Which of the following is NOT a key concept of radon measurement systems?

Spatial resolution is often not considered a key concept of radon measurement systems primarily because most dosimetry methods focus on capturing total exposure rather than analyzing variations in radon levels across different areas at a high resolution. The other options—counting efficiency, sensitivity, and the lower level of detection (LLD)—are vital components in evaluating the effectiveness and reliability of radon measurement devices.

Counting efficiency refers to a system's ability to accurately count the radon decay products, which directly impacts the accuracy of the measurement. Sensitivity indicates how well a measurement system can detect low levels of radon, thus affecting its capability to identify potentially hazardous conditions. The lower level of detection (LLD) signifies the smallest amount of radon that can be reliably measured, which is crucial for ensuring that measurements are meaningful, especially in areas where radon levels are commonly low.

In contrast, spatial resolution pertains more to the detail at which spatial variations can be detected, which does not significantly influence the overall measurement approaches typically used in radon assessment. Therefore, this element stands out as the least relevant to the core principles that define radon measurement systems.