Question

On an average, the number of neutrons and the energy of a neutron released per fission of a uranium atom are respectively

• A. \( 2.5 \) and \( 2 \text{ keV} \)
• B. \( 3 \) and \( 1 \text{ keV} \)
• C. \( 2.5 \) and \( 2 \text{ MeV} \)
• D. \( 2 \) and \( 2 \text{ keV} \)
• E. \( 1 \) and \( 2 \text{ MeV} \)

Hint:

Memorizing key constants in nuclear physics is crucial. For Uranium fission, remember: ~200 MeV of total energy is released per fission, ~2.5 average neutrons are produced, and these are "fast" neutrons with ~2 MeV energy each.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
This is a theoretical knowledge question based on the principles of nuclear physics, specifically regarding the induced nuclear fission of Uranium-235 (\({}^{235}\text{U}\)).

Step 2: Detailed Explanation:
When a slow (thermal) neutron strikes a \({}^{235}\text{U}\) nucleus, it makes the nucleus highly unstable (\({}^{236}\text{U}^*\)), causing it to undergo fission.
The nucleus splits into two lighter, intermediate-mass fission fragments.
To conserve nucleons, this process also emits several secondary neutrons.
Depending on the specific fission fragment pair created, 2, 3, or sometimes more neutrons are ejected. Experimental data shows that the average number of neutrons released per fission is approximately 2.47, which is conventionally rounded to \( 2.5 \) in general physics problems.
These newly born secondary neutrons are emitted with high kinetic energy. They are termed "fast neutrons".
The average kinetic energy of these fast neutrons emitted during fission is known to be approximately \( 2 \text{ MeV} \) (Mega-electron Volts).
Therefore, on average, \( 2.5 \) neutrons are released, each possessing an energy of roughly \( 2 \text{ MeV} \).

Step 3: Final Answer:
The correct pair of values is \( 2.5 \) and \( 2 \text{ MeV} \), corresponding to option (C).