Commissioning of the Beta Secondary Standard (BSS2) Speaker / Author: R.W. Thoka* Co-author: S. Jozela* * National Metrology Institute of South Africa (NMISA), Private Bag X 34, Lynnwood Ridge, Pretoria, 0400, South Africa E-mail: wthoka@nmisa.org, Phone: +27 12 841 3302, Fax: +27 12 841 3367 Abstract The National Metrology Institute of South Africa (NMISA) Dosimetry Section through NMISA s recapitalization procured a beta secondary standard irradiation facility type 2 (BSS 2). Beta radiation consists of electrons generated in the nucleus by the decay of a neutron into a proton (or vice versa), an electron and a neutrino. The calibration quantities are H'(0.07) and Hp (0.07) as operational quantities to estimate the local skin dose and H'(3) and Hp (3) as operational quantities to estimate the dose to the lens of the eye. The system was commissioned by verification of the personal dose equivalent (Hp (0.07), and software calculated time was checked for correctness using a calibrated stopwatch. An electronic personal dosimeter (EPD) was used to measure the personal dose equivalent (PDE) for 85 Kr and 90 Sr + 90 Y using polymethylmethacrylate (PPMA) and water phantoms with the results for both irradiation medium presented on this paper. Keywords: Beta radiation, Beta Secondary Standard, Dose Equivalent. 1. Introduction The dosimetry section has been using the irradiation system developed by National Physical Laboratory (NPL) with two beta emitting radioactive sources, 85 Kr and 90 Sr + 90 Y. The system has been mainly used for irradiation of Thermoluminiscent Dosimeters (TLDs) for the industry. TLDs irradiated by the laboratory are mostly used for monitoring radiation exposures to radiation workers. The TLDs are irradiated in a tissue equivalent slab phantom in a field of 85 Kr and 90 Sr + 90 Y beta radiation sources. The system has been maintained through the calibration of the beta sources at 5 yearly intervals at the primary standard laboratory and also use of EPDs for checks in between calibrations. As the irradiator was already old and some of the safety features were no longer functioning, posing a huge safety risk to employees, a decision was made to replace the system. This resulted in a purchase in 2014 of the Beta Secondary Standard 2 (BSS 2) system traceable to the National Metrology Institute of Germany, Physikalisch-Technische Bundesanstalt (PTB). The system offers capability to measure absorbed dose to tissue using beta standards. The beta secondary standard irradiation facility type 2, BSS 2 is an example of the concept of a calibrated secondary standards built as complete irradiation facilities including the instruments necessary to perform air pressure, temperature and humidity corrections. It uses long lived and high-purity radiation sources to produce a secondary reference radiation field and includes a traceable calibration. It consists of three sources of the nuclides 147 Pm, 85 Kr and 90 Sr + 90 Y according to series 1 of ISO 6980-1[1]. The measuring quantities are Hp (0.07), Hp (3) and H'(0.07). The physical reason for the inclusion of Hp (3) without any additional calibration is that the depth-dose curves are nearly identical for the sources of the BSS 2 [2].
The new software also gives detailed information on the photon contribution from the source and due to bremsstrahlung generation. The complete irradiation facility provides, after traceable calibration, the actual dose or doserate value corrected for the influence of air pressure, temperature and humidity [1]. 2. Verification Requirements Once a BSS 2 system with sources traceable to primary standard laboratory together with the recommended beta secondary standard chamber has been purchased, an SSDL is ready for calibrating beta instruments. However before any irradiation facility is used, verification, using a suitable survey instrument or even an electronic personal dosimeter with the capability of beta radiation measurement is recommended. The aim of this work is not to perform a validation but only to assure the constancy of the field as well as detecting defects in the sealing of the beta sources. Since in our case we do not have a beta secondary standard, an EPD was used for verification of the dose and/or dose rate produced by the beta sources purchased with the system. The recommended reference instrument to be used for calibration of beta detecting instrument by secondary standard laboratories is a secondary standard ionisation chambers. This chamber fulfils the requirement of ISO 6980-2 on secondary standard instruments for beta radiation [3]. The calibration setup should strictly follow the terms given for the calibration facility as recommended by ISO 6980-2. The BSS 2 is equipped with a turntable and necessary distance adjustment tools to perform the required mechanical set up to ensure correct positioning. Reference radiation fields are described in ISO 6980 parts 1 to 3 [4, 5, 6] and series 1 secondary reference radiation field is used in this report as recommended on the ISO 6980-1. Series 1 consists of three sources of the nuclides 147 Pm, 85 Kr and 90 Sr + 90 Y and beam flattening filters. The measuring quantities are Hp(0.07), Hp(3) and H'(0.07). Hp(0.07) is the only quantity measured in this paper. 