Antimatter Lecture at UCL

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.

Discovery

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).

Positron Interactions

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.

Positronium Ionisation

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:

Harnessing Positrons

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.

Questions

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.

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Particle Physics Lectures at UCL

I’ve just got back from a particle Physics Masterclass at UCL. Here’s a brief outline of what we did and what I thought.

After a number of technical difficulties involving the projector, the morning programme began. Interestingly, two of the lecturers were using Macs and the lecturer on distributed computing was running Linux with what looked like a GNOME desktop. Sadly for Linux supporters like me, it subsequently crashed apparently owing to the wifi (so cafe wireless at school isn’t that bad after all), after which he either did a really fast XP install or dual booted to XP.

1100 The LHC, ATLAS @ CERN

Dr Mario Campanelli, a researcher at CERN as I understand from his intro, gave us a brief talk on the LHC and the detectors. We got a brief description of the various different particles and a run-down on how precisely aligned the LHC’s parts had to be (0.1mm), leading to its being underground; why singularities produced in it weren’t going to swallow the earth and KILL US ALL (black holes would quickly evaporate in a puff of radiation), and besides cosmic rays hitting the Earth’s atmosphere create such singularities all the time – we’d be long gone by now if those were the Earth-swallowing type of black hole; the setup of the tubes; and a bit on how the detectors work. There was a lot specifically about CERN that I didn’t know before and that hadn’t been mentioned so I think we all found this particularly interesting.

He also said as a sidenote that apparently CERN would have closed over winter anyway owing to electricity costs, so the schedule wasn’t as badly set back by the ‘minor’ ‘meltdown’ (i.e. like 27 Kelvin) as the media make it out to be. Or maybe that’s his CERN researcher pride speaking

1200 Search for neutrinos in Antarctica

We were then told about the tremendous difficulties faced by scientists attempting to find neutrinos. When neutrinos interact with matter they form a cone of Čerenkov radiation consisting of blue light and radio waves projected in the shape of a cone caused (as I understand) by charged particles moving faster than the speed of light in the given medium. The research brought the scientists to the icy region of Antarctica, attempting to detect radio transmissions caused by neutrinos interacting with ice which carries radio waves well. The search went from water to ice to salt as media for neutrinos to interact with, and as yet neutrinos have never been detected except from two occasions: our sun and a supernova in 1987 (or thereabouts).

1230 Distributed Computing

This was more or less about how to process the 5 PB of data emerging from the LHC while in operation. The talk touched on supercomputers, showing us pics of CRAY supercomputers from ye olden dayes and more modern cloud computing centres. The capacity of distributed computing is enormous, as demonstrated by projects such as SETI@HOME and Folding@HOME.

After lunch:

1430 Hands-on

This consisted firstly of looking at simulated data from realistic particle collider experiments. We used Atlantis (software) and data from ATLAS (i.e. looking at particle traces and detector readings and unintelligible graphs of logs of angles against logs of other angles in some crazy units against GeV) and learnt to recognise different types of W and Z particle decays. I personally thought it was quite exciting and certainly eye-opening to be using the same software as researchers at CERN are using to analyse their data. However, realistic as the graphs and charts seemed and authentic-looking as they were, we successfully identified a Higgs Boson trace which the lecturers did not seem at all surprised about. Realistic indeed…

The day concluded with a video conference with some research labs in the US. As with all video conferences, the quality left something to be desired, but it was interesting if a little disheartening to watch the other side rip apart our conclusions from data and ridicule us as inefficient British people! In the end we ended up discussing in some depth differences in education systems between the US and the UK (apparently they start at 7:30 and finish at 2pm but were envious of our almost 2hr lunch breaks) before the sound quality totally disintegrated and nothing was left but an IRC channel!

Overall, I certainly got something out of the day. Although we didn’t really discover all that much new in terms of the theory behind particle Physics thanks to fairly thorough AS teaching, there was a lot I learnt about the practical side of particle colliders and detectors. More importantly, lunch was quite sublime (surprisingly so for pub food).

