Neutron activation of Ag with a Pu/Be n-source
In this lab exercise you will use a neutron source. It consists of Beryllium metal (powder) mixed with 450 GBq (12 Ci) of 238Pu, an α-emitter. When the α-particles hit the Be-nuclei the following nuclear reactions take place:
The source emits 3 •107 neutrons per second. Most often we want to know the number of neutrons that hits our target, the neutron flux. That is, the number of emitted neutrons per unit area and time. The neutrons emitted form the source have an average energy of 4-5 MeV and are called fast neutrons.
Thermal neutrons have and average energy of 0.025 eV and an energy distribution comparable to that of gas molecules at room temperature. At this low energy the probability of neutron capture is great. So we want to reduce the fast neutrons from our source to thermal energies.
Since neutrons are neutral they do not lose their energy in electrostatic interactions. Rather they lose energy in collisions with other nuclei. The most efficient transfer of energy is when the colliding neutron and nucleus have the same mass. So, materials containing a lot of hydrogen are good materials for moderating fast neutrons, two examples are paraffin and water.
Thermal neutrons will move in all directions, because of the collisions with the nuclei in the moderator. They can then be considered a gas, filling the moderator, where the density decreases with the distance from the detector.
Nuclear reactions following a flux Φ of thermal neutrons of an isotope M of a given element is typically:
AM (n, γ) A+1M
The new isotope is of the same element, but with mass number increased with one. Since the mass number changes, the isotope may be radioactive.