Billions of Positrons Created in Laboratory

by Paul Gilster on November 18, 2008

Irradiate a millimeter-thick gold target with the right kind of laser and you might get a surprise in the form of 100 billion positrons, the antimatter equivalent of electrons. Researchers had been studying the process at Lawrence Livermore National Laboratory, where they used thin targets that produced far fewer positrons. The new laser method came about through simulations that showed a thicker target was more effective.

And suddenly lasers and antimatter are again making news. Hui Chen is the Livermore scientist behind this work:

“We’ve detected far more anti-matter than anyone else has ever measured in a laser experiment. We’ve demonstrated the creation of a significant number of positrons using a short-pulse laser.”

Image: Physicist Hui Chen sets up targets for the antimatter experiment at the Jupiter laser facility. Credit: Lawrence Livermore National Laboratory.

What’s happening here is that ionized electrons are interacting with gold nuclei, giving off energy that decays into matter and antimatter. It’s a method that has been studied before at Livermore, using the laboratory’s petawatt laser. An article by physicist Michael Perry explains the interactions that laser achieved, first produced about a decade ago:

The intense beam of Livermore’s Petawatt laser was powerful enough to break up atoms by causing reactions in their nuclei. Accelerated by the laser, electrons traveling at nearly the speed of light collided with nuclei in a gold foil target, producing gamma rays that knocked out some of the neutrons from other gold nuclei and caused the gold to decay into elements such as platinum. Gamma rays also zoomed in on a layer of uranium sitting behind the gold and split uranium nuclei into lighter elements. Before the Petawatt, all of these effects had been solely in the domain of particle accelerators or nuclear reactors.

Accelerated to energies exceeding 100 megaelectronvolts, the electrons in the gold targets produced high-energy x rays. These in turn decayed into pairs of electrons and their antimatter counterparts, positrons, in such large numbers as to possibly generate an electron-positron plasma, never before created in the laboratory. An intense beam of protons also turned up. Not only was the Petawatt the most powerful laser in the world, but, unexpectedly, it also was a powerful ion accelerator.

The petawatt laser was decommissioned in mid-1999. Its original development centered on the study of inertial confinement fusion, in which a pellet of fuel could be ignited through intense laser bombardment. Variations on inertial confinement fusion play interestingly through interstellar propulsion studies, including the massive Daedalus probe designed by the British Interplanetary Society, which would have used deuterium and helium-3 as its fodder for a trip to Barnard’s Star.

But the petawatt laser also opened up the possibility of using a laser to do things particle accelerators had been called upon to do in the past. And now we’ve moved significantly beyond the earlier results — the petawatt experimenters detected roughly 100 antimatter particles compared to Chen’s one million. And note this: Chen’s number refers to particles that were directly detected, a result that produces an overall estimate of 100 billion positrons produced in the entire experiment.

We’ve looked in these pages at the possibility of harvesting naturally forming antimatter found in our own Solar System, even near the Earth, where cosmic ray interactions with the upper atmosphere produce small quantities. And the presence of antimatter near the center of our galaxy has been established, detectable because matter/antimatter annihilation produces gamma rays. The trick has always been that harvesting antimatter — or producing it in accelerators — yields small amounts at great expense. The latest work at least offers hope for more robust laboratory study of a material whose propulsive properties have long attracted interstellar theorists.

Just how significant a step this is remains to be seen, but I note what Peter Beiersdorfer, who works with Chen at Livermore, has to say:

“We’ve entered a new era. Now, that we’ve looked for it, it’s almost like it hit us right on the head. We envision a center for antimatter research, using lasers as cheaper antimatter factories.”

We’ll know more shortly, for Chen is presenting her work at the American Physical Society’s Division of Plasma Physics meeting that runs through Friday this week. A Livermore news release is available. Thanks to Centauri Dreams reader Leith for the heads-up on this work.

David November 18, 2008 at 13:20

He said cheaper anitmatter “Factories” That is a loaded term that implies we have mass prodcution ….
How much do we need to accelerate a small probe of say 1 kg to 0.1c?
We would then have interstellar travel!

Administrator November 18, 2008 at 16:11

David, unfortunately the amounts we’re talking about here are still tiny. I don’t have any figures for something in the 1 kg range, but re a much heavier interstellar probe, Robert Forward calculated that a 1-ton robotic probe moving at one-tenth the speed of light on a scientific mission to Alpha Centauri would require four tons of liquid hydrogen or other propellant, along with some forty pounds of antimatter. A two-stage mission that would decelerate at Alpha Centauri for extended exploration would require 770 pounds of antimatter and 24 tons of liquid hydrogen. Obviously nanotechnology changes the equation as we shrink the payload, but we’re still in needed of ramping up antimatter production by a huge amount.

