Conversion factors from “counts per minute” (measured value)
to Bq/cm2 and μSv/h under a condition described in the note below
Direct reading by the
counter after background
subtraction (cpm)
Note：This table is
applicable only for
surface contamination.
(29th July, 2015）
Surface activity Dose equivalent rate
(μSv/h)
(Bq/cm2)
0
0
0
10
0.04
0.0003
100
0.4
0.003
1,000
4
0.033
10,000
40
0.33
100,000
400
3.3
Note: Condition for conversion
•
•
•
•
It is assumed that the “cpm” data were measured with a calibrated large window-type GM counter of the effective area of
20 cm2 and the detector efficiency of about 40 %, by well trained personnel, by following ISO 7503-1, i.e., by holding the
counter at the distance of 5 mm from the contaminated surface for a time period that is more than 3 times of the time
constant of the counter.
The radionuclides and surface condition of the contaminated surface under measurement are assumed to be equivalent to
those during the calibration of the counter with a reference source. Based on this assumption, the source efficiency of the
reference source is used in the conversion.
Conversion to dose equivalent rate assumes that a flat surface is uniformly contaminated in a 20 cm-radius circle with Cs137 (of which the dose equivalent rate constant is 0.0927 μSv m2 MBq-1 h-1) at the surface activities listed in the seconde
column of the table.
The calculated dose equivalent rates are the values at the position of a 5 cm height from the center of the contaminated
circular area.
NATIONAL INSTITUTE OF
ADVANCED INSUSTRIAL SCIENCE AND TECHNOLOGY (AIST)