Dynamic neutron sources from the $^{207}$Pb($\gamma$,n) and $^{205}$Tl($\gamma$,n) reactions

Mono-energetic gamma-beams ($\Delta\sim 10$ eV) based on thermal neutron capture, in a nuclear reactor, using the V(n, $\gamma$) and Fe(n,$\gamma$) reactions, were utilized for generating fast neutron sources from lead and thallium respectively, via the $^{207}$Pb($\gamma$,n) and $^{205}$Tl($\gamma$,n) reactions. It so happened that one of the incident gamma-lines of the V source, E$_\gamma$ = 7163 keV, photoexcites by chance a resonance level in $^{207}$Pb, which emits neutrons at an energy of 423 keV. In a similar manner, the incident $\gamma$-line at E$_\gamma$ = 7646 keV of the Fe(n, $\gamma$) source photoexcites by chance a resonance level in the $^{205}$Tl isotope, which emits neutrons at an energy of 99 keV. The cross-sections for the neutron emission process were measured and found to be $\sigma$($\gamma$,n) = 35+/-6 mb and 107+/-17 mb respectively with intensities of the order of $10^4$ n/sec.