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Radioactive Decay of Substances
Radioactive decay is an important topic of the IIT JEE syllabus. Various topics covered under radioactivity like Alpha, Beta and Gamma rays assume great importance because questions are often picked up from these topics in JEE. It is important to master the topic to remain competitive in IIT JEE. Practical questions framed on this topic are simple as well as interesting. The JEE does not ask very tricky questions from these heads. Several topics are included under radioactivity but the prime ones are listed below:
All these topics have been discussed in detail in the coming sections. We shall give only a brief outline of each of these heads here:
Radioactivity is basically the process of decomposition of nuclei. The unstable atomic nuclei spontaneously decompose to form nuclei with a higher stability. This process which occurs or the attainment of stability is called radioactivity. The process leads to the emission of energy and particles which are called radiation. There can be two kinds of radioactivity- natural or induced radioactivity.
If N0 be the total number of atoms at t = 0, N be the total number of atoms left in the sample at time t then dN/dt will be the rate of disintegration.
The half-life for a given radioisotope denotes the time for half the radioactive nuclei in any sample to undergo radioactive decay. After two half-lives, the leftover portion will be one fourth of the original sample. Similarly, after three half-lives, one eighth of the original sample and so on. In other words, radioactivity may also be defined as the interval of time required for one-half of the atomic nuclei of a radioactive sample to decay or the time interval required for the number of disintegrations per second of a radioactive material to decrease by one-half.
Average Life or Mean Life
Alpha, Beta and Gamma rays
Alpha rays are the positively charged radiation. These rays are more massive than the beta rays and they contain a stream of positively charged particles, called alpha particles, which have the atomic mass of 4 and a positive charge of 2 (a helium nucleus). The emission of an alpha particle from the nucleus decreases the mass number of the nucleus by 4 and the atomic number reduces by 2. As in the reaction given below, the helium nucleus is the alpha particle.
23892U → 42He + 23490Th
View this video for more on alpha, beta and gamma rays
As discussed above, the penetration power of gamma rays is maximum, the below figure further clarifies this fact.
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