Every hour of every day, a thin cosmic rain of charged particles collides with the earth’s
atmosphere, some of which eventually reaches the surface.
Until recently, observing and measuring cosmic radiation was the domain of physicists
in fancy laboratories.
But now, thanks to a group of scientists at MIT and the National Centre for Nuclear Research
in Warsaw, even a dork in the garage like me can be a citizen particle physicist.
Hey I’m Grady and this is Practical Engineering.
Today we’re building a Muon Detector.
This video was sponsored by XOD - more on that later.
Behold the CosmicWatch Desktop Muon Detector - or at least the pieces of one.
This project was designed as an education tool for a “novice high school student,”
so I’m a little bit outside of my skill level.
But, who am I to deny you the incongruity a civil engineer soldering tiny components
to a circuit board while talking about cosmic radiation?
Before we get into the engineering behind the device, first we need to know a little
bit about cosmic rays (or at least our current understanding of them, because there is still
a lot of mystery behind their origin).
Spread throughout our galaxy, and indeed the entire universe, are stars.
On occasion, those stars explode creating supernovae, and when they do, they eject a
tremendous amount of interstellar material, also known as star stuff.
This material is traveling so quickly that it generates a shock wave of superheated plasma.
These shockwaves are believed to be the origin of most of the universe’s comic rays.
The superheated plasma accelerates the particles to unimaginable speeds, and some eventually
reach the earth.
When they slam into the earth’s atmosphere, they produce a slew of secondary particles
with crazy names like pions and kaons that eventually decay into muons that can survive
the trip through the atmosphere and even penetrate into the earth’s crust.
Scientists observe and measure muons and other cosmic radiation to learn more about the universe
with fantastically complex and expensive equipment, but this detector opens that door to any student
or citizen with a soldering iron and a good magnifying glass.
As fun as it is to dive into particle physics, the coolest part of the CosmicWatch project
is the engineering.
The device uses interesting components and clever circuity to make it possible to detect
and count these cosmic rays.
And it all starts with the scintillator, a piece of plastic with a very special ability
to absorb the energy of a radiation particle and re-emit that energy as light.
As a muon passes through the scintillator, a burst of light is created.
It’s not enough to see with the naked eye, but it can be detected by the attached photomultiplier,
which is essentially a super-sensitive solar panel capable of measuring even just a single
photon.
The photomultiplier converts the burst of light from the scintillator into an electric
signal.
But, this signal is extremely short - less than a microsecond - which is hard to detect.
The CosmicWatch uses an Arduino to measure the signal, but it can only take a measurement
about once every 6 microseconds.
You can see how easy it would be for the arduino to miss the Muon pulses.
So the CosmicWatch includes a peak detector circuit to amplify and stretch out the signal
so that it can be detected by the arduino.
This shot from the oscilloscope shows the output from the peak detector in yellow.
If I zoom in on the time scale, you can see how short the actual pulse from the photomultiplier
was.
Once the arduino detects a pulse, it sends a signal to this LED to let you know.
The arduino can count the amplitude and number of pulses to measure the average detection
rate, it can record each pulse on a memory card, and it can even send the data over USB
to your computer.
I built two detectors, which makes it possible to measure the direction of a particle and
helps eliminate false triggers from other types of radiation.
When operated in coincidence, a muon is only recorded if it was detected by both devices
at the same time.
In this shot, the bottom detector is the slave which only blinks if it detects a muon at
the same time as the master above.
Now that the detectors are assembled and working, it’s time to do some science, and there
is a lot of science that can be done here.
This is such a great educational tool, because the measurements are so simple.
Most of the experiments you can do are really asking the same question: does this particular
parameter affect the rate and or intensity of cosmic rays detected?
Spencer published some very cool experiments he used to test out the detector, including
how the rate changes at ground level vs. down in a mine and how much the detection rate
increases during a flight on an airline.
But, you know how much I like to make cool graphs, so I also designed a few of my own
experiments to test out.
First I know that Muon formation happens in the atmosphere, and I also know that some
atmospheric properties like temperature and stability change through the course of the
day.
So I hypothesized that there might be a measurable difference in muon detection between day and
night.
To test this out, I left the detectors running in the same spot for 24 hours.
I started the count at 6:30am when I left for work and reset at 8:00pm to leave it overnight.
The rates and measured intensities were almost identical, suggesting that, if there is a
difference in detection rate between day and night, it is only a small effect.
The null hypothesis prevails.
Next I wanted to test how the direction affected the detection rate.
You can leave these detectors blinking on your desk, but it’s still hard to imagine
the cosmic rays passing through your personal space if you don’t know what direction they’re
coming from.
My guess was that most would come from directly overhead because it’s the most direct path
through the atmosphere.
I set up the detectors one on top of each other for a day, then side by side for a second
day.
My results agreed with Spencer’s that the detection rate from side to side was about
half of that from straight up.
This chart shows the probability that a muon would exceed a certain amplitude, and you
can see that the measurements from the horizon had more low-energy detections than from straight
overhead.
My last experiment, obviously, needed to be related to concrete because I said I was going
to keep making videos about concrete and then made a muon detector instead.
My hypothesis was that layers of concrete would provide some shielding and attenuate
the detection rate.
So, I left the detectors in coincidence mode running in my car and parked on a different
level of the parking garage at work for three days.
Measuring only the particles coming from straight up, there was a small but obvious reduction
in the detection rate for each layer of concrete in the parking garage above my car.
I love this project because it takes something that is not just invisible, but maybe unknown
to most people, and makes it so tangible and approachable.
If you’re an educator, this is an awesome tool for exploring the scientific method because
the experimental design is fairly simple, the data collection is easy, and the subject
matter is fascinating.
Huge thanks to Spencer and the other folks associated with CosmicWatch who developed
this awesome device and helped me with this video.
Check out the link to their website in the description.
Thank you for watching, and let me know what you think!
Getting started in electronics can be intimidating at first.
If you’re using microcontroller boards in your projects like the Arduino in the Muon
Detector, you not only have to learn about circuits but also about writing code.
With XOD, you don’t need to be a programmer to use a microcontroller.
With its visual programming language and well-written tutorials, you can get started on awesome
projects with almost no learning curve.
Building programs visually with nodes makes controlling your electronics projects fun.
XOD is open source, and best of all, it’s completely free.
To get started, go to XOD.io or just click the link in the description below.
Thank you for watching and let me know what you think!