Tuesday, 17 March 2015

Ludlum Model 3 & Ludlum 44-3 Scintillation Probe

February 2014:

I spotted a rather sorry looking Ludlum Model 3 Survey Meter on ebay UK, the pictures showed it was in very poor condition.

The auction pictures are shown below, with the brown rust and aluminium oxide showing around the battery compartment it clearly indicated there was battery leakage.

Generally though it looked complete, my only concern was there was no picture of the front of the Ludlum 44-3 probe. As this is a scintillation probe the Sodium Iodide crystal used is especially vulnerable to moisture and or damage to the mylar light blocking film, so i did have a concern that the cover was missing and there was no picture of the sensor end.

The ebay auction was a BiN / Make Offer, so placed an offer in for about 60% under the asking price which was accepted.

What i found in the battery compartment was worse than i had expected:


Thankfully though with some basic testing, this involved connecting the power to by HP 6632B bench power supply. At 3v DC the meter powered on and gave a good 900v on the probe output. After altering the voltage to 420v and making a quick connection to my Mini Instruments MC71 Geiger Muller tube the meter was proven to be working. Date codes on the ICs indicate it was manufactured around 1992.

Testing: powered the meter from my bench power supply and made
a crude connection to one of my geiger muller tubes.

After complete disassembly, removing all the switches, circuits, meter movement i was able to remove the batteries and clean out as much of the corrosion as possible.

Thankfully the separate battery compartment on these Ludlum meters saved the rest of the components as it kept all the battery corrosion inside the battery compartment. Although the battery terminals are not serviceable now i am hoping i can get replacement terminal inserts from Ludlum. For the time being i have two Energizer Lithium AA batteries powering the meter located inside the main box.

Near the end of disassembly.

Remains of the two 'D' cell batteries. Going from the age of the meter and the old style Ever Ready zinc chloride batteries i suspect they had been in the meter for at least 10 years.

The same could not be said for the Ludlum 44-3 probe. The scintillation crystal (Thallium doped Sodium Iodide) was very off colour with blotches of something on it (they are moisture sensitive). The photomultiplier tube and all the internals of the probe were wet and the photomultiplier itself also appeared dead both in darkness and in light. The probe was a total loss, i have kept all the related hardware (aluminium tube, mountings etc) with the aim i might make some kind of probe in the future.

The photomultiplier tube was an EMI 9902KB and has been disassembled in this video: Teardown: EMI 9902KB Photomultiplier

Sodium Iodide Crystal.

 Photomultiplier Tube

In addition to all this the probe cable was completely broken. In the ebay pictures it showed the cable intact but in fact the cable had been ripped out of the Ludlum C connector at both ends, they had just been pushed into the connectors for the photos!

In the process of remaking the cable.


Main PCB with IC references.

So overall i am happy, the meter is working fine and is capable of driving both Geiger Muller tubes and scintillation probes (which use photo-multipliers, driven at a higher voltages) so i will be experimenting with scintillation probes in the future.
Not the cleanest, but it's working!

I may consider sand-blasting the case and having it re-painted.