Radon-beryllium neutron source used by E. Fermi & associates, 1933-34

Object EM*N-08430 consists of radon gas and a beryllium rod, enclosed in a glass tube, all enclosed in an exterior brass cylinder. This apparatus was used in 1934-35 by Enrico Fermi and coworkers in producing slow neutrons in investigations on induced radioactivity.

The brass rod pulls apart; inside is a glass tube sealed at both ends. Inside one end of the glass tube is a small sealed glass pod, prevented from sliding by wads of cotton. Inside the pod is a mass of black granules; the tube glass is discolored purple in this pod region. The outer brass rod/tube has internal pads at each end.

History

In their attempts to excite and transform atomic nuclei, physicists were limited throughout the 1920’s to bombarding atoms with particles, chiefly alpha particles, spontaneously emitted by sources consisting of naturally radioactive substances, for example radium. (Alpha particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus.) Disadvantages of using alpha particles included the limited supply and great expense of radium and similar substances, as well as the limited energy and uncontrollability of these spontaneous radiations. The situation was overcome by the use of neutrons, first discovered by James Chadwick in 1932. (See Chadwick ionization chamber replica; object ID no. - - - -) Chadwick recognized evidence of the particle in I. and F. Joliot Curie’s description of phenomena resulting from the bombardment of beryllium by alpha particles. Although the husband and wife team missed the neutron discovery, their continuing investigations of the bombardment of light elements by alpha particles led them in 1934 to recognize that in this process radioactivity was being induced artificially in the target nuclei. (See Joliot-Curie apparatus replica; object ID no. EM*N-09624.2)

Based on the above discoveries, Enrico Fermi at the University of Rome immediately inferred that if alpha particles could induce artificial radioactivity, neutrons should do so - - and far more readily. He quickly gathered his group of young coworkers to help him exploit the field thus opened. However Fermi left his position in Rome in 1938 to escape Italian racial laws that affected his wife, Laura Capon, who was Jewish. He took a position at the University of Chicago where he led development of a sustainable nuclear reaction - central to what became the atomic bomb.

Basic principles of Fermi’s neutron source

When a radioactive element that emits alpha particles is mixed with a light element such as beryllium, neutrons are emitted because many of the alpha particles are absorbed by the nuclei of the light element. The radon-beryllium mixture in the tube of object no. EM*N-08430 was used as a source of neutrons by Fermi and his associates at the University of Rome in 1934-35 for their investigations of neutron-induced radioactivity, which showed that nuclear reactions could be produced in almost all elements by bombarding them with neutrons.

In Fermi’s neutron source, radon gas (Rn 222) was bled from a solution of radium (Ra 226) and collected on and around the beryllium metal at one end of the tube. The radon decays with a half-life of 3.8 days to lead (Pb 210) by way of two short lived alpha-emitting isotopes. The three alpha particles resulting from the decay of the isotopes interact with the beryllium (Be 9) to produce neutrons.

Currently not on view