Year : 2018 | Volume
: 41 | Issue : 3 | Page : 107--109
Role of standards in radiation protection
Radiation Standards Section, Radiation Safety Systems Division, Bhabha Atomic Research Centre, Trombay, Mumbai - 400 085, Maharashtra, India
M S Kulkarni
Radiation Standards Section, Radiation Safety Systems Division, Bhabha Atomic Research Centre, Trombay, Mumbai - 400 085, Maharashtra
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Kulkarni M S. Role of standards in radiation protection.Radiat Prot Environ 2018;41:107-109
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Kulkarni M S. Role of standards in radiation protection. Radiat Prot Environ [serial online] 2018 [cited 2019 Jan 18 ];41:107-109
Available from: http://www.rpe.org.in/text.asp?2018/41/3/107/245803
Today, the radiation technology is being extensively used in all spheres of life; be it agriculture, food irradiation, industry, medical, space and basic research, use of radiation is inevitable. Radiation sources are extensively used in hospitals for diagnostic and therapeutic applications in medicine and in the industry for nondestructive testing. Therefore, its measurement as well as reliable estimate is crucial to ensure an improvement in our safety preparations and protection philosophies. Standards for ionizing radiation play a vital role in realizing, implementing and demonstrating compliance with the regulatory requirements for safety in radiation protection practices.
The standard can be an object, system, or experiment that bears a defined relationship to a unit of measurement of a physical quantity. The Bureau International des Poids et Mesures (BIPM) maintains the international reference standards for dosimetry and activity measurements. These standards are used in the BIPM key comparisons program. In India, National Physical Laboratory, New Delhi is recognized as the National Metrology Institute to maintain standards for all physical quantities. However, ionizing radiation metrology has been delegated to Radiation Standards Section, RSSD, BARC, Mumbai. It has also been recognized as the Designated Institute for ionizing radiation metrology in India. BARC maintains number of national standards for ionizing radiation and disseminates these standards to the users and also continuously updates them to achieve better accuracy. These standards include primary standards, secondary standards and working standards for radiological quantities, radioactivity, neutron, and high-dose dosimetry. Radiation standards developed and maintained at BARC are linked to international standards through various international intercomparison programs. The ionizing radiation standards maintained at BARC are broadly categorized into Radiological Standards, Radionuclide Standards, Neutron Standards, Chemical Dosimetry Standards, and Protection Level Standards. These standards are used for providing traceability and technical guidance to the users in developing national infrastructure through recognized calibration laboratories in DAE and private sector for the periodic calibration of large number of radiation monitoring instruments. BARC has also been recognized as a Secondary Standard Dosimetry Laboratory (SSDL-BARC) by International Atomic Energy Agency (IAEA)/WHO. Under this aegis, quality audits are being conducted since 1976 for assessing the dosimetric status of all the radiotherapy centers, nuclear medicine centers, and radiation processing facilities (RPF) in India.
BARC has developed and maintained primary, secondary, and tertiary standards for neutron source yield and neutron fluence rate measurements. All the neutron measurements in the country are traceable to the National Lab (BARC) through these standards. The national lab has a primary standard for thermal neutron fluence rate measurement also. It has various neutron spectrometers and standard neutron sources. Manganese sulfate bath system is the primary standard for neutron source yield measurement. It consists of a stainless steel tank of 1 m diameter filled with saturated solution of manganese sulfate. When a neutron source is suspended inside the tank, the emitted neutrons are moderated and absorbed by the inactive manganese and will become active, and the induced activity is measured and correlated to the neutron source strength. This standard was intercompared with similar standards of the world.
