When counting Cr-51 red blood cell samples in a scintillation well counter, what should the photopeak window be set at?

When counting Cr-51 red blood cell samples in a scintillation well counter, the photopeak window should be set around 320 keV. This is because the most significant gamma emission from Chromium-51 occurs at 320 keV. Setting the window to the energy level that corresponds to the primary photopeak ensures that the detector is sensitive to the photons being emitted, allowing for accurate measurement of the radioactive decay of the Cr-51 present in the samples.

Having the photopeak window correctly set aligns with the principles of radionuclide detection, where sensitivity and specificity are essential for obtaining reliable counts. This correct setting minimizes background noise and allows any counts observed to be more representative of the actual radioactivity within the sample.

The other energy levels listed do not correspond to the primary emission from Cr-51 and thus would not provide accurate counting of the red blood cells labeled with this radioisotope.