APMP supplementary comparison of absorbed dose rate in tissue for beta radiation

Transcription

1 APMP supplementary comparison of absorbed dose rate in tissue for beta radiation BIPM KCDB: APMP.R(I)-S2 Technical Protocol N. Saito and M. Kato National Metrology Institute of Japan (NMIJ), Tsukuba, Ibaraki JAPAN 19 October

2 1. Introduction The objective of this supplementary comparison is to establish the degrees of equivalence for the absorbed dose rate at 0.07 mm tissue depth for beta radiation [1] of the participating NMIs and to support the mutual recognition of calibration certificates for these qualities. The beta radiation fields of the radionuclides Sr-90/Y-90, Kr-85 and Pm-147 will be used for the comparison. Two transfer chambers will be calibrated by each of the participating laboratories for some radiation fields selected previously. The NMIJ/AIST is the pilot laboratory. 2. Participants The participants are listed in Table 1. Table 1. Participants in the comparison. Participant Institute Country Contact person ( ) 1 INER Taiwan Chien-Hau Chu (chchu1@iner.gov.tw) 2 KRISS Korea Chul-Young Yi (cyyi@kriss.re.kr) 3 Malaysian Nuclear Agency Malaysia Taiman Bin Kadni (Taiman@nuclearmalaysia.gov.my) 4 NMIJ Japan Masahiro Kato (masahiro-katou@aist.go.jp) Norio Saito (norio.saito@aist.go.jp) 5 OAP Thailand Vithit Pungkun (vithit@oaep.go.th) 3. Comparison procedure 3.1 Transfer chambers Two parallel-plate chambers of different type are to be used as transfer standards for the comparison, as listed in Table 2. The signal connections of the chambers are a tri-axial BNT plug for the Magna chamber and a BNC with a 4-mm banana plug for the CE-II chamber. The reference point of these chambers is the front surface of the detector case (NOT the surface of the thin film entrance window). A collecting voltage listed in Table 2, supplied at each laboratory, should be applied. Both polarities should be used for each chamber and the mean current evaluated. This voltage should be applied at least 30 minutes before starting measurements. A pre-irradiation of at least 30 minutes should also be made before any measurements. The leakage current should be measured before and after each measurement. Photographs of each chamber are presented in appendix B. Plastic cover of each chamber for protecting its entrance window shall be removed during the measurements. The Magna chamber is supported by its stem, 100 mm in length, 19 mm wide and 10 mm deep (in the beam direction). The CE-II is likewise supported by its cylindrical stem, 300 mm long and 12 mm in diameter. All stated dimensions are indicated in the photographs. The chambers are transported in an air tight box of around 500 mm height, 635 mm width, 305 mm depth, and weighing about 8 kg. It is important to note that the suggested connection for the Magna chamber requires the electrometer to operate in floating mode, the collecting electrode being at the polarizing potential. If the electrometer of the participant does not permit this, then the polarizing voltage must be applied to the outer electrode of the tri-axial connector, with the middle electrode connected to ground. In this case, care must be taken to avoid accidental contact with the connector. 2

3 Table 2. Technical data for the transfer chambers. Supplier EXRADIN OYOGIKEN Model Magna CE-II Serial number D Outer diameter (mm) Outer depth (mm) Diameter of the 79 collecting electrode (mm) Depth of the collecting volume (mm) Window foil material Window thickness Conductive Kapton film Al coated polyethyleneterephtalate inch, 25 m, 3.86 mg/cm mg/cm 2 Volume (cm 3 ) Cable length (m) 1.2 Stem length (mm) Stem size or diameter 19 x in diamter (mm) Cable connection Tri-axial (BNT) Signal: BNC HV: banana plug Outer electrode Ground Ground Middle electrode Inner electrode 250 V (both polarities) Current measurement (at polarizing potential) NG Current measurement (Ground) banana plug NG 400 V (both polarities) 3.2. Reference conditions The reference conditions for the chamber calibrations are as follows: 1. Field size at the reference point: larger than 15 cm in diameter. 2. The variation of the dose rate over the field size at the calibration distance should be less than ± 5 % for 90 Sr + 90 Yand 85 Kr, and less than ± 10 % for 147 Pm. To achieve this, the sources should be used with suitable beam-flattening filters. Examples of the constructions of the beam-flattening filters are shown in ISO :2006 [3]. If the variation is larger than ± 5 % for 90 Sr + 90 Yand 85 Kr, and ± 10 % for 147 Pm, corrections should be applied during the calibration of instruments and these factors given in the comparison report. 3

