Ludlum Model 3 Survey Meter

[dexters_lab]

Yea, this might need some work 

thankfully the battery compartment is isolated from the rest which doesn't look too bad inside

[tautech]

OOOHH 

Great candidate for the thread: "Don't you hate it when..."

Whats under the little Cal plate?

C'mon EE porn pics...?

[dexters_lab]

It's a Ludlum Model 3 with a Ludlum 44-3 Scintillation Probe (Sodium Iodide), as you will be able to see from the pics it dates from around 1993. The everready zinc carbon batteries look about that old too!

so i do wonder if i should have left this one on ebay to rot... the meter is in a bad way, lots of corrosion everywhere, the 'C' type connectors on the cable fell off when i took it out of the box and the probe seems to be dead.

I spent the afternoon disassembling it so i could be sure i wasn't going to get filthy every time i went near it!

The meter is completely disassembled now and the old batteries removed, the meter itself is working, after adjusting the voltage it worked quite happily with my Mullard MX-145 GM tube.

The probe i have disassembled, the crystal is, well it looks like something has started living on it (probably moisture damage) and the foam in the other end of the tube was actually wet! I was surprised to find zero environmental protection on the probe, no gaskets or anything. The photomultiplier tube, which is a Thorn Emi 9902KB (made in the UK!) seems to be dead. I had a look around it and could not see any sign of a getter but also could not see where one might have been fired either so maybe they dont have them? Given PM tubes are so simple i wonder if it's lost vacuum - any ideas to test them? I have been around and checked the obvious like the divider network and a single HV cap. Everything checks out.

I kinda put money on the crystal not working but i was hoping the PM tube was working

@tautech: access to the 10 turn pots for HV adjustment and calibration of the ranges.

[Fraser]

Ugh nasty !

You do have a little project on your hands there. I can confirm that the scintillator crystal is toast. It is very hydroscopic even in air. The scintillator in your unit is severely water damaged.

As to testing the PM tube, sorry in my experience they just work. I have no experience of PM tubes failing so cannot help on that front. I have bought a few Geiger counters and Dosimeters of recent but mine are all either GM or Si detectors.  From what I have heard, Scintillator probes are a Russian Roulette game as the Scintillator crystals are very prone to discolouration with age due to moisture ingress. Discolouration ruins accuracy. I have stayed away from them as a result.

Good Luck with your project..... I am not sure that I would have the patience !

Aurora

[dexters_lab]

the PM tube is a puzzle, i have stripped, cleaned and re assembled. i will test it again.

i do have my own plan for a new homebrew scintillation probe using a big 3" PM tube and a plastic scintillator like this:

http://www.ebay.co.uk/itm/3-Scintillation-Detector-Kit-Includes-most-everything-you-need-instructions-/261757080883?pt=LH_DefaultDomain_0&hash=item3cf1f01533

the working meter will be perfect for use with it

[Fraser]

Nice kit !

I have seen the plastic scintillators and wondered if they work as well as the traditional 'Crystal' types. If they are not hydroscopic that would be a great advantage.

Aurora

[xrunner]

Everything my mom owns that has batteries ends up like that thing. 

[GK]

I've cut open defective/low count rate Bicron scintillation assemblies which have the PMT and crystal encapsulated/potted inside and air tight stainless steel tube and have found white, milky crystals. So there is a mechanism for crystal degradation besides the absorption of water from the air.


Not sure if you know this already, but you should never power up a PMT in ambient or excessively bright light. If you did that with your survey meter probe it's probably thoroughly dead now if it wasn't to start with. See third question from the top:

http://www.et-enterprises.com/photomultipliers/frequently-asked-questions

Incidentally, this is a good resource on the application of PMTs:

https://www.hamamatsu.com/resources/pdf/etd/PMT_handbook_v3aE.pdf
 
 

[SeanB]

I have seen lots of stuff with those battery problems.  At least you got a nice meter movement out of it.......

