Radiat. Phys. Chem. Vol. 42, Nos 4—6, pp. 58 1—584, 1993 0146-5724/93$6.00 + 0.00Printed in Great Britain. All rights reserved Copyright 0 1993 Pergamon Press Ltd

ACCREDITEDDOSEMEASUREMENTSFOR VALIDATIONOF RADIATION STERILIZED PRODUCTS

ArneMillerHigh DoseReferenceLaboratory

RisøNationalLaboratory,DK 4000 Roskilde,Denmark

ABSTRACT

The activitiesandservicesof theaccreditedRisø High DoseReferenceLaboratoryaredescribed. The laboratoryoperatesaccordingto theEuropeanstandardEN45001 regardingOperationof TestingLaboratories,and it fulfills the requirementsof beingableto delivertraceabledosemeasurementsfor controlof radiation sterilization.

The accreditedservicesinclude:1. Irradiationof dosimetersand test sampleswith cobalt-60gammarays.2. Irradiationof dosimetersand test sampleswith 10 MeV electrons.3. Issueof and measurementwith calibrateddosimeters.4. Measurementof thedosimetricparametersof anirradiation facility.5. Measurementofabsorbeddosedistribution in irradiatedproducts.

The paperdescribestheseservicesand theproceduresnecessaryfor their execution.

Introduction

The term accreditationis new to many in thedosimetrybusiness. According to thedictionaryaccreditedmeans: to put into a reputableor outstandingcategory; consider, recognize,oracclaimas ngh~fi~llypocessingan uncontestedstatus.

Measurementsof absorbeddosewhich areusedto documenta radiation sterilization processmustpossessseveralproperties:

- themeasurementmustbe traceableto a nationalstandard- theuncertaintyof themeasurementmustbe known- the influenceof environmentalconditions on themeasurementresultmust be

known.

Absorbeddosemeasurementsfrom an accreditedlaboratoryfulfills theserequirements.To beaccreditedmeansthat someone(an accreditingorganization)haslookedinto thedocumentationandproceduresof the laboratoryandhas foundthat thelaboratoryoperatesaccordingtointernationalstandards.

The RisøHigh DoseReferenceLaboratoryoperatesaccordingto EN 45001, GeneralCriteriafor theOperationof TestingLaboratories,and theaccreditingorganizationhasassessedtheactivitiesof the laboratoryaccordingto anotherstandard:EN 45002,GeneralCriteria for theAssessmentof TestingLaboratories. The certificatesissuedby anaccreditedlaboratorywillhavethesamevalidity as certificatesfrom a primarynationallaboratory,but in order for thisfact to be recognizedinternationally,it is importantthat theaccreditingorganizationitself isrecognizedinternationally. In Denmarktheaccreditingorganizationis theDanishAccreditationScheme(DANAK), which is operatedby theDanishInstituteof FundamentalMetrology.DANAK iS a memberof WECC, WesternEuropeanCalibration cooperationandof WELAC,WesternEuropeanLaboratoryAccreditationCooperation.

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Signatoriesof the WECC agreementare (1992):Denmark DanishAccreditationScheme,DANAKFinland Centrefor Metrology andAccreditationFrance BureauNationalde Mdtroligie, BNMGermany DeutscherKalibrierdienst,DKDItaly Serviziodi Taraturain Italia, SITNetherlands NederlandseKalibratieOrganisatie,NKOSweden SwedishBoard for TechnicalAccreditation,SWEDACSwitzerland SwissCalibrationService,SCSUnited Kingdom NationalMeasurementAccreditationService,NAMAS

Signatoriesof the WELAC agreementare(1992):Denmark DanishAccreditationScheme,DANAKFrance RéseauNational d’Essais,RNENetherlands NederlandseStichtingvoor de ErkenningvanLaboratoriaen

Inspectieinstellingen,STERLABSweden SwedishBoard for TechnicalAccreditation,SWEDACUnited Kingdom National MeasurementAccreditationServices,NAMAS

Servicesof the Risø High DoseReferenceLaboratory.HDRL

The dosimetricservicesto which the HDRL hasbeenaccreditedare the following:

1. Irradiation ofdosimetersand testsampleswith cobalt-60gammarays.

Doserange : largerthan 1 kGy.Uncertainty’ : Type A: 2.4% (Combined:3.6%)

Type B: 2.8%

Irradiation facility : Cobalt-60 gammacell. Sourcestrength: approx.3 kCi.Irradiationtemperature: 10-80°C,thermostated.Traceability: The doserateof the gammacell is calibratedby irradiationof

dichromatereferencedosimetersissuedandmeasuredby theNationalPhysicalLaboratory(NPL), UK.

2. Irradiation ofdosimetersand sampleswith 10 MeVelectrons.

Doserange: 5-50kGy (higherdosesmaybe given simplyby increasingthe numberofirradiations).

Uncertainty: Type A: 2.6% (Combined: 4.4%)Type B: 3.6%

Irradiationfacility: 10 MeV linear accelerator.Irradiationtemperature: dependson dose,cannotbecontrolled.Traceability: Foreach irradiation, thedose is measuredby calorimeters,the responseof

which havebeencalibratedby irradiationat the 10 MeV acceleratoratNPL.

