Abstract. The 7-radiation emitted after thermal neutron capture in isotopically enriched 58Ni and 6~ was measured at the ILL high flux reactor by means of Ge/NaI detectors operated in Compton suppression and pair spectrometer mode. The neutron binding energies were determined as B.(59Ni) = 8999.15(23) keV and Bn(61Ni) = 7820.07(20) keV; some 95% of the total y-ray fluxes through s9,61Ni were assigned. The y-ray strength functions of the primary transitions and the level densities are discussed. 21.10.Ma; 23.20.Lv; 25.40.Lw; 27.50.+e
PACS:
MotivationThe 7-ray decay of the capture state(s) populated in thermal neutron capture reactions is considered to be a nonselective process and thus enables one to establish "complete" level schemes up to about half the neutron binding energy B, and within the spin window bound by s-wave capture and primary dipole transitions. Previous detailed studies of light and medium nuclei [1,2] have shown that the assignment of new states is essentially limited by the increasing level density and decreasing phase space of primary transitions, for given capture crosssections and spectrometer resolution and efficiency.Previous (n, ~) studies in the even-mass nickel targets [3] have shown deviations from this behaviour insofar as the primary spectra are dominated by very few strong transitions to low-lying negative-parity states which account for some 70%-90% of the primary flux. This direct mechanism thus dominates the capture process and leaves correspondingly little flux to higher excited states; this flux was not fully resolved up to now. With the high neutron flux available at the ILL reactor and a high resolution pair spectrometer we hoped to be able to identify the missing portions of the primary spectra Supported by Deutsches BMFT of the 5s'6~ 7) reactions, to identify new excited states and make spin-parity assignments. This information, together with the results from charged-particle induced reactions then should allow us to derive level densities and ~-ray strength functions in the semi-magic Z=28 nickel isotopes, and compare them to findings for N ~_ 20, 28 and 50 neutron magic nuclei [4][5][6][7][8][9].This present work complements our previous studies of the 6~Ni(n, ~)63Ni and 6ZNi(2 n, 7)64Ni reactions [10] and the GRID lifetime measurements in 59'61Ni by U1-big et al. [11]. In this latter work, assumptions concerning the feeding mechanism of several excited states had to be inferred which now can be checked with the present data. Ulbig and co-authors [11,12] also performed extensive shell model calculations of the negative parity states in 59Ni.
Experimental procedureThe experimental procedure followed the method applied in our previous neutron capture studies on 31p, s~ 86'87'aasr and 62Ni targets [5-8, 10, 13] where further details of the experimental set-up and data analysis are given. First the 58Ni target, consisting of ca. 130mg of metallic powder (isotopical enrichment 99.8%), was exposed to a thermal neutron flux of 5.5.1014 neutrons/cm 2 s near the core of the ILL h...
