By Jim Hill
Before beginning this project, download and read through message #28814 in GeigerCounterEnthusiasts Page 2 Titled "Consolidated INSTRUCTION DATA AND TIPS FOR 2" PLASTIC PROBE KIT " and read through the instructions posted there.
In the kit from GEO you will find the following:
1- RCA 6655 photomultiplier tube [PMT]
1- 14 pin socket for the PMT (installed in an aluminum box with a circuit board and associated components)
1- 10 Meg Ohm resistor (the tails are bent)
11- 4.7 Meg Ohm resistors
1- Plastic Scintillator block [Scint] (square or round depending on which kit you ordered)
1- Packet of optical coupling compound
1- Instruction sheet
You will need to supply the following to assemble the PMT and Scint block
1- Roll of high quality vinyl electrical tape (3/4 in wide if you can find it)
1- Roll of plumbers Teflon tape (3/4 in wide if available)
1- Roll of Scotch tape
1- Bottle of rubbing alcohol (Isopropyl alcohol IPA)
1- Pair of surgical gloves (blue ones without talc)
5- Assorted wet or dry (black, waterproof) sandpaper from the auto supply store, cut into quarters, the narrow way. 120, 360, 600, 1200, 2000 grit (in the painting supplies area)
1- Bottle of Megiar 's PlastX plastic auto window polish or other polish specifically made for plastic windows (auto supply store)
2- Microfiber polishing cloths (auto supply store)
*For Those wanting to polish the sides of Bismuth Germanate (BGO)*
1- Piece of smooth grain leather (for BGO)
1- Small container of Cerium Oxide, Tin Oxide, Barnsite, Titanium Dioxide or other polishing powder suitable for glass or gemstones (for BGO)
If you intend to put the tube assembly into a housing, you will also need a tube of cardboard, PVC, or brass or--? to enclose and protect the tube assembly.
Preparing the Components:
Prepare the PMT by removing the plastic coating on the front of the tube. It is firmly stuck to the front so carefully scrape up an edge with your fingernail (NO TOOLS) until enough is raised to get a grip on. Pull the plastic coating off and discard it. Clean he front of the tube with rubbing alcohol (Isopropyl alcohol) and a soft cloth. Be careful not to scratch the tube front surface. The end of the tube should look like clear caramel (tan). If it is clear or white, the tube has leaked air and is defective (Contact GEO for a replacement tube). Verify that there is MU Metal wrapped around the tube by finding the seam where it overlaps (inside the black tape wrapping). If you cant find the overlap, gently pull back the black wrap at the front edge. It should bend slightly and stay bent.
If all this went as described, the PMT is prepared and should be wrapped in the protective bubble wrap it came in and set aside.
The Photomultiplier tube
14 PIN SOCKET
Prepare the 14 pin socket by removing it from the box in which it is mounted. Unsolder all the wires and components connecting the tube socket to the box. Save the .01 uF 2KV capacitors for reuse in the voltage divider and the BNC connector along with the ground lug and nut.
You can now remove the socket from the box. (Mine was cemented in with epoxy and I had to scrape the stuff off with a knife. Some are held in with screws and clips and are easier to remove).
With the socket free of the box, remove the printed circuit board and associated wires from the socket and clean up the solder tabs.
Wire up the socket voltage divider circuit per the schematic supplied with the kit, using one of the .01 uF @ 2000V capacitors you salvaged from the original socket circuit.
Also I installed at this time a 6 in section of mini coax cable for the future connection to a BNC female connector. If you are going to run coax directly out of the completed scint housing, install a 36 in length.
p; The photomultiplier socket as received
The photomultiplier socket prepared for wiring
Plastic Scintillation Block
To prepare the Scint block, you will have to sand the rough-cut side surfaces with finer and finer sandpapers until they are smooth and then polish them.
