^Moderator

The neutrons produced in fission of ²³⁵U nuclei have average KE about 2 MeV. Such neutrons are called fast neutrons. These fast neutrons have more tendency to escape instead of triggering another fission reaction. Slow neutrons are more efficient in inducing fission in ₉₂U²³⁵ nuclei than fast neutrons. By the use of a moderator, the fast neutrons are slowed to thermal velocities i.e. velocities » 2200 m/s & energies » 0.0235 eV, it is same as that of atoms and molecules at room temperatures, such slow moving neutrons are called thermal neutrons. Light target are better moderators. The commonly used moderator are water, heavy water (D₂O), graphite and beryllium. About 25 collisions with deutrons (present in heavy water) or 100 collisions with carbon or beryllium are sufficient to slow down a neutron from 2 MeV to thermal energies.

A good moderator must have:

1. low atomic weight
2. should collide elastically with neutrons.
3. should not absorb the neutrons

^γ-decay

Most radioisotopes, after an alpha decay or a beta decay, leave the daughter nucleus in an excited state, these excited nuclei make a transition to a state of lower energy by emitting a photon. These photons are charge less, mass less & high energy electromagnetic waves (of the order of million electron volt) & are called the gamma rays.

_ZX^A (unstable nuclei) → _ZX^A (stable nuclei) + γ

^α-decay

Consider the following decay

As a nucleus decays due to internal force of repulsion, there is no net external force on it, hence in any nuclear reaction linear momentum must be conserved.

Before disintegration, the nucleus can be assumed to be at rest, so the total momentum was zero. After disintegration let it be m_av_a & m_D v_D for  alpha particle & daughter nuclei respectively. To conserve linear momentum the total vector momentum must still be zero i.e.  m_av_a + m_Dv_D = 0 or m_av_a = -m_Dv_D

i.e. momentum of a particle must be equal & opposite to that of daughter’s nucleus.

In magnitude, m_av_a = m_Dv_D

As mass of alpha particle is much lighter than thorium, thus the lighter α particle carries off most of the energy in the form its KE (about 98% of the total KE).