Gamma-ray spectroscopy in 61Ni-levels below 2.2 MeV in excitation

States in 61Ni were populated using the 58Fe(α,nγ)61Ni reaction. Gamma-gamma coincidence measurements at beam energies of 8.0, 8.5 and 12.0 MeV were used to determine the decay scheme. Excitation energies, gamma-ray branching ratios, angular distributions, linear polarizations and mixing ratios for levels below 2.2 MeV in excitation were measured at α particle energies of 7.8, 11.0 and 13.0 MeV. The analysis of the angular distributions and linear polarizations allowed spin and parity assignments of Jsup(π) = 7-/2, 7-/2, 5-/2, 9-/2, 9-/2, 5-/2, 7-/2, 9+/2 and 11-/2 to be made for the levels at 1015, 1454, 1609, 1807, 1987, 1997, 2018, 2121 and 2129 keV respectively. Electromagnetic multipole mixing ratios were determined for many of the gamma-ray transitions. Centroid shifts were also extracted from angular distributions taken at bombarding energies of 7.5, 7.8, 11.0 and 13.0 MeV. The mean lifetimes of the 908, 1132, 1186, 1454, 1609, 1729, 1807, 1987, 1997, 2018 and 2124 keV levels were deduced using the Doppler-shift attenuation method to be 1500 +- 400, 450 +- 30, 140 +- 20, 1200 +- 290, 370 +-30, 99 +- 8, 1600 +- 250, 960 +- 240, 72 +- 6, 580 +- 60 and 84 +- 6 fs respectively. The results are compared with recent shell-model calculations. (author)