Quantum Diaries

Hello again everyone. It has been an incredibly long time (on blog-world timescales) since my last post, but I am glad to say I am back and plan on stepping up the contributions again. Why have I been absent? Well let me recap….

I think I had mentioned that the DRAGON Group at TRIUMF were going to be running an exciting radioactive beam experiment at the beginning of the summer, one that I was looking forward to describing to you all. Well, as all the experimental physicists here will know, things don’t always go to plan (think LHC magnets) and yes, we had our very own failure of a crucial piece of hardware: the TRIUMF ISOL target that creates the radioactive beam. Although our problem was very small in scope compared to the LHC magnet crisis, it still has the effect for us of canceling weeks or even months of beamtime, depending on the particular circumstances.  ISOL targets are extremely complex combinations of high-energy particle beams, hot atom radiochemistry, and black magic! Accepting that every now and then the target will fail is part of the job. After all, producing beams that no-one else in the world can produce is a complicated and touchy business. 

In our case, all was not lost. While the lab rearranged its schedule quickly to replace the target with another that would service other radioactive beam experiments (different targets for different beams), we ran our ‘plan B’ experiment: using a very high intensity oxygen-17 beam using our newly-commitioned ‘Supernanogan’ offline ion source. We measured the fusion of oxygen-17 with helium, in an experiment destined to investigate the synthesis of heavy elements in rotating massive stars very early in the universe. The experiment was successful and analysis is underway.  

Then, I vanished into thin air on my annual holiday to Andalucia in Spain, where I found it extremely difficult to get internet access. Ultimately this is a good thing, you see, physicists are by nature (experimental physicists at least) very comfortable with technology and more than able to work remotely, whether it be on a beach, in the pub, in an airport,……its true. However, good internet access is key, as much of the work is in the form of discussion between collaborators, checking data, sending files, etc etc. Therefore many physicists find themselves working on holiday, something their partners (unless also physicists) probably loathe. This time in Spain I was stuck – no internet access and when I did have it it was touchy at best. So I had a lovely holiday instead, and forgot all about physics…or almost….

Andalucia has a rich scientific history, something that the western world is not nearly enough aware of. While I was there I started thinking about books I had read about the Moorish conquest of Spain in the year 711 CE, and reading some of my Father’s books on the subject. On returning from Spain I did some internet research and found a great documentary by the BBC on ‘Science and Islam’, which touches on some of the history of Andalucia. The documentary was narrated by a friend and colleague Prof. Jim Al-Khalili, from the University of Surrey. It is a fantastic documentary and I urge everyone to see it. I plan to make some posts (with pictures) on Andalucia at a later date and discuss some of the interesting scientific history it has given us. Jim Al-Khalili has also made a series called ‘Atom’, which very nicely lays down the history of atomic and nuclear physics and the beginnings of particle physics, starting from the first real experimental evidence of ‘atoms’: Brownian motion, and Einstein’s interpretation of it.

This series also got me interested because it contains stories of the often hot-headed personalities in physics and their battles with the establishment or each other. For example, I did not fully appreciate that Feynman Diagrams, something ubiquitously used in particle physics today, were zealously attacked when a young Richard Feynman introduced them and the methodology associated with them to a galaxy of physics stars in 1948 including Bohr, Dirac and Oppenheimer. Spurred on by this story I’m delving back into QED, something I have not touched since my university days. I’m starting with Feynman’s 1949 paper in Physical Review entitled ‘Space-Time Approach to Quantum Electrodynamics’. Very interesting indeed when you put it in its historical context.

I had a fascinating thought (to me anyway) today, which is more in the realms of science fiction than real science, but nevertheless entertaining.

It concerns the possibility of advanced, intelligent life, elsewhere in the Universe. Back in the early days of radio astronomy, and later with the SETI program, the consensus was that any alien civilization worth their salt would use radio signals to attempt to communicate with some unknown other civilization like ourselves, out there in the void. After all, radio signals are relatively easy to generate, albeit requiring some reasonably advanced technology to generate signals powerful enough to reach far off places with enough detectability for our perhaps primitive instruments.

With some excitement concerning periodic cosmic radio signals leading to the discovery of exotic stellar objects like pulsars, rather than the ‘Hello World’ of some advanced Galactic neighbor, the fascination with the search for extraterrestial intelligence using radio waves seems to have diminished a little.

My thought was as follows. Let’s suppose that a civilization much more advanced than ourselves has resolved to start trying to make contact with other civilizations. But let us suppose also that with this advancement comes a certain amount of inherit wisdom and the benefit of hindsight as to how other civilizations, perhaps less advanced than themselves, might be developing. Perhaps they know then, first of all, that such a civilization might still be embroiled in a culture fractured by religious schisms, war and instability, in which case they would be reluctant to assume contact should their sudden appearance cause shock and upheaval, potentially leading to the destruction of that civilization or at least causing one great religious war – maybe because they themselves almost reached that level of self-destruction at one stage in their development due to religious conflict. So, then, not to contact us too early, but how to know?

