1、21GA/Agenda 19.231Calibration of Radiation Instruments Used in Radiation Protection and Radiotherapy in MalaysiaTaiman Bin Kadni (taimanmint.gov.my)Secondary Standard Dosimetry Laboratory (SSDL)Malaysian Institute for Nuclear Technology Research (MINT)Bangi, 43000 Kajang, MalaysiaAbstract External r
2、adiation quantity can be measured in terms of exposure, air kerma, absorbed dose, dose equivalent, ambient dose equivalent and directional dose equivalent by using radiation measuring instruments. Radiation survey instruments are usually used to determine the amount of leakage radiation from the rad
3、iation sources such as radioactive sources and x-ray machines and hence protect workers from exposure to ionizing radiation. The outputs of Cobalt-60 teletherapy units and medical linear accelerators (linac) at radiotherapy centers are determined by using dosimeters. Radiation measuring instruments
4、such as survey meters and dosimeters need to be calibrated to ensure that they give accurate and correct reading with a certain uncertainties and to comply with the regulations imposed by the relevant authority. In Malaysia, calibration of radiation measuring instruments is legally required under th
5、e Radiation Protection (Basic Safety Standard) Regulation, 1988. The Secondary Standard Dosimetry Laboratory (SSDL) of the Malaysian Institute for Nuclear Technology Research (MINT) is the national focal point for the calibration of measuring instruments used in radiation protection and radiotherapy
6、. The main radiation facilities include a Cobalt-60 teletherapy unit, constant potential x-ray system with a 320 kV tube, gamma calibrators, panoramic gamma irradiators, PTB beta secondary standard and an Americium-241/Be neutron calibrator. Reference standard dosimeters traceable to the Primary Sta
7、ndard Dosimetry Laboratory (PSDL) and the IAEA Dosimetry Laboratory are used in the calibration procedures. Dosimeters used in radiotherapy can be calibrated in terms of air kerma and absorbed dose to water to comply with an International Code of Practices for dosimetry i.e. IAEAs Technical Report S
8、eries number 277, 381 and 398. The laboratory has been accredited by the Department Standard of Malaysia (The Malaysian accreditation body) as a calibration laboratory for radiation survey meters and therapy dosimeters. Keywords : Calibration, dosimeter, radiation survey instrument, radiation source
9、s, standardsINTRODUCTIONExternal radiation quantity can be measured in terms of exposure, air kerma, absorbed dose, dose equivalent, ambient dose equivalent and directional dose equivalent by using radiation measuring instruments. Various kinds of radiation measuring instruments such as survey meter
10、s, area monitors, personal dosimeters, contamination-monitoring instruments are used in irradiation facilities for radiation protection purposes. Implementation of the proper monitoring requires radiation-monitoring survey meters e.g. ionization chambers, Geiger-Muller (GM) counters and scintillatio
11、n counters to be calibrated in terms of dose equivalent quantities. Area dosimeters or dose ratemeters should be calibrated in terms of the ambient dose equivalent, H*(10), or the directional dose equivalent, H(0.07)1. In radiotherapy centers, the radiation dosimeters are normally used for determina
12、tions of the output of Cobalt-60 teletherapy units and medical linear accelerators (linacs) and should be calibrated in terms of exposure 2, air kerma 3 or absorbed dose to water 4,5. Radiation measuring instruments need to be calibrated to ensure that they give accurate and correct reading with a c
13、ertain uncertainties and to comply with the regulations imposed by the relevant authority. They should be calibrated annually 6,7 or after major repair. In Malaysia, calibration of radiation measuring instruments is a legal requirement under the Radiation Protection (Basic Safety Standard) Regulatio
14、n, 19888. The SSDL-MINT was established in 1980 and is a member of the IAEA/WHO Network of SSDLs. The laboratory has acquired the status of national standard laboratory with the basic aim of improving accuracy in radiation dosimetry in the country. It is also the national focal point for the calibra
15、tion of radiation measuring instruments used in radiation protection and radiotherapy. More than 1500 radiation instruments are normally calibrated every year. The laboratory has also the responsibility to ensure that the calibration services provided by the laboratory follow internationally accepte
16、d metrological standards. This is achieved by calibrating the laboratorys protection and therapy levels dosimeters against those in the Primary Standard Dosimetry Laboratories (PSDLs) or the International Atomic Energy Agency (IAEA) or by participating in the international comparison on dosimetry me
17、asurements programmes. 21GA/Agenda 19.232CALIBRATION FACILITIESa. Irradiation facilitiesThe calibration of radiation instruments used for radiation protection and radiotherapy requires appropriate irradiation facilities capable of providing air kerma rates up to approximately 1Gy/min. The SSDL-MINT
