Next Wednesday, Prof Wade Allison will be at the University of Surrey giving an evening Institute of Physics lecture based on his book "Radiation and Reason".
The talk is at 7pm in lecture theatre M, and is free to attend. Please turn up for a 7pm start, if you wish to come. I will summarise what he says here afterwards.
Here is a facebook event page for it, if you wish to register interest, but there's no obligation to.
Wednesday, 17 November 2010
Wednesday, 3 November 2010
Nuclei in Semiconductors
Semiconductors are substances who electronic structure is such that they are neither good electrical conductors nor insulators, but whose conduction properties may be altered by various means, chemical and physical to produce materials which can do all sorts of amazing things. Things like transistors, and all the technology that comes from them, things like solar cells, things like cheap LED lighting, like lasers for BlueRay players - like a lot of the neat things that get developed by my colleagues in the Advanced Technology Institute here at the University of Surrey.
Usually, people interested in semiconductor materials are not terribly concerned with the nuclei that hold the electrons in place, except that the nuclei have to be the right element, say Silicon, in order to have the right number of electrons and so the right electron structure. It's not always the case, though. One cutting edge of semiconductor research involves using quantum "spins" to make quantum computers. Spin is a kind of quantum angular momentum - to do with things rotating - though in the quantum world things don't have to rotate to have angular momentum. In spin-based semiconductor research (or "spintronics"), one tries to manipulate the orientation of a spinning electron to store information, rather than by presence or absence of a charge. This is one of the promising ways of creating a quantum computer.
Once one starts to deal with electron spins, however, the nuclei can start becoming interested. Silicon is element number 14, with 14 protons in each nucleus, and 14 electrons around a neutral silicon atom. Silicon comes in three naturally occurring isotopes, though: Si-28 with 24 protons and 24 neutrons but also Si-29 and Si-30 with one and two extra neutrons respectively. Si-28 and Si-30 have no nuclear spin, so they don't interfere with spin-based quantum computers, but Si-29 (like all odd-numbered isotopes) has a non-zero nuclear spin, and their presence in naturally occurring silicon causes the quantum computer states to decay, or "decohere". The solution? Make isotopically-enriched silicon, without the natural Si-29. It turns out not to be that easy to either make a sample sufficiently isotopically pure, or even get rid of other contaminants, as a paper published last week, cited below, shows. Surprising sometimes where nuclear physics issues pop up.
Witzel, W., Carroll, M., Morello, A., Cywiński, L., & Das Sarma, S. (2010). Electron Spin Decoherence in Isotope-Enriched Silicon Physical Review Letters, 105 (18) DOI: 10.1103/PhysRevLett.105.187602
Usually, people interested in semiconductor materials are not terribly concerned with the nuclei that hold the electrons in place, except that the nuclei have to be the right element, say Silicon, in order to have the right number of electrons and so the right electron structure. It's not always the case, though. One cutting edge of semiconductor research involves using quantum "spins" to make quantum computers. Spin is a kind of quantum angular momentum - to do with things rotating - though in the quantum world things don't have to rotate to have angular momentum. In spin-based semiconductor research (or "spintronics"), one tries to manipulate the orientation of a spinning electron to store information, rather than by presence or absence of a charge. This is one of the promising ways of creating a quantum computer.
Once one starts to deal with electron spins, however, the nuclei can start becoming interested. Silicon is element number 14, with 14 protons in each nucleus, and 14 electrons around a neutral silicon atom. Silicon comes in three naturally occurring isotopes, though: Si-28 with 24 protons and 24 neutrons but also Si-29 and Si-30 with one and two extra neutrons respectively. Si-28 and Si-30 have no nuclear spin, so they don't interfere with spin-based quantum computers, but Si-29 (like all odd-numbered isotopes) has a non-zero nuclear spin, and their presence in naturally occurring silicon causes the quantum computer states to decay, or "decohere". The solution? Make isotopically-enriched silicon, without the natural Si-29. It turns out not to be that easy to either make a sample sufficiently isotopically pure, or even get rid of other contaminants, as a paper published last week, cited below, shows. Surprising sometimes where nuclear physics issues pop up.
