Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Stimulated mutual annihilation

02.05.2014

How to make a gamma-ray laser with positronium

Twenty years ago, Philip Platzman and Allen Mills, Jr. at Bell Laboratories proposed that a gamma-ray laser could be made from a Bose-Einstein condensate (BEC) of positronium, the simplest atom made of both matter and antimatter (1). That was a year before a BEC of any kind of atom was available in any laboratory.


Schematic of the stimulated annihilation process in the positronium gamma-ray laser. Time sequence of frames, running top to bottom, suggests how some "seed photons" from spontaneous annihilation of a few Ps atoms will stimulate subsequent Ps annihilations, resulting in a pulse of 511 keV gamma rays

Credit: JQI/Schewe

Today, BECs have been made of 13 different elements, four of which are available in laboratories of the Joint Quantum Institute (JQI) (2), and JQI theorists have turned their attention to prospects for a positronium gamma-ray laser.

In a study published this week in Physical Review A (3), they report detailed calculations of the dynamics of a positronium BEC. This work is the first to account for effects of collisions between different positronium species. These collisions put important constraints on gamma-ray laser operation.

The World's Favorite Antimatter

Discovered in 1933, antimatter is a deep, pervasive feature of the world of elementary particles, and it has a growing number of applications. For example, about 2 million positron emission tomography (PET) medical imaging scans are performed in the USA each year. PET employs the antiparticle of the electron, the positron, that is emitted by radioactive elements which can be attached to biologically active molecules that target specific sites of the body. When a positron is emitted, it quickly binds to an electron in the surrounding medium, forming a positronium atom (denoted Ps).

Within a microsecond, the Ps atom will spontaneously self-annihilate at a random time, turning all of its mass into pure energy as described by Einstein's famous equation, E = mc2. This energy usually comes in the form of two gamma rays with energies of 511 kiloelectronvolts (keV), a highly penetrating form of radiation to which the human body is transparent. Platzman's and Mills' gamma-ray laser proposal involves generating coherent emission of these 511 keV photons by persuading a large number of Ps atoms to commit suicide at the same time, thus generating an intense gamma-ray pulse.

The Simplest Matter-Antimatter Atom

Ps lives less than a microsecond after it is formed, but that is a long enough for it to demonstrate the distinctive properties of an atom. It is bound by the electric attraction between positron and electron, just as the hydrogen atom is bound by the attraction between the proton and the electron. Figure 2 shows the spatial distribution of electron and positron density in Ps as given by the solution of the Schroedinger equation. The sharp central cusp is the place where the electron and positron meet, and annihilate. The electron density there is about four times the average density of the conduction electrons in copper wire.

Stimulated Annihilation

The electron and positron each has an intrinsic spin of ½ (in units of the reduced Planck constant). Thus, according to quantum mechanics, Ps can have a spin of 0 or 1. This turns out to be a critical element of the gamma-ray laser scheme. The 511 keV photons are only emitted by the spin-0 states of Ps, and this takes place within about 0.1 nanosecond after a spin-0 state is formed. The spin-1 states, on the other hand, last for about 0.1 microsecond, and only decay by emission of three gamma rays (for reasons of symmetry).

What we will get here is a process, stimulated annihilation, that is analogous to the stimulated radiation process at the heart of laser operation. Thus, when a pulsed beam of positrons is directed into a material, a random assortment of Ps states is created; the spin-0 states annihilate within the nanosecond, and the spin-1 states live for another nanosecond. During this latter time, the spin-1 states serve as an energy storage medium for the gamma ray laser: if they are switched into spin-0 states, which constitutes the active gain medium that generates the fast pulse of 511 keV gamma rays. Part of the JQI theorists' work involves the modelling of the most likely switch for this process, a pulse of far-infrared radiation. They find several switching sequences that approach the optimal condition of switching all spin-1 states to spin-0 states in a time short compared to the annihilation lifetime.

Positronium Sweetspot

Platzman and Mills pointed out that the Bose-Einstein condensate is a form of "enabling technology" for the Ps gamma-ray laser. This is because its low temperature and high phase-space density make coherent stimulated emission possible: in an ordinary thermal gas of Ps, the Doppler shifts of the atoms would suppress lasing action. This introduces another degree of complexity, which is explored in detail for the first time by the JQI team. A Ps BEC will only form when a threshold density of Ps is attained.

That density depends upon the temperature of the Ps, but it is likely to be in the range of 1018 Ps atoms per cubic centimeter, which is about 3% of the density of ordinary air. At that density, collisions between Ps atoms occur frequently, and state-changing collisions are of particular concern. On the one hand, two spin-1 Ps atoms can collide to form two spin-0 atoms; this process limits the density of the energy storage medium. On the other hand, two spin-0 Ps atoms can collide to form two spin-1 atoms; this process limits the density of the active gain medium. Using first-principles quantum theory, the JQI team has explored the time evolution of a Ps BEC containing various mixtures of spin-0 and spin-1 Ps atoms, and has found that there is a critical density of Ps, above which collision processes quickly destroy the internal coherence of the gas.

