This discussion shall supply you with information on radiation quantities and units, particularly on German regulations that were decisive for the design of our instruments. We already mentioned various measuring quantities in the General Notes on our Products, however without explaining details. Now we try to catch up on this subject.
This is anything but a thorough and comprehensive treatise on radiation quantities, nor will it be scientifically accurate down to the last detail. It is the attempt of a clear explanation of an admittedly complex subject. Moreover, we shall restrict ourselves to the application of our instruments.
We shall discuss photon radiation only. Photon radiation is a generic term covering both X-radiation and gamma radiation. X and gamma radiation are electromagnetic radiation, like radio waves and visible light are, but with much shorter wavelengths (traditionally X and gamma rays are characterized by energy in keV or MeV, not by wavelength, although this would equally be possible). X rays are created when fast electrons are stopped in matter like, for example, an electron beam in an X-ray tube or in a CRT (cathode ray tube). The maximum X-ray energy is equal to the energy of the electrons that were stopped. For example, an X-ray tube operated at a voltage of 100 kV accelerates the electrons to an energy of 100 keV. The X-radiation generated by that tube covers an energy spectrum ranging from zero to 100 keV (continuously, but with varying intensity). On the other hand, gamma rays originate from the decay of atomic nuclei. Gamma radiation does not form a continuous spectrum, but consists of one ore more »lines« that are characteristic for the decaying nucleus. For example, the gamma radiation following the decay of Cs-137 has an energy of 662 keV. Apart from their origin, X and gamma rays are the same, and therefore are joined by the term »photon radiation«.
Background information: What has wavelength to do with energy? Wavelength »L« (visible light as an example: L = 400 to 700 nm) and frequency »F« (FM radio as an example: F ~ 100 MHz) of electromagnetic radiation are linked to each other through »c«, the velocity of light: c = L x F. Max Planck discovered that the energy of electromagnetic radiation is not distributed evenly but divided into small packages of energy E = h x F, where the Planck constant »h« is a fundamental physical constant. These packages are called photons (»light particles«). Their energy is E = h x F = (h x c) / L. This means that wavelength, frequency and energy are equivalent quantities to specify electromagnetic radiation.
Furthermore, we shall focus on »strongly penetrating« photon radiation, that is photon radiation with energies not lower than 15 keV.
Remark on notation: We avoid subscripts because they disturb line spacing. For example, we use Hx and Ka instead of Hx and Ka.
Back in 1928 the second ICRU (International Commission on Radiation Units and Measurements) congress defined the »Roentgen« to measure the »quantity of X-radiation«. The definition of that quantity, also known as »exposure« or »exposure dose«, is based upon the X-radiation's ability to produce electric charge in air. That definition was extended to gamma radiation in 1937, that is to photon radiation in general. In 1928 the symbol for the Roentgen unit was defined as »r«, and was changed to »R« in 1962 (surprisingly enough, you can still meet the »r«). The SI unit of exposure is C/kg of air, which converts to R as follows:
1 R = 2.58 E-4 C/kg (Coulomb per kilogram of air, that is charge per mass of air).
The SI unit C/kg is hardly used in practice. Instead, the SI system prefers a quantity called »air kerma«, see the comments on absorbed dose now to follow.
In 1957 ICRUdefined: »Absorbed dose of any ionizing radiation is the energy imparted to »matter by ionizing particles per unit mass of irradiated material »at the place of interest«. Note that, unlike the definition of »exposure dose, this definition is neither restricted to a »particular radiation type nor to a particular absorbing material. »The traditional unit is »rad« (Radiation Absorbed Dose), and the SI »unit is »Gy« (Gray):
100 rad = 1 Gy = 1 J/kg (Joule per kilogram, that is energy per mass)
For photon radiation in air you can easily convert Exposure to Absorbed Dose by applying the appropriate factor:
Absorbed Dose [rad] = C x Exposure [R], where C = 0.877 rad/R (in air)
In water or tissue, C is within the range of 0.94 to 0.98 rad/R for photon energies ranging from 100 keV to 3 MeV. In other materials other values for C apply. If you further wish to convert absorbed dose from rad to Gy, simply divide by 100:
Absorbed Dose [Gy] = Absorbed Dose [rad] / 100.
Within the SI system, absorbed dose in air measured in Gy is called air kerma (Ka). The word kerma means »kinetic energy released per unit mass« or »kinetic energy released in matter«. Air kerma is the common SI replacement for exposure because these two quantities only differ by a constant factor over a very wide range of photon energies:
Ka [Gy] = 0.00877 Gy/R x exposure [R].
Air kerma can be regarded as the basic SI quantity. If conversion factors for other SI quantities such as dose equivalents are reported, those conversion factors usually refer to air kerma.
Exposure and Absorbed Dose are general scientific quantities; they are not primarily related to protection of persons against radiation. With regard to radiological protection, we need a quantity that measures the biological effect on tissue. One might suppose that the absorbed dose in tissue could serve that purpose. In fact, it does in the case of photons and electrons (beta particles). For other radiation types, however, the same amount of absorbed dose has a different biological effect. Therefore, RBE Dose (Relative Biological Effectiveness Dose) was introduced in the 1950s. The RBE Dose is the Absorbed Dose in tissue measured in rad multiplied by a quality factor Q (formerly called RBE factor) accounting for the biological effect of different types of radiation. The corresponding traditional unit is »rem«.
