How to Build a Muon Detector

A ring-imaging Cherenkov detector (RICH) is not something you’d find in most universities. Designed to identify subatomic particles, it’s an expensive piece of equipment. So what do you do when you want a RICH detector in a less-than-RICH university? You have to think outside the box.

The story of our detector starts during my sophomore year, when particle physicist David McKee joined my university’s teaching staff. When I was a junior, I had to design my own research project for an advanced lab. Shemaiah Khopang, who had been my lab partner since our freshman year, teamed up with me and we chose to work under Dr. McKee to make a RICH detector. We decided to use commercial off-the-shelf products. Our goal was to come up with a design that could be made available for other universities that, like us, wanted to study the muon energy flux.

To do this, we would need a camera, a data acquisition system, a transparent medium with an appropriate index of refraction, and a lot of math.

The muons we hoped to spot raining down upon our creation start as cosmic rays, high-energy particles that arrive from outside our planet’s atmosphere. When cosmic rays encounter the atmosphere, they produce secondary particles, including muons, which constantly strike Earth from all different directions at nearly the speed of light. A muon (µ) is an unstable subatomic particle with a mean half-life of approximately 2.2 µs.
What is really cool about muons is that they are one of only a few secondary particles from cosmic rays that actually reach sea level. If you were to hold out your hand, one muon will pass through it, on average, every second.

Cherenkov radiation occurs when a subatomic particle shoots through a medium faster than light moves through that particular medium. Detect Cherenkov radiation, and you can measure the energy and angle spectrum.

To create Cherenkov radiation, we needed a transparent medium with an index of refraction high enough to create Cherenkov radiation but low enough to avoid total internal reflection, which we calculated to occur at n = 1.41. We chose magnesium fluoride as our medium. Magnesium fluoride has an index of refraction of n = 1.37, and although it is a salt, it will not dissolve in Missouri-level humidity.

To spot the radiation, we chose a Sony A3000 camera. It has a removable lens and a powerful complementary metal-oxide semiconductor (CMOS) image sensor. In order for our design to work, the CMOS image sensor needed to be placed flush against the magnesium fluoride to eliminate errors that would be caused by Cherenkov radiation traveling through air, which has a lower index of refraction, n = 1.003. With the medium flush against the sensor, a muon should hit the magnesium fluoride, create a cone of photons emitting at Cherenkov angle θ, and hit the CMOS detector creating an ellipse or “ring” of light. If a gap of air is left in between the detector and medium, then we would end up with an additional angle φ added to θ because of the index of refraction of air. This effect would most likely lead to the photon ring missing the detector all together. Another problem that could arise is if the magnesium fluoride has a slight angle with respect to the CMOS. In this case, a possible outcome could be that our ellipse would be elongated or of a different shape than expected. Thus, our goal was to capture the ring of photons at an angle θ solely dependent upon a muon’s energy and angle, as well as the index of refraction determined by the magnesium fluoride.

The layout of the Sony A3000 is such that the CMOS image sensor is located deep within the camera base. The only way to get the CMOS flush with the magnesium fluoride was to completely disassemble the camera, modify it, and reassemble it. Seems simple enough, right? Wrong! Once we got the camera taken apart and the magnesium fluoride installed, we had to put the device back together, which was difficult. Because of the thickness of the magnesium fluoride, about 5 mm, we would need longer screws. Not that big of a problem, you might think. But the screws needed to be 1.6 mm in diameter. That was a problem. Apparently this particular diameter of screw is not readily available in the United States, or anywhere else as far as we could tell.

Luckily, Shamrock Bolt and Screw, a local hardware store, was able to get what were apparently the last 39 1.7-mm screws in the United States. Thank goodness they worked for our application, even though they were oversized in diameter by 0.1 mm.

As we struggled with this technical challenge, we had a deadline fast approaching. I was to present our research at the 2015 April Meeting of the American Physical Society. But we had nothing to show, just a seemingly failed engineering attempt.

With only two weeks left to obtain the “Cherenkov images” that I had applied to present, it was time to ask for more help. Physics major Chase Garrett had expressed interest in our project and decided to join our team. We worked in-between classes, late into the night, and even on weekends. Eventually we got the camera built and functioning properly!

Taking pictures of Cherenkov radiation is a bit different than snapping shots of your child playing in a soccer game. To capture the image, we needed complete darkness. We obtained this darkness by wrapping the camera about six times in black felt, turning off the lights, and sticking our hands inside the felt blanket to click pictures. We played around with exposure times and videos. Eventually we decided to stick with a 10-second exposure time. After 30 minutes of taking snapshots at these exposures and analyzing the images, we found exactly one photon ring.

We were ecstatic! Our one photon ring was all we had to bring to APS, but let me tell you, that felt way better than no photon rings. The presentation went really well. Just attending my first APS conference was an out-of-this-world experience. I learned so many things in that weekend that can’t be taught at school.

I recommend conferences such as this one to any undergraduates planning on attending graduate school. The lectures were educational, but the real education was given to us by SPS sessions arranged just for undergrads. Advice was given to us about graduate school and career choices. Being able to hang out and spend one-on-one time with grad students and ask them for advice will prove most helpful moving forward in my education, I am sure (see the article on p. 31 for more).

Moving forward, we plan to work on improvements to our design and gather more data. We will extract the angle of incidence, judged by the shape of the photon ring. We will work out the Cherenkov angle, judged by the size of the ring. Eventually, we will be able to analyze the muon energy flux.

We will then compare our data to that of large facilities. One of our future goals for this equipment is to transition this project into tomography by attempting to view cave hollows.

I plan to attend graduate school but am currently unsure as to which subfield I will end up studying. I originally wanted to do theoretical astrophysics to find out what the universe is made of and possibly help future generations find a way to travel through space. I’ve thought about doing particle astrophysics research that will further educate humanity about the particles that meet our atmosphere and the roles these particles play in our universe. I’ve considered nuclear physics to come up with some new clean energy that will help cut down on carbon dioxide emissions. And I’ve thought about being a geophysicist to try to come up with an economical solution to replenishing our groundwater.

I just hope that, whichever direction I choose, one day I can do something to help and further the knowledge of mankind. This project was a great first step in that direction. //