Maybe they also know, that the detection of radio waves is not necessarily the best sign of technological advancement. After all, radio waves were already discovered, studied, adapted and in use well before the age of computers, atomic weapons, the understanding of particle and nuclear physics. They might also know that at some point after the discovery and manipulation of radio waves our civilization would develop atomic then thermonuclear weapons, arriving at the stage where we have the ability to completely destroy the inhabitability of our own planet (having been there themselves), and would necessarily go through a century or so of equilibrating until the world had finally reached a peaceful and balanced enough stage to cope with signals from extraterrestial life.

Again, pure speculation, but they might also predict that we would have environmental problems due to the onset of the petrochemical age, the successor of an industrial revolution, that would go hand-in-hand with some of the developments needed for a nuclear age (assuming of course that these neighbors were also inhabitants of a class M planet once abundant in carbon-based fossil fuels). 

Detecting our ability in advanced space travel could be one criterion for contact, such as the buried monolith on the Moon in Arthur C Clarke’s 2001 A Space Odyssey was meant to signal to the aliens who placed it there. But the aliens would have of course, have had to have been here. 

So they scratch their heads (if they have them) and say ‘what signals could this civilization not detect and interpret until they are past the nuclear age, and at the stage where they have built up a physics-based model of the universe which is globally accepted by their population, such that they would be funded to build large enough projects to detect them?’

What about a beam of neutrinos, notoriously difficult to detect, requiring advanced knowledge of nuclear and particle physics, flavor-changing, industrial know-how for the huge-scale detectors? This is an option, however just like they would know that we knew about the existence of pulsars and AGN things that produce localized and periodic radio bursts, maybe they also know that we know about supernovae and other events that produce neutrinos. The same could be said of gamma rays, as we live in a constant swarm of them, bursts also coming from ‘natural’ events that we’re just discovering.

The one type of thing that might work, are ultra high-energy cosmic rays, such as those recently discovered (well, 1962) and now being studied by the Pierre Auger Observatory in Argentina, some of them seemingly from a localized source (as far as I have read). They would know that we know about the GZK cut-off. They would know that we would know the immense power required to accelerate these charged particles. Assuming that they themselves could build something powerful enough to accelerate particles to the 5 x 10¹⁰ GeV needed to exceed the GZK cutoff (but essentially giving them a ‘sphere of influence’), then these signals might be a great choice. This is pure science fiction of course, but it is interesting to entertain these suggestions. Building an accelerator some 7 million times more powerful than the LHC is no mean feat! (although the first accelerated beam nuclear physics experiment was performed on an accelerator some 8 million times less energetic than the LHC). However, if one day we actually harness some form of abundant energy, and eventually gain that capability, maybe a future Earth SETI program might involve firing ultra high-energy cosmic rays at class M planets and seeing if our ‘service is returned’!

Of course, the conventional hypotheses for the sources of these UHECRs include objects such as hypernovae, active galactic nuclei with supermassive black holes, or gamma ray bursts. However the alien explanation would make for a great sequel to Sagan’s Contact…..

It’s Saturday morning and I’m enjoying a well-earned but short-lived break, sipping coffee, Philip Glass on the stereo, about to head out to gym and some shops. 

This week has been hectic with the start of Radioactive Ion Beam (RIB) delivery. I am currently working with the TUDA Group from the UK here at TRIUMF taking some data on elastic scattering of fluorine-18 on protons, in inverse kinematics, using the ISAC-II superconducting accelerator. Everything on our end is working beautifully. The entire TUDA electronics and data acquisition system, consisting of large racks with power supplies, screens, cables and computers, was moved from its usual home in ISAC-I to ISAC-II, along with TUDA itself, a large vacuum chamber accommodating several highly-segmented high-precision silicon detectors. The move went well, and the calibration data we have taken with oxygen-18 looks great! 

Life isn’t always kind however. Turns out that the fluorine-18 beam is not reaching the intensities required for the experiment due to some spurious problems with the production target. Anyone who has ever worked with an ISOL (Isotope Separation On-Line) system will tell you how finicky the production targets can be sometimes. Extracting high-intensity beams is a tricky art, and that is what makes certain facilities around  the world including TRIUMF unique – their ability to produce certain  high-intensity RIBS that cannot be made anywhere else, and therefore the only place to do experiments with them. 

Although TUDA will still take valuable data, it is not at the intensity hoped for, which basically would mean the team will have to come back at a later date. All data is good data, however. We tend to be overly ambitious when requesting beamtime and asking for more than we need to produce something publishable. Always striving for the highest quality data that pushes the technology to the limits.  What is certain however, is that there definitely is not enough intensity to do the DRAGON fluorine-18 experiment, which nominally requires two orders of magnitude higher beam intensity than an elastic scattering experiment. So we have been thinking up contingency plans and what we might do is switch to an already-scheduled stabled beam experiment using oxygen-17, which is just as exciting an experiment. Then once the targets are behaving like they usually do again, we will run fluorine-18. 

