Positron luminescence outshines that of electrons

April 30, 2018 by Lisa Zyga, Phys.org feature
Differences between electron luminescence and positron luminescence for two different phosphors, ZnS:Ag and ZnO:Zn. Credit: Stenson et al. ©2018 American Physical Society

In old cathode ray TVs, a picture is generated when an electron beam excites a phosphor screen, causing the phosphor to radiate light. Now in a new study, researchers have found that a beam of positrons (positively charged anti-electrons) incident on a phosphor screen produces significantly more luminescence than an electron beam does.

When the researchers began their research, they expected the applications to be primarily utilitarian: mainly, to understand the differences between using positrons and electrons when performing experiments with phosphor screens as diagnostics. However, the differences were much more interesting than they expected, which may extend the potential applications to areas such as designing new diagnostic systems as well as learning more about the properties of luminescent materials.

The scientists, E. V. Stenson, U. Hergenhahn, M. R. Stoneking, and T. Sunn Pedersen, at the Max Planck Institute for Plasma Physics, among other institutes, have published a paper on their comparison of positron and electron in a recent issue of Physical Review Letters.

In their experiments, the researchers compared the luminescence excited by a positron with that excited by an for two different phosphors (ZnS:Ag and ZnO:Zn). For both phosphors, the overall results were similar. As the beam energy increased from zero, the amount of positron-induced luminescence rose rapidly, while the amount of electron-induced luminescence increased much more gradually. Above a certain level of beam energy, both types of luminescence grew in a linear fashion at approximately the same rate. So at very high beam energy levels, the difference between positron- and electron-induced luminescence became negligible.

Instead, the most striking difference occurred at the lower beam energy levels. For example, in order to produce the same amount of luminescence that is produced by an electron beam with several thousand electron volts of energy, a positron beam required only a few tens of electron volts (eV) for ZnS:Ag, and for ZnO:Zn, even less than 10 eV. As the researchers explain, the huge difference arises from positron annihilation, which produces greater numbers of excited states in the phosphor materials.

As positrons can be used as a tool for learning about phosphors, and phosphors can be used as a tool for learning about positrons, the researchers expect that the results will be interesting for both areas.

"For researchers who look at positrons incident on materials, it's the positrons that are the object of interest," Stenson told Phys.org. "In this case, a screen is seen as just a tool for learning about the positrons—for example, something as simple how many of them you have available as ingredients for making anti-atoms or matter-antimatter plasmas.

"For other researchers, it is the other way around, where positrons are a tool for learning about some material. One can do this, for example, by looking at how long it takes positrons to annihilate with a particular solid or liquid or gas, at what angles positrons scatter off a material, or the energy spectrum of electrons that are emitted from a material in which a positron beam annihilates."

Because of the striking difference between electrons and positrons, the results also offer a new tool for understanding the properties of luminescent materials in general.

"Luminescent materials have a long history, having been used for decades in things like cathode ray tubes, and they are still being developed for a variety of new applications," Stenson said. "Luminescent materials have applications ranging from consumer goods (displays, afterglow materials) to specialized detectors (gas sensors, scintillators) to nanoparticles used for cancer treatment."

Stenson explained that, despite these materials having such a long history, there are still open questions about important aspects of the physics of luminescent . These questions include the structure of luminescence centers, the excitation and relaxation pathways for cathodoluminescence, and the origin of the 'dead voltage'—that is, why electrons with less than a keV or two of energy don't produce detectable luminescence in many phosphors.

"I expect that further studies of positron-induced luminescence (for example, comparing the spectrum of light produced by low-energy positrons vs. high-energy positrons vs. high-energy electrons) will be a valuable means of investigating these open questions, especially when combined with other approaches that are already in use for studying ," Stenson said.

Explore further: Spinning electrons yield positrons for research

More information: E. V. Stenson, U. Hergenhahn, M. R. Stoneking, and T. Sunn Pedersen. "Positron-Induced Luminescence." Physical Review Letters. DOI: 10.1103/PhysRevLett.120.147401

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12 comments

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Beckler
1 / 5 (1) Apr 30, 2018
Would this mean if I plugged in my old CRT backwards it would be much brighter?
andyf
5 / 5 (2) Apr 30, 2018
For some reason, this doesn't seem at all surprising. After all, each positron will annihilate with an electron on the screen producing a pair of 0.511 MeV gamma rays in addition to any phosphorescence.

