Fun with LaBr3

[Steven Sesselmann]

Hi all,

Today I have been having fun building and testing a small 1" x 1" LaBr3 detector. After seeing the 3.1% resolution I'm not sure if I will ever be happy with NaI(Tl) again :)

While running the background spectrum I didn't see much of the internal radiation from this crystal, but that might be because it's relatively small.

This one is for a client, so unfortunately I won't be able to hold on to it.

The basic construction is;

1" x 1" LaBr3 Crystal
1.5" PMT
Single SHV connection
Gold anodised aluminium housing with black cap.

Resolution came in at a pleasing 3.1% and linearity was good, but I had to run it at 400V, lower than normal voltage.

The price of these crystals (especially the bigger ones) puts them out of reach for most amateur scientists, but if anyone is interested I can supply LaBr3 at competitive prices.

Steven

Encapsulated Crystal

Holding Encapsulated Crystal

Detector parts

3D printed Sleeve

Housing and label

LaBr3 Spectrum

Linearity check

linear-good.png (51.41 KiB) Viewed 117 times

PS: Note on Linearity

I found this quote in a paper by Saint Gobain, this is consistent with my findings today.

LaBr3:Ce scintillator has 1.6 times the light output and is more than 10 times faster than NaI(Tl). This can produce
non-linear effects in the pmt. A simple calculation shows that the instantaneous charge pulse is about 25 times that
of NaI(Tl). This is based on the ph ratio of 1.6 and multiplied by the faster time factor of 250ns/16ns specifically:
25X = 1.6X(250/16).
The non-linearity manifests itself in two ways. First, the FWHM of a peak is better than expected at that energy and
second, the position of higher energy peaks will be at lesser pulse height than expected from a linear extrapolation.
It is possible to verify that this is occurring by decreasing the HV by about 100V and observing an improvement in
linearity and a correspondingly a slight decrease in FWHM. In order to keep these undesirable effects to a minimum,
Saint-Gobain Crystals is selecting pmts that have superior linearity properties. Often these are 8-stage pmts.

[Steven Sesselmann]

I need to correct my statement about intrinsic background in the LaBr3 crystal.

There is actually some quite prominent peaks in the BG spectrum, initially I thought the La138 gamma at 1435 keV was K40 but it just happens to correlate closely. There is also an x-ray at 32 keV that correlates to the Cs137 x-ray, then there are three prominent peaks above 1500 keV apparently from alpha decay.

The background subtraction feature in PRA is very helpful here.

It has been an interesting learning curve for me.

1"x1" LaBr3 Background

LaBr3 Background.png (21.83 KiB) Viewed 101 times

[luuk]

Hi Steven,
Nice result with your detector and that with a standard Adit pmt, you should try it with a pmt with a sba or uba cathode I expect you can get maybe around 2.4% with that.
I was surprised that you did not saw any background but your second post, was more realistic.
You will also see a large signal from the beta’s from around 0 to 500 KeV.
But a very nice detector for sure, Nice!
Luuk

[Steven Sesselmann]

Thanks Luuk,

I must experiment with some of those ultra and super bialcali PMT's. The background LaBr3 spectrum confused me because at first I thought I was seeing K40 at 1460 and Tl208 at 2614.

Next week I will be making a 40 x 40 mm CeBr3 detector, should be interesting to see how it compares.

Steven

[luuk]

Hi Steven,
With a sba cathode you can expect an average improvement in resolution from 0.5% than with a standard pmt.
Luuk

[Conor Whyte]

Love to do a price comparison with CeBr3 to LaBr3. I am really interested to making a small homerolled scintillation detector with either medium.

Already have a very small CsI/Na scintillation detector that resolves low energy quite well.

[luuk]

Hi Conor,
The price is more or less the same for both crystals.
luuk