[jareklupinski]

currently trying to make a flip-dot display (think https://theartistbreakfast.com/works/spectral-flashbacks-ser...) but with some kind of radiation detector behind each pixel that will flip the dot when something passes through

i made a small 3x3 proof of concept using more expensive geiger tubes, and their really long 'z-axis' lengths made 'traces' happen very often, like a persistent cloud chamber

trying to find a reliable semiconductor (read:cheaper) method i can scale to an arbitrary number of pixels, but something seems to happen in between the bench and the wall :(

[DoctorOetker]

How many pixels would you aim for?

I have considered

* scanning a linear array of BPW34 photodiodes, in a similar spirit to a scanner to cover a plane, each photodiode going to its own "MCA circuit" (TIA->cheap audio codec like those from Everest Semi). Either direct measurement of generated charge pulses or covering the photodiode with phosphor on aluminum foil or so

* cloud or bubble chamber (cloud chamber is less dense and will generate fewer events, so probably bubble chamber): instead of needing a large 2D or 1D array of parallel circuits, we image and track generated charged particles and use the trajectory starting end (less curved) to determine the source direction!

* consider X-ray crystallography, an incoming straight beam can diffract in many directions on a monocrystal. rotating say a silicon wafer, and measuring the incoming photon energies with one or more photodiode/MCA circuits we can assign a source likelihood distribution by keeping track of the orientation of the monocrystal. akin to sparse sampling but instead of masks its diffraction patterns.

If you have better ideas or variations in mind, let me know!

[jareklupinski]

for resolution, i'm only limited by whatever the bus company is throwing away that day ;) but i'd love to come up with an easy-to-produce design that can work per-pixel for arbitrary spacing

yup after seeing #2 at a museum and learning about the chemistry needing to keep it running, i started looking for something like #1, but i'm not able to get any PIN diode circuit properly reporting events with consistency compared to my GM tubes :(

#3 sounds fun! i'd like to turn this into something i can open-source and hang on a wall, emitting x-rays might make it a hard sell :P

after reading about https://en.wikipedia.org/wiki/Single-event_upset#SEUs_and_ci... , i'm wondering if it's possible to use a suitably dense FPGA in a complicated enough design that any failure must be due to cosmic effects? monitoring ECC might not be enough https://en.wikipedia.org/wiki/ECC_memory#Concept

[DoctorOetker]

For your PIN diodes, what was the depletion region width/thickness of the Intrinsic layer?

Do you have access to a high bandwidth oscilloscope? do you observe the expected exponentially decaying pulses? Sound like you could debug your circuit to find out what is happening.

For higher energies on would want to use a thicker intrinsic region, one approach I have considered would be to use a distant aperture, so that the direction of incoming rays is known, and then tilt the photodiode so that the rays can experience a much longer path in the intrinsic region (so that when a photon generates a high energy electron, the stopping length can be attained without clipping / aliasing the energy resolution as much). Basically tilt the photodiode so that its plane is closer to parallel (or exactly so). There is a trade off between cross section (fewer events) and maximum energy measurable, one can compensate for the lower cross section by having more photodiodes.

all 3 proposals would be passive, including #3, so it wouldn't emit X-rays, just detect them and build up a self-consistent picture that explains the observation statistics for each event (with that energy and time/orientation of the silicon wafer).

[jareklupinski]

i was just using BPW34s :) i have a batch of different photodiodes that i was planning on swapping and comparing but i can't even make these work reliably...

currently limited by my 20MHz scope (and free time...), but i saw the expected pulses after they were drawn out long enough by the amplification circuit to validate it was working on my desk (https://physicsopenlab.org/2020/06/15/cern-diy-particle-dete...), but i think my issue is shielding the diodes without introducing noise?

this guy has to actually shave away some part of the diode to make it work? https://hackaday.io/project/204159-geigerwatch-a-sensitiv-ra...

[DoctorOetker]

sounds like you were nearly there

> this guy has to actually shave away some part of the diode to make it work? https://hackaday.io/project/204159-geigerwatch-a-sensitiv-ra...

Thats because he also wants to measure alpha and beta particles directly, if you are satisfied with high energy photons you wouldn't need to do that