Pions are the lightest mesons, with mass some 270 times that of the electron. Charged pions decay typically into a muon and a neutrino or antineutrino. This makes pion beams useful for producing beams of neutrinos, which physicists use to study those elusive particles. In a medical application during the late 20th century, accelerator centers installed “biomedical beam lines” to test pions for cancer therapy. In these experiments, pions attached themselves to atomic nuclei within cancer cells. The nuclei would literally explode, delivering a “pion star” of cancer-killing nuclear debris. Unfortunately, results were not as encouraging as hoped, and enthusiasm for this technique has waned.

The lifetime of charged pions is 26 ns. The length of an accelerator’s biomedical beam line, from the point where pions are created to the patient, could be at most about 
a. 800 m long.
b. 80 m long.
c. 8 m long.
d. 80 cm long.