Definitions

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 most recent set of protection quantities was 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:

$$H_T = \sum_{R} {w_R \cdot D_{T,R}}$$ (1)

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, $\overline{Q}$, at a depth of 10 mm in the ICRU sphere (for the definition of the ICRU sphere see section 3.2:

$$\overline{Q} = \frac{1}{D} \int_{L}{Q(L) \cdot D(L) \cdot dL}$$ (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: Values for radiation weighting factors recommended in ICRP Publication 60 [3].

RADIATION	$w_R$
Photons	1
Electrons and muons	1
Neutrons:
E$<$10 keV	5
10 keV$<$E$<$100 keV	10
100 keV$<$E$<$2 MeV	20
2 MeV$<$E$<$20 MeV	10
E$>$20 MeV	5
Protons, other than recoil protons (E$>$2 MeV)	5
$\alpha$ particles, fission fragments, heavy nuclei	20

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

$L$ in water	$Q(L)$
keV/$\mu$m	(with $L$ in keV/$\mu$m)
$<$10	1
10$\div$100	0.32$\cdot$$L-$2.2
$>$100	$\frac{300}{\sqrt{L}}$

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:

$$E = \sum_{T}{w_T \cdot H_T}$$ (3)

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

TISSUE or ORGAN	$w_T$
Gonads	0.20
Red bone marrow, Colon, Lung, Stomach	0.12
Bladder, Breast, Liver, Oesophagus, Thyroid	0.05
Bone surface, Skin	0.01
Remainder	0.05

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.