A blank is a measurement of what in radon assessment?

In radon assessment, a blank refers specifically to an unexposed device. This type of measurement is crucial in radon testing as it establishes a baseline to determine if there is any contamination or interference in the test results. By using an unexposed device, technicians can assess background measurements and ensure the accuracy of the radon levels detected by the exposed devices. This practice helps to confirm reliability and validity in the testing process, allowing for a more precise understanding of radon concentrations in a given environment.

The other options focus on different aspects of testing but do not accurately define what a blank represents in the context of radon assessment. For instance, using two or more devices simultaneously pertains to comparative accuracy but does not define a blank. A duplicate would indicate a repeated test for confirmation, while a test not involving decay speaks more to the mechanics of the radon measurement process itself rather than providing a baseline measurement like a blank does.