Delayed coincidence lifetime measurements of ⁵⁶Co excited states

Abstract

Numerous studies have been done on the ⁵⁶Co nucleus which hasone proton less than the magic number 28 and one neutron in excess, to determine how closely the shell model predictions explain its behavior. The spins and parities of ⁵⁶Co excited states populated by ⁵⁶Ni decay have been interpreted in terms of one-particle-one-hole shell model configurations, then more recently the importance of two-particle-two-hole configurations have been demonstrated. The lifetimes of ⁵⁶Co excited states have been measured in this work by the delayed coincidence method to investigate whether two-particle-two-hole configurations result in some degree of deformation in this nucleus leading to a collective effect. The lifetime of only one state and upper limits for two others are previously known for this nucleus.

The results for the half-lives of ⁵⁶Co excited states obtained are: T½ = 1.52 ± 0.04 nsec for the 1.45 MeV level, T½ = 0.07 ± 0.01 nsec 2 for the 970 keV level, and T1 > 0.02 nsec for the 158 keV level. The half-life for the 1.45 MeV level and that for the 158 keV level are consistent with values for an E2 and M1 transition, respectively. The relatively long half-life obtained for the 970 keV level suggests that the transition deexciting this state is a retarded M1 transition, and that two-particle two-hole admixtures can be attributed to the 970 keV level resulting in some degree of core polarization leading to a collective effect and resulting in an E2 admixture to this transition. Malik and Scho1z have shown that a marked degree of deformation results from the Corio1is coupling in the 1f7/2 region (⁵⁶Co is an f7/2 proton hole and a P3/2 neutron state).

⁵⁶Ni is produced from the ⁵⁴Fe(a,2n)⁵⁶Ni reaction on natural iron foil and the nickel separated from cobalt activities and iron by ion exchange chemical separation with a decontamination factor of almost 10³. The relative intensities of ⁵⁶Ni gamma rays are also measured. The delayed coincidence lifetime set-up makes use of l½-in. diam, x 1-in.-thick plastic scintillator and a 3/4-in.-diam. x ½-in.-thick NaI(Tl) detector with associated circuitry; resolution is around 0.8 nsec at 800 keV. The lower limit for mean lives that are measurable is set by the instrumental drift of around 16 psec.