Would love to know what 100 billion positrons translates to in micrograms, but I’m sure it isn’t much! The AIMStar Oort Cloud probe, designed for a fifty year mission, would use 30-130 micrograms of antimatter itself. Given all that, this is exciting work because it is making laboratory study of antimatter that much easier, leading (let’s hope) to breakthroughs down the road.

Hans Bausewein November 18, 2008 at 16:44

10 E-10 microgram.

or you need 10 billion times more for a single microgram.

http://en.wikipedia.org/wiki/Positron

hiro November 18, 2008 at 16:54

100 billion positions ~ 10^-10 microgram. This is terrible; I still hope we have enough antimatter around 2050s ( or sooner) to send a small probe to the Centauri system.

Ronald November 18, 2008 at 17:12

From Wikipedia: “The mass of a stationary electron is approximately 9.1 × 10^-31 kilograms”

I.e. 100 billion = 10^11 amounts to only about 9 x 10^-20 kg or 9 x 10^-11 microgram.

Still incredibly little.

djlactin November 19, 2008 at 1:56

just did some rough calculations.

assuming complete annihilation by interacting with an equivalent mass of normal matter, 9×10^-11 kg of positrons would release E = mc^2 = 3.6 x 10^-3 kg m^2 s^-2 (= 3.6×10^-3 N m = 3.6 x 10^-3 J) of energy (half due to the positrons and half due to the normal matter). Using kinetic energy E = mv/2, then v = sqrt(2E/m) and this would be enough to accelerate 1 kg to 0.085 m/s, or 1 microgram to 85 m/s.

This is hardly a useful quantity for propulsion purposes, and I also see several other objections. First, I have neglected: 1) all losses during propulsion (which are unavoidable due to thermodynamics); 2) energy required to contain the positron ‘gas’; 3) energy required to produce the positrons (which will certainly exceed their energy content — more thermodynamics).

My question: If you are thinking about accelerating probes, why go through all the intermediate steps (production, containment, burning), when you have a petawatt laser? 1 PW = 10^15 J/s; in one second, this is 2.8 x 10^17 more than the antimatter equivalent energy produced in the experiment that you described.

This brings me to a ‘peta’ idea of mine: “simply” use the laser to propel the probes.

By the way: “ionized electrons”?!

Usman November 19, 2008 at 2:04

@Hans Bausewein, @hiro

To be honest I don’t understand whether centauri dreams was the right site for this news. Because space exploration is generally not the priority of particle physicists and though we can speculate on using antimatter for propulsion I am certain we will never have a viable propulsion method based on antimatter. It would be too costly. Other methods such as fusion ramjets, solar sails or breakthrough propulsion physics are more promising.

My point simply is; not to give particle physicists the credit they don’t deserve. If they really were dreamers as have been many people who tirelessly have pushed back the frontiers of space exploration it won’t be in its current state.

James M. Essig November 19, 2008 at 10:07

Hi Folks;

It would be interesting if the PetaWatt laser could fire say 10 to 1,000 pulses per second for an entire year at Gold Foil in a vacuum. The number of positrons generated would be, I presume, as great as (10 EXP 3)[3.1 x (10 EXP 7)](10 EXP 11) = 3.1 x (10 EXP 21). Although this would still be roughly only 10 EXP -7 grams, it might be enough to tease out any novel behavioral effects of macroscopic quantities of antimatter that are not detectable in microscopic or quantum scale quantities of positrons.

Perhaps some such novel behavioral effects of the positrons could be based on the mirror image qualities of such positrons with respect to that of the electrons.

Even though experiments have shown, if I am not mistaken, that antimatter particles react to gravity in the same manner that normal matter particles do, perhaps in macroscopic quantities at roughly STP conditions, some novel gravatitational effects of antimatter could be revealed.

Any subtle or yet be detected assymetry in the qualities of positrons with respect to electrons might also be accentuated in any such macroscopic quantities of positrons.

I believe a company called Positronics Research, LLC at http://www.pr-llc.com/, is studying the production of stable positronium atoms wherein a positronium atom would consist of a stable bound state of an electron and a positron wherein the seperation of the two particles would be far enough apart so as prevent pair annihilation. A large supply of such fuel could in theory be detonated like nitroglycerin by shocking it or otherwise perturbing it in order to destabilize the fuel.

I can imagine such positronium fuel being an excellent fuel for a pulsed rocket like the Orion Project style craft concept researched in the 50s and 60s.

Even though antimatter production may be limited to a fraction of one gram per year at best in the years to come, perhaps novel effects can be produced in terms of gravitational effects, other novel space time distortion effects, etc., with these very limited supplies of antimatter.

