neutrons can easily pass thick lead partitions but struggle to pass through water and other materials with?

Question

high hydrogen content.

What is the reason( employing only classical mechanics argument)?

Answer 1

There is no classical reason.

The reason is purely quantum mechanical.

It is because the collision cross section of a hydrogen nucleus to a neutron (that is the apparent cross section for collision purposes - the nuclues is more or less a point) is large, and certainly much larger than other nuclei such as lead. This is not the result you would get classically - it can only be derived quantum mechanically.

Answer 2

Actually it is very easy to explain the elastic scattering cross section with classical theory.

When you look at a collision between two masses, m1 at v1, and m2 at rest. You will see that the maximum energy loss for m1 will occur when m1=m2 and when m1<>the mass of a neutron so when a neutron hits lead it bounces off with the same velocity that it had before.

There are other cross sections to consider in shielding like absorption but these require quantum considerations.

Answer 3

Epidavaros is correct. The scattering cross section is purely quantum mechanical and consists of coherent and incoherent cross sections. The cross sections are dependent upon the neutron's velocity, so the numbers given are for thermal neutrons. The scattering cross sections can very different for different isotopes of the same element. For hydrogen ^1H, the scattering cross section is 82.03 barns, whereas for deuterium ^2H the scattering cross section is 7.64 barns and for tritium ^3H the scattering cross section is 3.03 barns. For comparison, the scattering cross section of lead is 11.118 barns. Therefore, neutrons will pass through heavy water (D2O) more easily than through lead.

Answer 4

because hydrogen reacts with the neutrons to make various isotopes whilst lead doesnt

Answer 5

molecular activity is high