Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: chris on 25/08/2016 08:06:49
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What is a cyclotron? What does one do with it, and how does it work?
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Cyclotron (https://en.wikipedia.org/wiki/Cyclotron)
Essentially it is a very old and by today's standards inefficient method of accelerating charged particles (usually electrons). It uses a static magnetic field to cause the charged particle to go in a circle and two separated metallic half cylinders to which an RF signal is applied to accelerate the particle. As long as the RF signal has the right frequency the charged particle will always enter one of the half cylinders at the right time to be accelerated. Each pass makes the radius of the charged particle's orbit larger and eventually it will be ejected from the machine at high speed. The high speed particle can then be fired at targets to generate X-rays or other accelerated charged particles like at the LHC (though at much lower energies).
Today's particle accelerators work very differently and at much higher energy.
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Thanks. Very clear.
What sorts of things can it be used to make? I had heard that people use them to make materials for imaging studies. Is that true?
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I believe they were the source of the U235 used in the Hiroshima bomb
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I believe they were the source of the U235 used in the Hiroshima bomb
I recommend you to read "Surely You're Joking, Mr. Feynman!" to give you an overview of how the bomb was made while having fun.
spoiler alert : they use membrane gas separator to enrich the Uranium.
On the other hand, Tsar bomba used high speed centrifuge which was not yet feasible during development of the Little Boy.
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I had heard that people use [cyclotrons] to make materials for imaging studies. Is that true?
Technetium 99m (https://en.wikipedia.org/wiki/Technetium-99m) is used in imaging studies, and it was first discovered in experiments with cyclotrons - the cyclotron accelerated protons which were used to bombard a Molybdenum target.
Most commercial Mo 99 production today occurs in high-flux research reactors. However, it is also possible to produce these radioisotopes in modern particle accelerators (https://en.wikipedia.org/wiki/Particle_accelerator#Nuclear_physics_and_isotope_production), much like their original discovery.
I believe they were the source of the U235 used in the Hiroshima bomb
The U235 was already in the uranium ore as it was mined. They just needed to separate it from the (much more common) U238.
Several methods of isotope separation were tried in the Manhattan Project, and one of them, the Calutron (https://en.wikipedia.org/wiki/Calutron) was based on the Cyclotron.
The Cyclotron aims to accelerate the ions to very high energy, by spinning them around millions of times in the magnetic field. However, isotope separation does not need high energies, but just aims to physically separate the isotopes; half a circuit (180 degrees) is enough to do this.
Today we call these devices Mass Spectrometers.
What sorts of things can it be used to make?
Modern particle accelerators are used to produce ion beams for cancer treatment and silicon chip manufacture. They are used in experiments attempting to produce new elements in the periodic table (atoms heavier than Uranium). Bending the beam produces X-Rays for research purposes, and by using "wiggler" magnets, produce a tunable laser. You can even use them to make Higgs Bosons (if you have enough energy, like the LHC).
See: https://en.wikipedia.org/wiki/Particle_accelerator
Essentially it is a very old and by today's standards inefficient
With a fixed magnetic field, the spinning particles inside the Cyclotron eventually gain enough energy that they go flinging out of the machine. Because the bend radius is small (eg 0.1-3m), the accelerated particles lose a lot of energy to X-Ray radiation. That is good for producing a continuous beam of moderate energy in a small space.
However, modern accelerators increase the strength of the magnetic field in step with the energy of the particles, so that the particles stay on a circular track with a large diameter (eg 10m to many km=lower losses from X-Rays). That is good for producing pulsed beams of much higher energies than you could achieve with a cyclotron and just a single magnet.
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Essentially it is a very old and by today's standards inefficient method of accelerating charged particles (usually electrons). etc.
That is incorrect. Cyclotrons are widely used to treat cancer. In fact there's one at Mass General.
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That is incorrect.
Not at all. Although I guess I could have been more technical in that the LHC is actually considered a subclass of cyclotron called a synchrotron but the design and operation of a synchrotron is so different from the earlier cyclotrons most people (especially the non-technical people) consider them different types of machines.
Cyclotrons are widely used to treat cancer. In fact there's one at Mass General.
You might have a point if I had said that cyclotrons aren't used today but that isn't what I said. Just because something is inefficient by current standards and old does not imply it is no longer used. For example, vacuum tubes are generally less efficient than solid state transistors but are still used because they have other advantages. Similarly bipolar junction transistors are usually less efficient than MOSFETs but they also are still used because again they have other advantages.
Cyclotrons happen to have advantages if you want to use them to make a proton beam to fire at a tumor in a cancer patient. They can be made much smaller and more compact than a synchrotron and it is generally easier to get the charged particles into and out of a cyclotron than a synchrotron. Whenever you need a compact source of relativistic or near relativistic charged particles currently your best choice is a cyclotron. However, there are medical synchrotrons as well because there are some advantages and disadvantages to the output of both types of accelerators. In the future there are other means of acceleration usually involving lasers that may take over as the best compact source.
If you're looking to collide particles at experimentally relevant energies or need a high brightness energy tunable x-ray source currently your best options are a synchrotron or a linear accelerator. To be completely precise the synchrotron and linear accelerator are the best options for more reasons than just efficiency.
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I think that there is a misprint in the Wikpedia article
"Alpha racetracks had produced 88 kilograms of product with an average enrichment of 84.5 percent, and the Beta racetracks turned out another 953 kilograms enriched to 95 percent by the end of the year"
Should this have been 95.3 kilograms ?
I know that at least ten "little boy" bombs were constructed but never used perhaps this figure was leaked for political purposes
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Not at all.
Total nonsense. You're backpedaling here. You claimed
Essentially it is a very old and by today's standards inefficient method of accelerating charged particles
Which is quite wrong. Now enough of your attempts to make it appear as if you didn't make a serious mistake. I'm quite tired of people such as yourself backpedaling.
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For anyone interested here is a brief history of particle accelerators. (http://www.slac.stanford.edu/pubs/beamline/27/1/27-1-panofsky.pdf) The cyclotron was actually in use before the linear accelerator and was the first type of accelerator to use resonant acceleration instead of direct acceleration via a static electric field. It is basically the oldest type of resonant accelerator.
Also the energy stored in an RF cavity (such as the dees in a cyclotron) is linearly proportional to the volume of the cavity. In a cyclotron the RF cavity is basically the entire volume of the accelerator which includes areas where the charged particles never go. Approximating a cyclotron as a right circular cylinder the volume is approximately V = π*r^2*h (where r is the radius and h is the height). Thus the energy required to run a cyclotron increases roughly quadratically with the radius of the cyclotron assuming constant RF intensity and the beam itself occupies lower percentage of the overall volume as the radius increases.
For a synchrotron or a linear accelerator the RF cavities can be constructed to be not much wider and taller than the beam size. Additionally, the RF cavities can be limited to small acceleration sections followed by sections for beam focusing and steering. If we approximate a synchrotron as a rectangular ring the volume is approximately V = π*h*(2*r*t - t^2) where r is the radius to the outside of the ring, t is the ring thickness and h is the height. Even if the entire ring was an RF cavity the volume of the cavity only increases linearly with the radius of the cavity. Thus at any given size and for the same field intensity the RF cavity of a synchrotron will always be smaller and therefore consume less energy than a cyclotron. Further, the RF cavities of a synchrotron are well matched to the size of the beam meaning that a far greater percentage of the RF cavity of a synchrotron is occupied by the beam (and doing useful acceleration) than a cyclotron of the same radius. In other words cyclotrons are inefficient compared to more modern accelerators like synchrotrons.