ASTM B567-98(2003)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation:B 567–98(Reapproved 2003)
Standard Test Method for
Measurement of Coating Thickness by the Beta Backscatter
Method
This standard is issued under the fixed designation B 567; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the Department of Defense.
1. Scope suitably small interval of time divided by that interval of time
is called “activity.” Therefore, in beta backscatter measure-
1.1 This test method covers the beta backscatter gages for
ments, a higher activity corresponds to a greater emission of
the nondestructive measurement of metallic and nonmetallic
betaparticles.Theactivityofaradioactiveelementusedinbeta
coatings on both metallic and nonmetallic substrate materials.
backscatter gages is generally expressed in microcuries (1
1.2 The test method measures the mass of coating per unit
µCi = 3.7 3 10 disintegrations per second).
area, which can also be expressed in linear thickness units
3.1.2 aperture—the opening of the mask abutting the test
provided that the density of the coating is known.
specimen. It determines the size of the area on which the
1.3 Thetestmethodisapplicableonlyiftheatomicnumbers
coating thickness is measured. This mask is also referred to as
or equivalent atomic numbers of the coating and substrate
a platen, an aperture plate, a specimen support, or a specimen
differ by an appropriate amount (see 7.2).
mask.
1.4 Beta backscatter instruments employ a number of dif-
3.1.3 backscatter—when beta particles pass through matter,
ferent radioactive isotopes. Although the activities of these
they collide with atoms. Among other things, this interaction
isotopes are normally very low, they can present a hazard if
will change their direction and reduce their speed. If the
handled incorrectly. This standard does not purport to address
deflections are such that the beta particle leaves the body of
the safety issues and the proper handling of radioactive
matter from the same surface at which it entered, the beta
materials. It is the responsibility of the user to comply with
particle is said to be backscattered.
applicable State and Federal regulations concerning the han-
3.1.4 backscatter coeffıcient—the backscatter coefficient of
dling and use of radioactive material. Some States require
a body, R, is the ratio of the number of beta particles
licensing and registration of the radioactive isotopes.
backscattered to that entering the body. R is independent of the
1.5 This standard does not purport to address all of the
activity of the isotope and of the measuring time.
safety concerns, if any, associated with its use. It is the
3.1.5 backscatter count:
responsibility of the user of this standard to establish appro-
3.1.5.1 absolute backscatter count—the absolute backscat-
priate safety and health practices and determine the applica-
ter count, X, is the number of beta particles that are backscat-
bility of regulatory limitations prior to use.
tered during a finite interval of time and displayed by the
2. Referenced Documents
instrument. X will, therefore, depend on the activity of the
source, the measuring time, the geometric configuration of the
2.1 International standard:
measuring system, and the properties of the detector, as well as
ISO 3543: Metallic and Nonmetallic Coatings—
the coating thickness and the atomic numbers of the coating
Measurement of Thickness—Beta Backscatter Method
and substrate materials. X is the count produced by the
3. Terminology
uncoated substrate, and Xs, that of the coating material. To
obtain these values, it is necessary that both these materials are
3.1 Descriptions of Terms:
available with a thickness greater than the saturation thickness
3.1.1 activity—the nuclei of all radioisotopes are unstable
(see 3.1.12).
and tend to change into a stable condition by spontaneously
3.1.5.2 normalized backscatter—the normalized backscat-
emitting energy or particles, or both. This process is known as
ter, x , is a quantity that is independent of the activity of the
radioactive decay. The total number of disintegrations during a
n
source, the measuring time, and the properties of the detector.
The normalized backscatter is defined by the equation:
ThistestmethodisunderthejurisdictionofASTMCommitteeB08onMetallic
X 2 X
and Inorganic Coatings and is the direct responsibility of Subcommittee B08.10 on 0
x 5
n
X 2 X
Test Methods.