The main conclusion of the JQI work is that the critical density is greater than the threshold density, so that there is a "sweet spot" for further development of a Ps gamma-ray laser. Dr. David B. Cassidy, a positronium experimentalist at University College London, who was not one of the authors of the new paper, summarizes it thusly:

"The idea to try and make a Ps BEC, and from this an annihilation laser, has been around for a long time, but nobody has really thought about the details of how a dense Ps BEC would actually behave, until now. This work neatly shows that the simple expectation that increasing the Ps density in a BEC would increase the amount of stimulated annihilation is wrong! Although we are some years away from trying to do this experimentally, when we do eventually get there the calculations in this paper will certainly help us to design a better experiment."

###

(1) "Possibilities for Bose condensation of positronium", P. M. Platzman and A.P. Mills, Jr., Physical Review B vol. 49, p. 454 (1994)
(2) The Joint Quantum Institute is operated jointly by the National Institute of Standards and Technology in Gaithersburg, MD and the University of Maryland in College Park.
(3) "Spinor Bose-Einstein Condensates of Positronium," Yi-Hsieh Wang, Brandon M. Anderson, and Charles W. Clark, Physical Review A, vol. 89, p. 043624 (2014) , published online 28 April 2014 at http://link.aps.org/doi/10.1103/PhysRevA.89.043624 DOI: 10.1103/PhysRevA.89.043624

Yi-Hsieh Wang, yhw1110@umd.edu
Brandon Anderson, brandona@umd.edu
Charles W. Clark, charles.clark@nist.gov
Press contact at JQI: Phillip F. Schewe, pschewe@umd.edu, 301-405-0989. http://jqi.umd.edu/

Phillip F. Schewe | Eurek Alert!

Further reports about: BEC Bose-Einstein JQI Quantum collisions equation limits nanosecond photons rays temperature threshold

More articles from Physics and Astronomy:

nachricht LIGO confirms RIT's breakthrough prediction of gravitational waves
12.02.2016 | Rochester Institute of Technology

nachricht Milestone in physics: gravitational waves detected with the laser system from LZH
12.02.2016 | Laser Zentrum Hannover e.V.

All articles from Physics and Astronomy >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Production of an AIDS vaccine in algae

Today, plants and microorganisms are heavily used for the production of medicinal products. The production of biopharmaceuticals in plants, also referred to as “Molecular Pharming”, represents a continuously growing field of plant biotechnology. Preferred host organisms include yeast and crop plants, such as maize and potato – plants with high demands. With the help of a special algal strain, the research team of Prof. Ralph Bock at the Max Planck Institute of Molecular Plant Physiology in Potsdam strives to develop a more efficient and resource-saving system for the production of medicines and vaccines. They tested its practicality by synthesizing a component of a potential AIDS vaccine.

The use of plants and microorganisms to produce pharmaceuticals is nothing new. In 1982, bacteria were genetically modified to produce human insulin, a drug...

Im Focus: The most accurate optical single-ion clock worldwide

Atomic clock experts from the Physikalisch-Technische Bundesanstalt (PTB) are the first research group in the world to have built an optical single-ion clock which attains an accuracy which had only been predicted theoretically so far. Their optical ytterbium clock achieved a relative systematic measurement uncertainty of 3 E-18. The results have been published in the current issue of the scientific journal "Physical Review Letters".

Atomic clock experts from the Physikalisch-Technische Bundesanstalt (PTB) are the first research group in the world to have built an optical single-ion clock...

Im Focus: Goodbye ground control: autonomous nanosatellites

The University of Würzburg has two new space projects in the pipeline which are concerned with the observation of planets and autonomous fault correction aboard satellites. The German Federal Ministry of Economic Affairs and Energy funds the projects with around 1.6 million euros.

Detecting tornadoes that sweep across Mars. Discovering meteors that fall to Earth. Investigating strange lightning that flashes from Earth's atmosphere into...

Im Focus: Flow phenomena on solid surfaces: Physicists highlight key role played by boundary layer velocity

Physicists from Saarland University and the ESPCI in Paris have shown how liquids on solid surfaces can be made to slide over the surface a bit like a bobsleigh on ice. The key is to apply a coating at the boundary between the liquid and the surface that induces the liquid to slip. This results in an increase in the average flow velocity of the liquid and its throughput. This was demonstrated by studying the behaviour of droplets on surfaces with different coatings as they evolved into the equilibrium state. The results could prove useful in optimizing industrial processes, such as the extrusion of plastics.

The study has been published in the respected academic journal PNAS (Proceedings of the National Academy of Sciences of the United States of America).

Im Focus: New study: How stable is the West Antarctic Ice Sheet?

Exceeding critical temperature limits in the Southern Ocean may cause the collapse of ice sheets and a sharp rise in sea levels

A future warming of the Southern Ocean caused by rising greenhouse gas concentrations in the atmosphere may severely disrupt the stability of the West...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

Symposium on Climate Change Adaptation in Africa 2016

12.02.2016 | Event News

Travel grants available: Meet the world’s most proficient mathematicians and computer scientists

09.02.2016 | Event News

AKL’16: Experience Laser Technology Live in Europe´s Largest Laser Application Center!

02.02.2016 | Event News

 
Latest News

LIGO confirms RIT's breakthrough prediction of gravitational waves

12.02.2016 | Physics and Astronomy

Gene switch may repair DNA and prevent cancer

12.02.2016 | Life Sciences

Using 'Pacemakers' in spinal cord injuries

12.02.2016 | Medical Engineering

VideoLinks
B2B-VideoLinks
More VideoLinks >>>