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Why do sieverts account for the different biological effects of radiation types?

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detector output connected to a measuring circuit — radiation measurement Figure 1: (A) A simple equivalent circuit for the development of a voltage pulse at the output of a detector. R represents the resistance and C the capacitance of the circuit; V(t) is the time (t)-dependent voltage produced. (B) A representative current pulse due to the interaction of a single quantum in the detector. The total charge Q is obtained by integrating the area of the current, i(t), over the collection time, t_c. (C) The resulting voltage pulse that is developed across the circuit of (A) for the case of a long circuit time constant. The amplitude (V_max) of the pulse is equal to the charge Q divided by the capacitance C.

The sievert (Sv) is used to measure radiation dosage because it accounts for the relative biological effectiveness (RBE) of different types of ionizing radiation. While a physical dose—measured in grays—quantifies the energy absorbed per unit of mass, it does not reflect the varying degrees of biological damage caused by different radiation forms.12

The biological impact of radiation depends on how densely it ionizes matter along its path, which is determined by the radiation's energy, mass, and charge. For example, densely ionizing particulate radiation, such as alpha particles, creates clustered molecular damage that is often irreparable, making it more harmful to living tissue than the same physical dose of X-rays or gamma rays. Because the probability of injury—such as DNA damage or cell death—is higher when molecular damage is concentrated at critical sites, the sievert provides a standardized unit that reflects this increased risk, ensuring that different radiation types are compared based on their actual biological impact rather than just their energy transfer.345