Question 5: Define and explain half-life of a radioactive element.
Definition of the half-life of a radioactive substance
Time during which half of the number of nuclei of a radioactive material decays is called half-life of the material. It is denoted by T.
Suppose there are 10000 atoms of a radioactive substance present at a certain instant of time. Then,
- 5000 atoms of the sample will decay in the first half-life. 5000 will remain.
- 2500 atoms of the sample will decay in the second half-life and 2500 will remain.
- 1250 atoms of the sample will decay in the third half-life and 1250 will remain.
And so on and so forth.
- number of atoms will decay in the first half-life and N0/2 number will survive.
- of atoms will decay in the second half-life and the same number will survive.
- number of atoms will decay in the third half-life and the same number will survive.
Mathematical Expression for half-life
From the law of radioactivity, we know that
N0 is the initial number of radioactive atoms and N is the number of atoms at any time t.
Now if during a half-life, N = N0/2 and t = T, then put in the above equation,
Cancel N0 on both sides,
Now taking logarithm on both sides,
Now log2 = 0.693 and loge = 1, therefore, the above equation becomes,
This equation gives the half-life of a radioactive substance. Thus if we know the value of λ for a certain radioactive material we can determine the half-life of it.
Measurement of half-life
The half-life of a radioactive material can be determined by finding the activity of the given radioactive sample. A count-rate meter (such as GM counter) is used to do this. When a nuclear radiation enters the tube of the counter, a pulse is recorded. Let N pulses are recorded in time t, then the rate of activity,
Now using equation (1), the half-life of the sample can be found.