3.3 Radiation Units: Activity, Dose, Equivalent Dose

1. Activity (\(A\))

**Activity** is a measure of the rate at which radioactive nuclei decay. It is the number of spontaneous nuclear transformations (decays) per unit time.

• SI Unit: The **Becquerel (Bq)**, which is defined as one decay per second ($1 \text{ Bq} = 1 \text{ s}^{-1}$).
• Conventional Unit: The **Curie (Ci)**, which is based on the activity of one gram of Radium-226. The conversion factor is:
  $$1 \text{ Ci} = 3.7 \times 10^{10} \text{ Bq}$$

2. Absorbed Dose (\(D\))

**Absorbed Dose** is the amount of energy deposited by ionizing radiation per unit mass of a material. It is a physical quantity that measures the energy absorbed, regardless of the biological effects.

• SI Unit: The **Gray (Gy)**, which is defined as one Joule of energy absorbed per kilogram of material ($1 \text{ Gy} = 1 \text{ J/kg}$).
• Conventional Unit: The **Rad (rad)**, with the conversion factor:
  $$1 \text{ Gy} = 100 \text{ rad}$$

3. Equivalent Dose (\(H\))

**Equivalent Dose** is a biological quantity that accounts for the fact that different types of radiation have different biological effects for the same amount of absorbed energy. It is calculated by multiplying the absorbed dose by a radiation weighting factor ($W_R$).

• SI Unit: The **Sievert (Sv)**, defined as one Joule of energy absorbed per kilogram, weighted for the type of radiation ($1 \text{ Sv} = 1 \text{ J/kg} \times W_R$).
• Conventional Unit: The **Rem (rem)**, with the conversion factor:
  $$1 \text{ Sv} = 100 \text{ rem}$$

The radiation weighting factor ($W_R$) is a dimensionless value that reflects the relative biological effectiveness of different radiation types (e.g., $W_R$ for photons and electrons is 1, while for alpha particles it is 20).

4. Effective Dose (\(E\))

**Effective Dose** is a further refinement of equivalent dose that accounts for the different sensitivities of various tissues and organs to radiation. It is calculated by summing the equivalent doses to different organs, each multiplied by a tissue weighting factor ($W_T$).

It is a single value used to represent the overall health risk from radiation exposure to the entire body. The unit for effective dose is also the **Sievert (Sv)**.