# Why particle acceleration collisions don't cause explosions?

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1. Nov 14, 2015

### xpell

Please be patient with my ignorance. :)

I have just learnt in the LHC's own website ( http://lhc-machine-outreach.web.cern.ch/lhc-machine-outreach/beam.htm ) that their proton beams traveling at 0.999999991c carry about 362 MJ of energy, equivalent to 77.4 kg of TNT. These beams seem to be composed of 2808 bunches of protons, so I assume each bunch packs 128 kJ ---about 7 times the muzzle energy of a .50 BMG bullet.

Sure my question is easy and possibly silly, but I'm puzzled. :) Why doesn't the impact of these beams against a target cause something similar to a 77.4 kg-TNT equivalent explosion, or at least the effect of an ultra-rapid fire, super-heavy machine gun shooting 2808 projectiles at point-blank range, please?

(If it's not the case, please apply to any high-energy particle accelerator "shooting" at targets, or to accelerators of heavier ions, if you don't mind.)

Last edited: Nov 14, 2015
2. Nov 14, 2015

### BvU

As you might guess "it's not the case": the beams aren't sent to a fixed target but are made to collide with each other. Even though the 16 micron transverse dimension doesn't seem all that big, most of the particles in a bunch go through the other bunch without anything happening. Only a few hadrons actually collide head-on.

But if the beam is lost for some reason, it can sure cause a lot of damage !

3. Nov 14, 2015

### Staff: Mentor

The beam dump has been designed very carefully to absorb this energy without exploding. The actual absorber material is at the end of a tunnel with a length of 750 meters, so (a) the bunches spread out a bit and (b) it is possible to direct the bunches to different places at the actual beam dump. In addition, the particles are extremely fast, so unlike a bullet they go deep into the target material and their energy is distributed within several meters of the target material, not all in the first centimeters.
In total, the beam energy is distributed over several tons of graphite. It gets really hot each time the beam is dumped, but it stays solid and does not explode.

More details

4. Nov 14, 2015

### xpell

Yes, I remembered it later, that's why I added that sentence. :) But there are some other types of high-energy accelerators (LINACs?) which actually "shoot at a target", aren't there?

Got it, thank you both very much, I was really curious about this! :) By the way, do they use graphite because it is the best "stopper" or because it is the best "deccelerator" ("moderator"?)? I mean, if the beam was stopped too fast (with, I don't know, maybe lead or concrete?), this could cause the destructive effect that the beam dump is intended to avoid, am I right?

5. Nov 14, 2015

### Staff: Mentor

There are. They have much lower energies in the beams, however. If the secondary particles are needed (e. g. positron production with a high-energetic electron beam), the target needs a very good cooling. Sometimes rotating disks are used, sometimes liquid targets are used to improve cooling.

Lead would probably heat up too much, and its melting point is too low. Heat capacity and activation of the material are other things that have to be taken into account. The beam dump will get radioactive over time, and different materials lead to different activation levels.

6. Nov 14, 2015

### xpell

Thank you very much again. :) Yes, I was reading about this just now. It seems it's difficult to create a good "proton shield" and there is neutron production and proton/neutron activation and a lot of other phenomena, as far as I'm understanding!

7. Nov 14, 2015

### Staff: Mentor

Right.
Every proton or ion beam with at least a few MeV per nucleon will release neutrons via spallation. The neutrons get captured in atoms, making some of them radioactive. The direct target gets the largest dose, of course. If possible, materials are chosen that don't get activated much, like (pure) aluminium, silicon, some plastics, ...

8. Nov 15, 2015

### Orodruin

Staff Emeritus
A linac does not necessarily collide the beams with a stationary target. All it means is that the acceleration beamline is straight, unlike at the LHC where the beams are circulated. Linear colliders consist of two linacs accelerating two particle beams to collide with each other.

9. Nov 15, 2015

### xpell

Thanks, Orodruin. :) My point was that there are particle accelerators that actually "shoot at targets", but I appreciate this info too. ;)

10. Nov 15, 2015

### BvU

Thanks to you, xpell, for bringing this up ! It's an interesting subject, taken for granted by most physicists working at the experiments. I found this treatment on how to survive throwing away 350 MJ in a few microsecond time frame.

