BUILDING THE CDV700Pro Geiger-Mueller / Scintillation Counter. July 2005, David Prutchi, Ph.D.

Transcription

1 BUILDING THE CDV700Pro Geiger-Mueller / Scintillation Counter July 2005, David Prutchi, Ph.D.

2 David Prutchi s CDV700 Pro George Dowell (A.K.A. K0FF, GeoElectronics) started the trend of modifying the Electro Neutronics CD V-700 model 6b into a Lionel CD V-700 model 6b clone. In George s words: The Electro-Neutronics CD V-700 model 6b has had a bad reputation for 40 years. On the other hand, the Lionel CD V 700 model 6b is regarded by many as the best circuit ever. Surprisingly, of all the variations in CD V 700's I have seen, including Victoreen's various models, and the Antons, there is no circuit more similar to the Lionel's than the ENi! At least it is similar enough to make a logical transformation that is really significant electronically, but easy to do. In my opinion the physical layout of the ENi is better than that of the Lionel, so the merger of the Lionel circuit concept into the ENi mechanical layout really does give the best of both worlds. By performing the simple and inexpensive K0FF LENi mod, you can create a Lionel clone from an ENI.

3 David Prutchi s CDV700 Pro The CDV700 Pro is based on K0FF s LENi, but adds the following features: Preamplifier to make it compatible with scintillation probes (GM tubes can still be used) Selectable, regulated bias voltage (900V or 1,200V) for connection to GM tubes and photomultiplier tubes. A blinking indicator warns of the high voltage selection. Noise-reduction circuitry eliminates hum. Internal piezo clicker. Power input jack saves batteries when powered from car or AC-operated power supply. 8-digit digital counter.

4 CDV700 Pro Controls Integrator capacitor reset Meter response selector (Fast/Slow) Probe type selector (GM tube or scintillator) External 3VDC power input Headphone jack Blinking indicator warns of 1,200V HV selection Probe connector

5 CDV700 Pro Displays 8-digit count totalizer Analog rate meter

6 -15V HEADPHONE OUTPUT AMPHENOL 75-PC-1M J1 1 C2 R1 27k 0.05uF D1 30PIV SILICON D2 30PIV SILICON R2 10k Q2 PNP R6 19k R3 1.9k SW1B OFF X100 X10 X1 OFF X100 X10 X1 SW1C D3 30PIV SILICON C uF 50V + -15V R8 150k C1 200uF 3V R7 8.2k R5 39k - A + M1 50uA METER R4 35k C5 0.01uF 2kV D7 30PIV GERMANIUM PNP Q3 L1 C uF 50V Geiger Mueller Tube COAX 24mH 150 Ohm HIGH VOLTAGE R12 2.7M BT1 1.5V BT3 1.5V BT2 1.5V BT4 1.5V SW1A OFF N/A C6 25uF 25V -3V -15V + D4 30PIV SILICON D6 15.5V R13 1k Q4 PNP T ENI CD V-700 TRANSFORMER D5 1350PIV SELENIUM C8 0.01uF 2kV Schematic of unmodified Electro Neutronics CD V-700 Model 6-b Geiger-Mueller Survey Meter David Prutchi, July 2005

7 ENi CD V-700 model 6b PCB component location

8 Schematic Diagram of David Prutchi s CDV700 Pro

9 Step 1: Convert the ENi CD V-700 into a LENi (almost ) Study the schematic diagram of the original ENi CD V-700 and identify the components on the PCB. Disconnect the GM probe. Follow George Dowell s instructions for modifying the power supply section: Remove Zener diode CR6 (sometimes 2 diodes in series) and discard, it will not be used. Remove R13 and reinstall it in series with the base lead going to the oscillator transistor. Add a.0022μf 50V capacitor between the base and collector of transistor V4 (Q4). Substitute CR5 (D5) by a modern >5kV silicon diode. I use a Fuji ESJA53-20A, 20kV, 0.5A, 5mA diode Substitute C8 by a modern 3kV capacitor. Substitute C5 by a 3kV capacitor. Replace R13 by a 1.8MΩ and a 3.3MΩ in series.

10 To Probe R12 3.3M -3V R17 1.8M + C6 47uF 60V D4 1N4001 C T D5 15kV Silicon C8 0.01uF 3kV Q4 2N404A R13 1k ENI CD V-700 TRANSFORMER

11 Check your progress Check that your DMM has a 10MΩ input impedance when set to a range suitable for measuring 200VDC (use a second multimeter to measure the resistance across the input terminals of your DMM). String 9 10MΩ resistors in series to make a X10 probe: Body of plastic ballpoint pen contains 9 10MΩ resistors in series Dab of epoxy Silicone-coated probe cable Wire-wrapping pin Banana plug Use this probe to measure the voltage across C8. With no GM tube connected, you should measure between ~1,300 and ~1,900VDC (130 to 190V on your multimeter).

12 Step 2: Build the Zener Stack Build the Zener-diode stack using 9 1N V Zeners and 2 NTE5081A 24V Zeners. Use piece of perfboard as a substrate. Place 1 layer of Kapton insulation and add C15 to the back side of the board. Wrap the assembly with Kapton tape. R17 1.8M D13 24V D14 24V D5 15kV Silicon D12 150V D15 150V C8 0.01uF 3kV D11 150V D16 150V D10 150V D17 150V C uF 3kV D9 150V D18 150V

13 Step 3: Connect the Zener Stack Mount the Zener-diode regulator board onto the CD V-700 PCB. Make a small hole on the edge of the CDV s PCB and connect the ground end of the regulator board directly to the ground line on the CDV PCB: Make hole and connect directly to GND Check your progress: If you power the instrument, the voltage across C15 should be ~1,200V and ~900V if you short the cathode of D17 to GND.

