The mass spectrum of cationic boron clusters, ${B_N}^{+}$ ($N=5-20$), after photoexcitation demonstrates that radiative cooling is an important, though often neglected, process in determining the relative stability of small and isolated particles. The observed intensities in mass spectra suggest that ${B_5}^{+},{B_{11}}^{+},{B_{13}}^{+}$, and ${B_{15}}^{+}$ are particularly stable clusters, consistent with density-functional theory calculations. Quantitative agreement, however, is only obtained if radiative cooling is included in the analysis. All clusters are found to radiate on microsecond timescales, suggesting recurrent fluorescence as the dominant photon emission process.