[Crater-team] Albedo muons?

[Mark D Looper]

Hello--

	Sorry to be getting to this so late; during the telecon for which Andrew prepared this PPT I was at the Geant4 Space Users Workshop, and between that and AGU I'm a bit behind on, well, everything.  Haven't even cleared the floor to put our Christmas tree up yet!

	This is the first time I've seen a plot with the "mystery swoosh" pointed out.  In brief, I think it is due to a coincidence of two delta ray electrons inside the D6 detector, rather than to albedo muons.

	In my albedo simulations, all the muons (and pions) coming up from the lunar surface have decayed by the time they reach 50 km altitude.  Half to 2/3 of the electrons and positrons above 100 MeV at 50 km are the decay products of these unstable particles.  Referring to

https://cosmic.lbl.gov/SKliewer/Cosmic_Rays/Muons.htm

the lifetime T of a muon is 2.2 microseconds, so c * T is 660 meters.  To extend this mean travel distance to 50 km would require a time dilation factor gamma of about 76, or an energy of about 8 GeV.  The same reference says that the mean energy of muons created in atmospheric showers at Earth is not much below this, about 6 GeV; however, these particles are created in a very forward-peaked shower (less than one degree wide), so we could get such high-energy forward muons only from the lunar limb due to GCRs striking the surface at an extremely shallow angle.  At 50 km altitude the limb is 420 km away, so such muons would need eight times more time dilation (i.e., energy) to make it up to the spacecraft in reasonable numbers.  Upgoing muons would have to be the result of nuclear disintegration, like the neutrons, and would have a softer spectrum.  Anyway, as I noted, I do not see any evidence of undecayed muons at 50 km altitude in my simulation results from any direction.

	I do not have access to the 1985 Smart & Shea document referred to in the PPT, but I suspect that by "most common" they mean "in downgoing terrestrial atmospheric showers."  The energy loss of 2.2 MeV / (gm/cm^2) quoted is about what you would expect for 6 GeV muons, well into the relativistic rise above the minimum-ionizing velocity; Figure 1 of

http://pdg.lbl.gov/2016/AtomicNuclearProperties/adndt.pdf

shows the curve of dE/dx vs. energy, with the minimum-ionizing value being the same as for electrons and protons, and indeed any Q = +/- 1 particle.  Anyway, if the "mystery swoosh" is indeed from a single kind of particle traversing first D6 and then D4, for a subset of these particles to deposit a lot more energy in D4 than in D6 toward the end of the track implies that such particles must be close to stopping in D4, like the 60 MeV protons that define the extreme end of the albedo proton track.  I didn’t find any general range-energy tables for muons, but at

http://nopr.niscair.res.in/bitstream/123456789/25092/1/IJPAP%2052%281%29%207-12.pdf

a muon of about 11 MeV will stop in a column density of lead equal to that of window + D6 + D5 + TEP + D4, about 3.6 gm/cm^2; the number might be a factor of two or so different for the lower-Z materials of CRaTER, but clearly we are well below the relativistic range here, so the muons that would have to make up a track like this (rather than being lumped down near the origin with near-minimum-ionizing deposits in both detectors) would have decayed away far below LRO.

	I suspect that we are seeing the large energy deposit in D4, coupled with energy deposit around 0.6 MeV in D6, due to a primary GCR traversing D4 but missing D6, with two delta rays generated somewhere along the path (possibly in the TEP at a location near D5/D6) that strike D4 and deposit twice the minimum-ionizing energy deposit.  This is the same thing that produces the heavy tracks parallel to the axes but with only 0.3 MeV deposited by a single delta ray, in the PHA crossplots in the PPT.  You'll note that this single-delta track is weaker paralleling the D6 axis (primary particle deposits large energy in D6, delta ray deposits small energy in D4) than paralleling the D4 axis; this is because most such events are due to energetic penetrating GCRs, and they are mostly coming from above (some come from slightly below, between 90 degrees off nadir and the lunar limb at 76 degrees for 50 km altitude).  Since delta rays predominantly go forward, it is more likely that a GCR coming from above will traverse D4 and send a delta ray down to D6 than traverse D6 and send a delta ray up to D4; the effect would be compounded when two delta rays must reach the other detector, which would explain why we see this "mystery swoosh" parallel to the D4 axis but not the D6 axis (i.e., albedo-like rather than GCR-like).

	My simulations from 2010 do not appear to have enough statistics to show this effect, so I can't point at it in a synthesized PHA crossplot.  However, I have seen exactly this effect in both simulations and data for the RPS sensor aboard the Van Allen Probes, and I will be on the lookout for it when I am able to re-do my simulations of CRaTER sensor response with more and faster cluster cores and with the current version of Geant4.  At present, though, I think that albedo muons are not the cause, and indeed couldn't be observed from the LRO orbit.  Sorry to be the Grinch here...

--Mark Looper

Mark D. Looper
Space Sciences Department
The Aerospace Corporation
M/S M2-260
P.O. Box 92957
Los Angeles, CA 90009-2957
Mobile: 310-529-3406
Voicemail: 310-336-6302

On 11/28/18, 8:51 AM, "Crater-team on behalf of Andrew Jordan" <crater-team-bounces at lists.sr.unh.edu on behalf of ajordan at guero.sr.unh.edu> wrote:

    Hi folks,
    
    I've attached a few slides for today's CRaTER telecon.
    
    Thanks,
    Andrew
    

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