Comments on radiation quantities and units with regard to our instruments

Last modification: 11 December 2003.
Author: Dr. Wilhelm Buttler.

This discussion shall supply you with information on radiation quantities and units, particularly with German regulations that were decisive for the design of our instruments. This is anything but a thorough and comprehensive treatise on radiation quantities, nor will it be scientifically accurate down to the last detail. Nevertheless, we shall try to explain some basic concepts of radiological protection, and we hope this discussion will answer some frequently asked question like, for example, “how to migrate from Roentgens to Sieverts ?”.

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). 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”.

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 H_x and K_a.

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)

In 1957 ICRU defined: “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)

absorbed dose [rad]  =  C  x  exposure [R],   where C = 0.877 rad/R (in air)

absorbed dose [Gy]  =  absorbed dose [rad]  /  100.

Ka [Gy]  =  0.00877 Gy/R   x   exposure [R].

Exposure and absorbed dose are general scientific quantities; they are not primarily related to protection of humans 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 1950's. 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.

RBE-Dose [rem]  =  C  x  exposure [R]  (C  <=  0.98 rad/R)

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. Without dose equivalents one would need individual limiting values (expressed, for example, in rad or Gy) for all types of radiation.

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”, but 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

A good example for that confusion 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 rather confusing - notation for the fact that

H*(10) [Sv]  =  1.20  x  Ka [Gy]   (for Cs-137).

Now that we are aware of distinguishing quantities and units, we may discuss different dose equivalent quantities (unfortunately there are more than one) that are all measured in Sv.

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 reached. In Germany Hx was replaced by SI quantities such as H*(10) on 01 August 2001 (but may still be used in Germany under certain conditions for another ten years, that is until 01 August 2011). Hx was hardly accepted internationally. Nevertheless, we have to discuss Hx because it affected the design of our instruments.

Most instruments available in the 1980's 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].

Ambient dose equivalent H*(10) (also measured in Sv) is a quantity now widely accepted. It is similar to Hx except that it accounts for absorption and scattering of radiation by the human body. The definition of H*(10) simulates the human body by a phantom (the ICRU sphere, a sphere of 300 mm in diameter made of tissue equivalent material). H*(10) is the dose equivalent at a depth of 10 mm inside that sphere. Hx converts to H*(10) as follows:

H*(10)  =  f1(E)   x   Hx

H*(10)  =  f2(E)   x   Ka,   where f2(E)  =  f1(E)  /  0.877

Instruments designed for H*(10) measure the dose (rate) at some place in air (not at the body of a person). First recall why we measure a dose (rate) at some place: We do so because we need to estimate the dose a person would receive if he/she would be at that place for some time. Formerly we did so by measuring RBE-Dose (rate) at that place. However, this measurement did not account for changes to the radiation field caused by the body of person who finally went to that place. If we wished to account for that fact, we would have to put our instrument into a phantom and then read the instrument. This is exactly what H*(10) instruments simulate: they have an energy dependence a traditional instrument embedded in the ICRU sphere would have. In air, the reading of an H*(10) instrument divided by the reading of a traditional instrument is the factor f1 in Table 1.

Whereas instruments designed for Hx could also measure exposure (in R), instruments designed for H*(10) cannot. Therefore, H*(10) instruments cannot use a unit other than Sv.

Directional dose equivalent H'(0.07, Omega) (also measured in Sv) is the dose equivalent produced in the ICRU sphere at a depth of 0.07 mm, that is skin dose. With strongly penetrating radiation, skin dose will not significantly contribute to effective dose. Therefore, directional dose equivalent is only important in case of weakly penetrating radiation, such as alphas, betas with energies lower than 2 MeV, and photons with energies lower than 15 keV. Since we do not currently offer instruments for directional dose equivalent, we shall not further discuss this quantity.

