Standards for radiation protection and diagnostic radiology at the IAEA dosimetry laboratory, F Pernicka

Tags: calibration, IAEA, radiation protection, IAEA Dosimetry Laboratory, physical quantities, ionization chamber, radiotherapy, ISO, DOSIMETRY LABORATORY, secondary standards, Member States, quality assurance, radiation processing, diagnostic radiology, Medical Radiation Physics, calibration services, measurement, Safety Standard, ICRU Publication, Consultative Committee for Ionizing Radiation, ICRP Publication, Dose Equivalents, External Radiation Sources, ICRU Report, uncertainty, radiation measurements, air pressure, Pergamon Press, BIPM, Current measurement, electrometer, the International Basic Safety Standards, INTERNATIONAL COMMISSION, ionizing radiation, laboratories, quantities, International Measurement System, Standard Dosimetry Laboratories, SSDL Network, calibration purposes, mammography examinations, INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION, Radiation Sources, radiation protection instruments, INTERNATIONAL ORGANIZATION FOR STANDARDIZATION, ionization
Content: STANDARDS FOR RADIATION PROTECTION AND DIAGNOSTIC RADIOLOGY AT THE IAEA DOSIMETRY LABORATORY F. Pernicka, P. Andreo, A. Meghzifene, L. Czap, R. Girzikowsky Dosimetry and Medical Radiation Physics Section, IAEA 1. INTRODUCTION International Standardization in dosimetry is essential for the successful exploitation of radiation technology. The IAEA dosimetry programme is focused into services provided to Member States through the IAEA/WHO Network of Secondary Standard Dosimetry Laboratories (SSDLs), to radiotherapy centres and radiation processing facilities [1]. Radiation protection quantities defined by ICRU [2, 3] and ICRP [4] are used to relate the risk due to exposure to ionizing radiation to a single quantity, irrespective of the type of radiation, which takes into account the human body as a receptor. Two types of quantities, limiting and operational, can be related to basic physical quantities which are defined without need for considering specific aspects of radiation protection, e.g. air kerma for photons and fluence for neutrons. The use of a dosimeter for measurements in radiation protection requires a calibration in terms of a physical quantity together with a conversion from physical into protection quantities by means of a factor or a coefficient. Diagnostic radiology has become the largest contributor to the exposure of the public from man-made ionizing radiation. This is partly due to an enormous development in imaging technologies over past decades that allows to detect many diseases in their early stages thus increasing the probability of a successful treatment. Such systems require comprehensive quality assurance programmes. The physical aspects of any quality assurance programme in diagnostic radiology can be divided into two basic groups: (i) image quality assurance and (ii) radiation protection of patients and staff. Both activities within groups have to be balanced so that the optimal situation is achieved when the
XA9952469 probability of obtaining correct diagnosis is maximized while the patient exposure is minimized. In practice, this requires the measurement of a number of technical and physical parameters of the system and/or procedure that can influence the resulting image and dose. Due to the variety of imaging techniques, the dose descriptors may also vary from one technique to another. In one case it is an air kerma-area product (KAP) that is used to describe the patient exposure, in another case it is the entrance surface dose, etc; as in the case of radiation protection quantities, these dose descriptors require a calibration of the instrument in terms of basic physical quantities. It is one of the main tasks of the SSDLs to disseminate units of the basic physical quantities through appropriate instrument calibration. 2. THE IAEA/WHO NETWORK OF SSDLs The IAEA Dosimetry Laboratory is the central laboratory of the SSDL Network, establishing the link between the users and the International Measurement System. The SSDL Network presently includes 69 laboratories and 6 SSDL National Organizations in 58 Member States; the Network also includes 20 affiliated members, mainly PSDLs, ICRU, BIPM, and other international organizations. The SSDL Network has the responsibility to assure that the services provided by the laboratory members follow internationally accepted metrological standards. At present, this is achieved by providing traceable calibrations for therapy, radiation protection and diagnostic radiology instruments by the IAEA. The traceability is accomplished first with the dissemination of calibration factors for ionization chambers from the BIPM or PSDLs through the IAEA. As a second step, follow-up programmes and dose quality audits (intercomparisons using ionization chambers and TLDs) are implemented for the SSDLs to assure that the standards transmitted to users in Member States are kept within the levels required by the International Measurement System. Data collected between 1985-1998 show on the average that approximately 8% of the laboratories conduct radiotherapy calibrations only, 12% conduct radiation protection calibrations only, and nearly 80% of the laboratories do both type of calibrations. Introduction of quality assurance
12
programmes for diagnostic radiology in many countries requires the calibration of a large amount of measuring equipment; some SSDLs have already started such calibrations while others are considering to start this activity soon. 3. IAEA STANDARDS FOR RADIATION PROTECTION AND DIAGNOSTIC RADIOLOGY The Dosimetry and Medical Radiation Physics Section provides the programmatic responsibility, supervision and manpower required for the measurements at the IAEA Dosimetry Laboratory, where all the equipment is located. This consists of a set of reference radiation beams and instruments for the calibration of ionization chambers and radiation detectors for radiotherapy, radiation protection and diagnostic radiology, thermoluminescence dosimetry (TLD) systems, electron spin resonance (ESR) equipment, and ancillary equipment. Besides, the laboratory has access to two 60Co gammacells for calibration of dosimeters used for radiation processing. The layout of the calibration rooms is shown in Figure 1.
