Question

The mean free path of a 5 eV neutron in vacuum is closest to (Life time of neutron is about 10³ sec)–

• A. 10 km
• B. 100 km
• C. 1,000 km
• D. 10,000 km

Answer 1

Answer: (D)

10,000 km

Answer 2

The mean free path in vacuum is the distance the neutron travels in its lifetime, from generation to decay. Lifetime of the neutron is about 10³ s. As its energy 5 eV is much less than its rest energy 940 MeV, non-relativistic approximation may be used and its velocity is

v == c=$\sqrt { \frac { 2 \times 5 \times 10 ^ { - 6 } } { 940 } }$× 3 × 10⁸ = 10⁴ m/s.

Thus S = vt = 10⁴ km