I will post some pictures of the TUDA system anyway, and the operations center etc. It is an impressive setup that always produces great data. I am biased however, as my thesis project was in part commissioning TUDA with it’s first radioactive beam experiment way back in 2001 😉

On another note, I am currently reading a very entertaining book by Bill Bryson called ‘A Short History of Nearly Everything’, chronicling  science’s greatest celebrities-who-never-were, faux pas, grudges, failures, tragedies, as well as of course the stuff we all know about, the Einsteins and the Newtons. It also takes a look into scientific culture and attempts to characterize who ‘scientists’ are, all with Bryson’s unique and entertaining style. One thing that I noticed in the book however, was his incredulity, and his attempt to induce incredulity in the reader, at the great expense of scientific endeavor, illustrated by examples such as the great Superconducting Super Collider debacle, upgrades to Fermilab etc, citing the ‘billions’ of dollar costs of construction of these places as well as the ‘hundreds of millions’ of dollars for upkeep. This makes me think back to when the LHC had its little ‘glitch’ last year, and the scores of ignorant comments from certain users on the BBC news website exclaiming what a great waste of money the project is, with its ‘5 billion’ price tag.

I despair at how people cannot put this all into its correct context. A grand physics project like the LHC will take a large amount of money (to you and me) to construct, and run. However, this money does not go into a proverbial ‘black hole’ like the mini- ones the cranks think we’re all going to create. This money goes to, amongst other things, the salaries and therefore kid’s educations, family lives of thousands of scientists, technicians, administrators, cleaners, janitors,… (and not even what would be considered a ‘high salary’ by any means in the corporate world, in fact pretty paltry on that scale, even for the scientists). It also goes into contracts for local and international commercial enterprises to build and install equipment: welders, circuit manufacture, cabling, plumbing, electricians, construction, suppliers of all kinds. It also goes into education. By assembling some of the brightest minds in the world into these grand projects, and with money available for ‘communications and outreach’, children from all over the world are given the opportunity to see first hand the cutting edge of scientific enquiry and learn from the experts in the field, student bursaries are made available, the fostering of the next generation of great scientists is nurtured. And all this….as well as finding out the fundamental secrets of how our universe works, akin to first discovering how gravity works, or discovering radioactivity, or the tons of spin-off developments that come from the cutting edge and trickle down into society like, amongst others: cancer treatment, MRI and PET scans, GPS, CDs and DVDs, iPhones, the World-Wide Web,…..

So you see, a huge economy and advancement of knowledge and technology, and a large human community is supported by this money. A few billion dollars to start with and some hundreds of millions to keep going. Lets contrast that with some other figures.

1. $700 billion dollars in bail out funds to US banks. Much of that squandered by executives and the CEOs, not a lot of it going to the lowly office staff who have families to support that were laid-off in cost-cutting exercises.  
2. Over $500 billion dollars for the Iraq War, not counting the human cost of all those who died on all sides.
3. $7 billion dollars. Amount of insurance claimed by owner of the Twin Towers after 9/11. 
4. $135 billion dollars. Equivalent cost of Apollo Program.

So we can see, that the cost of something like the LHC, or a modest enterprise like TRIUMF, is really nothing, compared to the costs listed above, which (excluding Apollo Missions) have questionable value at best. The value for money of scientific enquiry can be illustrated by the success of the Hubble Space Telescope. Sure it cost a hell of a lot of money, but it continues to exponentially advance our knowledge of our Universe in ways Einstein and Newton could only have dreamed of. 

To answer the criticism of one naysayer on the BBC comments page regarding the LHC, that the money could be better spent helping the poor: Fine, we should be giving money to the poor ANYWAY, but an extra 1 dollar for every poor person on the planet, while maybe making a bit of a difference for a day or two, would be more destructive coming out of the budget of the largest scientific experiment ever attempted than from say, the salaries of the top-paid 1000 executives of the richest corporations, or by shaving a tiny amount of all the superpowers’ military budgets. 

To answer another criticism, ‘what good the LHC does for someone like me right now?’: stop being so selfish, the science performed now, apart from giving us knowledge about our very existence, will bear fruit for our future generations, and while it might not make petrol cheaper for you right now or enable you to have a cheaper iPod or a bigger TV or a voice activated channel changer so you don’t have to get your lazy butt up from that reclining seat, it may pave the way for realistic human interplanetary space travel, new energy technology, new cures for disease, new ways to enhance our lives and our place in the cosmos as well as have a meaningful and self-aware existence in it.

And to that, I sign off 🙂

ISAC Control Centre

TUDA Electronics and Chamber

TUDA and beamline

Data!

TIGRESS Array with part of SHARC chamber at centre

Sometimes, living and working abroad to follow your career passion has its difficulties, specifically concerning the people you leave behind. Of course you make new friends and associates, but your old friends and your family are the constants in your life that make it hard to be afar from. My Mother and sister live 7,000 km from me, and 8 time zones away in Edinburgh, while my Father lives even further in southern Spain and 9 time zones away. My greatest friends also all still live in Edinburgh. This makes it difficult to see everyone as often as I would like, as I very often have to make a choice between the UK or Spain each year, whether to see my Father or my Mother and Sister. Of course nowadays with Skype and other such devices, it is easy to talk face-to-face on a regular basis, and in fact I probably have more talk time with my parents than I did when I lived in the same city as them. However there is no freedom to have the kind of spontaneous family and friend activities that  one likes. 

