Irodov Problem 6.274
Physics · mechanics · modern-physics
Problem
To activate the reaction (n, a) with stationary B11 nuclei, neutrons must have the threshold kinetic energy Tth = 4.0 MeV. Find the energy of this reaction.Intuitive explanation
The key insight is that threshold energy accounts for the minimum kinetic energy needed to create products and conserve momentum—at threshold, the products move together as one unit in the lab frame. Since you know the threshold kinetic energy of the incoming neutron, you can use conservation of energy and momentum to work backward and find the Q-value (reaction energy). The Q-value tells you whether energy is released (exothermic) or absorbed (endothermic) in the nuclear transformation.
Intuitive explanation
The key insight is that threshold energy accounts for the minimum kinetic energy needed to create products and conserve momentum—at threshold, the products move together as one unit in the lab frame. Since you know the threshold kinetic energy of the incoming neutron, you can use conservation of energy and momentum to work backward and find the Q-value (reaction energy). The Q-value tells you whether energy is released (exothermic) or absorbed (endothermic) in the nuclear transformation.