s 0
Current edition approved Oct. 1, 2003. Published October 2003. Originally
approved in 1972. Last previous edition approved in 1998 as B 567 – 98.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
B 567–98 (2003)
4.3 The curve expressing coating thickness (mass per unit
where:
area) versus beta backscatter intensity is continuous and can be
X = count from the substrate,
subdivided into three distinct regions, as shown in Fig. 1. The
X = count from the coating material, and
s
X = countfromthecoatedspecimen,andeachcountisfor normalized count rate, x , is plotted on the X-axis, and the
n
the same interval of time. logarithm of the coating thickness, on the Y-axis. In the range
Because X is always$X and# X , x can only take values 0# x # 0.35, the relationship is essentially linear. In the
0 s n n
between 0 and 1. (For reasons of simplicity, it is often range 0.35#x # 0.85, the curve is nearly logarithmic; this
n
advantageous to express the normalized count as a percentage means that, when drawn on semilogarithmic graph paper, as in
by multiplying x by 100.) Fig. 1, the curve approximates a straight line. In the range
n
3.1.5.3 normalized backscatter curve—the curve obtained 0.85# x #1, the relationship is nearly hyperbolic.
n
by plotting the coating thickness as a function of x . 4.4 Radiation other than the beta rays are emitted or
n
3.1.6 beta particles—beta particles or beta rays are high- backscattered by the coating or substrate, and may be included
speed electrons that are emitted from the nuclei of materials in the backscatter measurements. Whenever the term backscat-
undergoing a nuclear transformation. These materials are ter is used in this method, it is to be assumed that reference is
called beta-emitting isotopes, beta-emitting sources, or beta made to the total radiation measured.
emitters.
3.1.7 coating thickness—in this test method, coating thick-
ness refers to mass per unit area as well as geometrical
thickness.
3.1.8 dead time or resolving time—Geiger-Müller tubes
used for counting beta particles have characteristic recovery
times that depend on their construction and the count rate.
Afterreadingapulse,thecounterisunresponsivetosuccessive
pulsesuntilatimeintervalequaltoorgreaterthanitsdeadtime
has elapsed.
3.1.9 energy—it is possible to classify beta emitters by the
maximum energy of the particles that they release during their
disintegration. This energy is generally given in mega-
electronvolts, MeV.
3.1.10 equivalent (or apparent) atomic number— the
equivalent atomic number of an alloy or compound is the
atomic number of an element that has the same backscatter
coefficient as the material.
3.1.11 half-life, radioactive—for a single radioactive decay
process, the time required for the activity to decrease by half.
3.1.12 saturation thickness—the minimum thickness of a
material that produces a backscatter that is not changed when
the thickness is increased. (See also Appendix X1.)
3.1.13 sealed source or isotope—a radioactive source
sealedinacontainerorhavingabondedcover,thecontaineror
cover being strong enough to prevent contact with and disper-
sion of the radioactive material under the conditions of use and
wear for which it was designed.
3.1.14 source geometry—the spatial arrangement of the
source,theaperture,andthedetectorwithrespecttoeachother.
4. Summary of Test Method
4.1 When beta particles impinge upon a material, a certain
portion of them is backscattered.This backscatter is essentially
a function of the atomic number of the material.
4.2 If the body has a surface coating and if the atomic
numbers of the substrate and of the coating material are
sufficiently different, the intensity of the backscatter will be
between two limits: the backscatter intensity of the substrate
and that of the coating. Thus, with proper instrumentation and
if suitably displayed, the intensity of the backscatter can be
used for the measurement of mass per unit area of the coating,
which, if the density remains the same, is directly proportional
to the thickness. FIG. 1 Normalized Backscatter
B 567–98 (2003)
5. Significance and Use deviation,theonlywaytodeterminethemeasuringprecisionis
to make a large number of measurements at the same coated
5.1 The thickness or mass per unit area of a coating is often
location on the same coated specimen, and calculate the
critical to its performance.
standard deviation by conventional means.
5.2 For some coating-substrate combinations, the beta back-
scatter method is a reliable method for measuring the coating
NOTE 1—The accuracy of a thickness measurement by beta backscatter
nondestructively.
is generally poorer than the precision described in 6.1, inasmuch as it also
5.3 The test method is suitable for thickness specification depends on other factors that are described below. Methods to determine
the random errors of thickness measurements before an actual measure-
acceptance if the mass per unit area is specified. It is not
ment are available from some manufacturers.
suitable for specification acceptance if the coating thickness is
specified and the density of the coating material can vary or is
7.2 Coating and Substrate Materials—Because the back-
not known.
