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THE AM BULLETIN BOARD => Technical Forum => Topic started by: Opcom on February 11, 2018, 01:43:50 AM

Title: question for some of the scientisis here, not really AM..
Post by: Opcom on February 11, 2018, 01:43:50 AM
This was inherited from my dad's estate.. buried in the garage. He was a partner in an analytic instrument company for a few years. IIRC test tubes with blood in them would be slipped into the 'well' on the end of this tube and read for radioactivity.

Maybe this would be good for checking meters at a little distance, where there's a stack of gear or a box of meters I don't want to have to go though one at a time.

It's a 6199 photomultiplier with a NaI(Tl) crystal on its face. From what I can find, the tube is sensitive at 400nm, so it's UV but also has some sensitivity over the visible range.The crystal glows UV when a particle hits it. It might be permanently sealed in the metal cylinder but the end caps are attached with bolts. At one end, a lined cylindrical 'well' passes through the end. I guess this would be right through the crystal.

This tube is new, never used, to my knowledge. I'm wondering if it can be used to check old meters to see if they are Radium type. I'm told the PMT+crystal setup is much more sensitive than a Geiger tube. If so, no need to get right next to each meter, but I hope it should indicate at some distance from a stack of gear.

What would this be good for today? Since it isn't open head-on to the crystal and PMT, it would seem to be hard to make a sensitive radiation detector out of it. The end cap is bolted on, but it may or may not be removable because the well is potted in place- maybe the whole end of it is. I'm kinda short on test tubes full of radioactive blood or radioactive anything at the moment but it would be fun to see how it would react to an old aircraft instrument with a radium dial.

Here's the spec on it from the packing sheet:

1.5" dia. x 1.5" thich Thallium activated Sodium Iodide crystal (X-AF66) hermetically sealed in an Aluminum housing, optically coupled to a 1.5" RCA 6199 photomultiplier tube with an external shield. Well linter 0.700" I.D. and well liner thickness to be 0.010" similar to PQ-73-920-1. Resolution to be 12% or better for Cesium-137.
Type 6MBAF6/1-1/2
SHIP BY 5-25-77 to Analite Lab Inc.
One for $300 EACH.

With it was Geiger counter, seems to work. About 30 counts per minute here in Dallas.

So, anyone have an idea what to do with this PMT scintillation detector thingie?

Title: Re: question for some of the scientisis here, not really AM..
Post by: w3jn on February 11, 2018, 09:27:19 AM
Useful for gamma spectroscopy.  You need a multichannel analyzer though.  here's a homebrew project:

Title: Re: question for some of the scientisis here, not really AM..
Post by: k7mdo on February 11, 2018, 10:32:05 AM
It sounds like the "heart" of what was a well counter for gamma scintillation counting.

If so it was designed to be bathed in liquid nitrogen and surrounded in a lead well to reduce spurious noise.

Packard made these well counters that I used extensively in medical research.

Best guess, 73, Tom

Title: Re: question for some of the scientisis here, not really AM..
Post by: kb4qaa on February 11, 2018, 10:46:28 PM
Sounds like a candidate for eBay, or otherwise you need to do some digging to find Forums for people interested radioactivity.   To the right person you might get sale.

Title: Re: question for some of the scientisis here, not really AM..
Post by: Opcom on February 13, 2018, 02:40:36 AM
Great ideas! Someone may have use for it. I should try to test it before trying to sell it.
In the application it was encased in a thick cast-lead shield. It was not cooled. I hope one of the shields might be around in a box somewhere.

That article's very interesting. I may be able to fire up the tube since it only needs a voltage divider and some low current HV. The datasheet for the tube is simple enough, has a schematic.

Title: Re: question for some of the scientisis here, not really AM..
Post by: PA0NVD on March 02, 2018, 06:09:29 PM
I did design equipment with NaI(Tl) crystals and PMT's, mainly gamma cameras. These had big crystals with 19 or 38 PMT's and a circuit to make a pic out of the information. The output was summed in resistive networks to get an X and Y information where the scintillation took place. Doing so you can make a pic of e.g. a radioactive organ like a thyroid or hart (after injecting a radio isotope like Tc99
When you take the crystal off, look if it is completely clear and not yellow. Yellow means that the housing leaks and that moist has entered, this ruins the spectrum of the output and the energy can't be defined good anymore.  It still works but has a wide energy spectrum. NaI crystals are highly hygroscopic.
Geider Muller tubes can't measure energy
NEVER cool them, do not subject to rapid temperature changes, the crystal may break.
Couple the crystal to the face of the PMT using a very clear silicone grease that is transparent for UV (NO AIR INCLUSIONS)
The output of the PMT can be coupled to a charge amp with a time const of about 10 uSec (a simple RF transistor with e.g. 15 pF from base to collector parallel to a resistor to give approx 10 uSec time. The amplitude of the pulse is proportional to the energy of the gamma entering the crystal. You can make 1 usec pulses using a shorted 500 uSec delayline parallel to the output. This to get rid of the DC component.
With a PC you can measure the amplitude and make a display of the entering gamma energy which is an indicator of the isotope used. Y axis is number of pulses, X axis is energy, pulse height
Take care that the PMT / crystal combination is COMPLETELY closed to light, the PMT can see single photons!!
How to connect the PMT can be found at internet, You need a VERY clean 0.5 - 2 kV (depending on the PMT type) where the voltage is the gain of the PMT, Current consumption is micro amps, The dynodes are connected to a resistive divider to get every dynode a higher voltage if it is closer to the anode.  e.g the first (closest to the sensitive cathode wich should be ground) dynode 100 V, the second 200 V etc.  E.g. a string of 500 k Resistors.
Very funny to experiment
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