Increases in Neutron Yield of ²³⁹Pu-Be(α,n) Sources—II

Neutron yields of ²³⁹Pu-Be(α,n) sources are increasing due to the buildup of ²⁴¹ Am, an alpha emitter, from the beta decay of ²⁴¹ Pu. The yields and heat outputs of four sources, in which PuBe₁₃ was formed during fabrication, have been followed for several years. Rates of increase in neutron yields at t = 0 vary from 0.3 to 1.9%/yr for initial ²⁴¹ Pu contents from 0.10 to 0.70%. For three of these sources the increase in neutron yield follows the radioactive decay of the constituents. For the fourth, the increase in neutron yield is greater than expected. A possible explanation is offered.