M.F. Desrosiers, J.M. Puhl, and S.M. Seltzer

National Institute of Standards and Technology, Gaithersburg, MD, USA

Send any questions by email to marcd@nist.gov

1. Introduction

The NIST Ionizing Radiation Division welcomes your participation in an investigative comparison of national standards for absorbed dose to water from Co-60 gamma radiation at the dose levels used in radiation processing. The comparison will cover the range from 1 kGy to 30 kGy and use NIST alanine dosimeters as the transfer dosimeters [1, 2]. When this pilot comparison is complete, the outcome will be used to inform a full-scale CCRI(I) comparison that will then be held in early 2009 under the CIPM MRA.

The last comparison of the high-dose standards for absorbed dose to water from 60Co gamma radiation among the primary dosimetry laboratories operating standards and services was in the late 90’s. Organized by the Bureau International des Poids et Mesures (BIPM) and conducted by the National Institute of Standards and Technology (NIST, USA) and the National Physical Laboratory (NPL, UK), the previous comparison included also the Istituto Nazionale di Metrologia delle Radiazioni Ionizzanti (ENEA-INMRI, Italy), the Physikalisch-Technische Bundesanstalt (PTB, Germany), the National Institute of Metrology (NIM, China), and the International Atomic Energy Agency (IAEA, Vienna). Because it does not offer a high-dose service, the BIPM took part at a single dose level (1 kGy) to provide a direct link to the international reference for absorbed dose to water in Co-60. The comparison was recently published [3] and found that there was a general level of agreement between the institutes at three dose levels: 5 kGy, 15 kGy, and 30 kGy. The agreement was well within the expanded uncertainty for each institute.

An examination of the level of agreement reveals suggestions of dose-dependent trends in the data. Though these trends were within the uncertainties, it was thought they might be related to dose-dependent trends historically found in NIST-NPL comparisons (unpublished data). For the past few years, NIST has conducted experiments to learn the root cause of these differences that has culminated with new findings that could explain/reconcile the agreement between high-dose dosimetry laboratories. Evidence of an alanine dosimetry dose-rate dependence that is dependent on the absorbed dose has been documented, and a manuscript describing this work has been published in the NIST Journal of Research [4]. It is possible that this previously unknown effect impacted the calibration of high-dose sources maintained by national laboratories and contributed to the discrepancies observed in this comparison. The dose rates for Co-60 sources within a laboratory’s calibration scheme and/or the protocol for calibration of these sources will be required to assess the equivalence of future comparison data. To this end, a prerequisite to participation in this proposed comparison will be an explanation of the calibration scheme/protocol and the dose rates (with detailed uncertainty statements) of all Co-60 sources involved.

2. Measurements

The NIST alanine dosimeters for use in the Co-60 source comparisons are supplied in watertight cylindrical holders nominally 12 mm in diameter and 29 mm in length; each vial contains four alanine pellets. The relative standard uncertainty of absorbed-dose estimates derived using the NIST dosimeters is 1.0 %. A detailed protocol for the comparison will be issued in advance to the participating institutes. Each institute (other than the BIPM if they choose to participate) will be sent ten alanine transfer dosimeter vials from the NIST. Of each ten, two will remain unirradiated (as control dosimeters) and two will be irradiated to each of four nominal dose levels; 1 kGy, 5 kGy, 15 kGy and 30 kGy. Irradiations at all institutes will attempt to be complete within a short time period (e.g., two weeks). The dosimeters will be returned immediately to the NIST with information on the average irradiation temperatures, but no information on absorbed-dose estimates.

The irradiation geometry will not be specified in detail in the protocol; it is preferred that each irradiating institute use their normal arrangement. This policy will permit the absorbed-dose estimates to be representative of those routinely disseminated by each institute, rather than modified for the purpose of the comparison.

3. Reporting the Results

Dosimeters from the participating laboratories will be measured at the NIST according to established procedures [2] and compared to the NIST calibration. These data will be disseminated to the participating laboratories within 30-60 days after the measurements for comment. Follow-up discussions will be conducted to resolve any outstanding issues. A report on the comparison may be published. Participating laboratories will have the option to not be included in the publication if they so choose.

References

1. Radiation Processing Dosimetry Calibration Services and Measurement Assurance Program, J.C. Humphreys, J.M. Puhl, S.M. Seltzer, W.L. McLaughlin, M.F. Desrosiers, D.L. Bensen, M.L. Walker, NIST Special Publication 250-44, March 1998.

2. Ionizing Radiation Division Quality manual for Calibration Services, Procedure 12, http://www.physics.nist.gov/Divisions/Div846/QualMan/Procdures/Procedure12v301.pdf

3. CCRI supplementary comparison of standards for absorbed dose to water in 60Co gamma radiation at radiation processing dose levels,” D.T. Burns, P.J. Allisy-Roberts, M.F. Desrosiers, V. Yu. Nagy, P.H.G. Sharpe, R.F. Laitano, K. Mehta, M.K.H. Schneider, Y. Zhang, Radiat. Phys. Chem. 75 (2006) 1087-1092.

4. An Absorbed-Dose / Dose-Rate Dependence for the Alanine-EPR Dosimetry System and Its Implications in High-Dose Ionizing Radiation Metrology, M.F. Desrosiers, J.M. Puhl, S.L. Cooper, J. Res. Natl. Inst. Stand. Technol. 113 (2008) 79-95.

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