3. Equipment and Methods 3.1 BSS 2 System Components The following standards and equipment were used during the measurements with the BSS 2 system: 85 Kr source, 90 Sr + 90 Y source and 147 Pm source, each source is mounted on a separate shielded holder with a shutter to fit in the source stand. Three beam flattening filters, one for each of the three sources with filter identity cards according to their matching sources. The filters are held in place by means of four wires with metal tags that fit into grooves in the metal support rods. Irradiating device; a source stand mounted on a linear guiding. Fixed distances (11, 20, 30 and 50 cm) can be seen on a linear guiding. Beam axis pointer that can be screwed into the source position (in the absence of the source). The pointer is 300 mm long and 4 mm in diameter steel rod that passes through the centre of the guide. Control unit; the control unit contains all electronic components necessary to operate the BSS2 system including the sensor for atmospheric pressure. A separate sensor for measuring temperature and humidity is connected to the control unit using a cable. The
control unit is connected to the linear guiding or source stand by a special cable (maximum length 5 m). Built-in software to calculate the desired dose or dose rate. A Perspex phantom measuring 30 cm x 30 cm x 15 cm. A water phanthom. 3.2 Methods A calibrated Thermo Scientific, Mk 2.3 EPD with energy response from 250 kev to 1,5 MeV and an accuracy of 20% for 90 Sr + 90 Y and 85 Kr was used. This type of EPD was unable to detect radiation from 147 Pm as this radionuclide has an energy of 224.1 KeV so its use was only limited to 90 Sr and 85 Kr. The time calculated by the software was verified using a hand calculated time from the calibration certificate of the radioactive sources. A hand timer was simultaneously used with the system time to check for correctness. Once the operation correctness was verified, an EPD was irradiated with a personal dose equivalent Hp(0.07) of 3 msv, 5 msv and 10 msv. The irradiation were performed at different mediums, a 30 cm x 30 cm x 15 cm PPMA and the 30 cm x 30 cm x 15 cm ISO calibration water phantom which complies with the standard ISO 4037 part 3, which is suitable for calibrations and type tests of personal dosimeters, because they measure the quantities Hp(0.07) and Hp(10). The irradiations were performed at 30 cm from the source at a 0º incident angle. The irradiation was repeated 10 times to check repeatability and reproducibility. 4. Uncertainty of Measurements on the BSS 2 System The complete irradiation facility provides, after traceable calibration of the radioactive sources, the actual dose or doserate value corrected for the influence of air pressure, temperature and humidity. It also gives the associated uncertainty determined according to the Guide to the Uncertainty in Measurement (GUM) and includes extensive quality control to assure every parameter used for the determination of the dose or doserate value [7]. This however depends on the level the facility will operate. To determine the absolute dose rate, the user needs calibrated thermometer, barometer and the humidity measuring device. These must then not be part of the facility if they cannot be independently calibrated. For this work, attention has only been paid on the acceptance of the measured personal dose equivalent in conforming to the 20% accuracy of the EPD. 5. Results Tables 1 and 2, show personal dose equivalent results for Kr-85 and Sr-90 in two different mediums. Column 1 shows the given Hp (0.07), columns 2 and 4 show the measured Hp (0.07) and columns 3 and 5 show the percentage difference. Figures 1 to 4 show the repeatability and/or reproducibility of the measurements taken. The verification of the system timer with a model 500 Stopwatch manufactured by Sanji also yielded satisfactory results.
Dose Equivalent (msv) Table 1: Personal dose equivalent EPD results on the ISO water phantom for BSS2 system ISO Water Phantom Medium 85 Kr 90 Sr + 90 Y Hp(0.07) % Given Hp(0.07) (msv) Hp(0.07)(mSv) % Difference (msv) Difference 3,0 2,82 6,63 3,35 11,40 5,0 4,73 5,61 5,61 12,05 10,0 9,43 5,87 11,89 18,73 Table 2: Personal dose equivalent EPD results on the PPMA phantom for BSS 2 system PPMA Phantom Medium Given Kr 90 Sr + 90 Y Hp(0.07) Hp(0.07) % Difference Hp(0.07)(mSv) % Difference (msv) (msv) 3,0 2,83 6,12 3,24 7,66 5,0 4,76 5,11 5,32 6,19 10,0 10,29 2,80 10,70 6,92 Repeatability Chart 85 Kr PPMA 13 12 11 10 9 8 7 6 5 4 3 2 1 0 0 1 2 3 4 5 6 7 8 9 10 11 # of Measurements 3 msv 5 msv 10 msv UL 10 msv UL 5 msv LL 5 msv UL 3 msv LL 3 msv Figure 1. A plot showing three personal equivalent doses Hp(0.07) 3 msv, 5 msv and 10 msv with upper and lower limits of ± 20 % for 85 Kr in PPMA phantom. The maximum standard deviation across the doses was 0,07 msv, achieved at 10 msv.
