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There are two types of quantities defined for use in radiological protection: protection quantities (defined by the ICRP and used for assessing the exposure limits) and operational quantities (defined by the ICRU and intended to provide a reasonable estimate for the protection quantities).

The following is based on the set of protection quantities recommended in ICRP Publication 60 [3]: it includes the tissue or organ equivalent doses (H_T ) and the effective dose (E). The equivalent dose H_T in a tissue or organ T is given by:

where D_{T, R} is the average absorbed dose from radiation R, in tissue T, w_R is the radiation weighting factor for radiation R and the sum is performed through all kind of radiation that constitue the radiation field considered. Table 1 gives the values of radiation weighting factors as recommended by ICRP. For radiation types and energies that are not included in table 1 an approximation to w_R can be obtained by the calculation of the average quality factor, Q, at a depth of 10 mm in the ICRU sphere (for the definition of the ICRU sphere see section 3.2:

where D(L)dL is the absorbed dose at 10 mm between linear energy transfer values of L and L + dL and Q(L) is the corresponding quality factor. The relation between Q and L (recommended by ICRP 60) is shown in table 2.

Table 1: Values for radiation weighting factors recommended in ICRP Publication 60 [3].

Table 2: Relation between L in water and the quality factor Q, as recommended by ICRP Publication 60 [3].

The effective dose E is the sum of the weighted equivalent doses in all the tissues and organs of the body. It is given by the expression:

where H_T is the equivalent dose in tissue or organ T, w_T is the weighting factor for tissue T and the sum is performed on all tissue and organs involved in irradiation. Table 3 gives the values of tissue weighting factors as recommended by ICRP.

Table 3: Tissue weighting factors recommended in ICRP Publication 60 [3].

The protection quantities H_T and E are not directly measurable [4], but may be related by calculation to the radiation field if the condition or irradiation are known. The only way to estimate H_T and E is to measure the radiation field outside the body and to convert it to H_T and E using previously calculated conversion coefficients.

The mean absorbed dose D_{T, R} is a quantity that cannot be evaluated experimentally, therefore operational quantities [5] (defined in terms of the quality factor Q) should be used. The operational quantities are intended to provide a reasonable estimate of the protection quantities, the goal is that the value of the appropriate protection quantity is less than that of the corresponding operational quantity.

For strongly penetrating radiation the appropriate operational quantity for area monitoring is ambient dose equivalent. The ambient dose equivalent H*(d) at a point in a radiation field is the dose equivalent that would be produced by the corresponding expanded and aligned field in the ICRU sphere at a depth d, on the radius opposing the direction of the aligned field. The recommended value of d for penetrating radiation is 10 mm. The dose equivalent at other depths may be considered when the dose equivalent at 10 mm provides an unacceptable underestimate of the effective dose.

Giuseppe Battistoni; INFN, Milano
Stefan Roesler; CERN, Geneva

Last updated: 10th of October, 2008