Guidance needed - PMT with a Harshaw/Filtrol NaI(Tl) X'tal

[99tito99]

Hi Folks: I'm a newbie, just did my intro a few hours ago.

I recently obtained a PMT with a Harshaw/Filtrol NaI(Tl) X’tal, and have some questions and need some guidance. On the Har/Fil label is: Type:8SAF38M/2A-X, Serial: DB-618, NAI(TL). The X’tal has a well running through (ID ~18mm, tube thickness ~0.14mm). The PMT has 14 pins (see picture of PMT and schematic of voltage divider, which it came with it).

I built a simple RC lowpass/Pre-Amp running the audio into my PC sound card. I am using a Ludlum Model 177 as my HV source and running the PMT at 800 volts. I have used both PRA8 and Theremino 4.5 for data acquisition and can’t seem to get good data from PRA. Attached is a spectra run yesterday of a Chinese thoriated lantern mantle.

So:

1) Is there any technical info about this PMT/X’tal combo (I have found none)?
2) Based on the divider schematic is it logical and correct that pin 13 is a focusing electrode?
3) Is this divider the best design? Can it be improved?
4) Is 800 volts appropriate?
5) Thoughts on the spectra

Any input will be most appreciated. Cheers, Mark

[Steven Sesselmann]

Mark,

The divider circuit you attached is not typical, usually resistor R1 between cathode and dynode 1 is around twice the resistance of R2 to R11 or it is a variable resistor. This resistor regulates the potential between the cathode and dynode 1, and therefore the PMT gain.

The results you are getting in Theremino indicate you are on the right track. I do recommend you use PRA when you are in the process of trouble shooting and discovering the limits of your equipment.

Here is a post in this same forum, which I started some time ago, and which generated a bit of input from others too.

viewtopic.php?f=13&t=6187&hilit=divider#p34631

Steven

http://www.gammaspectacular.com

[99tito99]

Hi Steven,

Thanks for your observation about R1 and the link to the previous discussion on PMT dividers. From the web I have amassed a small stack of papers, data sheets and discussions, many from this forum trying to bootstrap my knowledge base. Somehow I missed the link you provided.

Cheers, Mark
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PS
Interestingly, I have read some of your previous posts and just realized you’re the beeresearch guy, and last week I received one of your GS-1100A units, but have not had time to fiddle with it yet. I wanted something a bit more portable then my Ludlum 177. I will report back when I get some results.
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[Jerry Biehler]

I wouldnt mess with the dynode string. I do have a hamamatsu PMT modules that use 1:1 all the way across so it is not unheard of. Second guessing the people that built this probe is kind of silly.

[George Dowell]

Who did build the base? George Dowell

[George Dowell]

My improved 14 pin base for MCA work. The option is to double the value of the bottom resistor, but this arrangement works very well, as long as the power supply can handle the load.

George Dowell

[99tito99]

Thanks George for your design idea, but for now I am not going to fiddle with the divider as Jerry advises.

Steven noted that my divider circuit was not typical and that would be because I drew it incorrectly. The corrected is shown below.

I have hooked up my new BeeResearch GS-1100A and will generate new spectra and will report back in a day or so.

Cheers, Mark
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[George Dowell]

That's better and is the standard wiring we use on hand held, portable , battery operated instruments, to keep from loading down their HV power supply. Good for general purpose work, but the improved schematic is better for MCA, if the MCA can handle the current draw.

George Dowell

[99tito99]

Hi George, I redrew your divider with a 2xR between 13 and 14. Did I get this right?

A couple of questions:

• On your design the resistor at 11 is 220 KΩ about half that of the others, but I have seen designs in which it is 2X or more (see page 3 on attached pdf). What is the reason for one or the other?

• Also on your design the cap from 9 to 14 is 0.01 µF, but I have seen designs in which it is 0.047 µF. Again, what is the reason for one or the other?

Cheers,
Mark
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