Witzel, W., Carroll, M., Morello, A., Cywiński, L., & Das Sarma, S. (2010). Electron Spin Decoherence in Isotope-Enriched Silicon Physical Review Letters, 105 (18) DOI: 10.1103/PhysRevLett.105.187602
Thursday, 30 September 2010
Nuclear archaeology
Nuclear Physics can help in the world of archaelogy, helping to understand artefacts created long before humankind knew anything about atomic nuclei. I heard a story on Radio 4's Today programme yesterday about the discovery of the skeleton of a bronze age teenage boy near Stonehenge who came from the Mediterranean.
The story mentioned "geochemical" analysis, but the main interviewee mentioned nothing about the nuclei behind the story. What they really discovered was that the isotope ratios of both Oxygen and Strontium isotopes were more characteristic of someone growing up in a Mediterranean environment than a British one.
Heavy oxygen isotopes in water molecules tend to fall more readily as rain when a cloud is cooling, and when it gets colder and colder it tends to be more and more depleted in heavy oxygen. The ratio of Oxygen-18 to Oxygen-16 can be used therefore as a reasonable guide to temperature (and it is used, for example, in measuring the historical temperature of the earth by looking in ice cores from Greenland).
By looking at the oxygen isotope ratio in tooth enamel, which is grown during childhood, evidence of the climate one experienced while growing up can be found.
The other clue comes from the presence of Strontium. Strontium occurs all over the world in ores, but its occurrence, like with all elements varies across the world as a result of chance geological events. Strontium makes its way into the local food chain and then substitutes for calcium in bones, it being in the same chemical group of the periodic table. Looking at the strontium isotopes can then be correlated with where one grew up. This all adds up to the ability to determine where a boy from 1550 BC grew up!
More details can be found on the British Geological Survey's website.
The story mentioned "geochemical" analysis, but the main interviewee mentioned nothing about the nuclei behind the story. What they really discovered was that the isotope ratios of both Oxygen and Strontium isotopes were more characteristic of someone growing up in a Mediterranean environment than a British one.
Heavy oxygen isotopes in water molecules tend to fall more readily as rain when a cloud is cooling, and when it gets colder and colder it tends to be more and more depleted in heavy oxygen. The ratio of Oxygen-18 to Oxygen-16 can be used therefore as a reasonable guide to temperature (and it is used, for example, in measuring the historical temperature of the earth by looking in ice cores from Greenland).
By looking at the oxygen isotope ratio in tooth enamel, which is grown during childhood, evidence of the climate one experienced while growing up can be found.
The other clue comes from the presence of Strontium. Strontium occurs all over the world in ores, but its occurrence, like with all elements varies across the world as a result of chance geological events. Strontium makes its way into the local food chain and then substitutes for calcium in bones, it being in the same chemical group of the periodic table. Looking at the strontium isotopes can then be correlated with where one grew up. This all adds up to the ability to determine where a boy from 1550 BC grew up!
More details can be found on the British Geological Survey's website.
Tuesday, 24 August 2010
NIF video
To follow up the previous post about the NIF, I notice in a tweet from @lasers_llnl that they have a pretty cool-looking 3D movie of the facility online. Must make some 3D glasses...
Tuesday, 17 August 2010
The Conference Excursion
I'm back from California now, and enjoyed the Nuclear Structure 2010 conference. I'll probably blog about more of the talks - I particularly enjoyed George Dracoulis' on Tantalum-180 - but not this evening. Instead, I want to talk about the conference excursion.
For those that don't have the pleasure of going to scientific conferences, let me explain the conference excursion. Not every conference has one, but often one afternoon of a week-long conference will involve taking a trip somewhere of interest near to the conference venue. It doesn't have to be somewhere of relevance to the conference topic. It could be a local place of historical interest, or something like that. The excursion is partly an excuse to socialise with the other people at the conference, which is an important part of the purpose of getting all the attendees together at a conference. Indeed physicists sometimes need help in socialising, and these events can lead to useful discussions and collaborations. Of course the excursions are also partly for fun.
