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Significance and Use
4.1 Gamma, electron, and X-ray (bremsstrahlung) facilities routinely irradiate a variety of products such as food, medical devices, aseptic packaging and commodities (see ISO/ASTM Practices 51204 and 51431). Process parameters must be carefully controlled to ensure that these products are processed within specifications (see ISO 11137, Section 2.3). Accurate dosimetry is essential in process control (see ISO/ASTM Guide 51261). For absorbed dose measurements to be meaningful, the combined uncertainty associated with these measurements must be estimated and its magnitude quantified.
NOTE 1 - For a comprehensive discussion of various dosimetry methods applicable to the radiation types and energies discussed in this guide, see ICRU Reports 14, 17, 34, 35 and Refs (5, 6).
4.2 This guide uses the methodology adopted by the International Organization for Standardization for estimating uncertainties in dosimetry for radiation processing (see 2.3). ASTM traditionally uses the terms of precision and bias where precision is a measure of the extent to which replicate measurements made under specified conditions are in agreement and bias is a systematic error (see ASTM Terminologies E 170 and E 456, and Practice E 177). As seen from this standard, components of uncertainty are evaluated as either Type A or Type B rather than in terms of precision and bias. Error is different from Type A and Type B components of uncertainty.
4.3 Although this guide provides a framework for assessing uncertainty, it cannot substitute for critical thinking, intellectual honesty, and professional skill. The evaluation of uncertainty is neither a routine task nor a purely mathematical one; it depends on detailed knowledge of the nature of the measurand and of the measurement method and procedure used. The quality and utility of the uncertainty quoted for the result of a measurement therefore ultimately depends on the understanding, critical analysis, and integrity of those who contribute to the assignment of its value.
4.4 Process requirements may necessitate establishment of a target uncertainty, which provides a point of reference for evaluating whether the calculated value of uncertainty is acceptable for the process under consideration.
4.5 Results of an uncertainty assessment may be used to aid in the evaluation of the statistical control in the given application. Controllable components of uncertainty may be ranked by comparison to total uncertainty. This ranking may be used to identify areas for corrective action to reduce the total uncertainty.
1.1 This guide defines possible sources of uncertainty in dosimetry performed in gamma, X-ray (bremsstrahlung), and electron irradiation facilities and offers procedures for estimating the resulting magnitude of the uncertainties in the measurement of absorbed dose using a dosimetry system. Basic concepts of measurement, estimate of the measured value of a quantity, "true value", error, and uncertainty are defined and discussed. Components of uncertainty are discussed and methods are given for evaluating and estimating their values. How these contribute to the standard uncertainty in the reported values of absorbed dose are considered and methods are given for calculating the combined standard uncertainty and an estimate of expanded (overall) uncertainty. The methodology for evaluating components of uncertainty follows ISO procedures (see ). The traditional concepts of precision and bias are not used in this document. Examples are given in five annexes.
1.2 This guide assumes a working knowledge of statistics. Several statistical texts are included in the references
This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
ASTM ISO / ASTM51707-05, Standard Guide for Estimating Uncertainties in Dosimetry for Radiation Processing, ASTM International, West Conshohocken, PA, 2005, www.astm.orgBack to Top