ASNT

Table 8 Conversion to SI radiographic units

Traditional Unit

Symbol

Multiply by

Resulting SI Unit

SI Symbol

3.7 × 10 10

Curie

Ci

becquerel

Bq

37

gigabecquerel

GBq

rad a

10 –2

Rad

gray

Gy

10

milligray

mGy

10 –2

Rem

rem

sievert

Sv

10

millisievert

mSv C/kg

2.58 × 10 –4

Roentgen

R

coulomb per kilogram

258

microcoulomb per kilogram

µC/kg

a. The abbreviation rd may be used for radiation absorbed dose where there is possibility of confusion with radian (rad), the SI unit for plane angle.

which is one disintegration per second. Because billions of disintegrations are required in a useful source, the multiplier prefix giga (10 9 ) is used and the unit is normally seen as a gigabecquerel (GBq). Coulomb per Kilogram Replaces Roentgen . The unit for quantity of electric charge is the coulomb (C), where 1 C = 1 A × 1 s. The original roentgen (R) was the quantity of radiation that would ionize 1 cm 3 of air to 1 electrostatic unit of electric charge (ESU), of either sign. It is now known that a roentgen is equivalent to 258 microcoulombs per kilogram of air (258 µC/kg of air). This corresponds to 1.61 × 10 15 ion pairs per 1 kg of air, which has then absorbed 8.8 mJ (0.88 rad). Gray Replaces Rad . The roentgen (R) was an intensity unit, but was not representative of the dose absorbed by material in a radiation field. The radiation-absorbed dose (rad) was first created to measure this quantity and was based on the erg, the energy unit from the old CGS system. In the SI system, the unit for radiation dose is the gray (Gy). The gray is useful because it applies to doses absorbed by matter at a particular location. It is expressed in energy units per mass of matter or in joules per kilogram (J/kg). The mass is that of the absorbing body. Sievert Replaces Rem . The SI system’s unit for the dose absorbed by the human body (formerly rem for roentgen equivalent man; also known as ambient dose equivalent, directional dose equivalent, dose equivalent, equivalent dose, and personal dose equivalent) is similar to the gray, but includes quality factors dependent on the type of radiation. This absorbed dose has been given the name sievert

The speed of light is a physical quantity, but can be used as a unit of measure. Unified Atomic Mass Unit. The unified atomic mass unit (u) is 12 –1 of the mass of the atom of the nuclide carbon 12; 1 u = 1.660 538 7310 –27 kg with a combined standard uncertainty of ±1.3 × 10 –34 kg (Taylor 2001; Mohr and Taylor 2000). Radiation Measurement Because of existing practice in certain fields and countries, the International Committee for Weights and Measures (CIPM, Comité Internationale des Poids et Mesures) permitted the units given in Table 8 (curie, roentgen, rad, and rem) to continue to be used with the SI until 1998 (Taylor 1995; Taylor 1998; Taylor 2001). However, these units must not be introduced where they are not presently used. The National Institute of Standards and Technology strongly discourages the continued use of curie, roentgen, rad, and rem (Taylor 1995; Taylor 1998; Taylor 2001). The American National Standards Institute (ANSI), the American Society for Testing and Materials (ASTM), the Institute of Electrical and Electronics Engineers, the Interna- tional Organization Standardization (ISO), and ASNT all support the replacement of older English units with SI units. Becquerel Replaces Curie. The original unit for radioactivity was the curie (Ci), simply the radiation of one gram of radium. Eventually all equivalent radiation from any source was measured with this same unit. It is now known that a curie is equivalent to 3.7 × 10 10 disintegrations per second. In SI, the unit for radioactivity is the becquerel (Bq),

CHAPTER 1

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Part 4