Geiger Counter/ scintillation detector
A device that can measure radiation using a variety of different detection instrumentation. An adjustable high voltage supply, PMT pre amp, counting circuitry, digital display, and PC interfacing systems are included. It is compatible with many different tubes, including geiger muller tubes, PMT based scintillators, or even Boron 10 lined proportional tubes.
Shortly after I first made the wind bottle CRT, My dad started to get concerned about radiation. Even though I was sure there was absolutely no significant amount of radiation being produced, and even if there was it was very low energy, but I had to get quantitative experimental data. The only problem was I had nothing capable of measuring radiation. Luckily I did know someone who’s main hobby was radiography, and he agreed to help me. He brought out a Ludlum scintillation unit, and I was intrigued by it to say the least.
The unit had an a digital readout, adjustable timer for total dose counts, and adjustable power bias and amplifier settings to allow for compatibility with various different scintillation tubes.
After that, I knew I wanted to make my own unit, but I wanted to be able to detect basically any type of radiation. Pretty much the only way to do this is to use a lot of different types of tubes with it.
I decided that to start designing with the probe. I could have bought an SBM-20 very cheaply, and been done, but the first thing that I wanted to be able to do was measure background radiation, something that Geiger tubes are not the best at doing, although I wanted to return to it, I decided to start with a scintillation tube.
I chose to base the probe around the R1307 photomultiplier tube, coupled to some scintillation plastic. I chose this tube because of the large front view window, and because its peak sensitivity is at about 350 nm, that matched the scintillation plastic’s emissions.
The PMT has a board built into it with the divider circuit built in, which is a definite time saver. I did have to pick apart all the connections on the board myself, as no schematics were available. next is the power supply based on a CCFL inverter, and the amplifier for the PMT.
Beginning work on the probe construction, the first thing I had to do was polish the sides of the scintillation plastic. They were very rough from machining, and needed to be more reflective for the plastic to be most effective.
To get the sides smooth I sanded the sides progressively with 60,120, and 400 grit sand paper before rubbing it with firm pressure on a brown paper bag laid flat on my work bench. Despite not having the best selection of sand paper, it worked pretty well. Not perfect, but surprisingly good.
Next I had to make some modifications to the PMT’s board. I began unsoldering the resistors in the divider circuit, which actually proved pretty difficult (they were bent over the pad making desoldering difficult), and replaced them with higher value resistors, and rewired a few things using the following schematic that I found after searching the part number on my PMT.
Above is the board with the original resistors
The 10meg resistors (only 8 were needed).
Since the resistors were so difficult to desolder, I eventually removed the entire board to finish making all the modifications marked on the schematic.
Above you can see all the resistors in the divider have been replaced.
I made the rest of the necessary modifications and…
I replaced the board on the PMT which turned out to be a less than trivial task. Since I had to clip the leads to remove the board, they became too short to support the board, so I had to solder some longer leads to them prior to replacing and re soldering the board.
Above: The tube ready for coupling to the scintillation plastic.
The scintiallation plastic came with quite a bit of optical coupling compound, which I made good use of. I applied a small amount of the compound to the face of the PMT to help make a good connection to the plastic.
Above: I prepared the plastic by wrapping it in paper to keep the adhesive on the electrical tape from affecting the plastic, and then I wrapped both the tube and plastic with black electrical tape excluding the sides that will be coupled.
I then put the plastic on the PMT window, and used a gentle swirling motion to evenly spread the optical compound without creating bubbles. I used a few long pieces of tape to hold the scintillator to the PMT, and then finished wrapping both of them with black tape.
I decided to use a paint can to act as an enclosure for the detector because it was large enough to hold the PMT, and it was also metal which means it is easy to ground and can effectively shield the detector from electrical noise.
I wrapped the scintillation detector (now barely short enough to fit into the paint can in some recycleable packaging material digikey always uses in their shipments. I made sure to fold a little over the front of the scintillation tube to help protect it.
After drilling a hole in the lid of the can, I tightened a female BNC connector into the hole, and soldered the ground and voltage wires to it in their respective spots (ground outside of the connector and power+signal inner pin). I then taped around the connector with black tape on the inside to stop any light leaks.
I closed up the can, which concluded the construction of the detector.
Now It is time to work on the amplifier, power supply and counter electronics.