Internal Bremsstrahlung Spectrum Accompanying $1S$ Electron Capture in Decay of ${\mathrm{Fe}}^{55}$, ${\mathrm{Cs}}^{131}$, and ${\mathrm{Tl}}^{204}$

The internal bremsstrahlung spectra in coincidence with the $K$ x rays resulting from $1S$ electron capture in ${\mathrm{Fe}}^{55}$ and ${\mathrm{Cs}}^{131}$ have been measured. The results are in good agreement with Martin and Glauber's revised calculations including relativistic and screening effects. The shape of the coincidence spectra exhibits a maximum at the predicted energy and then drops off in intensity toward the low-energy region. This is markedly different from that of the total bremsstrahlung spectrum, which rises continuously toward the low-energy region. In the case of ${\mathrm{Cs}}^{131}$, the shape of the bremsstrahlung spectrum pertaining to the radiative capture of $P$ electrons as obtained by subtracting from the total spectrum the spectrum due to $S$-electron capture is also in good accord with that predicted. An estimate of the total probability for bremsstrahlung production during $1S$ electron capture as obtained by comparing the corresponding bremsstrahlung spectrum with the x-ray intensity gives (1.5±0.8)×${10}^{-5\mathrm{}}$ for ${\mathrm{Fe}}^{55}$ and (1.4±1)×${10}^{-5\mathrm{}}$ for ${\mathrm{Cs}}^{131}$, which is in fair agreement with the theoretical prediction including relativistic corrections. The end point of the $1S$ electron capture bremsstrahlung spectrum of ${\mathrm{Tl}}^{204}$ was also measured and found to be 310±10 kev.