14 Step 4: Add Biasing Circuit Add R14, R15, R16 and C9. Consult George Dowell s instructions on modifying the CD V-700 PCB to accommodate these components. R16 22k SW1A OFF -3V N/A R14 1.2k C9 0.1uF 50V R15 3.3k

15 Step 5: Modify Metering Circuit Remove CR7 (D7). Connect the emitters of the metering transistors in parallel and route them to the -3V line. Consult George Dowell s instructions on modifying the CD V-700 PCB. Replace L1 with an 18kΩ 1/2W resistor (R18 in the CDV700 Pro schematic). Modify the circuit to insert a 10Ω 1/2W resistor between the anode of D4 and the -15V line feeding the metering circuit. Replace C6 by a 47μF 60V electrolytic capacitor. Replace C1 by two 100μF capacitors. Leave the negative terminal of one of these capacitors open so that it can be connected to the front-panel time-constant selection switch.

16 Step 6: Mount Probe BNC Connector Remove and discard the sealing nut through which the GM probe cable passed. Tap this hole using a 3/8 diameter 32 tpi pitch tap (McMaster-Carr 25705A64 ). Mount a non-isolated bulkhead BNC connector (Jameco 71589) on this tapped hole.

17 Step 7: Modify Front Panel Drill front panel (use casting marks on back side) to accommodate the extra switches, connectors, piezo speaker and LED:

18 Step 8: Wire the Front Panel to PCB Wire the instrument using clean, new cable with insulation for the appropriate voltage rating. Route cables next to the enclosure and keep connections as short, direct and clean as possible. Use high-voltage test lead wire between the PCB and the center terminal of the probe BNC. Use good heat-shrink tubing to dress all switch connections. Keeping things tidy will really pay off later. Once the PMT preamplifier is added, noise will creep into the system if you don t pay attention to your wiring habits.

19 Step 9: Test your Almost LENi Measure the voltage at the BNC connector using the high-impedance X10 probe: You should read ~1,200V with the probe selector switch in the Scintillator position. The blinking LED should light up and blink. You should read ~900V with the probe selector switch in the GM position. The LED should remain off. Turn the probe selector switch to the GM position. Connect a GM probe (e.g. the original CD V-700 probe that has been connectorized). The unit should produce background clicks and be able to detect the radiation emitted by the operational check source.

20 Step 10: Install the Digital Totalizer Use a nibbling tool to cut a 68mm x 33mm rectangular hole on the enclosure bottom to accommodate the Veeder-Root A Totalizer. Connect the totalizer to the circuit and verify that the counter advances once for every click

21 Step 11: Build the Preamplifier Build the preamplifier circuit on a small piece of prototyping board. Keep wires short and the circuit neat and organized. -15V D25 1N914 R20 1M R21 1M Q5 2N3906 R22 33k R23 10k Q6 2N3906 Q7 2N3904 R25 10k + 100uF C14 C uF C16 0.1uF C11 10pF D24 1N914 PREAMPLIFIER BOARD R24 10k + C13 10uF R26 10k Gain R27 1k

22 Step 12: Mount the Preamplifier Unsolder C5. Mount the preamplifier board directly onto the CD V-700 PCB as shown in the pictures and wire C5 and C12 between the preamplifier board and the CD V700 PCB: C5 C12 GND C16 C17-15V Add the ground wire and bypass capacitors C16 and C17.

23 Step 13: Complete Assembly Add a 2-D-cell plastic battery to make space for the preamplifier circuit. Make some mounting holes on the PCB and mount it using ¼ nylon spacers. Connect a scintillation probe (next section), set the probe selector switch accordingly and trim R26. CONGRATULATIONS! You have completed modifying the ENi CD V-700 model 6b into a CDV700 Pro!

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25 David Prutchi s Scintillation Probe for the CDV700 Pro Thick cardboard tube ~2 ID XP2102 PMT Voltage divider 8 x 2 dia NaI(Tl) scintillation crystal in steel jacket 2 OD PVC

26 Index-coupling gel or adhesive Photonis XP2102 Photomultiplier Tube Cathode Grid DY1 DY2 DY3 DY4 DY5 DY6 DY7 DY8 DY9 DY10 Anode NaI( Tl) Crystal R1 10M R2 10M R3 10M R4 10M R5 10M R6 10M R7 10M R8 10M R9 10M R10 10M R11 10M R12 10M C1 C2 C3 C4 0.01uF 0.01uF 0.01uF To CDV700 Pro J uF 3kV 1 2 Schematic of Scintillation Probe for "CDV700 Pro" David Prutchi, July 2005

27 Other Scintillation Probe Ideas Charlie Thompson s paint can scintillator: pson15/paintcanscint.html Charlie Thompson s Hamamatsu R1307 conversion circuit: pson15/r gif

28 APPENDIX A compilation of material by George Dowell on modifying the ENi CD V700 model 6b into a LENi

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32 K0FF s Zener Diode Board

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