Absorbed, equivalent and effective dose

Ionising radiations — $\alpha$, $\beta$, $\gamma$ rays, neutrons, X-rays — interact with matter by transferring energy to the atoms they pass through, stripping electrons from them (ionisation). When the targets are living cells, the biological consequences depend both on the amount of energy deposited and on the type of radiation. To quantify them, three successive quantities are defined, each refining the previous one.

Principle — Dosimetry

• Absorbed dose $D = E/m$: energy deposited per unit mass of irradiated tissue. Unit: gray (Gy = J/kg). It is a purely physical quantity.
• Equivalent dose $H = w_R\cdot D$: weights the dose with a factor $w_R$ that depends on the type of radiation ($w_R = 1$ for $\beta$ and $\gamma$; $w_R = 20$ for $\alpha$; $w_R \sim 5$–$20$ for neutrons). Unit: sievert (Sv).
• Effective dose: a weighted sum over the various tissues, accounting for their different radiosensitivity (bone marrow is more critical, bones less so).

The key step is from gray to sievert. Two identical absorbed doses in joules per kilogram can have very different biological effects: an $\alpha$ particle deposits its energy over a very short path, concentrating the damage, and this is why it is twenty times more harmful than a $\gamma$ ray of the same energy. The factor $w_R$ translates physics (gray) into biology (sievert).

Orders of magnitude

Gy = J/kg (physics). Sv = Gy $\cdot\, w_R$ (biology).

• Acute lethal dose: $\sim 5\;\text{Sv}$.
• Annual background dose (Italy): $\sim 2{,}4\;\text{mSv}$.
• Chest X-ray: $\sim 0{,}1\;\text{mSv}$.
• Rome–New York flight: $\sim 0{,}05\;\text{mSv}$.

Collegamenti

Argomenti: Fisica nucleare Concetti: Decadimento radioattivo

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