RBE Dose [rem] = Q x Absorbed Dose in tissue [rad], where Q = 1 for photons and electrons.
For neutrons or alphas Q may range up to 20. Both rem and rad represent energy per mass, so Q has no unit. Rem means »Rad Equivalent Man« indicating that one rem is the amount of any type of radiation that has a biological effect on human tissue equivalent to the effect of one rad of photon radiation. In other words, one rem of any type of radiation causes the same biological damage as one rad of photon radiation. If we use that equation for photon radiation (Q = 1) and replace Absorbed Dose in tissue with Exposure, we obtain
RBE Dose [rem] = C x Exposure [R] (C <= 0.98 rad/R)
Since C is close to 1, Exposure Dose is a good estimate for RBE Dose originating from photons. It even slightly overestimates RBE Dose. This reflects many years' practice to take the reading of instruments indicating Exposure in R as the RBE Dose of photons in rem.
The advantage of an »equivalent« dose is obvious: If a person is exposed to different radiation types, you may add the equivalent dose values of those radiations to get the total amount of biological effect. This total amount, the »effective« dose, decides whether permissible limits are exceeded or not. It took a long time for the Dose Equivalent to be widely accepted. In 1957 ICRU mentioned the RBE Dose and the rem as »recognized symbols«, however did not define them as recommended quantity and unit. In the time thereafter a lot of work (and, unfortunately, a lot of modifications) on the dose equivalent concept was performed as documented in many publications of ICRU and ICRP (International Commission on Radiological Protection). One of the results is the SI unit »Sv« (Sievert) to replace the rem as unit for the Dose Equivalent:
1 Sv = 100 rem = 1 J/kg
Now we have to confuse you a little bit. We just learned that 1 Sv = 1 J/kg. When discussing Absorbed Dose (see above), we mentioned that also 1 Gy = 1 J/kg. This means that 1 Sv = 1 Gy. From this equation one might conclude that Dose Equivalent is equal to Absorbed Dose. However, that conclusion is not correct at all. You cannot conclude that two quantities are equal just from the fact that they are measured with the same unit.
Background information: A quantity and its unit are different things. For example, think of the distance between two cities. That distance depends on the type of transportation (air, road, rail). The type of transportation is the measuring quantity, and the unit is, for example, miles. When specifying a distance in miles, the type of transportation (the quantity) needs to be specified, too, if the specification shall be unambiguous. A common way to manage this problem is to »invent« new units clearly allowing to conclude the quantity from. Such new units would be »air miles« or »road miles« in our example. Electrical engineering makes full use of this method. In order to specify various voltages such as direct, alternating or peak-to-peak voltage, units such as VDC, VAC, Vpp have been created although the official unit is just V like Volt.
The distinction between a quantity and its unit was not always clearly observed in the radiation business (in the early days even by experts), and we feel that among users a lot of confusion arose from neglecting this distinction.
A good example for confusing a quantity and its unit is the popular equation »Sv/Gy = 1.20 (for Cs-137)«. Since we just learned that 1 Sv = 1 Gy, how can then be Sv/Gy = 1.20? The answer is that »Sv/Gy = 1.20« is a short - but incorrect - notation for the following fact:
H*(10) [Sv] = 1.20 x Ka [Gy] (for Cs-137).
Correctly the popular equation reads »H*(10)/Ka = 1.20 (for Cs-137)«, because 1.2 is the ratio of the quantities, not the ratio of their units. Ka is air kerma, and H*(10) is a dose equivalent quantity that will be discussed later.
Now that we are aware of distinguishing quantities and units, we may discuss different dose equivalent quantities (unfortunately there are more than one) which are all measured in Sv.
Photon Dose Equivalent Hx
Photon dose equivalent Hx (measured in Sv) is a quantity introduced in Germany in 1980. It became the legal quantity in Germany on 01 January 1986. Hx was an interim solution because at that time no international agreement on dose equivalent quantities had been achieved. In Germany Hx was replaced by SI quantities such as H*(10) on 01 August 2001. Hx was not accepted internationally. Nevertheless, we have to discuss Hx because it affected the design of our instruments.
Most instruments available up to the early 1980s were designed for Exposure (rate) and calibrated in R(/h). The question arose whether we could still use them to measure dose equivalent. The answer was: Yes, we can, because Exposure is a good estimate for the dose equivalent of photons in tissue. Therefore Hx was defined as
Hx [Sv] = 0.01 Sv/R x Exposure [R]
Since this conversion does not depend on photon energy, Hx and Exposure are strongly related quantities; they just differ by the factor 100. This is why some of our instruments (some 6150AD models) allow the user to select either R or Sv as the unit. Basically Hx was not really a new quantity. It was more the old quantity Exposure in a new wrapping.
Why the denomination Hx? Probably because the letter H was already internationally designated for the Dose Equivalent quantities, however there was not yet agreement which suffixes or indexes to append to that letter (H*(10), Hp(10), ...). Consequently the magic »unknown x« had to serve as the index.
»New« Dose Equivalent Quantities
The new quantities distinguish, in all combinations, area and personal monitoring, as well as strongly and weakly penetrating radiation. This makes a total of four quantities: