Generally, the personal dose equivalent, Hp(d), is an operational quantity for individual monitoring. According to the ICRP, the personal dose equivalent is defined as:
ICRP Publication 103:
“The dose equivalent in soft tissue (commonly interpreted as the ‘ICRU sphere’) at an appropriate depth, d, below a specified point on the human body.”
The personal dose equivalent is given the symbol Hp(d). Two common operational quantities for individual monitoring defined by the ICRP are:
- Personal dose equivalent, Hp(0.07). The Hp(0.07) dose equivalent is an operational quantity for individual monitoring to assess the dose to the skin and the hands and feet.
- Personal dose equivalent, Hp(10). The Hp(10) dose equivalent is an operational quantity for individual monitoring to assess the effective dose.
As can be seen, various depths can be used. The personal dose equivalent, Hp(d), can be assessed indirectly with a thin, tissue-equivalent detector (radiation dosimeter) worn at the body’s surface and covered with an appropriate thickness of tissue-equivalent material. The specified point, d, is normally taken where the radiation dosimeter is worn.
For assessment of superficial organs and the control of equivalent dose, depths of 0.07 mm for skin and 3 mm for the eye lens are employed, and the personal dose equivalents for those depths are denoted by Hp(0.07) and Hp(3), respectively. Hp(0.07) is also called the shallow dose equivalent.
Hp(10) with a depth d = 10 mm is chosen to assess deep organs and control of effective dose. Hp(10) is also called the deep dose equivalent. Suppose the personal dosimeter is worn on a position of the body representative of its exposure, at low doses and under the assumption of a uniform whole-body exposure. In that case, the value of Hp(10) provides an effective dose value sufficiently precise for radiological protection purposes. Neutron and gamma radiations contribute to both deep and shallow doses, but beta radiation is completely absorbed in the skin and contributes to shallow doses only.
The SI unit of Hp(d) is the sievert (Sv). Unit of sievert was named after the Swedish scientist Rolf Sievert, who did a lot of the early work on dosimetry in radiation therapy. For all types of external radiation, the operational quantities for individual monitoring are defined based on an equivalent dose value at a point in a simple phantom, the ICRU sphere, which is a sphere of tissue-equivalent material (30 cm in diameter, ICRU (soft) tissue with density: 1 g/cm3, and mass composition: 76.2% oxygen, 11.1% carbon, 10.1% hydrogen, and 2.6% nitrogen).
As was written, operational quantities are measurable, unlike an effective dose, and instruments for radiation monitoring are calibrated in terms of these quantities. In monitoring, the values of these operational quantities are taken as a sufficiently precise assessment of effective dose and skin dose, respectively, if their values are below the protection limits. Numerical links between operational quantities and effective dose are represented by conservative conversion coefficients, which are defined for a reference person. In most practical situations, dosimeters provide reasonable approximations to the personal dose equivalent, Hp(d), at least at the location of the dosimeter. It must be noted that the personal dose equivalent generally overestimates the effective dose. On the other hand, this procedure is valid only at low doses and under the assumption of a uniform whole-body exposure. However, this procedure might not be sufficient for high personal doses approaching or exceeding the annual dose limit or in strongly inhomogeneous radiation fields.
See also: Dose Limits
Dose limits are split into two groups, the public and occupationally exposed workers. According to ICRP, occupational exposure refers to all exposure incurred by workers in the course of their work, with the exception of
- excluded exposures and exposures from exempt activities involving radiation or exempt sources
- any medical exposure
- the normal local natural background radiation.
The following table summarizes dose limits for occupationally exposed workers and the public:
According to the recommendation of the ICRP in its statement on tissue reactions of 21. April 2011, the equivalent dose limit for the eye lens for occupational exposure in planned exposure situations was reduced from 150 mSv/year to 20 mSv/year, averaged over defined periods of 5 years, with no annual dose in a single year exceeding 50 mSv.
Limits on effective dose are for the sum of the relevant, effective doses from external exposure in the specified period and the committed effective dose from intakes of radionuclides in the same period. For adults, the committed effective dose is computed for a 50-year period after intake, whereas for children, it is computed for the period up to age 70. The effective whole-body dose limit of 20 mSv is an average value over five years, and the real limit is 100 mSv in 5 years, with not more than 50 mSv in any year.
Occupational Exposure – Effective Dose
In most situations of occupational exposure, the effective dose, E, can be derived from operational quantities using the following formula:
The committed dose is a dose quantity that measures the stochastic health risk due to an intake of radioactive material into the human body.