I was a bit nervous being first up on the night, which clearly made me talk really fast. I was wondering how I'd chopped a good 10-15 seconds out of my talk! Well, now I know.
I was inspired to talk about this topic for two reasons: firstly, my PhD was in designing particle accelerators for particle therapy (including proton therapy) which directly uses the beam from an accelerator to treat certain types of cancer. The second reason was that I thought the results which come up when you Google this weren't very satisfactory... Although there is one quite interesting video getting scientists to give their initial thoughts on the effects of putting your hand in the LHC beam
Now down to some science - where did the crazy numbers I gave in my talk come from?
Let's start with how much energy is dumped in your head when the LHC proton beam goes through it. Usually, I would get this data from NIST, but if you visit their website, you'll notice that the data for protons only goes up to about 10 GeV in energy (that is 700 times lower in energy than the nominal LHC proton beam energy). So what do we do?
Let's pretend your head is made of muscle, if you have a look at the graph below of proton energy loss (stopping power) in muscle that I got from the NIST website, you can see that it kind of levels-off at high energies. I took the value of energy loss to be 2.5 MeV/cm (this assumes the density of the head is something like 1.0 g/cm^3, which is the same as water, so pretty reasonable).
This is the big 'assumption' I had to make as the data simply isn't available for higher energies!
If the head is about 20cm thick (another assumption), each proton loses 50 MeV of energy each time it passes through the head. In joules, this turns out to be 8.011E-12 J, so not much, really!
Now, in the LHC beam there are 1.15E+11 protons per bunch, and 2808 bunches per beam. So this means that the total energy loss per bunch is 0.9213 J and therefore per beam is 2586.88 J - this is a lot!
The LHC beam is travelling so fast that in one second it can go around the 27km ring about 11,000 times. So if you put your head in the beam for one second, the total energy absorbed by your head would be: 2586.88 x 11,000 = 28,455,659 J. (Here I have assumed that you only get in the way of one (not both) beams, and that the change in position of the beam losing 10% of it's energy would not lose the whole beam in the machine, which might be unrealistic, but not as unrealistic as the idea of putting your head in the beam anyway...)
But Joules don't mean much to anyone, so lets calculate the absorbed radiation dose in Gray (Gy). We know that a dose of 5 Gy will lead to death within 14 days, and this is equivalent to receiving 375 J for a 75kg adult.
So if we divide our energy absorbed (28,455,659 J) by 375 J, we find out that:
Putting your head in the LHC beam for one second would kill you not just once, but 75,882 times.
If there's one thing I can leave you with, though, it's that proton therapy using smaller accelerators providing much lower doses can (and does!) treat certain types of cancer much more effectively than X-ray radiotherapy. I encourage you to look it up - although for the sake of keeping this post from being thesis-length, I won't describe it here now. Maybe that will have to wait for a future post...