September 25, 1985

CPG 4-1, Volume 6, CHG 1
CD V-700M

     There are operational problems with the Victoreen CDV-700 Models 6 and 6A instruments when they are modified to a CD V-700-M. Because of this, two modified instruments were evaluated by EMSTF.
     As a result of this EMSTF Report No. 73-GMI-29, it is requested that all CDV-700-M instruments be examined to determine the type of end-window GM tube installed in the probe. Requisition replacement Amperex 200 NB GM tubes, Sort Code 50720, for instruments that have an EON-type 6210S GM tube installed.
     The following test report is a result of previously mentioned test.
1. References:
    a. Defense Civil Preparedness Agency Report No. STRATCOM/FY73/5, Item No.
4.1, dated 1 May 1973.
    b. Letter from the State of Michigan RADEF Maintenance Officer dated 1 May
1973, describing operational problems with the Victoreen CDV-700 Model 6 and Model 6-4
when modified to a CDV-700-M .

2. The Defense Civil Preparedness Agency (DCPA) was contacted by the referenced letter in regards to operational problems with Victoreen CDV-700 Models 6 and 6A instruments when modified to a CDV-700-M with an end-window probe. Michigan was requested to furnish two (2) such modified instruments to this Facility for evaluation and the instruments were received on 7 May 1973.

3. In response to the referenced request, this Facility performed an initial inspection of these instruments which revealed that the modification had been performed correctly and that they failed to respond properly on the X-100 range as outlined by the referenced letter. The instruments were recalibrated in a 4.0 mR/h y field using a 490 Ci 137Cs-137Ba source collimated in free air and attentuated with tungsten filter. Subsequent tests showed that both instruments still swamped out or jammed at ~ 20 mR/h causing the instrument's meter to indicate zero. The instrument parameters were tested and it was found that the input trigger sensitivities were at the design level of 250 mV and also that the high-voltage outputs were at acceptable levels of 925V and 940V. The GM tubes furnished this Facility with the defective instruments were of EON Type 6210S. These EON GM tubes are the tubes bought with the probe and are normally supplied with it for the CDV-700-M modification. The only end-window GM tubes in the Facility stock were Amperex Model 200 NB tubes. Two (2) of the Amperex tubes were substituted for the EON tubes. Satisfactory operation of the two (2) CDV-700-M instruments was now possible. Further testing of the EON tubes showed that the output GM pulse height into the modified Victoreen CDV-700 Models 6 and 6A showed the height to be ~300 mV at y exposure rates of ~1 mR/h. On the other hand, the Amperex pulse height was ~1.5V under the same conditions. Therefore, the problem occurs because as the y exposure rate increases the EON tube pulse height drops below the 250m V trigger level of the Victoreen CDV-700 Models 6and 6A causing them to swamp out and causing the meter
reading to return to zero.
4. The Anton CDV-700 Model 6 and the Lionel Model 6B instruments were tested to determine if the EON Type 6210S tube would operate in these instruments since the input trigger sensitivities are designed at 500mV in both of these models. The results of the tests showed that both of these instruments will detect y exposure rates of up to 50 mR/h. However, at exposure rates above 55 mR/h both instruments swamped out. The reason for the better performance of the Anton CDV-700 Model 6 and the Lionel Model 6B is that their input impedance is higher than the input impedance of the Victoreen CDV-700 Models 6and 6A which puts less of a load on the tube raising the input pulse height to> 0.5V at ~1 mR/h. A Victoreen CDV-700 Model 6B instrument was also tested with an EON tube and found to swamp out at 50 mR/h. The reason this instrument operates better than the Victoreen Models 6and 6A is the result of a slightly lower input trigger sensitivity of 200mV.
5. Since all of the CDV-700 makes and models are operating on a marginal basis with the EON Type 62l0S GM tube, it is recommended that it be replaced by the Amperex Model 200 NB GM tube in all CDV-700-M probes. The EON Type 6210S tube should be placed in a special issue code by Defense General Supply Center for use only in the NEC scaler systems. The Amperex 200 NB tube should be immediately issued as a replace- ment tube for all CDV-700-M instruments.

CP 4-1 Pages 26-1 thru 26-3