Neutron activation analysis

Samples arrive in the supplied vials and are assayed as received. Nothing is dissolved, extracted, or counted sphere by sphere.

Samples are irradiated in a 2 MW research reactor. The neutron flux converts a small fraction of each stable isotope label into a short-lived radioactive form, in place, inside the sealed vial. Samples then sit for a short decay period before counting, which lets the short-lived background activity from sodium and chloride in the tissue die away while the label signal remains.

Each sample is then counted on a high-purity germanium (HPGe) gamma spectrometer. Every label emits gamma rays at its own characteristic energy, so a single count resolves and quantifies all the labels in a sample at once, however many were injected. Counting is non-destructive: the sample survives it intact and can be recounted or archived.

What comes back is the activity measured for each label in each sample, reported as disintegrations per minute. Given the tissue masses and the reference withdrawal, those figures are converted to blood flow in mL/min/g.