4 3. Air temperature, pressure and relative humidity of T = K, P = kpa and h = 65 %. 4. The calibration coefficients for the transfer chambers should be given in terms of the absorbed dose in tissue per charge, in units of Gy C The values of the half-lives to be used are: ( ) days for Sr-90/Y-90 (3915 3) days for Kr-85 ( ) days for Pm Calibration and Source-to-filter distances are listed in Table 3. Table 3 Calibration distances and source to beam-flattening filter distances. Radionuclide Calibration distance / (cm) Source-to-filter distance / (cm) Sr-90/Y Kr Pm Measurements This comparison is performed for the calibration of an ionization chamber in terms of the absorbed dose rate in tissue, t (0,07; α ). The current measured by the chamber at laboratory ambient conditions should be corrected to the current at reference conditions (pressure: kpa, temperature: K, rel. humidity: 65 %) in the collecting volume. D Each participant should calibrate the transfer chamber in several beta reference fields of the radionuclides Sr-90/Y-90, Kr-85 and Pm-147 for the incidence angle of 0. As a minimum, all participants must take part with Sr-90/Y-90 (Table 4). Table 4 Measurement qualities. All participants must take part with the quality labelled A. The label B denotes optional qualities for the comparison. Radionuclide Incidence angle Label of the quality Sr-90/Y-90 0 A Kr-85 0 B Pm B The calibration coefficient of the transfer chamber N in mgy/(h A) is given as the following equation: N D I c t (0,07; I c α ) / 2, where t (0,07; α ) is the conventional true dose rate at a depth of 0.07 mm in tissue in mgy/h for the angle of incidence α, measured with a primary standard and corrected to the ambient air conditions during the calibration measurements, and I + c and I - c are the ionization current in amperes at the two polarities, corrected to reference conditions. We note that the symbol I tc in the MS-Excel sheet (see section 3.6) is used to denote the ionization current before correction to reference conditions. D The comparison reference value C E (N n ) will be determined at each beta energy E as the weighted mean of the calibration coefficients N n of the participants. The weighting factor for each calibration factor N n will be its reciprocal of the variance, i.e. (1/u Nn ) 2. These reference 4

5 values C E (N n ) will be used to determine the degree of equivalence for each participating laboratory Structure of comparison There will be a star-shaped circulation of the transfer chambers between the NMIJ and the participants. The NMIJ will pay for the transport from NMIJ to the airport of the participant s country and, for the return, from Narita airport in Japan to the NMIJ. Each participant will pay for the transport from the airport of the participant s country to their institute and, for the return, from their institute to Narita airport. The participants should pay for the import custom duties. The chambers should stay at the participants site for no longer than 1 week. After each participant s calibrations, the NMIJ will perform chamber constancy checks by recalibrating both transfer chambers at all of the qualities. These data will form the basis of the uncertainty estimate for chamber stability entering in the data analysis Comparison schedule The comparison is scheduled to commence in July 2011 and the proposed schedule is shown in Table 5. Table 5. Proposed schedule for the comparison. Date of chambers Participant leaving NMIJ for participant Malaysian Nuclear Agency KRISS INER OAP 7-Jan Jun Jul Oct-2013 Measurement period at the laboratory 21- Jan-2013 to 25- Jan Jun-2013 to 21-Jun Jul-2013 to 2-Aug Nov-2013 to 15-Nov-2013 Date of chambers leaving participant for NMIJ 28- Jan Jun Aug Nov-2013 Notes: 1. The time allowed for measurements for each participant is one week. 2. Allowance is made for a transportation time for the chambers of about two weeks. 3.6 Submission of calibration results It is expected that all participating laboratories will submit their calibration results within 6 weeks of calibration. An MS-Excel sheet will be provided by the pilot laboratory in which information on the participants radiation qualities, primary standards and calibration results can be submitted. 3.7 Evaluation of measurement uncertainty All participating laboratories are required to evaluate the uncertainty of their calibration coefficients as Type A and Type B according to the criteria given in the Guide to The Expression of Uncertainty in Measurement issued by the International Organization for Standardization (ISO) in The Type A uncertainty is obtained by the statistical analysis of a series of observations; the Type B uncertainty is obtained by means other than the statistical analysis of series of observations. The participating laboratories submit to the 5

