Alpha particles

Here's a photograph of a little demonstration cloud chamber:

To the right in the chamber, diagonally across the picture, there's a tungsten welding rod. These rods work better if traces of other substances are added to them and this one has a small amount of thorium oxide. Thorium is a naturally occurring radioactive substance so these welding rods are very slightly radioactive, not enough to endanger the lives of welders - obviously! - but enough to be interesting in the cloud chamber.

Thorium is chemical element number 90. It has chemical symbol Th. The isotope we find in nature is ²³²Th, "thorium-232". From webelements.com we find that its half-life is 14 billion years - it stays radioactive for a long, long time, but at a low level.

Look at the two misty streaks heading left from the welding rod. Each of these is the path of a single alpha particle, a nucleus of the element helium, ejected forcibly from a thorium nucleus when it spontaneously decays. As it travels through the air in the chamber the fast-moving alpha particle knocks electrons out of atoms, leaving a trail of ions behind it. The chamber, cooled well below room temperature, contains alcohol vapour as well as air. Each ion becomes the centre for the formation of a little droplet of liquid alcohol and the path of the alpha particle becomes visible as a string of liquid droplets. The cloud chamber, Nobel-prize winning invention of the Scot CTR Wilson, doesn't quite show us individual electrons and ions but it does let us see where they go and how they move.

Let's do some sums. Alpha-particles emitted in radioactive decay all have more or less the same energy, depending only on which radioactive nucleus we started from. There's a table at this link, telling us that an alpha particle emitted by ²³²Th has an energy of 3.8111 MeV.

The eV - "electron-volt" - is the convenient unit of energy in the subatomic world. We need 14.5 eV of energy to completely liberate an electron from a nitrogen atom and make it into an ion - ionise it; 13.2 eV for an oxygen atom. The average energy of a photon of sunlight is about 2 eV. "M" stands for "mega": one million. So just one of these ²³²Th alpha's will ionise hundreds of thousands of atoms.

(Yes, we call them "alpha's" in a familiar way. Maybe even α's, using the Greek symbol. They're our friends.)

As an α travels away from the nucleus that spawned it, through the air of the laboratory, it loses energy by knocking electrons out of atoms. Each ion created robs the α of roughly 30 eV. How far can it go in air?

The answers to such questions are well known, even available online. From the NIST ASTAR program, available online, we learn that a 4 MeV α stops in a column density of 0.003 grams per square centimetre of air. We need to think about how much material the α meets, and this is best expressed as the amount of material in a cylinder of 1 cm² area. This hypothetical cylinder should include 0.003 grams. If it's very tenuous, like the air we might meet in the stratosphere, the cylinder has to be very long. At sea level where the air is much denser, it will be shorter. But it needs to include 0.003 grams. Air at sea level has a density of just over 0.001 grams per cubic centimetre, so the α will travel (0.003 grams per cm²)÷(0.001 grams per cm³) = 3 cm - slightly over an inch, just as we see in the picture.

In solar flares, ions sometimes get accelerated to energies of MeV and beyond, by strong electric fields, not in radioactive decay. The same ideas apply. How far will they go? How much will they heat up the solar atmosphere in the process? In the cloud chamber we can see much of the relevant physics, even for this remote and exotic application.

Neutrons

One can never read too many books about neutrons so I'm tackling Neutrons, Nuclei and Matter by James Byrne. Neutrons do everything: weak, strong, electromagnetic, and of course gravity. Everybody who remembers high school chemistry knows that the nucleus of the atom is made up of protons and neutrons. Hit a nucleus hard enough, with a proton for instance, and neutrons may be liberated. A neutron on its own is radioactive: unleash a cloud of neutrons and half of them will have disintegrated into other things inside about 10 minutes. Neutrons can penetrate deeply into substances that are opaque even to X-rays and are so useful as probes that the UK government has spent 100s of millions of pounds building the ISIS neutron source (I know - an unfortunate name nowadays).

In solar flares, individual ions are accelerated to high energies. They collide with other ions and produce all sorts of by-products, including neutrons. Some of these escape from the Sun. If we detect them with spacecraft experiments they can tell us about the events of the flare. Even light takes eight minutes to get here from the Sun so the neutrons need to be pretty high speed if they're not going to decay long before they reach Earth. When they decay they glow faintly in X-rays; maybe we'll be able to detect this radiation.

