Question

If the rest mass of an electron or positron is 0.51 MeV, then the kinetic energy of each particle, in the electron- positron pair prochred by a γ- photon of 2.42 MeV, will be?
A. 0.3 MeV
B. 1.9 MeV
C. 0.7 MeV
D. 1.5 MeV

Answer 1

Hint: See that part of 2.42 MeV, some of the energy will be consumed in the formation of the electron-positron pair and rest will be given as kinetic energy.

Complete step-by-step answer:
Let us analyze the given data: the energy of the γ-photon is 2.42 MeV.

Now it is mentioned that an electron-positron pair produced from this γ-photon will consume 0.51 MeV of energy in process production. So the remaining energy is given by

E¹ = 2.42 − (0.51 × 2)
    = 2.42 − 1.02
E¹ = 1.4 MeV

As this is the remaining energy
∴ It will be equally divided in both the particles as both have the same mass i.e. $\dfrac{1.4}{2}$ = 0.7 MeV

∴ Kinetic energy in each electron – positron pair is 0.7 MeV.

The correct option is (C).

Note: You can note that there will not be any extra energy in the system. Therefore some of it will be consumed in the production and the remaining will only be distributed as the energy of the particles.