Saturday, July 28, 2018

Scintillation Probe Kit

Prologue: A New Geiger Counter...

With the arrival of a new Geiger counter with a x1000 scale it was finally time to assemble the scintillation probe kit I purchased from Iradinc in 2014. The probe only required 900V so the stage was set.

The first hassle was to clear off enough space on my decrepit workbench.

The Kit: Some Assembly Required...

The pieces of the puzzle (click to enlarge)
Left to right: 3" diameter 2.25" plastic crystal; Hamamatsu R6233 Photomultiplier tube; tube cap; BNC connector; voltage divider parts; silicone grease; huge diameter heat shrink tubing.

Additional materials (click to enlarge)
Isopropyl alcohol, spray adhesive, electrical tape — all as recommended by the GeigherCounters group brain-trust. I still haven't found any 99% isopropyl alcohol locally but I did get some 90%...

The first step was to wire up the resistor string on the base of the PMT. My eyesight has been going downhill for years and this was somewhat of a challenge. The markings on the tiny resistors were very hard to make out. I was very nervous about plunging in and naturally I managed to scratch the insulation off of one. Tom was kind enough to send me some spares so I dodged that bullet.

PMT voltage divider complete (click to enlarge)

BNC connector attached (click to enlarge)

At this point it was time to glue the cap to the PMT. Unfortunately, the silicone rubber I had bought turned out to be the type that gives off acetic acid while curing. This damages electronic components.

I had specifically searched for and bought what I thought was "neutral cure" but somehow managed to screw that up. Yet another delay as I waited for the correct adhesive to arrive (once again I couldn't find any locally).

Cap glued to PMT (click to enlarge)
The plastic crystal is highly fluorescent and glows beautifully under 365nm UV light.

Crystal fluorescing. In a dark room it's quite bright. (click to enlarge)

Perfect fit (click to enlarge)
I did not bother to polish the crystal. I was too nervous at this point to be able to without mucking things up even more. A voice of experience told me not to sweat it so...

Next up was attaching the crystal to the PMT. I carried the bag of silicone coupling compound to the workbench, went to get something else, sat down and... the silicone grease was nowhere to be found on my cluttered workbench. AUGHHH!

Fortunately I had some more in another PMT wiring kit but my nerves were wearing thin. I proceeded with taping the crystal to the PMT and quit for the night. The next day I wrapped the entire assembly with layers of electrical tape and glued the front scrim on.

I took everything into a windowless bathroom and sealed the door. I powered up the Geiger counter and was highly relieved to hear lots of background counts. The relief was well-earned.

Taped and working. (click to enlarge)

It took me a couple of weeks to reach a point where I was ready to finish things up with the very large piece of shrink tubing. My job was a bit sloppy but I was satisfied with the result when I heard background counts again. Mission accomplished.

Heat shrink shrunk. (click to enlarge)

By the way, if you happen to see a small plastic bag labelled "Silicone" anywhere please let me know.

A big thanks to Tom Hall for this kit and his support. A very special thanks to K0FF without whose help none of this would have been possible let alone gotten done. Bravo.


I found the missing silicone grease. It was on my workbench all along amidst the bubble lights. Look closely. Very closely.

Closer. Closer... (click to enlarge)
Right there the entire time. I told you my eyesight is deteriorating.