RADIATION PROTECTION QUANTITIES AND UNITS

EQUVIALENT

DOSE

The equivalent dose H is the absorbed dose multiplied by a dimensionless radiation weighting factor, WR which expresses the biological effectiveness of a given type of radiation

To avoid confusion with the absorbed dose, the SI unit of equivalent dose is called the sievert (Sv). The old unit was the “rem”

The SI unit: sievert (Sv)

HT (Sv) = WR x D (Gy)

Traditional (old) unit:

rem (roentgen equivalent man)

HT (rem) = WR x D (rad)

1 Sv = 100 rem

RADIATION

WEIGHTING

FACTORS

Radiation type and energy range

WR

Photons (X-rays and gamma-rays) all energies Electrons, all energies Neutrons <10 keV 10-100 keV >100 kev to 2 MeV 2-20 MeV >20 MeV Protons >20MeV Alpha-particles, fission fragments

1 1 5 10 20 10 5 5 20

To account for differences in LET when measuring theeffect of radiation, each type of radiation has beenassigned a radiation weighting factor (WR).

This was done by measuring how much of each radiationtype it took to produce the same biological effect as 200-keV X rays.

As shown, all photons, beta particles do the sameamount of damage.

Thermal neutrons do somewhat more damage, and fastneutrons and alpha particles are extremely damaging.

EFFECTIVE

DOSE

The equivalent dose in each organ and tissue is

multiplied by a tissue weighting factor, WT, and the

results are summed over the whole body to give the

effective dose E

E=∑WT HT

TISSUE

WEIGHTING

FACTORS

Organ/Tissue WT Organ/TissueWT

Bone marrow 0.12 Lung 0.12

Bladder 0.04 Liver 0.04

Bone surface 0.01 Oesophagus 0.04

Brain 0.01Salivary

Glands0.01

Breast 0.12 Skin 0.01

Colon 0.12 Stomach 0.12

Gonads 0.08 Thyroid 0.04

Liver 0.05 Remainder 0.12

These are multipliers used for radiation protection purposes to account for the different sensitivities of the organs and tissues to the induction of stochastic effects of radiation.

The relationship between the probability of the stochastic effect and equivalent dose varies with the tissue irradiated.

Tissues which are at higher risk from radiation will have higher weighting factors (WT)

The sum of the tissue weighting factors is equal to 1.

COLLECTIVE EFFECTIVE DOSE

The collective dose refers to the amount of radiation received by a group of people.

It is calculated by multiplying the average effective dose received by the number of persons exposed.

The collective dose is expressed in person-sieverts (person-Sv).

S=∑Ei Ni

where Ei is the average effective dose in the population subgroup i and Ni is the number of individuals in the subgroup.

COMMITTED DOSE

The quantity HT(tf), defined as:

HT (tt)=∫t0+tf

t0 HT (t) dt

Where t0 is the time of intake and HT (t) is the equivalent dose rate at time t in organ or tissue T

“ t “is the time elapsed after an intake of radioactive substances.

When t is not specified, it will be taken to be 50 years for adults and to age 70 years for intakes by children.

ANNUAL LIMIT OF INTAKE

ALI is the derived limit for the amount of radioactive material taken into the body of an adult worker by inhalation or ingestion in a year.

ALI is the smaller value of intake of a given radionuclide in a year by the "reference man" that would result in a committed effective dose equivalent (CEDE) of 5 rems (0.05 sievert) to any individual organ or tissue.

DERIVED AIR CONCENTRATION

The DAC is the activity concentration in air in Bq/m3 of the radionuclide considered which would lead to an intake of ALI assuming a breathing rate of the Reference Worker of 1.2 m3 /hr and an annual working time of 2000 hoursThen the DAC is given by: DAC=ALI/2400

PERSONNEL DOSE EQUVIALENT

The personal dose equivalent ,Hp(d), is the operational quantity for individual Monitoring

It is the dose equivalent in soft tissue (ICRU-sphere) below a specified point on the body at an appropriate depth ‘d’

This quantity can be used for measurements of superficial and deep organ doses,depending on the chosen value of the depth in tissue.

The depth d is expressed in millimetres, and ICRU recommends that any statement of personal dose equivalent should specify this depth.

For superficial organs, depths of 0.07 mm for skin and3 mm for the lens of the eye are employed, and the personal dose equivalents for those depths are denoted by Hp(0.07) and Hp(3), respectively.

For deep organs a depth of 10 mm is frequently used, with the notation Hp(10).

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