I know I said this was going to be about everyday stuff, but in a few months, this will be everyday for me. Starting in June, I'll be getting proton radiation therapy at Massachusetts General Hospital as part of my cancer treatment. As a physicist, I'm excited by the idea of having protons fired into my brain, and I decided to look into some of the details of the cyclotron that will be providing the protons.
For those unfamiliar with the term, a cyclotron is similar to a supercollider, but only accelerates particles, rather than smashing them into each other. It is built as a ring (hence 'cyclo-') with a series of magnets around the circumference. The magnets gradually accelerate the particles as they circle the ring, until they get to the desired speed. Generally, the particle speed is given in terms of how much energy the particle has. The MGH cyclotron gets the protons up to 235 MeV, where MeV is mega electron volts, the energy contained in a million electrons after accelerating through one volt of potential difference. We can convert this to a velocity using the relativistic kinetic energy equation:
As far as relativistic particles go, this isn't stunningly fast, but it's certainly fast enough to get some neat effects. If there were a clock on the protons that we could read, we would see 4 seconds tick by for every 5 seconds that passed on our clocks. According to the specs of the MGH cyclotron, it takes 800 turns around the loop to get up to the correct speed. I couldn't find exactly how big the MGH ring is, but a similar cyclotron has a diameter of 6.6 meters. Some quick algebra gives the total distance traveled as 16.6 kilometers. According to us, that trip will take 92.2 microseconds, but if you were riding on the proton, it would only seem like 73.8 microseconds.
Another interesting property of the protons is their Bragg peak. This is the depth at which the proton deposits the majority of its energy in the material it's traveling through (in this case, my brain). This plot compares the Bragg peaks for protons and photons (from Wikipedia):
Notice that the photon (used in typical radiation treatment) peaks almost immediately, and stays relatively high for a significant distance. The proton, however, peaks sharply at a specific depth, then drops to zero almost immediately. This is the main advantage of proton therapy over traditional radiation treatments – very little is irradiated aside from what is targeted, reducing side effects.
I hope you haven't been put off by the cancer talk – I'll start on more everyday things tomorrow.
That proton/photon graph is awesome! I wonder how I could make use of tightly targeted radiation in my own daily life.
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