Neutron activation and sample handling

When an object is exposed to a neutron beam some of the neutrons will be absorbed, which is also called captured, by the atom nuclei in the object. This often creates unstable isotopes, which decays rapidly by emitting gamma radiation. This process of making a material radioactive is called neutron activation. The decay of the unstable isotopes can create a stable isotope, which will not cause any further radiation, or it can create new unstable isotope. The activation of a sample depends on both the amount of neutron radiation it is exposed to, the energy of the neutrons, and the elemental composition of the sample. Some elements capture neutrons very effectively and they will become more radioactive than other materials when subjected to the same amount of neutron radiation. Many elements decays rapidly into stable isotopes and the sample is then no longer radioactive. In case the decay slow or goes into further unstable isotopes it may take a while before the sample is no longer active. So the degree of activation does not only depend on the materials ability to capture neutrons but also the isotopes that is created and what they in turn decay into and at which rate.

The rate at which an isotope decays is called the radiation half-life and is the time it takes for the radiation level to be reduced to half the original amount of radiation. If the isotope does not decay into further unstable isotopes the half-life is also the time it takes for half the radioactive nuclei to become stable. It is important to note that the radiation half-life is not a measure of the radiation intensity, but a rate that describe for how long a material is radioactive, the intensity of the radiation depends on the number of unstable isotopes that has been created and the number of unstable isotopes that they in turn decay into. Of cause a short half-life also means that the material will release more radiation per second, but it will “cool” faster.

Handling Neutron Activated Samples

So what does neutron activation imply mean for a neutron experiment? There are two things, the first and most general is the health risk. After a sample has been probed by a neutron beam, it is activated, so it is very important not to take it out of the neutron instrument right after a measurement with your bare hands. Instead you should wait for an appropriate amount of time dependent on the instrument and sample, and then measure the radiation of the sample with a radiation detector, which is an improved Geiger counter. If the radiation dose equivalent is below 100 μSv/h it is safe to handle, but limiting ones exposure is always a good idea so one should not have contact with an active sample more than necessary. If the radiation is above 100 μSv/h the local radiation protections team should be contacted and the sample will be stored until it has “cooled” to a level were it is safe to handle. The time it needs to be stored depends on the radiation half-life of the isotopes that are created in the sample and the half-life of any unstable isotope they decay into. The half-life does not depend on the atom number of the element, else heavier elements would not exist, but on the specific isotope. However, heavier elements tend to have a longer decay chain before they end up at a stable isotope. So a sample that is made of for example gold could be very radioactive after it was exposed to a neutron beam and it may have to be store for several years, while an organic sample of either plastic or proteins would have cool within minutes or even seconds after the exposure. So the health risk depends very much on the sample and which materials it is made from, so this should always be considered when planing a neutron experiment. Also the state of the sample is important if it is a power for instance, it can be dangerous if it is accidentally inhaled since it will then cause internal exposure to radiation if it has been activated in the neutron beam.

The second thing about neutron activation in relation to experiment is actually beneficial. When an isotope decays the energy or wavelength of the emitted gamma ray depends on which isotope decayed. So by activating a sample with a neutron beam and measuring the wavelengths of the resulting gamma radiation, we can actually find the elemental composition of the sample. This is called neutron activation analysis and is very useful when looking at cultural heritage items with neutrons, for example as part of an imaging analysis of an archeological find. One drawback of neutron activation analysis is that we only learn the type and quantity of elements present in the material and not its structure, but it is still a simple and useful experiment.