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Physics Lectures at the IoE

On Wednesday I was shipped off along with the rest of the L8 Physics population to a series of Physics lectures. There were five lectures, some of which caused some controversy among us. Here’s my take on matters:

The first talk was on avalanches. In short, the lecturer gave a quick run through of how avalanches occur, introducing concepts of fluid dynamics and elucidating key points about avalanches, including the different ‘flavours’ on offer (as he described it) and various methods of protecting against them. I personally think this was quite a good talk – his explanations of how avalanches form were clear and accessible to anyone who happened to be listening. There were some breathtaking shots of scenery and amazing clips of avalanches in full flow, and I really felt interested in what he had to say. His exposition of defence mechanisms against avalanches were a nice resolution – the case study of Iceland was well chosen and I enjoyed being led through the story of how various attempts to divert avalanches failed and the (sometimes) ingenious systems which were eventually adopted. He even showed us a clip of a simulation of an avalanche that used ping pong balls which looked a great deal of fun to conduct! Some of my peers dismissed it as a distinctly unscientific lecture, but my opinion is that it was more a discussion of the engineering problems faced by inhabitants of avalanche-prone areas and an interesting supplementary brief of the different types of avalanche as a means to better understanding of possible defence systems rather than an attempt to get an audience to understand the Bernoulli Principle.

We were then treated to almost an hour of someone talking about dark matter. Well, someone talking about general cosmology. Well, someone chatting about general cosmology, with a mention of dark matter at the end. My credentials entitle me to absolutely no authority whatsoever to comment on this lecture: the greatest height I’ve achieved in terms of credibility is becoming the Physics editor of the Science magazine at St Paul’s – an impressive-sounding title, but ultimately one of little significance. In essence, I’m a layman commentator; however I felt the lecturer failed to do such a promising topic justice. Dark matter is a hugely exciting topic – so much theory goes into it and it’s a conjecture which springs from measurements which seem to fly in the face of believable science – it’s on the border between Physics and fantasy and a controversial subject. However I felt the style in which the talk was delivered was contrived in such a way that those who already knew and understood the stuff the lecturer was talking about ended up bored since explanations and mathematics were distinctly lacking, while those who didn’t know much about this area of science got confused as almost none of the assertions were justified or explained in much detail and much knowledge and understanding was incorrectly assumed. It was almost like one of those horrific ’science appreciation’ lectures which I so abhor and I felt the topic was betrayed by such an exposition. Perhaps the subject was ill-chosen for such an audience – a lecture on dark matter and dark energy pitched at AS-level students is doomed from the start: none of the interesting mathematics is yet accessible and everything falls apart and becomes a jumble of words.

I was horrified in the following lecture to discover that at least one member of the audience couldn’t tell the time and another didn’t know the speed of light, evidence to support my point about dark energy being a poorly chosen topic for the audience. This next lecture was about time travel and this time, despite the apparently difficult subject matter, the topic was unravelled very skilfully with appropriately chosen and utilised equations. I particularly enjoyed the way the lecturer managed to get the most rowdy member of the audience to volunteer to sit inside a (wire and fabric?) Tardis for five minutes and subsequently declare his experience ‘impressive’ – skilful manipulation on the part of the lecturer and congratulations to her. Again the now *very* familiar Lorentz (I’ll get it right this time) factor appeared (though tragically obscured by the Tardis) to explain forwards travel in time relative to a stationary object. Some of the reasoning was still circular, for example defining the limit of the speed of light as, well, the limit of the speed of light, although to be fair, with the level of mathematics available to members of an audience who cannot tell the time, the Lorentz factor (the reciprocal of a radical, oh horror) was probably already too great a stretch. But overall I thought this lecture was a well thought-out summary of the concepts of time travel and the paradoxes involved (both Grandfather and twins).

