ReactionZr90(n, n′)Zr90m

The cross section for the excitation of the 2.3-MeV metastable state in the ${\mathrm{Zr}}^{90}(n, n^{\prime }){\mathrm{Zr}}^{90m}$ reaction has been measured from threshold to 5 MeV. An irradiate-count cycle was used in which a Zr sample was first bombarded by monoenergetic neutrons and then transferred by a pneumatic transport system to a shielded counting station where the 2.3-MeV gamma rays from the decay of the metastable state were measured. The half-life of this state was found to be $T_{\frac{1}{2}}=0.801\mathrm{}\pm 0.005$ sec. The excitation curve has a threshold at 2.3 MeV and increases monotonically to 300 mb at 5 MeV. In addition to the threshold rise there are also marked increases in cross section that begin at 2.75 and 4.4 MeV. These increases are due to states at these energies that cascade through the 2.3-MeV state. The state at 2.75 MeV is probably the hitherto missing 4- (four-minus) state of the ($p_{\frac{1}{2}}{g}_{\frac{9}{2}}$) configuration and it is essentially equal in energy to the 3- (three-minus) state at 2.75 MeV that has been observed in other reactions.