18、has four irradiation rooms or bunkers to accommodate radiation sources and to perform calibration of radiation protection survey instruments and therapy level dosimeters. The dimensions of the irradiation rooms were approximately 9m x 4.8m x 3.2m high. The floor, walls and ceiling are concrete. The
19、design of the irradiation rooms is in accordance with the relevant national and international safety regulations. The shielding of the rooms are sufficient to ensure that the radiation doses to the staff and the public are kept as low as reasonably achievable and that the given dose limits are not e
20、xceeded. The irradiation rooms are equipped with radioactive and x-ray sources that can be operated remotely from the control room. The control room also has monitors coupled to video cameras in the irradiation rooms. The irradiation rooms are equipped with calibration benches running on a pair of r
21、ails to accommodate measurements at various distances from the sources and hence calibrate the radiation measuring instruments. The ionization chambers or radiation survey instruments are positioned on the calibration benches where they can be positioned at the required distances from the source. Th
22、e calibration distance is determined using a telescope or laser. Laser alignments are also installed in each irradiation room to ensure that the radiation survey meters and dosimeters are placed in the center of the radiation beam during the measurements. The irradiation rooms are also provided with
23、 appropriate safety equipment and accessories such as warning lights for each irradiation unit, door interlocks and continuously operating radiation monitors. b. Radiation sourcesi. X-radiationCalibration at photon energies below 300 keV is usually carried out using an x-ray system. SSDL-MINT has cr
24、eated a series of x-ray beam qualities which were selected to match the beam qualities offered by the International Bureau of Weight and Measures (BIPM) and based on ISO Narrow Spectrum Series. The Yxlon constant potential x-ray systems type MG 325 with a 320 kV tube is used to generate ISO narrow s
25、pectrum series x-ray reference radiation 9 for the calibration of radiation protection instruments. The beam characteristics are shown in table 1, where the values of the mean energies have been adopted from the ISO document. X-rays generated between 100 kV and 250 kV in accordance with the BIPM the
26、rapy qualities 10 are used for calibration of therapy level dosimeters. Table 1. ISO Narrow Spectrum Series X-ray Beams at the SSDL-MINTRadiation qualityMean energyTube potentialAdditional filtration mmHVL mmkeV kV Al Cu Sn Pb CuN-40 33 40 4.0 0.2 - - 0.09N-60 47 60 4.0 0.6 - - 0.25N-80 65 80 4.0 2.
27、0 - - 0.60N-100 83 100 4.0 5.0 - - 1.15N-120 100 120 4.0 5.0 1.0 - 1.78N-150 117 150 4.0 - 2.5 - 2.58N-200 164 200 4.0 2.0 3.0 1.0 4.13N-250 207 250 4.0 - 2.0 3.0 5.34N-300 248 300 4.0 - 3.0 5.0 6.33Table 2. Therapy Qualities X-ray Beams at the SSDL-MINTAdditional filtration mm HVL mmTube potential
28、kV Al Cu Al Cu100 3.5 - 4.05 -135 1.0 0.3 - 0.50180 1.0 0.6 - 1.01250 1.0 1.6 - 2.5121GA/Agenda 19.233ii. Gamma radiationRadionuclide sources are used to perform calibration at photon energies above 300 keV. There are several gamma sources i.e. Caesium-137, Cobalt-60 and Americium-241 with different
29、 activities available to provide radiation standards for protection and therapy level dose rates. Two irradiation rooms are equipped with two collimated gamma irradiators model OB 85 which consists of 137Cs (740 GBq) and 60Co (37 GBq) and two panoramic gamma irradiators model OB 34. The panoramic ga
30、mma irradiator contains a total of seven 137Cs and 60Co gamma sources with activities ranging from 3.7 MBq to 7.4 GBq that produce uncollimated radiation fields. Table 3 shows the specifications of protection level gamma sources used to calibrate radiation survey instruments. The reference gamma rad
31、iation fields are determined in terms of air kerma rate. A teletherapy 60Co unit model Eldorado 8 is used to provide national standards for air kerma and absorbed dose to water at therapy level and calibration service for therapy level dosimeters. Its activity was 114.2 TBq (3,086 Ci) on November 15
32、, 1995. Table 3. Protection level gamma sourcesGamma irradiatorNuclide Activity range No. of sourceCovered air kerma rate rangeSource distance mCs-137 7.4MBq-7.4GBq 4 0.49 Gy/h 1.59 mGy/h 0.5 2.5OB 34Co-60 3.7MBq-370MBq 3 0.29 38.94 Gy/h 0.5 2.5Cs-137 740 GBq 1 16.53 Gy/h 150.48 mGy/h0.5 5.0OB 85Co-
33、60 37 GBq 1 5.76 Gy/h 5.54 mGy/h 0.5 5.0OB 6 Cs-137 74 GBq 1 0.22 14.56 mGy/h 0.5 4.0 Am-241 11.1 GBq 1 5.08 119.92 Gy/h 0.4 2.0iii. Beta radiationThe beta radiation sources of the Pm-147, Kr-85 and Sr-90/Y-90 nuclides of the PTB Beta Secondary Standard (BSS 2) developed by the Physikalisch-Technisc
