How many photons do I need to see a pulse with a photodiode (PD)?
OK mostly I hoping someone who has done this estimate before will confirm my numbers. (ping Phil =93tar and feather him=94 Hobbs.. or any others) The estimate is loose, I=92ll not worry about factors of pi or two.
This idea started with Jan=92s use of a PD to look at a tritium light. I started wondering if I could use a PD to see pulses from some radioactive decay with a scintillator. So assume I have a pulse of light that is much shorter than the response time of my PD. I=92ll characterize the PD by some =91gain=92 R and some 3dB frequency f. The pulse of light makes a charge Q, which lasts for a time t. (t
Alpha/phosphor flashes are visible, and easily detected with a PMT with mediocre coupling efficiency. I'd imagine that a PD could detect them without a lot of difficulty, as long as you funnel the light in properly.
But Phil will know better.
--
John Larkin Highland Technology, Inc
jlarkin at highlandtechnology dot com
http://www.highlandtechnology.com
Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser controllers
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On a sunny day (Thu, 19 Apr 2012 09:20:57 -0700 (PDT)) it happened George Herold wrote in :
Cool, some things I do know: Pulses from a scintillation crystal are not really that short, maybe 50 to >1000 ns range? I have some data on BGO crystals, will have to look it up.
I have a paper tha tgoogle fails to find from 2009: BGO and CsI(Tl) light output tests by Cesar Martin IEAP University of Kiel
its a pdf, I can email tit to you if you want, this is a cut and paste from it:
density(g/cm 3) µ-1* hygroscopic m a x (nm ) index refr. photons/M eV decay tim e (ns) B i 4G e 3O1 2 7.13 1.11 no 480 2.15 700 60 7500 300 total= 8200 Bi 4G e 3O1 2 (170K) 7.13 1.11 no 480 2.15 24000 2000 CsI(Tl) 4.51 2.43 no 540 1.80 59000 680 5400 3340 total=64400 Ta b le 1 . P ro p erties o f B G O a n d C s I(T l)
Sure, we do this in a number of physics detectors. First, you need to know the light output of the scintillator. Then, you need a VERY quiet charge-integrating amplifier. Pretty much everything we use uses a large-area JFET, such as the InterFet IF9030, a 1 mm square JFET, running at about 4 mA drain current. Depending on the scintillator characteristics, you will get some form of a double-exponential tail pulse, likely with a Tr of maybe 50 - 100 ns and a tail that extends for several us.
If you are looking for charged particles a thin scintillator will work, but you will probably need to be in a vacuum chamber to allow the particles to travel without interference with air. For Gamma rays a thicker chunk of scintillator that is a dense as possible is best to have the highest probability of absorbing the photon. Bismuth Germanate (BGO) is great, but some of the old standbys like Sodium Iodide and Cesium Iodide work fine.
This is all a lot easier to do with a photomultiplier tube, but then you have high voltages and power dissipation in the dynode resistor divider to deal with.
We have used 1 cm square Hamamatsu PIN diodes with Plexiglas (acrylic) light guides coupled with either silicone grease (messy) or some optical clear RTV (somewhat permanent). We usually wrap the entire contraption with teflon tape as a non-specular reflector. Larger PDs can capture more light, but the capacitance kills you. Smaller PDs have lower capacitance, but the optics get difficult to guide the photons.
You really do need a sensitive preamp. The signals out of a PD are really TINY, and the large capacitance makes simple amps suffer from a noise gain problem, so you have to start out with a low-noise front end.
The PD is essentially working with the same signal as the photocathode of a PMT has, although the PD does a better job of capturing the photons. So, you are starting off with a signal maybe a million times smaller with the PD, compared to the PMT output.
I'd guess that an alpha would make a decent signal slamming directly into a photodiode, as long as there isn't a window or anything in the way. 1 MEV or whatever could produce a lot of pairs. That would be easy to test, with a smoke detector source and some bare photodiode.
I tried making an electroscope, with the hope that a smoke detector source would make enough ions to eventually discharge it. It collapsed the leaves instantly.
Hey: an alpha source and an air gap, essentially an ion chamber, would probably resolve discrete events.
Do you have Knoll's book, Radiation Detection and Measurement?
--
John Larkin Highland Technology, Inc
jlarkin at highlandtechnology dot com
http://www.highlandtechnology.com
Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro acquisition and simulation
The eye is a remarkable detector, but that's partly because it's pixellated. You can see femtowatts with your eye pretty easily, but not with a large PD.
