ASNT

relationship, roentgen converts to millisieverts on a one-to-ten basis. Exposure charts were often made by using curie minutes at a source-to-film distance in inches squared. This was written Ci/min/in. 2 . Exposure charts made in SI use gigabecquerel minutes for a source-to-film distance in centimeters squared, where 1 Ci/min/in. 2 = 50 GBq/min/cm 2 . Table 9 lists some of these compound units.

(Sv), but its dimensions are the same as the gray; that is, 1 Sv = 1 J/kg. Compound Units Exposure to ionizing radiation could be measured in roentgens with an ionization chamber that, when placed 1 m (39 in.) from the radiation source, provided necessary information — one roentgen per curie per hour at one meter (R/Ci/h at 1 m), for example. The numbers, however, had limited physical meaning and could not be used for different applications such as high-voltage X-ray machines. The roentgen per hour (R/h) was used to designate the exposure to an ionizing radiation of the stated value. Because the radiation received from 1 R/h was considered about equal to 1 rem, the relationship is approximated as 1 R/h = 0.01 Gy/h = 10 mGy/h. A previously popular unit, roentgen per curie per hour at one meter (R/Ci/h at 1 m), is expressed in SI units as millisievert per gigabecquerel per hour at one meter (mSv/GBq/h at 1 m), such that 1 mSv/GBq/h at 1 m = 3.7 R/Ci/h at 1 m. In this

Table 9 Compound radiographic units

Traditional

Multiply

Resulting

Unit

by

SI Unit

R/Ci/h at 1 m Ci/min/in. 2

0.27

mSv/GBq/h at 1 m

GBq/min/cm 2

50

R/min a R/min b

0.01 0.01

Gy/min Sv/min

2.58 × 10 –4

R

C/kg

a. Absorbed dose. b. Dose absorbed by human body.

CHAPTER 1

36

Part 4