Building an XRF detector with CsI(Tl)
Posted: 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.
Steven
* References taken from Saint Gobain - Efficiency Calculations for Selected Scintillators
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.
Steven
- 2 mm Csi(tl)
- Window transmission
- NaI(Tl) absorption curves
- CsI(Tl) absorption curves
- Csi-tl.png (83.16 KiB) Viewed 9427 times