So this year I am in luck. It just so happens that I will take a vacation to Spain in June for two weeks to stay at my Father’s house, where my Sister will come down to join us. This will be the first time I have seen my Father in the flesh for two years. Also, a few days later one of my good oldest friends from Scotland will join us for his vacation. It doesn’t end there. My Mother just happens to be vacationing in the North of Spain for a week at the same time, so I will also get to see her for a couple of days! There’s more: my friend from Jordan will be in Spain at the same time and would like to join us also. I feel an embarrassment of riches all of a sudden. So long have I felt that I don’t see all of the people I miss enough, and they are all going to be packed into the same two weeks! Amazing!

I may also be extending my stay to work on some particle detector tests in Madrid, with a group of collaborators, if they manage to book beamtime at the small accelerator there, so I could very well end up staying for up to 30 days in Spain, half vacationing, half working. I am very privileged to work in a job which is international by its nature and requirements. Physicists need to travel all over the world as members of collaborations and groups all working towards a common goal. In fact we travel less than the typical business traveler, but we may go to a more diverse range of countries to attend meetings and conferences where we share the latest knowledge, hatch plans to improve existing experiments or come up with novel ones. The internationality of science is what makes it tick – everyone brings different skills and ideas to the table. Governments pledge funding for different aspects of projects. The LHC is a good example, as is TRIUMF, where we receive scores of international scientists visiting us each year to help advance our knowledge in certain areas of physics. 

All this said, I do enjoy travel not connected to work, where I can dissociate myself from work for at least….oh a few days, and enjoy the warm pleasantness of the country where I was born. I am very much looking forward to it.  🙂

It’s been a while since my last post as it’s all go here at the lab right now in preparation for a busy month of running.
Desperately trying to finish off the analysis of our last radioactive beam fusion experiment (the fusion of magnesium-23 with hydrogen in novae), I’m also in preps for three other experiments coming up imminently.

The first, a test experiment, starts on monday when we intend to implant stable aluminium into very thin carbon targets to prove that we can eventually use this method to create radioactive aluminium targets. This is a fairly trivial exercise but we’re short-staffed to do this simple job so it will be a busy few days.
Soon after, TRIUMF’s ISOL system will start up with a silicon-carbide production target and a ‘FEBIAD’ (Forced Electron Beam Ion-Arc Discharge) ion source to produce an intense radioactive fluorine-18 beam for two astrophysics experiments. The first of these is an elastic scattering experiment designed to probe quantum energy levels in neon-19 (which is what is made when fluorine-18 fuses with hydrogen) of astrophysical interest. The second is a DRAGON experiment where we will measure directly the fusion of fluorine-18 with hydrogen, and experiment that has not been done before due to lack of an intense enough beam.
So expect radio silence from me at least until mid-next week, but then I shall be blogging about the daily activities of the fluorine-18 experiments and you will hopefully get an insight into the drama, banality, trials, fortunes, etc…of a nuclear physics experiment! 🙂

I am what you might call an ‘Eco-back seat driver’, in that I have admirable ideals and intentions when it comes to looking after the environment but maybe don’t do as much as I really really could if I just put a little more effort into it. There are others in my life who are paragons in that way – they lead by example and really cast allusions about the way things might be if everybody acted like them. One thing I do like to do is brainstorm – to throw ideas around about ways we can solve the energy crisis, pollution, reign in the corporate obliviousness to the degradation of the environment and usher in a new era of rational responsibility – but sometimes, given the wasteful and distracting but unavoidably political nature of the subject, I often meet my critics who come from ‘the other side of the fence’, if indeed there is one. 

One thing I always say, however, to those people who may argue on points such as humankind’s impact on the atmosphere and the correlations with global temperature rises etc, is the following: Regardless of how much greenhouse gas we put into the atmosphere, and how many new non-fossil sources of energy (excluding renewables) we turn to to wean us away from our Middle Eastern (and Canadian, South American,..) crude addiction  (I talk about ‘the West’ here), we still use too much energy in the developed world per capita, and there is no excuse because we have a fantastic range of technologies now that can help us to tighten our energy budget and reduce the impact of each human on their surroundings. 

So I was happy to be partially vindicated in the form of a New Scientist article by Astrophysicist Eric Chaisson in which he argues that the impact caused by our energy usage (and wastage) alone, is enough to cause a large and serious part of the environmental problem that we need to solve in addition to reducing greenhouse gases. 

We have to think about the really wasteful parts of our lives. We now have the technology to build and retrofit homes with smart materials that optimize insulation and utilize the houses natural environment to help conserve energy. It would be possible to run an entire home’s energy supply under a smart computer monitoring system that switches off lights when no-one is in the room, keeps heat confined to the most used areas, stores excess energy and new energy generated by solar or wind on the building (this has been proven to work in inner cities as well using the huge wind currents that are generated on the upside of tall buildings). If only we could all fit our homes this way. No more incandescent light bulbs – there really is no excuse to continue to use Edison’s originally great but extremely wasteful invention from the 19th century (giving Coolidge’s later addition of Tungsten filament credit of course), regardless of how ‘distasteful’ some people might find the spectrum of light emitted from energy efficient bulbs. With new LED technology whole rooms can be lit on a fraction of the cost of normal bulbs. 

