Discrete radioactive particles are tiny solid bits that carry specific radionuclides. They come from materials that become radioactive and can shed off as particles. In nuclear settings, used nuclear fuel and components that have become activated by neutron exposure—such as corrosion products on reactor surfaces—can form and release small particles containing radionuclides. These particles are a form of contamination that can be inhaled or ingested, which is why they’re considered discrete radioactive particles. Natural background radiation, medical x-ray devices, and household smoke detectors all relate to radiation in different ways but not as discrete particles. Natural background is mainly photons (and some radon in the air), x-ray devices emit photons, and smoke detectors use sealed sources that don’t release particles under normal use. So the source that produces discrete radioactive particles is nuclear fuel and activated corrosion products.

Rem expresses the dose equivalent a person receives from radiation, factoring in the different biological effects of various radiations. It’s not a measure of energy deposited per kilogram (that’s absorbed dose, in gray), nor a measure of how fast the material decays (that’s activity, in becquerels), and it isn’t simply a specific organ dose without considering tissue sensitivity. Historically, rem, short for roentgen equivalent man, was used to express how much radiation exposure a person would effectively experience, with sievert now serving as the modern equivalent unit.

Distance is fixed at 30 centimeters when applying the dose-rate criteria for Radiation Area definitions. Measuring at 30 cm provides a consistent, conservative boundary to decide if an area could expose workers above the allowed rate. Because dose rate falls off with distance, using this close-in, standardized distance makes it easier to classify areas reliably across different sources and setups. The other distances aren’t used for this specific criterion, so they don’t fit the definition.

TEDE represents the total radiation dose a person receives from all sources, by adding together the dose from external radiation and the internal dose from radionuclides that have entered the body. External sources are radiation that comes from outside the body, such as gamma rays or X-rays, while internal sources come from radioactive materials taken into the body (inhaled, ingested, or absorbed) and emitting radiation from inside. The internal dose is often described as the committed effective dose, reflecting how radionuclides behave in the body over time and deposit energy in tissues. Because TEDE combines both external and internal contributions, it provides the overall picture of exposure. That’s why the correct choice is that TEDE relates to external and internal sources. If you focus only on external or only on internal sources, you’d miss part of the total dose. External-only in most cases isn’t accurate because internal contamination can be a significant or even dominant part of the dose in many settings.