If the sides are very rough and have deep grooves in them, start with 120-grade wet or dry. (Use a piece of plate glass to back up the sandpaper) Wet the sandpaper with water and a drop of liquid dish soap and sand away. Pull the Scint block toward you with downward pressure (holding the end of the abrasive paper to keep it from slipping). After a few strokes, rinse the sandpaper and the scint block to remove the sanded off plastic. Wet the paper again with the water and detergent. Do not let the plastic build up on the surface of the paper! Sand until all the marks are removed from the sides of the block. If yours is a round block, pull it toward you and rotate it at the same time.
Change to 360-grade sandpaper and repeat the process, sanding wet until all the marks from the 120 grade are removed. Work your way through all the grades of sandpaper in order. When you are finished with the 2000 grade the edges should be almost clear.
Polishing comes next and requires you to clean everything to remove ALL the grit from sanding from the glass plate, counter top, scint block and your hands. Fold the microfiber cloth in half and then apply a stripe of Megiars on it. Draw the scint block toward you over the polish with downward pressure. Continue until the sides are polished and reflect light internally. When the scint is polished, you can pick it up by the polished sides and not see where your fingers are holding it. Wash the scint block with soap and water and dry it carefully with a soft cloth. Inspect it with care and if there are any scratches visible, polish them away.
When you are satisfied, wrap it carefully and put away for now.
The plastic scintillation block polished
BGO Scintillation Crystals (For those of you using this material)
Be careful, the crystals are brittle and will shatter if dropped
Polishing the sides of BGO scint crystals requires a slightly different process than polishing the plastic material. The work must be done a hard flat surface such as a piece of ¼ in plate glass.
All the ones I polished had a surface finish equal to about 360 grade wet or dry sanding finish so I started there, because I wanted to be sure there weren't any scratches on the existing surfaces. There are small chips on the corner edges of most of the crystals and this process will not remove them.
All four sides are ground, each in order, narrow ones first, then the wide ones (old telescope mirror maker's practice - not sure why). Grind them wet, with a drop of dish detergent as with the plastic scint. Be careful not to snag the abrasive paper with the corner of the crystal. When satisfied with the finish from 360, wash and dry EVERYTHING to prepare for 600. It only takes 1 grain of coarser abrasive on the finer one to scratch the surface. Proceed in the same way through 1200 and 2000 grades.
To prepare BGO for polishing, again clean the whole work area to eliminate all grit particles. Dampen the leather polish surface with water and sprinkle on some polish. Smear the polish over the surface with your fingers (it should be about the consistency of toothpaste). Rub the BGO crystal back and forth on the paste and leather until the surface dries (most of the polish happens just before the leather dries out- you will feel the drag increase). Continue on all the surfaces 'till they are shiny bright.
Wash he crystal and wrap in Kleenex or other soft clean material and put away.
Assembling the Scintillation block to the PMT
Lay out the polished scint block, PMT, Teflon tape, and electrician's vinyl tape on a clean towel on your work surface. Put on the surgical gloves and wipe them down with IPA. Clean the scint block with soap and water on a soft cloth, rinse, wipe dry and set aside. No fingerprints or other oily contaminants should be visible. Take a clean water glass and make 12 or so thin strips of Scotch tape (3/16 in wide or so, not critical) and attach the ends to the water glass so they are easily accessed.
Set the scint block upright on one of its flat ends on the folded towel. Cut a strip of Teflon tape an inch or so longer than the width if the crystal and drape it across the middle. Cut two more and put them on each side, with a ¼ in overlap and tape them together. Continue until the upper surface is covered with the white tape. No Scotch tape should contact the scint block. Cut a piece of cardstock bigger than the scint block, put it on the Teflon tape on top of the scint block and turn the assembly over. Pull the center strip gently up the side of the scint block and then pull up the one beside it and tape them together so that they are smooth.. Do not tape to the scint block, only tape to the Teflon itself. Continue around the assembly until all the ends are taped to each other. Now take the roll of Teflon tape, attach the end to the already applied Teflon tape and carefully wind it on, completely covering the side of the scint block. Make sure the tape overlaps at least ¼ in on each turn and do not let it overlap onto the remaining flat surface. Tape the end down to its self with the Scotch tape. Turn the assembly over and make sure no gaps have developed, and look into the scint block end to make sure no Scotch tape is visible on the surface.