You'll probably find more in the cern yellow reports - there's a multitude of them. The overview ones are fairly legible, e.g. here and here

11. Nov 15, 2015

### xpell

You're very welcome, BvU! I've hated having a question but not a (rational) answer since I was a kid. :D Now I have it for this question that was puzzling me. :)

12. Nov 17, 2015

### nikkkom

I suspect beam dump gets significantly radioactive rather soon. Think about it. Lethal (for humans) dose of radiation, when converted to energy units (Joules), is actually trivial - it won't heat up the body by even 0.1 degree C. Here, poor beam stop absorbs amount of energy on the order of the speeding train car, and heats up to 700 C (IIRC). Even though spallation neutrons consume not all but a fraction of this energy, it's still lots of energy, which will be gradually released during beta-decay of activation products.

I noticed that it's very hard to find a specific information about that. CERN probably expects a bunch of neo-luddites getting predictably hysterical if they learn just how many mSv/h this beam stop cylinder gives off, so I guess CERN minimized exposure to this particular info to the bare minimum.

13. Nov 17, 2015

### Staff: Mentor

Most energy gets released as heat.
The studies exist, and should be public and open access (as everything published by CERN). They might be technical, and not of general interest to the public or even the accelerator and particle physics community (it is a unique system), so they might be a bit hard to find. The TDR for the LHC is probably a good starting point.

14. Nov 17, 2015

### nikkkom

Yes, they exist, and I found some info: LHC beam stop is a 7 meters long, 1 meter wide graphite cylinder, shielded by concrete blocks at the sides. Even photos of it can be googled up.
It is used approximately once or twice a day to absorb the LHC beam.
This info I did find.
What is conspicuously missing is, for example, "how much mSv/h (or rem/h) of gamma radiation beam stop emits now, after a few years of such usage?"

15. Nov 18, 2015

### Astronuc

Staff Emeritus
Ref: https://lhc-mp-review.web.cern.ch/lhc-mp-review/documents/LHC-DR-LBDS.pdf
See section 17.3.6 Beam Dump Absorber Block TDE, last paragraph, Activation
[40] L.Bruno, S.Peraire, Design Studies of the LHC Beam Dump, LHC-PROJECT-NOTE-196, 1999.

Some other background - http://lsag.web.cern.ch/lsag/BeamdumpInteraction.pdf

http://www.irpa.net/irpa9/cdrom/VOL.1/V1_13.PDF - puts radiation levels (from induced activation) at several tens of μSv/h in the dump caverns (outside the dumps), but doesn't elaborate on decay. In the dump material, inside the shielding, I would expect some orders of magnitude higher.

See - IV. Induced Radioactivity, and Figure 5 for dose rates as function of time (from an iron sample) and Figure 7 - http://www.aesj.or.jp/publication/pnst002/data/358-364.pdf

From Symmetry Magazine - http://www.symmetrymagazine.org/article/december-2007/protecting-the-lhc-from-itself

16. Nov 18, 2015

### BvU

Just because you can't find something straightaway doesn't mean it's "conspicuously missing" or "exposure is minimized to the bare minimum". There is Physics with a capital P at the frontier of human knowledge and there is nuts-and-bolts physics to make it possible. It's just less glamorous and doesn't make the front pages of the tabloids. It's good that you remind the community of its existence by posting this thread. As it was good that Simon van der Meer got a Nobel prize for something that made the $\bar p$ colliders possible to facilitate the $P$hysics.

17. Nov 18, 2015

### Staff: Mentor

The referenced study ("[40]") can also be found at CDS.

As I said, all those studies exist, and they are publicly available. "it is a cylinder with that size" is NOT a study, that is a very short description made for the general public that is not interested in the temperature at the left side of absorber 34b located 50 meters after kicker 13 (made-up numbers).

18. Nov 18, 2015

### nikkkom

The point is, I did not find it not only "straight away". I looked quite hard.

Why do you assume I'm an idiot?

Astronuc found it now. Thanks!

19. Nov 18, 2015

### BvU

I didn't make that assumption -- I know no more of you than what I read in this thread --, but I did find your wording somewhat tendentious (#12, #14). And even after so many years it put me in a defensive mode. Did you miss my compliment in post #16, or were you in turn in an excited state before encountering that ?

Ah I see, the compliment was meant for the OP. That's not so bad.