Level lifetimes in the stable Zr nuclei: Effects of chemical properties in Doppler-shift measurements

The lifetime of the second excited 2⁺ state of 94Zr at 1671 keV was determined with the Doppler-shift attenuation method (DSAM) following the (n,n^′γ) reaction on scattering samples of Zr metal and ZrO² of natural isotopic abundance. The measured lifetime, τ=368-23+27 fs, is considerably longer than the previous measurement, as are measured lifetimes of other levels in 94Zr. However, lifetimes of low-lying, low-spin states in 90^,91^,92^,96Zr were also determined and found to be in good agreement with measurements employing non-DSAM methods. Possible reasons for the discrepant lifetimes measured for 94Zr are discussed and focus on the chemical properties of the scattering sample, specifically, the effects of using amorphous materials, or those composed of small particles as scattering samples. These properties have been investigated by x-ray powder diffraction and scanning electron microscopy. The effects of using these materials for lifetime measurements were investigated by employing, as scattering samples, Zr(OH)⁴ (a known amorphous material) and ZrO² with various crystalline domain sizes.