Anyway the prompt for this posting was not so much the wee neutrons themselves as the Preface to Neutrons, Nuclei and Matter, which begins by discussing George Bernard Shaw, looks back into scientific history, references several poets and includes a quote from Shelley, who Byrne is sure "would have understood what many of our legislators and educators appear to have forgotten: that science is concerned first and foremost with revealing the secrets of nature, and scientists have more in common with artists than they have with accountants, politicians or lawyers." What a great start!

Positronium

Science continues to find that homeopathy doesn't work. I know homeopathy has sincere adherents who don't think about scientific evidence in the same way as the rest of us. I admire those people who are constantly in debate with them and I'm glad my own area of science is a long way away from this sort of topic. So I was quite taken aback to come across this account of positronium homeopathy.

Positronium is a sort of an atom but a very exotic one, with only a fleeting existence. Like an atom of hydrogen it consists of two elementary particles with equal and opposite electric charges. Like an atom of hydrogen one of these particles is an electron, familiar (as far as any subatomic particle is "familiar") as a constituent of the atoms around us. Unlike a hydrogen atom the positively charged particle is a positron. A positron has the same mass as an electron but a positive, rather than a negative charge. It is the electron's antimatter counterpart, a sort of evil twin. When an electron and a positron come together they cancel one another out (the word used is "annihilate") and the mass of both particles is converted into energy, in the form of light, according to Einstein's most famous relation E = mc².

Antimatter sounds like something out of science fiction (and one of its most famous occurrences must certainly be in the propulsion unit of the Starship Enterprise). Nonetheless it exists. The first discovery of a positively charged electron was in 1932 by the American scientist Carl Anderson. Now they are put to work every day in our hospitals (know somebody who's had a PET scan? Guess what the "P" in "PET" stands for).

Natural processes produce positrons, as do events in man-made particle accelerators. My own interest focuses on the energetic events of solar flares, in which positrons are sometimes produced. Once they exist, positrons don't hang around for very long. There's usually an electron for them to annihilate with. But before annihilating, for a ten-millionth of a second or less, the positron and the electron orbit one another as an atom of positronium. I like to think of them in a sort of dance, eyeball to eyeball, each recognising in the other its equal, its opposite and its imminent extinction. Finally they annihilate one another, producing a spectrum line in gamma-rays that we can detect and learn from.

Composed only of two very simple objects, the positronium atom gives very precise tests of the quantum theory of electromagnetism, so it has been studied in great detail in the laboratory. It lets us test very precisely elements of our basic understanding of the world.

Matter and anti-matter can both exist but there is more matter than anti-matter. If there were equal amounts of both, they would have annihilated each other very early in the history of the Universe. The Universe would be filled with light but there wouldn't be atoms and molecules i.e. ourselves. So the question, "why is there more matter than anti-matter" is a fundamental question about the Universe.

After all this I'm both sad and a bit cross to discover talk of "positronium homeopathy". In its yin-and-yang components, its brief existence, its annihilation, and the matter-antimatter asymmetry at which it hints, positronium is a very beautiful and exotic aspect of the physical world. There's more than enough real, deep beauty and weirdness there without making up a lazy, unsubstantiated fairy story.

Brazil (3): A solar physicist takes a taxi ride

On the day I arrived a colleague kindly met me at the airport. On a Tuesday he could not bring his car (traffic congestion control measure) so we took a taxi to my new lodgings. I guess the phrase "South American big city taxi ride" will conjure up a particular, hair-raising image. Stereotypes concocted from a great distance so often turn out to be wrong; not this time. I'm sure we travelled at 60 miles an hour through dense traffic in busy streets. Over and over again we cut in front of other cars at what felt like well after the last possible minute to take some sliproad or junction. Our driver's eye and reflexes were fantastic. We'd have been in ten crashes at home but nobody even tooted their horn. Evidently drivers expect such craziness of each other. I said out loud, "this is exciting".

Here's a São Paulo taxi crossing the Avenida Higienópolis not far from Mackenzie University. The camera has frozen it but I expect it was travelling at high speed and dodging from lane to lane. I actually took this photo because I like the name, "Higienópolis". It also applies to the whole area: "hygiene town" and reflects how nice and clean it was, compared to Downtown, when they started expanding the city in this direction. It's still appropriate.

Much later it struck me that the taxi is a nice metaphor for some of the science that triggered my visit. The gas of the Sun's atmosphere is sitting quietly, atoms moving around randomly in all directions, with a range of speeds but none of them going too much above the average speed. Then from somewhere above them comes an electron or an ion at enormous speed, like one of those São Paulo taxis heading from the airport into the city centre. Unlike the taxis it has no skilful driver, only speed, and it just crashes into all the rest of the traffic, sending them flying in all directions with much greater speeds than they had before; heating them up and making them glow in various ways. Exactly why they glow holds our detailed attention. But of course we'd really like to know, "who ordered the taxi?" And why was it moving so quickly?

