Which radiation unit is used to measure exposure in this context?

The appropriate unit for measuring exposure in the context of radiation is the Roentgen, which quantifies the amount of ionization produced in air by X or gamma radiation. It's particularly pertinent when discussing exposure levels in radiological scenarios.

The Rontgen Equivalent Man (REM) is actually a unit that takes into account the biological effects of absorbed radiation, factoring in the type of radiation and its impact on human tissue. While it is relevant in discussions of dose equivalence, it does not directly measure exposure, which is specifically linked to the initial interaction of radiation with matter, typically air.

The Curie measures radioactivity based on the number of decay events per second in a sample, which is distinct from exposure measurement.

The Gray, on the other hand, measures absorbed dose—the amount of energy deposited in a material by ionizing radiation. Like REM, it pertains to the effects of radiation rather than the exposure level itself.

Hence, the correct unit for measuring exposure is the Roentgen, as it addresses the direct interaction of ionizing radiation with air, leading to ionizations that can be quantified and utilized in safety assessments.