The neutron unbound ground state of (25)O (Z=8, N=17) was observed for the first time in a proton knockout reaction from a (26)F beam. A single resonance was found in the invariant mass spectrum corresponding to a neutron decay energy of 770_+20(-10) keV with a total width of 172(30) keV. The N=16 shell gap was established to be 4.86(13) MeV by the energy difference between the nu1s(1/2) and nu0d(3/2) orbitals. The neutron separation energies for (25)O agree with the calculations of the universal sd shell model interaction. This interaction incorrectly predicts an (26)O ground state that is bound to two-neutron decay by 1 MeV, leading to a discrepancy between the theoretical calculations and experiment as to the particle stability of (26)O. The observed decay width was found to be on the order of a factor of 2 larger than the calculated single-particle width using a Woods-Saxon potential.

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The transition rates for the 2(1)+ states in (62,64,66)Fe were studied using the recoil distance Doppler-shift technique applied to projectile Coulomb excitation reactions. The deduced E2 strengths illustrate the enhanced collectivity of the neutron-rich Fe isotopes up to N = 40. The results are interpreted using the generalized concept of valence proton symmetry which describes the evolution of nuclear structure around N = 40 as governed by the number of valence protons with respect to Z ≈ 30. The trend of collectivity suggested by the experimental data is described by state-of-the-art shell-model calculations with a new effective interaction developed for the fpgd valence space.

Six pigeons were trained on a delayed red-green matching-to-sample task that arranged four delays within sessions. Matching responses intermittently produced either 1.5-s access to food or 4.5-s access to food, and nonmatching responses produced either 1.5-s or 4.5-s blackout. Two phases were conducted: a signaled phase in which the reinforcer magnitudes (small and large) were signaled by houselights (positioned either on the left or right of the chamber), and an unsignaled phase in which there was no correlation between reinforcer magnitude and houselight position. In both phases, the relative frequency with which red and green matching responses produced food was varied across five values. Both matching accuracy and the sensitivity of performance to the distribution of reinforcers for matching responses decreased with increasing delays in both phases. In addition, accuracy and reinforcer sensitivity were significantly lower on signaled small-reinforcer trials compared with accuracy and sensitivity values on signaled large-reinforcer trials and on both types of unsignaled trials. These results are discussed in the context of research on both nonhuman animal and human memory.