I bet the screen degrades rather rapidly.
andyf
5 / 5 (2) Apr 30, 2018
Only if it was using DC supplies. AC doesn't matter which polarity you use.


That's wrong in every respect.

LED Guy
1 / 5 (1) Apr 30, 2018
I'll agree with andyf. Most of the CRT phosphors work rather well as scintillators and can be excited by UV light. The extra energy from gamma rays would explain everything rather nicely.

At higher beam energies, the "surplus" energy from positions would be minimized with respect to the total energy.

Crystal defects and surface states explain the "dead energy" with electrons. Near the surface of a phosphor particle, crystal defects and surface states results in non-radiative electron relaxation. An electron beam has to have enough energy to penetrate into a particle and still be able to cause impact ionization.

Nothing new here except that someone has a rather expensive replacement for the electron gun for an outdated display technology.
andyf
5 / 5 (1) Apr 30, 2018
An electron beam has to have enough energy to penetrate into a particle and still be able to cause impact ionization.


Agreed, whereas a positron only has to wander up close enough to be attracted by an electron in the phosphor before the 0.511MeV gammas are inevitable.

Low energy electrons are repelled by the electron cloud and can't trigger phosphorescence. Hence the 'dead voltage'.

Nothing new here except that someone has a rather expensive replacement for the electron gun for an outdated display technology.
and created a screen that is a source of gamma rays.

At least CRT's only produced X-rays in the early days.
adam_russell_9615
not rated yet Apr 30, 2018
It seems to me that they are graphing luminescence against attractive screen bias, and positrons have attraction to the screen even when the screen has zero bias. So some of the attraction is due to the positron charge. IOW its really the same luminescence but there is attraction even to an unpowered screen so the graph starts earlier.
Hyperfuzzy
1 / 5 (3) Apr 30, 2018
Be careful, you may start a nuclear reaction! Think dummies! Charge only exist as diametrical spherical fields of Values of E! That's it! There are no particles! Only these immortal fields; which, we'd like to keep stable! Only one high speed charge allowed to navigate the negative shielding provided by mono-polar orbiters!
Hyperfuzzy
1 / 5 (3) Apr 30, 2018
Threshold, volume (pV = NRT) and Charge Count, Total Energies, i.e negative energy, positive energy (polar causality), collisions at higher energies, i.e. acceleration! git it P(n*Collision/sec) an distribution levels/ increasing or decreasing? anyway, common sense
Thorium Boy
5 / 5 (1) Apr 30, 2018
How come they didn't know this 60 years ago?
granville583762
4 / 5 (4) May 01, 2018
For the benefit of mankind
How using Albert Einstein's Noble prize award winning research on the photoelectric effect had he and his like minded colleagues had in mind to convert the 0.5MeV gamma-rays into electricity for the benefit of mankind instead of his infernal bomb spewing gamma-rays and relativistic 14MeV neutrons everywhere, because now we have a cathode ray tube playing havoc with our retina's with nothing to show for it except an electricity bill.
milnik
not rated yet May 01, 2018
Again, it is about ignorance of the way in which matter is formed and its relation to the substance (aether) from which matter forms. What is the difference in the properties of the positron and the electron? What is luminescence? Neither electron nor positron could act without Aether, and luminescence was a phenomenon that arose when the positron or electron enters the neutrons contained in that "coating" on the screen.
Hyperfuzzy
5 / 5 (1) May 03, 2018
For the benefit of mankind
How using Albert Einstein's Noble prize award winning research on the photoelectric effect had he and his like minded colleagues had in mind to convert the 0.5MeV gamma-rays into electricity for the benefit of mankind instead of his infernal bomb spewing gamma-rays and relativistic 14MeV neutrons everywhere, because now we have a cathode ray tube playing havoc with our retina's with nothing to show for it except an electricity bill.

Shhh, don't to give away the A bomb!

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