External Exposure - External Contamination

External Dose Uptake

External exposure is radiation from outside our body that interacts with us. In this case, we analyze exposure from gamma rays predominantly since alpha and beta particles generally constitute no external exposure hazard because the particles generally do not pass through the skin. For example, the radiation source can be a piece of equipment that produces the radiation, like a container with radioactive materials or an x-ray machine. In radiation protection, there are three ways how to protect people from identified external radiation sources:

Principles of Radiation Protection – Time, Distance, Shielding

Limiting Time. The amount of radiation exposure depends directly (linearly) on the time people spend near the source of radiation, and the dose can be reduced by limiting exposure time.
Distance. The amount of radiation exposure depends on the distance from the source of radiation. Similar to heat from a fire, if you are too close, the intensity of heat radiation is high, and you can get burned. If you are at the right distance, you can withstand it without any problems, and it is comfortable. If you are too far from a heat source, the insufficiency of heat can also hurt you. In a certain sense, this analogy can be applied to radiation also from radiation sources.
Shielding. Finally, shielding must be used if the source is too intensive and time or distance does not provide sufficient radiation protection. Radiation shielding usually consists of barriers of lead, concrete, or water. There are many, many materials, which can be used for radiation shielding, but there are many, many situations in radiation protection. It highly depends on the type of radiation to be shielded, its energy, and many other parameters. For example, even depleted uranium can be used as good protection from gamma radiation, but on the other hand, uranium is absolutely inappropriate for shielding of neutron radiation.

As was written, it is crucial whether we are exposed to radiation from external or internal sources, similar to other dangerous substances. Internal exposure is more dangerous than external exposure since we carry the radiation source inside our bodies, and we cannot use any of the radiation protection principles (time, distance, shielding).

ionizing radiation - hazard symbol — ionizing radiation – hazard symbol

Radiation Exposure

In general, radiation exposure is a measure of the ionization of air due to ionizing radiation from high-energy photons(i.e., X-rays and gamma rays). Radiation exposure is defined as the sum of electrical charges (∆q) on all the ions of one sign produced in the air when all the electrons, liberated by photons in a volume of air whose mass is ∆m, are completely stopped in the air.

Radiation exposure is given the symbol X. The SI unit of radiation exposure is the coulomb per kilogram (C/kg), but in practice, the roentgen is used. The roentgen, abbreviated R, is the unit of radiation exposure. In the original definition, 1 R means the number of X-rays or γ-radiation required to liberate positive and negative charges of one electrostatic unit of charge (esu) in 1 cm³ of dry air at standard temperature and pressure (STP).

Absorbed and Equivalent Dose

In radiation protection, a sievert is a derived unit of equivalent dose and effective dose. The sievert represents the equivalent biological effect of depositing a joule of gamma rays energy in a kilogram of human tissue. Absorbed dose is defined as the amount of energy deposited by ionizing radiation in a substance. The absorbed dose is given the symbol D. The absorbed dose is usually measured in a unit called the gray (Gy), derived from the SI system. The non-SI unit rad is sometimes also used, predominantly in the USA.

For radiation protection purposes, the absorbed dose is averaged over an organ or tissue, T. This absorbed dose average is weighted for the radiation quality in terms of the radiation weighting factor, w_R, for the type and energy of radiation incident on the body. The radiation weighting factor is a dimensionless factor used to determine the equivalent dose from the absorbed dose averaged over a tissue or organ. It is based on the type of radiation absorbed. The resulting weighted dose was designated as the organ- or tissue equivalent dose:

Radiation weighting factors - current - ICRP — Table of radiation weighting factors. Source: ICRP Publ. 103: The 2007 Recommendations of the International Commission on Radiological Protection

An equivalent dose of one sievert represents that quantity of radiation dose that is equivalent to specified biological damage to one gray of X-rays or gamma rays. A dose of one Sv caused by gamma radiation is equivalent to an energy deposition of one joule in a kilogram of tissue. That means one sievert is equivalent to one gray of gamma rays deposited in certain tissue. On the other hand, similar biological damage (one sievert) can be caused only by 1/20 gray of alpha radiation (due to high w_R of alpha radiation). Therefore, the sievert is not a physical dose unit. For example, an absorbed dose of 1 Gy by alpha particles will lead to an equivalent dose of 20 Sv. This may seem to be a paradox. It implies that the energy of the incident radiation field in joules has increased by a factor of 20, thereby violating the laws of Conservation of energy. However, this is not the case. Sievert is derived from the physical quantity absorbed dose but also considers the biological effectiveness of the radiation, which is dependent on the radiation type and energy. The radiation weighting factor causes the sievert cannot be a physical unit.

Contamination versus Radiation

Radioactive contamination consists of radioactive material that generates ionizing radiation and is the source of radiation, not radiation itself. Anytime radioactive material is not in a sealed radioactive source container and might be spread onto other objects, radioactive contamination is possible. Radioactive contamination may be characterized by the following points:

Radioactive contamination consists of radioactive material (contaminants) that may be solid, liquid, or gaseous. Large contaminants can be visible, but you cannot see radiation produced.
When released, contaminants can be spread by air, water, or mechanical contact.
We cannot shield against contamination.
We can mitigate contamination by protecting the integrity of barriers (source container, fuel cladding, reactor vessel, containment building)
Since contaminants interact chemically, they may be contained within objects such as the human body.
We can eliminate contamination by many mechanical, chemical (decontaminate surfaces), or biological processes (biological half-life).
It is of the highest importance which material is the radioactive contaminant (half-life, mode of decay, energy).

