Positron–neutrino correlations in 32Ar and 33Ar decays: Probes of scalar weak currents and nuclear isospin mixing

The positron–neutrino correlation in the 0⁺→0⁺ β-decay of ³²Ar was measured at ISOLDE by analyzing the effect of lepton recoil on the shape of the narrow proton group following the super-allowed decay. Our result is consistent with the standard model prediction;for vanishing Fierz interference we find a=0.9989±0.0052±0.0039. Our result leads to improved constraints on scalar weak interactions. The positron–neutrino correlation in ³³Ar decay was measured in the same experiment;for vanishing Fierz interference we find a=0.944±0.002±0.003. The ³²Ar and ³³Ar correlations, in combination with precision measurements of the half-lives, super-allowed branching ratios and beta endpoint energies, will determine the isospin impurities of the super-allowed transitions. These will provide useful tests of isospin-violation corrections used in deducing |V_ud| which currently indicates non-unitarity of the KM matrix. This revised version was published online in July 2006 with corrections to the Cover Date.     

One-neutron knockout from Ne isotopes

One-neutron knockout reactions of ^24–28Ne in a beryllium target have been studied in the Fragment Separator (FRS), at GSI. The results include inclusive one-neutron knockout cross-sections as well as longitudinal-momentum distributions of the knockout fragments. The ground-state structure of the neutron-rich neon isotopes was obtained from an analysis of the measured momentum distributions. The results indicate that the two heaviest isotopes, ²⁷Ne and ²⁸Ne, are dominated by a configuration in which a s_1/2$s_{1/2}$ neutron is coupled to an excited state of the ²⁶Ne and ²⁷Ne core, respectively.     

Properties of the He ground state from He neutron knockout

The unbound nucleus ⁷He, produced in neutron-knockout reactions with a 240 MeV/u ⁸He beam in a liquid-hydrogen target, has been studied in an experiment at the ALADIN-LAND setup at GSI. From an R-matrix analysis the resonance parameters for ⁷He as well as the spectroscopic factor for the ⁶He(0⁺) + n configuration in its ground-state have been obtained. The spectroscopic factor is 0.61 confirming that ⁷He is not a pure single-particle state. An analysis of ⁵He data from neutron-knockout reactions of ⁶He in a carbon target reveals the presence of an s  -wave component at low energies in the α+n$\alpha +\mathrm{n}$ relative energy spectrum.     

Deformation of the N=Z nucleus 76Sr using beta-decay studies

A novel method of deducing the deformation of the N=Z nucleus 76Sr is presented. It is based on the comparison of the experimental Gamow-Teller strength distribution B(GT) from its beta decay with the results of quasi-random-phase approximation calculations. This method confirms previous indications of the strong prolate deformation of this nucleus in a totally independent way. The measurement has been carried out with a large total absorption gamma spectrometer, "Lucrecia," newly installed at CERN-ISOLDE.     

Determination of equilibrium constants of strong acidic ion exchange resins by potentiometric titrations

A new method for the determination of ion exchange equilibrium constants is developed. A technique based in the application of potentiometric titrations is used in the study of non selective strong acidic resins. Potentiometric titrations are carried out without constant ionic strength, so a mathematical treatment is developed in order to take into account the estimation of the activity coefficients and the liquid junction potentials. The ion exchange thermodynamic equilibrium constants for two resins, Dowex CM-15 and Dowex C650, between the proton form and the alkaline metals at 25 degrees C are given.     

Potentiometric and spectrophotometric study of the acid-base equilibria of Cr(VI) at 25°C and different ionic strengths of aqueous KNO3

The hydrolysis of the chromate ion has been studied using a potentiometric-spectrophotometric automated titration system, titrating basic K₂CrO₄–KNO₃ solutions with previously standarized HNO₃–KNO₃ solutions. The temperature was kept constant at 25°C and the ionic strengths were 0.5, 1.0 and 2.0 mol-dm^–3. The resulting titration curves can be interpreted in terms of two equilibria involving the formation of HCrO ₄ ^– (aq) and Cr₂O ₇ ^2– (aq). A spectrophotometric batch study was also carried out in order to obtain the thermodynamic constant for the HCrO ₄ ^– (aq) formation as well as the spectrophotometric parameters at low ionic strengths. The proposed stoichiometric constants for the two reactions and the molar absorptivities for the three species at different wavelengths and ionic strengths are also given.     

Study of the complexation and precipitation equilibria in the system Cr(VI)-Cu(II)-H2O

A potentiometric and spectrophotometric study of the system Cr(VI)–Cu(II)–H₂O has been performed. The formation of the complex CuCrO₄ is suggested and the corresponding thermodynamic formation constant has been calculated (log ₁₁ ⁰ = 3.27 ± 0.11). The study of the precipitation equilibrium between Cr(VI) and Cu(II) has also been carried out. The results indicate the formation of a mixed precipitate with formula CuCrO₄·2Cu(OH)₂ whose corresponding thermodynamic solubility product is p K _so ^o =48.6±0.2.     

Determination of ion exchange equilibrium constants of strongly acidic resins with alkaline-earth metals by means of the potentiometric titrations technique

A recently developed methodology for the determination of ion exchange equilibrium constants has been applied to ion exchange systems of 1:2 stoichiometry. Potentiometric titrations with variable ionic strength were carried out. Ionic medium titrations were performed for the estimation of the liquid junction potential. The modified Bromley's methodology and the Wilson model were used for the estimation of the activity coefficients of the species in the aqueous and resin phase, respectively. A modification of the Henderson equation is used for the estimation of liquid junction potentials in the mixtures including 1:2 electrolytes. Equilibrium constants for the H(+)/M(2+) (M=Mg, Ca, Sr and Ba) exchange systems in the strongly acidic resins Dowex CM-15 and Dowex C650 were studied.