Comma-seperated values in scintillator table

Scintillation crystals, PMTs, voltage dividers etc...
tim.hbn
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Comma-seperated values in scintillator table

Post by tim.hbn » 17 Sep 2021, 09:08

Hi Everyone

My question is about the table on Page 11 of the following URL:

https://indico.cern.ch/event/774281/con ... cindet.pdf

If you look at the table, you will notice that for CsI(Tl) and Baf2, the "Decay time" and "Light yield" boxes have two comma-seperated values instead of just one value. This confuses me. Does anyone here know what this actually means?

How can you tell which of the two comma-seperated values you need to take account of if you are building a gamma spectrometer using CsI(Tl) or Baf2?

Thank you very much.

Kind regards

Tim
Last edited by tim.hbn on 17 Sep 2021, 19:44, edited 1 time in total.

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GigaBecquerel
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Re: Comma-seperated values in scintillator table

Post by GigaBecquerel » 17 Sep 2021, 19:23

The two numbers are two different decay times.
Many scintillators have more than one decay mode, often with different wavelengths, intensities and of course decay times.
A scintillator with a fast and slow decay constant tf and ts will have a decay according to If*e^(-t/tf)+Is*e^(-t/ts), If and Is being the fast and slow intensities, with If+Is = 1, the total light output.
The different intensities depend on the material itself, but also how the crystal is excited. Gammas tend to preferentially excite fast components, while massive particles tend to excite slower components.
This makes it possible to discriminate between different radiation, and is often used to do so. One nice example are CLYC scintillators, which are sensitive to neutrons and gammas, and both particles can be well separated with puse shape discrimination, which just looks at the different fractions of If and Is.

tim.hbn
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Re: Comma-seperated values in scintillator table

Post by tim.hbn » 17 Sep 2021, 19:44

Hi Lukas

Thank you very much for your very informative reply.

Does this mean that, for example, gamma spectroscopy using CsI(Ti) uses the 40000 Light yeald value and alpha spectroscopy using CsI(Ti) uses the 25000 Light yeald value?

Thank you very much.

Kind regards

Tim

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GigaBecquerel
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Re: Comma-seperated values in scintillator table

Post by GigaBecquerel » 17 Sep 2021, 20:10

Close, but no.
There will always be both fractions, but how much of which one changes.
https://www.berkeleynucleonics.com/clycce
This shows a graph for CLYC, with the energy on one axis and fraction ratio on the other.
The difference is small, but measurable!

Knowing which fraction is dominant is just a thing of literature.
For Gamma in CsI:Tl Knoll gives: 0.68 µs (64%), 3.34 µs (36%), meaning that ~2/3 of the light will be in the fast fraction.

tim.hbn
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Re: Comma-seperated values in scintillator table

Post by tim.hbn » 17 Sep 2021, 22:22

Hi Lukas

Thank you very much for this.

I would be very grateful for further clarification about something.

According to the table that I linked to, the light yealds for CsI(Tl) are as follows:

Fast: 40000 ph/MeV
Slow: 25000 ph/MeV

My interpretation (perhaps wrong) of what you are saying suggests that the total light yeald for CsI(Tl) is equal to (40000 x 0.64) + (25000 x 0.36) = 34600 ph/MeV.

Is this correct?

Thank you very much.

Kind regards

Tim

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GigaBecquerel
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Re: Comma-seperated values in scintillator table

Post by GigaBecquerel » 18 Sep 2021, 00:17

No, it's equal to Fast plus Slow, since If and Is are added in the formula above.
That's around 6.5*10^4 ph/MeV, and the values roughly correspond to the 64 and 36% from Knoll.
the numbers are not exact, and will vary from crystal to crystal, with imperfections and different concentrations

tim.hbn
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Re: Comma-seperated values in scintillator table

Post by tim.hbn » 18 Sep 2021, 03:31

Hi Lukas

Thank you very much for this.

Does this mean that if, hypothetically, in perfect conditions, an MPPC was able to count every individual visible photon emitted by a CsI(Tl) crystlal when a 1 MeV gamma photon is detected by the crystal, it would count 65000 photons?

Thank you very much

Kind regards

Tim

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GigaBecquerel
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Re: Comma-seperated values in scintillator table

Post by GigaBecquerel » 18 Sep 2021, 03:45

I will leave it to you as homework to figure out how many photons can be counted, if your crystal emits 65000 photons, and your theoretical counting system is 100% efficient ;-)

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Re: Comma-seperated values in scintillator table

Post by tim.hbn » 18 Sep 2021, 04:14

Hi Lukas

> "if your crystal emits 65000 photons"
I think you seem to be saying that the answer is yes.

Thank you very much indeed for all your help with this.

Kind regards

Tim

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Re: Comma-seperated values in scintillator table

Post by tim.hbn » 20 Sep 2021, 22:46

Hi Lukas

I would be very grateful if you could let me know which particular piece of literature authored by Knoll you have refered to in this thread. I suspect it will probably have some very interesting information in it.

Thank you very much

Kind regards

Tim

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