The two irradiation rooms are equipped with the radionuclide and x-ray sources that are operated remotely through their respective control panels located in the control room. This room also contains monitors coupled to video cameras in the irradiation rooms. The ionization chambers or other radiation protection instruments are positioned on the calibration benches where they can be moved into a required distance from the source. Their position is fixed at the calibration distance using a telescope. All high voltage (HV) generators are situated in one room and they are interconnected with the respective x-ray tubes by HV cables. The control room also accommodates a system for measuring the ionization current and/or charge, that consists of electrometers, digital voltmeters, capacitors, bias supplies, barometer and thermometers. Measurements are computer controlled via an IEEE-488 interface using a LabView application to collect data from ionization chambers and monitors; the computer is also used for a basic evaluation of the measurement. The data are shared with other computers through a local network.
MammographyX-rays 122 m x 4 m x 2.9m Figure 1. Layout of the IAEA Dosimetry Laboratory calibration rooms 13
3.1. REFERENCE RADIATION Secondary standards of basic physical quantities for radiation protection and diagnostic radiology are realized at the laboratory through appropriate reference radiation beams and instruments. The laboratory system of reference radiation include collimated 137Cs and 60Co photon beams produced by a mobile irradiator Buchler OB 85, x-ray beams produced by a Philips MG 164/324 x-ray unit and mammography x-ray beams produced by a Senographe DMR unit. A detailed description of the various sources of reference radiation used at the IAEA for calibrations of radiation protection instruments is given in Appendix I. In addition to these reference collimated beams, the laboratory also has a panoramic Buchler OB 34 irradiator. It contains a total of seven 137Cs and 60Co y-sources with activities ranging from 3.7 MBq to 7.4 GBq that produce uncollimated radiation fields. This unit is mainly used for routine checks of radiation protection and environmental monitors. 3.1.1. 137Cs and 60Co gamma ray beams The photon beams from the 137Cs and 60Co sources in the Buchler OB 85 irradiator are collimated with a circular collimator; the resulting diameter of the beams is 750 mm at a distance of 3 m from the source. The instrument to be calibrated is positioned with its reference point on the beam axis and its response compared with that of the reference ionization chamber using the substitution method. Both instruments are positioned in the beam using an alignment system consisting of a. calibration bench, a laser beam and a telescope. The calibration set-up for the Buchler OB 85 irradiator is schematically shown in Figure 2.
'uCoor I J ' C s source
lonization chamber
(Drawing not to scale) Figure 2. Calibration set-up used at the IAEA Dosimetry Laboratory for the calibration of radiation protection secondary standards in 137Cs and 60Co reference gamma ray beams. 3.1.2. ISO narrow spectrum series x-ray beams The two x-ray tubes of the Philips MG 164/324 x-ray unit are used to generate ISO narrow spectrum series x-ray reference radiation [5]. Their beam characteristics are shown in Table I, where the values of the mean energies have been adopted from the ISO document (they have not been established from spectrometry measurements). The laboratory has recently acquired a new high purity germanium (HPGe) spectrometry system that will be used for spectral evaluation of all x-ray reference beams at the laboratory.
