A. Van Geert

Performance of HPGe detectors in the temperature region 2–77 K

Abstract Planar HPGe detectors designed for detection of charged particles in low-temperature nuclear orientation experiments were tested in the temperature range between 2 and 77 K. Various detector characteristics were studied. The detectors gave good and stable results for registration of both charged particles and γ-rays at temperatures close to liquid helium temperature. Performance of the detectors in strong magnetic field was also tested.

Helicity structure in low- and intermediate-energy weak interaction

Abstract We report on the status of the measurements of the longitudinal polarization of positrons emitted by polarized 107In and 12N nuclei. Present results yield a lower limit of 306 GeV/c2 (90 % CL) for the mass of a possible W gauge boson with predominantly right-handed couplings, if interpreted in the framework of the manifest left-right symmetric model. In addition, the helicity structure of the weak interaction is discussed on the basis of experimental results at low and intermediate energies in the leptonic, semileptonic and hadronic sectors. Whereas in the minimal manifest left-right symmetric extension of the Standard Model the results from collider experiments are superior to results from other experiments, experiments in the three sectors of the weak interaction yield complementary information on the helicity structure of the interaction if interpreted in general left-right symmetric models.

The behaviour of HPGe detectors operating at temperatures below 77 K

Abstract Planar detectors from high-purity germanium single crystals designed for the detection of charged particles in low-temperature nuclear orientation experiments were tested in the temperature range 4.5–77 K. Various detector characteristics were measured. The distortions in spectra observed at the lowest temperatures are discussed. Good performance of the detectors in real experimental conditions was obtained.

Perpendicular induced magnetic moments of Ag in Ag/Fe multilayers

Abstract We have studied the induced magnetic moments of Ag in Ag/Fe multilayers with low temperature nuclear orientation. The experiments show that there are induced magnetic moments in the Ag spacer and these induced magnetic moments do not lie in the plane of the multilayer for 2 and 4 ML of Ag at low external field.

Evidence for perpendicular magnetic anisotropy of Tb in Tb/Fe multilayers

The magnetic moment direction of Tb in an multilayer has been studied by the low-temperature nuclear orientation technique in external magnetic fields applied along and perpendicular to the sample plane. A perpendicular magnetic anisotropy of the Tb magnetic moments has been found at zero external magnetic field and the mean misalignment angle has been estimated to be about with respect to the sample plane normal. The external magnetic field applied along the sample plane turned the Tb magnetic moments to the plane, without reaching any saturation even at . A possible structure of misalignment within the Tb layer is discussed.

Calorimetric single particle detection in high magnetic fields at low temperatures

High resolution calorimetric detection of single particles in magnetic fields up to 8 T at temperatures <0.1 K is reported. A careful study of heavily doped Si and Ge thermistors reveals an extremely large positive magnetoresistance in the variable range hopping conductivity regime. This phenomenon is used to optimize the impedance and the temperature sensitivity of a calorimetric detector. Initial experiments show an energy resolution full width at half maximum of 800 eV for 62.5 keV conversion electrons and an energy threshold of <400 eV.

A study of the induced magnetic hyperfine fields in Fe/Ag multilayers

Near perpendicular magnetic hyperfine fields in 110mAg have been observed in Fe/Ag multilayers. These fields are studied by low temperature nuclear orientation (LTNO) in multilayers [Ag(x ML)/Fe(y ML)]20, with (x,y) monolayer (ML) values of (2,10), (3,9), (5,10) and (3,18). The 110mAg γ-ray anisotropy was measured as a function of applied magnetic field parallel to the multilayer. The average induced hyperfine field of Ag is significantly influenced by the quality of the multilayer as measured by X-ray diffraction.

Study of the non-linear I-V characteristics of thermistors in the variable range hopping conduction at low temperatures and in high magnetic fields

Abstract We present preliminary results concerning the non-linear conduction in the variable range hopping regime. The dependence of this non-linearity on the temperature and on the applied magnetic field is investigated.

Ag hyperfine fields at the interface in Fe/Ag multilayers

Abstract Magnetic hyperfine fields in the Ag of Fe/Ag multilayers have been observed with low-temperature nuclear orientation for multilayers [Fe(xML)/Ag(yML)] with (x,y) values of (9,3), (18,3) and (10,5). The γ-ray anisotropy was measured as a function of the external applied magnetic field. The hyperfine fields from the measurements are then compared with those fields derived by the model developed by M.B. Stearns.

Tb magnetic moment behaviour in a Gd single crystal: nuclear orientation study

Low-temperature nuclear orientation has been used to study the behaviour of the Tb magnetic moment in a Gd + 1 at.% Tb single crystal. The -ray anisotropy of several lines was monitored in the temperature range of 5-50 mK and in the external magnetic field range of 0-8.5 T. The field was applied either along the a- or along the c-axis of the Gd single crystal. In the case of parallel to the a-axis the Tb magnetic moment was found to be almost aligned along even at very low fields, as expected because the hexagonal basal plane is an easy magnetization plane for the Tb ion in terbium metal. On the other hand, when is along the c-axis the Tb magnetic moment rotates very slowly towards the c-axis. Even at the Tb magnetic moment is not along the direction of . This is in contrast to the fast saturation of the gadolinium ions in pure Gd. Apparently, the Tb-Gd exchange interaction is in competition with the crystal field acting on the Tb ion in a hexagonal lattice, our data indicates that at along the c-axis the angle between the Tb magnetic moment and the applied field is of the order of .