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American National Standard N13.11-2009

Personnel Dosimetry Performance - Criteria for Testing


This American National Standard provides a procedure for testing the performance of dosimetry systems (i.e., the hardware, the software, and the processor supplying the services, or in some cases the user of the services) for the purpose of monitoring personnel exposure to ionizing radiation. In 1973, the Conference of Radiation Control Program Directors appointed a task force with state and federal participation. The purpose of this task force was to implement the Conference recommendation for the establishment of a program for testing of personnel dosimetry performance throughout the United States on a continuing basis. After 10 years of development and pilot testing, the first standard was issued in 1983 as ANSI N13.11-1983. That standard formed the basis of the Nuclear Regulatory Commission's (NRC) test program administered by the National Voluntary Laboratory Accreditation Program (NVLAP). The Department of Energy (DOE) modified ANSI N13.11-1983 for use at DOE facilities and, in 1986, issued DOE/EH-0027 as the standard which formed the basis of DOE's test program known as the DOE Laboratory Accreditation Program (DOELAP).

By ANSI policy, standards must undergo periodic review and modification if necessary.

After extensive review, the first modification of ANSI N13.11-1983 was issued in 1993 as HPS N13.11-1993. The Working Group for that effort felt that it was not possible to write a useful standard without considering how the standard is applied in the real world and, consequently, had hoped to produce a product which would be acceptable to both DOE and NRC, but was unable to do so. The current version, HPS N13.11-2001, has been developed by a Working Group convened in 1996 and working through 2000. The overarching objective of this Group was to produce one standard to suit the needs of both the DOE and the NRC (and the states) for their DOELAP and NVLAP programs. This may prove to be a prudent objective not only because it makes sense for the United States to have a single dosimeter performance test program, but also because the DOE and NRC have recently been involved in a pilot project to simulate NRC regulation of DOE facilities. Other Group objectives were to simplify the standard, to reduce the number of test dosimeters required, and to take no action which would prevent or preclude the use of electronic dosimeters or other future technology. This Group was made up of people with widely varied backgrounds, including DOELAP assessors and NVLAP assessors, dosimeter vendors, government, military, national standards laboratories, and nuclear power. The Group held multiple meetings over a period of about three years to create this second revision of the original standard. The Working Group identified about a dozen major issues which were considered in the writing of this version of the standard. The following describes how the Group implemented the resolution of many of the issues. Some of these items are considered in detail in the appendices, which we hope the readers find convenient and useful.

It is recognized that, historically, N13.11 has been somewhat of a hybrid of a type test and a periodic test. This version can also be generally perceived in that way. Most criticisms of such a hybrid test involve high cost to the test participant. It is suggested that if there were two separate tests (i.e., type and periodic), there would likely be a high "front-end" cost and a lowering of periodic costs. The Working Group was sensitive to the cost issue and has helped the situation by reducing the number of test categories from nine to six. The Group endeavored to cut the number of dosimeters required for testing while making the test comprehensive enough for needs of both DOELAP and NVLAP. For example, economy while retaining a good test of dosimeter capability can be achieved by selecting general options within the nonmixture categories. It is a bit of an oversimplification to state it this way, but the nine categories in HPS N13.11-1993 have been reduced to six in this standard by combining the two accident photon categories, combining the two "regular" photon categories, and embedding the angular category within the photon category. The same angles used in the previous version of the standard are retained in this version. Over 70 x-ray beam codes have been approved for use in performance testing subject to energy and angular restrictions within various categories and availability at the performance test laboratory. The beta category has been modified to allow future use of 85Kr as an alternative to 204Tl. The advantages are that, while functionally about the same, 85Kr's half-life is longer and its available activities are greater than 204Tl's. The narrow spectrum series (i.e., NS20, NS80, 241Am, 137Cs, and 60Co), particularly appropriate sources for testing needed capabilities at certain DOE facilities, has been added as an option in the photon category. An unmoderated 252Cf source has been added as an option in the neutron/photon mixture category.

