discovered_01_2013

FOCUS// The HZDR Research Magazine WWW.Hzdr.DE 06 07 cleaner on the positrons," explains Wagner. After all, positrons are the electrons’ sister particles with largely identical properties except for the fact that they are positively instead of negatively charged. But since opposite electrical charges attract, positrons are being sucked in by the defect’s partial negative charge. Once the positrons have made it to the defect, the positively charged atomic nuclei in the area repel the tiny, positively charged positrons, encapsulating them. This "captivity," however, isn’t a permanent one: If a positron encounters an electron, the fate of these two oppositely charged but otherwise largely identical elementary particles is sealed and they dissolve in two or three bursts of energy that rapidly dissipate in different directions at the speed of light in the form of hard radiation. "We are calling this mutual annihilation," explains Wagner. Positron life time But the physicist is only marginally interested in the energy of the annihilation flashes, which is known to be 511 kilo electron Volts (keV) for each of the two resulting flashes. "Instead, we analyze the positrons’ lifetime - the time it takes from their creation to their annihilation," explains Wagner. Their typical lifetime lies somewhere between one tenth and one hundredth of a billionth of a second and depends, to a large extent, on defects in the material. What do these defects look like and how many are there? Which is why, upon closer scrutiny, the time distribution until positron de- radiation can tell scientists a thing or two about the nature of the defect. As such, the positrons turn out to be a good way of analyzing vacancies and other defects, even though they won’t allow you to actually see them. But what is the source of positrons produced at the HZDR’s EPOS facility? One important hint is built into the acronym, which stands for ELBE Positron Source or the Positron Source at the Electron Linear Accelerator for Radiation of High Brilliance and Low Emittance (or, as it were, ELBE for short). If this electron beam hits solid material, the tiny particles begin to decelerate. In the process, they emit energy in the form of X-ray radiation. Contrary to how the mutual annihilation of positrons and electrons is able to generate two X-ray flashes, a tiny quantum package of this bremsstrahlung in a vacuum is capable of producing a pair consisting of one electron and one positron. But this only works if a nearby atomic nucleus SISTER PARTICLES: Positrons are similar to electrons, although they carry a positive rather than a negative charge. They are attracted by excess electrons in the material defects and are thus the perfect tool for defect analysis. Photo: Frank Bierstedt

discovered_01_2013

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