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Radiation Units: Becquerel, Gray, Sievert

In radiation news, units like 'sievert', 'gray' and 'becquerel' get mixed up. Yet each answers a different question: how active is the source (Becquerel), how much energy was deposited in tissue (Gray), and what is its biological effect (Sievert). We explain all three as a chain, clearly.

Primer · for everyone· ~7 min read· Sourced

One news story says "so many microsieverts", another "so many becquerels", and for most people they all mean the same blurry "amount of radiation". Yet these units measure different things. The easiest way is to think of them as a chain: one that starts at the source and reaches the biological effect in tissue. Becquerel is the start of the chain, Sievert the end.

Becquerel (activity)

Becquerel (Bq) measures how active a radioactive source is: how many atomic nuclei decay per second.1 1 Bq = 1 decay per second. This is a property of the source — we are not yet talking about any tissue or any effect. For example, how many Bq a nuclear-medicine drug has describes the source's strength; but on its own it does not tell you what it will do to a person.

Gray (absorbed dose)

The second link in the chain: when radiation reaches a tissue, it deposits some energy there. This deposited energy is the absorbed dose, with the unit Gray (Gy): 1 joule per kilogram (1 Gy = 1 J/kg).1 Gray answers "how much energy entered this tissue?". Now we are talking not about the source but about the effect in tissue. (The unit often used in medicine is the milligray, mGy, a thousandth of a Gray.)

Sievert (effect)

But the same amount of energy (Gray) causes different biological harm depending on the type of radiation and which organ it entered. An alpha particle, even depositing the same energy, is far more destructive than an x-ray. To account for this, the absorbed dose is multiplied by a radiation weighting factor → equivalent dose; and summed with a tissue weighting factor for organs' different sensitivity → effective dose. Both have the unit Sievert (Sv).2 Sievert answers "what is the biological meaning of this dose?" — and it is the unit we actually use in radiation safety. (Because the weighting factor is 1 for x-rays and gamma, in medicine 1 mGy ≈ 1 mSv; for alpha the factor is much higher.)

A chain of three

Source → tissue → biological effectBecquerel (Bq)how active is the source?(decays per second)Gray (Gy)how much energy in tissue?(1 J/kg)Sievert (Sv)what is the biological effect?(equivalent/effective dose)sourceabsorptioneffect / risk
Becquerel measures the source's strength, Gray the energy entering tissue, and Sievert its biological effect. Radiation safety usually speaks in Sieverts.2
In one sentence
Becquerel = how active the source is · Gray = how much energy entered the tissue · Sievert = what its biological effect is. The "dose" in the news is usually in Sieverts.
Related articles
What Is Radiation? · Types of Radiation · Why Does Dose Matter? · Natural & Background Radiation

References

  1. Bushberg JT, Seibert JA, Leidholdt EM, Boone JM. The Essential Physics of Medical Imaging, 3rd ed. Lippincott Williams & Wilkins, 2011. Bölüm 3: soğurulan doz Gray (Gy) = 1 J/kg; eşdeğer doz ve efektif doz Sievert (Sv); aktivite Becquerel (Bq) = saniyede bir bozunma.
  2. ICRP Publication 103. The 2007 Recommendations of the ICRP — eşdeğer doz (radyasyon ağırlık faktörü wR) ve efektif doz (doku ağırlık faktörü wT). icrp.org
  3. İlişkili: Radyasyon Nedir? · Doz Neden Önemli? · Radyasyondan Korunma
Note: This content is for education; for clinical decisions or regulatory compliance, consult a qualified medical physicist and current regulations.

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