Particle Detectors

Since neutrons are uncharged, their detection must depend on an interaction with matter which produces energetic charged particles. There are several nuclear reactions initiated by neutrons which result in charged particles. One of the most useful for slow neutrons is the reaction in which a neutron is incident on a boron nucleus. This reaction produces a lithium nucleus and an alpha particle, both of which are rapidly moving. Note that it is the boron isotope of mass 10, with a natural abundance of about 20%, that is required for this reaction and that the alpha particle is simply the nucleus of the helium atom. The boron is usually incorporated in the gas molecule BF₃ (boron trifluoride), which can be used as the gas in a proportional counter, which is much like a Geiger counter. The difference is simply that the voltages used are lower so that the discharge does not spread disruptively along the whole central electrode with the result that the electrical signal coming from the tube is proportional to the number of ions produced. The signals are much smaller than from a Geiger tube and require more amplification but the signal produced by the lithium nucleus and alpha particle, both of which are heavily ionizing, is relatively large and easily distinguishable. For fast neutrons, the probability of this boron reaction becomes very low so that other methods are required. A useful technique is to use a proportional counter filled with hydrogen. Fast neutrons colliding with the protons in hydrogen produce energetic protons which produce a signal from the counter.