Radiation is all around us, from the sun to our electronic devices. But what exactly is radiation, and how do we measure it? In this blog post, we will explore the different units of radiation and what they mean. We will also provide some examples to help you understand how these units are used in practice.

**Activity vs. Exposure**

Before we can discuss the different units of radiation, we need to understand the difference between activity and exposure. Activity is a measure of the number of radioactive decays that occur in a given amount of time. Exposure, on the other hand, is a measure of the amount of radiation that is absorbed by a person or object.

**Activity**

The unit of activity is the becquerel (Bq). One becquerel is equal to one radioactive decay per second. For example, if a sample of radioactive material has an activity of 10 Bq, it means that 10 radioactive decays are occurring in that sample every second.

**Absorbed Dose (Exposure)**

The absorbed dose is a measure of the amount of radiation that is absorbed by a person or object. The unit of absorbed dose is the gray (Gy). One gray is equal to one joule of energy absorbed per kilogram of mass. For example, if a person receives an absorbed dose of 1 Gy, it means that their body has absorbed 1 joule of radiation energy for every kilogram of their body weight.

**Example 1**

Let's say that a person is exposed to a source of radiation that has an activity of 10 Bq. If the person is exposed to this radiation for 1 hour, they will receive an absorbed dose of 0.01 Gy. This is because 10 Bq is equal to 10 radioactive decays per second, and 1 hour is equal to 3600 seconds. Therefore, the person will be exposed to 10 * 3600 = 36000 radioactive decays. Since 1 Gy is equal to 1 joule of energy absorbed per kilogram of mass, and the person weighs 70 kg, the person will absorb 36000 * 1 / 70 = 0.514 joules of radiation energy. Therefore, the person's absorbed dose will be 0.01 Gy.

**Example 2**

Now, let's say that a person is exposed to a source of radiation that has an activity of 100 Bq. If the person is exposed to this radiation for 1 hour, they will receive an absorbed dose of 0.1 Gy. This is because 100 Bq is equal to 100 radioactive decays per second, and 1 hour is equal to 3600 seconds. Therefore, the person will be exposed to 100 * 3600 = 360000 radioactive decays. Since 1 Gy is equal to 1 joule of energy absorbed per kilogram of mass, and the person weighs 70 kg, the person will absorb 360000 * 1 / 70 = 5.14 joules of radiation energy. Therefore, the person's absorbed dose will be 0.1 Gy.

**Equivalent Dose (Exposure)**

The equivalent dose is a measure of the biological damage caused by radiation. The unit of equivalent dose is the sievert (Sv). One sievert is equal to one gray multiplied by a weighting factor that accounts for the type of radiation and the sensitivity of the tissue being exposed. For example, if a person receives an equivalent dose of 1 Sv, it means that their body has absorbed 1 joule of radiation energy that has been weighted for the type of radiation and the sensitivity of the tissue being exposed.

**Effective Dose (Exposure)**

The effective dose is a measure of the overall health risk from radiation exposure. The unit of effective dose is also the sievert (Sv). The effective dose is calculated by multiplying the equivalent dose by a weighting factor that accounts for the different organs and tissues in the body and their sensitivity to radiation. For example, if a person receives an effective dose of 1 Sv, it means that their body has absorbed 1 joule of radiation energy that has been weighted for the type of radiation, the sensitivity of the tissue being exposed, and the different organs and tissues in the body.

**Example**

Let's say that a person is exposed to a source of radiation that has an activity of 100 Bq. If the person is exposed to this radiation for 1 hour, they will receive an equivalent dose of 0.1 Sv. This is because 100 Bq is equal to 100 radioactive decays per second, and 1 hour is equal to 3600 seconds. Therefore, the person will be exposed to 100 * 3600 = 360000 radioactive decays. Since 1 Sv is equal to 1 joule of energy absorbed per kilogram of mass, and the person weighs 70 kg, the person will absorb 360000 * 1 / 70 = 5.14 joules of radiation energy. The weighting factor for the type of radiation and the sensitivity of the tissue being exposed is 1, so the equivalent dose is 0.1 Sv.

The effective dose is then calculated by multiplying the equivalent dose by a weighting factor that accounts for the different organs and tissues in the body and their sensitivity to radiation. The weighting factor for the whole body is 1, so the effective dose is also 0.1 Sv.

**Take-Home Messages**

- Radiation is all around us, from the sun to our electronic devices.
- The different units of radiation are activity, absorbed dose, equivalent dose, and effective dose.
- Activity is a measure of the number of radioactive decays that occur in a given amount of time.
- Absorbed dose is a measure of the amount of radiation that is absorbed by a person or object.
- Equivalent dose is a measure of the biological damage caused by radiation.
- Effective dose is a measure of the overall health risk from radiation exposure.