Adding a Scint Probe

I've been putting in a lot of effort into this development and this page may have changed radically since you last viewed it
I've been working towards two goals:
  • Short term - To be able to use the Geiger Kit with a Scint Probe for the purpose of counting
  • Long term - In addition to counting, to be able to use the Geiger Kit with a Scint Probe for Gamma Spectroscopy.
I accomplished the counting goal by increasing the Kit's max HV and adding the "ScintCon" board described below.

Using the kit for Gamma Spectroscopy requires a bit more work . . .
  • The HV must be cleaned up - as it comes from the kit there is to much ripple.
  • The signal should be processed to make it suitable for the sound card input for PRA and Theremino MCA software.
  • The signal going back to the kit for counting must be normalized so the varying pulse heights can produce a uniform interrupt at the MCU.
    It will be nice to have level discrimination for this, so that minimum and maximum pulse heights can be selected for counting. 
The current status is described below.

Now the disclaimer! This is a work in progress, and it is mainly based on my experience with the two scintillation probes - a Saint Gobain 1.12X1.12M3/1.12L  and a SE International RAP47. YMMV. However, if you do try it, I would be very interested to hear your results.

The HV: 
The current GK-B4 kits with the v4.0.2 PCB (and above) have increased the HV to ~960V open output.
If you have an older kit, you can modify it with a few parts to get the same ouput. See the bottom of this page for instructions.

With a 120MΩ RAP47 attached, the max voltage remains near ~940V. With a 11MΩ Saint Gobain scint attached, it drops to ~820V. To further gauge the maximum current available I made a chain of 10M resistors and measured the voltage drop with the resistors across the HV. Starting at 940V open circuit I got the following readings . . .
 120M 110M 100M 90M 80M 70M 60M 50M 40M 30M 20M 10M
 957V 956V 954V 951V 948V 944V 940V 936V 929V 913V 885V 775V
 











(It's possible to achieve much higher voltages by doing this mod and adding the voltage multiplier described in this schematic. I've gotten 1820V open circuit this way. However, the voltage falls to 910V when the 11M scint is attached. I guess you don't get something for nothing and the voltage multiplier creates more voltage at the expense of current.)

Since the Geiger Kit was originally designed to drive GM tubes, a 4.7MΩ anode resistor is built into the board. When scintillation probes are used, this resistor must be bypassed. The v4.0.2 PCBs (and above) have a jumper block to bypass the anode resistor. Install this jumper when using scintillation probesCaution: With the anode resistor bypassed, the HV from the kit can give you a nasty little bite! It should command a bit of respect. Be sure to use the anode resistor again when switching back to GM tubes.

A Quick Check with a Scope:

Before going further, it's a good idea to check the results with your probe with a scope if you have one.
 If the HV is not able to drive your scint, it's better to find out as soon as you can.

This is a scope shot of background that I first got from my scintillation probe. The scope input is taken from a 1M load resistor on the positive side and then through a .001uF cap to the scope. See hookup diagram below. 



First attempt - ScintCon v2:
Scintillation probes don't so much put out a voltage but rather a charge to a capacitor. In order for the kit to get countable pulses from the scint, some signal processing had to be done. The solution to this was the original ScintCon kit, which used an Op Amp. In addition, since the Geiger kit uses "cathode sensing" for GM tubes a modification to the board was required in order to get clicks from the scint. 

The original ScintCon kit wasn't my best work. However it did work, and a few brave souls are using it. If you are interested in the schematics and other details for the ScintCon v2 they are in the Build Instructions for the kit here. 


Going Forward - ScintCon v3 & v4:
The goal is to use the Geiger Kit with a Scint Probe for Gamma Spectroscopy. I decided to use circuits from Theremino's PmtAdapter to clean up the HV coming from the kit, and condition the pulse. My thanks to the Theremino group for their design posted here.

ScintCon v4 is the same as v3 with some mistakes corrected.
 
ScintCon v4is intended to . . .
  • Filter the kits HV so it can be used as an HV source for Gamma Spectroscopy 
  • Add the signal conditioning circuit used by Theremino with pulse shaping and output to the sound card.
  • Feed back to the Geiger Kit from the scint for pulse counting with no board mods to the kit required.
  • Include an adjustable "window discriminator" for counting scint pulses by the Kit 
The connection to the Geiger Kit involves only 4 wires - +/- 5V and the HV anode and cathode screw terminals.
A HV supply other than the kit's can be used and only use the kit for counting.
The PCB is designed so that it will easily fit in a Hammond box with BNC connectors installed.


I received the first small batch of PCBs and put one together. I'm still evaluating it. . 
As a scint probe counter it seems to work pretty well with the kit.
However for spectroscopy with both Theremino and PRA it's disappointing. However, I'm not the sharpest knife in the drawer when it comes to SA and I'm hoping there is something that can be improved in my setup - soundcard, settings etc. 



This scope shot shows the output from the "raw pulse" output on the ScintCon board. Despite using the Theremino HV filter, I'm getting ~88mV of ripple. Using a 470k load resistor the signal peaks at ~2.75V with a thorite sample.

Is this signal to noise ratio too high for spectroscopy? I'm not sure. 

It seems that there is a good distribution in pulse height, but the spectra are not very good.





Here is a sample spectrum I got with Th232 and PRA. With Theremino it's even worse.
If you spot what I might be doing wrong please let me know.






Here is the current schematic.

Here are the preliminary Build Instructions, with more information on v4. 

In addition to the poor spectra, the v3 board has the ground plane too close to the pads and get arcing on humid days. So v3 is just another batch of ice scrapers. I have a v4 batch on order and depending on how it performs I will decide on when and how to make these kits available.


Test Results:
The chart below shows the results I got with the ScintCon v2 for the RAP47 and Saint Gobain (Bicron) scints.
I will make another if newer versions changes things. (click for larger image)



      
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