The problem is the initial cost. Much as we would like to modify our homes to be the tightest, most environmentally friendly abodes around, we cannot afford it. So how to solve the problem. Well, here’s a crazy idea: How about the government and the power companies put out a massive effort to fit all Canadian (this applies to other countries too) homes with smart energy-saving systems in the next 15 years. The money can be loaned to the homeowner, who will still have the choice of a competitive market of devices and systems. These systems will be built to last. The government and companies will earn their money back in the following way – by simply continuously raising the price of electricity as the usage drops as more homes are converted (different price for different houses in their stages of development). Thus the owner sees no reduction in their bill, but is using perhaps one third of the power they usually do, and therefore can have a healthier conscience about the planet! The strain on the national grid will decrease as well as demand drops. Anyway, this was a crazy idea almost thought up on the spot, which may have gaping holes in it. I’d be interested to hear however what crazy ideas other people have thought up about this most important problem that scientists can make a great impact on if given the resources and green light!

Nuclear fusion probabilities are extremely small! In a typical Dragon experiment where we fire radioactive nuclei at a gas target, sometimes only 1 in every 10,000,000,000,000 particles undergo a fusion reaction. Thus, in order to collect a handful of reaction products in order to tell how such a reaction would occur in a stellar environment, one needs to fire a hell of a lot of particles at the gas target; in this case at least 1,000,000,000,000,000 (ten to the 15th power, or 1 x 10^15) of them. Now, you can either do this by producing a low intensity beam of these radioactive nuclei, and run for a very, very long time in order to get this large number, or you can create a very intense beam and run for a shorter timescale. Since time is finite and expensive (we share beam time with other experiments, like any large science lab) and intensity is difficult to achieve more often than not a compromise is reached and a typical Dragon experiment may last 3-4 weeks of continuous running (yes we do nightshifts) with radioactive beam intensities of between 1 x 10^7 and 1 x 10^9 particles per second.

Artists impression of a Classical Nova, where a compact White Dwarf star receives transferred hydrogen-rich material from a less-evolved orbiting companion which has expanded into its Red Giant phase. The material provides fuel and the conditions necessary to initiate a thermonuclear explosion on the White Dwarf surface, leading to the synthesis of some heavier types of nuclei and the ejection of material into space.

How does such an experiment work? Well first we must describe what occurs in the particular type of nuclear fusion reaction we are interested in. (Warning: some complicated concepts approaching…)

Inside a stellar plasma, nuclei are moving fast, at typical velocities of around 0.1% the speed of light in stars like our sun, and up to a couple of percent the speed of light for hotter scenarios like x-ray bursters. The particles do not all have the same velocity, but instead exhibit what is known as a Maxwell-Boltzmann, or thermalized distribution, indicating that the ensemble of particles have reached thermal equilibrium. Thus, two individual particles may approach one another at a range of different relative velocities, or kinetic energies, and in the simplest cases the probability that they will fuse is dependent on this relative velocity for two reasons: firstly, these particles behave in some ways like waves, and have a characteristic wavelength relative to the other particle which is dependent on the velocity – the lower the velocity, the bigger the wavelength, and the higher the probability of interaction since the wavelength defines a kind of geometric area of overlap between the approaching particles (think of firing a billiard ball at a marble, and then at another billard ball – the latter will be easier to hit); the second effect is that of the repulsive electrostatic field between similarly charged particles (we are assuming the particles are charged), giving rise to the opposite effect – the probability drops off exponentially at low approach energies (velocities). This is called the penetrability. However, things are made more complicated by the presence of quantized ‘energy levels’ in the final nucleus (the one that is made by fusion) that exhibit themselves as ‘resonances’ in the probability, or regions at a specific energy where the probability of fusion suddenly skyrockets to values much higher that if considering penetrability alone (for purists I defer a complex discussion of ‘direct capture’ to bound final states for now).

The DRAGON recoil spectrometer for astrophysics, TRIUMF

Ok, I hope you got that – it’s a little complex – but the point is there are these ‘resonances’, and we are currently incapable of coming up with a precise enough theory of atomic nuclei that can predict both the strength and position of all these resonances at stellar energies (because a theory of nuclei with more than a few nucleons from first principles is extremely difficult due to the many-body nature of the problem). Therefore, we must measure  the strength and energy of the resonances in the laboratory in order to determine the total, energy-dependent fusion probability.

Good. That is the hard bit over now.   So, how do we measure the strength of these little resonances using our radioactive beam and a gas target? Well, we start with our beam, which for the purposes of discussion we’ll assume is made up of particles of all the same velocity. They are fired at Dragon’s famous ‘windowless’ gas target, which means that the beam particles, traveling in vacuum, suddenly meet a region of gas at constant pressure, pass through it, then leave into a region of vacuum again without passing through any thin ‘windows’ to confine the gas. This is achieved by using powerful pumps (the kind used on big ships) to quickly recirculate the escaping gas back into the central target region, maintaining a constant pressure there. As our particles pass through the gas, they lose energy due to many small collisions with electrons in the gas, so that the average velocity of particles leaving the gas target will be a bit lower than the original velocity. The beam energy (velocity) is chosen, so that the resonance that we want to measure the strength of occurs at an energy that corresponds to an energy somewhere between the original beam energy and the exist energy of the particles after the target (let’s assume we roughly know where a resonance is). When a beam particle, on its way through the gas, passes through the resonance energy, the probability of reaction shoots up and some of these particles will fuse. Most beam particles will NOT fuse, however, and pass straight through. That is why I gave the number above of only 1 in every 10,000,000,000,000 particles fuse. 