If a taxi crashes into a car all that happens is car and taxi get mangled and some fragments go flying off in all directions. When we look more closely at the collisions in the Sun's atmosphere things get a bit stranger. Most of the time no actual damage is done. A very fast-moving proton crashes into a slow-moving proton. Afterwards the slow-moving one is going faster than before, because it's had a big dunt, but both protons are still protons. No lumps are knocked off them, both are still intact. But if the incoming proton is going fast enough, things get weirder, a bit like a surreal, Looney Tunes version of such a car-crash. The two protons crash into each other, there's a bit of a blur, but afterwards, different stuff comes out and moves off. It's as though the taxi crashed into the car and then, afterwards, the car shot off in a new direction and not just the original but two new taxis also get spat out the far end of the collision, only different sizes and maybe colours from the original (protons don't have a "colour". But in the "Big book of science" type books I read as a kid they were always drawn as little red billiard balls, red for positive electric charge. Electrons were blue (negative) and neutrons were green.)

Let's stick with the taxi metaphor. A fast red taxi crashes into an identical, slow-moving red car. Afterwards we'd expect a red taxi and a red car to come shooting out of this collision. Instead we get a red taxi, a green car and a funny wee yellow, sort of half-size taxi. Before our eyes, the funny new car travels a short distance then changes into something else different again.

The taxi metaphor's running out of steam. Here's what happens: accelerated to very high energy by an uncertain process, a proton collides with another proton. After the collision some of the energy of the proton has been transformed into a new particle, called a pion. After about one hundred millionth of a second the pion changes into a different, less massive particle called a muon, and finally the muon decays and spawns an electron or a positron (the antimatter counterpart of the electron). The electron or positron emits gamma-rays (like X-rays only even more penetrating) which we detect with instruments above the Earth's atmosphere on satellites. Once we detect those gamma-rays we can follow this sequence of events backwards to learn about the protons that were accelerated in the first place.

My colleagues here in Brazil lead the world at observing very short wavelength radio waves from the Sun. Recently they've begun to explore the wavelengths between radio and infra-red, what is called Terahertz radiation. This is a frontier and they have made some discoveries that we don't completely understand yet. The stories I've described above might be part of understanding Terahertz radiation in solar flares and that's a big part of why I'm in Brazil.

Since arriving I've had other taxi rides that were not nearly as exciting as that first one. Was the driver thinking of proton-proton collisions and pions? That question will probably go unanswered but we can be more optimistic about the reasons that solar flares glow in Terahertz radiation.

Phew

busy, busy weeks. Seven hours of teaching per week, all squeezed into a 26 hour period that starts at 19.30 on a Wednesday. Two more hours will be added soon (I know, from outside academia seven hours may not sound like a lot, but each hour of face to face contact needs more hours of preparation). Also:

• a school pupil spent a week doing work experience with me. Very rewarding for me, I think new and exciting for her
• we had our first Cosmic Way public event, a mix of cosmic ray and subatomic physics, solar-terrestrial interactions, and Scottish science history, hopefully made accessible for all sorts of people. Watch that website, by the way - more to come. Special mention for the music by Drew Mulholland.
• we were visited by Christian Monstein, from ETH Zurich, who helped us to establish a wee radio telescope looking for bursts of radio waves from the Sun. Our telescope uses Christian's Callisto receiver, and contributes its data to the e-Callisto network. Here's our first solar radio burst - doesn't look like much, I know - we'll open it up in other blog postings. Here is Christian's report of its setting-up
• the Centre for Open Studies had its official launch event, a very nice day with taster sessions on many of our subjects (yes, including Astronomy), and with representatives from many of friends in other organisations, like the Astronomical Society of Glasgow
• I took a couple of posters to the Royal Astronomical Society discussion meeting on solar radiophysics, one on our e-Callisto node and the other on the work Aline Dinkelaker and I carried out together, asking: "do solar flares behave like avalanches or cascades?"

And of course all the other jobs that go with the start of the teaching year and with keeping the research going, some just as interesting as those I've listed. Too much blogging - back to work.

Solar flare prediction?

Maybe I lost some friends with the Spice Girls blog. Let's turn to more familar topics.

