Portuguese groups at ISOLDE - CERN Ulrich Wahl Instituto Tecnológico e Nuclear (ITN) Sacavém Centro de Física Nuclear da Universidade de Lisboa (CFNUL) ISOLDE and Nuclear Solid State Physics Portuguese experiments at ISOLDE: history, people, collaborations Examples: - position-sensitive electron detector development - emission channeling lattice location of Fe in Si - PAC investigation of O and F in High-Tc Hg-1201 Conclusions amd outlook

The ISOLDE on-line isotope separator Available beams: ~ 600 isotopes of ~ 70 elements mass separation target and ion source 60 keV acceleration 1 GeV protons radioactive ion beams

ISOLDE Experiments 2003 37 Experiments 300 Users 96 Institutes Experiments involving Portuguese groups: typically around 10-12 shifts per year i.e. ~3% of ISOLDE availability ISOLDE Experiments 2003 Particle and Astro-Physics 13% Solid State Physics 22% Biology/Medicine 5% Atomic Physics 15% Weak Interaction and Nuclear Physics 46% 37 Experiments 300 Users 96 Institutes 22 Countries 375 8h-shifts of radioactive beams

Experimental techniques used with radioactive isotopes at ISOLDE Portuguese groups: emission channeling (EC) perturbed angular correlation (PAC)

History of e--g PAC and EC at ISOLDE-CERN 1986 e-g PAC at ISOLDE IS01/13 spokesperson J.C.Soares 1994 Test of new PAC probes IS-P66 spokesperson J.G. Correia e--g PAC 197mHg, 73Ge, 127Cs, 80Br 1997 IS360 Studies of high-Tc superconductors with radioactive isotopes ~2006 spokesperson J.G. Correia (ITN & CFNUL) 1998 IS368 Lattice location of transition metals and rare earths in semiconductors ~2006 spokesperson U. Wahl (ITN & CFNUL, then IKS Leuven) 2000 IS390 Studies of colossal magnetoresistive oxides with radioactive isotopes ~2005 spokesperson V. Amaral (U Aveiro) 2001 New improved electron emission channeling pad detectors and development of self-triggering front-end electronics Cooperation agreement between ITN, CFNUL and CERN EP-ATT Group (now: PH-TA1)

Network of groups involved KU Leuven (Belgium) Prof. André Vantomme 2 PhD students: Bart De Vries Stefan Decoster LEUVEN CFNUL Lisbon Prof. José C. Soares Prof. Manuel. R. Silva 2 PhD students: Elisabete Rita Ana C. Marques 1 Diploma student: Sonia Dias Dr. J. Guilherme Correia Dr. Ulrich Wahl Dr. Heinz Haas (retired) ITN Sacavém University of Porto Prof. J. Pedro Araújo 1 Masters student: Tania Mendonça Univ. of Kwazulu Natal (South Africa) Prof. Krish Bharuth-Ram University of Aveiro Prof. Vitor Amaral Dr. Mario S. Reis 1 PhD student: Armandina Lopes 1 Diploma student: João C. Amaral University of Göttingen (Germany) Prof. Hans Hofsäss (future collaboration) CERN Dr. Peter Weilhammer (detector development)

Funding: mainly from national sources KU Leuven (Belgium) CFNUL Lisbon ITN Sacavém Fund for Scientific Research - Flanders (dedicated project on emission channeling) Portuguese Foundation for Science & Technology (dedicated CERN project on PAC & emission channeling) Univ. of Kwazulu Natal (South Africa) University of Porto University of Aveiro University of Göttingen (Germany) CERN Portuguese Foundation for Science & Technology (dedicated CERN project on magnetic oxides) Federal Ministry of Education and Research (dedicated project on emission channeling)

¬ Position-sensitive Si pad detectors for Emission Channeling Collaboration: ITN, CFNUL & CERN PH-TA1 group Future detectors: pads self-triggering Þ 5 keV trigger Þ 10 kHz readout ¬ 73As in ZnO low electron energy [0001] [-1102] Cooled 2oC In a collaboration between the emission channeling group from ITN and CERN’s EP-ATT division, a new position-sensitive Si pad detector was put operational. The detector can be cooled to –30°C and exhibits better noise behavior than previous position-sensitive detectors for emission channeling. This allows to decrease the energy for electron EC measurements to below 40 keV. One of the first experiments to be carried out with the new detector was the lattice location of As in the II-VI semiconductor ZnO by means of the 43 keV conversion electrons of the isotope 73As. The experimental patterns showed that implanted As occupies multiple lattice sites in ZnO. Perspectives for the future are to implement self triggering pad detectors which allow to detect electrons with energies as low as 5 keV at count rates up to 10 kHz. Arsenic occupies multiple sites in ZnO

Lattice site changes of implanted 59Fe in Si as-implanted: mainly displaced substitutional Fe annealed at T=300°C: mainly tetrahedral interstitial Fe annealed at T=800°C: mainly ideal substitutional Fe Fe is gettered by at least two types of vacancy-like defects Following release to interstitial state re-gettering occurs at a different gettering center on ideal substitutional sites U. Wahl et al., accepted by Phys. Rev. B

O PAC F Oxygen & Fluorine Configurations in Hg1201 (High-Tc) PAC Radioactive ions Experiment h ~ 0.20 2500 5000 w(Mrad/s) 50 100 EFG Simulations O PAC PAC h ~ 0.0 50 100 2500 5000 w(Mrad/s) The Perturbed Angular Correlation (PAC) technique uses the fact that the angular correlation of two successively emitted gamma rays shows a time dependent perturbation in the presence of external nuclear fields. This perturbation is a function of the interaction between the quadrupole moment of the nucleus and the external Electric Field Gradient (EFG). The EFG is generated by the charge distribution in the probe neighborhood and acts as the finger print of the responsible defect. The experimental data is compared with first principle simulations of the EFG generated by different defect configurations. 199mHg is the probing atom/isotope. Hg1201 is the first member of the family of high-Tc superconductors with highest critical temperature (~125K). The “simulation” pictures show projections of the lattice along the Hg planes. The top spectrum assigns the position of dopant oxygen atoms in the Hg planes (PRB 61 (2000) 11769). The bottom spectrum assigns the Fluorine configuration, where F atoms have entered the Hg planes, but, unlike O, ordering along [100] or [010] lines [(b)-configuration]. Configuration (a) is excluded due to the big EFG asymmetry parameter h=0.34 (to be published). These studies aim to correlate the injection of charge carriers into the Cu superconducting planes with the position of oxygen and fluorine in the lattice. PAC F Electric Field Gradient  dopant configuration fingerprint HgBa2CuO4+d Tc > 92K J.G. Correia et al., submited to Phys. Rev. B

Conclusions and outlook: Currently and near future (till ~2006): IS368: Lattice location in semiconductors, e.g. Si, Ge, SiGe, diamond, III-V, nitrides, II-VI, ZnO… electrical doping, transition metals, rare earths, diluted magnetic semiconductors IS360: PAC in High-Tc superconductors IS390: Magnetism in manganites (CMR) After 2006: New emission channeling proposal(s) applying… new position-sensitive detectors (self-triggering pad, CCD) - on-line experiments with short-lived isotopes - low energy electron emitters (Auger electrons?) Þ a new variety of elements (e.g. Mg, Co, Ni, Zn, Ge) will be available for emission channeling