The rem is a unit to measure?

The rem is a unit used to measure the dosage of ionizing radiation received by living tissue. It takes into account both the type of radiation and its potential biological impact on human health.

Radiation of atomic material like radium, uranium,thorium, indium, another radioactive sources. It also used to measure x-rays from x-ray Devices.

Hope this helps.

Biological effect of ioniozing radiation.