To-sterilize-90-of-a-given-cell-population-a-dose-of-10-Gy-is-needed-with-the-250-keV-x-ray-beam-while-only-2-Gy-is-needed-with-beam-type-A-What-is-the-RBE-for-beam-type-An-A-RBE-=-5n-B-RBE-=-2n-C-RBE-=-10n-D-RBE-=-1

The RBE (Relative Biological Effectiveness) is a measure of the effectiveness of a given radiation type in causing biological damage compared to a standard reference radiation. In this case, the reference radiation is the 250 keV x-ray beam.

The formula for calculating RBE is:

RBE = D (reference dose) / D (test dose)

where D is the dose required to achieve a certain biological effect (in this case, sterilization).

From the information given, we know that:

• The reference dose (D) for the 250 keV x-ray beam is 10 Gy.
• The test dose (D) for beam type A is 2 Gy.

Plugging these values into the formula, we get:

RBE = 10 Gy / 2 Gy = 5

Therefore, the answer is A. RBE = 5.