The above radioactive decay processes often result in the changed nucleus, the daughter product, having an excess internal energy. Studies of the energy of radio­active emissions — and of the energy necessary to produce ‘artificial’ radioactive nuclei — have led to an understanding of the internal energy of the nucleus. It seems that if nuclei are stimulated in some way they cannot absorb just any quantity of energy. A nucleus can exist only with certain discrete internal energies. The state with the least internal energy is the most stable and is called the ground state of the nucleus.

If the nucleus is in some other discrete energy level above ground state it is said to be ‘excited’. The nucleus may then return to the ground state, or some other intermediate excited state, by giving off gamma (?) radiation. This is electromagnetic radiation occupying the very high frequency short wavelength end of the electromagnetic spectrum. It is therefore akin to X-radiation but even more penetrating. The energy of the emitted у ray will be exactly — and, for a given element, uniquely equal to the energy difference between the initial and final states of the excited nucleus; this is the basis of gamma spectro­scopy. There is no change, of course, in either the mass number or atomic number of the nucleus. (See also Section 1.5.4 of this chapter.)

To summarise: if in radioactive decay a daughter product is formed in an excited state, as is very often the case, the particulate radiation emissions are accompanied by у radiation as the nucleus drops to a lower or ground state.

T4 = logn(2)/X = 0.693/X = 0.693т

After five half lives only about 3% of the original radioactive nuclei remain, Fig 1.5. Values for half lives for particular isotopes range from fractions of a second (Be-8 : 3 x 10-16 s) to millions of years (U-238 : 4.5 x 109 years). The neutron is also an example of a radioactive substance; when it is freed from a nucleus it is a /3 emitter, and therefore trans­forming to a proton, with a half life of 10.8 minutes.

The unit of radioactivity used to be the Curie (Ci) and was originally defined in terms of the rate of activity of radium, the first radioactive substance studied. It was subsequently redefined to be 3.7 x 1010 nuclear disintegrations per second. The Curie has now been replaced by a new unit, the becquerel (Bq), which is exactly equal to 1 disintegration per second.