6 pilot laboratory their detailed uncertainty budgets (with relative standard uncertainties, k = 1). An MS-Excel sheet will be provided by the pilot laboratory in which the participants can detail the uncertainty. The sheet should be submitted together with the calibration results. 3.8 Comparison report The pilot laboratory will prepare a draft report for circulation to all participants for comments and discussion of the results. A revised final report will be submitted to the APMP/TCRI Chairman and to the CCRI(I). After the agreement of the APMP and the CCRI(I), the report will be published as the Technical Supplement in Metrologia. In addition, the comparison report will be sent to the BIPM for inclusion in the key comparison database (KCDB). 4. References [1] International Organization for Standardization. ISO , 2004 Nuclear energy -- Reference beta-particle radiation -- Part 2: Calibration fundamentals related to basic quantities characterizing the radiation field [ [2] Schötzig, U. and Schrader, H. Halbwertszeiten und Photonen-Emissionswahrscheinlichkeiten von häufig verwendeten Radionukliden PTB-report Ra-16/6 (2000) [3] International Organization for Standardization. ISO , 2006 Nuclear energy -- Reference beta-particle radiation -- Part 1: Methods of production 4 Guide to the Expression of Uncertainty in Measurement, International Organization of Standards, Switzerland (1995) 6

7 Appendix A Addresses of the participants Pilot laboratory NMIJ National Metrology Institute of Japan NMIJ National Institute of Advanced Industrial Science and Technology, AIST Quantum Radiation Division Umezono, Tsukuba, Ibaraki Japan contact person: Masahiro Kato Tel.: Fax: masahiro-katou@aist.go.jp Norio Saito Tel.: Fax: norio.saito@aist.go.jp Participants INER Institute of Nuclear Energy Research Health Physics Division No.1000, Wunhua Rd., Jiaan Village, Longtan Township,Taoyuan County, 32546, Taiwan(R.O.C.) Contact person : Dr. Chien-Hau Chu Tel: EXT7673 Fax: chchu1@iner.gov.tw KRISS Center for Ionizing Radiation Korea Research Institute of Standards and Science (KRISS) 267 Gajeong-ro, Yuseong-gu, Daejeon Korea Contact person : Dr. Chul-Young Yi Tel: or Fax: cyyi@kriss.re.kr Nuclear Malaysia Malaysian Nuclear Agency (Nuclear Malaysia) Bangi, Kajang, Selangor Darul Ehsan, Malaysia Contact person: Mr. Taiman Bin Kadni Tel.: ext 1260 or (direct) Fax: taiman@nuclearmalaysia.gov.my 7

8 OAP Office of Atoms for Peace 16 Vibhavadi Rangsit Road, Laadyaw, Chatuchak, Bangkok Thailand Contact person: Mr.Thongchai Soodprasert Tel: ext Fax: thongcha@oaep.go.th 8

9 Appendix B Photographs of the chambers EXRADIN Magna 19 mm 100 mm 10 mm Oyogiken CE-II 300 mm 12 mm Center of the chamber 9