Model 78 and 78-1
Technical Manual
Section 8
Ludlum Measurements, Inc.
Page 8-5
March 2021
G
ENERAL
D
ETECTOR
S
ETUP
I
NFORMATION
The operating point for the instrument and probes is established by setting the probe
voltage and instrument sensitivity (
HV
1,
HV
2, and
DISC
).
The two energy-compensated, Geiger-Mueller detectors supplied with the Model 78
operate at:
HV
1 = 550 Vdc,
HV
2 =550 Vdc, and
DISC
at 50 mV.
E
XPOSURE
R
ATE
C
ALIBRATION
To calibrate the Model 78 to exposure rate after setting the HV and DISC
potentiometers, start with the following values for DT (Dead Time, Switch Position 1),
CC (Calibration Constant, Switch Position 2), and SC (Series Constant Switch Position
3). Note that the SC is left at zero for the lower range tube, but is used for the high
range tube's calibration.
For exposure rates:
mR/hr: DT = 75 x 10
-6
, CC = 645 x 10
5
, SC = 0000
R/hr: DT = 40 x 10
-6
, CC = 100 x 10
4
, SC = 300
For dose-equivalent rates (Sv/h Firmware 27201n07):
µSv/h: DT = 75 x 10
-6
, CC = 645 x 10
4
, SC=0000
mSv/h: DT = 40 x 10
-6
, CC = 100 x 10
3
, SC=300
For exposure rate calibrations, use a calibrated
137
Cs source, and set the Model 78 to
mR/hr. Place the detector at the following points:
mR/hr
R/hr
2
2
8
8 *adjust CC
20* adjust CC
20
80
80 *adjust DT
200
200
800* adjust DT
800 *adjust SC
