Student Guide - Neutron Activation of Ag with a Pu/Be n-source
(written for the RoboLab NAA exercise)
Neutron Source
For this lab exercise a neutron source is needed. If you are using the RoboLab remote experiment facility the neutron source in Oslo is a Pu/Be source. This source consist of powdered beryllium metal mixed with 350 GBq (9,6 Ci) of 238Pu, an α-emitter. When the α-particles hit the Be-nuclei the following nuclear reactions take place:
9Be(α,n)12C
The source emits 2.8•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 at the location of the material we irradiate. The neutrons emitted form the source have an average energy of 4-5 MeV and are called fast neutrons.
Thermal Neutrons
Thermal neutrons have an 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 much higher for most elements compared to using high-energy neutrons. Therefore 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. When used for this purpose, the paraffin or water is usually refered to as "the moderator".
Thermal neutrons will move in all directions, because of the collisions with the nuclei in the moderator. Thus, they can then be considered a gas, filling the moderator, where the density decreases with the distance from the detector. There will therefore be an optimal distance between the n-source and the sample, where the neutrons have been efficiently moderated but the "n-gas" is not too diluted. The type of moderator, n-source, geometrical construction of the irradiating facility, etc. will influence this optimal sample position, but in most cases it will be between 3-7 cm away from the source.
Neutron Activation
Nuclear reactions following a flux φ of thermal neutrons of an isotope M of a given element is typically:
MA (n, γ)M+1A
The new isotope is of the same element, but with the mass number increased by one. We refer to this as n-capture, since the new isotope has "captured" an extra neutron. Since the mass number changes, the isotope produced in the reaction may be radioactive - which is the whole point of n-activation. In this way the amount of the element in the sample can accurately be determined even at at very low concentrations, we then would call it Neutron Activation Analysis (NAA). It is also a common way to produce radionuclides for use as e.g. tracers.
If you look in your nuclear chart, you will find two stable isotopes of silver: 107Ag and 109Ag. For each, both a metastable state and the ground state of the daughter will be produced (as you can see from the cross section - it is given as the sum of two numbers, indicating the cross section for forming the metastable state (first number) and ground state (last number).
I.e. for n-activiation of natural silver we will get: 108mAg, 108Ag, 110mAg, and 110Ag.