Frequency of neutron versus gamma emission by a radioactive source Given gamma and neutron emission spectra for a radioactive source, say PuBe-239 for example, is there a way to calculate what fraction of total particles emitted are neutrons or gammas? If not, what's a good ballpark estimate? 99% are gammas, 1% neutrons?
 A: The Particle Data Group's reference page about common radioactive sources gives
$$\begin{array}{ccccc}
\text{source} &
\text{particle} & \text{energy (MeV)} & \text{branching fraction}
\\\hline
\rm^{241}Am
& \alpha & 5.443 & 13\% \\
& \alpha & 5.486 & 84\% \\
& \gamma & 0.060 & 36\% \\
& \text{Np x-rays} & & 38\% \\
\hline
\rm^{241}Am/Be
& \rm n & \sim4\rm\,MeV & 60\times10^{-6} \\
& \gamma & 4.43 & 40\times10^{-6} \\
\end{array}
$$
Apparently, some (but not all) of the americium alpha decays are accompanied by a gamma-ray cascade, but the most intense of those gamma rays is relatively low energy. In an AmBe source, the cross section for the $\rm^{9}Be(\alpha,n)2\alpha$ reaction is about about twice as high for the branch of that reaction with an associated gamma as for the branch without the associated gamma, and those gamma rays are much higher-energy than the primary gamma rays from the americium decay.  However, both cross sections are very small.
So from  a sealed AmBe source (which doesn't allow the primary alpha particles to escape), nearly all of the radiation particles will be low-energy gamma rays and x-rays, but the higher-energy gamma rays and the neutrons will be about equally common.  A PuBe source will behave in broadly the same way.
(I know people who have used the 4 MeV photons from a PuBe or AmBe source as "triggers" to tell a data acquisition system to look for a neutron.)
