Radiation - Absorbed Dose Converter
Convert Gray (Gy), Rad, Milligray (mGy), Microgray (µGy), Centigray (cGy), Millirad (mrad).
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Relative Value
*Diagram shows values relative to the selected base unit (Gray).
Unit Information
What is a Gray (Gy)?
The Gray (symbol: Gy) is the SI derived unit of absorbed dose of ionizing radiation. One Gray is defined as the absorption of one joule of radiation energy per kilogram of matter (1 Gy = 1 J/kg). It is named after the British physicist Louis Harold Gray, a pioneer in radiation biology.
What is a Rad?
The Rad (Radiation Absorbed Dose) is an older, non-SI unit of absorbed dose. One rad is defined as the dose corresponding to the absorption of 100 ergs of energy per gram of any material (1 rad = 0.01 joules/kg = 0.01 Gy). While largely replaced by the Gray, the rad (and millirad) may still be encountered in older literature or some specific contexts in the US.
What is a Centigray (cGy)?
A centigray is one-hundredth of a Gray (1 cGy = 0.01 Gy). It is numerically identical to the rad (1 cGy = 1 rad), which made the transition from older units to SI units easier in some medical physics contexts.
What are mGy, µGy, nGy?
These are standard SI submultiples. A milligray (mGy) is 10⁻³ Gy, a microgray (µGy) is 10⁻⁶ Gy, and a nanogray (nGy) is 10⁻⁹ Gy. These smaller units are used for measuring low-level environmental radiation and diagnostic imaging doses.
What is a Kilogray (kGy)?
A kilogray is one thousand Grays (1 kGy = 1000 Gy). This is a very large dose, used in industrial applications like material modification and sterilization of medical equipment or food.
What is a Millirad (mrad)?
A millirad is one-thousandth of a rad (1 rad = 1000 mrad). It is a very small unit of absorbed dose, sometimes used in specifying low-level exposures in older documents.
Formulas
1 Gray (Gy) = 100 Rad
This is the direct conversion factor between the SI unit (Gray) and the older CGS unit (Rad).
1 Rad = 0.01 Gray (Gy)
To convert from rads to grays, divide by 100.
1 Gray = 1 Joule/Kilogram
The fundamental definition of the Gray in terms of energy per unit mass.
1 Centigray (cGy) = 1 Rad
This numerical equivalence is very convenient and is often used in medical physics.
Equivalent Dose (Sv) = Absorbed Dose (Gy) × Wr
To find the biological effect, the absorbed dose is multiplied by a radiation weighting factor (Wr) to get the equivalent dose in Sieverts (Sv).
Key Reference Points
- A chest X-ray delivers an absorbed dose to the chest of about 0.1 milligrays (mGy).
- A mammogram delivers an absorbed dose of about 0.4 mGy to the breast tissue.
- A typical head CT scan delivers a dose of about 2 mGy to the brain.
- A single radiotherapy fraction for cancer treatment is often around 2 Grays (Gy).
- A dose of 5 Gy to the whole body is lethal to 50% of the population within 30 days (LD50/30).
- Food irradiation for sterilization might use doses up to 25 kilograys (kGy).
- The average annual absorbed dose from natural background radiation is about 2.4 mGy.
- A transatlantic flight can result in an absorbed dose of about 0.05 mGy from cosmic radiation.
- A dose of 10 kGy is sufficient to sterilize medical equipment.
- The absorbed dose rate near the Chernobyl reactor core shortly after the accident was extremely high, many Grays per hour.
Did You Know?
Absorbed dose (Gray, Rad) is the total energy deposited. Absorbed dose rate (e.g., Gray per hour, Gy/h) is how quickly that energy is deposited. Both are important. A high dose delivered over a short time (high dose rate) can have different biological effects than the same total dose delivered over a long period (low dose rate).
The biological effect of absorbed radiation depends not only on the dose but also on the type of radiation (alpha, beta, gamma, neutrons) and the sensitivity of the tissue irradiated. This is why units like Sievert (for equivalent dose) are used.
Astronauts in space are exposed to higher levels of cosmic radiation than people on Earth. Measuring and mitigating this radiation dose is a significant concern for long-duration space missions.
TLDs are common passive radiation dosimeters. They contain crystals that absorb radiation energy and then release it as light when heated. The amount of light emitted is proportional to the absorbed dose.
Food can be irradiated with high doses of gamma rays (measured in kilograys, kGy) to kill bacteria, molds, and insects, extending shelf life and improving safety. This process does not make the food radioactive.
Different types of cells in the body have different sensitivities to radiation. Rapidly dividing cells, such as those in bone marrow and tumors, are generally more radiosensitive than cells that divide slowly, like nerve cells. This principle is exploited in radiotherapy.
In 1987, a radiotherapy source was stolen from an abandoned hospital in Brazil. People were fascinated by the glowing blue cesium-137 powder, leading to severe contamination and several deaths. It highlights the danger of unsecured radioactive sources and the importance of dose assessment.
We are all constantly exposed to natural background radiation from cosmic rays, the ground (terrestrial radiation), and within our own bodies (from elements like potassium-40). The average annual dose from these sources is typically a few milligrays (mGy).
A convenient fact is that 1 Rad is exactly equal to 1 centiGray (cGy). This made the transition from older units to SI units easier in some medical contexts, as the numerical values for typical doses often remained the same.
The primary mechanism by which ionizing radiation damages living cells is by causing breaks in DNA strands. A dose of 1 Gray to a cell can cause thousands of DNA base damages and dozens of double-strand breaks.
Electronics intended for use in high-radiation environments (like space or nuclear reactors) must be 'radiation-hardened'. This involves designing circuits that can withstand a high total absorbed dose without failing.
This advanced form of radiotherapy uses beams of protons or other heavy ions. These particles deposit most of their energy at a specific depth (the Bragg peak), allowing a high dose to be delivered to a tumor while sparing surrounding healthy tissue.
The 'Lethal Dose, 50/30' (LD50/30) is the absorbed dose of radiation expected to cause death in 50% of an exposed population within 30 days. For humans, this is estimated to be around 4-5 Grays of whole-body radiation without medical treatment.
The Chernobyl disaster released a massive amount of radioactive material, resulting in high absorbed doses for first responders. The dose measurements in Gray and Rad were critical for medical treatment and understanding the long-term health consequences.
A Fricke dosimeter is a chemical dosimeter that measures absorbed dose by observing the chemical change (oxidation of ferrous ions to ferric ions) in an aqueous solution when it is irradiated.
The Linear No-Threshold (LNT) model is a model used in radiation protection which assumes that the long-term, biological damage caused by ionizing radiation is directly proportional to the dose, even at very low levels. This is a conservative model for setting safety standards.
Researchers are exploring the use of nanoparticles (like gold) in radiotherapy. When injected into a tumor, these high-atomic-number materials can increase the localized absorbed dose from X-rays, enhancing the therapeutic effect while sparing healthy tissue.
The rad is defined as 100 ergs of energy per gram. The erg is the CGS unit of energy, a very small amount equal to 10⁻⁷ Joules. The Gray, being 1 Joule/kg, is a much larger and more practical unit.