Competition of isoscalar and isovector proton-neutron pairing in nuclei
Abstract We use shell model techniques in the complete pf shell to study pair correlations in nuclei. Particular attention is paid to the competition of isoscalar and isovector proton-neutron pairing modes which is investigated in the odd-odd N = Z nucleus 46 V and in the chain of even Fe-isotopes. We confirm the dominance of isovector pairing in the ground states. An inspection of the level density and pair correlation strength in 46 V, however, shows the increasing relative importance of isoscalar correlations with increasing excitation energy. In the Fe-isotopes we find the expected strong dependence of the proton-neutron isovector pairing strength on the neutron excess, while the dominant J = 1 isoscalar pair correlations scale much more gently with neutron number. We demonstrate that the isoscalar pair correlations depend strongly on the spin-orbit splitting.
Observation of the proton emitter 11657 La 59
, , The quantum tunneling and emission of a single constituent nucleon provide a beautifully simple and unique window into the complex properties of atomic nuclei at the extreme edge of nuclear existence. In particular, for odd-odd proton emitting nuclides, the associated decay energy and partial half-life can be used to probe the correlations between the valence neutrons and protons which have been theoretically predicted to favour a new type of nuclear super ﬂ uidity, isoscalar neutron-proton pairing, for which the experimental “ smoking gun" remains elusive. In the present work, proton emission from the lanthanum isotope 11657 La 59 , 23 neutrons away from the only stable isotope 13957 La 82 , is reported. 116 La nuclei were syn-thesised in the fusion-evaporation reaction 58 Ni( 64 Zn, p 5 n ) 116 La and identi ﬁ ed via their proton radioactivity using the mass spectrometer MARA (Mass Analysing Recoil Apparatus) and the silicon detectors placed at its focal plane. Comparisons of the measured proton energy ( E p = 718 ± 9 keV) and half-life ( T 1/2 = 50 ± 22 ms) with values calculated using the Universal Decay Law approach indicate that the proton is emitted with an orbital angular momentum l = 2 and that its emission probability is enhanced relative to its closest, less exotic, odd-even lanthanum isotope ( 11757 La 60 ) while the proton-emission Q -value is lower. We propose this to be a possible signature for the presence of strong neutron-proton pair correlations in this exotic, neutron de ﬁ cient system. The observations of γ decays from isomeric states in 116 La and 117 La are also reported. 48mm, segmented into 72 vertical strips in the y plane and 192 horizontal strips in the x plane, giving a total of 13824 quasipixels. A second layer of with a thickness of 500 μ m was located directly behind the DSSD to veto punch-through events due to uncorrelated high-energy light charged particles and to detect β particles escaping after leaving a partial energy signal in the DSSD. Five clover HPGe detectors were placed surrounding the focal plane of MARA in a close-packed geometry and used for the measurement of delayed γ rays following recoil implantations and/or charged particle decays detected in the DSSD. All detector signals were time stamped using a global 100 MHz clock and recorded independently by the triggerless data acquisition system 63 . The data were analyzed online and of ﬂ ine using the GRAIN software package 64 . The energy calibration of the DSSD was accomplished using a standard mixed-isotope ( 241 Am, 244 Cm, 239 Pu) α radioactive source with its three main α -energy peaks, as well as in-beam data for the known proton decay of 109 I 50 , its α -decaying daughter 108 Te, and the α -emitters 109 Te and 110 I 65 . The 109,110 I and 108,109 Te ions were produced in reactions using a higher beam energy of 370MeV in a brief run after the main experiment, see the corresponding characteristic proton and alpha decay peaks in Fig. 4. relative comparison as follows
Monte Carlo simulations of magnetovolume instabilities in anti-Invar systems
We perform constant pressure Monte Carlo simulations of a spin-analogous model which describes coupled spatial and magnetic degrees of freedom on an fcc lattice. Our calculations qualitatively reproduce magnetovolume effects observed in some rare earth manganese compounds, especially in the anti-Invar material YMn2. These are a sudden collapse of the magnetic moment which is connected with a huge volume change, and a largely enhanced thermal expansion coefficient.