Dosimetry

Radiation dosimetry is the calculation of the absorbed dose in matter and tissue resulting from the exposure to ionizing radiation. It is a scientific subspecialty in the fields of health physics and medical physics that is focused on the calculation of internal and external doses from ionizing radiation.

Dose is reported in gray (Gy) for the matter or sieverts (Sv) for biological tissue, where 1 Gy or 1 Sv is equal to 1 joule per kilogram. Non-SI units are still prevalent as well, where dose is often reported in rads and dose equivalent in rems. By definition, 1 Gy = 100 rad and 1 Sv = 100 rem.

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Radiation effects on living tissue

The distinction between absorbed dose (Gy) and dose equivalent (Sv) is based upon the biological effects of the radiation in question and the tissue and organism irradiated. For different types of radiation, the same absorbed dose (measured in Gy) may have very different biological consequences. Therefore, a radiation weighting factor (denoted w_r) and tissue/organ weighting factor (W_T) have been established, which compare the relative biological effects of various types of radiation and the susceptibility of different organs.

Organ dose weighting factors

By definition, the weighting factor for the whole body is 1, such that 1 Gy of radiation delivered to the whole body (i.e. an evenly distributed 1 joule of energy deposited per kilogram of body) is equal to one sievert (for photons with a radiation weighting factor of 1, see below). Therefore, the weighting factors for each organ must sum to 1 as the unit gray is defined per kilogram and is therefore a local effect. As the table below shows, 1 gray delivered to the gonads is equivalent to 0.25 Gy to the whole body—in this case, the actual energy deposited to the gonads, being small, would also be small.

Radiation weighting factors

By definition, x-rays and gamma rays have a weighting factor of unity, such that 1 Gy = 1 Sv (for whole-body irradiation). Values of w_r are as high as 20 for alpha particles and neutrons, i.e. for the same absorbed dose in Gy, alpha particles are 20 times as biologically potent as X or gamma rays.

Dose versus activity

Radiation dose refers to the amount of energy deposited in matter and/or biological effects of radiation, and should not be confused with the unit of radioactive activity (becquerel, Bq). Exposure to a radioactive source will give a dose which is dependent on the activity, time of exposure, energy of the radiation emitted, distance from the source and shielding. The equivalent dose is then dependent upon the weighting factors above. Dose is a measure of deposited dose, and therefore can never go down—removal of a radioactive source can only reduce the rate of increase of absorbed dose, never the total absorbed dose.

The worldwide average background dose for a human being is about 3.5 mSv per year [1], mostly from cosmic radiation and natural isotopes in the earth. The largest single source of radiation exposure to the general public is naturally-occurring radon gas, which comprises approximately 55% of the annual background dose. It is estimated that radon is responsible for 10% of lung cancers in the United States.

Measuring dose

There are several ways of measuring doses from ionizing radiation. Workers who come in contact with radioactive substances or may be exposed to radiation routinely carry personal dosimeters. In the United States, these dosimeters usually contain materials that can be used in thermoluminescent dosimetry (TLD) or optically stimulated luminescence (OSL). Outside the United States, the most widely-used type of personal dosimeter is the film badge dosimeter, which uses photographic emulsions that are sensitive to ionizing radiation. The equipment used in radiotherapy (linear particle accelerator in external beam therapy) is routinely calibrated using ionization chambers.

Dose standards

Because the human body is approximately 70% water and has an overall density close to 1 g/cm³, dose measurement is usually calculated and calibrated as dose to water. National standards laboratories such as the NPL provide calibration factors for ionization chambers and other measurement devices to convert from the instrument's readout to absorbed dose. The standards laboratories operate a Primary Standard, which is normally calibrated by absolute calorimetry, the warming of substances when they absorb energy. A user sends their Secondary Standard to the laboratory, where it is exposed to a known amount of radiation (derived from the Primary Standard) and a factor is issued to convert the instrument's reading to that dose. The user may then use their Secondary Standard to derive calibration factors for other instruments they use, which then become tertiary standards, or field instruments.

The NPL in the UK operates a graphite-calorimeter for absolute photon dosimetry. Graphite is used instead of water as its specific heat capacity is one-sixth that of water and therefore the temperature rises in graphite are 6 times more than the equivalent in water and measurements are more accurate. Significant problems exist in insulating the graphite from the laboratory in order to measure the tiny temperature changes. A lethal dose of radiation to a human is approximately 10–20 Gy. This is 10-20 joules per kilogram. A 1 cm³ piece of graphite weighing 2 grams would therefore absorb around 20–40 mJ. With a specific heat capacity of around 700 J·kg^-1·K^-1, this equates to a temperature rise of just 20 mK.