Additional niters x-ray tube| Monitor chamber
Diaphragm 1 I
lonization chamber
Field size 0 240 mm Figure 3. Calibration se-upfor ISO x-ray beamqualities at the IAEA Dosimetry Laboratory. The x-ray beams are collimated with a set of circular diaphragms, the resulting diameter of the beams is 240 mm at a distance of 3 m from the focus. The calibration set-up for the x-ray beams is shown in Figure 3. As for the Buchler OB 85 irradiator, the substitution method is used for the calibration of instruments. The output of the xray machine is monitored using a transmission ionization chamber PTW 786-073 and the measurements are corrected for fluctuations.
14
TABLE I. ISO NARROW SPECTRUM SERIES X-RAY BEAMS AT THE IAEA DOSIMETRY
LABORATORY
Radiation Mean
Tube
Added filtration
1S'HVL
quality energy potential
[keV]
[kV] [mm Al] [mm Cu] [mm Sn] [mm Pb]
[mm Al] [mm Cu]
N-40
33
40
1.00 0.22
-
-
2.70
N-60
48
60
1.00 0.59
-
-
0.24
N-80
65
80
1.00
1.85
-
-
0.59
N-100
83
100
1.00 5.30
-
-
1.15
N-120
100
120
1.00
5.00
1.00
-
1.74
N-150
118
150
1.00
-
2.50
-
2.40
N-200
164
200
1.00
2.00
3.00
1.00
4.06
N-250
208
250
1.00
-
2.50
2.50
5.21
N-300
250
300
1.00
-
3.00
5.00
6.19
3.1.3. Mammography x-ray beams
During its meeting in 1996, the SSDL Scientific Committee had recommended extending the experience of the IAEA in the field of standardization at radiotherapy and radiation protection level for the SSDL Network, to the field of diagnostic radiology x-rays.
As a first step, a mammography x-ray unit was acquired because of the importance of mammography examinations world-wide. The mammography reference x-ray qualities at the laboratory are generated by a GE Senographe DMR unit. A high frequency HV generator is used to power the x-ray tube with a useful range of 22-49 kV.
The Senographe DMR is a clinical unit equipped
with a Statorix x-ray tube, model M52.2/GS412-
49, having a dual track rotating anode. One track
is of molybdenum and the other of rhodium.
Electrons emitted from the cathode can be
focused into a small and/or large focus, but only
the large focus (0.3 x 0.3 mm) is used for
calibration purposes. The inherent filtration of
the tube is 0.8 mm of beryllium. The unit is
equipped with a filter wheel that has three
interchangeable
filters
made
of
aluminium (1 mm), molybdenum (0.03 mm) and
rhodium (0.025 mm). The unit arm with the tube
has been fixed in a horizontal position and
adapted to the existing calibration set-up of the
x-ray calibration room. This arrangement allows
positioning the measuring equipment on the
existing calibration bench and also makes use of
the available alignment system. The calibration set-up is shown in Figure 4. Seventeen beam qualities have been established for tube voltages between 23 kV and 40 kV that are equivalent to the NIST mammography calibration beams [6]. The beam parameters are given in Table II.
Diaphragm
lonization chamber
(Dtawing not to «cala)
Field size 0100 mm
Figure 4. Calibration set-up used at the IAEA Dosimetry Laboratory for the calibration of mammography secondary standards.
3.2. REFERENCE INSTRUMENTS Ionization chambers and other equipment are calibrated at the IAEA Dosimetry Laboratory mainly in terms of air kerma free in air. Reference conditions are T=20.0 °C, P=101.325 kPa and R.H.=50%. Calibrations are either made for a system composed of a detector (ionization chamber) plus a readout instrument (electrometer) or for a detector only. All calibrations are performed by the substitution method, comparing the response of the detector to be calibrated with that of a reference instrument.
