An Investigation of Bremsstrahlung by Means of the Nuclear Isomerism of Indium

The metastable state of ${\mathrm{In}}^{115}$ has been used as a detector of a narrow energy band in the continuous x-ray spectrum produced by a monoenergetic electron beam on gold targets. The thin target x-ray isochromat ($h\nu =1.04\mathrm{}$ Mev) has been investigated in the neighborhood of the short wave-length limit. This isochromat is finite at the threshold and constant for at least 400 kev. This in agreement with Guth's theory and verifies the assumption that the excitation of ${\mathrm{In}}^{115*}$ is a line absorption. Both the thin target and thick target x-ray excitation curves for ${\mathrm{In}}^{115}$ have been obtained in the region from 1.0 to 2.6 Mev. The thin target curve exhibits a step-like character from 1.0 to 1.9 Mev. The thick target curve below 2.0 Mev exhibits straight line segments as expected from the thin target curve, but above 2.0 Mev there are no straight line segments, in good agreement with the thin target data and theory. Activation levels have been found at 1.04±0.02 Mev and at 1.42±0.02 Mev. The over-all cross section (per electron incident on a 34 mg/${\mathrm{cm}}^2$ gold target) is ${10}^{-34\mathrm{}}$ ${\mathrm{cm}}^2$ in the region of the threshold. Evidence is presented for the existence of a lower activation level between 0.8 and 0.9 Mev with an over-all cross section (for a similar target) of the order of ${10}^{-36\mathrm{}}$ ${\mathrm{cm}}^2$.