As a former particle physicist and a current Raspberry Pi aficionado, I was instantly intrigued when I saw the Hackaday description of Marco Reps' CosmicPi build. This set me off on a bit of a quest to understand the state of DIY muon detectors.
Goals / Purpose
One important question to start with is "What would I want a muon detector for?" So far, I've come up with only about 3 answers. Nearly all of the DIY projects I've found seem to focus on the detector as a "teaching lab," i.e. a hands-on project for undergrads or high-schoolers to learn about building detectors and perhaps about the basics of cosmic rays. Here, the emphasis is just on detecting invisible particles (muons) and perhaps on measuring the change in muon flux as a function of declination angle or with altitude.
The second potential goal of such DIY detectors would be to crowd-source some kind of detector array for air showers. Initially, I understood such "citizen science" to be the main goal for the CosmicPi project. This is explicit in James Devine's TEDx talk, and I believe it is still a long-term hope for the CERN team, but they seem to have given up on this in the near term. The problem is that this would require a conveniently purchasable kit that could be assembled by hobbyists and sufficient uptake of these kits to find a reasonable density of detectors in any given geographic area in order to reconstruct cosmic air showers with some accuracy. Although I might well be interested to participate in such an effort, it doesn't seem likely that anyone would get useful physics measurements out of an organic, ragtag adoption of these devices.
That leaves the third potential goal for such devices, which would be muon tomography. There have been a number of research projects aiming at muon tomography, most famously with the effort to find undiscovered chambers in the Great Pyramid, but also with the MU-RAY project looking at Mt Vesuvius for magma chambers. It's not clear what hidden chambers one could find in your own house, but perhaps some interesting applications might come up. However, this does increase the complexity of the detector requirements. In particular, one would need decent angular resolution on the muon's direction, and this would either require a significant number of channels (i.e. multiple scintillators, scintillating fibers, or perhaps CCDs) or else a narrow aperture and a way to step across a useful field of view (which would then necessitate very long exposure times).
Survey of Prior Art
There are quite a lot of projects one can find in this general space, even though none of them truly reach the target of a DIY home-built detector. My list doubtless leaves out many projects, but hopefully captures the most relevant ones.
CosmicPi
The initial reference I found was to the CosmicPi design, which came from a group at CERN. There is a recorded webinar from Dec 2020 that gives a good summary of the current state of their project. My understanding is that this is primarily now an outreach program for schools around CERN, with the CosmicPi team building some 1 or 2 dozen units (per year?). The designs have been open-sourced, allowing anyone else to build their own unit, in theory, but there are no comprehensive kits available for sale (and parts and PCBs would get more expensive in smaller lots). In the webinar, James Devine indicated the parts cost currently running about $375 (350 CHF), which is probably too high for the casual hobbyist.
CosmicWatch
It turns out that the CosmicWatch project at MIT is very similar to CosmicPI and also focused on student outreach, but perhaps coming from a slightly earlier generation of technology. It does not integrate a Raspberry Pi, but instead supports a serial port readout to an unspecified computer. Also, the CosmicWatch detector holds a single 5cm x 5cm scintillator, rather than the pair of 10cm x 5cm scintillators in the CosmicPI. Hence, one would need 2 of the $100 CosmicWatch units to do a similar coincidence trigger as on the CosmicPi design. Part of the difference in expense could also be from the blue-to-green wavelength-shifting fiber (WSF) in the CosmicPi design versus the straight blue design in CosmicWatch. It's unclear how big a difference in efficiency the WSF makes. I think the main concern is to use WSF with larger scintillators where the blue light has a short absorption length compared to the size of the scintillator.
OpenPhysicsLabs.org Muon Telescope
This description appears to be an independent effort very similar to the CosmicWatch effort (the paddles here are 5.7 cm squares vs 5 cm squares in CosmicWatch). There is some hint at using this for tomography, but it seems more geared towards students with, for example, a measurement of muon flux as a function of declination angle (and also the discussion about East-West asymmetry, which is a pretty interesting subtlety).
Muon Telescope at GSU
The project website seems unfinished, but there are some additional details on the github site. The concept uses 2 (or more?) paddles of 14.5 cm x 14.5 cm scintillator with a loop of WSF milled into the paddle to gather light into a silicon photomultiplier (SiPM) detector mounted on the corner of the paddle. It's unclear if this is aimed at a low-cost DIY project, since the machining and assembly seems fairly involved. It's also unclear how useful a resolution one would get from 14.5 cm paddles separated by 1 m or so.
MU-RAY project at Mt Vesuvius
This is not a DIY project (and not at all cheap!), but it demonstrates what is needed for useful muon tomography. The detectors are 1 m2 of triangle extrusions of plastic scintillator with wavelength-shifting fibers running down the centers. There are a total of 128 channels in each of 3 planes. A lot of good details are available in this talk.
MITO
MITO has a clever design where position info is extracted from a single scintillator block via timing of 4 PMTs at each of the corners. This is an interesting technique that could perhaps be applied at a smaller scale for a DIY project to reduce parts cost while still getting decent angular resolution.
Comparison of Technologies
For tomography, the main figure of interest is the angular resolution on a muon track. For a 2-paddle detector, this is something like the width of an element a divided by the distance between the paddle planes b. Obviously, things get easier if there are multiple channels next to each other in each of the planes, giving a wider aperture of varying muon angles that can be observed. The smaller that a gets, the closer the other detector plane can be and still give the same resolution a / b. However, the muon flux also decreases as a decreases, so there would be a need to increase the exposure time correspondingly as one shrinks a. One advantage of a cheap DIY muon detector, though, is that it can easily be run for very long exposure times.
An alternative to multiple scintillator blocks would be to use scintillating fibers. These are often embedded into scintillator blocks (such as the triangle extrusions of the MU-RAY detector), but they could also be used just as ribbons of multiple fibers. The downside, however, is that each fiber would need its own SiPM, so there is likely a significant cost with even 16 or 32 fibers.
An affordable alternative might be to use CCD camera chips with a light-proof cover over the lens. This provides a very large number of pixels with a very small a. However, the alignment of 2 or 3 camera planes with sufficient accuracy to reconstruct muon trajectories would likely be a significant technical challenge. Perhaps the alignment could be determined empirically with enough "calibration" tracks through (at least) 3 planes/CCD chips.
There have been a number of efforts to use cell-phone cameras as crowd-sourced muon detectors, so I presume the technology is roughly viable as a muon detector (and it clearly has a cost advantage over scintillator and SiPMs). I am currently experimenting with the Raspberry Pi camera to see how well this works in practice. One big concern is the size of the camera being 100x smaller than the typical scintillator paddle (5x5 mm vs. 5x5 cm), requiring correspondingly longer exposure times.
- DECO paper about cell-phone camera CCDs. Also, sensorcast.org group. Android-only app at present.
- CRAYFIS project - another cell-phone effort. Also a paper that analyzes the number of phones needed to make a decent air-shower collector.
- Cosmic Ray App - similar app, but iOS only.
