Rationale

Section 1.4 Negative: “Beta back scatter instruments employ a number of different radioactive isotopes. Although the activities of these isotopes are normally very low, they can present a hazard if handled incorrectly. This standard does not purport to address the safety issues and the proper handling of radioactive materials. It is the responsibility of the user to comply with applicable State and Federal regulations concerning the handling and use of radioactive material. Some States require licensing and registration of the radioactive isotopes.” The last sentence is a bit inappropriate and would be better expressed as: “Such regulations may require licensing and registration of radioactive sources.” Section 2.1.1 Negative: “activity—the nuclei of all radioisotopes are unstable and tend to change into a stable condition by spontaneously emitting energy or particles, or both. This process is known as radioactive decay. The total number of disintegrations during a suitably small interval of time divided by that interval of time is called “activity.” Therefore, in beta backscatter measurements, a higher activity corresponds to a greater emission of beta particles. The activity of a radioactive element used in beta backscatter gages is generally expressed in microcuries (1 µCi = 3.7×10 4 disintegrations per second).” The mention of µCi conflicts with section 1.5 that only SI units are contained within this standard. I would propose instead: “… The activity of a radioactive element used in beta backscatter gages is generally expressed in kilobecquerels (37 kBq (1 µCi) = 3.7×10 4 disintegrations per second).” There also being the need to modify section 1.5 as follows: “Units—The values stated in SI units are to be regarded as standard. The values given in parentheses after SI units are provided for information only and are not considered standard.” Section 2.1.5.1 Negative: “absolute back scatter count—the absolute backscatter count, X, is the number of beta particles that are backscattered during a finite interval of time and displayed by the instrument. X will, therefore, depend on the activity of the source, the measuring time, the geometric configuration of the measuring system, and the properties of the detector, as well as the coating thickness and the atomic numbers of the coating and substrate materials. X0 is the count produced by the uncoated substrate, and Xs, that of the coating material. To obtain these values, it is necessary that both these materials are available with a thickness greater than the saturation thickness (see 2.1.12).” This definition fails to consider that not all backscattered beta particles are detected. Therefore, propose the following modification: “… the absolute backscatter count, X, is the number of beta particles that are backscattered and detected during a finite interval of time and displayed by the instrument. …” Section 6.1.1 Comment: “Radioactive disintegration takes place randomly. Thus, during a fixed time interval, the number of beta particles backscattered will not always be the same. This gives rise to statistical errors uncertainty inherent to radiation counting. In consequence, an estimate of the counting rate based on a short counting interval (for example, 5 s) may be appreciably different from an estimate based on a longer counting interval, particularly if the counting rate is low. To reduce the statistical error uncertainty to an acceptable level, it is necessary to use accounting interval long enough to accumulate a sufficient number of counts.” I would suggest “uncertainty” is a more current appropriate term to use in this section and number of others. Note 1 Negative: “NOTE 1—The accuracy of a thickness measurement by beta backscatter is generally poorer than the precision described in 5.1, in as much as it also depends on other factors that are described below. Methods to determine the random errors of thickness measurements before an actual measurement are available from some manufacturers.” The reference to section 5.1 above seems inappropriate since section 5.1 doesn’t really address precision. Same also for section 6.15 Section 6.3.1 Comment: “Despite the collimated nature of the sources used in commercial back scatter instruments, the back scatter recorded by the detector is, nearly always, the sum of the backscatter produced by the test specimen exposed through the aperture and that of the aperture plate(n). It is, therefore, desirable to use a material with a low atomic number for the construction of the platen and to select the largest aperture possible. Measuring errors Measurement bias will be increased if the edges of the aperture opening are worn or damaged, or if the test specimen does not properly contact these edges.” I would suggest “bias” is a more current appropriate term to use in this section and number of others. Section 8 (Referee Test) Comment: At this point in time I’m wondering if the focus on NIST SRMs is still appropriate versus standards from ISO/IEC 17025 accredited vendors that are traceable to the SI. Table X2 Negative: At this point in time, I think we can drop the legacy “Radium D + E” terminology. Table X2 Comment: Isotope or Source Approximate HalfLife, years Carbon Symbol Emax, MeV C-14 0.16 5750.7 5700 Promethium Pm-147 0.22 2.8 2.6 Thallium Tl-204 0.77 3 3.8 Lead-210 – bismuth-210 Pb-210 – Bi-210 1.17 1.16 19.4 22.2 Strontium – yttrium Sr-90 – Y-90 2.27 2.28 28.8 Ruthenium - rhodium Ru-106 – Rh-106 3.54 1 Adjustments to some of the data above based on following reference, but there is also the choice of using the stated value with uncertainty provide in the source below: Laboratoire National Henri Becquerel, Nuclear database. Decay Data Evaluation Project (DDEP). http://www.lnhb.fr/home/nuclear-data/nuclear-datatable/