Solar flares, the subjects of my own research, were in the news recently. A group of scientists in the USA believe they have a new solar flare prediction technique. I thought I'd discuss this new idea here, mostly because it was in the news but also because it's pretty wild! If correct - and that's probably a big "if" - its implications go a long way beyond satellite engineering to genuinely new physics.

A solar flare is an explosion on the Sun, a sudden release of energy from the Sun's magnetic field. Patches of the Sun's surface brighten up briefly, typically for some minutes, and radio waves, X-rays and ultraviolet light are emitted. A big flare is usually accompanied by a cloud of material expelled from the Sun into interplanetary space: a Coronal Mass Ejection (CME). When a CME arrives at Earth it can trigger the sort of magnetic storms that give us beautiful displays of the Northern Lights, but may also shut down power lines and electronic communications. CME's can be a threat to satellites and technologies that rely on them: telecommunications, GPS, etc.

A group of American researchers in Purdue University have announced a possible new method for predicting flares, maybe more than a day before they actually occur. Our technologically sophisticated society is more and more reliant on satellites and telecommunications so there is a lot of interest in flare prediction. So far such forecasts are of the "20% chance of an X-class flare in the next 24 hours" type. You can find examples at e.g. the Solar Influences Data Center in Brussels. It would be extremely useful if there was a technique that could say, "there will be an X-class flare 24 - 28 hours from now".

Professors Jenkins and Fischbach, the Purdue researchers, have been studying radioactive decay. Suppose we have a lump of a radioactive substance, say 1 kg of silicon 32 (which we write ³²Si - not the usual sort of silicon found in bathroom sealants and breast implants, but a radioactive isotope). We switch on a Geiger counter, a radiation detector (for younger readers: here's a great video demonstrating a Geiger counter in action. They were often seen in Cold War era science fiction, e.g. when the scientists in The Thing from Another World find the crashed alien spaceship). Normally the counter goes 'click' roughly 10 times per minute (on average; some minutes it will be 8, or 9, or 11, or 12; more rarely 3, say, or 20; much more rarely 0 or 30; etc.). This represents the normal, 'background' level of radiation, due mostly to tiny quantities of radioactive substance found everywhere. If we bring the counter near to the ³²Si it starts to click more rapidly. This tells us that this substance is radioactive, possibly dangerous if we get too close. We can go further and count the clicks. Then we can calculate how rapidly nuclei of ³²Si change into something else, i.e. decay. Different substances decay more or less rapidly. Some are gone in microseconds, some last for billions of years. ³²Si decays rapidly enough to give reasonable numbers of clicks in the counter, but not so rapidly that it's all gone before the experiment is over.

The rate of decay is a property of the nucleus of the atom of the particular isotope. We believe it doesn't depend on anything else: how hot the stuff is, presence of other substances, magnetic fields.... We don't expect the decay rate ever to vary. We believe it will be the same, for the same substance, everywhere in the Universe. Professors Fischbach and Jenkins find that the rate of decay of certain substances seems to vary during the year. The number of clicks per minute in the Geiger counter (for example) varies very slightly, at about the tenth of a percent level; slightly greater than average in January and slightly less in July. More than one group of scientists has made this discovery so it looks like it's real - but it's definitely unexpected!

Earth's distance from the Sun is not quite constant, varying by about 3% over the course of a year. Fischbach and Jenkins note that the decay rate of ³²Si is greatest when Earth is closest to the Sun: could radioactive decay on Earth be influenced by something to do with the Sun? They go on to speculate that almost massless, subatomic particles called neutrinos might be the means for the Sun to influence radioactivity here on Earth. Vast numbers of neutrinos continually escape from the nuclear furnace of the Sun's deep core. They are detected - with difficulty - here on Earth, allowing us to confirm our ideas of what happens deep inside the Sun. Neutrinos would have to behave in ways we don't presently know about for slight variations in their numbers to have such effects on laboratory radioactivity. So this is either wrong, or extremely interesting!

Maybe the rate of decays is actually constant - as almost all nuclear scientists would expect - and the detectors, rather than the ³²Si itself, behave slightly different in winter and in summer. Some scientists have proposed detailed explanations along these lines. It seems to me that this might be checked by making measurements north and south of the equator. We might expect them to be six months out of phase. I don't know if anybody has done this.

Fischbach and Jenkins go further, speculating that it is solar magnetism - sunspots and flares - that influences radioactive decay rates here on Earth. On a couple of occasions they claim to have seen a decrease in the rate of decays starting 39 hours before a major flare and this is what they think could give an early warning of flares. Suppose the neutrino mechanism is correct. Some neutrinos would indeed be produced in a major flare, at the same time as the gamma-rays that can be detected by spacecraft near Earth. Their numbers would be absolutely tiny, however, compared to the number continually being produced in the Sun's core. A solar flare is a huge event by earthly standards, but it involves a miniscule portion of the Sun's enormous bulk.

