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discovered_01_2013

discovered 01.13 FOCUS WWW.Hzdr.DE actually takes up this X-ray quantum’s impulse: At the HZDR, this job is performed by a Tungsten sheet. The resulting positron beam consists, just like the ELBE electron beam, of short pulses between five and ten trillionth of a second in duration and that, compared with other positron sources, are sharply delimited. This means the researchers know more exactly how much time it takes the positrons to be created and determine their lifetime with greater precision than with the help of other positron sources. "In short, the defects can be analyzed much more precisely with the help of EPOS positrons than at other facilities," explains Reinhard Krause- Rehberg of the Martin Luther University in Halle, Germany, who was instrumental in EPOS’ development. Fast processors, efficient catalysts, and robust materials There is a wealth of materials that are needed for cutting- edge technologies and whose structure can be explored and, in the end, even improved with the help of positrons. One example is silicon dioxide, a key component of modern-day computer processors - even though moving electrical charges through this material is decidedly difficult. This, however, significantly slows down the switching mechanisms that take place continuously and at a high frequency inside a processor. In fact, if one were able to lower the dielectric constant, a processor would be notably faster. Material scientists are able to make this happen by incorporating tiny holes approximately one millionth of a millimeter in diameter directly into the material. This produces a kind of microsponge with a significantly lowered dielectric constant. And these tiny holes can again be nicely examined with the help of positrons and their annihilation. Membranes with this type of microsponge structure play a critical role in catalysts that are key components of many different chemical reactions. However, they only work if the tiny holes are interconnected to permit the flow of fluids. And again EPOS’ positron beam allows for a close analysis of these defects, which in turn allows scientists to conclude whether the membrane will perform its job reliably. The energy of the X-ray flashes that are produced in the course of annihilation is not always exactly 511 keV. As such, if the positron meets an electron that orbits around an atomic nucleus, depending on the atomic nucleus and orbit, this electron has a precisely measurable energy. This energy, however, is also taken up by the annihilation flashes, which changes their keV values. Using these new keV values, the researchers are able to analyze the chemistry in the defect’s immediate vicinity to obtain valuable information about the material they are studying. EPOS could potentially also play an important role in an energy source that has been under development for a number of years now: nuclear fusion. Here, light-weight atomic nuclei melt together into heavier ones and, in the process, give off massive amounts of usable energy. Scientists all around the world are still puzzling over the specifics underlying this fusion reaction. What they do know is it results in neutron production. Over time, these rather heavy uncharged elementary particles shoot defects into the steel wall of such a reactor, which means the metal atoms have a harder time gliding past each other and the steel becomes brittle and could, in extreme cases, break or tear. With the help of EPOS positrons, however, Andreas Wagner is able to study the origin of these kinds of defects. This knowledge could help with developing a type of long-lasting steel that won’t get brittle as quickly in the face of high neutron flow in these types of fusion reactors. ELBE positrons could thus play a key role in the development of a number of important future materials. UNDER SCRUTINY: The HZDR facility for positron experiments offers scientists Andreas Wagner (right) and Maik Butterling the option of a much more precise analysis than do other positron sources. Photo: Frank Bierstedt CONTACT _Institute of Radiation Physics at HZDR Dr. Andreas Wagner a.wagner@hzdr.de

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