Neutron Telescope is the primary standard for fast neutron fluence rate measurement. It consists of a proton radiator and charged particle detector CsI(Tl) placed inside a vacuum chamber. The efficiency of this system has been evaluated regarding angle subtended by the radiator and the detector. This system has been established for 14 MeV neutrons. Precision Long Counter is the secondary standard for the neutron yield measurement as well as fluence rate measurement. It consists of a long BF3 counter placed inside a special moderating assembly consisting of two concentric cylinders separated by a neutron absorber. It has a constant efficiency for a long range of energies. STAG is the primary standard for thermal neutron fluence rate. It consists of six Am-Be neutron source embedded in a graphite pile. The thermal neutron fluence rate in the central cavity has been standardized using gold and manganese cross sections. The value has been intercompared with similar standards of the world.
Protection Level Gamma Standards
BARC maintains reference radiation fields that are required for calibrating of radiation protection monitoring instruments which form the backbone of the radiation monitoring programme for harnessing the benefits of nuclear energy and ionizing radiation. These instruments are type-tested and periodically calibrated at standard reference radiation fields to ensure their healthy working condition and fitness for their intended use. The laboratory is equipped with necessary infrastructure-collimated radiation beams, linear distancing system, and remote viewing facility have been established and maintained as per the recommendations of ISO-4037:1. The reference radiation fields are standardized using a 100.9 cc cylindrical graphite walled ion chamber which is maintained as a reference standard, as per ISO 4037:2, for the calibration of protection level radiation monitoring instruments. RSSD carries out periodic calibration of radiation protection monitoring instruments, which is once in 2 years, as well as the type test which involves various tests, to study the variation of the calibration factor with influencing quantities such as linearity of response, energy response, angular dependence, and overload characteristics. RSSD periodically participates in the IAEA Quality Audit for radiation protection level calibrations since year 2001, and the results of the quality audit are well within the acceptance limit (±7%) for the participating laboratories.
Radionuclides are used in many different applications such as nuclear medicine, carbon dating, tracer application, nuclear power plant operation survey, food processing, and sterilization. In all these fields, there is a requirement of accurate measurements of radioactivity. The primary, secondary and working radioactivity Standards maintained at BARC are given below.
The primary standards such as liquid scintillation-based 4 πβ (LS)-γ coincidence counting system, proportional counter-based 4 πβ (PC)-γ coincidence counting system are directly applicable to all radionuclides decaying by the simultaneous emission of two or more radiations in prompt succession, such as β-γ, α-γ, electron capture-γ, γ–γ, γ-x, etc. Other primary standard for radioactivity measurement is CIEMAT/NIST efficiency tracing technique using commercially available liquid scintillation spectrometer. These standards are used for measurement of activity of pure β, β-γ, and α-γ emitting radionuclides. Large area windowless proportional counter for standardization of large area beta sources (ISO 8769:2010) as emission rate standard. These standard sources are used for the calibration of contamination monitors (BS EN 60325:2004). These standards provide results with uncertainty <1% at coverage factor k = 1.
The secondary standards maintained in the laboratory are 4 πγ ion chamber and HPGe gamma spectrometer for standardization of gamma-emitting radionuclides. These standards provide results with uncertainty of ~3%–5% at coverage factor k = 1. 4 πγ ion chamber is used for standardization of radionuclides extensively used in nuclear medicine such as I-131, Tc-99m, F-18, Lu-177, etc. These calibrated radionuclides standards are used for calibration of “dose calibrator” used in nuclear medicine departments (IAEA-TECDOC-602, 1991, Technical Report Series-454, 2006). “Dose Calibrator” is used as the Working Standard for standardization of radionuclides in nuclear medicine centers. This standard provides results with uncertainty 3%–5% at coverage factor k = 1.
In radiation dosimetry, traceability and accuracy of radiation measurements are very important especially in radiotherapy where the success of patient treatment is dependent on the accuracy of the dose delivered to the tumor. The main aim of the radiation therapy is to deliver an accurate dose (within ±5%) to the tumor volume for better cure rates. To achieve this level of accuracy in dose delivery IAEA jointly with the WHO setup, a Network of SSDLs in 1976 and BARC was recognized as SSDL for India. SSDL-BARC has been designated by the competent national authorities to provide a necessary link in the traceability of radiation dosimetry to the international standards for various users within the country. SSDL-BARC establishes and maintains radiological standards, provides calibration services to the hospitals for their dosimeters, and conducts thermoluminescent dosimetry postal dose quality audit for therapy centers.
