J.G. Marques

Final Analysis and Results of the Phase II SIMPLE Dark Matter Search

We report the final results of the Phase II SIMPLE measurements, comprising two run stages of 15 superheated droplet detectors each, with the second stage including an improved neutron shielding. The analyses include a refined signal analysis, and revised nucleation efficiency based on a reanalysis of previously reported monochromatic neutron irradiations. The combined results yield a contour minimum of σp=5.7×10(-3)  pb at 35  GeV/c2 in the spin-dependent sector of weakly interacting massive particle (WIMP) proton interactions, the most restrictive to date for MW}≤60  GeV/c2 from a direct search experiment and overlapping, for the first time, with results previously obtained only indirectly. In the spin-independent sector, a minimum of 4.7×10(-6)  pb at 35  GeV/c2 is achieved, with the exclusion contour challenging a significant part of the light mass WIMP region of current interest.

First Results of the Phase II SIMPLE Dark Matter Search

We report results of a 14.1   kg d measurement with 15 superheated droplet detectors of total active mass 0.208 kg, comprising the first stage of a 30 kg d Phase II experiment. In combination with the results of the neutron-spin sensitive XENON10 experiment, these results yield a limit of |a(p)|<0.32 for M(W)=50 GeV/c² on the spin-dependent sector of weakly interacting massive particle-nucleus interactions with a 50% reduction in the previously allowed region of the phase space, formerly defined by XENON, KIMS, and PICASSO. In the spin-independent sector, a limit of 2.3×10⁻⁵ pb at M(W)=45 GeV/c² is obtained.

Electron emission channeling with position-sensitive detectors

Abstract Electron emission channeling allows direct lattice location studies of low doses of radioactive atoms implanted in single crystals. For that purpose the anisotropic emission yield of conversion electrons from the crystal surface is measured, most conveniently by use of position-sensitive detectors. We discuss characteristic features of this method, including quantitative data analysis procedures, which are achieved by fitting simulated two-dimensional emission distributions for different lattice sites to the experimental patterns. The capabilities of this approach are illustrated by the case of rare earth atoms (Er, Tm, Yb) in Si, where we were able to do lattice location experiments down to implanted doses which are 150 times lower compared to previous RBS studies.

Radiation damage formation and annealing in GaN and ZnO

The radiation damage formation upon low temperature ion implantation and neutron irradiation has been compared for GaN and ZnO. Both materials exhibit strong dynamic annealing effects during implantation, even at 15 K, leading to high amorphisation thresholds. The damage build-up with fluence was found to proceed in a similar way for GaN and ZnO, both showing two saturation regimes below the amorphisation level where, over wide fluence regions, the damage level increases only very slowly. For low fluences the damage accumulation rate is similar for both materials. For higher fluences, on the other hand, GaN shows considerably higher damage levels and finally collapses into an amorphous structure while ZnO remains single crystalline up to the highest fluence of 7×1016 Ar/cm2. Neutron irradiation produces similar defects as ion implantation but within the entire sample while the defect density is much lower. The main effect of irradiation on the structural properties of GaN is an expansion of the c-lattice parameter. Optical properties are significantly deteriorated after irradiation and only recover partially after annealing. ZnO does not suffer such a pronounced change of the lattice parameters but reveals a strong deterioration of the surface, possibly due to blistering and exfoliation. At the same time the optical properties are less affected than for GaN. The near band edge emission is partly quenched but recovers to a large extend after annealing while broad defect bands are observed below the bandgap for irradiated samples, before and after annealing.

Annealing behavior and lattice site location of Hf implanted GaN

Abstract The defect recovery and lattice site location of Hf implanted into GaN single crystalline epilayers were studied combining RBS/channelling and hyperfine interactions measurements. The RBS/channelling measurements performed after implantation of 5×1014 Hf+ cm−2 at 100 keV show that nearly all the implanted ions were incorporated into substitutional sites of the GaN lattice. The damage produced by the implantation recovers almost completely after one hour annealing at 900°C and all the Hf ions then occupy substitutional sites. The hyperfine interaction measurements were performed with the radioactive 181Hf/181Ta probe, after implantation of 181Hf to a fluence of 5×1012 Hf+ cm−2 with 80 keV. These measurements show that the defect recovery occurs in the 600–800°C annealing temperature range.

