric fields that cause lightning in space.
The way scientists created this space was to shoot fields of lead nuclei at each other at an incredibly high speed – almost the speed of light.
Intimate magic
When lead nuclei collide, there is a strong nuclear force and they eventually annihilate completely. But nuclei usually have near defects, and they only touch each other with an electric force.
The strength of the electric field decreases very rapidly as you move away from an object that has an electric charge (such as a proton). But over very short distances, even a small charge can create a very strong field.
So when one lead core simply grazes over the other, the electric field between them is greater. The rapidly changing distance between the nuclei causes them to vibrate and sometimes shed some protons. If one of them spits out exactly three protons, the lead nucleus has turned into gold.
Counting protons
So if you’ve turned a lead atom into gold, how do you know? In the ALICE experiment, they use special detectors called zero-degree calorimeters to count protons captured in lead nuclei.
They cannot see the gold nuclei themselves, so they only know about them indirectly.
About the author
Ulrik Egede is Professor of Physics at Monash University.
This article was originally published by The Conversation and is reprinted under a Creative Commons license. Read the first article.
ALICE scientists estimate that, as they collide in beams of lead nuclei, they produce about 89,000 gold particles per second. They also observed the production of other elements: thallium, which is what you get when you take one proton from lead, as well as mercury (two protons).
The alchemical problem
Once the lead nucleus has changed by losing protons, it is no longer in the perfect orbit that keeps it spinning inside the vacuum tube of the Large Hadron Collider. In a matter of microseconds it will collide with the walls.
This effect causes the beam to decrease significantly over time. So for scientists, the production of gold in collisions is actually more of a nuisance than a blessing.
However, understanding this alchemy of error is important for understanding the experiments — and for designing larger experiments for the future.
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