  Photon branching ratios are critical input data for activities such as
nuclear materials protection and accounting because they allow material
compositions to be extracted from measurements of gamma-ray intensities.
Uncertainties in these branching ratios are often a limiting source of
uncertainty in composition determination. Here, we use high statistics, high
resolution (~60-70eV full-width-at-half-maximum at 100 keV) gamma-ray spectra
acquired using microcalorimeter sensors to substantially reduce the
uncertainties for 11 plutonium (238Pu,239Pu,241Pu) and 241Am branching ratios
important for material control and accountability and nuclear forensics in the
energy range of 125 keV to 208 keV. We show a reduction in uncertainty of over
a factor of three for one branching ratio and a factor of 2{3 for four
branching ratios.
