Key Concepts in Evaluating Overall System Uncertainty

Carroll S. BrickenkampNVLAP Program Manager

Panasonic Users Meeting

June 2001

Uncertainty & Traceability

• “Uncertainty” underlies definition of “traceability”• unbroken chain of measurements, with

stated uncertainty of measurement results at each link in chain

• To conform to ISO 17025, accredited testing labs must estimate uncertainty

Uncertainty for Testing Labs

• “Testing labs shall have and shall apply procedures for estimating uncertainty of measurement.”

• “In certain cases, the nature of the test method may preclude rigorous, metrologically and statistically valid, calculation of uncertainty of measurement.”• “…identify all components of uncertainty

and make reasonable estimation...”

Does Test Method Specify Limit?

• “…where a well recognized test method specifies limits to the values of the major sources of uncertainty…and specifies the form of presentation of calculated results, the lab is considered to have satisfied this clause by following the test method and reporting instructions.”

ANSI/HPS N13.11-1993

• Tolerance Level = 0.3 = uncertainty in interpretation of dose equivalent (or absorbed dose) in vicinity of maximum permissible levels under fixed irradiation geometries and fixed lab ambient conditions

• Tolerance level on bias plus standard deviation = 0.5 for all but accident categories

• Tolerance level on bias plus standard deviation = 0.3 for accident categories

ANSI/HPS 13.11-1993Test Category Test Irradiation

RangeTolerance Level

DeepTolerance Level

ShallowAdditional Limits

on B and SI Accidents, low-e

photons0.1 to 5 Gy 0.3 No test None

II Accidents, hi-ephotons

0.1 to 5 Gy 0.3 No test None

III Low-e photonsA general

B hi-e techniques

0.3 to 100 mSv 0.5 0.5 0.35

IV hi-e photons 0.3 to 100 mSv 0.5 No test 0.35

V Beta particlesA Hi-e only

B Low-e onlyC General

1.5 to 100 mSv No test 0.50.350.35none

VI Photon mix 2 to 50 mSv 0.5 0.5 0.35VII Beta-photon mix 2 to 50 mSv 0.5 0.5 0.35VIII Neutron-photon

mix1.5 to 50 mSv Total 0.50

Neutron 0.50No testNo test

0.35none

IX angles ofincidence, photons

1 to 50 mSv 0.5 0.5 None

ANSI/HPS 13.11-2001Test Category Test Irradiation

RangeTolerance Level

DeepTolerance Level

ShallowPerformance

Quotient LimitApplied?

I Accidents photonsA General

B 137CsC M150

0.1 to 5 Gy 0.3 No test No

II PhotonsA General

B Hi-eC Med-e

D Narrow Spectrum

0.3 to 100 mSv 0.4 0.4 Yes

III BetasA General

B Hi-eC Low-e

1.5 5o 100 mSv No test 0.4 Yes

IV Photon MixA GeneralB IIB + IICC IIB + IID

0.6 6o 100 mSv 0.4 0.4 Yes

V Beta/Photon MixA GeneralB IIB + III

2 to 100 mSv 0.4 0.4 Yes

VI Neutron/Photon MixA GeneralB 252Cf + II

C 252Cf(D2O) + II

1.5 to 50 mSv 0.4 No test No

ANSI/HPS N13.11-2001

• |B| + S L• L = 0.3 for accident cat I

• L = 0.4 for cat II - VI

• Performance Quotient Limit (PQL) for cat II, III, IV, V:• |Pi|> L for only 1 of 15 dosimeters tested

ANSI/HPS N13.11-1993• Sources of error specifically not included

in performance tests• Geometry of radiation incidence• Ambient temperature before, during, after

irradiations up to time of processing• Ambient humidity• Time intervals between dosimeter issue,

irradiation, and processing

ANSI/HPS N13.11-1993• Additional sources of error specifically

not included in performance tests• Exposure to visible, UV• Position of dosimeter on body• Bias introduced by processor’s awareness

of being tested

• Are any of these MAJOR SOURCES of uncertainty?

Guidance on Uncertainty

• ISO “Guide to the Expression of Uncertainty in Measurement”• See NIST Website for NIST Tech Note

1297http://physics.nist.gov/cuu/Uncertainty/index.html

Steps in obtaining an “Expanded Uncertainty”

• Type A evaluation of uncertainty• statistical analysis of series of

observations

• Type B evaluation• evaluation by means other than statistical

analysis

Steps in obtaining an “Expanded Uncertainty”

• Combined uncertainty (uc) either:• Combine Type A and B by root mean square (if

independent sources), or• Combine Type A and B by sum (if sources of error

are related)

• Coverage Factor (k = 2 for 95 % confidence level)

• Expanded Uncertainty (U = kuc)

Sources of Uncertainty (from ISO 17025)

• Sampling (part of standard and part of process)• Incomplete definition• Imperfect realization of the definition• Inadequate knowledge or measurement of

environmental effects• Instrument resolution • Values assigned to measurement standards• Approximations in measurement method• Variations in repeated observations under

apparently identical conditions

What other standards might contain useful guidance?

• ASTM E 1956-98 : “Standard Practice (SP) for Use of Thermoluminescence-Dosimetry (TLD) Systems for Radiation Processing”

• ASTM E 1707-95 “Standard Guide (SG) for Estimating Uncertainties in Dosimetry for Radiation Processing”

See Also:

• ASTM E 668-00 “SP for Application of Thermoluminescence-Dosimetry (TLD) Systems for Determining Absorbed Dose in Radiation-Hardness Testing of Electronic Devices”

• ASTM E 1261-00 “SG for Selection and Calibration of Dosimetry Systems for Radiation Processing”

Sources of Uncertainty (ASTM E 1707)

• Uncertainty in absorbed dose received by the dosimeters during system calibration• Response of primary or reference standard

• 137Cs source calibrated dose value (type A & B)

• Irradiation time• Fading correction (type A & B) readout time after

irradiation must be controlled

• Decay corrections• Non-uniformities in standard radiation field• Corrections for attenuation and geometry

• directional dependence

Sources of Uncertainty

• Analysis of dosimeter response• Intrinsic variation in dosimeter response

• time between prep and readout • temperature, humidity, ambient light dependence • energy dependence • absorbed dose rate dependence

• Variation in thickness of individual dosimeters• measurement of thickness of individual dosimeters

• Variations in readout equipment

Also mentioned in N13.11-93

Also mentioned in N13.11-93

Sources of Uncertainty

• Fit of data to calibration curve• Variation in response of dosimeters

• reproducibility of individual dosimeter response (type A)

• Analytical function used in fit• determination of calibration curve (type

A & B)

Sources of Uncertainty in Measurement of Absorbed Dose

• Variation in response of a group of dosimeters irradiated to same dose level

• Dosimeter system analytical instrumentation • variation in absorbance reading

• Irradiation source decay correction for given data

• Calibration of radiation source used to irradiate dosimeters during calibration

Sources of Uncertainty in Measurement of Absorbed Dose

• Dosimeter response dependence on geometrical effects

• Dosimeter response dependence on differences in energy spectrum between calibration and wearer’s exposure

• Calibration of dosimetry system analytical equipment

Also mentioned in N13.11-93

Sources of Uncertainty

• Routine use of dosimeters in facility• Deviations in environment from

calibration conditions

• Reproducibility in placement of dosimeter on wearer

• Orientation of dosimeters to source of radiation

Also mentioned in N13.11-93

Also mentioned in N13.11-93

Where Do We Go from Here?

• Problem is worthy of a group approach• What information, data, estimates do

you have for the different components?

• Which are significant and which are not?

• How would you like to see information disseminated?