ASTM_B_567_-_98_2014.pdf

Designation:B56798(Reapproved 2014)Standard Test Method forMeasurement of Coating Thickness by the Beta BackscatterMethod1This standard is issued under the fixed designation B567;the number immediately following the designation indicates the year oforiginal adoption or,in the case of revision,the year of last revision.A number in parentheses indicates the year of last reapproval.Asuperscript epsilon()indicates an editorial change since the last revision or reapproval.This standard has been approved for use by agencies of the U.S.Department of Defense.1.Scope1.1 This test method covers the beta backscatter gages forthe nondestructive measurement of metallic and nonmetalliccoatings on both metallic and nonmetallic substrate materials.1.2 The test method measures the mass of coating per unitarea,which can also be expressed in linear thickness unitsprovided that the density of the coating is known.1.3 The test method is applicable only if the atomic numbersor equivalent atomic numbers of the coating and substratediffer by an appropriate amount(see 6.2).1.4 Beta backscatter instruments employ a number of dif-ferent radioactive isotopes.Although the activities of theseisotopes are normally very low,they can present a hazard ifhandled incorrectly.This standard does not purport to addressthe safety issues and the proper handling of radioactivematerials.It is the responsibility of the user to comply withapplicable State and Federal regulations concerning the han-dling and use of radioactive material.Some States requirelicensing and registration of the radioactive isotopes.1.5 The values stated in SI units are to be regarded asstandard.No other units of measurement are included in thisstandard.1.6 This standard does not purport to address all of thesafety concerns,if any,associated with its use.It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2.Terminology2.1 Definitions of Terms Specific to This Standard:2.1.1 activitythe nuclei of all radioisotopes are unstableand tend to change into a stable condition by spontaneouslyemitting energy or particles,or both.This process is known asradioactive decay.The total number of disintegrations during asuitably small interval of time divided by that interval of timeiscalled“activity.”Therefore,inbetabackscattermeasurements,a higher activity corresponds to a greateremission of beta particles.The activity of a radioactive elementused in beta backscatter gages is generally expressed inmicrocuries(1 Ci=3.7 104disintegrations per second).2.1.2 aperturethe opening of the mask abutting the testspecimen.It determines the size of the area on which thecoating thickness is measured.This mask is also referred to asa platen,an aperture plate,a specimen support,or a specimenmask.2.1.3 backscatterwhen beta particles pass through matter,they collide with atoms.Among other things,this interactionwill change their direction and reduce their speed.If thedeflections are such that the beta particle leaves the body ofmatter from the same surface at which it entered,the betaparticle is said to be backscattered.2.1.4 backscatter coeffcientthe backscatter coefficient ofa body,R,is the ratio of the number of beta particlesbackscattered to that entering the body.R is independent of theactivity of the isotope and of the measuring time.2.1.5 backscatter count:2.1.5.1 absolute backscatter countthe absolute backscat-ter count,X,is the number of beta particles that are backscat-tered during a finite interval of time and displayed by theinstrument.X will,therefore,depend on the activity of thesource,the measuring time,the geometric configuration of themeasuring system,and the properties of the detector,as well asthe coating thickness and the atomic numbers of the coatingand substrate materials.X0is the count produced by theuncoated substrate,and Xs,that of the coating material.Toobtain these values,it is necessary that both these materials areavailable with a thickness greater than the saturation thickness(see 2.1.12).2.1.5.2 normalizedbackscatterthenormalizedbackscatter,xn,is a quantity that is independent of the activityof the source,the measuring time,and the properties of the1This test method is under the jurisdiction ofASTM Committee B08 on Metallicand Inorganic Coatings and is the direct responsibility of Subcommittee B08.10 onTest Methods.Current edition approved May 1,2014.Published May 2014.Originallyapproved in 1972.Last previous edition approved in 2009 as B567 98(2009).DOI:10.1520/B0567-98R14.Copyright ASTM International,100 Barr Harbor Drive,PO Box C700,West Conshohocken,PA 19428-2959.United States1 detector.The normalized backscatter is defined by theequation:xn5X 2 X0Xs2 X0where:X0=count from the substrate,Xs=count from the coating material,andX=count from the coated specimen,and each count is forthe same interval of time.Because X is always X0and Xs,xncan only take va