That's OK - I expect I will have hold of it eventually, and it is probably the only one I would ever want. I do still have the PMT to play with.
We stay with it, though, even for USA folk, it is the largest cost chunk, proportionately, of all the bits (so far). We may revise that idea when it comes to the costs of the physical constructions of what goes around the AM241 stuff, and how the radiation lands on some steel offered. You wanted some lead bits in there too!
Keeping an eye out for other silicon PIN diodes of reasonable area, (betrayed by their capacitance), is something we should do. As I understand it, one needs more than simple area. The other thing needed is thickness, so that the photon actually gets absorbed instead pf passing right through. Diffusion thickness PIN diodes could reasonably easily be made. Getting up a thick enough epitaxial layer, that was pure enough was apparently uneconomic by slow diffusion. The technology moved on, and purer silicon and GaAs became available. Larger area diodes with the required absorption characteristics are still a very minority thing - but maybe we can get lucky. All the amplifier circuits and software can stay pretty much the same.
I did stumble a bit on the simulations. A perfectly well formed exponential rise pulse looks OK at 100nA, but starts to look a weird mess at 2nA.
I have to be reasonably convinced a build is going to work, and very definitely, it has to get actually built and the hell wrung out of it before I would expect anyone else to run with it on my say-so. When I do get hold of it, it's going to be winter breadboard time. I will be getting in some A/D converter stuff, probably one that uses SDA to command transfer of full 8 or 16 bits parallel at a time into GPIO. That will be at the A/D speed maxed out. Probably enough to get, (at the fastest), about 20 samples out of a 4uS complete event. It means about 200kHz to 500kHz bandwidth amplifier, and 2Msps to 5Msps A/D converter. The actual events would likely be slower. If we were counting detected "pulses", it would be Hz, to maybe a few hundred Hz, or some few kHz, but when they happen, we hopefully know their energy.
What we have is between about 5,000 and 60,000 electrons building up in a "pulse", being spread into a capacitance of between 50pF and 300pF, (depending on reverse bias), while at the same time being "leaked away" by a 40Meg ohms noisy resistance, and some few pA into (or out of) the charge amp . That little tickle of electrons in and out, onto the (tiny) junction of the TIA amp, provokes a feedback current to force the voltage between the inputs to zero. The feedback current, through the feedback resistance around the op-amp, gives us the first stage TIA gain.
In your vision of the gadget, do you have any preferences as to what batteries it carries? My notion is to have some button cells, or small batteries, enough to work the sensitive front end. It would be convenient if that also provided the bias, which can be between 2V to 10V. I thought the "back-end", A/D current, and data transfers could be powered from the receiving computing device.
- - - - - - - - - - - - - - - -- - - - -
My machine "fun" stuff is more or less mothballed for the present.. The profiles for the outhouse/shop/hideout are going up. If I get very cheeky, I can nudge it to (in USA units) 18' x 12'. I may have to settle for 16' x 10', but I am perishing the thought!
It's odd, but who would have thought that an extra 2ft would seem like acres!
I see some of the other shops folk have on HM. Compared to what I hope for, even maxed out as hard as I can push my luck with it, they seem gargantuan!
We stay with it, though, even for USA folk, it is the largest cost chunk, proportionately, of all the bits (so far). We may revise that idea when it comes to the costs of the physical constructions of what goes around the AM241 stuff, and how the radiation lands on some steel offered. You wanted some lead bits in there too!
Keeping an eye out for other silicon PIN diodes of reasonable area, (betrayed by their capacitance), is something we should do. As I understand it, one needs more than simple area. The other thing needed is thickness, so that the photon actually gets absorbed instead pf passing right through. Diffusion thickness PIN diodes could reasonably easily be made. Getting up a thick enough epitaxial layer, that was pure enough was apparently uneconomic by slow diffusion. The technology moved on, and purer silicon and GaAs became available. Larger area diodes with the required absorption characteristics are still a very minority thing - but maybe we can get lucky. All the amplifier circuits and software can stay pretty much the same.
I did stumble a bit on the simulations. A perfectly well formed exponential rise pulse looks OK at 100nA, but starts to look a weird mess at 2nA.
I have to be reasonably convinced a build is going to work, and very definitely, it has to get actually built and the hell wrung out of it before I would expect anyone else to run with it on my say-so. When I do get hold of it, it's going to be winter breadboard time. I will be getting in some A/D converter stuff, probably one that uses SDA to command transfer of full 8 or 16 bits parallel at a time into GPIO. That will be at the A/D speed maxed out. Probably enough to get, (at the fastest), about 20 samples out of a 4uS complete event. It means about 200kHz to 500kHz bandwidth amplifier, and 2Msps to 5Msps A/D converter. The actual events would likely be slower. If we were counting detected "pulses", it would be Hz, to maybe a few hundred Hz, or some few kHz, but when they happen, we hopefully know their energy.
What we have is between about 5,000 and 60,000 electrons building up in a "pulse", being spread into a capacitance of between 50pF and 300pF, (depending on reverse bias), while at the same time being "leaked away" by a 40Meg ohms noisy resistance, and some few pA into (or out of) the charge amp . That little tickle of electrons in and out, onto the (tiny) junction of the TIA amp, provokes a feedback current to force the voltage between the inputs to zero. The feedback current, through the feedback resistance around the op-amp, gives us the first stage TIA gain.
In your vision of the gadget, do you have any preferences as to what batteries it carries? My notion is to have some button cells, or small batteries, enough to work the sensitive front end. It would be convenient if that also provided the bias, which can be between 2V to 10V. I thought the "back-end", A/D current, and data transfers could be powered from the receiving computing device.
- - - - - - - - - - - - - - - -- - - - -
My machine "fun" stuff is more or less mothballed for the present.. The profiles for the outhouse/shop/hideout are going up. If I get very cheeky, I can nudge it to (in USA units) 18' x 12'. I may have to settle for 16' x 10', but I am perishing the thought!
It's odd, but who would have thought that an extra 2ft would seem like acres!
I see some of the other shops folk have on HM. Compared to what I hope for, even maxed out as hard as I can push my luck with it, they seem gargantuan!