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.

Tuesday, October 25, 2016

Craig Anderton's Multiple Identity Filter

Craig Anderton's Multiple Identity Filter™ originally appeared in a series of ariticles in Contemporary Keyboard in 1979. It is a CEM3320-based VCF with each of the four stages operating semi-independently. Each stage has...
  • Three Filter Modes: Lowpass, Highpass, Allpass
  • Two frequency ranges (10:1 ratio)
  • Series/Parallel Switching
Add to that voltage-controlled resonance, straight/filtered signal mixer w/phase inverter (for phasing, changing bandpass to notch, etc.) and more. Craig was in top form—this is one tricked-out VCF.

Here is the article: MIF.pdf

Here is a mirror on Mediafire in case the link above doesn't work for you. I've been told that some web browsers (or their extensions) warn that Mediafire is a dangerous site. It's not but the page does have a lot of extraneous links and other junk that can make it confusing to navigate. To start the actual download of the pdf file click on the big green "DOWNLOAD" button that also has the file size in smaller letters: MIF pdf

Update 2017.2.26

Note that this design is not optimal. More work has to be done to get the stages to "behave" properly. This has been discussed on the Synth DIY list on multiple occasions. There is a searchable archive here: SDIY Archive

And there's some very good news: a clone of the CEM3320 has been fabbed. It's available here and the price is very reasonable. The datasheet is here.

Wednesday, February 24, 2016

The Oscilloscope Artist

The Oscilloscope Artist originally appeared in the November, 1975 issue of Popular Electronics. It creates all sorts of fascinating moving geometric patterns on an oscilloscope screen.

All you need is a low bandwidth 'scope with horizontal and vertical (XY) inputs.

Update (2018/02/16): David Dixon spotted a really stupid typo in the original pdf file that was shared below. This has been fixed and the latest version is now 1.2.1. All other (unattributed) copies floating around the Internet are of the earlier, incorrect version.

A reconstruction of the complete article is in a pdf file. All the original parts are still available!

The original block diagram is a bit confusing so I redrew it. Oscillators A & D create a Lissajous figure baseline from triangle/square waves. Oscillators B & C are multiplied and summed with A & B.

To make things interesting, C is 90° out of phase between the multipliers. All the oscillators are sync'd to A, which is ~60Hz. All of this produces complex patterns which move and shift.

I've also used it with a laser projection system with X and Y galvos and it works although the corners aren't too sharp.

Friday, November 4, 2011

Halloween 2011

Another Halloween come and gone. I'm rather worn out from setting it all up and taking it all back down.

Click to enlarge



One bulb blown

Everyone said it looked great.

Sunday, August 21, 2011

Room temperature diamagnetism with pyrolitic graphite

Updated 2011.09.01

I just acquired some pyrolitic graphite and powerful neodymium magnets from United Nuclear. Pyrolytic graphite has the highest diamagnetism of any room temperature material [see correction below]:
Diamagnetism is the property of an object which causes it to create a magnetic field in opposition to an externally applied magnetic field, thus causing a repulsive effect. Specifically, an external magnetic field alters the orbital velocity of electrons around their nuclei, thus changing the magnetic dipole moment. According to Lenz's law, these electrons will oppose the magnetic field changes provided by the applied field, preventing them from building up. The result is that lines of magnetic flux curve away from the material.
Here are some small pieces of pyrolitic graphite on top of four 0.5" square neodymium magnets:

Click to enlarge
This is not magnetic repulsion. If that were the case the pieces of graphite would simply fly off the magnets. The magnetic field is strongest at the edges of the magnets so the largest piece is trapped in the center; it's pushed inward to the point of least magnetism. If perturbed it will snap back to this same point and orientation. The smaller pieces are standing on edge because they are repelling the magnetic field on both sides and can't fall over. If one is pushed over it will immediately snap back into a vertical position.

In this view you can clearly see the largest piece levitating and the smaller pieces standing on edge. The medium size piece is tilted slightly. This may be because its thickness is slightly uneven, which would make the repulsion slightly stronger on the more massive side. This calls for more experimentation.

Click to enlarge
Needless to say this behavior is completely unintuitive and surprising. When pushed the pieces react in a totally unexpected way. It's as if they've fallen into a sort of bizarre magnetic well. Here's a video showing this behavior in real time.

Superconductors have an even more powerful diamagnetic effect but require liquid nitrogen to cool them sufficiently.

I'll have more on this topic in the future as I've also acquired some bismuth, which has the highest room temperature diamagnetism of any metal.

Correction (2011.08.31)

Here's the difference between bismuth and pyrolitic graphite (without the math):

"The most strongly diamagnetic material is bismuth, although pyrolytic carbon may have a [lower] susceptibility in one plane."

So there you have it. I played around with some bismuth the other day and it was quite interesting. Stay "tuned"...

Friday, August 19, 2011

Crookes tube first test

My Crookes tubes showed up today and I had time to get a couple of quick shots of magnetic deflection in action with my invisible magnet:


Thursday, August 18, 2011

Induction coil—another new toy

I have a bunch of HV stuff coming in this week, starting off with this induction coil:

Click to enlarge
I got it from I'm going to be powering Crookes tubes with it so I didn't pay the extra money for something bigger. Once you get over 15-20kV X-rays become a hazard. I have enough problems already. Like the fact that my Crookes tube order is sitting in a facility in Michigan and not moving. So much for Priority Mail.