NEET Physics Notes Optics-Radioactivity-Concept of Radioactivity
Concept of Radioactivity
Radioactivity is the phenomenon of spontaneous emission of radiations by heavier nucleus. Some naturally occurring radioactive substances are uranium, thorium, polonium, radium, neptunium etc. In fact, all elements having atomic number Z > 82 are radioactive in nature.
Radiations emitted by radioactive substances are of three types, namely
α – Particlesare positively charged particles with charge 
and mass 
. Thus, a-particles may be considered as helium nuclei (or doubly charged helium ions). Ionising power of α – Particles is maximum but, their penetrating power is minimum.
β – Particles are negatively charged particles with rest mass as well as charge same as that of electrons. But origin of β – Particles is from the nucleus. Their ionising power is lesser than that of a-particles, but speed as well as penetrating power is much greater than that of a-particles. Generally, 
γ – rays are electromagnetic radiations of extremely short wavelengths. Thus, γ – rays travel with the speed of light. Their ionising power is least but penetrating power is extremely high. These are not deflected either in an electric or a magnetic field. Proton was discovered through artificial disintegration of nitrogen by a-particles as follows
Law of Radioactive Decay
According to Rutherford-Soddy’s law for radioactive decay, “The rate of decay of a radioactive material at any instant is proportional to the quantity of that material actually present at that time.” Mathematically,
Here, X is a proportionality constant, known as the decay constant (or disintegration constant). Unit of X is s-1 or day-1 or year-1 etc. It can be shown that number of nuclei present after time t is given by 
here, N0 = number of nuclei present at time t = 0. Again number of nuclei decayed in time t will be 
Activity
The activity of a radioactive substance is defined as the rate of disintegration (or the count rate) of that substance. Mathematically, activity is defined as
Different Units of Activity
Units of activity are
- 1 becquerel = Bq = 1 disintegration per second (SI unit)
- 1 curie = 1 Ci = 3.7 x 1010 Bq
- 1 rutherford = 1 Rd = 106 Bq
Half-Life Period
Half-life period of a radioactive sample is the time, in which half of the particles of the sample initially present, gets disintegrated. Alternately, it is the time in which, activity of the sample falls to one-half of its initial value.
The half-life period is related to decay constant X as
After n half-lives, the quantity of a radioactive substance left intact (undecayed) is given by
Mean Life Period (T)
- Mean life of a radioactive sample is the time, at which both N and R have been reduced to
or e-1 (or 36.8%)
of their initial values. It is found that
- Half-life T1/2 and mean life x of a radioactive sample are correlated as,
