Radiation Detection

Science Summaries

Here is a scan of the schematic diagram #NA-0221 I found inside this Civil Defense Victoreen/ Jordan nuclear radiation CD V-750 Dosimeter Charger.
Four V-138 test unit dosimeters are pictured, as well as the top of the Model CDV-750, Model No. 5b, for identification purposes. The schematic is a JPEG, so you can download it, and print it out. I have added a parts list. Purchased on eBay for $20.

1. Nuclear Radiation Detection Summary - for general information
2. Geiger Counters Summary - for descriptions and pricing
3. Geiger Counter Data Compared - for sensitivity comparisons
4. Ludlum Radiation Product Summary - detection devices and probes
5. Victoreen CDV-700 6B Detector Summary - True Geiger Counter (antique)
6. Victoreen CDV-715 1A Detector Summary - Ion chamber detector (antique)
7. Victoreen CD V-750 5B Dosimeter Charger - Charges CD quartz dosimeters
          1. CD V-750 Photograph
          2. CD V-750 Schematic Diagram
          3. CD V-750 Test Voltages
          4. CD V-750 Dosimeter Notes
          5. CD V-750 Parts List
8. Uranium Ore Summary - common types of radioactive ore

Voltages

Nominal

Mine

BATT.

-1.48 Volts

-1.54 Volts

Duracells are higher.

TP1

52 Volts

45 Volts

Need to tune T1?

OUTPUT

57 - 220 VDC

45 - 200 VDC

According to manual

All voltages referenced to ground.
All voltages measured with instrument operating.
Voltages may vary ±20% from instrument to instrument.

These units were poorly designed. The output voltage depends upon the ON Switch being activated. The problem is that DURING the removal of the dosimeter, the output voltage is CHANGING. The fix was to retrofit these units with the 0.035µFd (C3) across the output voltage. The result is that the output voltage varies with the SPEED of removal of the dosimeter.

Using one of these old chargers is more of an art than a science. You must develop a "feel", or technique, when removing the tested unit, after you have set the voltage which produces zeroing of the mR scale. It takes me about 20 tries for each unit.

The problem is that the charging circuit turns off before the charging contact is removed. Quick withdrawal results in a reading that is too HIGH (150 mR) (V too LOW) - withdraw more slowly. Slow withdrawal results in a reading that is too LOW (50 mR) (V too HIGH) - withdraw more quickly.

Dosimeter Ranges by Model # (made by Bendix, Landsverk, and Victoreen (later Arrow-Tech)
V-138 - 200mR (only 200,000 made)
V-725 - 5R (Arrow-Tech, 1990's)
V-730 - 20R
V-740 - 100R
V-742 - 200R (both the 730 and 740 were replaced by the 742) Over 3 million made. Very plentiful.
V-746 - 600R (forget about it, you are finished)

Notes: The voltage scale above is NOT linear. The measured voltages were averaged over my 4 test units.
The RED area above shows how the calibration varies, based upon dosimeter removal time.
Note that the CD V-138 dosimeter is 1,000 times more sensitive than the CD V-742.
200 milliREMs vs. 200 REMs.

Experiment
I set 4 Bendix V-138's to 100mR (mid-scale). I then placed the 4 V-138s in my lead pig (container) with my uranium ore for 10 hours. The average reading went up to 160mR, which equates to 6mR per hour (160 - 100, divided by 10). In order to get my maximum recommended human daily limit of 2mR, I need to handle my ore samples for only 20 minutes.

In order to see the same deflection on my V-742 dosimeter units, it would take 10,000 hours, or almost 14 months. It's no wonder that I thought that my CD V-742's were broken. The plentiful V-742 units are useful only if an atomic bomb detonates next door. Which, of course, was the idea. They are utterly useless for low dose exposures and measurements.

 Resistors

 Semiconductors

R1

10M Ohm Pot. 1/4W 30%

Q1

QT-0033 Transistor

R2

3.9M 1/2W 10%

CR1

QD-0021 Diode

R3

27K Ohm 1/2W 20%

 Miscellaneous

 Capacitors

T1

TX-0002 Transformer

C1

0.002µF

SW1

ON-off switch

C2

0.002µF

I1

#131 Lamp

C3

0.035µF @ 400V

K1

Zeroing knob

Although the author has tried to be as accurate as possible, errors are possible.

I could not find an online schematic diagram like the above. So, I made this Dosimeter Charger schematic web page, to help myself understand the technology. I hope that it helps you.

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