Single photon counting pixel detectors vs CCD

[Mishra]

Hello,

I need to learn as much as I can about single photon counting pixel detectors (producing a signal for each photons hitting the detector) e.g. PILATUS. I do not understand how this type of detectors counts single photons as opposed to integrating the energy of multiple photons (CCD, MOS, CMOS...). By which mechanism does one reach the single photon counting mode ?

 

[Baluncore]

With the Dectris PILATUS the photons being counted are X-rays. There is no optical scintillation plate or electron multiplier. Each X-ray photon triggers a current flow in the silicon detector. The magnitude of that current pulse is a measure of X-ray energy. Integrated electronics can apply threshold limits and count the X-ray photons detected.

Use more google.
https://en.wikipedia.org/wiki/Pilatus_(detector)
https://www.dectris.com/search.html?keywords=pilatus

 

[anorlunda]

Your question might be better suited to the quantum mechanics forum.

But you also might be asking about this

http://en.m.wikipedia.org/wiki/Single-photon_avalanche_diode

 

[Mishra]

Thank you Anorlunda, the question is probably more related to quantum and solid states physics you are right. I will go back to the physics department!
@Bulancore: This does not tell me the fundamental difference between a CCD and a single photon counting pixel. I know how to use google thank you...

 

[mfb]

Detectors for single photons in the visible light need some integrated preamplification, like the avalanche diodes or photomultipliers.
Those do not work with too much light, on the other hand (they get saturated), where a conventional CCD is better.

 

[Mishra]

Hi,

thank you for your answer.

I am more focused on X-ray detectors, but the physics is probably the same. From what I understand from CCD, each photon will produce an electrophoton. After a certain readout time the photonelectrons of each pixels are carried over to the signal converter. Whereas in single photon counting mode, each photoelectron will give a distinct signal (dirac peak).

Are you saying that the avalance is the key here? Some sort of amplification of the photoelectron combined with a very short readout time ?

 

[Baluncore]

This does not tell me the fundamental difference between a CCD and a single photon counting pixel.

I am sorry you failed to understand. One integrates charge over time, but you cannot tell the energy of the photons or how many photons were integrated to make up the total. The other counts the pulses of charge that conform to a programmed energy window.

 

[Mishra]

Hi again Baluncore,

I see that failed to express my question in a clear manner. Reading my post again makes me understand your answer now.
What I was asking is how does this device count each individual photon? I would like to understand the physics of these pixels as I already know what they do.

ps: I am not really focused on knowing their energy (as the X-rays are produced in a monochromatic fashion by a synchrotron).

 

[Baluncore]

The physics of the silicon sensor used for each pixel ?
Or;
The parallel electronics behind every pixel of the silicon sensor that does the digital counting in real time as the X-ray photons are converted to charge ?

 

[Mishra]

Well, I'm interested in knowing how those pixels count individual photons (vs. integrating energy of multiple photons). What makes those detectors work in single photon mode ?

Is is because of the electronics short readout time? This seems odd as the X-rays flux is gigantic in synchrotron sources, I don't imagine any electronics device having such a short readout time.

 

[Baluncore]

Each pixel would be connected to the ASIC discriminator front-end through a transmission line. All ASIC counters in the array would have a common clear and an enable control to gate the entire array of pixel counters. The individual counters accumulate the photon count while the array is enabled, then the counts are read out later.

If the x-ray conversion to charge was low impedance and wideband then the pulse energy discriminator and counter on each pixel detector could maybe count at several hundred of pulses per microsecond using today's ASIC technology. The expect the digital electronics could operate at up to about 1GHz.

 

[Mishra]

So basically, there is not much physical differences between those two types of detectors (MOS CCDs energy integration vs. CMOS in single photon mode), the readout time is just shortened by integrating ASIC to each pixels ?
I'll try to find out if ~1GHz is enough but I doubt the flux through a pixel would be higher than that. Such read time should allow for individual photon counting.

This is really amazing stuff and so was your answer, thank you very much!

You seem to know a lot about this so I would have an other question about CCDs:
It seems that MOS were design to control electronic current in the n-channel by changing the voltage gate (thus creating a transistor ?). In the case of photo sensitive usage (MOS arrey, or CCD), the idea is to create photoelectrons is the depletion site (created by the gate voltage). Once these electrons+holes are created, the holes go to the "ground". The electrons are traped by the insulating oxide and stay in the depletion site.

My question would be: once the photoelectrons are trapped in the depletion site, the CCD device needs to "carry" them to the serial array (reader). How does it do that? Just by changing the gate potentials (phase lock)? That would mean that this gate allows to get rid of the holes and to transfert the electrons? Everything I could read on the subject seems to say that.

That would mean that the n-channel is useless in this case? (edit: the n-channel is also called "source and drain")
ps: Sorry for my lack of proper language but I am a physicist and have very poor knowledge in electronics.

 

[Baluncore]

What do you mean by “readout time” ?

google ' x-ray photon counting detectors '

http://scitation.aip.org/docserver/fulltext/aapm/journal/medphys/40/10/1.4820371.pdf?expires=1430208442&id=id&accname=guest&checksum=CD8BA83825841D33EBC10C2C91F73772

https://en.wikipedia.org/wiki/Charge-coupled_device