Perturbed gamma-ray angular correlation (PAC) is a materials characterization technique that senses the electromagnetic environment of a radioactive probe nucleus, using the interaction of the nucleus's electromagnetic moments with the electric and magnetic fields at the nuclear site. This interaction is observed through the changes it causes in the angular distribution of cascade gamma-rays emitted in the radioactive decay of the tracer. PAC observes such characteristics as crystalline electric fields, internal magnetic fields, vacancy and impurity diffusion, and fluctuations. We have applied PAC to metals, alloys, biological molecules, and ceramics, and are presently investigating the static and dynamic characteristics of crystallographic and magnetic phase transitions in perovskite oxides and alloys.
At UMBC we have a computer-controlled PAC spectrometer using high-speed
BaF2 scintillators. The sample temperature is controlled
using a closed-cycle helium refrigerator capable of maintaining the sample
at any temperature between 10 K and room temperature, a liquid helium cryostat
for temperatures below 10 K, and a selection of resistance furnaces for
temperatures above room temperature. Facilities for sample preparation include
a fully equipped radioisotopes laboratory, an argon arc furnace, and a Lindberg
tube furnace with capability of heating samples in vacuum or in gaseous
atmospheres at electronically controlled temperatures up to 1500 C.
A. T. Motta, G. L. Catchen, S. E. Cumblidge, R. L. Rasera, A. Paesano, Jr., and L. Amaral, "Defects and magnetic hyperfine fields in ZrFe2 investigated using perturbed-angular-correlation spectroscopy," Physical Review B 60, 1188 (1999).
R. L. Rasera and G. L. Catchen, "Mn-site hyperfine fields in LaMnO3 and NdMnO3 using perturbed-angular-correlation spectroscopy," Physical Review B 58, 3218 (1998).
R. L. Rasera and G. L. Catchen, "Perturbed Angular Correlation (PAC) Spectroscopy as a Tool for the Study of Ferroelectrics," Ferroelectrics 150, 151 (1993).
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