Muon Telescopes...Attack! Part I

The last time I was in Rapid City I got a chance to visit my friend's physics lab at SDSM&T. Mark Hanhardt showed me his work detecting incoming particles generated from cosmic ray interactions with the atmosphere. He does this with an uber-sensitive photomultiplier tube that can measure individual photons. When I was there, I noted the suspicious absense of duct tape. That is simultaneously comforting and disturbing as his work will eventually become part of the research lab at the Deep Underground Science and Engineering Lab at the Sanford Homestake Laboratory in Lead, SD.

Mark's Lab - Note how time and space are visibly distorted there. Two photomultiplier tubes are visible on the left, middle - they are the gold colored glass bulbs with grey bases in the box. 

Mark, the FLOEAS resident particle physicist, had a response to the DUSEL muon flux shielding questions that came up a while back:


I have two notes about the work you did to calculate the mwe: First of all, I may have just been really tired when I read one of your calculation emails, but I don't think that comparing the relative densities of different types of rock to water will work.  I'm probably overthinking this, but when we are talking about muons, we are talking about charged particles, so it's not just the mass of the barrier, but also the charge distribution, structural lattice, and other atomic considerations. Then again, as I type that out, I think those things may smooth out enough over large distances that maybe my objection is moot.  Anyway, my second note would be this: I think that the muon rate is so easy to measure empirically that it's entirely possible that when people do mwe estimates, they simply let it be equal to some close, round value of the depth of rock (adjusted by experience) until someone actually measures it with a muon telescope or other apparatus to update the estimate. 


Yes, muon flux can be measured directionally.  I have been so focused on trying to finish my PMT work, that I have had no time to study the muon telescope I will be installing at Sanford Lab in January, but as I understand the proposed design it will primarily consist of two separate scintillation/PMT devices placed tens of meters apart in the line in which we expect the muons to travel.  (The underlying concept is that a muon is confirmed detected when we get a coincident signal in both detectors.)  As such, I expect that by placing these two devices along different lines we can measure muons incident from a different direction.  Given good enough time resolution, I also believe I can tell if the muon is traveling parallel or antiparallel to my device-line, although it's something I don't think my professor has considered since we will probably just assume that the bulk of the muons are traveling downward.


Mark is a great guy. I hope he finishes his physics degree, because he is my friend and not because of the monster it will make him. Only getting a PhD and becoming a tenured professor could make his ego any bigger, but his belligerence is boundless.

So, scintillators and photomultiplier tubes are arranged to detect incoming muons. The scintillators are pieces of plastic that emit a flash of light when struck by ionized particles, such as muons. The flash of light is detected by the  photomultiplier tube, converted to an electrical pulse, and amplified hugely to a voltage that can be measured by an oscilloscope. There are two sets of these scintillator/PMTs separated by some distance, tens of meters as Mark says. When an incoming particle strikes both scintillators (nearly simultaneously), scientists know it is a muon as opposed to some other type of radiation - say, decaying thorium in the nearby rocks which would only strike one scintillator. This apparatus is called...wait for it...a muon telescope!

Mark is saying that if one were to place two sets of these detectors at right angles, you could tell the difference between cosmic rays incoming from the atmosphere and random flux. This would help answer my question about heterogeneous muon flux because of changes in topography or overlying geology. The only catch is you would have to rotate this big, 30+ foot long apparatus inside a relatively small underground cavern long enough to gather enough muon measurements. I would even argue that the perpendicular apparatus is not necessary to measure flux.

That's it for this one. I have a bunch of detailed questions about how this stuff works and attempt to answer it in the next post, Part II.