34、he Bundesanstalt (PTB), Germany are used to calibrate radiation survey instruments. The radiation qualities are in compliance with the series of standards ISO 698011. The sources are calibrated by PTB at the specified distances in terms of absorbed dose rate to tissue surface, directional dose equiv
35、alent rate and personal dose equivalent rate. The dose rate at the calibration distance is between 7 Gy/s and 38 Gy/s. The irradiation procedure is controlled by a personal computer which stores the calibration data for the source used. The specifications of beta sources have been summarized in tabl
36、e 4.Table 4. Beta Secondary Standard SourcesSource 1 2 3Radionuclide Pm-147 Kr-85 Sr-90/Y-90Nominal activity 3.7 GBq 3.7 GBq 460 MBqBeam flattening filter Yes Yes NoMean beta energy (MeV) 0.06 0.24 0.8Calibration distance (cm) 20 30 11, 20, 30 & 50Reference date 07.02.2003 27.02.2003 28.03.2003Half-
37、life (years) 2.62 10.72 28.8 yearsiv. NeutronThe collimated neutron calibrator model OB 26 manufactured by Buchler GmbH, Germany which consists of 185 GBq Americium-241/Beryllium neutron source is available to carry out calibration of neutron survey instruments and personal dosimeters. The neutron e
38、mission rate quoted by the manufacturer was 1.1 x 107 n/s on July 20, 1984. c. Dosimetric equipments21GA/Agenda 19.234The SSDL-MINT is equipped with secondary standard dosimeters and working standard dosimeters to provide exposure and air kerma standards used for calibrating radiation survey instrum
39、ents and air kerma and absorbed dose to water standards for calibrating therapy level dosimeters. The secondary standards for radiation protection are based on 1,000 cm3 and 10,000 cm3 spherical ionization chambers, LS-01 and LS-10 designed and manufactured by PTW, Germany and the Austrian Research
40、Center, Austria respectively. The chambers are calibrated in terms of air kerma at 137Cs, 60Co and a number of x-ray beam qualities at the IAEA Dosimetry Laboratory and the Austrian Research Center Seibersdorf. A 0.6 cm3 ionization chamber NE 2571 has been selected as reference chamber for the calib
41、ration of therapy level dosimeters. The chamber had been calibrated in terms of absorbed dose to water at 60Co and air kerma at 60Co and x-ray beam qualities at the IAEA Dosimetry Laboratory. The ionization current from the ionization chambers is measured with a PTW-Unidos Universal Dosimeter and Di
42、gital Currentintegrator DCI 8500 with calibration traceable to the National Metrology Laboratory, SIRIM Berhad. The leakage current for the system ionization chamber plus electrometer is considered negligible. The stability of the chamber plus electrometer system is checked at regular intervals usin
43、g a 90Sr and 241Am check source. Working standard dosimeters used for protection and therapy levels at SSDL are calibrated against secondary standard dosimeters. They are routinely used for the calibration of radiation protection survey instruments and therapy level dosimeters. CALIBRATION OF RADIAT
44、ION INSTRUMENTSa. Radiological protection instrumentsRadiation protection survey instruments such as ionization chamber, Geiger-Muller counter, scintillation counter and solid-state detector are calibrated in terms of exposure, air kerma, dose equivalent and ambient dose equivalent as well as direct
45、ional dose equivalent. Reference photon radiation selected from ISO Standard 4037-19 are used for calibration of radiation survey instruments and for determination of their energy response. For the x and gamma radiation qualities, the conversion coefficients given in ISO Standard 4037-312 were used
46、for conversion from air kerma to ambient dose equivalent, H*(10) or directional dose equivalent, H(0.07). Reference conditions are temperature = 20.0 0C, pressure = 1013.25 mbar and relative humidity (RH) = 50 %. H*(10) and H(0.07) are new operational quantities for area monitoring for external radi
47、ation sources introduced by ICRU Report 39 1 and intended mainly for the measurements of strongly penetrating and weakly penetrating radiations respectively. Calibrations are performed either by the substitution method (comparing the response of the instrument to be calibrated with that of a referen
48、ce standard instrument) or simultaneous method (both instrument to be calibrated and reference standard instrument are placed in the radiation beam at the same time and irradiated together). These methods are normally used when the radiation survey meters are calibrated at x-ray beams. Calibration i
49、s performed in a known radiation field when the survey meters are calibrated against gamma, beta and neutron beams 7,13. Standardization of 137Cs, 60Co, and 241Am gamma sources (protection level) at various distances using reference standard dosimeters are performed once a year with deviation of less than 2%. Calibration with beta radiation is performed using beta secondary standard sources which the absorbed dose rates at particular distances of each sources i.e. 90Sr/90Y, 85Kr and 147Pm have been determine
