Radioactivity

In radioactive processes, particles or electromagnetic radiation are emitted from the
nucleus. The most common forms of radiation emitted have been traditionally classified as
alpha (a), beta (b),  and  gamma  (g )  radiation. Nuclear  radiation occurs  in  other  forms,
including the emission of protons or neutrons or spontaneous fission of a massive nucleus.
Of the nuclei found on Earth, the vast majority is stable. This is so  because almost
all short-lived radioactive nuclei have decayed during the  history  of  the Earth. There are
approximately 270 stable isotopes and 50 naturally occurring  radioisotopes (radioactive
isotopes). Thousands of other radioisotopes have been made in the laboratory.











Radioactive decay will change one nucleus to another if the product nucleus has  a
greater nuclear binding energy than the initial decaying nucleus. The difference in binding
energy (comparing the before and after states) determines which decays are energetically
possible and which are not. The excess binding energy appears as  kinetic energy  or  rest
mass energy of the decay products

The Chart of the Nuclides, is a plot of nuclei as a function  of  proton  number,  Z,  and  neutron  number,  N .  All  stable  nuclei  and  known radioactive nuclei, both naturally occurring and manmade, are  shown  on  this  chart, along with their decay properties. Nuclei with an  excess  of  protons  or  neutrons in  comparison

with the stable nuclei will decay toward the stable nuclei by changing protons into neutrons

or neutrons into protons,  or  else  by  shedding  neutrons  or  protons  either  singly  or  in

combination. Nuclei are also unstable if they are excited, that is, not in their lowest energy

states.  In this case  the  nucleus  can  decay  by  getting  rid  of  its  excess  energy  without

changing Z or N by emitting a gamma ray.

Nuclear decay processes must  satisfy  several conservation laws, meaning that the

value of the conserved quantity after the decay, taking into account all the decay products,

must  equal the  same  quantity  evaluated  for the  nucleus  before  the  decay.  Conserved

quantities include  total  energy  (including  mass),  electric  charge,  linear  and  angular

momentum, number of nucleons, and lepton number (sum  of  the  number  of  electrons,

neutrinos, positrons and antineutrinos—with antiparticles counting.

The number of nuclei in a sample that will decay in a  given interval of  time is
proportional to the number of nuclei in the sample. This condition leads to radioactive decay
showing itself as an exponential process, as shown in Fig. 3-2. The number,  N,  of  the
original nuclei remaining after a time t from an original sample of N0
 nuclei is
N = N0
e
-(t/T)
where T is the mean lifetime of the parent nuclei. From this relation, it can be shown that t1 / 2
= 0.693T.