The 10 B(a; n) 13 N reaction is studied as an activation process in a variety of solid boron-containing neutron shielding materials. The source of a-particles is the neutron capture reaction 10 B(n; a) 7 Li. Samples of boron carbide, boron oxide, and boron nitride are irradiated with thermal neutrons and the rate of 13 N production is determined. 13 N promptly decays, emitting a positron. This positron efficiently annihilates with electrons in the material and the resultant 511 keV gamma ray is detected. For each of the above-mentioned materials, the rate of 13 N production is (1–2) · $10^10$ per captured neutron.