The excursion at Nuclear Structure 2010 certainly counts as a physics excursion, and also a fun one. We went to NIF: the National Ignition Facility, based at Lawrence Livermore National Laboratory not too far from San Francisco. The facility is being built to make small pellets of Hydrogen-2 and Hydrogen-3 nuclei fuse together to give off energy, just as they might do in a future fusion reactor and just as they actually do in thermonuclear weapons.
The US has a stockpile of thermonuclear weapons (the phrase used to describe hydrogen bombs, which work by fusing hydrogen isotopes together as opposed to nuclear fission bombs used in anger in WW2), but it has agreed that it will no longer test them in either atmospheric or underground explosions. However, it would like to understand that they are being well-maintained and still functional - something that I suppose it not obvious when you have a rather hi-tech device which you have built and then left on the shelf for many years. The way around the test-ban is to build a kind of controlled thermonuclear bomb, and that is what the NIF is. They certainly make no secret that the driving purpose of the facility is weapon "stewardship" but given that the weapons already exist and will continue to do so, it seems that they have managed to build something that allows quite a bit of interesting basic physics research to take place, piggybacked onto the weapons programme.
To get to see the facility, alien attendees at the conference had to get security-checked months in advance, and thankfully I passed the tests (though I don't know what I was being tested for). So last Wednesday, I boarded a bus in Berkeley, showing my passport before I even got on, and we drove to the lab. We stopped at the security gate for a while, and were escorted to the badge office to get our temporary badges. The whole procedure was generally taking so long that I feared we would have an hour-long trip to the security office and a 10-minute tour of the facility. We were all enjoying joking about it, though, which I was also nervous wouldn't endear us to the facility people... but all was well.
The building in which NIF sits is a rather ugly warehouse-looking place, but inside it is impressively hi-tech. I've been to a few facilities (as a mere theoretical physicist, I don't actually see inside labs all that often) and I think it's fair to say that I've never seen one so sparkling, shiny, sophisticated and hi-tech. The fusion will take place by having a tiny capsule of H-2 and H-3 that you could hold in your fingertips, placed at the focus of a couple of hundred laser beams - the most powerful in the world, which will collapse the capsule causing compression and heating and then nuclear fusion. They haven't started fusion runs yet, but have fired the lasers at non-fusion pellets and everything looks good so far.
The current set up is such that the pellet is placed very carefully in a big spherical chamber into which the lasers are beamed. They will be able to make one explosion every four hours or so when it is all up and running. If they want to actually get fusion energy out of this, they said that they need a rate of 10 Hz - i.e. 10 explosions per second. That seems quite ambitions to me, but I expect that they will learn some important things about the hydrodynamics of fusing hydrogen plasma, which is really what is needed. Certainly the nuclear physics reactions are well enough understood.
After the tour, and I wish I could show you pictures but cameras were verboten, we were treated to cookies and a talk about the basic science that might come out - about matter at the extremes of density - and the promise that the majority of the experiments would be unclassified. Then it was time to head back to the bus, and back to Berkeley. Back from the borders of the sunny desert to the perennially cloudy Bay Area...
For those that don't have the pleasure of going to scientific conferences, let me explain the conference excursion. Not every conference has one, but often one afternoon of a week-long conference will involve taking a trip somewhere of interest near to the conference venue. It doesn't have to be somewhere of relevance to the conference topic. It could be a local place of historical interest, or something like that. The excursion is partly an excuse to socialise with the other people at the conference, which is an important part of the purpose of getting all the attendees together at a conference. Indeed physicists sometimes need help in socialising, and these events can lead to useful discussions and collaborations. Of course the excursions are also partly for fun.
The excursion at Nuclear Structure 2010 certainly counts as a physics excursion, and also a fun one. We went to NIF: the National Ignition Facility, based at Lawrence Livermore National Laboratory not too far from San Francisco. The facility is being built to make small pellets of Hydrogen-2 and Hydrogen-3 nuclei fuse together to give off energy, just as they might do in a future fusion reactor and just as they actually do in thermonuclear weapons.
