The ancient quest to change lead into gold, a process known as chrysopoeia, fascinated alchemists for centuries. They were inspired by the similar density of the two metals, but it wasn’t until much later that science revealed lead and gold are different elements, making chemical transmutation impossible, according to CERN.
The discovery of nuclear physics in the 20th century showed that heavy elements could change into others through radioactive decay or by being hit with neutrons or protons in a lab. Gold has been created this way before, but the ALICE team at CERN used a new method involving near-miss collisions between lead nuclei in the LHC, according to the press release.
When lead nuclei collide at extremely high energies in the LHC, they can form a state of matter called quark-gluon plasma. This is a hot, dense material believed to have filled the universe just after the Big Bang. However, most of the time, the lead nuclei only pass close to each other without actually touching.
In such near-miss events, their powerful electromagnetic fields can cause interactions between photons and nuclei, opening up new possibilities for scientific research.
The electromagnetic field around a lead nucleus is especially strong because it contains 82 protons, each with a positive charge.
When these nuclei move at nearly the speed of light, their electromagnetic fields become compressed into a thin shape, creating a brief pulse of photons. The brief pulse can trigger a process called electromagnetic dissociation, in which a photon excites the nucleus and causes it to lose a few protons or neutrons. To make gold, which has 79 protons, three protons must be knocked out of a lead nucleus.
“It is impressive to see that our detectors can handle head-on collisions producing thousands of particles, while also being sensitive to collisions where only a few particles are produced at a time, enabling the study of electromagnetic ‘nuclear transmutation’ processes,” said Marco Van Leeuwen, ALICE spokesperson, in the press release.
The ALICE team used a special detector called a zero-degree calorimeter (ZDC) to count how many photon-nucleus interactions led to the loss of zero, one, two, or three protons, along with at least one neutron. Such losses correspond to the creation of lead, thallium, mercury, and gold, respectively.
While gold production is less common than thallium or mercury, the LHC currently creates gold at a rate of about 89,000 nuclei per second during lead-to-lead collisions at the ALICE experiment site.
However, the gold nuclei are highly energetic and quickly break apart into smaller particles after hitting the LHC’s beam pipe or other equipment. The gold exists for just a split second.
During the LHC’s Run 2 from 2015 to 2018, about 86 billion gold nuclei were created at the four main experiments, which amounts to only 29 picograms, far less than what would be needed for even a tiny piece of jewelry. With recent upgrades, Run 3 has almost doubled the amount of gold produced, but it is still trillions of times too little for practical use, CERN said.
“Thanks to the unique capabilities of the ALICE ZDCs, the present analysis is the first to systematically detect and analyse the signature of gold production at the LHC experimentally,” said Uliana Dmitrieva of the ALICE collaboration.
John Jowett, also of the ALICE collaboration, added, “The results also test and improve theoretical models of electromagnetic dissociation which, beyond their intrinsic physics interest, are used to understand and predict beam losses that are a major limit on the performance of the LHC and future colliders.”