scatter intensity depends on the atomic numbers of the sub-
strateandthecoating,therepeatabilityofthemeasurementwill
6. Instrumentation
depend to a large degree on the difference between these
atomicnumbers;thus,withthesamemeasuringparameters,the
6.1 In general, a beta backscatter instrument will comprise:
greater this difference, the more precise the measurement will
(1)aradiationsource(isotope)emittingprimarilybetaparticles
be.As a rule of thumb, for most applications, the difference in
having energies appropriate to the coating thickness to be
atomic numbers should be at least 5. For materials with atomic
measured (seeAppendix X2), (2) a probe or measuring system
numbers below 20, the difference may be reduced to 25 % of
with a range of apertures that limit the beta particles to the area
the higher atomic number; for materials with atomic numbers
of the test specimen on which the coating thickness is to be
above 50, the difference should be at least 10 % of the higher
measured, and containing a detector capable of counting the
atomicnumber.Mostplasticsandrelatedorganicmaterials(for
number of backscattered particles (for example, a Geiger-
example, photoresists) may be assumed to have an equivalent
Müller counter (or tube)), and (3) a readout instrument where
atomicnumbercloseto6.(AppendixX3givesatomicnumbers
the intensity of the backscatter is displayed. The display, in the
of commonly used coating and substrate materials.)
form of a meter reading or a digital readout can be: (a)
7.3 Aperture:
proportional to the count, ( b) the normalized count, or (c) the
coating thickness expressed either in thickness or mass per unit 7.3.1 Despite the collimated nature of the sources used in
area units. commercial backscatter instruments, the backscatter recorded
by the detector is, nearly always, the sum of the backscatter
7. Factors Affecting the Measuring Accuracy produced by the test specimen exposed through the aperture
andthatoftheapertureplate(n).Itis,therefore,desirabletouse
7.1 Counting Statistics:
amaterialwithalowatomicnumberfortheconstructionofthe
7.1.1 Radioactive disintegration takes place randomly.
platen and to select the largest aperture possible. Measuring
Thus, during a fixed time interval, the number of beta particles
errors will be increased if the edges of the aperture opening are
backscattered will not always be the same. This gives rise to
worn or damaged, or if the test specimen does not properly
statisticalerrorsinherenttoradiationcounting.Inconsequence,
contact these edges.
an estimate of the counting rate based on a short counting
7.3.2 Because the measuring area on the test specimen has
interval (for example, 5 s) may be appreciably different from
to be constant to prevent the introduction of another variable,
an estimate based on a longer counting interval, particularly if
namely the geometrical dimensions of the test specimen, it is
the counting rate is low. To reduce the statistical error to an
essentialthattheaperturebesmallerthanthecoatedareaofthe
acceptable level, it is necessary to use a counting interval long
surface on which the measurement is made.
enough to accumulate a sufficient number of counts.
7.4 Coating Thickness:
7.1.2 At large total counts, the standard deviation (s) will
7.4.1 In the logarithmic range, the relative measuring error
closely approximate the square root of the total count, that is
is nearly constant and has its smallest value.
s5 X; in 95 % of all cases, the true count will be within
=
7.4.2 In the linear range, the absolute measuring error,
X 6 2s. To judge the significance of the precision, it is often
expressed in mass per unit area or thickness, is nearly constant,
helpful to express the standard deviation as a percentage of the
which means that as the coating thickness decreases, the
count, that is, 100 X/X, or 100/ X. Thus, a count of
= =
relative measuring error increases. At or near x = 0.35, the
100 000 will give a value ten times more precise than that
n
relativeerrorsofthelinearandlogarithmicrangesareaboutthe
obtained with a count of 1000. Whenever possible, a counting
same. Thus, the relative error at this point may, for most
interval should be chosen that will provide a total count of at
practical purposes, be used to calculate the absolute error over
least 10 000, which corresponds to a statistical error of 1 % for
the linear range.
the count rate. It should be noted, however, that a 1 % error in
the count rate can correspond to a much larger percentage error 7.4.3 In the hyperbolic range, the measuring error is always
large because a small variation in the intensity of the beta
in the thickness measurement, the relative error depending on
the atomic number spread or ratio between coating and backscatter will produce a large variation
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