It Came Out of Niels Bohr’s Nuclear Well

This story originally appeared in the Health Physics Society's Newsletter.

In the basement of Niels Bohr’s Institute for Theoretical Physics in Copenhagen was a well (Levi 1986)—two meters across, three meters deep, a spiral staircase leading to the bottom. And what better place than the Institute for Theoretical Physics? It was here that George Gamow developed the quantum mechanical explanation for alpha particle escape from the nuclear well.

It was also at the Institute that Gamow drafted his theory regarding beer: Carlsberg is the best (Gamow 1970). When J. Carl Jacobsen, founder of the Carlsberg Brewery, died, he left his money to the Danish Academy of Sciences, and it was a Carlsberg Fellowship that paid for Gamow’s stay at the Institute. Thanks to beer, alphas tunnel out of the nucleus.

Jacobsen’s will also stipulated that his palatial mansion should be occupied by Denmark’s most accomplished scientist. The first to take up residence was Niels Bohr who had previously occupied an apartment at the Institute. Bohr derived great pleasure from the fact that he no longer had to both live and work in the same place (Aaserud 1985)—not surprising when you consider that his new home was a mansion smack dab next to the Carlsberg brewery (with beer on tap?). Could this also explain Bohr’s often difficult to understand mumbled speech?

Bohr’s old friend George de Hevesy was more of a workaholic and not nearly as eager to leave the Institute and go home as Bohr. Once, while hurrying down the Institute stairs, Hevesy passed his assistant, L. Hahn exclaiming "Herr Hahn, Herr Hahn, you are lucky. You have your rabbits [Hahn’s experimental animal]—I must go home to my family" (Levi 1985).

Good friend that he was, Hevesy organized a very special present from the Danish people in honor of Bohr’s fiftieth birthday: 100,000 kroner to purchase 600 milligrams of radium. Of course, Hevesy knew that a theoretician like Bohr had no use for radium. That’s why the present was so special. Hevesy intended to use the radium himself—to generate neutrons with which he could produce artificial radioisotopes (e.g. P-32).

To produce neutrons, the radium had to be intimately mixed with powdered beryllium and the unsavory task of grinding up the beryllium fell on Otto Frisch, an Austrian physicist working at the Institute (Frisch 1979). The powdered beryllium was mixed in a solution of the radium, the solution dried, and the residue sealed in several small capsules attached to long handles.

Where these neutron sources should be kept was obvious. Could there be a more appropriate place for neutrons than the bottom of a nuclear well? And it was in this well, with these neutrons, that Hevesy’s coworker, Hilde Levi, activated Carl Auer von Welsbach’s dysprosium - the experiment that accidentally led to the technique of neutron activation analysis (for that and other stories concerning de Hevesy, read Four Tales of George de Hevesy).

When not otherwise in use, the neutron sources were stored in a flask of carbon disulfide. Over a couple of weeks an n,p reaction with S-32 would produce a usable quantity of P-32 (Levi 1985). With this P-32, Hevesy conducted some of the first tracer studies using artificial radionuclides: determinations of phosphorous metabolism in humans and the turnover of various organic compounds (e.g., ATP) in the body.

One day, a technician was in the well changing out the carbon disulphide when disaster struck—the bottle containing the fresh solution slipped and broke! She quickly scrambled up the stairs, a rising cloud of poisonous explosive vapors nipping at her heels. Frisch made an [insert your own adjective] attempt to enter the well holding his nose, but was forced back (Frisch 1979). And as he escaped back up the stairs, he had visions of the nails in his shoes striking a spark that would blow him right through the roof. Finally, cooler heads prevailed and a pump was installed to expel the vapors. It was a slow process and Frisch, whose apartment was more or less directly above the well on the top floor of the Institute, had a troubled sleep that night: "I went to bed in a fatalistic mood. But the next morning I was still there." (Frisch 1979).

Much later, long after the carbon disulphide vapors had dissipated, Otto Frisch would venture once again into the well—in search of something never before seen by the eyes of man or woman.

His search began as a result of a mysterious letter in 1938 from Otto Hahn to Lisa Meitner, Frisch’s aunt. Hahn asked Meitner if she could offer some fantastic explanation for the apparent production of barium in the uranium he had bombarded with neutrons. "Would it be possible" Hahn asked, "that uranium 239 bursts into a Ba and a Ma [Tc]?"(Stuewer 1985). By employing the liquid drop model of the nucleus developed by Gamow and Bohr, Meitner and Frisch demonstrated that a splitting of the uranium nucleus to produce barium was theoretically feasible. Furthermore, they calculated that the products of this splitting would have exceptionally high kinetic energies. But their fantastic explanation seemed far too fantastic for some at the Institute. So, to satisfy himself as well as the doubters, Frisch decided to experiment (Frisch 1979). Down he went into the well (Friday, 13th, January 1939)—to measure the energy of the particles emitted by uranium as it was exposed to neutrons. By positioning the uranium next to a proportional chamber, exposing the uranium to neutrons, and viewing the pulses from the chamber with an oscilloscope, Frisch performed a visual pulse height analysis! Word quickly spread throughout the Institute about the unusual events in the well, and one of those who decided to see for himself was the biologist William Arnold (Arnold 1996; Ermanc 1989). At first, all Arnold saw on the oscilloscope screen was a series of small pulses produced by uranium’s alpha particles. But then, at Frisch’s suggestion, he picked up one of the neutron sources by the handle and put it next to the uranium. The world changed! Huge pulses began appearing—pulses far larger than anything produced by the alpha particles—pulses produced by the fragments of the splitting uranium nuclei!

Later that day, Frisch tracked Arnold down and said something to the effect "You’re supposed to be some kind of biologist. What is the term you use to describe dividing bacteria?" Arnold replied, "Binary fission." Frisch then asked if the word "fission" would suffice and Arnold agreed that it would.

And that’s how fission, alpha particles, neutron activation analysis and Otto Frisch came out of the nuclear well.

References

• Arnold, W. Personal communication; 1996.
• Aaserud, F. Niels Bohr as a Fund Raiser. Physics Today. October:38-46; 1985.
• Ermenc, J. Atomic Bomb Scientists. Meckler; Westport; 1989.
• Frisch, O. What Little I Remember. Cambridge University Press; Cambridge; 1979.
• Gamow, G. My World Line. Viking Press; New York; 1970.
• Levi, H. George de Hevesy. Adam Hilger Ltd.; Bristol; 1985.
• Levi, H. Semicentennial Lecture, Modern Trends in Activation Analysis. 7th International Conference. June 23, 1986.
• Stuewer, R.H. Bringing the News of Fission to America. Physics Today. October:48-56; 1985.