Page 567 - Instrumentation Reference Book 3E

Introduction 549

p = 19.3

0.01

0.005

-
/// I I I A 1
0.1 0.2 0.3 0.4 0.50.6 0.8 1 2 3 4 56789
Gamma-ray energy (Mev)
Figure 23.1 Thickness needed to attenuate a narrow gamma-ray beam byafactor of 10.
To reduce this to as small a value as possible, then S, 23.1.3 Accuracy/precision of measurements
17, or t or all three of these variables should be The precision or reproducibility of a measure-
increased to as high a value as possible. In many ment is defined in terms of the to repeat
cases,, however, the time available for measurement measurements of the Same quantity. Precision is
is short. This is particularly true on high-speed pro- expressed qualltitatively in terms the standard
duction lines of sheet matel<al, where only a few deviation, ~, from the average value obtained by
milliseconds may be available for the measurement. repeated measurements. In practice it is deter-
It can now be seen how measurement time, col-
limation, detector size, and absorber thickness may mined by statistical variations in the rate of emis-
sion from the radioactive source, instrumental
affect the error in the measurement. The shorter instabilities, and variations in measuring con-
the measurement time, the greater the degree of ditions,
collimation, the thicker the absorber and the smal- The accuracy of a measurement is an expression
ler the detector, the greater will be the source activ- of the degree of correctness with which an actual
ity required to maintain a constant error. A larger measurement yields the tme value of the quantity
source will be more expensive, and in addition its being measu.ed, It is expressed quantitatively in
physical size may impose a limit On the activity terms of the deviation from the tme value of the
usable. Bearing in mind that a source radiates meall of repeated measurements. The accuracy of
equ& in all directions, only a very small fraction a measurement depends on the precision and also
call lbe direcied by collimation for useful measure- on the accuracy of calibration. If calibration
meat: the rest is merely absorbed in the shielding is exact, thell in tile limit, accuracy and precision
necessary to protect the source.