When counting Cr51 red blood cell samples in a scintillation well counter, the window should be set around which photopeak?

Cr-51 (Chromium-51) is a beta-emitting radioisotope used in nuclear medicine primarily for labeling red blood cells. When these labeled red blood cells are counted in a scintillation well counter, it is important to set the energy window around the specific photopeak emitted by Cr-51.

The primary photopeak for Cr-51 is located at approximately 320 keV, which corresponds to the gamma radiation emitted when the isotope decays. This energy level is significant because it allows for the detection of the gamma rays produced during the decay of Cr-51 in the scintillation counter, thereby generating accurate counts of the labeled red blood cells. Setting the window around this energy level maximizes detection efficiency while minimizing interference from other gamma emissions, leading to more precise measurements.

The other options represent different energy levels associated with other isotopes or decay processes unrelated to Cr-51's 320 keV peak. Thus, focusing on the correct energy level ensures that the counting reflects only the activity from the Cr-51 labeled red blood cells, essential for proper assessment in clinical applications.