Personal dose equivalent Hp(10) (also measured in Sv) is the dose equivalent in tissue at a depth of 10 mm in the body (not in the ICRU sphere) at the location where the personal dosimeter is worn. From this definition follows that Hp(10) includes the effect of the body on the radiation field, that is absorption and scattering. Dosimeters designed for Hp(10) have to account for that absorption and scattering. Since they are worn on the body (allow some clothing in between), they have always been exposed to radiation scattered from the body, even before Hp(10) was introduced. One might suppose that personal dosimeters measured Hp(10) from the very beginning, just without knowing. In fact, this is true to some extent.

Consider a personal electronic dosimeter designed for Hx such as our ALADOS. It is specified for an energy range from 70 keV to 3 MeV. Within that energy range, response is more or less flat (referred to Hx, that is, free in air). What happens to that response if the ALADOS is worn on the body? Response increases because radiation scattered from the body adds to indication. This means the ALADOS will measure Hp(10), although not perfectly, because part of the scattered radiation has an energy too low to be detected, and because the location of the detector is not equal to the place of interest (10 mm in the body). Hp(10) also requires the response to drop at low energies because of the absorption of the 10 mm tissue layer covering the place of interest. However, that absorption is only significant at energies below approx. 30 keV. At such low energies, the ALADOS is almost blind anyway, and therefore easily meets that requirement.

How to verify that a personal dosimeter measures Hp(10)? One would have to measure response while the dosimeter is worn on the body. Since volunteers for that purpose are rare, the definition of Hp(10) has been extended to the slab phantom (a square slab dimensioned 300mm x 300mm x 150mm, PMMA walls, filled with water). The slab phantom may also be used to calibrate dosimeters to Hp(10). However, once the effect of the slab phantom is known for a particular dosimeter type, dosimeters of that type may be calibrated free in air again if the correction factor according to the phantom effect is applied.

We checked the suitability of the ALADOS for Hp(10). In the energy range from 70 keV to 200 keV, referred to Hx, it overresponds by up to +20%, whereas referred to Hp(10) using the slab phantom, it underresponds by up to -20%. At energies above 200 keV, it meets Hp(10) even better than Hx. This means that, within an energy range of 70 keV to 3 MeV, and allowing for an error of +/-20%, the ALADOS is equally suited for Hx and Hp(10). We did not yet check directional response referred to Hp(10). However, this is not expected to reveal major changes; maybe referred to Hp(10) the useful energy range would start at 80 or 90 keV instead of 70 keV.

Personal dose equivalent Hp(0.07) is similar to Hp(10) except that it refers to weakly penetrating radiation (skin dose). Hp(0.07) dosimeters are calibrated on rod phantoms simulating fingers, arms, or legs.

With photon radiation, comparing the SI quantities Hp(10) and H*(10) (both measured in Sv) with the traditional quantity exposure (measured in R) gives the following basic results:

Introducing personal dose equivalent Hp(10) has only little influence on practical personal dosimetry. Hp(10) explains what personal dosimetry really is rather than modifying it. Except at low energies, most personal dosimeters calibrated in R will be suited to measure Hp(10); just multiply their reading by 0.01 Sv/R. The same is true for personal dosimeters calibrated in rem as long as those rems come from photon radiation.

Ambient dose equivalent H*(10) measured at some place in air shall estimate Hp(10) a person would receive at that place. Unlike exposure meters, H*(10) meters need to simulate the absorbing and scattering effect of the human body through their energy response. The absorbing effect is only significant at energies lower than approx. 30 keV. The scattering effect is most important at intermediate energies and may lead to an increase of up to 50%. At higher energies, say 200 keV or more, the scattering effect is 20% or less, and can be neglected at very high energies. If you would like to measure H*(10) with an exposure meter calibrated in R, first multiply the reading by 0.01 Sv/R. Then apply a correction factor depending on the energy of the radiation (factor f1 in Tables 1 and 2). If you have no idea about the energy, use a correction factor of 1.5; this will almost certainly overestimate H*(10).