From here on the physics is simple: conservation of linear momentum ensures the heaver fusion product, or ‘recoil’, has the same momentum as the original beam particle had (since the gas is effectively at rest relative to the beam). This is until the recoil, which has been ‘excited’ into a particular quantum state by the collision, emits a high-energy photon, or gamma ray. This gamma ray, because it is massless, results in only a small ‘kick’ being given to the recoil, in any direction, with the result that the recoils are confined to a thin conical range of directions around the original beam direction.  In order to know that fusion has taken place, we could measure the gamma rays using some sort of detector. This we do using an array of crystals of ‘BGO’ (Bismuth Germanate), which almost completely surround the gas region, and give us an electrical signal when a gamma ray interacts with them. However, many gamma rays come from all directions from natural background radiation, and especially from the radioactive beam itself which results in the intense emission of gamma rays. Thus looking for some particular gamma rays in a forest of others is not the best way to go – we need to also detect the recoil particle. This is where the ‘needle in a haystack’ analogy comes in.  To detect one recoil particle in every 10,000,000,000,000 beam particles, the ‘needle’ in the ‘haystack’, we need a pretty good filter. The recoil particles have the same momentum on average as the beam particles, so the main way to tell them apart is by mass, the recoils being around 1 atomic mass unit heavier. Therefore we pass them through the ‘Dragon’, the recoil ‘spectrometer’, which consists of a series of electric and magnetic fields designed to filter out the beam particles and leave only the recoils.

The main filtering occurs in a section called an electric dipole, a kind of constant electric field between two electrodes, in the case some 200,000 volts in strength, which causes the slightly heavier recoil particle to be bent in a circular path with a slightly smaller radius than the lighter beam particles (this is because the bending angle is greater the lower the velocity), therefore the lighter beam particles are ‘underbent’, and usually end up being stopped by a plate while the heavier recoils keep on going. 

The recoils can then be detected in a series of instruments which measure their speed, energy, and atomic charge, much like in the discussions of particle physics in previous posts only using specialized nuclear physics instrumentation for the lower energy regime. If we choose to accept only particles which come within a given time window of a gamma ray detected at the BGO array, we have what we call a ‘coincidence’ and this rejects nearly all background events. Thus, by counting the recoil particles at the end of the Dragon, and knowing how many particles we injected, we measure the ‘strength’ of the particular resonance in that reaction. We would do this for all known resonances in the reaction (at different energies) and add up their contributions to get a number for the total reaction rate as a function of temperature. This is the number that is given to the astrophysical modelers who would then investigate how this affects a particular scenario like a classical nova explosion. Usually an individual rate can really change quite drastically the final conditions of the star (e.g. the material it ejects into space) and this is why it is important to measure these nuclear reaction.

So that was quite the essay, but I hope it provides enough background that when I begin to talk about running experiments (which will occur in a few weeks’ time) people will have a decent idea of what the point in all this is! I will in future try to imbed a short movie clip into the post, as well as add some pictures of Dragon. Until next time…cheers!

Before I continue with my physics posts on how we measure stellar fusions reactions in the laboratory, I’d like to make a small detour into another area of Canadian cultural territory based on my experiences this Easter weekend: Ice Hockey!

Yes, this weekend marked my first ever experience of a live game of Canada’s national pastime (note: officially lacrosse is apparently the national sport but hockey is the heart and soul of Canadians they will tell you) at a stadium here in Vancouver. The game was not an NHL game, but a ‘West Coast Conference’ game consisting of the Vancouver Giants and The Spokane (Washington) Chiefs. I knew hockey games were spectacular, and so as I entered the stadium I was not surprised by the melee of smiled-up fans, families, small kids, adults, septuagenarians, hot dog stands, t-shirt shops, beer kiosks, pizza places, drummers, police, people carrying lots of beer and lots of rock music.

Vancouver Giants vs Spokane Chiefs, Pacific Coliseum

As I took my seat, I even wasn’t surprised by the electric but jovial atmosphere in the Pacific Coliseum, the stadium where the Vancouver ‘Canucks’ used to play before they moved the the larger ‘GM Place’ stadium where all the biggest music acts come to play. I have been to football (soccer for all you Americans 😉 ) matches before and know what 10,000 hyped-up fans sound like. I was not however, ready for several things I was about to witness during this hockey game. 

First off, the players were all aged at between 17-20 in this particular league, and my Canadian chums informed me that this was ‘real hockey’, where the stakes were higher (these kids are all jostling for a rare spot in the big-time, big-money NHL) and it was a dog-eat-dog attitude. Thus the testosterone levels were off the scale and the passion and voracity with which these kids played resulted in some spectacular collisions, fouls, smashes, trips, slides and very near fights! The sounds in the arena: the scraping of the ice, the slap of the puck, the smack of the barriers when two hotheads race each other full speed in a do-or-die battle for the puck, are incredibly exciting at a live game and something I did not appreciate watching the game on TV.