A four-detector spectrometer for e−−γ PAC on-line with the ISOLDE-CERN isotope separator

Abstract A four-detector e−−γ spectrometer has been installed on-line with the ISOLDE isotope separator. The spectrometer consists of two magnetic lenses for detection of conversion electrons, and two BaF2 scintillators for γ-ray detection. The spectrometer has been equipped with a 20 kV pre-acceleration system which enables detection of conversion electrons down to 2 keV. Implantation and measurement can be performed simultaneously on a large temperature range by heating or cooling the sample holder. The advantages of using the e−−γ PAC technique on-line at ISOLDE are discussed.

Magnetization and magneto resistance in Fe-ion-implanted Cu and Ag thin films

Abstract Diluted granular films of CuFe and AgFe (iron content ⩽ 2%) were produced using 57 Fe ion implantation on Cu(Ag) films previously grown by laser ablation. Conversion electron Mossbauer spectroscopy shows that the implanted Fe forms either very small clusters (up to a few atoms) or large iron α-phase particles. These structural characteristics directly reflect on the magnetization, which exhibits ferromagnetic-like behaviour at room temperature (due to large clusters) superimposed by a significant paramagnetic contribution at low temperatures due to the small clusters. We observe deviations from strict superparamagnetic behaviour due to non-negligible local anisotropy effects at low temperatures and low fields. The Kondo effect is particularly enhanced in the CuFe films which have higher concentration of isolated Fe atoms and small sizes clusters. The magnetoresistivity Δ ϱ / ϱ of our films is dominated (for 0 ⩽ μ 0 H ⩽ 15 T) by a linear term in H , attributed to GMR-like effect from spin-dependent scattering when an electron passes between adjacent large and small clusters. At low fields we observe instead Δ ϱ / ϱ ∝ H 2 , due to the usual GMR effect between large clusters, during the alignment of their easy axes. The relevant physical differences (structural, magnetic and magnetoresistive) observed in our ion-implanted diluted Fe films, with respect to the concentrated granular films, are critically analysed.

Incorporation of the transition metal Hf into GaN

The perturbed angular correlation (PAC) technique was applied to study the incorporation of the transition metal Hf into GaN after implantation. To this end the PAC probe 181Hf(181Ta) was implanted into epitaxial Wurtzite GaN layers (1.3 μm on sapphire) with an energy of 160 keV and doses of 7× 1012 at/cm2. PAC spectra were recorded during an isochronal annealing programme, using rapid thermal annealing (RTA) and furnace annealing, in the 300–1000ºC temperature range. After implantation the spectra show a damped oscillation corresponding to a quadrupole interaction frequency (QIF) of νQ= 340 MHz for 30% of the probe nuclei. Annealing up to 600ºC reduces the damping of this frequency without an increase of the probe atom fraction fs in these sites. Above 600ºC fs grows rapidly until after the 900ºC RTA step more than 80% of the Hf probes experience a well defined QIF due to the incorporation of Hf on undisturbed sites of the hexagonal GaN wurtzite lattice. An interaction frequency of νQ= 340 MHz is derived. RTA and furnace annealing yield similar results for annealing up to 800ºC, where the undisturbed fraction reaches about 60%. Then RTA at higher temperatures increases this fraction, while furnace annealing leads to a decrease down to 22% after annealing at 1000ºC. To our knowledge this is the first time that a transition metal probe like Hf is incorporated to such a large extent into a semiconductor lattice.

MICROSCOPIC STUDIES OF IMPLANTED 73AS IN DIAMOND

Abstract In this work we present results on the lattice location and the microscopic surroundings of As implanted into diamond. A mixture of the isobars 73 Se and 73 As was implanted to a dose of 1.0 × 10 14 at/cm 2 with 60-keV energy. Complementary γ-e − Perturbed Angular Correlations (PAC), Emission Channeling (EC) and RBS/channeling (RBS/C) measurements were performed for the same sample, after full decay of 73 Se to 73 As. After annealing at 1400 K the EC and RBS/C spectra show that more than 50% of the As nuclei reside in substitutional positions, although some residual damage is still seen within the implanted range. On the other hand, the PAC data show that significant annealing of the lattice damage occurred only in the vicinity of 30% of the implanted As atoms, and that these have remaining defects in their neighborhood.

73Ge: a new high resolution PAC probe

Long lens electron spectrometers were used to make electron-gamma and electron-electron PAC measurements for the 5/2+ 13.3 keV state of73Ge. Sources of73As were produced in the decay of73Se implanted at the CERN/ISOLDE facility. The magnetic interaction in nickel was determined as ωL=74.2(7) Mrad s−1 and the quadrupolar frequency in antimony asvQ=19.7(2) MHz. The nuclear moments derived are μ=−1.08(3) μN and |Q|=0.70(8) b.