Struggling with LYSO

[luuk]

Hi Silviu,
I got a question you say that the result with more than one LySo crystal is not good, can you tell me how you coupled this crystal with each other are they separately packet or other?
I have got great results in the past with 4 and more LySo crystals res. @662KeV 7,5%- 8%
Luuk

[stamasd]

I tried 2 crystals, each was individually wrapped in PTFE tape then they were put next to each other on top of the PMT with coupling compound, covered with light-insulating tape and heat-shrink tubing on top. The background activity was much higher than expected from the sum of 2 individual crystals because I expect each crystal is scintillating not only at its own background but also detects its neighbor's. And the resolution for 137Cs was poorer than what the last picture shows. I don't have the data to show anymore because I deleted it and that detector has since been dismantled and rebuilt with one crystal only (which I used to generate the data in the last plot).

The data from the graphs at the beginning of this thread was obtained with the same PMT and an array of 5x5 crystals, all individually wrapped.

[luuk]

Hello Silviu,

I would advice to try to put two “bare” pixels together and than wrap them in ptfe sheet,
If you have two pixels each separately wrapped the pmt will see two detectors and if the light output is not exactly the same you will get a “double” peak or a wide peak.
If you put them bare together you can even try to put some couple compound between the connected surface, the peak should be more as one, even if the light output is no exactly the same, ofcource you can do that 4 or more pixels as well.

Luuk

[stamasd]

I have the detector configured now with one pixel only. I should not (and indeed examining pulse shapes I do not) have problems with wide pulses. At 90-100cps all pulses appear correctly shaped.

One pixel only appears reasonably sensitive. However energy resolution is poor and the signal is overwhelmed by the background. I'm not sure how putting more pixels together will improve that.

[luuk]

Hi Silviu,
If you look here : viewtopic.php?f=5&t=31
You can see the test I did with LySo about 2.5 years ago maybe it gives an indication what is possible.
Hope that it can help you.

Luuk

[stamasd]

I had actually read that post a while ago. I am using the same PMT (Hamamatsu R9420).

TBH my MCA/software isn't designed for use with PMT-based detectors (it's optimized for neutron detection). Maybe that is my problem. I'll try again when my GS2000Pro gets here.

[stamasd]

Hm, what a difference a (not so) little change makes.

(hint: the spectra below were done with sources of LYSO, 137Cs and KCl respectively - but with a different detector. The LYSO was used only in one test as a check source, not as a detector)
The same detection electronics and software were used, but the detector is a Hamamatsu R10133 PMT with a 2.5inch NaI(Tl) crystal attached. Count time was 5 minutes only each.

[stamasd]

I rebuilt the probe with 2 LYSO crystals stuck together, coupled to each other by silicone compound and wrapped in teflon tape together as suggested by luuk.

I also played with different settings on my acquisition system, using a higher sampling rate than before and also varying the cutoff for pulses (#samples). I got somewhat better results, but it still begs some questions.

The first graph is the probe alone, basically counting its internal background
The second is same but with a 137Cs source
The third is 2 minus 1.

I can see the 661kEv peak of the cesium much better now.

The main question is, what is the peak that I see in about the same place on the background count alone? It's not exactly in the same place, the maximum of that peak is around channel 368, and the Cs peak is around 402. But it's pretty darn close. I don't think it's either of the 176Lu peaks - I think those are squished together between 160 and 280 on the background graph. But I have no explanation for the peak at 368.

[luuk]

Hi Silviu,

My guess is that maybe you see the 511 KeV?
Luuk

[stamasd]

Hm. But 511keV from what? I have no source of positrons nearby. And when I analyze the LYSO crystal with a NaI probe there is no 511keV peak.

Also, the tests above were done inside a lead castle with inner copper lining. It's about 3cm of lead and 0.7mm copper.