Students Rock Neutron Under UC Berkeley Campus

In an underground vault closed by six-foot concrete walls and accessed by a 25-ton concrete and steel door at Berkeley, University of California, students rock neutrons on a new theme: the one more suited to production isotopes for geological dating, forensic science, diagnosis and treatment at the hospital.

Dating and forensic science rely on a jet of neutrons to convert atoms into radioactive isotopes, which betray the chemical composition of a substance, helping to trace a gun, for example, or to reveal the substance. age of a rock. Hospitals use isotopes produced by neutron irradiation to kill tumors or to detect diseases such as cancer in the body.

For these applications, however, only nuclear reactors can produce a sufficiently strong neutron sputtering, and there are only two in the west of Mississippi.

As an alternative, a team made up of UC Berkeley students built a relatively inexpensive table-top neutron source to be reproduced and possibly portable, as well as capable of producing a more restricted range. neutron energies, minimizing the production of unwanted radioactive byproducts.

"No matter what hospital in the country could have this item, it could build it for a few hundred thousand dollars to produce local medical isotopes of very short duration." lift up to the patient, "said Karl van Bibber. a professor of nuclear engineering from the University of Berkeley who supervises students perfecting the device. "It has applications in geochronology, neutron activation analysis for forces of order – when the FBI wants to determine the provenance of a sample as evidence, for example – neutron radiography , to look for cracks in the aircraft parts of a small convection oven, I think it 's great, it makes us excited.

Researchers at the University of Berkeley have now demonstrated that the High Flux Neutron Generator (HFNG) can produce "boutique" neutrons, neutrons in a very narrow energy range, that can be used to accurately date fine-grained rocks virtually impossible to date by other techniques using radioisotopes. . The study will be published this week in the journal Progress of science.

"This will increase the dating capacity of fine-grained materials, such as the clay minerals associated with ore deposits, including gold and lava flows," said Paul Renne, professor in residence at the University of Ottawa. University of Berkeley, Department of Earth and Planetary Sciences. Director of the Berkeley Geochronology Center. "This device could also allow us to examine the most primitive objects of our solar system – the calcium / aluminum rich inclusions found in some types of meteorites – which also have a very fine grain. "

As they point out in the new article, the researchers used the neutron generator to determine the age of fine-grained lava from the 79 BC Vesuvius eruption, which buried the Roman city of Pompeii. The date they calculated was as accurate as the answer given by a comprehensive 1997 study using state-of-the-art argon-argon dating of samples irradiated in a nuclear reactor.

"It allows you to do things that were not possible otherwise," said Renne.

The long way to merging workstations

For decades, Renne was looking for better ways to irradiate rock samples. Stanley Prussin, who died in 2015, had recently discussed a possible method. This technique involves the fusion of two deuterium atoms, which are isotopes of hydrogen. , to produce helium-3 and a neutron. These neutrons have an energy – about 2.5 million electron volts – ideal for irradiating rocks in order to achieve argon-argon dating, one of the most accurate methods used today. # 39; hui.

The argon-argon dating is based on the fact that about one in 1,000 potassium atoms in the rock is the 40 potassium radioactive isotope, which decays in argon 40 with a half-life more than a billion years old. By using neutrons, scientists convert a portion of the stable potassium, 39 potassium, to argon 39, and then measure the Ar 40 to Ar 39 ratio in the sample to calculate its age.

Rock samples must now be irradiated on nuclear reactors, but the reactors produce very energetic neutrons that can remove the argon atoms from the sample – a particular problem for microscopic grain rocks – and produce also unwanted radioactive elements. Both effects make the calculation of age more difficult.

The HFNG avoids these two problems because neutrons account for one tenth of the energy of those of a nuclear reactor and have a narrower energy range while maintaining a high neutron flux.

"Eliminating the problem of hindsight, as well as reducing interference reactions, is huge," said Renne. "But the radiological aspects are also improved."

"We realized that the beauty of this thing is that you do not write it everywhere and that you do not create a radiological problem," said van Bibber, Shankar Sastry Chair for Leadership and Leadership. innovation. "You actually have a modest number of neutrons, but by bringing the target closer to the point source – which counts, the neutron flux at the sample level is very high."

The first device for creating neutrons by deuterium-deuterium fusion (DD) was designed 10 years ago by the Renne team, which included plasma physicist Ka-Ngo Leung, formerly of Lawrence Berkeley National Laboratory. (Berkeley Lab). But their prototype languished until van Bibber became interested in 2012, shortly after his appointment to the leadership of the UC Berkeley Nuclear Engineering Department. To house the fusion generator, van Bibber took over a concrete vault, formerly used for experiments on the campus nuclear reactor, which was once under the current Soda Hall, although it is found in a large underground room in the basement of Etcheverry Hall – until the closure of the reactor in 1987 and its withdrawal.

The generator uses about 100,000 volts to accelerate the ionized deuterium atoms to a titanium metal cathode. The deuterium accumulates on the cathode in a thin layer which then serves as a target for the other incoming ions. When melting deuterons, a neutron is produced in a wide beam that irradiates the sample located at about a third of an inch.

Over the years, van Bibber has recruited numerous undergraduate, graduate and postdoctoral fellows to help make the neutron generator a reality. One of them, Max Wallace, a changing student, an aspiring young man interested in nuclear forensics, was astonished at the access he had to such a machine.

"It's rare to be able to work with radioisotopes as an undergraduate student," said the former software engineer. "I learned to do a lot of things late at night, wearing gloves and goggles to measure radiation, taking samples, performing security checks, and running the software." I would learn something in my nuclear physics course, then I would come here to work on a direct application of it. "

For Mauricio Ayllon Unzueta, a fourth-year nuclear engineering student, the experience he gained in developing the neutron generator has led to a new project at Berkeley Lab: designing a variant of HFNG that can be used on the ground. ground. Activate neutrons in soils to measure carbon content – essential information if society hopes to sequester carbon in soils to mitigate the effects of climate change.

"Over three generations of graduate students, we have transformed this system that was just working into a high-performance neutron generator," van Bibber said.

According to Renne, Daniel Rutte, a postdoctoral researcher in geology at Berkeley at Renne and the head of the BGC laboratory, played a crucial role in designing and implementing the first dating experience.

"Daniel was literally the key player to demonstrate that it would work for Ar-Ar geochronology," he said.

Rutte's goal is to develop new methods and instruments to better understand the processes of the Earth, especially the deformation of the earth's crust, which occurs by slow creep or rapid rupture resulting in earthquakes .

"To understand the long-term deformation of the Earth's crust, I date the old ruptures stored in the disk," said Rutte. "The neutron generator will contribute to progress in this area by expanding the range of materials we can date."

With the continued help of students, van Bibber and Renne are waiting to make the neutron generator more compact and produce a more intense neutron sputtering, which would make it more useful for geochronology, as well as for other specialized uses. Researchers at the UC Berkeley Space Science Laboratory have already shown interest in using these neutrons to test electronic equipment to determine how it would survive in the radioactive environment of the planet. 39; space. High energy neutrons could be used for neutron radiography, which can complement X-ray radiography to image inside dense objects, such as metals.

"The goal has always been to test Paul's dream of whether we could use a very compact, low-voltage device to irradiate with neutrons," van Bibber said. "We have now shown that any university can have a neutron source for the argon-argon dating technique."