Structure of Atoms and Nuclei — Physics STD 12 Science — Question
Maharashtra BoardEnglish MediumSTD 12 SciencePhysicsStructure of Atoms and Nuclei4 Marks
Question
What are alpha, beta and gamma decays?
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Answer
(a) A radioactive transformation in which an $\alpha$-particle is emitted is called $\alpha$-decay.
In an $\alpha$-decay, the atomic number of the nucleus decreases by $2$ and the mass number decreases by $4$ .
Example : ${ }_{92}^{238} U \rightarrow{ }_{90}^{234} T h+{ }_2^4 \alpha$
$Q=\left[m_u-m_{T h}-m_a\right] c^2$
(b) A radioactive transformation in which a $\beta$-particle is emitted is called $\beta$-decay.
In a $\beta^{-}$-decay, the atomic number of the nucleus increases by $1$ and the mass number remains unchanged.
Example : ${ }_{90}^{234} Th \rightarrow{ }_{91}^{234} Pa+{ }_{-1}^0 e+\bar{v}_e$ where $\bar{v}_e$ is the neutrino emitted to conserve the momentum, energy and spin. $Q=\left[m_u-m_{T h}-m_\alpha\right] c^2$
In a $\beta^{+}$-decay, the atomic number of the nucleus decreases by $1$ and the mass number remains unchanged.
Example: ${ }_{15}^{30} P \rightarrow{ }_{14}^{30} S i+{ }_{+1}^0 e+v_e$
where $v_e$ is the neutrino emitted to conserve the momentum, energy and spin. $Q=\left[m_p-m_{S i}-m_e\right] c^2$
[Note : The term fi particle refers to the electron (or positron) emitted by a nucleus.]
A given nucleus does not emit $\alpha$ and $\beta$-particles simultaneously. However, on emission of $\alpha$ or $\beta$-particles, most nuclei are left in an excited state. A nucleus in an excited state emits a $\gamma$-ray photon in a transition to the lower energy state. Hence, $\alpha$ and $\beta$-particle emissions are often accompanied by $\psi$-rays.
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