Ionizing radiation is formed by high-energy particles (photons, electrons, etc.) that can penetrate matter and ionize (to form ions by losing electrons) target atoms to form ions. Radiation exposure is the consequence of the presence nearby the source of radiation. Radiation exposure as a quantity is defined as a measure of the ionization of material due to ionizing radiation. The danger of ionizing radiation lies in the fact that the radiation is invisible and not directly detectable by human senses. People can neither see nor feel radiation, yet it deposits energy into the body’s molecules. The energy is transferred in small quantities for each interaction between radiation and a molecule, and there are usually many such interactions. Unlike radioactive contamination, radiation may be characterized by the following points:

Radiation consists of high-energy particles that can penetrate matter and ionize (to form ions by losing electrons) target atoms. Radiation is invisible and not directly detectable by human senses. It must be noted that beta radiation is indirectly visible due to Cherenkov radiation.
Unlike contamination, radiation cannot be spread by any medium. It travels through materials until it loses its energy. We can shield radiation (e.g., by standing around the corner).
Exposure to ionizing does not necessarily mean that the object becomes radioactive (except for very rare neutron radiation).
Radiation can penetrate barriers, but a sufficiently thick barrier can minimize all effects.
Unlike contaminants, radiation cannot interact chemically with matter and cannot be bound inside the body.
It is not important which material is the source of certain radiation. The only type of radiation and energy matters.

External Contamination

External contamination means that radioactive material has been deposited on surfaces (such as walls and floors). It may be loosely deposited, much like ordinary dust, or firmly fixed by a chemical reaction. This distinction is important, and we classify surface contamination based on how easily it can be removed:

Free Contamination. In the case of free contamination (or loose contamination), the radioactive material can be spread. This is surface contamination that can easily be removed with simple decontamination methods. For example, if dust particles containing various radioisotopes land on the person’s skin or garments, we can clean it up or remove clothes. Once a person has decontaminated, all particulate radioactivity sources are eliminated, and the individual is no longer contaminated. Free contamination is also a more serious hazard than fixed contamination because dust particles can become airborne and easily ingested, leading to internal exposure to radioactive contaminants. Although almost all contaminants are beta radioactive with accompanying gamma emission, alpha contamination is also possible in any nuclear fuel handling area.
Fixed Contamination. In the case of fixed contamination, the radioactive material cannot be spread since it is chemically or mechanically bound to structures, and normal cleaning methods cannot remove it. Fixed contamination is a less serious hazard than free contamination and cannot be re-suspended or transferred to the skin. Therefore the hazard is usually an external one only. On the other hand, it depends on the level of contamination. Although almost all contaminants are beta radioactive with accompanying gamma emission, alpha contamination is also possible in any nuclear fuel handling area. Unless the level of contamination is very severe, the gamma radiation dose rate will be small and external exposure will be significant only in contact with, or very close to, the contaminated surfaces. Since beta particles are less penetrating than gamma rays, the beta dose rate can be high only at contact. A value of 1 mSv/h at contact for a contamination level of 400 – 500 Bq/cm² is fairly representative.

References:

Radiation Protection:

Knoll, Glenn F., Radiation Detection and Measurement 4th Edition, Wiley, 8/2010. ISBN-13: 978-0470131480.
Stabin, Michael G., Radiation Protection, and Dosimetry: An Introduction to Health Physics, Springer, 10/2010. ISBN-13: 978-1441923912.
Martin, James E., Physics for Radiation Protection 3rd Edition, Wiley-VCH, 4/2013. ISBN-13: 978-3527411764.
U.S.NRC, NUCLEAR REACTOR CONCEPTS
U.S. Department of Energy, Nuclear Physics and Reactor Theory. DOE Fundamentals Handbook, Volume 1 and 2. January 1993.

Nuclear and Reactor Physics:

J. R. Lamarsh, Introduction to Nuclear Reactor Theory, 2nd ed., Addison-Wesley, Reading, MA (1983).
J. R. Lamarsh, A. J. Baratta, Introduction to Nuclear Engineering, 3d ed., Prentice-Hall, 2001, ISBN: 0-201-82498-1.
W. M. Stacey, Nuclear Reactor Physics, John Wiley & Sons, 2001, ISBN: 0- 471-39127-1.
Glasstone, Sesonske. Nuclear Reactor Engineering: Reactor Systems Engineering, Springer; 4th edition, 1994, ISBN: 978-0412985317
W.S.C. Williams. Nuclear and Particle Physics. Clarendon Press; 1 edition, 1991, ISBN: 978-0198520467
G.R.Keepin. Physics of Nuclear Kinetics. Addison-Wesley Pub. Co; 1st edition, 1965
Robert Reed Burn, Introduction to Nuclear Reactor Operation, 1988.
U.S. Department of Energy, Nuclear Physics and Reactor Theory. DOE Fundamentals Handbook, Volume 1 and 2. January 1993.
Paul Reuss, Neutron Physics. EDP Sciences, 2008. ISBN: 978-2759800414.

See above:

Radiation Protection