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 keV
• B. 3 and 1 keV
• C. 2.5 and 2 MeV
• D. 2 and 2 keV
• E. 1 and 2 MeV

Hint:

In a nuclear reactor, we use "moderators" like heavy water or graphite specifically to slow these 2 MeV fast neutrons down to "thermal" energies (\(\approx 0.025 \text{ eV}\)) to sustain a controlled chain reaction.

Answer 1

The Correct Option is C

Concept: Nuclear fission involves the splitting of a heavy nucleus, like \(^{235}\text{U}\), into smaller fragments when struck by a neutron.
• Neutron Multiplication: Each fission event doesn't release a fixed whole number of neutrons; rather, it varies. On average, \(2.5\) neutrons are emitted per fission event.
• Energy of Fission Neutrons: The neutrons released are "fast neutrons," meaning they possess very high kinetic energy.

Step 1: Identify the average neutron count.
Scientific data across many fission events of Uranium-235 shows that for every one neutron absorbed, the resulting split releases between 2 and 3 neutrons, averaging out to approximately 2.5.

Step 2: Identify the energy scale.
Nuclear reactions involve energy changes on the scale of Mega-electron volts (MeV), unlike chemical reactions or thermal movements which are in the eV or keV range. The kinetic energy of these prompt neutrons is typically around 2 MeV.