Radiation Safety

Radiation is another word for the emission or transmission of energy through waves or particles. There is radiation everywhere around us in everyday life both from electromagnetic waves and particles, and it can be very beneficial such as the radiation from the Sun without which life could not exist. When the particle or waves interacts with matter it deposits the energy, if the energy is high enough it can break down or alter molecules which is radiation damage. For organic tissue like in the human body, this damage can destroy the cells or causes mutations that may e.g. for example evolve into cancer. In a neutron facility, there are places with elevated radiation levels, but with a few simple precautions which we describe below, it is a very safe working environment.

Radiation Types

There is a number of different kinds of radiation depending on which type of particle or wave that is being referred to. The two that are most important to be familiar with at a neutron facility is gamma radiation and of course the radiation of the free neutrons themselves. We will describe these two types of radiation in more detail below.

Gamma radiation is photons with a very short wavelength, less than 10-11 m (0.1 Angstrom), which also means that they carry energy on the order of keV 10 to the power of minus 11 meters, or 0.1 Angstrom, which also means that they carry energy on the order of kilo-electronvolts which is a lot for photons. They are produced in nuclear reactions when an unstable isotope (for instance after neutron capture) decays, and so gamma radiation is present in the processes involved in neutron science. Gamma radiation causes random cellular damage throughout the body and can penetrate deeply into most materials since they are photons and thus don’t have a mass or charge. This means that it is difficult to make shielding against gamma radiation and thus a lot of effort goes into safety measures against gamma radiation at neutron research facilities.

Neutron radiation is of cause present at a neutron facility since we are using free neutrons as a probe for our sample investigation. Neutron radiation is very hazardous for organic tissue since it interacts strongly with the hydrogen which is the most abundant element in the body. When neutrons interact with the tissue at can either be absorbed by the tissue and create unstable isotopes (neutron activation) that then decays and send out gamma radiation, or it can kick an atom so hard that it loses an electron (ionization). When the last processes happen to a hydrogen atom it moves a short distance through the tissue where it will ionize other molecules, coursing further tissue damage, the neutron itself is slow but can continue to move through the tissue until it is absorbed or exits the body. Luckily there are very effective ways to stop neutrons, and even a couple of meters of air is enough to reduce the health risk considerably. The main concern of neutron radiations is the activation of materials where unstable isotopes are created. A sample exposed to neutrons will become radioactive for a while depending on which materials it consists of.

Safety Measures

There are several radiation protection measures to consider when working with neutrons both in terms of shielding of high-radiation areas but also in terms of personal protection by limiting your exposure to radiation, monitoring your personal dosage and always follow the safety regulations.

Constructional safety measures

Shielding is of cause the best way to avoid exposure since it prevents the radiation from reaching the areas where people are working. It often evolves a lot of concrete and elements that block radiation depending on what type of radiation is a concern. The most important shielding factor for gamma radiation is the atomic number, the larger, the better. Thus hydrogen (in e.g. water) would not offer much protection whereas lead would.

For neutrons, the cross-sections for neutron interaction (and thus the shielding capacity) of a material varies through the periodic table and even differ between isotopes of the same element. This lead does not shield neutrons very efficiently whereas hydrogen (in e.g. water) will actually offer good protection. However, materials such as Boron and Cadmium are even better to absorb neutrons and are thus often used for shielding in the neutron instrument areas where the materials scientists work close to the neutron beam. All these considerations aside, since concrete is such a cheap and practical material to work with it is the most commonly used material for shielding of all radiation in most of the neutron facilities. However, they have to use very thick blocks of concrete around the target and guide areas for example, as plain concrete does not shield either neutrons or gamma rays very efficiently.

The neutron instrument areas need to be accessible to the scientists and therefore cannot be completely sealed off by concrete blocks. Instead, an interlock system has been installed which ensures that the neutron beam is shut off if the door to the area is open.

Personal safety measures

When talking about exposure there a two kinds, external and internal. External exposure is radiation from sources that are outside the body and internal exposure is radiation from radioactive materials that has entered the body. With external exposure, there are two important factors to minimize when trying to avoid radiation damage to the body: the level of radiation in the area and the time you are exposed. Areas with background radiation above the natural background radiation dose equivalent (~3 mSv/year) are always clearly marked and ranked by the level of the radiation. Most of the areas of a neutron research facility do not have elevated radiation levels and are not marked Some areas, mainly in the instrument caves, have a radiation level of up to 2 mSv/h dose equivalent and it is enough protection to just avoid being there too long. Very few areas have a radiation level of above 100 mSv/h microsievert per hour dose equivalent and it is enough protection to just avoid being there too long. Very few areas have a radiation level of above 100 microsievert per hour and they require special radiation protection gear and special permission to enter.

Internal exposure is another problem entirely. Like external exposure the amount of radiation is important, but with internal exposure, the radioactive material has entered the body, this means that you cannot limit the exposure time since it stays in the body. For this reason, it is strictly forbidden to eat, drink or smoke anywhere close to the instruments in a neutron research facility.

The last personal safety measure is monitoring. Radiation damage is cumulative, so it is important to know how much total radiation a person has been exposed to. For this reason, scientists and workers always wear personal dosimeters when they are in a neutron facility. The dosimeters are read by the radioprotection team at regular intervals and will inform the scientist if he or she received any radiation dosage. Neutron research facilities are very well safety regulated and monitored, so for a materials scientist that only visits neutron facilities to do his experiment, the largest dose of radiation he receives in connection with the experiment would actually be on the airplane going to/from the facility.