H. Ho

Pre-equilibrium alpha emission accompanying deep-inelastic16O+58Ni collisions

Abstractα particles were measured in coincidence with projectile-like reaction products (oxygen and carbon) produced in deep-inelastic16O+58Ni collisions at about 6 MeV/N bombarding energy. The kinematic analysis of the HI andα energies measured as a function ofΘα gives strong evidence for a sequential process: the target-like fragments are excited by the deep-inelastic collision and undergo subsequentα decay. In contrast, the angular correlations show a pronounced forward peak, indicative of direct or pre-equilibriumα emission. The emission time for the latter is estimated to be of the order of 2×10−21 s. To resolve this conflict of co-existing statistical and direct features of the pre-equilibrium emission, the concept of a hot spot is proposed. From the angular correlation and from theα multiplicities, a local temperature ofT−3.5 MeV is deduced which agrees well with the temperature derived from the shape of theα spectra. The spot size is estimated to be 1/5 of the sphere.

Temperatures, barriers, and level densities of highly excited nuclei with A ≈ 160☆

Abstract From coincidence measurements between heavy residues, light particles, and γ -rays, the excitation excitation energy dependence of the temperatures, barriers, and nuclear level density parameters for nuclei with A ≈ 160 has been determined. The temperature increases with excitation energy in the range of 100 to 400 MeV consistently with a nuclear level density parameter a increasing from essentially a = A /8 to a = A /13. The emission barrier is lower than predicted by either spin dependent or temperature dependent theoretical calculations.

Alpha emission in coincidence with fission-like fragments from Cl+Ag reactions at 10 MeV/n

Energy and angular correlations between alpha particles and fission-like fragments in the reaction of35Cl+Ag at 10 MeV/n have been measured. The coincident yield is dominated by two mechanisms: sequential emission from both fragments (Mα=0.137±0.014) and alpha emission from the composite system prior to scission (Mα=0.124±0.012). The observation of nuclear shadowing for sequential emission from the fragments allowed us to estimate the “lifetime” of the 218 MeV excited143Gd composite system as (1–3)×10−20s. It is argued that this rather long time reflects the slow evolution in shape of the composite system and might be termed “sticking time,” since statistical-model lifetimes are typically an order of magnitude shorter. Observed also are small contributions of nonequilibrium emission in the forward direction (Mα=0.005±0.001) and a component displaying the characteristics of emission from the neck (Mα=0.010±0.002).

Shadowing of α-emitting fragments observed in 313 MeV32S+93Nb collisions

Energy- and angular correlations ofα particles in coincidence with deeply inelastic fragments from32S+93Nb reactions at 313 MeV were measured and the assumption of sequentialα emission from the fragments tested. In the direction of the detected light fragment too fewα particles from the heavy fragment are observed. This pronounced anomaly is ascribed to nuclear shadowing of theα particles by the proximate light fragment.

Angular momentum dissipation in16O +58Ni collisions investigated by observation of light particles andγ rays

A systematic study of angular momentum dissipation in deep-inelastic heavy ion collisions is carried out by combining two experimental methods: the angular correlation ofα particles and protons in coincidence with deeply inelastic light fragments; and the first two moments of theγ-ray multiplicity. The unified analysis of these data, using a quantum mechanical treatment of the particle anisotropies and statistical decay calculations, yields most probable fragment spinsI0=10.8±1.1ħ and 13.4±1.1ħ for58Ni and62Zn, respectively, close to the sticking limit. For the width of the vector spin distribution assumed in our analysis to be equal in all three directions we obtainσ=3.9±1.3 ħ and 2.9±1.8ħ for58Ni and62Zn, respectively. These values correspond to most probable alignment parameters 〈Pzz〉=0.66±0.19 and 0.82±0.17 for58Ni and62Zn, respectively. As only very few partial waves of this light system undergo deep-inelastic scattering,σ directly relates to spin fluctuations, and is compared to asymptotic values from transport theories.

Fragment spin distribution in 400 MeV40Ar+93Nb collisions investigated by light-particle anisotropies

In and out-of-plane angular correlations between light particles and projectile-like fragments from deeply inelastic collisions of 400 MeV40Ar+93Nb were measured. At backward angles, the equilibrium emission of a particles and protons from the target-like fragment was identified. By analysing the out-of-plane angular correlations in the framework of a semi-classical statistical model, an average spin of 30±2ℏ and an average alignment ofPzz=0.80±0.15 for the target-like fragment were deduced.

Shadowing of sequential α-emission in deep-inelastic 32S + 93Nb collisions: A new probe of fragment lifetimes

Abstract Energy and angular distributions of products with 3⩽Z⩽22 from 32 S + 93 Nb reactions at 10 MeV/N were measured and the cross section for deep-inelastic reactions was deduced. Energy and angular correlations between α-particles and deep-inelastic fragments were measured and analysed in terms of sequential emission from the detected projectile-like fragment ( M α = 0.30± 0.06) and sequential emission from the target-like fragment α = 0.90±0.10). For sequential emission from the target-like fragment shadowing was observed due to the proximity of the two fragments at the time of emission. Three-body Coulomb trajectory (TBCT) calculations give a lifetime of (1–3)×10 −21 s for the target-like fragment, in good agreement with statistical-model calculations. It is shown that the shadow effect is sensitive to the charge/size of the projectile-like fragment as predicted by the TBCT calculations. In the forward hemisphere nonsequential emission of beam-velocity α-particles is observed α = 0.20±0.06). At backward angles another component is observed α = 0.05±0.01) which appears to be emission from the neck region of the scissioning fragments. It is shown that the two nonsequential emission processes have a different dependence on the exit-channel mass split.

Energy dependence of proton emission in 4He and 12C induced reactions between 25 and 84 MeV/u☆

Abstract Spectra of protons emitted after bombardment of different targets with α -particles of 25 MeV/u and 43 MeV/u and with 12 C ions of 25 MeV/u and 84 MeV/u have been measured. The data are described within a “moving source” parametrization. The extracted source velocities and temperatures and their energy dependence are discussed.

Large shape deformations of compound nuclei 161Ho, 160Ta disclosed by their α-emission pattern

Abstract α paricles measured in coincidence with fission fragments in the fusion-fission reaction 200 MeV, 254 MeV 37Cl+124Sn→ 161Ho (A, B) and 318 MeV 28Si+141Pr→169Ta (C) exhibit an angular correlation with respect to the spin axis which is by a factor of 4–18 less anisotropic than expected for spherical nuclei. Derived quadrupole deformation parameters |δ|≌1 ( A ) , |δ|=0.8±0.1 (B) for the Ho composites at angular momenta I≌80 h are well in the regime of superdeformation. Shapes tend to be more compact at still higher temperature and spin, |δ|=0.5±0.1 (C).

Evidence for Coulomb-field induced light-particle emission during scission

Abstract Energy and angular correlations of protons and α particles were measured in coincidence with fission fragments in the reactions 200 MeV, 254 MeV 37Cl + 124Sn and 318 MeV 28Si + 141Pr. The light-particle yield is dominated by statistical emission from composite nucleus and from fission fragments, but α spectra also display the characteristics of evaporation during scission. We give Coulomb-field induced light-particle emission as a novel and hitherto only consistent explanation of this emission process.