# Example in the book I don't understand fully

In a nuclear reactor, neutrons lose energy by making collisions with nuclei of atomf of the materials that maay be present in the core of the reactor. If a neutron of mass mn has initial kinetic energy of 5.0 MeV,how much kinetic energy will it lose if it makes head on elastic collision with nucleas of lead mpb = 206mn, carbon mc = 12 mn or hydrogen mh = mn?

The book solves this question by getting the final speed in terms of elastic collision.
I understand all the example but what I don't understand is we here used elastic collision but isn't elastic collision defined when there is no loss of kinetic energy ? so why then we would use elastic collision in this case ? I know its stated in the problem,but what I don't understand is that kinetic energy is defined for elastic collision to be when there is no loss in kinetic energy thats what I don't understand about this problem.

ehild
Homework Helper
Elastic collision means that the sum of the kinetic energy of the colliding particles is the same before and after the collision. The problem ask how much kinetic energy the neutron loses.

ehild

Andrew Mason
Homework Helper
In a nuclear reactor, neutrons lose energy by making collisions with nuclei of atomf of the materials that maay be present in the core of the reactor. If a neutron of mass mn has initial kinetic energy of 5.0 MeV,how much kinetic energy will it lose if it makes head on elastic collision with nucleas of lead mpb = 206mn, carbon mc = 12 mn or hydrogen mh = mn?

The book solves this question by getting the final speed in terms of elastic collision.
I understand all the example but what I don't understand is we here used elastic collision but isn't elastic collision defined when there is no loss of kinetic energy ? so why then we would use elastic collision in this case ? I know its stated in the problem,but what I don't understand is that kinetic energy is defined for elastic collision to be when there is no loss in kinetic energy thats what I don't understand about this problem.
Just to expand a bit on what ehild has said, the neutron loses a certain amount of energy but the nucleus that it collides with gains that same amount of energy. Assume the total kinetic energy is conserved. If you apply conservation of momentum and conservation of energy to the problem you will be able to figure out how much the neutron loses.

Can you see from the answer why water is used as a moderator ie. to slow fast neutrons down to thermal energies?

AM

Oh I see thank you guys. Yeh @ andrew I can see because they share since the mass of hydrogen materials is close to the mass of the neutron so it will be the one which slow it down the most :).
Thanks guys it this makes perfect sense to me now.