Positrons are shining brighter
Luminous materials have long been in daily use, e.g. in TV screens or PC monitors and in science for investigating plasmas and particle or antiparticle beams. Whether particles or antiparticles are involved – if these impinge on a luminous material they excite it to luminescence. It was hitherto not known, however, that the light emission with electrons is much lower than with positrons, their antiparticles. This was discovered by Dr. Eve Stenson at Max Planck Institute for Plasma Physics (IPP) in Garching and Greifswald while preparing experiments with matter-antimatter plasmas.
“If antimatter were not so difficult to produce, one could hope for an era of high-luminosity, low-voltage displays where luminous screens are excited not by electrons but by positrons”, is how Dr Eve Stenson comments her discovery, albeit with a twinkle in her eye. “But this is, unfortunately, not feasible”. Nevertheless, there may be a future for positron-induced luminescence.
First it must be clarified, states Eve Stenson, why positrons excite luminous matter much more strongly than equally fast electrons. Both electrons and positrons transfer their kinetic energy to the luminous matter on impact. Such collisions raise the electrons of the luminous matter from a lower to a higher energy level. When they drop back again, energy released is emitted as light – the material luminesces at the impacted site.
In the case of an impacting positron, however, there is a second effect: After it has relinquished its energy in the luminous material the positron can annihilate itself with an electron, its antiparticle. This leaves a cavity in the sea of electrons of the luminous material into which other electrons from higher energy levels can fall, this giving rise to a repeated emission of light. This accounts for the higher light emission of positrons. “This second light could, however, also yield information on the material properties of the luminous substance and the mechanism of luminescence”, states Eve Stenson. As she points out, although luminescent substances and luminous screens have been in use for decades – in TV sets, displays, signposts, physical sensors or as nanoparticles in medicine – important physical details of their behaviour have not yet been clarified.
The different effects of electrons and positrons were discovered by Eve Stenson when calibrating the luminous screen on a particle trap that can store electrons or else positrons. She was astonished to find that the two types of particles resulted in two completely different shapes of curves: Positrons with an energy of a few tens of electron volts generate in the zinc sulphide or zinc oxide fluorescent screens investigated by Eve Stenson as much light as electrons with several thousand electron volts. “In order to understand this, I was suddenly whisked on an unscheduled detour from plasma physics deep into solid-state physics”. For she was confronted by the fact that the luminescence yielded by electrons and positrons had hitherto apparently never been compared for low energies, although both kinds of particles were being routinely detected with luminescent screens.
The particle trap forms part of an experimental setup now being put together by a team headed by Professor Dr. Thomas Sunn Pedersen at IPP. It is about to produce the first ever matter-antimatter plasma composed of electrons and positrons.
Isabella Milch, Max-Planck-Institut für Plasmaphysik
Foto: IPP, Eve Stenson