‘Uncertaintiesare specifiedat 2 standarddeviations(approximately95% confidencelevel) accordingto the nomenclatureby the BIPM (Giacomo,1981), who in lack of betternamesspecified- Tyoe A uncertaintiesthat can be evaluatedby statisticalmethodsapplied to a seriesofrepeatedmeasurements.- Type B uncertaintiesthat areevaluatedby othermeans,e.g. by estimation.

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3. Issueof and measurementwith calibrateddosimeters.

3.1 Dosimetersareissuedfor irradiationat acustomer’sirradiationfacility. The dosimeterswill thenbe returnedto the HDRL for measurement.Thefollowing dosimetersareusedin this way:

a) Dichromateliquid ampoules(Sharpeet al., 1985; ASTM, 1991).Doserange: 10-40kGyUncertainty: Type A: 2.6% (Combined3.4%)

Type B: 2.2%

Traceability: The dosimetersarecalibratedby irradiation in theHDRL gammacell.

b) FWT-60 film dosimeters(ASTM, 1988), sealedin vapour tight pouches.

Doserange: 15-100kGy (510 nm)1.5-30kGy (605 nm)

Uncertainty: Type A: 3.6% (Combined: 4.8%)Type B: 3.2%

Traceability: The dosimetersarecalibratedby irradiation in the HDRL gammacell.

3.2 Dosimetersarealso issuedfor irradiationand measurementat the customer’sfacility.Thesedosimetersarewaterand graphitecalorimeters(Miller andKovacs, 1985, 1990).

Doserange: 3-15 kGy (graphite)10-50kGy (water).

Uncertainty:Graphite Water

Type A: 1.8% 3.2%Type B: 3.0% 3.6%Combined: 3.4% 4.8%

Traceability: The graphitecalorimetersareirradiatedat the 10 MeV electronaccelera-tor atNPL andtheir responseis comparedwith that of the NPL standardcalorimeter. The watercalorimeterresponseis calibratedby comparisonwith the graphitecalorimeterresponseat simultaneousirradiations.

4. Measurementofthe dosi,netricparametersofan irradiation facility.

The dosimetricparametersof an irradiationfacility andtheir relationto otherfacility parame-ters must be measuredduring facility qualification,alsocalledcommissioning(CEN, 1991).TheHDRL is accreditedto do thesemeasurementsby using thedosimetersmentionedearlier.

The measurementsthat will becarriedout dependto someextenton the natureof the facility,but will typically consistof the following:

For gammafacilities: Additionally for electronaccelerators:a) Dosedistributionin referenceproduct. d) Electron beamenergy.b) Nominal doseatdifferent dosesettings. e) Electron beamwidth.c) Calibrationof routinedosimetersystem. f) Electronbeamspot size.

5. Measurementofabsorbeddosedistribution in irradiatedproduct.

As apart of productqualification(CEN, 1991), it is neededto measurethe dosedistribution intypical productpackagesas theywill be irradiatedin the facility, in order to ensurethatallpartsof the productwill receiveat leastthe stipulatedminimumdoseand will not receiveadosehigherthanapredeterminedlevel. Also an evaluationof the uncertaintylimits of themeasureddosevalues mustbe carriedout.

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The HDRL is accreditedto carry out thesemeasurementswith the useof thedosimeters

mentionedin section3.

InternationalAccentance

The RisøHigh DoseReferenceLaboratoryfulfills anecessaryrequirementof providingcertified, traceablemeasurementsto the radiationprocessingindustry. Internationalacceptanceof HDRL certificatesand reportsis at presentestablishedin mostWesternEuropeancountries,and work is in progressthroughthe DanishAccreditationSchemeandits membershipof theWesternEuropeanCalibrationCooperationto widen the acceptance.

References

ASTM (1988), E 1275-88StandardPracticefor Use of aRadiochromicFilm DosimetrySystem. American Society for Testing andMaterials, 1916 RaceStreet,Philadelphia,PA.USA.

ASTM (1991), E 1401-91 StandardPracticefor Use of a DichromateDosimeterSystem.American Societyfor TestingandMaterials, 1916 RaceStreet,Philadelphia,PA, USA.

CEN (1989), GeneralCriteria for the Operationof TestingLaboratories,EN 45001. EuropeanCommitteefor Standardization,Brussels.

CEN (1991). Sterilizationof medicaldevices. Methodsfor validation androutine monitoringof sterilizationby irradiation. prEN 552: Requirements,prEN 553: Guidance. EuropeanCommitteefor Standardization,Bruxelles.

Giacomo,P. (1981),News from theBureau Internationaldes Poidset Mesures. Metrologia 17,69-73.

Miller, A. andKovacs, A (1985), Calorimetryatindustrial electronaccelerators.NuclearInst.andMethodsin PhysicsRes. BlO/il, 994-997.

Miller, A., Batsberg,W. and Karman, W (1988), A new radiochromicthin-film dosimetersystem. Rad. Phys. Chem. 31, 491-496.

Miller, A, and Kovacs,A. (1990), Applicationof calorimetersfor routine andreferencedosimetryat 4-10MeV industrial accelerators.Radiat.Phys. Chem. 35, 774-778.

Sharpe,P.H.G., Barrett, J.H., Berkeley, A.M. (1985),Dichromatesolutionsas areferencedosimeterfor usein industrial irradiation plants. mt. J. Appi. Radiat. Isot. 36, 647-652.