The US has a stockpile of thermonuclear weapons (the phrase used to describe hydrogen bombs, which work by fusing hydrogen isotopes together as opposed to nuclear fission bombs used in anger in WW2), but it has agreed that it will no longer test them in either atmospheric or underground explosions. However, it would like to understand that they are being well-maintained and still functional - something that I suppose it not obvious when you have a rather hi-tech device which you have built and then left on the shelf for many years. The way around the test-ban is to build a kind of controlled thermonuclear bomb, and that is what the NIF is. They certainly make no secret that the driving purpose of the facility is weapon "stewardship" but given that the weapons already exist and will continue to do so, it seems that they have managed to build something that allows quite a bit of interesting basic physics research to take place, piggybacked onto the weapons programme.
To get to see the facility, alien attendees at the conference had to get security-checked months in advance, and thankfully I passed the tests (though I don't know what I was being tested for). So last Wednesday, I boarded a bus in Berkeley, showing my passport before I even got on, and we drove to the lab. We stopped at the security gate for a while, and were escorted to the badge office to get our temporary badges. The whole procedure was generally taking so long that I feared we would have an hour-long trip to the security office and a 10-minute tour of the facility. We were all enjoying joking about it, though, which I was also nervous wouldn't endear us to the facility people... but all was well.
The building in which NIF sits is a rather ugly warehouse-looking place, but inside it is impressively hi-tech. I've been to a few facilities (as a mere theoretical physicist, I don't actually see inside labs all that often) and I think it's fair to say that I've never seen one so sparkling, shiny, sophisticated and hi-tech. The fusion will take place by having a tiny capsule of H-2 and H-3 that you could hold in your fingertips, placed at the focus of a couple of hundred laser beams - the most powerful in the world, which will collapse the capsule causing compression and heating and then nuclear fusion. They haven't started fusion runs yet, but have fired the lasers at non-fusion pellets and everything looks good so far.
The current set up is such that the pellet is placed very carefully in a big spherical chamber into which the lasers are beamed. They will be able to make one explosion every four hours or so when it is all up and running. If they want to actually get fusion energy out of this, they said that they need a rate of 10 Hz - i.e. 10 explosions per second. That seems quite ambitions to me, but I expect that they will learn some important things about the hydrodynamics of fusing hydrogen plasma, which is really what is needed. Certainly the nuclear physics reactions are well enough understood.
After the tour, and I wish I could show you pictures but cameras were verboten, we were treated to cookies and a talk about the basic science that might come out - about matter at the extremes of density - and the promise that the majority of the experiments would be unclassified. Then it was time to head back to the bus, and back to Berkeley. Back from the borders of the sunny desert to the perennially cloudy Bay Area...
Wednesday, 11 August 2010
Superheavy in Berkeley
I'm in Berkeley, California, attending the Nuclear Structure 2010 conference. There have been a few talks on superheavy elements (roughly those heavier than found on the earth, so heavier than Uranium). This is hardly any wonder since Berkeley is home of the Lawrence Berkeley National Laboratory, where superheavy element creation was pioneered.
Krzysztof Rykaczewski presented a talk about the recently-announed discovery of element 117. Like all superheavy nuclei, it is made by reacting together two lighter nuclei: In this case Calcium-48 (20 protons, 28 neutrons) and Berkelium-249 (97 protons, 152 neutrons). This is the most obvious choice, since Calcium-48 is the most neutron rich stable light nucleus that there is and then one needs to match with the right number of protons in the other nucleus to make the one you're interested in. The tough thing about this is that Berkelium has a half life of around 320 days and is itself a superheavy nucleus that has to first be made in a lab. They made it at Oak Ridge, Tennessee, where they placed (also superheavy, or at least transuranic) Americium (element 95, widely used in household smoke detectors) and Curium (element 96) samples in a nuclear reactor for 250 days, where they absorbed neutrons and underwent beta decay until heavier elements had been created, including the Berkelium, which was separated by chemical means.
They made a total of 22mg of Bk-249 which they then turned into a target which they shipped to Russia (which turns out to require quite a bit of paperwork) to the nuclear physics lab at Dubna. Here they installed the Berkelium target onto which the Ca-48 beam impinged. They had a total of 3g of Ca-48 to work with. It's not as rare as Berkelium, but it's pretty rare, and Russia have it all. They ran the experiment for several months, and in that time made a positive identification of the isotopes of Z=117 with N=176 and N=177. When there has been independent verification of the discovery, the group will be invited to name the new element.
