Walls that are considered to be secondary barriers should be:

Secondary barriers in radiation protection are essential for minimizing exposure to secondary radiation, often derived from scatter and leakage radiation produced in diagnostic imaging environments. The primary function of these barriers is to ensure that individuals not involved in the imaging process are kept safe from excessive radiation exposure.

Selecting the appropriate lead thickness for secondary barriers is crucial. The effective thickness is a function of several factors, including the type of radiation being produced and the anticipated levels of exposure. The standard for secondary barriers typically falls at a minimum lead thickness of 0.8 mm. This thickness is effective in providing adequate shielding against the lower energy scatter radiation common in radiographic procedures, resulting in a significant reduction in exposure risk.

In addition to the lead thickness, the height of the barrier is also important. A height of 2.1 meters (7 feet) is considered adequate for secondary barriers, as it is generally tall enough to protect individuals who might be standing in typical positions near the imaging equipment.

Thus, the correct answer reflects both the appropriate lead thickness and height necessary to ensure the safety of individuals from secondary radiation in a clinical setting.