(Reuven Ramaty was the guru of nuclear processes in solar flares. I was once at a meeting where Reuven was very scornful of suggestions that neutrino rates on Earth were being influenced by flares, for exactly this reason - not nearly enough neutrinos ).

To make neutrinos we would need energetic ions. In flares, ions do indeed gain high energies and some neutrinos would result. But the same ions would definitely make gamma-rays as well (very energetic, penetrating radiation like X-rays). We would detect this gamma-radiation at the same time as we think the neutrinos are being produced, at the moment with the NASA Fermi Gamma-ray Space Telescope. Anybody can look for gamma-rays before big solar flares with the RHESSI data broswer. They aren't there, at least not easy to see in these measurements. This is even interesting: it lets us rule out some ideas for solar flares that involve accumulating energy in the form of energetic ions before the flare happens.

There are some other comments that might be made about predicting flares a day and a half before they happen, by any means at all. Flares result because the turbulent flow of gas in the Sun's outer layers twists and stresses the magnetic field. This is a noisy process with random elements; I'm not convinced that even big flares are inevitable this far in advance. But there are other people who could comment more expertly then me.

I think the measurements these gentlemen start from are really interesting, because diferent people have obtained similar results at different times. They may be pointing to a matter of detail in how radioactivity is measured, technically interesting but little more. They might be pointing to some aspect of radioactivity, neutrinos etc. that hasn't previously been properly understood. This would be very exciting! I'm very sceptical about the suggested connection to flares and magnetic activity, however. I'll be keeping an eye on this topic to see how it plays out.

May 21: pub!

It's 21 May, a very significant date. In my PhD thesis I analysed X-ray data from three solar flares, on 10 April, 21 May and 5 November 1980. The data had been collected by the Hard X-ray Imaging Spectrometer (HXIS) on the SMM satellite.

At that time we believed that hard (penetrating) X-rays from the Sun would come from the ends of magnetic loops (like those shown in the accompanying picture of the Sun in ultraviolet light), where they meet the dense solar atmosphere. HXIS showed us for the first time where on the Sun the hard X-rays were coming from. Sure enough these three flares displayed X-rays coming from pairs of points on the Sun, presumably the two end points of the loops involved: "footpoints". It would have been a big surprise if X-rays had come from above the surface - although in due course this too was seen. In Glasgow we weren't the first people to look at these footpoint observations but we tried to see what more they could tell us about the workings of flares.

The 21 May flare was one of the most intensively studied at that time. It was a big (X-class) flare, it did lots of different, interesting things and was inspected by several leading edge instruments, like HXIS. So many people wrote articles on aspects of this flare that it eventually played the starring role in its very own review article, where two famous solar astronomers summarised the various studies and drew them all together.

Many individual solar flares have been important in our developing understanding of these events: the first big flare seen in some new instrument, a flare that did something in a particularly simple way so that cause and effect seem clearer than in most cases, a flare that did something dramatic never seen before.

Over the decades there have been many such significant events. This was useful when I was young and frivolous. Today, for example, we could say, "it's the anniversary of the 21 May 1980 flare. We need to celebrate this - let's go to the pub!" In fact, if we scoured the solar physics literature we could probably find a solar flare to celebrate on most days of the calendar, especially now a couple of decades on: 13 January, 20 January, 23 February, 24 May, 3 June, 7 June, 14 July, 28 October....that's enough, you get the point, and I'm sorry so few of them have nice web resources and so many of the links are technical. Maybe that should be a wee job for somebody: "Flare of the day" blog. Anyway we never needed to wait very long to have an excuse for a wee pub visit, and if we really needed an excuse we could probably scour the literature and find somebody with their own wee solar observatory, lost and forgotten in the woods or clinging to some unvisited mountainside in some far-off and exotic land, who had observed a flare on that particular date.

Anyway, nowadays those pub visits are much rarer. We probably only used a famous solar flare as an excuse on a few occasions, to be honest - most of the time we didn't worry about excuses. But as I headed home on the bus this beautiful May day, I spotted lots of people sitting outside enjoying a beer or a glass of wine and I was glad to see that the X-class footpoint flare of 21 May 1980 is still celebrated vigorously.

Image: NASA Solar Dynamics Observatory AIA instrument ultraviolet image of solar loops from January 2012