Project: New Alpha, Beta & Gamma Geiger Tube SBT-11A

So with all the geiger counters and stuff i have here i realised i had no means to measure alpha particle radiation.

Detecting Alpha particles requires a Geiger Muller tube with a mica window, the mica window allows the retention of the gasses inside the geiger tube but also allow alpha particles to enter and create a count. Using metal or glass for the body of the tube prevents any alpha particles from entering the geiger tube and as such will only be Beta and Gamma sensitive.

After a little searching on ebay i found a seller in Ukraine selling new-old-stock of the SBT-11A tube, it's a flat rectangular shaped tube. It's sensitive to Alpha, Beta and Gamma and operates at around 320v. SBT-11A was ordered from any-devices.

A small metal project box by Hammond was used to house the tube to which i attached a monopod 'selfie stick'.

Total cost to build was about £50.

Hammond 27969P Metal Box

Fitting of the BNC connector.

Fitting the SBT-11A Tube

Connecting to the BNC. The SBT-11A has five connections, two are ground the other three pins are the anodes. There are actually four in this tube, the centre of the three pins connects to anodes 2 & 3. The outer two connect to anodes 1 and 4. So these should be connected in parallel.

Fitting a Tripod screw thread to the box and attaching the angled head from a selfie stick.

Attaching the selfie stick. This is a cheap Chinese stick from Ebay, it's telescopic and can extend to about 1 meter.

Testing the new probe on the Ludlum Model 3

Packard 1000 TR Liquid Scintillation Analyser - Part 1

So the teardown progresses!

Now the analyzer is in the shed i can take a look at this in more detail.

Overall Construction

The base of the unit is made from pressed steel which is thick and sturdy. The top cover is constructed from fiberglass with cutouts for the sample loading mechanism, keypad and LCD display.

The unit consists of many components, these i will remove and produce separate videos on through my YouTube channel.

PSU Board & Transformers

The main unit appears to operate from 110/120v AC, in the unit i have there is an additional transformer which i believe drops the 220/240v AC. A secondary transformer and power supply board supply the low voltages to the rest of the unit.

Motor Control Board

Controls power to the two motors used for loading and unloading the samples.

Photomultiplier Power Supply

Creates the high voltage supply for the two photomultipliers.

Mystery High Voltage Power Supply

Not sure what this is exactly for but its output is into the sample loading mechanism, looks like an ionizer of some sorts, maybe to eliminate static charge or dust on the sample vial or something i have not thought of yet. I will have to do more thinking and research into this.

Sample Loading Mechanism

A mechanical device operated by a motor to load and eject samples and insert the radioactive source into the sample chamber.

Sample Chamber & Photomultipliers

Metal constructed unit that allows the sample to enter a dark chamber. This entire part was surrounded with 80Kg of lead shielding to reduce the background radiation count rate.

CPU, Logic & Analog Boards

Total of four PCBs located on a metal structure to run the entire instrument. These also connect to two 25pin RS232 ports located on the side.

Teardown Video

After i had filmed this and in the process of editing the video i realised i may have exposed the photomultipliers to ambient light in the process. Given the light cap that covers the sample chamber had been slightly bent, the lead shielding was removed which would have provided some darkness to what should already be opaque covering and i was using it without the top over.

Thankfully after some testing at night in darkness it is registering some count but it does respond massively to a torch being pointed at the loading mechanism so there is a certain light leak somewhere. Hopefully there is no permanent damage.

Packard 1000 TR Liquid Scintillation Analyser - Lead Removal

So the first thing i needed to do with this analyser is to remove the lead shielding so i can move it into somewhere i can take a look properly at the guts and see how this machine actually works.

Thankfully only a couple of things needed to be removed to access the lead shielding.

You can see the large sections of lead shielding, with a sample loading mechanism mounted on top.

Thankfully the transport mechanism simply pulls off. There are two convenient threaded inserts to remove the top block. The part poking out is part of the sample loading mechanism.

Once the centre block has been removed the others can simply be lifted out.

I am not sure exactly where the Barium 133 source is located inside the loading mechanism but there is definitely something in there somewhere.

All the lead removed and the reaction chamber re-assembled. There was a total of 80Kg of lead removed.

Packard 1000 TR Liquid Scintillation Analyser - Teaser

So i saw this auction on ebay which looked interesting.

It's a Packard 1000 TR Liquid Scintillation Analyzer, which is used to generate a count and energy spectrum from radioactive erm, stuff.

Where this differs is the sample is suspended in a liquid and it's the liquid itself that is the scintillator. The light is then picked up by two photomultipliers. Because the scintillation substance is in contact with or dissolved within it it's more sensitive and accurate.

The seller is quite local to me and dispose of all sorts of IT, medical and science items, in fact it's the same place i acquired the Palflash 500 Flash Light Source.

The guys there had no idea what it was and actually asked me what it's for!

So it's currently sat in the back of my van, there is some serious lead shielding around the analysis chamber and the photomultiplier tubes. Thankfully the lead should be enough to cover the cost of buying it!

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:

Crusty!

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.

Reassembling.

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.