NCERT Solutions
Class 12 Physics
Nuclei

Q.13
The radionuclide 11C decays according to 1111C à 115B+ e+ + ν; T1/2 =20.3 min. The maximum energy of the emitted positron is 0.960 MeV. Given the mass values: m (116C) = 11.011434 u and m (116B) = 11.009305 u,
Calculate Q and compare it with the maximum energy of the positron emitted.
The given nuclear reaction is:
1111C à 115B+ e+ + ν
Atomic mass of, m (116C) = 11.011434 u
Atomic mass of, m (116B) = 11.009305 u
Half-life of ((116C) nuclei, T (1/2) = 20.3 min
Maximum energy possessed by the emitted positron = 0.960 MeV
The change in the Q-value (∆Q) of the nuclear masses of the (116C)
∆Q = [m (116C) – [m (116B) + me]] c2 (1)
Where, me = Mass of an electron or positron = 0.000548 u
c = Speed of light
If atomic masses are used instead of nuclear masses, then we have to add 6 me in the case of C11 and 5me in the case of 11B.
Hence, equation (1) reduces to:
∆Q = [m (116C) – m (116B) -2 me]] c2
Here, m (116C) and m (116B) are the atomic masses.
∆Q = [11.011434 − 11.009305 – (2 × 0.000548)] c2
= (0.001033 c2) u
But 1 u = 931.5
∆Q = (0.001033 × 931.5)
≈ 0.962 MeV
The value of Q is almost comparable to the maximum energy of the emitted positron.