[dexters_lab]

Nice kit !

I have seen the plastic scintillators and wondered if they work as well as the traditional 'Crystal' types. If they are not hydroscopic that would be a great advantage.

Aurora

Yes, shame i found it after i bought my PM tube and some random bit of plastic scintillator on ebay  The seller does sell the plastic on it's own, i might ask him if he'll do the kit without the PM tube. As far as i'm aware the plastic scintillator is immune from damage like water, although typical solvents would just like other plastics. They are not as sensitive though. http://www.crystals.saint-gobain.com/uploadedFiles/SG-Crystals/Documents/SGC%20BC400-404-408-412-416%20Data%20Sheet.pdf

I've cut open defective/low count rate Bicron scintillation assemblies which have the PMT and crystal encapsulated/potted inside and air tight stainless steel tube and have found white, milky crystals. So there is a mechanism for crystal degradation besides the absorption of water from the air.


Not sure if you know this already, but you should never power up a PMT in ambient or excessively bright light. If you did that with your survey meter probe it's probably thoroughly dead now if it wasn't to start with. See third question from the top:

http://www.et-enterprises.com/photomultipliers/frequently-asked-questions

that link provided some help maybe, thanks. No i didn't power it in the open, well not until i checked it first in it's enclosure. This answer does help though:

I have an old photomultiplier. How can I easily find out if it is in working order?

Apart from doing a visual test, there are no other tests other than electrical ones. Sometimes from mishandling, the vacuum within the photomultiplier is compromised, in which case it will not work. When you look at the window of the photomultiplier it should be difficult to see the internal structure because the photocathode appears a yellow/brown colour. If this is not the case and the window looks clear then the photocathode may be lost.

The front window is almost clear

I have seen lots of stuff with those battery problems.  At least you got a nice meter movement out of it.......

Sorry, when i said the meter is working i mean the whole thing! not just the analog meter! It's just the handheld probe that isn't.

[Stonent]

Can you give a list of the chips and transistors used on it?

I think I see a 555 and some CD4000 series chips, but I can't read the numbers very well.

[dexters_lab]

Can you give a list of the chips and transistors used on it?

I think I see a 555 and some CD4000 series chips, but I can't read the numbers very well.

with pleasure...

[dexters_lab]

well it's back together, i cleaned it up so it's arrested the rot, i may look at stripping it down again and repaint it but i dont have the time at the moment.

I'm trying to see if i can get spares from Ludlum of the battery terminals that live in the base, these need replacing before i can re-use the battery compartment. So in the meantime i'm running it off 2 AA lithium cells inside the box.

I have made a Ludlum 'C' connector to BNC cable (the 'C' connectors are a frickin PITA to solder up!) so i can attach it to my MX-145 Geiger Tube for testing and its working just fine.  So a working Ludlum is a good thing and will be perfect for use when i get my scintillator made as the meter can read upto 400,000 CPM

So yea, i'm disappointed the Ludlum 44-3 probe is dead, but i could re-use the hardware. The aluminium tube, end cap, bezel etc and make something new out of it. Maybe i can find a nice windowed GM tube to go into it.

I should say it's very nicely made inside, cabled stitched up, excellent soldering, the meter movement is also very slick and precise you can tell it is an expensive hand made unit. Still a current model at US$ 599.00

[Stonent]

There's a Youtube channel called Antiprotons that makes extensive use of a similar Ludlum with a NaI scintilator.  At idle with nothing near, it chirps away at around 2,000 CP/M.  Very gamma sensitive.

[dexters_lab]

thanks Stonent, yea i have been subbed to Toms channel for a while, he does have some nice kit.

The 44-2 i think is essentially the same as the 44-3 but has a much larger crystal in it

[dexters_lab]

i just had part of the photomultiplier apart.

all the dynodes after the 3rd one are blackened and show effects of heating. Looks like someone ran this in light and blinded it (wasn't me, damage was done before i had it)

This is what the 6th dynode looks like