Cubic defect clusters in fcc metals

The number of cubic defects in In-implanted copper, silver and platinum has been determined by decorating the cubic defects with He atoms. For this, the samples were post-implanted with He⁺ ions at subthreshold energies. Our results, obtained by DPAC measurements on¹¹¹In, are compared with data from Mössbauer spectroscopy and channeling experiments.     

Data reduction methodology for perturbed angular correlation experiments

Data reduction schemes currently used for time differential perturbed angular correlation measurements are evaluated in terms of (i) their relative effectiveness in eliminating irrelevant variables, including single counter efficiencies and spectrum time shifts; and (ii) their effectiveness in putting data in a form that can easily be fitted by theoretical correlation functions. It is pointed out that erroneous conclusions may be derived from improperly reduced data, but that properly analyzed experiments performed at four angles allow a good determination of both the time-dependent and time-independent parts of the correlation function. Correction of data for accidentals, source self-absorption and spectrum livetime differences are discussed.     

Magnetic hyperfine field of Ta in iron measured by means of the dpac method

The magnetic hyperfine field of Ta in iron was measured by means of DPAC using the 5/2⁺ state in¹⁸¹Ta. At room temperature, this field was found to be |H _hf(RT)|=6.08(7) T, which confirms the result of an earlier DPAC experiment. A hyperfine anomaly^5/2Δ^7/2=−4.6(23)% between the above state and the 7/2⁺ ground state of¹⁸¹Ta is deduced.     

Non-perturbative renormalisation and improvement of the local vector current for quenched and unquenched Wilson fermions

By considering the local vector current between nucleon states and imposing charge conservation, we determine its renormalisation constant and quark mass improvement coefficient for Symanzik O(a) improved Wilson fermions. The computation is first performed for quenched fermions (and for completeness also with unimproved fermions) and compared against known results. The two-flavour unquenched case is then considered.     

Formation of small vacancy clusters in tungsten around silver and indium impurities studied by PAC and THDS

Vacancy clustering at Ag and In impurities in W was investigated combining Thermal Helium Desorption Spectrometry (THDS) and Perturbed Angular Correlation (PAC) measurements. The results of these experiments are compared with each other and with those obtained from Monte Carlo calculations.     

Trap mutation in He-doped ion-implanted tungsten

In this work, we report on an investigation of the defect complexes InVe₂He _n in tungsten by means of PAC. The release steps InVe₂He _n InV₂He _n-1 + He were identified forn=1t–5. The dissociation energies, in the range of 2.9–4.5 eV, agree perfectly with the values that have been previously determined by means of THDS. The quadrupole frequency is about 122 Mrad/s forn=1,2 and about 101 Mrad/s forn=3,4. These values are smaller than the frequency of the undecorated vacancy (n=0). The vacancy mutates into a divacancy if it is filled with about 10 He atoms, giving rise to a quadrupole frequency of about 218 Mrad/s. This so-called trap mutation ultimately leads to bubble information. We observed at least three different bubble-associated quadrupole interactions.     

Soft-landing deposition of radioactive probe atoms on surfaces

We present a method to deposit a wide range of radioactive probe atoms on surfaces, without introducing lattice damage or contaminating the surface with other elements or isotopes. In this method, the probe atoms are mass-separated using an isotope separator, decelerated to 5 eV, and directly deposited on the surface. The method allows for performing hyperfine interactions experiments using trace amounts of radioactive probes located at surfaces and interfaces. The characteristics of the deposition method will be described, with particular attention to the deceleration stage of the isotope separator. The method was tested with perturbed angular correlation (PAC) experiments on the system In on Cu(17,1,1). The results are in agreement with molecular dynamics simulations of the deposition process. New developments, in particular the study of the self-diffusion of Ag atoms on Ag(100) by means of PAC with the ¹¹¹Ag probe, are briefly discussed. This revised version was published online in August 2006 with corrections to the Cover Date.