Dose Equivalent (msv) Repeatability Chart 90 Sr + 90 Y PPMA 14 12 10 8 6 4 2 0 0 2 4 6 8 10 12 # of Measurements 3 msv 5 msv 10 msv UL 10 msv UL 5 msv UL 3 msv LL 3 msv Figure 2. A plot showing three personal equivalent doses Hp(0.07) 3 msv, 5 msv and 10 msv with upper and lower limits of ± 20 % for 90 Sr + 90 Y in PPMA phantom. The maximum standard deviation across the doses was 0,18 msv, achieved at 10 msv.
Dose Equivalent (msv) Repeatability Chart 85 Kr Water 14 12 10 8 6 4 2 0 0 2 4 6 8 10 12 # of Measurements 3 msv 5 msv 10 msv UL 10 msv UL 5 msv LL 5 msv UL 3 msv LL 3 msv Figure 3. A plot showing three personal equivalent doses Hp(0.07) 3 msv, 5 msv and 10 msv with upper and lower limits of ± 20 % for Kr-85 in ISO water phantom. The maximum standard deviation across the doses was 0,16 msv, achieved at 10 msv
Dose Equivalent (msv) Repeatability Chart 90 Sr + 90 Y Water 14 12 10 8 6 4 2 0 0 2 4 6 8 10 12 # of Measurements 3 msv 5 msv 10 msv UL 10 msv UL 5 msv LL 5 msv UL 3 msv LL 3 msv Figure 4. A plot showing three personal equivalent doses Hp(0.07) 3 msv, 5 msv and 10 msv with upper and lower limits of ± 20 % for 90 Sr + 90 Y in ISO water phantom. The maximum standard deviation across the doses was 0,12 msv, achieved at 10 msv.
6. Discussions and Conclusions For the purpose of this work, the results achieved were satisfactory. An EPD was used to measure the personal equivalent dose on the two beta sources 85 Kr and 90 Sr + 90 Y, and all measurements taken were within the ± 20% limits which is the most accuracy an EPD used can manage. The results for 85 Kr in both mediums are well within the limits (maximum of 7 %) whereas the 90 Sr + 90 Y results even though still within the 20 % limits, they are closer to the upper limit. The reason for this needs further investigation as the measurement data taken for this work is insufficient to conclude on this. This is more important especially on deciding what phantom to use for customer work. The BSS2 system has thus been verified and can further be used as the previous beta system has been used, that is irradiating customers dosimeters. Output measurements will however be taken at least twice a month until enough data is achieved to make a conclusion on the reason of the performance of the two sources used, with 90 Sr + 90 Y giving results closer to the 20 % limit on the ISO water slab phantom. Measurements will also be taken on a 10 mm PPMA as is a recommendation from ISO 6980. This will allow for a better comparison of the suitability of phantoms. Further work will be embarked on to measure absolute dose rate with the extrapolation chamber. This will be in preparation of the participation on the inter-comparison planned for 2017. For work that needs to be performed with the system, the system uncertainty with routine quality control suffices. An investigation will also be made into whether there is a need to purchase a survey meter capable of measuring the energy range of the radioactive sources used. 7. Acknowledgements The authors of this paper would like thank NMISA management for providing the technical infrastructure and support.
8. References 1. P. Ambrosi, G. Buchholz, K. Helmstadter, The PTB Secondary Standard BSS 2 for radiation protection. 2. Brunzendorf, J. Determination of Depth-Dose Curves in Beta Dosimetry, Radiat. Prot. Dosim. 151, (2012) 203 210. 3. Behrens, et al, International comparison EUROMET.RI(I)-S2 of extrapolation chamber measurements of the absorbed dose rate in tissue for beta radiation (EUROMET project No 739): Final report, Metrologia 44 (2007), Techn. Suppl. 06003 (doi: 10.1088/0026-1394/44/1A/06003). 4. INTERNATIONAL ORGANIZATION FOR STANDARDIZATION, Nuclear Energy- Reference beta particle radiation Part 1: Methods of production, ISO 6980-1, Geneva (2006). 5. INTERNATIONAL ORGANIZATION FOR STANDARDIZATION, Nuclear Energy- Reference beta particle radiation Part 2: Calibration fundamentals related to basic quantities characterizing the radiation field, ISO 6980-2, Geneva (2004). 6. INTERNATIONAL ORGANIZATION FOR STANDARDIZATION, Nuclear Energy- Reference beta particle radiation Part 3: Calibration of area and personal dosimeters and determination of their response as a function of energy and angle of incidence, ISO 6980-3, Geneva (2006). 7. INTERNATIONAL ORGANIZATION FOR STANDARDIZATION, Uncertainty of measurement Part 3: Guide to the expression of uncertainty in measurement, ISO/IEC Guide 98-3, Geneva (2008).