Scintillation rate / SiPM

Scintillation crystals, PMTs, voltage dividers etc...
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sgt_bear
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Scintillation rate / SiPM

Post by sgt_bear » 12 Jun 2017, 02:58

Hello,

I'm currently in the beginning of collecting knowledge about silicon pmt tubes. This brings me to the questions how many pulses can a NaI or CsI Crystal pick up depending on size?

I already made the following estimations:

Background unshielded is around: 100cps

1uCi of Co-60 would be 37'000 disintegrations per second for each energy so
so for Co-60 1uCi there would be 74'000 disintegrations per second.

But i don't believe a crytal can pick up all of the rays, so the number of rays will be lower. Anyone knows how much lower?

Since SiPM are expensive, the size of the crystal would be around 10x10x30mm or so.

The problem with silicon photomultipliers is that they have very short pulses (1-5ns) which is out of the range of affordable electronics. If i know the maximum possible rate of impulses, there would be the option to stretch the pulses by hardware for example from 1 GHz down to 1kHz that could be handled by "normal" microcontrollers / fpgas.
- Jonathan from Switzerland

luuk
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Re: Scintillation rate / SiPM

Post by luuk » 13 Jun 2017, 02:07

Hi Jonathan,

If a source is 1μCi (37000Bq)it is for the complete source 1 or 10 different energies from that source that does not matter.
The source radiates in 4 Π so if in contact with a detector/crystal you can see only <50% in a well detector you might get ±90% efficiency.
The counter rate limit is not in the crystal but in the SiPm the amount of pixels limits the counter rate, when a pixel is activated(fires) it need a recovery to fire again when all the pixels are I use that is the limit of your detector.
A crystal can handle a few hundred of thousand counts per sec, depending in the material a NaI(Tl) decay 250ns is about 4 times faster then CsI(Tl) decay 1μs.
Other problem is that the detector gets none-linear when to many pixels are in use about 20% busy at the same time is acceptable to get a linear response.

Luuk

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Re: Scintillation rate / SiPM

Post by sgt_bear » 13 Jun 2017, 16:12

I guess that the rate is limited by the crystal in my case, the SiPM says "pixel recovery time = 12ns" and when the decay time is 250ns, its way greater.

So 37000/50% = 18.5k * 0.9 = 16.5k pulses per second. So i could stretch the pulses ALOT* by hardware to save on expensive microprocessors.

If the crystal would put out continous pulses 250ns long, 1ns pause this would result in around 4Mhz which will make electronics very expensive.
- Jonathan from Switzerland

luuk
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Re: Scintillation rate / SiPM

Post by luuk » 14 Jun 2017, 02:12

Hi Jonathan,

Can you tell me what exactly you want to do with the SiPm detector?
Because I don’t think you have to worry for counter rate problems, it all depends on the amount of pixels you have, if not enough then your SiPm will be saturated very quick but it all depends what you want to measure with it?
The 12ns recovery time is theoretically but when you use a SiPm the pixels will fire if there is enough signal before they are completely “charged” so the result is that it will give for the same energy a lower signal so the peak will be more wide and therefore the resolution worse.

Luuk

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Re: Scintillation rate / SiPM

Post by sgt_bear » 15 Jun 2017, 19:56

My goal is to build a pocket/transportable spectrometer. I want to do the pulse processing in a microcontroller/fpga instead of using the "sound-card" method.

Currently i want to use a SENSL60035, wich is 6*6mm and has 18980 microcells.
Or a Hamamatsu with 14000 microcells would be an option too.


Another thing will be to find a nice crystal...
- Jonathan from Switzerland

luuk
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Re: Scintillation rate / SiPM

Post by luuk » 17 Jun 2017, 07:26

Hi Jonathan,

I expect that with a portable system you don’t expect very high doses to measure so a relative low counter rate.
The sensl are really good we use them a lot they are reliable and need only about 27.5Volt to work in linear mode.
If you are looking for a crystal let me know what you are looking for e.g. material and size, and I maybe can help with it I have some detectors and crystals for sale and have to possibility to machine and tread them

Luuk

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