N. J. Stone

Incompressibility in finite nuclei and nuclear matter

The incompressibility (compression modulus) K0 of infinite symmetric nuclear matter at saturation density has become one of the major constraints on mean-field models of nuclear many-body systems as well as of models of high density matter in astrophysical objects and heavy-ion collisions. It is usually extracted from data on the Giant Monopole Resonance (GMR) or calculated using theoretical models. We present a comprehensive re-analysis of recent data on GMR energies in even-even Sn and Cd and earlier data on 58 ≤ A ≤ 208 nuclei. The incompressibility of finite nuclei KA is calculated from experimental GMR energies and expressed in terms of A −1/3 and the asymmetry parameter β = (N-Z)/A as a leptodermous expansion with volume, surface, isospin and Coulomb coefficients Kvol, Ksurf , Kτ and Kcoul. Only data consistent with the scaling approximation, leading to a fast converging leptodermous expansion, with negligible higher-order-term contributions to KA, were used in the present analysis. Assuming that the volume coefficient Kvol is identified with K0, the Kcoul = -(5.2 ± 0.7) MeV and the contribution from the curvature term KcurvA −2/3 in the expansion is neglected, compelling evidence is found for K0 to be in the range 250 < K0 < 315 MeV, the ratio of the surface and volume coefficients c = Ksurf/Kvol to be between -2.4 and -1.6 and Kτ between -840 and -350 MeV. In addition, estimation of the volume and surface parts of the isospin coefficient Kτ , Kτ,v and Kτ,s, is presented. We show that the generally accepted value of K0 = (240 ± 20) MeV can be obtained from the fits provided c ∼ -1, as predicted by the majority of mean-field models. However, the fits are significantly improved if c is allowed to vary, leading to a range of K0, extended to higher values. The results demonstrate the importance of nuclear surface properties in determination of K0 from fits to the leptodermous expansion of KA . A self-consistent simple (toy) model has been developed, which shows that the density dependence of the surface diffuseness of a vibrating nucleus plays a major role in determination of the ratio Ksurf/Kvol and yields predictions consistent with our findings.

On the absence of appreciable half-life changes in alpha emitters cooled in metals to 1 Kelvin and below

The recent suggestion that dramatic changes may occur in the lifetime of alpha and beta decay when the activity, in a pure metal host, is cooled to a few Kelvin, is examined in the light of published low temperature nuclear orientation (LTNO) experiments, with emphasis here on alpha decay. In LTNO observations are made of the anisotropy of radioactive emissions with respect to an axis of orientation. Correction of data for decay of metallic samples held at temperatures at and below 1 Kelvin for periods of days and longer has been a routine element of LTNO experiments for many years. No evidence for any change of half life on cooling, with an upper level of less than 1%, has been found, in striking contrast to the predicted changes, for alpha decay, of several orders of magnitude. The proposal that such dramatic changes might alleviate problems of disposal of long-lived radioactive waste is shown to be unrealistic.

Confirmation of parity violation in the gamma decay of Hf-180(m)

This paper reports measurements using the technique of On Line Nuclear Orientation (OLNO) which reexamine the gamma decay of isomeric 180Hfm and specifically the 501 keV 8 – 6+ transition. The irregular admixture of E2 to M2/E3 multipolarity in this transition, deduced from the forward-backward asymmetry of its angular distribution, has for decades stood as the prime evidence for parity mixing in nuclear states. The experiment, based on ion implantation of the newly developed mass-separated 180Hfm beam at ISOLDE, CERN into an iron foil maintained at millikelvin temperatures, produces higher degrees of polarization than were achieved in previous studies of this system. The value found for the E2/M2 mixing ratio, ǫ = -0.0324(16)(17), is in close agreement with the previous published average value ǫ = 0.030(2), in full confirmation of the presence of the irregular E2 admixture in the 501 keV transition. The temperature dependence of the forward-backward asymmetry has been measured over a more extended range of nuclear polarization than previously possible, giving further evidence for parity mixing of the 8 and 8+ levels and the deduced E2/M2 mixing ratio. PACS numbers: 21.10.HW,23.20.En,23.40.Bw,27.70.+q,29.30.Lw,150≤A≤189

Studies of weak electric quadrupole interaction in57,60CoFe using modulated adiabatic passage on oriented nuclei

The electric quadrupole interactions at57Co and60Co in co-diffused57,60CoFe single crystal have been measured using Modulated Adiabatic Passage on Oriented Nuclei (MAPON). The quadrupole splittings ΔνQ<100> are + 18(2) kHz for57Co and +9(2) kHz for60Co, corresponding to a principal electric field gradient (efg) tensorVzz=2.0(5)×1019 Vm−2 and 2.5(6)×1019 Vm−2, respectively, in broad agreement with previous MAPON results for CoFe. The distributions of the efg’s are very similar for the two isotopes, verifying that previously reported differences in58CoFe and60CoFe could be attributed to different host preparations. The measurement of such weak efg’s which are not spectroscopically resolved, allows determination of new nuclear electric quadrupole moments, not accessible by other techniques. Applications to other systems are discussed.