Question

Two common units of radiation used in newspaper and news magazine articles are the rad and rem. What does each measure? Which would you use in an article describing the damage an atomic bomb would inflict on a human population? What relationship does the gray have with these units?

Short answer

Rad measures absorbed dose; rem measures biological effect. Use rem for human damage. 1 gray = 100 rad.

Step-by-step solution

Understanding the rad

A 'rad' is a unit that measures the absorbed dose of radiation. It stands for 'radiation absorbed dose' and is used to quantify the amount of energy deposited by ionizing radiation in a mass of tissue. Specifically, 1 rad is equivalent to 0.01 joules of energy absorbed per kilogram of tissue.

Understanding the rem

The 'rem' measures the dose equivalent, which accounts for the different biological effects of various types of ionizing radiation. It stands for 'roentgen equivalent man' and is calculated by multiplying the absorbed dose (in rads) by a quality factor that is unique to the type of radiation (e.g., alpha particles have a higher quality factor than beta or gamma radiation).

Choosing the appropriate unit for damage analysis

In an article describing the damage an atomic bomb would inflict on a human population, the rem would be more appropriate. This is because the rem takes into account not just the energy deposited but also the biological impact of the radiation, which is essential for understanding health effects.

Relationship with the gray

The gray (Gy) is the SI unit for absorbed dose and is used internationally. It is defined as the absorption of 1 joule of radiation energy per kilogram of matter. There is a conversion factor between these units: 1 Gy = 100 rad. To convert rems to the SI unit sieverts (Sv), which is similar to grays for the purpose of biological impact, 1 Sv = 100 rem.

Key concepts

Rad and Rem

Rad and rem are two key units used in measuring radiation.
They often appear in media reports related to radiation exposure.

• A 'rad' (radiation absorbed dose) measures the amount of energy absorbed by a mass of tissue from ionizing radiation. This energy is absorbed per kilogram, where each rad equates to 0.01 joules.


• A 'rem' (roentgen equivalent man) accounts for both the absorbed energy and the biological effects of the radiation. It adjusts the rad value by a quality factor unique to the type of radiation to reflect its impact on living tissue.

Hence, the primary difference is that rad measures energy absorbed,
while rem adjusts this number to include the biological effects of different radiation types.

Absorbed Dose

The absorbed dose is a fundamental concept in radiation measurement.
It refers to the amount of energy from radiation deposited in a material.

• The unit for measuring absorbed dose is the rad in the conventional system, which directly indicates the energy absorbed in a particular tissue or substance.


• The absorbed dose is crucial in assessing the initial physical impact of radiation on cells and tissues.

Without this measurement, understanding the potential effects of radiation exposure on objects and living beings would be difficult.
While energy absorption is important, the specific biological effects also depend on the type of radiation.

Biological Effects of Radiation

The biological effects of radiation differ significantly based on the type of radiation involved.
That's why understanding radiation isn't just about measuring absorbed doses like rads.

• Different radiation types—such as alpha, beta, and gamma—interact with living tissue uniquely, requiring the use of a corrective concept known as dose equivalent, usually measured in rems.


• The dose equivalent considers not only how much radiation is absorbed but also how it affects living cells, leading to potential damage to DNA and other cellular components.

In scenarios like atomic bomb exposure, emphasizing the biological effects helps better assess potential health risks and necessary safety measures.

Gray (Gy) and Sievert (Sv) Conversion

Gray and sievert are the SI units used internationally to measure radiation.
They replace the older rad and rem units with more standardized measures.

• The gray (Gy) is used for absorbed dose, with 1 Gy equating to 100 rad. It offers a clear numeric indicator of energy deposited per kilogram of tissue.


• The sievert (Sv) measures dose equivalent similar to how rem functions, by including biological effects. 1 Sv equals 100 rem, maintaining consistency with older units.

Converting between these units is a straightforward process thanks to their simple numeric relationships.
This conversion enables easy translation between traditional and modern measurements in scientific and practical applications.