Short Communication An indirect method to measure the electric charge deposited on insulators during PIXE analysis

AbstractTotal charge deposited by a proton beam in an insulator during PIXE analysis has been indirectly determined using a Mylar filmcoated with cobalt. Elemental concentrations in the samples, pieces of volcanic glass, were obtained and compared to concentrationsdetermined by ICP OES on the same samples. The strong agreement between these results shows the accuracy of the charge determinedby this method. 2007 Elsevier B.V. All rights reserved. PACS: 34.50.Fa; 32.30.Rj; 29.27. aKeywords: PIXE; Thick target; Electric charge; Concentrations; Obsidian rock 1. IntroductionThe determination of elemental concentrations in a sam-ple using PIXE (Particle Induced X-ray Emission) requiresthe knowledge of the total charge deposited on the sampleduring the irradiation. In the case of thin samples thecharge is measured directly by a Faraday cup positionedbehind it, but when the sample is rather thick and it is madeof an electric insulator, a different method must be used[1,2]. When possible, some metal is deposited by evapora-tion on the surface of the sample and wired to a standardbeam-measuring device. However, there are samples wherethe application of this technique is not advisable or is notpermitted because it may introduce important modifica-tions to its structure, appearance, cultural or economicvalue. At some laboratories a thin metallic foil is positionedin the beam path and the charge is determined in terms ofthe number of beam particles elastically backscattered. Inthis case, additional equipment must be installed insidethe irradiation chamber.Here we present an alternative method to measure thecharge during PIXE analysis of an electric insulator, suchas bone, wood, seed, glass or a delicate archaeologicalpiece.2. The methodThe basic idea consists of producing characteristicX-rays from an element in a film positioned close to thesample under irradiation. The presence of the film shouldnot significantly affect the beam properties like energyand intensity, nor the X-ray spectrum emitted from theirradiated sample. This can be accomplished by holding athin mono elemental film at a few millimeters from the sur-face of the sample, so that the irradiation spots of the sam-ple and the film subtend the same solid angle. Then, thetotal charge is determined from a calibration functionobtained previously, between the total number of countsin a relevant peak of the element in the film and the totalcharge measured by a Faraday cup. The selection of the