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A radioactive substance emits radiations:
(a) $\alpha ,\beta $ and $\gamma $ simultaneously
(b) in the order $\alpha ,\beta \text{ and }\gamma $ one by one
(c) X-rays and $\gamma $ -rays
(d) $\alpha \text{ or }\beta $

Answer
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Hint: While a radioactive emission occurs, there is change in atomic mass, atomic number of the initial element. But in some decay reaction there is no change in atomic mass or atomic number, but simply emission of energy.

Complete answer:
- When a nucleus is transformed to one or more other elements by emitting radiation is called nuclear decay reaction or radioactive decay. The resulting daughter nucleus has a lower mass and is in lower energy than the parent nucleus.
- There are six different types of nuclear decay reactions and in each reaction a different kind of particle or energy is released. The common types of emission are alpha, beta and gamma emission. At a time only one decay reaction occurs.
- All the three, that is alpha, beta and gamma emission do not occur simultaneously and while gamma emission, no X-ray emission occurs.

For nuclear decay reaction, we use the notation ${}_{Z}^{A}X$ where X is the element, or alpha particle, beta particle, gamma, neutron, positron etc. And A is the atomic mass, Z is the atomic number of the particle. $\alpha $ particle is also helium nucleus and can be denoted as ${}_{2}^{4}H{{e}^{2+}}or{}_{2}^{4}\alpha $ and beta particle is usually free electron or an electron associated with an atom and is denoted as ${}_{-1}^{0}\beta $.
- In an alpha decay, the atomic mass of the initial element decreases by 4 and the atomic number decreases by 2. And the reaction is as follows
${}_{Z}^{A}X\to {}_{Z-2}^{A-4}{{X}^{'}}+{}_{2}^{4}\alpha $
Here new element X’ is formed.
- In a beta decay, there is no change in the atomic mass of the element, but the atomic number increases by 1 and the reaction is as follows
${}_{Z}^{A}X\to {}_{Z+1}^{A}{{X}^{'}}+{}_{-1}^{0}\beta $
- In a gamma decay, only energy is emitted and there is no change in the atomic mass or atomic number and the reactions is as follows:
${}_{Z}^{A}{{X}^{*}}\to {}_{Z}^{A}{{X}^{'}}+{}_{0}^{0}\gamma $ , here X* is the excited element and it releases energy in gamma radiation and becomes X’.
Thus, a radioactive substance emits either $\alpha \text{ or }\beta $ rays is the correct answer.

So, the correct answer is “Option (c) is correct.”.


Note:

For some decay reactions, there might be gamma emission along with an alpha decay or beta decay. But alpha and beta decay do not occur together.