CBSE Class 12 Physics 2017 Delhi Set 1 Paper

Question : 19 of 26

Marks: +1, -0

(i) A radioactive nucleus '

A

' undergoes a series of decays as given below:

{\A} {{ ──▸}}↖{α} {\A}_1 {──▸}↖{β} {\A}_2 {──▸}↖{α} {\A}_3 {──▸}↖{γ} {\A}_4

The mass number and atomic number of

A_2

are 176 and 71 respectively.
Determine the mass and atomic numbers of

{\A}_4

and

A

.
(ii) Write the basic nuclear processes underlying

β^{+}

and

β^{-}

decays.

Solution:

(i) Determining the mass and atomic number of

{\A}_4

and

{\A}

(ii) Basic nuclear processes of

β^{+}

and

β^{-}

decays
(i)

{\A}_4:

Mass Number : 172

Atomic Number : 69
A : Mass Number : 180
Atomic Number : 72
[Alternatively : Give full credit if student considers decay and find atomic and mass numbers accordingly

\;{ }_{72}^{180} {\A} {──▸}↖{α}{ }_{70}^{176} A_1 {──▸}↖{β^{+}}\; _{71}^{176} A_2 {──▸}↖{\α}

\; _{69}^{172} A_3 {──▸}↖{γ} \; _{69}^{172} A_4

Gives the values quoted above.
If the student takes

β^{+}

decay

\;{ }_{72}^{180} {\A} {──▸}↖{α}{ }_{70}^{176} A_1 {──▸}↖{β^{+}}\; _{71}^{176} A_2 {──▸}↖{\α}

\; _{69}^{172} A_3 {──▸}↖{γ} \; _{69}^{172} A_4

This would give the answers:

( {\A}_4: 172,69) ;( {\A}: 180,74)]

(ii) Basic nuclear process for

β^{+}

decay

p → n+{ }_1^0 e

+v

For

β^{-}

decay

n → p+{ }_{-1}^0 e+\ov{v}

[Note: Give full credit of this part, if student writes the processes as conversion of proton into neutron for decay and neutron into proton for decay.]

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