☢️ What’s the Difference Between Sievert(Sv) and Gray(Gy)?🤔

♦️ Introduction

Both Sievert (Sv) and Gray (Gy) are radiation-related measurement units. They’re named after two scientists who made great contributions in this field:

• Rolf Sievert – Swedish physician, pioneer of radiation protection.
• Louis Harold Gray – British physicist, specialist in radiobiology.

👉 Both use the SI definition of 1 joule per kilogram (1 J/kg).

The difference lies in what they describe and how we use them.

---

♦️ Definitions

• Gray (Gy) = absorbed dose：The amount of energy deposited in tissue per kilogram. A physical quantity, independent of biological effects.
• Sievert (Sv) = weighted dose：A radiation protection quantity. Absorbed dose is multiplied by:
  • Radiation weighting factor (w_R): accounts for type of radiation (e.g., X-rays = 1, alpha = 20).
  • Tissue weighting factor (w_T): reflects sensitivity of each organ (e.g., lens > muscle).

[ Formulas]

• Equivalent dose (H_T, Sv) = Σ w_R × D_T,R (Gy)
• Effective dose (E, Sv) = Σ w_T × H_T

---

♦️ A Simple Analogy

Imagine throwing a punch:

Punch the eye (sensitive tissue), and even a weak punch is devastating → high Sv.

Punch the buttock (more resistant), and a strong punch may not cause serious harm → lower Sv.

This is why the same 1 Gy of radiation can correspond to different Sv, depending on radiation type and organ.

---

♦️ Weighting Factors Explained

• Radiation weighting factor (w_R): accounts for how damaging the radiation is per unit of energy.
  • X-rays, gamma rays, beta particles: w_R = 1
  • Alpha particles: w_R = 20
  • Neutrons: energy-dependent values
• Tissue weighting factor (w_T): accounts for how sensitive each organ is to stochastic effects (e.g., cancer). The lens of the eye and bone marrow are much more sensitive than muscle or skin.

👉 So, Equivalent dose (H_T, Sv) = Σ w_R × D_T,R (Gy)

👉 Effective dose (E, Sv) = Σ w_T × H_T

♦️ Practical Points

• Gray (Gy): used in therapy, deterministic effects, and tissue reactions.
• Sievert (Sv): used in radiation protection, exposure limits, and risk management.
• Effective dose (Sv): an average whole-body risk indicator, useful for population or occupational exposure management. It’s not meant to predict exact cancer risk for an individual.

Gy is “how much radiation energy is absorbed.” Sv is “how risky it is, after considering radiation type and where it hits.”

📝 Summary

🔗 Related articles

📚 References & Links

Japanese Version