Physics Dawn of Modern Physics BUMHS 2022
PMDC Verified Question 33 of 52
Positron and electron annihilate into:
A
\( \alpha \)-rays
B
One \( \gamma \) ray photon
C
Two \( \gamma \) ray photons
D
X-rays
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option C: Two \( \gamma \) ray photons
Concept:

The annihilation of an electron and a positron must satisfy both the conservation of mass-energy and the conservation of momentum.

Formula:

$$ 2m_ec^2 = 2hf $$

Solution:

  • If they annihilate at rest, initial momentum is zero.


  • A single photon cannot have zero momentum (since \( p = E/c \)), so one photon cannot be produced.


  • Therefore, two gamma (\( \gamma \)) ray photons must be generated, firing off in exactly opposite directions to ensure total momentum remains zero.


Why other options are incorrect:

Alpha rays are massive helium nuclei. X-rays do not contain enough energy to account for the \( 1.02 \text{ MeV} \) rest mass conversion of an electron-positron pair.

Quality & Fidelity Assurance: Every question on BeambePrep is rigorously curated against the official PMDC syllabus with zero filler, zero out-of-syllabus content, and zero typos. When an authentic past paper originally contained a historical mistake or ambiguity from the examining board (such as UHS or NUMS), BeambePrep faithfully reflects the original paper while detailing the nuance and scientific consensus in the autopsy above.

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