Weighing in on the Creation of Heavy Elements

Researchers at CERN and Argonne Labs are bringing the study of the formation of heavy elements like gold, platinum and uranium from colliding neutron stars in deep space down to Earth.

A look inside the ISOLDE Solenoid Spectrometer at CERN (Argonne National Laboratory)

Whilst physicists are fairly confident that elements are forged in the nuclear furnaces that exist in the centre of stars, some ambiguity still exists around the creation of the heaviest elements — some like gold, platinum and uranium have become vitally important to us. These elements must, they believe, be formed in conditions even more extreme and violent than those found at the centre of stars. Clearly few events qualify, but neutron star mergers and massive supernova explosions most certainly would.

The investigation into this area of physics began to heat up last year with the discovery of the heavy element strontium around the site of a neutron star merger, and now this investigation has been brought down to Earth by Argonne National Laboratory researchers working at CERN.

The team used novel techniques developed at Argonne to study both the nature and creation of heavy elements. In turn, they hope to provide critical insights into the processes that operate in conjunction in order to create the exotic heavy nuclei. The study, published in the journal Physical Review Letters, could potentially redefine models of stellar events and our understanding of the early universe.

“One of the biggest questions of this century has been how the elements formed at the beginning of the universe,” says Ben Kay, Argonne physicist and lead scientist on the study. “We can’t just go dig up a supernova out of the earth, so we have to create these extreme environments and study the reactions that occur in them.”

During the course of their investigation, the scientists became the first in history to observe the structure of an atomic nucleus with fewer protons than lead — 82 — in conjunction with more than 126 neutrons — so-called “magic numbers” in the field of nuclear physics.

Beyond The Magic Numbers

The established “magic numbers” of atomic nuclei are 8, 20, 28, 50, and 126. At these numbers, nuclei have greater stability than other elements, in a way that is analogous to the stability granted to noble gases by filled electron shells.

The region of the nuclear chart populated by nuclei below 82 protons and above 126 neutrons is almost completely unexplored as these nuclei are incredibly difficult to produce. Investigation of the nuclei that lie in this zone is vital if researchers are to understand the rapid neutron-capture process — r-process — that researchers believe produce many of the universe’s heavy elements.

The r-process is able to occur in environments such as in the vicinity of neutron-star mergers and supernovae, due to the excess of free-neutrons. This allows nuclei to capture neutrons, in the process, creating new and heavier elements. Under ordinary conditions would decay before they have chance to soak up enough neutrons to become heavy elements— but in this neutron-rich atmosphere, that the process is accelerated.

In order to conduct their study, the researcher chose to focus on the mercury isotope ²⁰⁷Hg. The study of this isotope which consists of 80 protons and 127 neutrons, they believe, has the capacity to teach them about the properties of its neighbours in the zone beyond the nuclear magic numbers. It is these nuclei that are key in rapid neutron-capture.

The first problem the team needed to address is where to find equipment capable of creating a radioactive beam of mercury that could be used to investigate the structure ²⁰⁷Hg.

Having a blast at CERN

Conducting an experiment able to probe ²⁰⁷Hg required the creation of a high-energy beam of protons to be fired at a molten lead target. The resulting collisions produce hundreds of exotic and radioactive isotopes, from which the researchers sift out ²⁰⁶Hg nuclei from the other fragments. These nuclei then must be focused into a beam — the most energetic ever created at a nuclear accelerator facility — that is then blasted at a deuterium target.

The process described above could only be conducted at the HIE-ISOLDE facility at CERN in Geneva, Switzerland, with the deuterium target located inside the new ISOLDE Solenoidal Spectrometer (ISS) — a newly-developed magnetic spectrometer that the nuclear physicists used to detect instances of ²⁰⁶Hg nuclei capturing a neutron and thus transforming into ²⁰⁷Hg.

“No other facility can make mercury beams of this mass and accelerate them to these energies,” said Kay. “This, coupled with the outstanding resolving power of the ISS, allowed us to observe the spectrum of excited states in ²⁰⁷Hg for the first time.”

The key to artificially inducing neutron-capture is the deuterium target that the beam of ²⁰⁶Hg nuclei is slammed against. Deuterium is an isotope of hydrogen often referred to as “heavy hydrogen” as its nucleus consists of a proton and a neutron in contrast to the nucleus of “regular” hydrogen which is a lone proton.

The isotope of mercury ²⁰⁶Hg can capture a neutron from the deuterium target in the process causing the neutron to be separated from its proton partner, thus causing the proton to recoil. As these emitted protons hit the ISS detector, key information about the structure of the nucleus and how it is bound together can be extracted from their position and energy. As the structure and binding of these nuclei mitigate how the r-process proceeds, the team’s results impart important information about models of nuclear astrophysics.

Of course, physicists have for many years conducted investigations into atomic nuclei, which leads to the question, why has the investigation of heavy nuclei not been conducted before?

Hitting a feather with a cannonball

The difficulty that has limited these experiments in the past arises from the fact that hitting heavy targets with light ion beams is relatively straight forward, but hitting a light target with a heavy ion beam is markedly more difficult. The physics of these latter collisions become distorted and therefore more difficult to analyse.

“When you’ve got a cannonball of a beam hitting a fragile target, the kinematics change and the resulting spectra are compressed,” explains Kay.

This is the Miniball germanium array, which is using the first HIE-ISOLDE beams for the experiments described below (Julien Ordan /CERN)

The solution the team arrived at was the use of ISS which was inspired by HELIOS, a pioneering concept suggested by Argonne distinguished fellow John Schiffer built to serve as the lab’s helical orbital spectrometer. Since 2008, HELIOS has allowed exploration of nuclear properties that were previously impossible to study. Building upon this, CERN’s ISOLDE facility can produce beams of nuclei that complement those that can be made at Argonne.

“John Schiffer realized that when the collision occurs inside a magnet, the emitted protons travel in a spiral pattern towards the detector, and by a mathematical ‘trick’, this unfolds the kinematic compression, resulting in an uncompressed spectrum that reveals the underlying nuclear structure,” Kay concludes.

The first results from the experiment at CERN have seemingly confirmed the theoretical predictions made by current nuclear models. The team now intend to use the new capabilities and experimental technique to probe other nuclei beyond the magic numbers, thus acquiring a deeper insight into how the world around us was formed.