Abstract
A thin (20-kev) target of manganese on a tantalum backing was bombarded by monoenergetic protons of 2.97, 3.42, and 3.77 Mev from the Rockefeller electrostatic generator. Nuclear emulsion plates (Eastman NTB, 200 microns thickness) were mounted 15 cm from the target at 0\ifmmode^\circ\else\textdegree\fi{} and 90\ifmmode^\circ\else\textdegree\fi{} with respect to the incident proton beam. The resulting neutron spectra, obtained from the analysis of 2400 proton recoil tracks, show well-defined groups which correspond to excited states in ${\mathrm{Fe}}^{55}$ at 0.42, 0.94, 1.36, and 2.08 Mev. The group corresponding to the level at 1.36 Mev has an unusually large width indicating that this level is complex. The $Q$-value for the ground-state reaction was measured to be -1.05\ifmmode\pm\else\textpm\fi{}0.05 Mev, giving the threshold for neutron production as 1.07\ifmmode\pm\else\textpm\fi{}0.05 Mev. Levels in ${\mathrm{Fe}}^{55}$ at 0.935 and 1.41 Mev have been found by Deutsch and Hedgran from a study of the decay scheme of ${\mathrm{Co}}^{55}$. These levels correspond closely to two of the levels reported here.
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