A free-air ionization chamber is maintained as the primary standard for low and medium energy X-ray beam qualities (up to 300 kV). Standard beam qualities as per ISO-4037 guidelines have been generated for various applications. BARC has established diagnostic beam qualities for the calibration of instruments used in diagnostic radiology as per IAEA TRS-457 guidelines. A diagnostic range free-air ionization chamber developed for the same has been established as a standard for the diagnostic range (20–150 kV). A cylindrical ionization chamber is the reference standard for absorbed dose to water and air kerma at 60Co energy. This reference chamber has been calibrated against the primary standards available at BIPM, Paris. IAEA TRS-398 protocol is followed for the calibration of dosimeters belonging to almost all the radiotherapy centers in this country. RSS has participated in various intercomparisons under the aegis of Asia Pacific Metrology Programme and IAEA for the 60Co beam quality. A cylindrical graphite large volume ionization chamber is maintained as the reference standard for brachytherapy. The chamber is used to provide traceable calibration to more than 300 brachytherapy facilities in the country. IAEA TECDOC 1274 guidelines are followed for the calibration of well-type chambers. Calibration of well-type chamber is provided for high-dose rate 192Ir sources and low-dose rate 137Cs sources against this standard. RSS also provides traceable calibration for well-type chambers for 60Co HDR source using the reference standard calibrated at BIPM.
Beta dosimetry is carried out using an extrapolation chamber. ISO-6980 specifies the reference beta particle radiation fields regarding Series 1 and Series 2. A calibration facility has been designed and developed at SSDL-BARC for generating Series 1 and Series 2 reference fields as per international standards to calibrate beta measuring instruments. Calibration of beta skin dose monitors has been carried out against the reference standard extrapolation chamber. In addition to the existing standards, BARC is also working for the augmentation of new standards in the laboratory that includes a graphite calorimeter as a primary standard for absorbed dose measurement.
Chemical Dosimetry Standards
Standards for high dose are established and maintained in the laboratory for calibration and standardization of high-dose radiation processing applications. Fricke dosimeter is maintained as a primary reference standard using ISO/ASTM practice 51026:2015(E) BARC, and it is used for standardization of other dosimeters. Gamma chambers used for irradiation are calibrated using the Fricke dosimetric system as per ISO/ASTM 52116:2013(E). Alanine ESR using ISO/ASTM practice 51607:2013(E) (considered as gold standard in high-dose dosimetry applications) and glutamine using indigenously developed spectrophotometric readout method are used as transfer standard dosimeters in radiation processing. These standards provide results with uncertainty <3% at coverage factor k = 1 as per ASTM practice E 1707:1995.
The chemical dosimetry standards are used to carry out dose quality audit programs for all the radiation processing facilities (RPF) in India dealing with irradiation of food and allied products. Under this program, different routine dosimetry systems (indigenous and imported) are calibrated with standard dosimeters based on dose range and other parameters as per ISO/ASTM 51261:2013(E). In addition, ability of RPF to deliver and to measure accurate doses is verified by following standard protocols based on AERB guidelines and international recommendations as per Technical Reports Series No. TRS-409, ISO 11137-3:2017(en), ISO/ASTM 52628:2013(E) and ASTM E3083-17. Routine dosimeters used by RPF are ceric-cerous ISO/ASTM practice 51205:2017(E), dichromate ISO/ASTM practice 51401:2013(E), radiochromic film ISO/ASTM practice 51275:2013(E), perspex ISO/ASTM practice 51276:2012(E), radiochromic waveguide ISO/ASTM practice 51310:2004(E), Fricke ISO/ASTM practice 51026:2015(E), etc. These standards provide results with uncertainty up to 5% at coverage factor k = 1 as per ASTM practice E 1707:1995.