There’s nothing like particle physics to make you conscious that we exist in an infinite three-dimensional pinball sport. Throughout us, subatomic particles arc, collide, and barrel together with merry abandon. Some originate within our own bodies, others come from the far ends of the cosmos. However detecting this invisible tumult requires gear, which will be pricey. I wished to create a technique to detect at the least a number of the pinballs for lower than US $15.
My predominant purpose was to have a brand new educating instrument. I’m doing a Ph.D. within the Physics Institute III B at RWTH Aachen University, and I spotted such a detector would assist fulfill my educating obligations whereas tapping into my pursuits in physics, electronics, and software program design.
Luckily, I didn’t have to begin from scratch. Oliver Keller at CERN’s S’Cool Lab has created a DIY particle detector that depends on cheap silicon photodiodes to detect alpha and beta particles (helium nuclei and free electrons whizzing by the air, respectively) and estimate their vitality. Usually, photodiodes are used to answer gentle, such because the indicators utilized in fiber-optic communications. However a charged particle placing the photodiode may even produce a pulse of present, with higher-energy particles producing larger pulses. In observe, given typical circumstances and the sensitivity of the photodiodes, this primarily means detecting beta particles.
In Keller’s design, these pulses are amplified, transformed to voltages, and transmitted through a cable from an audio jack on the detector to the microphone enter of a laptop computer or smartphone. The information is then digitized and recorded.
A colleague of mine had constructed the CERN gadget, however I spotted there was room for enchancment. Passing the analog pulse sign by the size of an audio cable left the detector vulnerable to noise from numerous sources. As well as, the design requires its personal energy supply, within the type of a 9-volt battery. Other than the trouble of getting a separate battery, this additionally implies that if you happen to miswire the gadget, you’ll ship an unacceptable voltage into an costly smartphone!
Lowering Amplification Noise
I made a decision I might clear up these issues by bringing the digitization to the photodiodes. The nearer I may get it, the much less noise I’d must cope with. Noise-resistant digitized information may then be despatched through a USB connection, which may additionally provide energy to the detector.
The BetaBoard makes use of three varieties of printed circuit board: The quilt [top] and a physique board [middle] don’t have any circuit traces and are used to create a light-tight and electromagnetically shielded enclosure; the underside board hosts a photodiode detector array and an RP2040 microcontroller. James Provost
In fact, to digitize the sign from the photodiodes, I would want some onboard processing energy. I settled on the RP2040 microcontroller. Though it does have some recognized problems with its analog-to-digital converter, you possibly can work round them, and the chip has greater than sufficient compute energy in addition to a built-in USB controller.
In my first design of my so-called BetaBoard, I created a single printed circuit board populated with the RP2040, an array of photodiodes, and a set of low-noise amplifier built-in circuits. I wrapped the board in aluminum tape to stop gentle from triggering the picture detectors. The outcomes proved the idea, however whereas I’d eradicated the noise from the audio cable, I found I’d launched a brand new supply of noise: the USB energy provide.
Greater-frequency noise—over 1 kilohertz—from the USB connection comes from information and polling indicators flowing over the interface. Decrease-frequency noise originates within the AC energy provide for the host laptop—50 hertz right here in Europe. I filtered out the high-frequency noise by inserting a low-pass RC filter earlier than the amplifiers’ provide voltage pins and liberally utilizing capacitors in the remainder of the circuitry. Filtering out the 50-Hz noise in {hardware} is difficult, so my answer was to only combine a digital high-pass filter into the software program I wrote for the RP2040. ({Hardware} and software program information can be found from my Github repository.)
The software program additionally supplies a serial interface to the surface world: A human or a program can ship instructions through the USB cable and get information again. I wrote a Python script to report information and generate visualizations.
One other enchancment I made to my preliminary design was to remove the necessity to wrap the board in aluminum tape (or place it in a container, as in Keller’s authentic model).
To try this, I designed two different varieties of PCB with the identical exterior dimensions as the unique board, however with none circuitry. The primary kind has two massive cutouts: an open space over the photodiode array and amplifiers, and one other space over the RP2040 and its supporting circuitry. The photodiode cutout is surrounded by a broad metallic fill on the front and back of the PCB, with the fills related by vias. By stacking two of this sort of PCB on the circuit board containing the elements, I created an enclosure that gives shielding in opposition to electromagnetic interference.
A photodiode has a junction between positively and negatively doped areas, with a impartial depletion layer forming in between. Incoming gentle or charged particles [red line] creates cost carriers within the depletion area. This produces a spike in present between the doped areas. The peak of the spike is proportional to the vitality of the particle.James Provost
The second kind of PCB acts as a canopy for the stack, with a smaller cutout over the photodiode array, over which I positioned some black tape—sufficient to dam gentle however nonetheless enable beta particles to succeed in the photodiodes.
The consequence is a strong detector, albeit not essentially the most delicate on the planet. I estimate that the place a research-grade detector would register 100 counts per second from a given beta emitter, I’m getting about 10. However you can do significant measurements with it. My subsequent step is to present it the power to detect alpha particles in addition to beta particles, as Keller’s model can do. I may do that now by modifying a $10 photodiode, however I’m experimenting with methods to make use of the cheaper photodiodes utilized in the remainder of the design. I’m additionally engaged on the documentation in order that it may be utilized in classroom settings that don’t have the luxurious of getting the detector designer current!