If you make the PD sufficiently small, you can see something on the order of 100 electrons. I've been working on a pHEMT front end for a biochip application, which looks like it should be able to measure less than 100 electrons at the shot noise limit in a 70 MHz bandwidth. The bad news is that the input capacitance has to be less than 0.5 pF, or the noise starts to degrade.
So it all winds up depending on how small a voltage change you can see across your photodiode capacitance.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics
160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net
Maybe a rod of scintallator wave guide with a PD at each end. The capacitance question is where you have to calculate something. I think it's going to depend on the average particle flux. How many pulses per second. You'll have to trade off response time vs number of photons.
Yeah yeah.... well I'm not an expert, but I've got this nice book, "Building electro-optical instruments and making it all work." or something like that.
I don't want to make the most sensitive detector, just 'a' dectector.
Hmm OK 10^6.. so a pmt see's 1 photon with (say) 10% efficiency and a gain of 1000 to 10,000.
Then my PD sees a photon with ~ 100% efficiency and no gain so I need ~ 1000 to 10,000 of them... is that about right?
I really wanted to do this correlation trick too. A peice of scintalator with a PD on each end.
Neat! I'll have to read about smoke detectors.
No. It was Jan's tritium thing, that got me thinking about it. I used a Geiger-Mueller(sp) tube for an x-ray crystalography lab back when I was an undergrad. But I wasn't into electronics as much back then.
Re Knolls: $25 dollars for a used copy... Should I get the third editon?
=A0Ta b le 1 . P ro p erties o f B G O a n d C s I(T l)- Hide quoted text -
Hmm that was hard to read from my end. Does it list some sort of effeciency... how much light energy for a given particle energy? I think I'll order the Knolls book that John L. mentioned. Even if I never build anything it'll be a fun read.
Thanks Phil, If you can see something like 100, then I should be able to find 1000 without too many 'heroics'. You're probably not much more than an order of magnitude smarter than I am. (And you let us peek over your shoulder sometimes :^)
It's all about low capacitance. If your application works with 10 pF at
1 nV noise in 1 Hz, then to get the same sensitivity at 100 pF, you'd need 0.1 nV noise. That starts to get inconvenient.
The 100-electron gizmo has to have less than 0.8 pF capacitance on the input node, total. Should be just doable.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics
160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net
The problem is that the pulse from a single particle or photon hitting the scintillator is quite small. If you want to count each pulse, you need an amplifier that can bring these pulses above the amplifier's own noise level.
Well, PMTs have much better efficiency that that, they list it in the datasheet. And, after the photocathode converts the photon into a few photoelectrons, it then multiples it by at LEAST
10^6, often 10^7 or so.
No, 10,000 photodiodes won't help. Much better to use an ultra-quiet amplifier. Today, you might actually be able to use a super low-noise op amp for this. Certainly putting a single JFET on the input of a good op-amp would work. Depending on the scintillator material, the circuit may not need to be fast at all. The slower crystals have one advantage. Dark current from the detector or noise from the amp will have a different time signature from a pulse from the scintillator, so you can exclude those noise pulses.
There are app notes from Analog Devices and probably TI show how to add a JFET to an op-amp for lower noise.
Right, that is called direct detection, and has a significant inprovement in efficiency over the scintillator approach for charged particles. Depending on the particle energy, some windows may not be thick enough to stop the particles. We generally use detectors with no window. Tiny metal-cased detectors can have the window removed with care. You just saw off the whole front of the can. The big ones usually have an epoxy front cover poured into the package, these can be pretty thin.
Well, it depends on whether you want to measure the steady-state detector current or count each particle.
Uh, we built our first detector system with PDs and JFET amps back in
1994, and used it in physics experiments for about a decade. This had 96 detectors with 1 cm square Hamamatsu PIN diodes coupled to slabs of CsI scintillator with a Plexiglas light guide. Some of these units had a plastic scintillator glued to the front of the CsI. That is called a phoswich in the literature, it gives a different pulse shape for different particle types. By not only recording total energy, but also the ratio of early and late energy components, you cen separate out protons, deuterons, tritons, He4 (Alpha), Boron, etc.
We are now working a lot with silicon strip detectors, large-area silicon diodes with many strips of aluminization front and back to form pixels. We have detectors with 32 up to 128 strips/side, and read out the pulses with custom chips. Our chips have 16 channels each, with CMOS FET current-integrating amplifier, shaper (bandpass filter), peak detector, discriminator (detects arrival time of particle) and sparsifying readout logic. 64 chips fit into a motherboard and is read out with some FPGAs in a VME crate.
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