The pass/fail criteria is that the absolute value of the bias plus the standard deviation must be less than or equal to the tolerance level (i.e., |B| + S ≤ L). The value of L is 0.3 for the accident photon category and is 0.4 for all other categories. There is no separate limit on |B| and on S as there was in HPS N13.11-1993. However, there is for categories II through V, a performance quotient limit (commonly called the 10% rule) which is consistent with current practices of the International Organization for Standardization (ISO). This means that, regardless of the value of |B|+ S, a test participant cannot pass a category if more than 10% of the dosimeters tested in that category have a performance quotient, Pi, whose absolute value is > L (i.e., can't pass if more than 10% of the |Pi| > L). With 15 dosimeters per category, if 2 or more have |Pi| > L, the test participant fails that category. Under the previous versions of the standard, simulations showed that perhaps as many as 25% (i.e., 3 or 4 of 15) of the test dosimeters could fail and the category still be passed. That unacceptable situation, although rare, will be prevented by the performance quotient limit.

The dose range for the accident categories and "regular" categories have traditionally had a breakpoint at 100 mSv (10 rem). For consistency, the upper limit of the personal dose equivalent range for the photon mixture category and for the beta/photon mixture category has been increased to 100 mSv (10 rem). The upper limit of the neutron/photon mixture category remains at 50 mSv (5 rem) for practical irradiation time considerations. The lower limit of the personal dose equivalent range for the photon mixture category has been set equal to 0.6 mSv (60 mrem) to be consistent with the restriction that no single photon personal dose equivalent is allowed to be < 0.3 mSv (30 mrem). The lower limits of 2 mSv (200 mrem) and 1.5 mSv (150 mrem) remain for beta/photon mixtures and neutron/photon mixtures, respectively.

The conversion coefficients for photons used in this standard are those issued in the NVLAP Bulletin, Volume II, No. 1, DOSIMETRY (January 1995) and have been in use by NVLAP since January 1995. Because the international opinion was that backscatter corrections were inappropriate for dosimeter testing and because new data which allowed consistent treatment of all photon spectra became available, these coefficients replaced those published in HPS N13.11-1993.

The Working Group had much discussion about changing the neutron fluence to personal dose equivalent conversion factors to those currently advocated by the ISO. Because the ISO factors were calculated for a slab geometry making use of the latest alpha and proton stopping power information, such a change would result in technical consistency with the factors for the other types of radiation used in this standard. However, it was ultimately decided not to change the neutron factors because the existing factors are consistent with the neutron-related data used by the major radiation rules in the United States (i.e., 10 CFR 20 and 10 CFR 835) and because the magnitude of the change (about 20%) would likely prove to be a stumbling block for wide acceptance of this version of the standard.

For practical purposes, the polymethyl methacrylate (PMMA) phantom will continue to be used in this standard. In addition, the reference dose point (RDP) will continue to be at the phantom surface (i.e., the current NVLAP practice). However, the test lab will report the absorbed dose or personal dose equivalent at the sensitive elements of the dosimeter (i.e., the current DOELAP practice), if the test participant desires it and supplies the phantom surface-to-sensitive element distance. The Working Group believes that it is important to document this type of operational detail so that it is readily available to the test participant and has done so in Appendix E, Irradiating Laboratory Guidance.

The lower limit of detection (LLD) requirements have been removed from this version of the standard since it is more appropriately a one-time test. Consequently, LLD is mentioned in Appendix G on Type Testing. Since most dosimetry is done at or near background, test participants should determine LLD for their system.

In an attempt to be most helpful in addressing current issues, the Group has significantly modified the appendices in this version of the standard. The little-used tabular and graphical details associated with various radiation spectra have been omitted. In addition to Type Testing mentioned above, appendices on Electronic Dosimeter Testing and on Guidance for the Irradiating Laboratory have been added.

 

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