half-life (24,000 years) that it could not be detected. The first isotope of element 94 to be discovered was Pu²³⁸, made by direct deuteron bombardment in the Berkeley 60-inch cyclotron by Radiation Laboratory scientists Seaborg, McMillan, Kennedy, and Wahl; it had an α-decay half-life of 86.4 years, which gave it sufficient radioactivity so that its chemistry could be studied.

Having found these chemical properties in Pu²³⁸, experimenters knew ₉₄Pu²³⁹ would behave similarly. It was soon shown that the nucleus of ₉₄Pu²³⁹ would undergo fission in the same way as ₉₂U²³⁵ when bombarded with slow neutrons and that it could be produced in the newly assembled atomic pile. Researchers wished to learn as much as possible about its chemistry; therefore, during the summer of 1942 two large cyclotrons at St. Louis and Berkeley bombarded hundreds of pounds of uranium almost continuously. This resulted in the formation of 200 micrograms of plutonium. From this small amount, enough of the chemical properties of the element were learned to permit correct design of the huge plutonium-recovery plant at Hanford, Washington. In the course of these investigations, balances that would weigh up to 10.5 mg with a sensitivity of 0.02 microgram were developed. The "test tubes" and "beakers" used had internal diameters of 0.1 to 1 mm and could measure volumes of 1/10 to 1/10,000 ml with an accuracy of 1%. The fact that there was no intermediate stage of experimentation, but a direct scale-up at Hanford of ten billion times, required truly heroic skill and courage.

By 1944 sufficient plutonium was available from uranium piles (reactors) so that it was available as target material for cyclotrons. At Berkeley it was bombarded with 32-MeV doubly charged helium ions, and the following reactions took place:

This is to be read: plutonium having an atomic number of 94 (94 positively charged protons in the nucleus) and a mass number of 239 (the whole atom weighs approximately 239 times as much as a proton), when bombarded with alpha particles (positively charged helium nuclei) reacts to give off a neutron and a new element, curium, that has atomic number 96 and mass number 242. This gives off alpha particles at such a rate that half of it has decomposed in 150 days, leaving plutonium with atomic number 94 and mass number 238. The radiochemical work leading to the isolation and identification of the atoms of element 96 was done at the metallurgical laboratory of the University of Chicago.

The intense neutron flux available in modern reactors led to a new element, americium (Am), as follows:

The notation (n, γ) means that the plutonium absorbs a neutron and gives off some energy in the form of gamma rays (very hard x rays); it first forms ₉₄Pu²⁴⁰ and then ₉₄Pu²⁴¹, which is unstable and gives off fast electrons (β), leaving ₉₅Am²⁴¹.

Berkelium and californium, elements 97 and 98, were produced at the University of California by methods analogous to that used for curium, as shown in the following equations: