Making a scintillator

[lodovico]

Hi,
I think you are using LYSO crystal 4x4x20mm.
With a silicon photomultiplier attached to a only one crystal I obtain around 100cps. With 16 crystals you should obtain 100x16x60 = 96000cps
These values includes natural background plus lutetium activity :

Radioactivity of LYSO Crystal
Mass = Volume x Density = 0.42 x 2,2 x 7.4 = 2.6 g
Of which 24% * is Lutetium = 0.625 g
And ~ 2.5% is Lu-176 with a half life of 3.78 x 1010 years
There are 6.02 x 1023 x 0.625 x 0.025/176 = 5.34 x 1019 nuclei
The disintegration constant is ln2/(3.154 x 107 x 3.78 x 1010) = 5.81 x 10-19 s-1
hence λN = 31 cps or Bq

So I think your measures are correct.

Lodovico

[lodovico]

typo : 100cpsx16x60 = 96000cpm

[Sesselmann]

Wow, you guys are talking pretty high count rates here, be mindful of your sound card limitations if you are using PRA or Theremino.

At 48 kHz sampling your upper detection limit is 1000 cps, at 96 kHz 2000 cps etc..

For the detector being discussed here you want to be sampling at 192 kHz at least which most good sound cards can now do.

Steven

[AnomalySmith]

Hi everyone,

Thanks for details that match my experiment!

It seems difficult to exploit this configuration has mentionned before, I'm actually thinking of replacing the LYSO (which will serve to another project) into a plastic BC412 plastic scintillator which is surely better for general portable purpose.

Waiting for updating the mounting, is there something I should hear about those family's crystals ?
I already know the global characteristics are often bellow NaI(Tl) or organic ones, but better for neutrons and muons, is that right?

Is a new post needed? :)

[valentin]

Hi, the choice of scintillator depends on your requirements. Many plastic scintillators are unsuitable for spectoscopy, as only a small fraction of gammas will have their full energy converted into a photoelectron. However, if you just need count rate (like a geiger counter, but with a much higher detection efficiency), then a robust plastic might be the way to go. So read the data sheets carefully.

If you want to do serious spectoscopy "out in the field", you have to be aware of other difficulties as well. For example, I noticed how sensitive my NaI(Tl) based spectrometer is to slight temperature changes, which are very hard to deal with if you are outside. So in the end you might prefer high count rate over resolution to quickly discover interessting stuff that you can take home for further analysis.

Valentin

[AnomalySmith]

Hi valentin,

Yes it's exactly the way I want to use this probe :) I know the problem of temperature sensitivity of many crystals, especially for NaITl and its hygroscopicity too.
That's the reason I finally prefered plastics ones (since the LaBr3 is pretty expensive).
That also gave me an idea to do a little neutron discimination module/program. I saw some articles on this and it seems to be feasible quite easily.

Yvan.

[AnomalySmith]

Hi,

I finally took off the crystals from the PMT, good new: the pusle rate is now about 2/min max.
Its also respond correctly with a UV LED carefully pointed thought the different layers of tissu used to cover the whole (~350 000 cpm).

I guess the next step will be to wait the BC-412 crystal and find a proper enclosure. Since I haven't saw any thin aluminium tube with the needed inner diameter (about 43-50mm), I think to use PVC pipe but not sure if it alters the final measure.
Keeping in mind it's mainly for "countrate purpose", I feel it won't be a particular problem, right?

Last thing, I also want to replace the 3kV cap with lower rated voltage ones, is it possible? I mean for those which are between two dynodes, the voltage to take as reference is actually the "delta" voltage, isn't it? (like Dy8: 800V and Dy7: 700V = min: 100V max rated cap voltage?)

Thanks.

Yvan.

[Sesselmann]

There is no need to replace the 3kV caps with smaller ones unless you are trying to save space.

If you really get stuck for the enclosure, I have stock of these.
http://www.gammaspectacular.com/gamma-s ... art60x230y

Steven

[valentin]

Hi Yvan,

In principle, non-metal enclosures work, I even saw home made probes made with tape or heat shrink tubing. Keep in mind that the enclosure must be lightproof. A thin PVC pipe may not be enough, even if it "looks" opaque.

A metal enclosure connected to the GND provides an Equipotential around the photomultiplier, which is a good thing because PMTs are sensitive to electric fields. For a "deluxe" probe, you may even include a layer of soft magnetic alloy, such as Mu-metal, to shield against magnetic fields as well.

Valentin

[AnomalySmith]

Hi sesselmann and valentin,

There is no need to replace the 3kV caps with smaller ones unless you are trying to save space.

Yes because I'm trying to make a low-profil electronic module.

@valentin: Indeed, I allowed myself to use PVC (5mm thick) pipe, black painted because I actually own a Mu-metal sheet I plan to use for this probe.

In fact, I was worrying if neutrons were not too absorbed by the PVC (probably more than aluminium or other metal?) if I want to use it as a neutrons detector (with signal discrimination).

Again thanks for your help ;)

Yvan.