The next step is to cover the Teflon tape with two layers of electrical tape. Begin by wrapping the round side of the crystal with a layer of electrical tape, overlapping about ¼ in. Next cut enough electrical tape strips to cover the front face with the same overlap and cover that surface. Cut the same amount of strips and cover the front again, at right angle to the first layer. Then wrap a second layer of electrical tape on the round side.
You should now have a white layer of Teflon tape and two layers of opaque electrical tape on the scint block with none of it overlapping the remaining clear face. Clean the scint block face and the face of the PMT with IPA and set aside.
Squeeze a blob of optical grease the size of a small pea onto the scint block face and gently set the PMT face onto it. Swirl and rotate the PMT while gently pushing it down. This spreads the optical grease evenly and without bubbles over the whole face of both the parts. Align the two parts and stick them together with several strips of electrical tape. While maintaining that alignment, tape the two parts securely. Make sure there are two layers of electrical tape light proofing the joint. Inspect the PMT black coating and tape any spots that do not look light tight.
If you are going to install an alpha sensitive ZnS film, it will replace the front Teflon and electrical tape covering, but the side Teflon and electrical tape covering must remain. You must also cover the ZnS film with radfilm to exclude outside light (instead of the electrical tape). Put the ZnS surface facing out and do not put optical grease between the film and the scint block. The side of the scint block must still be covered and made light proof with the electrical tape as before. Assembly is the same as before, but be extra careful not to damage the radfilm surface. It must also be protected inside a housing of some sort after assembly (a short section of cardboard tubing will work).
The completed assembly with 'O' rings to support it in the housing
If your assembly is light tight it can be used without a protective housing if you insulate the voltage divider somehow, but a simple cardboard mailing tube with plastic end closures will protect the detector from accidents. Also check out the plastic fittings at your local home improvement store. ABS plastic plumbing pipe and fittings are black, light tight and allow betas and gammas through. Wrap the assembly in thin sheet upholstery foam, wrapped thick enough to hold it securely in the tube.
A detector assembled for alpha detection must have a screen of some sort in the front of the radfilm to keep it from being touched or damaged during use. A ring of the upholstery foam can be used to separate the screen and radfilm all held in place with a rubber "O" ring.
I like to do machine work so I made mine from aluminum tubing and caps (all made to screw together) so there are many ways to make a housing. (Look in GCE Photos section titled 'GEO's 2" scintillator for more photos.) Use your imagination and to quote GEO, "Have Fun"
The probe as assembled
Other photos of this project are in the PHOTOS section of the GeigerCounterEnthusiasts user group
*Postscript: What if no more KITS are available?*
There are PMTs available on eBAY and other surplus sources are out there. Even the PMT tubes from defunct 1950s vintage uranium prospecting scintillators can be used. Almost all PMT tubes will work with the voltage divider circuits from the GCE user group files, and information can usually be gotten about them from the manufacturers. The PMT tubes we use are the end sensing type, not the side sensing ones. GEO sells the scint blocks and optical coupling compound as separate items, and all the electronic parts are standard items available from electronics component sellers.
What? You say your just acquired PMT doesn't have a socket! I built one from a scrap of 1/8 in phenolic board and 14 female connectors scavenged from a square 24 pin electronic connector assembly. Find one with sockets that fit the pin diameter of your PMT, lay out the pattern on the insulating board of your choice, drill holes to fit and assemble your own. You can also solder the voltage divider circuit directly to the PMT tube pins.
My home built socket is on the left, a commercial one on the right
A thank you goes out to the following group respondents who, with their input and skilled electronic knowledge, made this project and the associated user groups possible.
George Dowell aka; GEOelectronics
Doug Moore aka: troglodite
Robert Atkinson aka: robert8rpi
And all the members who's names I have not been able to recall!
Again THANK YOU ALLJim Hill