There’s nothing like particle physics to make you conscious that we exist in an limitless three-dimensional pinball recreation. Throughout us, subatomic particles arc, collide, and barrel together with merry abandon. Some originate inside our personal our bodies, others come from the far ends of the cosmos. However detecting this invisible tumult requires tools, which may be expensive. I needed to create a method to detect no less than a number of the pinballs for lower than US $15.
My most important cause was to have a brand new instructing instrument. I’m doing a Ph.D. within the Physics Institute III B at RWTH Aachen College, and I spotted such a detector would assist fulfill my instructing obligations whereas tapping into my pursuits in physics, electronics, and software program design.
Fortuitously, 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 means of the air, respectively) and estimate their power. Usually, photodiodes are used to answer gentle, such because the indicators utilized in fiber-optic communications. However a charged particle placing the photodiode may also produce a pulse of present, with higher-energy particles producing larger pulses. In apply, 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 by way of a cable from an audio jack on the detector to the microphone enter of a laptop computer or smartphone. The info is then digitized and recorded.
A colleague of mine had constructed the CERN system, however I spotted there was room for enchancment. Passing the analog pulse sign by means of 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. Aside from the effort of getting a separate battery, this additionally signifies that in case you miswire the system, you’ll ship an unacceptable voltage into an costly smartphone!
Lowering Amplification Noise
I made a decision I’d resolve these issues by bringing the digitization to the photodiodes. The nearer I might get it, the much less noise I’d need to take care of. Noise-resistant digitized information might then be despatched by way of a USB connection, which might additionally provide energy to the detector.
The BetaBoard makes use of three varieties of printed circuit board: The duvet [top] and a physique board [middle] haven’t 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 wish some onboard processing energy. I settled on the RP2040 microcontroller. Though it does have some recognized issues with its analog-to-digital converter, you’ll be able to 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 forestall gentle from triggering the photograph 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.
Larger-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 hard, so my resolution 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 offers a serial interface to the skin world: A human or a program can ship instructions by way of the USB cable and get information again. I wrote a Python script to document information and generate visualizations.
One other enchancment I made to my preliminary design was to get rid of 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 sort has two giant 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 steel 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 towards 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 power of the particle.James Provost
The second sort 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 permit beta particles to succeed in the photodiodes.
The end result is a strong detector, albeit not essentially the most delicate on the earth. 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 offer it the power to detect alpha particles in addition to beta particles, as Keller’s model can do. I might 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 posh of getting the detector designer current!
