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:
and