Giant resonances and bound collective states observed in the scattering of 92.5-MeV electrons from the closed-neutron-shell nucleus Y89 between excitation energies from 2.0 to 55 MeV

92.5 MeV electrons were used to study in (89)Y the excitation range between 2 and 55 MeV. Above neutron threshold, broad electric resonances have been seen at 14.0 (63A(-1/3) MeV (E2, vT = 0) and 28 (125A-(1/3) MeV (E2, vT = 1). The total width of the isoscalar E2 resonance is (4.5 +/-0.4) MeV and its strength (56 =/- 6) % energy-weighted sum rule (E2, vT = 0). For the isovector E2 resonance only a minimal value of 7 MeV for the width can be given which is connected with (48 +/- 5)% of the isovector sum rule. The strength of the E1 resonance [T>7(104 =/- 10)% of the Thomas-Reiche-Kuhn sum rule] agrees well with (y,n) measurements, thus giving a check for the accuracy of the evaluating methods employed. A structure around 19-20 MeV, believed to be the T> part of the giant dipole resonance, carries (8 +/- 3)% of the E1 sum rule. In addition to these generally well known states, clustering of E3 strength, (13 +/- 2)% energy-weighted sum rule (E3, vT = 0), was found at 6.75 and 80.5 MeV; the enveloping line shape of these clusters was best described by a Breit-Wigner term. Other concentrations of strength include E3 at 2.6 MeV, (15 +/-3)% energy-weighted sum rule E3, (vT = 0), E2 at 4.0 MeV, (11 +/- 3)% energy-weighted sum rule (E2, vT = 0), and E3 strength at 13.5 MeV. No resonance other than the E1 was found around 17 MeV, thus ruling out in (89)Y the existence of a monopole state with 100% sum rule proposed for (90)Zr from (a,a') and (e,e') measurements. In contrast to heavy nuclei, no resonant 3hw E3 strength could be located

Disturbance patterns in southern Rocky Mountain forests

The pattern of landscape diversity in the Southern Rocky Mountains has been described as resulting from two superimposed vegetation pat- terns: the distribution of species along gradients of limiting factors, and patterns of disturbance and recovery within the communities at each point along the environmental gradients (Romme and Knight 1982). The previous chapter (D. H. Knight and W. A. Reiners, this volume) has emphasized the first pattern whereas this chapter emphasizes the role of natural disturbance in creating landscape patterns. Although human impacts on fundamentally natural disturbances such as fires and insect outbreaks are included, other chapters treat disturbances of exclusively human origin such as logging and road construction