Building an XRF detector with CsI(Tl)

Scintillation crystals, PMTs, voltage dividers etc...
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Steven Sesselmann
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Location: Sydney

Building an XRF detector with CsI(Tl)

Post by Steven Sesselmann » 05 Jan 2020, 17:04

When we want to measure x-rays we are only interested in the low energy photons especially those under 100 keV. We could use a typical 1.5 x 1.5" NaI(Tl) detector, but such a probe will also count gamma rays up to 3 MeV, which means the region of interest is only 3% of the spectrum.

The solution is to use a thin crystal, a 1 mm slither of NaI(Tl) will stop 100 keV x-rays while most of the the higher energy gamma will pass right through.

One of the problems we face when building such an x-ray detector is the attenuation factor of the aluminium housing, even a thin 0.5 mm layer of aluminium can attenuate the low energy x-rays we are interested in, so if we make a detector with an encapsulated 1 mm NaI(Tl) crystal the region below 20 keV may not be visible.

One solution to this problem is to use a detector with a Beryllium (Be) window, these are commercially available and quite expensive, and building one means working with Be which is quite carcinogenic if inhaled.

So I asked my crystal supplier if they could provide a thin slice of CsI(Tl), the benefit of using CsI(Tl) is that it is much less hygroscopic and does not need to be encapsulated. The polished crystal mounts straight to the PMT with a thin backing of white PTFE (Teflon) and some thin black plastic to block out light. The thin layer of plastic should be relatively transparent to x-rays, compared to Aluminium.

While CsI(Tl) is about twice the price of NaI(Tl), a thin 2 mm slice as the one I am holding in the picture below, is very affordable.

I am waiting for some PTFE to arrive before assembling and testing this probe, it will be a 2.0" x 0.1" GS-2001-CSI

Will post below once I have built and tested this one.

2 mm Csi(tl)
Window transmission
NaI(Tl) absorption curves
CsI(Tl) absorption curves
Csi-tl.png (83.16 KiB) Viewed 1318 times
* References taken from Saint Gobain - Efficiency Calculations for Selected Scintillators

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Re: Building an XRF detector with CsI(Tl)

Post by luuk » 06 Jan 2020, 06:47

Hi Steven,
Yes, a thin slice of CsI(Tl) can be used as xrf detector best example is the Rap-47 detector.
It is simple to build a detector like that, only the tricky part is you will need a good pmt that has a low noise level that is not always easy to find.
Simple and cheap aluminum house hold foil can be used as the entrance window.
I made a few detectors like that before some years ago and still have some slices CsI(Tl) from the test that I did, the lowest detectable energy will be around 10KeV although resolution is not very good at the lowest energies.
Have to look if I still have some test somewhere if I find them I shall post them here.

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Re: Building an XRF detector with CsI(Tl)

Post by isoenzyme » 06 Jan 2020, 11:07

Can't wait to see the spectra once you've finished this detector. Hope you'll post a price to make more if it works!
Christopher Lloyd
Salt Lake City, Utah (US)

Setup: Modified GS-STANDUP-20 with GS-USB-PRO
Primary Detector: GS 2" x 2" CsI(Tl)

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