Uranium 239, which may be formed as the result of U-238 absorbing a neutron, is radioactive and decays by 0 emission, with a half life of 23-j minutes, to neptunium 239. This also decays by (3 emission, with half life 2.3 days, to plutonium 239, an a emitter of half life 24 000 years.

The above may be written:

238U + n 239U ——

23 t m

/3

— 239Np ——

2.3d

— 239-Pu (2.4 x 104y)

absorption of neutrons of any energy including slow neutrons of very low energies. The uranium 238 is called a fertile material because the absorption of the neutrons, which we have seen previously it most readily does in the resonance capture peaks, leads to the formation of the fissile material Pu-239.

Similarly thorium 232 is also a fertile material because neutron absorption leads, via protactinium 133 t0 the fissile material uranium 233:

&

233-Th + n — 233-Th —————

22m

— 233-Pa ——

27.4d

— 233-U (1.6 x 10-у)

Thus the fertile materials U-238 and Th-232 yield the fissile materials Pu-239 and U-233 respectively.