After a Tesco lunch (I’m not an M&S snob) we were all made to feel stupid by an engineer demonstrating to us why our brains are rubbish and why we should think laterally. After getting every single question he posed wrong (fine, I exaggerate), I listened intently as he explained the fundamentals of lateral thinking. Lateral thinking is something which is unfortunately not taught or even encouraged at any level in the National Curriculum. Exam questions invariably involve some standard procedure, and if any other method is employed, the poor GCSE / AS / A2 marker will just get confused, have a good head bash and give you zero for that question. I actually distinctly remember being told in English lessons not to try to write something original in the exam – ‘by all means be creative when doing homeworks but just do something standard that ticks all the boxes for the exam’. I feel at this point inclined to thank one of the many variants of the imaginary creator of the universe to get planning permissions for a school as good as mine – I’d be willing to bet that schools which aim for their pupils to attain a C at GCSE level produce very linearly thinking students, so to speak.

The final lecture was by far the most entertaining – a pseudo-magician taking the piss out of real magicians and abusing the sciences, specifically a Van der Graaf generator, a Wimshurst (sp?) machine, a whip and a Barbie doll. What more needs saying?

Overall I felt it a highly stimulating series of lectures. Almost every discipline associated with Physics was covered, from engineering to cosmology to relativity to logical thinking. Although I suspect few people actually learnt anything particularly new and deep from any of the lectures (except perhaps the lateral thinking one), every lecturer had something new and interesting to say, and it was a great way to spend the middle of a week.

Large Hadron Collider at CERN flicks on

Today (10 Sept 2008) the Large Hadron Collider at CERN starts shooting a high-speed high-energy beam of protons around its 27 kilometres of high-tech, computer-controlled, 3.8m-wide, £5bn magnetic tubing after months of preparation, years of building and decades of planning. It is undoubtedly ‘an historic moment’ for science: it is the epitome of the progress of science, engineering and organisation that man has made over the centuries, and represents a huge step forwards for mankind.

If all goes well, it is hoped the collider will uncover the elusive Higgs Boson, dubbed informally ‘the God Particle’, the particle which gives mass to other particles. It is thought to give gauge bosons like the W and Z particles (responsible for the weak nuclear force) mass but not photons (EM radiation); the finding of this particle would unfortunately lose Stephen Hawking a bet of $100. The Elegant Universe also mentioned the possibility of the discovery of the graviton (a closed string free to move between branes), the thus far theoretical gauge particle of gravity, and indeed other particles, including ones providing evidence for supersymmetry, predicted by String Theory (another huge theory which may be proven or broken by the findings of the LHC).

If all goes wrong, according to the interesting but incorrect media-hyped grunts of pseudo-scientists, the Earth will be sucked into a black hole and/or be turned into a smouldering mass of ’strange’ (or is it ‘charmed’?) particles. I have lamented this issue before, and now the media have yet again taken the word of a qualification-less laymen and amplified it to the point that the fallacious argument resounds more strongly than the truth. Presumably an ignorant yet arrogant individual read somewhere that singularities might be produced in high-energy particle collisions at CERN. Presumably (s)he failed to read the next paragraph, which probably explained that such singularities would be so devoid of mass that they would evaporate almost instantly releasing comparatively innocuous particles in the process, and ended up writing a letter to the real scientists, who were almost certainly working hard to progress human understanding ever further beyond apparent boundaries and obstacles. The letter warned the scientists that ‘if the world ends [the pseudo-scientists] will kill them’. I think this sample of their intellect leaves not much more to be said.

Some wonder whether the LHC is just a waste of money. My response couldn’t be put better than Stephen Hawking’s: ‘Both the LHC and the space programme are vital if the human race is not to stultify, and eventually die out. Together they cost less than one tenth of a percent of world GDP. If the human race cannot afford that, it doesn’t deserve the epithet ‘human”.

So I hope that, whatever obstacles fly in the face of this audacious, ambitious and hugely important project, results will be published over the Winter, and great progress is made towards humans’ ultimate understanding of the universe – even if it means proving String Theory! As Stephen Hawking put it, ‘Whatever the LHC finds, or fails to find, the results will tell us a lot about the structure of the Universe’.

Stephen Hawking references taken from a BBC News Article