Light pulse and photodiode

PMTs are far better than any photodiode for very faint signals--like six orders of magnitude better, on an area basis.

Cheers

Phil Hobbs

Nope, not even close! You will definitely need true heroics to measure less than 10,000 electrons. The more compressed in time they come, the better they stand out from the noise. So, direct detection of charged particles has a tail time constant of about one us, typically. A fast scintillator like BGO compresses that, but the PD may stretch it out some. And, NaI is really slow, with a tail out to 5-10 us.

Our CMOS preamp has a noise floor of about 3000 electrons, and believe me, it is heroic to get there with CMOS. Not sure what the noise floor of our JFET amp was, but it was probably getting closer to that 1000 electrons on a good day. But, of course, that was with 1 cm square PD's, the smaller, lower capacitance you can get the better. And, of course, making the detector smaller reduces the probability of capturing a particle. There is NO free lunch!

Jon

YUP, that indeed is the rub. If you can arrange the optics to funnel all the light into a small photodiode, then it helps everything immensely. If you are forced for optical reasons to use a 1 cm square diode, for example, then everything gets far more difficult.

Jon

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No I meant 10,000 photons... sorry.

=A0Much better to use an ultra-quiet

Sure, thanks. I've been doing all this noise stuff lately. If my analysis is at all valid then you want as much bandwidth as possible and as big an R as you can afford.

George H.

Cool, Any good papers?

Here's something for 1969,

I certainly want to count individual events... I really would like to do two detectors and a correlation thing. Do you do that? Or how about this? A hunk of scintallator with six parabolic light pipes radiating along the Cartesian axes, focusing the light onto six photodiodes. I could collect almost all the photons and do a six way correlation. (This is mostly a 'light' pipe dream.)

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Sounds fun!

George H.

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That makes sense, you want the C as low as possible and then R as big as possible, except if it limits the time response.

I was looking at some paper that could see the muon background. That would be cool.

George H.

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Do you ever do any conical light pipes? We use to do that in the FIR.

George H.

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Oh, Well I'm probably not going to be able to really do this... unless I do it on my own time. But 3000 seems not too bad. I can do some simple tests with an LED.

George H.

Google put my message into a separate message.

What I wanted to say was: How about using several photodiodes, with each their own amplifier.

And then summing the amplified signal when it is larger and not so critical.

By the way, what are good components for this, photodiodes and op amps, PMTs are too hard to use for me

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Yeah a few PD's and then not only sum them, but also have a seperate threshold for each circuit. Then only 'detect' something when all the circuits fire... that's the 'correlation thing'. You'd be able to set the threshold closer to the noise level.

PMT's are actually easier, circuit-wise. But they have lots of other issues.

George H.

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I have no idea what do those crystals which emit light look like. But I wou= ld think light is emitted in every direction and is easier to collect with = several detectors than with one.

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Ahh no idea, but when I post from home on google groups, sometimes weird things happen.

But on an early post from you it said, "On 19 huhti, 19:20, George Herold wrote:"

Did you add that? or google?

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ould think light is emitted in every direction and is easier to collect wit= h several detectors than with one.

Yeah, I think that may be why they wrap white teflon tape around the x- tals. But Jon Elson knows much more than I. I wonder if some metal mirror type coating would be better.

George H.

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Some of these have been partly conical, or at least tapered. Often we have to adapt from a weird-shaped scintillator that pack together to form angled rings to the square PDs. So, the light guides end up with a bunch of facets. Our shop has gotten good at making some really complicated shapes.

Jon

There are plenty of papers, my boss is D. G. Sarantities, we have published a number of papers in Nuclear Instrumentation and Methods on some of these devices. There was the "Micro Ball" and the "Dwarf Ball".

Not generally. We are detecting multiple particles that shoot out of a nuclear reaction. Their time coincidence is pretty good at assuring this is a real signal.

Unless you want to detect cosmic rays, how do the particles reach the scintillator, except by passing THROUGH the photodiodes?

Until you have a $40K MOSIS run and the chips have some kind of design defect that we have to engineer our way around as best we can.

Jon

They look like chunks of glass, if they are well-made. All those detectors are competing for the same limited number of photons. So, it may be better to reflect all the light to one photodiode. We have found white, non-specular stuff like white paint or white Teflon plumbing tape works best at getting a reproducible number of the photons to the photodiode. Mirror-like reflectors tend to make the system sensitive to where the particle hits, so you detect a different signal amplitude depending on location, while the tape or paint just bounces them around and a pretty constant proportion of photons are detected.

Hey, I'm just revealing all our trade secrets!

Jon