Doctors use the radioactive isotope chromium-51 to label red blood cells in the human body. chromium-51 gives off relatively alpha particles that can be stopped by .

The given blanks can be filled with harmless and a sheet of paper.

Explanation:

Chromium-51 is a chromium’s radioactive isotope. It has an application in medical diagnosis, as it is used to mark RBCs for the determination of red cell volume or mass, sequestration studies, survival time, and for the determination of gastrointestinal bleeding, and is utilized to mark platelets to examine their survival.  

The chromium-51 gives off relatively harmless alpha particles, which can be even prevented by a sheet of paper. The alpha particles do not exhibit a high concentration of energy and are not very ionizing, as they possess a high mass. This makes them comparatively safer to function with.