Nuclear Resonance Fluorescence inAs75

When ${\mathrm{As}}^{75}$ is irradiated with the gamma rays from a gaseous source of ${\mathrm{Se}}^{75}$, strong resonance fluorescence from the 265-kev excited state is observed. The angular distribution of the resonance radiation has the form $W\left(\theta \right)=1+(0.15\mathrm{}\pm 0.02)P_2\left(cos\theta \right)$, indicating that the spin of the 265-kev level is larger than ½. If one assumes a spin value of $\frac{3}{2}$, the measured self-absorption of the resonance fluorescence leads to a mean life of (1.6±0.2)×${10}^{-11\mathrm{}}$ second for the 265-kev transition to the ground state of ${\mathrm{As}}^{75}$.