a little help understanding the issue....

[Marco75]

Hi Forum!
Two weeks ago I collected during a travel to USA, one PMT R9420 purchased from Tom of IradInc
This is the first time I try to build a scintillation probe, but I wanted to give a try even if I lack the expeience .
I am assembling the R9420 with a brand new Scionix crystal, a NAI 38x1mm beryllium window, especially made for Xrays (a special occasion through a friend). R9420 was suggested by Scionix as a good option for this crystal.

I connected the R9420 to a shv and the mu-metal as well to the ground. Tom provided the divider for the R9420 already developed for GS-USB-PRO (23 Mohm)
All is driven by the fantastic GS-USB-PRO.
As a source I am using a Cs137 , so considering the xrays at 32kev.

I am not getting any signal from the R9420, I think just noise. I am sure that coupling of PMT is correct . chassis of the detector is fully opaque to light
Coupling of crystal... well this is my first crystal. I think i did a correct job but maybe i need to re-check/ re-do. Maybe the layer of the optical grease is too thick?
The crystal is brand new and certified, just received from Scionix sdo this is not the source of the problem.
GS-USB PRO is working perfectly as you can see from the attached screenshot using a GS2020CSI probe
When I connect the R9420 (tested from 800 to 1500V) I didn’t get the typical signal and I just have noise generated within the first arb.
Attached you can find the test using the Csi2020 and the PMT9420 with the same source, always using the GS-USB-PRO and PRA

Any thoughts? Any suggestion?? How can I test that PMT is working ?

Thanks a lot for any suggestion you would like to provide.

Ciao
Marco


Post scriptum: I am trying to uplaod the screenshot of PRA with bopth PMT but the upload give s me an error. don't know why, I tried png and jpeg of different size

[Marco75]

here is the PRA screenshot with R9420:

[Sesselmann]

Marco,

It sounds like you are doing everything correctly.

I am familiar with the R9420 from Tom, I have a few in stock but for some reason I never used them, can't recall why or what voltage they normally run at.

It is an 8 stage PMT, meaning it might need a bit higher voltage than a 10 stage.

Since this is your first build, make sure that you never connect power to a PMT in daylight, as it will strip the cathode (Brown coating on inside of glass) in seconds.
Otherwise connect positive to anode ground to cathode, couple your crystal to the front of the PMT and seal the whole thing up in something light proof, preferably metal and grounded.

Magnetic shield around the PMT is great if you have it.

Use good shielded coaxial cable and keep the unshielded wires as short as possible.

With a 1 mm crystal you are mainly catching very small pulses, so they may indeed be drowned out by noise, so start at 700V and increase it by 100v at the time. The PMT should be able to operate as high as 1500V.

You will need as much gain as possible on your GS when using an X-ray detector, but make sure not to turn shape and volume to 100% as this will kill the opamp, the potentiometer full range is 15 turn, I recommend you set both Shape and Volume to 10 turns. Just turn the pots counter clockwise until you feel or hear a slight clicking sound then go the other way 10 turns.

Good luck and let us know how you go 👍

Steven

PMT

PCB

[Marco75]

thanks Steven for all the suggestions!! very appreciated. I will make some new tests in next days and for sure I will also make again the optical coupling between crystal and PMT (maybe the thickness of the grease is too high).
do you know a "test" to check if the PMT is working correctly? I don't think it has been damaged during shipment but it would be easier to check each component and be sure they work indipendently (the crystal is brand new so no problem). The only other component I have is divider+PMT
Marco

[iRad]

Hi Marco - I’m currently traveling in Italy, so sorry for the slow reply. Steven provided very good information (thanks Steven!). It would also be helpful if you provided a detailed picture of your finished voltage divider. Regards, Tom

[Marco75]

Hi Tom!!
enjoy your vacation and Italy and don't lose your time for me! no worries and no hurry we can be in touch later in case, but first I can do the tests suggested by Steven.
Anyway the voltage divider was already installed by you on the R9420 so I did not touch it. Also the mu-metal was already in place. I just soldered a shv connector to the red (one) and the black (two, one from mumetal) cables coming from the PMT
I will anyway take a picture
take care and enjoy your vacation ! weather will be nicer in next days, we had an anomalous decrease of temperatures in these days but everything should come back to normality soon
Marco

[Madmax]

Hi Marco,
Can you record audio signal using any audio recording app? Tune HV to minimum (counter-clockwise until click), start recording audio from USB device and slowly increase HV to, say, 1200 V during 1-2 minutes. Than please upload recording and i will try to suggest.
Good luck!

Max

[Marco75]

Thanks Max,
the audio signal is almost nothing and so you can see from the PRA's audio diagram.
From tomorrow I will be on business travel, I hope to have the time today to do all the tests suggested including a re-check of the optical coupling (someone told me that for xrays the thickness of the optical grease must be really very very thin), including your audio recording. Otherwise will be next week, just after my return.
Many thanks also to you for the kind support and help. very appreciated

Marco

[Marco75]

Hi Marco - I’m currently traveling in Italy, so sorry for the slow reply. Steven provided very good information (thanks Steven!). It would also be helpful if you provided a detailed picture of your finished voltage divider. Regards, Tom

here it is .
Don't be shocked by the "temporary" assembly :-)
Being my first assembly I would like first to see if it works and then build the metal chassis

light stop:
- for the body: mu-metal and black rubber (installed by you Tom).
- teflon and then black tape between pmt and crystal.
- on the back (black tape to close), here removed to show the divider.

during the test i also put a black thick fabric over the probe and source to stop light too

connection of divider:
- ground (black wires, one from mu-metal) to the SHV ground
- positive to the hot wire of SHV
i don't think there are problems here

Hoping to have all the time I am today doing the other tests.
TOM: do NOT waste time for me, enjoy your vacation in Italy. really, there is NO hurry here. I don't want you responding before you are back in USA :-)

[iRad]

Marco - A little rain in Florence on Tuesday morning, but weather is beautiful here in Italy right now. We are taking the high-speed train to Venice at this time.

Make sure there is absolutely no light at the back of the PMT. Your enclosure must be fully opaque, or the PMT will refuse to operate correctly. No pulses is typical when there is light getting to the PMT.

Ciao, Tom