As my Institute of Physics Branch colleague Alby notes, the same group are now in a position to name element 114.
The paper announcing the element, in Physical Review Letters, is available (to subscribers). Details below:
Oganessian, Y., Abdullin, F., Bailey, P., Benker, D., Bennett, M., Dmitriev, S., Ezold, J., Hamilton, J., Henderson, R., Itkis, M., Lobanov, Y., Mezentsev, A., Moody, K., Nelson, S., Polyakov, A., Porter, C., Ramayya, A., Riley, F., Roberto, J., Ryabinin, M., Rykaczewski, K., Sagaidak, R., Shaughnessy, D., Shirokovsky, I., Stoyer, M., Subbotin, V., Sudowe, R., Sukhov, A., Tsyganov, Y., Utyonkov, V., Voinov, A., Vostokin, G., & Wilk, P. (2010). Synthesis of a New Element with Atomic Number Z=117 Physical Review Letters, 104 (14) DOI: 10.1103/PhysRevLett.104.142502
Krzysztof Rykaczewski presented a talk about the recently-announed discovery of element 117. Like all superheavy nuclei, it is made by reacting together two lighter nuclei: In this case Calcium-48 (20 protons, 28 neutrons) and Berkelium-249 (97 protons, 152 neutrons). This is the most obvious choice, since Calcium-48 is the most neutron rich stable light nucleus that there is and then one needs to match with the right number of protons in the other nucleus to make the one you're interested in. The tough thing about this is that Berkelium has a half life of around 320 days and is itself a superheavy nucleus that has to first be made in a lab. They made it at Oak Ridge, Tennessee, where they placed (also superheavy, or at least transuranic) Americium (element 95, widely used in household smoke detectors) and Curium (element 96) samples in a nuclear reactor for 250 days, where they absorbed neutrons and underwent beta decay until heavier elements had been created, including the Berkelium, which was separated by chemical means.
They made a total of 22mg of Bk-249 which they then turned into a target which they shipped to Russia (which turns out to require quite a bit of paperwork) to the nuclear physics lab at Dubna. Here they installed the Berkelium target onto which the Ca-48 beam impinged. They had a total of 3g of Ca-48 to work with. It's not as rare as Berkelium, but it's pretty rare, and Russia have it all. They ran the experiment for several months, and in that time made a positive identification of the isotopes of Z=117 with N=176 and N=177. When there has been independent verification of the discovery, the group will be invited to name the new element.
As my Institute of Physics Branch colleague Alby notes, the same group are now in a position to name element 114.
The paper announcing the element, in Physical Review Letters, is available (to subscribers). Details below:
Oganessian, Y., Abdullin, F., Bailey, P., Benker, D., Bennett, M., Dmitriev, S., Ezold, J., Hamilton, J., Henderson, R., Itkis, M., Lobanov, Y., Mezentsev, A., Moody, K., Nelson, S., Polyakov, A., Porter, C., Ramayya, A., Riley, F., Roberto, J., Ryabinin, M., Rykaczewski, K., Sagaidak, R., Shaughnessy, D., Shirokovsky, I., Stoyer, M., Subbotin, V., Sudowe, R., Sukhov, A., Tsyganov, Y., Utyonkov, V., Voinov, A., Vostokin, G., & Wilk, P. (2010). Synthesis of a New Element with Atomic Number Z=117 Physical Review Letters, 104 (14) DOI: 10.1103/PhysRevLett.104.142502
Wednesday, 28 July 2010
229Th
Thorium-229 is one of my favourite isotopes. With the lowest-energy first excited state of any known nucleus, it's the isotope of choice for interacting directly with things such as lasers, atoms and molecules, whose characteristic energies are much smaller than normal nuclear transition energies. One of the most promising practical uses of 229Th is in making a new time standard which is more accurate than existing atomic clocks. My friend and fellow blogger Rob Jackson has just published a paper on the chemical side of implanting 229Th in material for possible use in such a clock standard. He's blogged about it here.
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