On Friday (24 April) I attended an open lecture at UCL on antimatter (“Antimatter in the Laboratory and beyond”). The lecturer was Dr Dan Murtagh from the Department of Physics and Astronomy, UCL, who is currently working on antimatter. The talk was split up into several main areas: the prediction of antimatter, creating, harnessing and detecting it, and applications for it, both current and future. My memory is poor and I only wrote down bits that I thought were interesting / useful to me, so I shan’t attempt to give a comprehensive summary; rather I’ll just put into a coherent and legible form my scribbles.
Antimatter was originally proposed by Paul Dirac as it came out of his equations. Originally he thought the antimatter equivalent of an electron would be like a proton and therefore have the same mass; eventually the positron was discovered by someone who noticed what seemed like a positively charged electron in his bubble chamber (a rudimentary particle detector). Thus was antimatter born.
Detection of Positrons
Positrons are created from Sodium-22, an artificial isotope which for some reason is only produced in one place in the world (somewhere in South Africa), and is also in relatively high demand owing to increased research on it. The situation appears to have some money-making potential… But anyway, when the positron stream is created, the positrons have mostly far too high an energy to be useful. So, like in nuclear power plants, a moderator is used in an attempt to slow down the positrons.
Moderators (mostly made out of tungsten) tend to have several problems. Positrons go into the metal and interact with matter, presumably mostly via EM forces, and can end up doing one of four things: they can end up coming back out the way they came as ‘thermal positrons’ (whatever they are; not very useful); they might get stuck in a hole where a tungsten ion is absent and end up annihilating, creating gamma photons (useless); they could come out the other side and somehow have gained energy or not lost enough (useless) or they might lose sufficient energy to be useful in most experiments (useful). Once they come out the other side they follow a complicated path through some apparatus as illustrated, guided by magnetic forces in a very similar way to the LHC.
(click to enlarge)
Essentially the positrons get channelled through various filters to clean up the beam (most of the process consists of getting rid of electrons) which itself ends up in a gas cell where various detection instruments (ion detectors, positron detectors and photonmultiplier tubes).
Positrons interact with normal matter in three different ways:
Positron Impact Annihilation
In other words:
This is however a rare occurrence – it requires a positron to occupy the exact same position as an electron and the probability of this happening makes annihilation almost negligible.
In other words:
A positronium is essentially a Hydrogen atom in which the proton is replaced with a positron – it’s an electron-positron pair. It has an average life of 143ns before the pair annihilates. Fortunately (from what I inferred) most positronium ‘atoms’ are moving fast enough to be relativistic, so scientists have just about enough time experiment with them, and, as discussed later, even in the Ps’ frame of reference, 143ns is enough time to do chemistry.
Speaking of atoms, there is some research called ATHENA taking place at CERN to attempt to create anti-hydrogen: an anti-proton/positron pair, an experiment which was discussed to some depth in the questions and which made another appearance when the lecturer was discussing traps.
The third interaction is impact ionisation, when the positron knocks the electron out of orbit:
As an aside, I was discussing the possibility of work experience at Imperial with a reader in Quantum Optics, and he showed me round the lab. As it turns out, his PhD students were/are also working on ion traps, though instead of with antimatter with entangled ions.
Anyway, onto positron traps. As it turns out, the method for trapping them involves cylinders held at different voltages. As shown in the diagram, the trap consists of a series of cylinders laid end-to-end, held at different voltages. The positrons are repelled by the walls of the cylinders by different amounts. Since E = QV, the positrons will be at lowest energy (preferred) when the cylinder voltage is 1V. An energy diagram is shown below the schematic showing clearly how positrons get trapped. The voltages shown are arbitrary and are there just to give an idea…
These traps are very effective but unfortunately occupy a substantial amount of room, thus are unfeasible for antimatter storage.
Returning to ATHENA, their method of creating anti-Hydrogen consists of using ion traps. Using a potential diagram, the idea is to trap anti-protons and anti-positrons in the same place. Positrons chase low potential while anti-protons chase high potential.
So positrons are being pulled up, trapped by the underside of the potential curve, while anti-protons are being pulled down, trapped by the top of the curve. Eventually the two types of particles interact to form anti-hydrogen. Unfortunately the anti-atoms produced are higly excited and since they are neutral, cannot be trapped by electro-magnetic means and end up hitting the side of the apparatus where they annihilate.
Uses of Antimatter
PET scanners (Positron Emission Tomography) bombard the subject with positrons (essentially like beta-plus radioactivity) and when annihilation takes place, the photons can be detected with gamma-ray detectors. It’s apparently used for detecting cancer and beta-plus emitters can be bonded to sugars which is also apparently useful. This strikes me as a somewhat dangerous procedure – bombarding a patient’s brain with beta-plus ionising radiation which itself produces enough gamma radiation to be imaged.
A futuristic and probably impractical use involves using antimatter as rocket fuel, something NASA are working on. And rather than producing antimatter, the rockets would need an antimatter harvester in the form of a massive satellite with rings of 30Km diameter positioned near Saturn’s rings which uses strong magnetic fields to gather antimatter. It all sounds rather … unlikely.
Anti-proton production involves colliding protons (a 2GeV beam) with metal which results in pair production, producing a proton/anti-proton pair.
I wondered how antimatter is supposed to interact with gravity. What I thought (as a sort of wild guess) was that if the Feynman model of antimatter as matter going backwards in time is correct, antimatter should attract itself going backwards in time, thus be observed by us as repelling itself going forwards in time. There’s also a theory that it doesn’t interact at all. As it turns out, a research group at CERN fired a (very long) anti-proton beam and found antimatter does indeed fall towards the earth; antimatter is attracted by matter.
I had read in the New Scientist about how lasers work – the interaction of electrons with holes and subsequent production of positronium which ends up annihilating if a photon passes releasing a coherent photon (stimulated emission). Apparently some scientists have managed to stabilise that Ps, turning it into ‘excitons’ which have a much longer life. I asked if this may be a potential form of storage. Since positronium is like an atom, it has a first ionisation energy (of about 6.8eV) which means if it were possible to somehow store Ps it would probably be feasible as positronium storage.
There were quite a lot of questions and it all ended up as a big discussion about ‘anti-chemistry’ (a research group at Riverside, CA are investigating that) and the possibility of antimatter galaxies.
All in all, the talk was, to me at least, highly interesting and thought-provoking. My friend had advised me that most of the actual talks, since they are public and pre-university students are in the audience, tend to be relatively non-mathematical and simple (and I was horrified at first when he declared E=mc2 was to be the only equation in the talk); but I felt that the talk itself probed the subject quite deeply and the experimental side was very new to most of us. And of course, the questions were an invaluable part of the experience.