Installation of the IMPAC technique in the Pelletron laboratory

Abstract

A special target chamber for the direct production & recoil implantation of radioactive probes for perturbed angular correlation has been constructed and tested. Feasibility studies were made by reproducing the known quadrupole frequencies at 77 Br and 111 In probes in metallic zinc. Preliminary results were obtained on the lattice site dependence of Cd hyperfine field in Pd 2 MnSn Heusler alloy with the PAC technique.The Perturbed Angular Correlation (PAC) technique based on the hyperfine (HF) interactions of radioactive probe nuclei is a very useful tool in the study of magnetic hyperfine fields (MHF) and electric field gradients (EFG) at crystalline sites. In these experiments one observes the interaction of an internal magnetic field with the nuclear magnetic dipole moment or that of the electric field gradient with the nuclear quadrupole moment of an intermediate state of a gamma-gamma cascade in the probe nucleus. The interaction leads to a change in the gammagamma angular correlation between the two emitted gamma rays, which can be observed by recording time differential coincidence spectra gg(t), with scintillation (BaF2) detectors arranged in a plane around the sample. The coincidence spectra show the exponential decay of the intermediate state modulated by a perturbation function G 22 (t), which in the case of a magnetic interaction is given by, where ω L = gµ N B hf / is the Larmor frequency, and g is the nuclear g factor and B hf is the magnetic hyperfine field.In the case of an electric quadrupole interaction, the corresponding perturbation factor is given bywhere the primary frequencies ω n and their amplitudes S 2n are related to the hyperfine splitting of the intermediate nuclear level and depend on the nuclear quadrupole frequency ω Q = eQV zz /4I(2I − 1) and the asymmetry parameter η = (V xx − V yy )/V zz where V xx , V yy and V zz are the elements of the EFG tensor in its principal axis system. As usual, V zz is the largest component of the EFG tensor and generally one uses the spin independent quadrupole frequency defined by ν Q = eQV zz /h, where Q is the nuclear electric quadrupole moment of the intermediate level.The known quadrupole moment of 0.83 b for the I = 5/2 + intermediate level of 111 Cd has been used to determine V zz . The effects of finite time resolution τ R of detectors and the distribution of EFG with a width δ are properly taken into account in the above equantion.From the perturbation pattern, the magnitude and the sign of the MHF, the magnitude, symmetry and the orientation of the EFG tensor or both may be determined. Very often, the hyperfine interactions corresponding to different crystalline sites are observed. In these cases, the perturbation factor G 22 (t) = i f i G i 22 (t) takes into account the fractional site populations f i and G i 22 (t) are the corresponding perturbation factors.

Study of the effect of disorder on the local magnetism in Heusler alloys

“…Since Pd is a constituent element of the alloys in the present experiment, the samples themselves served as the reaction targets during irradiation, and practically all the 111 In nuclei produced in the reaction stopped in the sample. 4 Measurements were carried out using a perturbed angular correlation ͑PAC͒ spectrometer consisting of four conical BaF 2 detectors with a conventional fast-slow coincidence electronic setup. The gamma cascade of 172-245 keV, populated from the electron capture decay of 111 In and having an intermediate level at 245 keV with spin I =5/2 + and T 1/2 = 85 ns, in 111 Cd was used to measure the hyperfine interaction.…”

Implantation of 111In-probe Nuclei with Nuclear Reactions 108Pd(6, 7Li, xn)111In using Pelletron Tandem Accelerator: Study of Local Magnetism in Heusler Alloys

“…Since Pd is a constituent element of the alloys in the present experiment, the samples themselves served as the reaction targets during irradiation and practically all the 111 In nuclei produced in the reaction stopped in the sample. The Heusler alloy samples were cut in the form of a 5 mm diameter disc with a thickness of õ1 mm and mounted in a special reaction chamber [4]. The irradiation times varied from 8Y10 h with an average beam current of the order of 400Y500 nA on the target.…”

Implantation of 111In in the Heusler alloys Pd2MnZ (Z=Sn,Sb,Ge,In) following heavy ion nuclear reactions: measurement of magnetic hyperfine field using PAC spectroscopy

“…Since the Heusler alloys chosen for the experiment all contain Pd as one of the component elements, they themselves served as reaction targets. The samples were cut in to small slices of about 5x5 mm 2 and 1mm thick and mounted in an especial reaction chamber [5] for irradiation with the 6 Li beam. The average recoil energy of 111 In ions being too small (∼1.7 MeV) they all stop in the relatively thick (∼1 mm) sample and get directly implanted in the Heusler alloy.…”