ASNT

observed. For example, the meanings of the prefix m (milli) and the prefix M (mega) differ by nine orders of magnitude. SI Units for Radiography The original discoveries of radioactivity helped establish units of measurement based on observa- tion, rather than precise physical phenomena. Later, scientists who worked with radioactive substances (or who managed to manufacture radioactive beams) again made circumstantial observations that were then used for measurement purposes. This practical approach was acceptable at the time, but a broader understanding of physics and the modern practice of using only one unit for a quantity has led to the modification of many of the original units (see Tables 7 to 9). In the SI system, radiation units have been given established physical foundations and new names where necessary. Physical Quantities Three physical quantities in particular are widely used as measurement units — the electronvolt (eV), the speed of light (c), and the unified atomic mass unit (u). Their precise values, however, are obtained experimentally. Electronvolt. The electronvolt is the kinetic energy acquired by an electron in passing through a potential difference of 1 V in vacuum; 1 eV = 1.602 176 462 × 10 –19 J with a combined standard uncertainty of 6.3 × 10 –27 J (Taylor 2001; Mohr and Taylor 2000). The electronvolt is accepted for use with SI. Speed of Electromagnetic Radiation. The quantity c represents the speed of light, that is, the speed

Table 6 SI prefixes and multipliers

Prefix yotta

Symbol

Multiplier

10 24 10 21 10 18 10 15 10 12

Y Z E P

zetta

exa

peta tera giga

T

10 9 10 6 10 3 10 2

G

mega

M

kilo

k

hecto a deka a deci a centi a

h

da

10

10 –1 10 –2 10 –3 10 –6 10 –9 10 –12 10 –15 10 –18 10 –21

d

c

milli

m

micro

μ n p

nano

pico

femto

f

atto

a z y

zepto yocto

10 –24 a. Avoid these prefixes (except in dm 3 and cm 3 ) for science and engineering.

decimeter, dekameter (or decameter), and hectome- ter are often avoided in scientific and technical uses of SI because of their variance from the 10 3 interval. However, dm 3 and cm 3 are commonly used. Note that 1 cm 3 is not equal to 0.01 m 3 . Nevertheless, in equations, submultiples such as centimeter (cm) or decimeter (dm) are often avoided because they disturb the convenient 10 3 or 10 –3 intervals that make equations easy to manipulate. In SI, the distinction between upper- and lower-case letters is meaningful and should be

of electromagnetic waves in vacuum; 1 c = 299 792 458 m/s exactly (670 616 629 mi/h).

Table 7 Physical quantities used as units Values of physical quantities are experimentally obtained and may only be approximated in SI Conversions are provided here for descriptive purposes

Physical Quantity

Symbol

Multiply by

SI Unit

SI Symbol

Electronvolt a

1.6 × 10 –19

eV

joule

J

2.99792458 × 10 8

Speed of electromagnetic waves in vacuum

c

meter per second

m/s

Unified atomic mass unit a,b

1.66 × 10 –27

u

kilogram

kg

a. Approved for use with SI. b. Mass of unified atomic mass unit is 12 –1 of the mass of the atom of the nuclide carbon 12.

CHAPTER 1

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Part 4