Mean Life of the 1.61-Mev Level ofMg25

Abstract

Nuclear resonance fluorescence techniques have been used to measure the mean life of the 1.61-Mev level of ${\mathrm{Mg}}^{25}$ and the 1.83-Mev level of ${\mathrm{Mg}}^{26}$. The exciting $\gamma$ radiation was obtained by bombarding metallic ${\mathrm{Mg}}^{25}$ and ${\mathrm{Mg}}^{26}$ targets with 4.0- and 4.4-Mev protons. For the ${\mathrm{Mg}}^{25}$ level, assumed to be ${\frac{7}{2}}^{+}$, the self-absorption of the resonance radiation gives $\tau =\left({{2.5}_{-0.4\mathrm{}}}^{+0.6\mathrm{}}\right)\times {10}^{-14\mathrm{}}$ sec. The angular distribution for the resonance scattering was found to be $1+(0.42\pm 0.03)P_2\left(cos\theta \right)+(0.03\mathrm{}\pm 0.003)P_4\left(cos\theta \right)$, where the errors given are statistical only. For other reasons it is believed that the correct coefficient of the $P_4$ term is approximately zero. For the ${\mathrm{Mg}}^{26}$ level, the apparent resonance scattering cross section combined with some previous estimates of slowing-down times for the excited nuclei gives $\tau =(7\mathrm{}\pm 3)\times {10}^{-13\mathrm{}}$ sec. Further evidence as to the collective nature of these nuclei and of ${\mathrm{Al}}^{27}$ is discussed. Support is given to the suggestion of the Chalk River group that the 1.61-Mev ${\mathrm{Mg}}^{25}$ and the 2.21-Mev ${\mathrm{Al}}^{27}$ levels are the ${\frac{7}{2}}^{+}$ second members of $K={\frac{5}{2}}^{+}$ rotational bands based on the ground states. For the ${\mathrm{Mg}}^{25}$ level, spin and parity ${\frac{7}{2}}^{+}$ is required to obtain agreement between the quadrupole transition probability from these measurements and that found by Coulomb excitation.