[en] We have made new measurements of the lifetimes at 4.29 K and 2.92 K in He I, and at 2.14 K and 1.67 K in He II, without and with an electric field. At 4.19 K we have a new value of the lifetime, 2.01 ± 0.0002 ns, leading to a Z_eff of 3.53 at zero field. In contrast to earlier reports, Z_eff decreases very slightly as the temperature is lowered and the density increases. The orthopositronium lifetime at 4.19 K is 98.1 ± 0.2 ns, at zero field, independent of field up to 18 kV cm^-1. At 4.19 K the free positron decay rate falls exponentially with increasing field to a value 5% below its zero-field value, the drop-off rate being 0.36 ± 0.04 cm amagat V^-1. From 28 through 50 V cm^-1 amagat^-1 the decay rate of the free component rises slowly as expected because of the increasing positronium fraction over the same range of electric field. In liquid argon there is positronium enhancement beginning at or below 3 V cm^-1 amagat^-1, which contrasts with 160 V cm^-1 amagat^-1 for argon gas. At least two lifetimes make up the long triplet component, suggesting different degrees of thermalization, or different states of positronium within the liquid. The effect of the electric field is to enhance the longer-lived, or more thermal component. The argon results suggest positron behaviour similar to that of electrons in the liquid phase, namely, such particles move in a conduction band subject to small effective scattering and diffusion cross sections. An analytic model in which the scattering length is an adjustable parameter yields the onset of positronium enhancement just below 3 V cm^-1 amagat^-1 when a_s = -0.043 a_o, corresponding to an elastic scattering and diffusion cross section of 0.034 π a_o². Consequences for positron thermalization are discussed. A model of positronium losing energy slowly in the liquid without bubble formation is suggested. (Author)

[en] The mass-symmetric reactions ⁵⁸Fe,⁵⁸Ni+⁵⁸Fe,⁵⁸Ni were studied at a beam energy of E_beam=30 MeV/nucleon in order to investigate the isospin dependence of fragment emission. Ratios of inclusive yields of isotopic fragments from hydrogen through nitrogen were extracted as a function of laboratory angle. A moving source analysis of the data indicates that at laboratory angles around 40 degree the yield of intermediate mass fragments (IMF close-quote s) beyond Z=3 is predominantly from a midrapidity source. The angular dependence of the relative yields of isotopes beyond Z=3 indicates that the IMF close-quote s at more central angles originate from a source which is more neutron deficient than the source responsible for fragments emitted at forward angles. The charge distributions and kinetic energy spectra of the IMF close-quote s at various laboratory angles were well reproduced by calculations employing a quantum molecular-dynamics code followed by a statistical multifragmentation model for generating fragments. The calculations indicate that the measured IMF close-quote s originate mainly from a single source. The isotopic composition of the emitted fragments is, however, not reproduced by the same calculation. The measured isotopic and isobaric ratios indicate an emitting source that is more neutron rich in comparison to the source predicted by model calculations. copyright 1998 The American Physical Society

[en] The ability to vary the isospin of projectiles through the use of unstable beams has expanded the playing field for nuclear reaction mechanism studies. The first of these experiments was a study of the degree of equilibration achieved in heavy-ion reactions. Recently work has been done on the isospin dependence of the collective flow. Work is currently underway to understand the Coulomb contributions to fragmentation. This talk will give an overview of these recent heavy-ion studies that have added new insight to traditional reaction mechanism studies by using the isospin degree of freedom. (author)

[en] Isotopically resolved intermediate-mass fragments and light charged particles have been detected from the reactions ⁴⁰Ar and ⁴⁰Ca with ⁵⁸Fe and ⁵⁸Ni at E_beam=33 and 45 MeV/nucleon. There is an angular dependence to the isotopic ratios. A moving source analysis shows that fragments emitted at Θ_lab=40 degree can be attributed primarily to a composite source while the fragments emitted at backward angles are primarily from a targetlike source. The results are compared to predictions of QMD, BUU, and GEMINI. QMD generally reproduces the charge distribution and energy spectra and has partial success with the isobaric ratios when the system is chemically equilibrated. All of the models have difficulty reproducing the isotopic ratios when the system is not chemically equilibrated. copyright 1997 The American Physical Society

[en] The longitudinal polarization of positrons emitted along and opposite to the nuclear spin direction has been measured in the decay of polarized ¹²N. The results are consistent with the Standard Model prediction. In manifest left-right symmetric models this measurement provides a lower bound of 310 GeV/c² at 90% CL on the mass of a possible right-handed gauge boson contributing to the electroweak interaction. In generalized versions of this model our measurement provides constraints complementary to those set by high-energy experiments

[en] We modify the method of Albergo et al. for determining the temperature of an excited nucleus from double ratios of isotope yields and present a statistical model which accounts for the population and decay of excited states of the emitted fragments. Nuclear temperatures are extracted using experimental ratios of isotopic yields of fragments from helium through carbon for the reactions ⁴⁰Ca + ⁵⁸Ni, ⁴⁰Ar + ⁵⁸Ni, ⁴⁰Ca + ⁵⁸Fe, and ⁴⁰Ar + ⁵⁸Fe at 33 MeV/nucleon projectile energy. Using the model we obtain consistent values for the temperature from various isotope combinations within the experimental error when accounting for the population and decay of the excited fragments. copyright 1996 The American Physical Society

[en] A new forward array has been designed and constructed using silicon technology. The detectors telescopes are Si followed by CsI that are read out by photodiodes. This combination allows for isotopic resolution through Z=6. The special edge-mounting of the silicon detector combined with the ring-like structure of the overall array has allowed for a design that minimizes dead area. (orig.)

[en] Published in summary form only

[en] We have made new measurements of the lifetimes at 4.19 and 2.92 K in HeI, and at 2.14 and 1.67 K in HeII, without and with electric field. The ortho-positronium lifetime is independent of field up to 20 kV/cm. At 4.19 K, there is no increase observed in the positronium fraction from zero field to 20 kV/cm, in strong disagreement with Manuzio and Rizzuto. Some additional positronium is, however, formed above 20 kV/cm, with a corresponding decrease in lifetime of the free component. At 4.19 K the free positron mean life rises sharply with increasing electric field to a constant value of about 2.14 ns above 5 kV/cm. The increase in the free positron lifetime with field is less marked at 2.92 K. In liquid argon measurements were made at the normal boiling point only, at fields up to 20 kV/cm, above which we experienced electrical breakdown. The ortho-positronium intensity is 7% at zero field and begins to increase at 2.5 kV/cm, reaching 13% at 20 kV/cm at which field value it was still increasing. The ortho-positronium lifetime displayed an 8% increase over this field range. For liquid nitrogen we repeated some of our earlier measurements at 77.3 K and made some measurements at 64.5 K so as to test models for positronium formation in the spur. A good fit to a simple exponential formula is obtained at both temperatures, yielding a single drop-off rate of 0.21 cm/kV. (orig.)

[en] Published in summary form only