nuclei Question 3

Question: Q. 3. Mx and My denote the atomic masses of the parent and the daughter nuclei respectively in a radioactive decay. The Q-value for a β-decay is Q1 and that for a β+decay is Q2. If me denotes the mass of an electron, then which of the following statements is correct?

(a) Q1=(MxMy)c2 and Q2=(MxMy2me)c2

(b) Q1=(MxMy)c2 and Q2=(MxMy)c2

(c) Q1=(MxMy2me)c2 and Q2=(MxMy+2me)c2

(d) Q1=(MxMy+2me)c2 and Q2=(MxMy+2me)c2

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Solution:

Ans. Correct option : (a)

Explanation: Let the parent nuclei zAX is radioactive atom and decay βas :

$$ \begin{aligned} { }{\mathrm{Z}} X^{A} & \rightarrow{ }{z+1} Y^{A}+{ }{-1} e^{0}+\bar{v}+Q{1} \ Q_{1} & =\left[m_{n}\left({ }{z} X^{A}\right)-m{n}\left({ }{z+1} Y^{A}\right)-m{e}\right] c^{2} \ & =\left[m_{n}\left({ }{z} X^{A}\right)+m{\varepsilon} Z-m_{n}\left({ }{z+1} Y^{A}\right)-(Z+1) M{e}\right] c^{2} \ & =\left[m\left({ }{z} X^{A}\right)-m\left({ }{z+1} Y^{A}\right)\right] c^{2} \end{aligned} $$

Let the nucleus =zXA radiate β decay

$$ \begin{aligned} & { }{z} x^{A} \rightarrow{z-1} Y^{A}+{1} e^{0}+v+Q{2} \ & Q_{2}=\left[m_{n}\left({ }{z} X^{A}\right)-m\left({ }{z-1} Y^{A}\right)-2 m_{e}\right] c^{2} \ & =\left[m_{n}\left({ }{z} X^{A}\right)+m{e} Z-m_{n}\left({ }{z-1} Y^{A}\right)-(Z-1) m{e}-2 m_{e}\right] c^{2} \ & =\left[m\left({ }{z} X^{A}\right)-m{e}\left({ }{z-1} Y^{A}\right)-2 m{e}\right] c^{2} \ & =\left(M_{x}-M_{y}-2 m_{e}\right) c^{2} \end{aligned} $$

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