Scrap Metal from the Proton Merry-Go-Round

by Nicholas Mee on August 11, 2020

Rutherford probed the structure of matter using alpha particles emitted by radioactive elements such as radium as projectiles to bombard his target materials. This technique enabled him to make great breakthroughs in understanding the structure of matter, including the discovery of the atomic nucleus and also one of its constituents—the proton. But in his annual address as President of the Royal Society in 1927, Rutherford called for something better, a way to generate higher energy particles for use as projectiles in physics experiments. He believed:

This would open up an extraordinary new field of investigation that could not fail to give us information of great value, not only in the constitution and stability of atomic nuclei but also in many other directions.

The American physicist Ernest Orlando Lawrence (1901-1958) would meet Rutherford’s challenge and build ever larger machines to accelerate protons and alpha particles to higher energies than had ever been available before. 

Ernest Lawrence

Lawrence was very much a hands-on physicist who didn’t mind getting his hands dirty, but unlike most physicists he was always smartly dressed in a suit and tie even when he had a screwdriver or a soldering iron in his hand.

The Proton Merry-Go-Round

Lawrence’s flash of inspiration came the year after Rutherford’s address while reading a paper by Norwegian physicist Rolf Wideroe in the university library at Berkeley near San Francisco. Inspired by Wideroe’s article Lawrence could see how to create a particle accelerator—a proton merry-go-round—as he described it. Developing this idea on an ever grander scale would become Lawrence’s lifework. His initial model was a five-inch diameter disc that would fit in the palm of one hand and would accelerate electrons to a modest energy of 80 keV. He called it a cyclotron. Lawrence would transform this machine into one of the most important instruments of the twentieth century.

A cyclotron consists of two hollow ’D’ shaped pieces of metal with a narrow gap between their straight edges, all sandwiched between the poles of an electromagnet, as shown in the diagram below. Charged particles, such as protons, move in a circle in a uniform magnetic field, and the faster they travel the larger the circle, but crucially the time taken to complete each circuit remains the same. Lawrence realised that an alternating electric field tuned to the orbital frequency would give the protons a kick each time they cross the gap. So if protons are injected close to the centre, they will spiral outwards with ever greater speed until they leave through an opening at the outer edge as a high-energy proton beam that can be directed at a target.

Diagram from Lawrence’s 1932 patent application for the cyclotron.

By 1931 Lawrence had an eleven-inch cyclotron that accelerated protons to 1.1 MeV. There was still some way to go to match the naturally-occurring projectiles available from radioactive decay. But Lawrence would steadily progress to ever larger and better machines throughout the 1930s. 

Big Science

By 1936 Lawrence had built a 37-inch cyclotron that could accelerate deuterons (heavy hydrogen nuclei) to an energy of 8 MeV and alpha particles to 16 MeV. Berkeley was now at the forefront of nuclear research with a machine generating particle beams that were more powerful than the radiation from any radioactive element. Fundamental discoveries would soon follow.

Lawrence’s 60-inch cyclotron completed in 1939, showing the huge 60-inch electromagnet sandwich to the left and the beam pipe for the high-energy particle beam exiting the cyclotron to the right.

Lawrence is recognised as the father of Big Science. He pioneered a new approach to research in which teams of engineers and physicists push the frontiers of knowledge in large-scale projects involving the design and construction of new and often expensive technology. Particle accelerator design has come a long way since Lawrence’s cyclotrons, but all circular accelerators including the Large Hadron Collider in Geneva trace their ancestry back to Lawrence’s machines. In 1939 Lawrence was rewarded with the Nobel Prize for Physics:

for the invention and development of the cyclotron and for results obtained with it, especially with regard to artificial radioactive elements.

As the Nobel Prize Awards Committee intimated rather prophetically, Lawrence’s machines would project Berkeley scientists into the position of world leaders in the creation of artificial elements, stretching the outer edges of the Periodic Table beyond uranium. The first new element created by one of Lawrence’s machines at Berkeley was discovered in a rather surprising way. It was tracked down ten thousand kilometres away by a wily Italian who knew the value of a good piece of scrap metal when he saw one.

Scrap Metal

Emilio Segrè

Italian physicist Emilio Segrè (1905-1989) visited New York with his wife Elfriede in the summer of 1936. She found the weather humid and stifling in the city, so they decided to cross the country by train to visit California where Segrè could see Lawrence’s new 37-inch cyclotron in action.

During his visit to the Berkeley Rad Labs, Segrè noticed there was lots of radioactive scrap lying around. In Italy the raw materials for nuclear research were in short supply, so Segrè collected several pieces of discarded cyclotron components, and Lawrence was happy for him to return home with them. 

Back in Italy Segrè took up a position at the University of Palermo in Sicily. A few months later, in February 1937, Segrè received a package from Lawrence containing a piece of molybdenum foil that had been used to protect one of the cyclotron’s magnets by deflecting particle beams out through the exit slot of the cyclotron. Segrè realised that this heavily irradiated sample of molybdenum offered him a great opportunity.

Mind the Gap

Mendeleyev had predicted the existence of an unknown element with similar chemical properties to manganese (Mn) that would fit beneath it in the Periodic Table. He provisionally named it eka-manganese. Following Henry Moseley’s explanation of atomic numbers as the charge on the atomic nucleus, it was clear that Mendeleyev’s eka-manganese was element number 43. But it was still nowhere to be found. Element 43 was one of just two gaps that remained in the Periodic Table with an atomic number lower than lead (element 82). 

Detail of the Periodic Table showing a gap for the element with atomic number 43, beneath manganese (Mn) and immediately after molybdenum (Mo).

Molybdenum (Mo) is element 42, so it slots into the Periodic Table one place in front of Mendeleyev’s eka-manganese. And this was where Segrè saw his chance. He realised there was a reasonable possibility that after undergoing the heavy particle bombardment in the cyclotron some molybdenum nuclei in the discarded foil would have absorbed an extra proton, increasing their atomic number by one, and transforming them into nuclei of the missing element number 43.

Technetium

Segrè enlisted Carlo Perrier (1886-1948), professor of mineralogy and an excellent analytical chemist, to help search for the new element. Within two months Segrè and Perrier had succeeded in isolating element 43 from the irradiated molybdenum foil and were ready to tell the world. They resisted the temptation to give their new element a name straight away, despite receiving suggestions that would celebrate both fascism and Sicily. A decade later, with their discovery firmly established, they chose the name technetium derived from the Greek for artificial, as this was the first artificially created element.

Technetium

In June 1938 Segrè visited Berkeley to follow up his research into technetium. While he was in America, however, Mussolini passed anti-Jewish legislation banning Jews from university positions in Italy, which meant that Segrè had no job to return to. With war in Europe on the horizon Segrè arranged for his family to join him in California and Lawrence offered him a position at Berkeley where he set about studying his new element.

Technetium would turn out to be very interesting and surprisingly useful. Every isotope of technetium is radioactive and the longest lived are technetium-97 and technetium-98, both of which decay with half-lives of 4.2 million years, so any technetium that might have been around when the Earth formed would have undergone radioactive transmutation long ago. 

Segrè was awarded the Nobel Prize in Physics in 1959. In a future post I will look at how it is that technetium, an element that does not naturally exist on Earth, has turned out to be so useful.



Further Information

There is more about Henry Moseley and atomic numbers in this post: Henry Moseley and the Nuclear Treasure Chest

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