I. Kovács

EXTERNAL-BEAM PIXE ANALYSIS OF SMALL SCULPTURES

Abstract Non-destructive analysis of precious art objects is an important tool to solve provenance problems or to facilitate restoration. External beam PIXE analysis is one of the most powerful and popular methods used in this respect. The paper summarises the external beam PIXE set-up at the Accelerator Laboratory in Budapest, and two selected applications are described. Different parts of a small Cambodian metal sculpture probably made in the 11th century were analysed. It was observed that the sculpture was composed of an iron core and an outer bronze shell. This sculptural technique was well known in the Middle-East but no indication has been found about its use in the Far-East, yet. Before its restoration paint traces on a wooden relief of St. Jerome made by an unknown artist in about 1600 were analysed. Different white and red paints were distinguished at selected points and the presence of titanium on a certain part of the relief was attributed to a repainting in modern times.

Pressure effects on the dark-adaptation of bacteriorhodopsin

The influence of hydrostatic pressure (340 MPa) on the dark-adaptation kinetics and the relaxation of dark-adapted bacterio-rhodopsin following a pressure jump (0.1 MPa --> 340 MPa) have been studied. We have also measured the temperature dependence of the equilibrium isomeric ratio of all-trans and 13-cis retinal in dark-adapted bacteriorhodopsin at 340 MPa. The results show that hydrostatic pressure affects both the dark-adaptation rate and the dark equilibrium isomeric ratio. With increasing pressure, the fraction of all-trans isomers decreases. The kinetics have been analyzed with a two-state model. The description of the pressure dependence using transition state theory is inappropriate for two reasons; (a) pressure changes the viscosity of the protein and its environment, and (b) pressure changes the population of conformational substates within either isomeric form of bacteriorhodopsin. The temperature independent ratio of all-trans and 13-cis isomers indicates that the all-trans and 13-cis conformations have the same conformational volume.

Charge motion in vacuum-dried bacteriorhodopsin

Abstract The photocycle and the corresponding electric signals from vacuum-dried bacteriorhodopsin were studied. For this purpose, oriented purple membrane samples were used. The absorption changes resulting from laser flash excitation and electric response signals were measured with a time resolution of 0.1 μs. Two intermediates absorbing at 630 and 412 nm were seen. The decay of the 630 nm form cannot be described by a single exponential. From the electric signals it can be calculated that the proton does not leave the membrane, and only undergoes a backward and forward motion with respect to the proton-pumping direction of the bacteriorhodopsin.

On the enhanced PIXE yields from thick iron fluoride targets

Abstract The enhancement effect observed by Peisach and his co-workers [Nucl. Instr. and Meth. B 75 (1993) 14] in thick target PIXE measurements on transition metal fluorides was demonstrated to occur on targets containing no fluorine at all. On the basis of the very characteristic background shape the enhancement was attributed to the charging up of the insulating target. Using a hot carbon filament both the charging and the enhancement were possible to eliminate. The time course of the whole phenomenon was also monitored.

PIXE determination of Pb in wine

Abstract The permissible level of the Pb content in wine varies in the range of 200–300 μg/l in different countries. These values are below the detection limit for standard PIXE spectrometry. Initiated by an intercomparison action of The International Measurement Evaluation Programme (IMEP, Geel, Belgium), an attempt was made to lower the detection limit of PIXE without introducing any additional chemical steps. Wine samples of 5 ml were gradually condensed below an infrared lamp and 8 μl aliquots were taken from time to time and pipetted onto filter paper. The samples were analysed by standard PIXE technique, with the analyte addition method used for calibration. A detection limit of about 50 ppb was finally achieved. Comparing this value to the permissible levels, this simple and fast technique can be used for screening purposes. As an application, the lead content of three Hungarian market wines was also measured.

Speeding up of internal excitation calculations and application to x‐ray analysis

An improved calculation method is described for the calculation of internally excited x-rays. The basis of the method is to express the internal excitation term of a point-like source by three parameters and to store their precalculated values in a look-up table. The intensities are taken from this table for the actual values of the parameters instead of calculating them repeatedly on the spot. Therefore, the computational work has been reduced significantly. The results of this calculation are compared with those obtained by standard methods and the speed increment was proved. The method was also tested and validated experimentally for several samples. Copyright