Also, there is a lot of rock music played. In fact, every time the whistle blows a rock song fades in until the moment play starts again. There’s Queen (we will rock you), which is the one I remember for sure, plus a host of other instantly recognizable classics, giving a rock concert feel to the whole occasion.

Next surprise was at the end of the first period. I knew something called a ‘Zamboni’ was about to come out and clean the ice, and I was ready to take a picture of this icon to send back to my friends in the UK. However, before that happened suddenly the ice became a hive of activity as assistants wheeled out equipment, music played and a bunch of little kids, must have been about 8 or so, sped out onto the ice in full hockey gear for a mini hockey match, cheerleaders danced onto the ice, and two giant remote-controlled airships rose above the rink in a destinationless and erratic advertising dance! This was a spectacle for sure. But it wasn’t over. The ice cleared, and suddenly a 4×4 shot out onto the ice driven by a guy in a business suit, and with a hot scantily-clad lady standing in the back with a kind of ‘foam bullet assault cannon’, starts firing big foam pellets into the crowd and at the airships!! I think the foam pellets must have had some merchandise or something inside them as I witnessed the crowd catching and opening them. After all that, the snail-like Zamboni seemed like an anticlimax!

mini-hockey, blimps and cheerleaders

The end of the second period did not disappoint either. This time instead of mini-hockey we were treated to a game of human skittles, where five 6-foot skittles (pins) were set up on the ice, and volunteers from the audience (or competition winners?) sat on a small tray and were propelled into the skittles by a giant catapult that spanned the entire width of the rink!! When all pins were not knocked down, the team mascots ( a giant ‘sailor’ and a beaver ) would propel themselves haphazardly into the remaining pins to help. I was almost in tears of laughter at the whole…spectacle…of the thing.

The next surprise was that the teams drew 2-2, and the game went into extra time. Now, apparently they had an extra full three 20-minute periods before someone scored the ‘golden goal’ and one (the away team unfortunately) team snatched victory, but I didn’t know because by this time, my friends and I, hungry and full of too much beer, had gone to find any place open at almost midnight to try and get a meal.

human skittles

Overall, the game was an amazing experience and one that was long overdue for me since I’ve lived here for 5 years already. I would recommend one of these junior league games to anyone. Great fun for all the family.

I’ve had a few requests to explain exactly what ‘Nuclear Astrophysics’ involves, for the layman. In my first post I mentioned that, since we can’t touch a star only observe its light, all we can do is to try and make a ‘little piece’ of a star in the laboratory to study. That is still a cartoon of what we really do. I will try to explain more clearly here and in future posts what is involved in studying the inner workings of stars.

Nuclear Astrophysics relates astronomical observations (top left) to models of exotic stellar phenomena (bottom left, classical nova) by studying the microscopic nuclear reactions involved (top right) by experiment (bottom right)

Stars are big balls of high temperature ‘burning’ gas held together by gravity, and mostly hydrogen and helium, the most abundant elements in the universe. Many of you will know that our sun generates energy by fusing together hydrogen nuclei (we talk about nuclei here since in the hot, dense plasma of stars electrons move freely rather than being bound in atoms) into helium nuclei. Although this is a rare occurrence, it is made possible by virtue of the incredible density and number of particles within the sun’s core, as well as the spooky but real world of quantum mechanics, which allows two protons to get close enough to ‘fuse’ whereas classical physics rules would prohibit them from doing so.

Some of you will also know that everything we and the planet Earth are made of, right up to the heaviest natural element uranium, is synthesized within successive evolutionary stages of stars through a series of nuclear fusion reactions. Stars can then spread this heavy material around by either ejecting it in powerful winds, or by exploding and spewing the material into space. (The latter would include a supernova, probably the most well known type of ‘exploding star’). 

Some of the stages of this ‘nucleosynthesis’ have been known in outline form for a long time, for example the carbon-nitrogen-oxygen cycle originally outlined in a paper by Hans Bethe in 1939, while the first step of the main set of nuclear reactions that generate energy in the sun was first properly treated in a 1938 paper by Bethe and Critchfield* but was first suggested by Eddington in 1920. The opening lines of Bethe and Critchfield’s paper are quoted here:

It seems now generally accepted that the energy production in most stars is due to nuclear reactions involving light elements. Of all the elements, hydrogen is favored by its large abundance, by its large internal energy which makes a considerable energy evolution possible, and by its small charge and mass which enable it to penetrate easily through nuclear potential barriers. Again, the most primitive is the combination of two protons to form a deuteron, with the positron emission:

                                                H + H = D + e⁺ 

Excuse the tech language there. It is important to realize that this was before the time of satellites, space exploration, rockets, the atomic bomb, electronics, computers. The transistor had only just been developed, and the field of nuclear reactions had only been alive for about 6 years since Cockroft and Walton’s accelerator experiment at the Cavendish Laboratory in 1932. Moreover, it had only been confirmed that the sun was composed mainly of hydrogen less than 15 year previously. However, the field of quantum mechanics was well established, and quite frankly, the scientists of the time were intellectual giants. It always amazes me that from so little information, and in so few decades, we went from not even knowing what atoms were to knowing what powers our own sun!

