Nothing reminds us like particle physics that we exist in an infinite three-dimensional pinball game. All around us, subatomic particles arc, collide, and merrily fly around. Some originate within our own bodies, while others come from the far reaches of the universe. However, detecting this invisible disruption requires equipment and can be expensive. I wanted to create a way to discover at least some pinballs for less than $15 USD.
My main reason was to get new material. I have a Ph.D. When I completed my PhD at the Institute of Physics III B at RWTH Aachen University, having a detector like this would help me fulfill my teaching duties while also using my interests in physics, electronics, and software design. I realized that it was.
Fortunately, we didn’t have to start from scratch. Oliver Keller of CERN’s S’Cool Lab has developed a DIY particle detector that uses inexpensive silicon photodiodes to detect alpha and beta particles (helium nuclei and free electrons flying through the air, respectively) and estimate their energies. has been created. Photodiodes are typically used to respond to light, such as signals used in fiber optic communications. But charged particles that strike the photodiode also produce current pulses, and higher-energy particles produce larger pulses. In practice, given typical conditions and the sensitivity of the photodiode, this means primarily detecting beta particles.
In Keller’s design, these pulses are amplified, converted to voltage, and sent via a cable from the detector’s audio jack to the microphone input of a laptop or smartphone. The data is then digitized and recorded.
A colleague of mine was building the CERN instrument and realized there was room for improvement. Passing an analog pulse signal through an audio cable makes the detector susceptible to noise from a variety of sources. Additionally, this design requires its own power source in the form of a 9-volt battery. Besides the hassle of having to get a separate battery, this means that if you wire your device incorrectly, you could be sending unacceptable voltages to your expensive smartphone.
Reducing amplification noise
I decided to solve these problems by digitizing photodiodes. The closer you get, the less noise you have to deal with. Noise-tolerant digitized data is transmitted via a USB connection, which can also power the detector.
BetaBoard uses three types of printed circuit boards. The cover (top) and body board (middle) are free of circuit traces and are used to create a light-tight and electromagnetically shielded enclosure. The bottom board contains a photodiode detector array and an RP2040 microcontroller. james provost
Of course, digitizing the signal from the photodiode requires some onboard processing power. I settled on the RP2040 microcontroller. There are some known issues with analog-to-digital converters, but you can work around them. The chip also has plenty of computing power and a built-in USB controller.
The first design of the so-called BetaBoard created a single printed circuit board with an RP2040, an array of photodiodes, and a set of low-noise amplifier integrated circuits. I wrapped the board in aluminum tape to prevent the light from triggering the photodetector. The results proved the concept, but although we eliminated noise from the audio cable, we discovered a new source of noise: the USB power source.
High frequency noise above 1 kilohertz from a USB connection comes from the data and polling signals flowing across the interface. Low frequency noise comes from the host computer’s AC power supply (50 hertz here in Europe). I filtered out high-frequency noise by inserting a low-pass RC filter in front of the amplifier’s supply voltage pin and using plenty of capacitors in the rest of the circuit. Filtering 50 Hz noise in hardware is difficult, so my solution was to integrate a digital high-pass filter into the software I wrote for the RP2040. (Hardware and software files are available in my Github repository.)
This software also provides a serial interface to the outside world. A human or program can send commands and retrieve data via the USB cable. Created Python scripts to record data and generate visualizations.
Another improvement I made to the original design was to eliminate the need to wrap the board in aluminum tape (or place it in a container like in Keller’s original version).
To do this, we designed two other PCBs with the same dimensions as the original board but without any circuitry. The first type has two large cutouts. One is an open area over the photodiode array and amplifier, and the other is over the RP2040 and its support circuitry. The photodiode cutout is surrounded by wide metal fillers on the back and front of the PCB, and the fillers are connected with vias. By stacking two PCBs of this type on top of the circuit board containing the components, we created an enclosure that provides shielding against electromagnetic interference.
A photodiode has a junction between a positively doped region and a negatively doped region, with a neutral depletion layer formed between them. Incident light or charged particles (red line) generate charge carriers in the depletion region. This causes a spike in current between the doped regions. The height of the spike is proportional to the energy of the particle. James Provost
The second type of PCB served as a cover for the stack and had a small cutout above the photodiode array, on which I applied black tape. This is enough tape to block light, but enough tape to allow beta particles to reach the photodiode.
The result is a robust, if not the most sensitive, detector in the world. I estimate that a research grade detector records 100 counts per second from a given beta emitter, but I get about 10 counts. However, you can use it to make meaningful measurements. My next step is to be able to detect alpha particles as well as beta particles, similar to Keller’s version. I could accomplish this by modifying the $10 photodiode, but I’m experimenting with ways to use the cheaper photodiodes used in the rest of the design. We are also working on documentation for use in classroom environments where there is no room for a detector designer to be present.