Thanks;

Jim

Robert Clark November 26, 2008 at 11:09

Jim’s pretty much correct about the mass. The mass of a positron, the same as the electron is 9.1X10^-28 gm, call it 10^-27 gm. So the mass produced would be 3×10^-6 gm, 3 micrograms, per year. Of course these petawatt scale lasers can’t fire as frequently as 1,000 times a second, but it is not too far out of the question they could be scaled up t o produce this much energy that quickly.
Note that this small amount of 3 micrograms is not too very far off the amounts envisioned for a propulsion method using electron-positron annihilation:

June 30th, 2005
Positron Drive: Fill ‘er Up For Pluto
Written by Fraser Cain

“Using chemically based propulsion systems, 55 percent of the weight associated with the Huygens-Cassini probe sent to explore Saturn was found in the probe’s fuel and oxidizer tanks. Meanwhile to hurl the probes 5650 kg of weight beyond the Earth required a launch vehicle weighing some 180 times that of fully-fueled Cassini-Huygens itself (1,032,350 kgs).
“Using Dr. Smith’s numbers alone – and only considering the maneuvering thrust required for Cassini-Huygens using positron-electron annihilation, the 3100 kgs of chemical propellant burdening the original 1997 probe could be reduced to a mere 310 micrograms of electrons and positrons – less matter than that found in a single atomized drop of morning mist. And with this reduction in mass the total launch weight from Canaveral to Saturn could easily be reduced by a factor of two.”

“According to Dr. Smith, “for many years physicists have squeezed positrons out of the tungsten targets by colliding the positrons with matter, slowing them down by a thousand or so to use in high resolution microscopes. This process is horribly inefficient; only one millionth of the positrons survive. For space travel we need to increase the slowing down efficiency by at least a factor of one thousand. After four years of hard work with electromagnetic traps in our labs, we are preparing to capture and cool five trillion positrons per second in the next few years. Our long-range goals are five quad-trillion positrons per second. At this rate we could fuel up for our first positron-fueled flight into space in a matter of hours.”
“While it is true that a positron-annihilation engine also requires propellent (typically in the form of compressed hydrogen gas), the amount of propellant itself is reduced to almost 10 percent of that required by a conventional rocket – since no oxidizer is needed to react with the fuel. Meanwhile, future craft may actually be able to scoop propellant up from the solar wind and interstellar medium. This should also lead to a significant reduction in the launch weight of such spacecraft.”
http://www.universetoday.com/2005/06/30/positron-drive:-fill-er-up-for-pluto-/

I wonder what this production method is for positrons Smith is talking about that he expects to create 5 trillion positrons per second over the next few years.

Bob Clark

Brandon Taylor April 27, 2009 at 12:35

Let’s not also forget that the positrons being produced are simply by blasting away at gold foil; research into nano-tweaked foils which will more efficiently distribute laser energy, increased scaling of laser emitters (which has been previously mentioned), and better energy efficiency of energy storage devices (especially as lasers are not always terribly efficient) will also increase the output of positrons. As for storage of these positively charged particles, I believe two concentric spherical shells with an appropriate dielectric in between them (spherical capacitor) would be sufficient to store them without expending much energy.

CosmicFudgefactor November 10, 2009 at 15:49

To lower the required onboard antimatter fuel I propose researching the interactions of antimatter with Bose Einstein Condensates.

It’s said that BEC in many ways behave as a single superatom due to overlapping wavefunctions etc,

what I would like to know just how far extends this superatrom behaviour?

Could it be possible that if we aim an antimatter particle at a cluster of atoms condensed into a BEC, the antimatter would not only annihilate a matching atom, but kill the whole entangled cluster.

It would be like killing 100.000 birds with one stone and shift the engineering/economical challenge from how much antimatter can you produce affordably to less tasking effort of bringing ordinary matter into BEC and add a grain of salt.

All of this highly speculative, but has anyone tried it?

Positron June 23, 2010 at 10:29

Another way to create antimatter positrons would be to create a positron emitter like sodium-22 or carbon-11 or nitrogen-13 or oxygen-15. The positrons could then be stored in a magnetic trap and reacted with electrons for energy. The only problem is that methods for creating these nuclei that I find tend to be endothermic. One way I can think of creating carbon-11 (I’m not sure if it will work) is nuclear fusion of lithium-6:
Li6 + Li6 -> C11 + n (neutron)
also a proton may be released instead of a neutron, so I’m not sure how efficient it will be, but I looked at the nuclear binding energy of lithium-6 and carbon-11 and it seems that the reaction is exothermic (31.994 MeV per Lithium-6 so 63.989 MeV total for reactants and 73.439 MeV nuclear binding energy for carbon-11 so 9.45 MeV released in the reaction).

Comments on this entry are closed.

{ 2 trackbacks }