Anyway, the point is all of energy generated in stars occurs from nuclear processes, and that it what drives the evolution of stars forward, synthesizing heavier and heavier elements. Most of the details of the reactions involving the light elements are known very well from a long 50+ years of study. However, the processes that take place in more exotic scenarios like dying stars, involving the heavier elements, are not so well known. It is the goal of the field of Nuclear Astrophysics to understand exactly how all the elements were made, and also to understand the exploding, dying stars that produce them.

Since we cannot make an entire star, you might think we can make a small piece of one. That too however is problematic. Instead we say to ourselves that we understand quite well the physics of hot gases, and to some extent the physics of magnetic fields and turbulent motion. We can create a computational model (now using powerful computers) of a star, whose properties such as composition, temperature, density etc depend on the energy deposited by individual nuclear reactions. Since it is extremely difficult to calculate what the ‘strength’ of each reaction is, we must measure them individually by experiment.

So in essence, we measure each nuclear reaction of importance and that becomes another ‘building block’ to add to our model of stars, each time refining the model. The model then gives us predictions, such as the luminosity of the star, or the synthesis of certain types of nuclei within it, and we can then compare what we actually see from the real stars (by a satellite-based telescope for example), to our model to see if we get it right. So by combining astronomy with these laboratory nuclear physics experiments, we can actually understand the microscopic goings-on in the stars of our universe, including the gigantic supernova explosions and powerful x-ray bursters (Google those)!!

What’s so special about what we do at TRIUMF? Well, in order to study a nuclear reaction directly, for example a proton fusing to some…oxygen let’s say, you would usually take some oxygen gas and fire a proton at it using a particle accelerator, observing the high-energy photons (gamma rays) that are produced in the process as your signal that fusion has taken place. However the vast majority of nuclear reactions that take place in stars involve radioactive nuclei that are so short-lived that we can’t make a ‘gas target’ out of them to fire protons (or anything else) at. Therefore we do things in reverse: we actually create the short lived particles in a high-energy nuclear reaction with TRIUMF’s cyclotron accelerator, then accelerate those exotic particles to the energy that they would typically have inside the star of interest, and then we hit a hydrogen (or helium) gas target with them, before they have the chance to decay, to observe the fusion. Thus we need a ‘Radioactive Ion Beam Facility’, which is what ISAC at TRIUMF is.

In the interests of keeping each post from stretching into an essay, I will stop here for now but next time, I will tell you about just how we perform the fusion reactions using the unique DRAGON instrument here at TRIUMF. Any questions feel free to leave a comment 🙂

A quick thought on the parallels between physics, and my other love, music. I have always considered myself a musician, although I am not as proficient as I’d like to be. There is a stereotype that physicists only listen to classical music, perhaps due to the appreciation of the underlying mathematical constructs, but although I am a huge classical fan, I was brought up with my Father’s vinyl collection: The Doors, The Beatles, Jimi Hendrix, Bowie, Dylan,… This has highly influenced the kind of music I appreciate today.
The main thing I wanted to share however, was an exchange I had with one of my closest friends who is a Jordanian composer, currently working on a theatrical production to celebrate the initiation of the ancient city of Petra into the ‘seven wonders of the world’. We often have lengthy conversations over some single malt about physics, and for a musician, the guy has an impressive collection of popular science tomes. In an email exchange recently, I posed to him a thought I’d had in reaction to some ‘anti-scientific’ exchanges I’d had on Youtube with a supporter of Creationism along the lines that ‘science destroys the beauty of nature’. Here is my email excerpt to my friend (excuse the maloquence!):

Recently I was debating with someone about scientific reductionism. Their argument was that science somehow belittles the beauty of nature because it seeks to understand reality in its microscopic and most fundamental form by the breaking down of its description into mathematics and laws etc. They argued that the beauty in life is in the mystery, in the not knowing, and that was the basis for a lot of the feeling of people with ‘faith’. I tried to argue back that science is not ‘reducing’ nature by understanding it, and in fact most scientists see beauty in nature and reality in the same way, if not stronger, than most people. 

Then I thought about the example of music. When you first hear an amazing song, you experience it totally and freely and let the sounds sink into your ‘soul’, but then we, as musicians, feel compelled to listen deeper, to understand just how the guitarist hit that pinch harmonic or bent that note a fraction of a second later than we’d expect, of how there are probably three, not two, keyboard tracks laid on top of one another and they’re just out of phase and recorded at different locations relative to the mics giving a certain ethereal dimension. Then we pick up the guitar and we try to mimic the music, and we take it and we make it our own, and we sing it to ourselves and maybe even perform it to others, but do we now think that the music is less beautiful because we have taken away the mystery? Do we appreciate it less because we understand the sound waves and the admixtures and exactly what we’d need to reproduce it? I don’t think so, because if true, then all musicians would be incapable of enjoying music, because every time they learned a song it would become banal, and they would lose their passion, but the passion stays, and that is what drives musicians, and also scientists, because there is always joy in understanding, and always another puzzle to solve. And physics is exactly that – it is the learning of the song of nature, with a mind to being able to perform it ourselves, and reproduce its notes, that we might discover other songs and other dimensions of melody not yet conceived.