15
3.2.1. Ionization chambers and electrometers The secondary standards for radiation protection are based on two 1000 cm3 spherical ionization chambers, LS-01 and HS-01 designed and manufactured at the Austrian Research Center at Seibersdorf (OFZS). The energy response of the LS-01 chamber is optimized for measurements of air kerma, Kair, while that of the HS-01 is optimized for the measurement of ambient dose equivalent, H*. The chambers are calibrated in terms of K^ or H* at 137Cs, 60Co and a number of x-ray beam qualities at PTB and BIPM every two years. The energy dependence of the air kerma calibration factor, NK, for the LS-01 chamber
(#114) based on the PTB calibration is shown in Figure 5. Reference ionization chambers used for calibrations of radiation protection instruments are listed in Appendix II. The ionization current from the ionization chambers is measured with a Keithley 6517 or Keithley 617 (for x-rays) electrometer. The leakage current for the system ionization chamber plus electrometer is considered negligible (typically 20-25 fA). The measured current is corrected for temperature and pressure dependence. No corrections for saturation and humidity are applied in the case of radiation protection calibrations, as they are insignificant.
TABLE II. MAMMOGRAPHY REFERENCE BEAMS AT THE IAEA DOSIMETRY LABORATORY
Radiation quality*
Tube potential
Added filtration
1st HVL
Homogeneity coefficient**
[kV]
[mm]
[mm Al]
[Al]
Mo anode
Mo/Mo23
23
0.03 Mo
0.272
0.78
Mo/Mo25
25
0.03 Mo
0.297
0.77
Mo/Mo28
28
0.03 Mo
0.328
0.78
Mo/Mo30
30
0.03 Mo
0.348
0.79
Mo/Mo35
35
0.03 Mo
0.382
0.80
Mo/Rh28
28
0.025 Rh
0.394
0.82
Mo/Rh32
32
0.025 Rh
0.423
0.83
Mo/Mo25x
25
0.03 Mo + 2 Al
0.582
0.98
Mo/Mo28x
28
0.03 Mo + 2 Al
0.611
0.94
Mo/Mo30x
30
0.03 Mo + 2 Al
0.663
0.96
Mo/Mo35x
35
0.03 Mo + 2 Al
0.699
0.84
Rh anode
Rh/Rh25
25
0.025 Rh
0.336
0.77
Rh/Rh30
30
0.025 Rh
0.414
0.76
Rh/Rh35
35
0.025 Rh
0.469
0.75
Rh/Rh40
40
0.025 Rh
0.514
0.76
Rh/Rh30x
30
0.025 Rh + 2 Al
0.797
0.92
Rh/Rh35x
35
0.025 Rh + 2 Al
0.854
0.88
'The beam codes are those used by NIST. They are a combination of the chemical symbol followed by the potential of the tube in kilovolts and a letter "x"for beams attenuated by 2 mm of aluminium. "The homogeneity coefficient is defined as the ratio of the 1" HVL to the 2"d HVL.
16
The stability of the chamber plus electrometer system is checked at regular intervals using a 90Sr check source. The ionization current of the LS01 chamber, measured during the period 19951999, has varied by ±0.3 % around the mean value. During each calibration provided by the IAEA, the air kerma at the reference point is measured with the secondary standard; this value is systematically compared with previous measurements as an additional check of the stability of the system. Results of the measured air kerma rates at the reference point during the period 1997-1999, corrected for the decay of the source are shown in Figure 6. All values measured are within ± 0 . 1 1 % for the 137Cs source and + 0.25 % for the 60Co source. The acceptance limit for these measurements is set at ± 2 standard deviations of all measurements; if a measurement falls outside this limit, the reasons are investigated and the measurement repeated.
1.00 CO O 0.98 0.96 -
o o° o o o
0 o°
0 - o -
new
I
100
1000
Energy (keV)
Figure 5. Energy dependence of the air kerma calibration factor, NKfor the LS-01 reference ionization chamber. A 6 cm3 ionization chamber Radcal 10X5-6M, has been selected as reference for the calibration of mammography equipment. The charge generated in the chamber during the duration of a pulse is collected by a capacitor and measured with a Keithley617 electrometer. The relatively large volume of this chamber allows its use for both entrance and exit beam measurements. The traceability of the beams is to be achieved through its calibration at a PSDL.
1.010
0.990 1/1/97
1/1/99
Figure 6. Measured values of the air kerma at the reference point corrected for source decay for the 137Cs(») and 60Co (o) beams of the Buchler OB 85 irradiator. 3.2.2. Ancillary equipment In addition to the reference radiation beams, ionization chambers and electrometers, the calibration of radiation protection and diagnostic radiology instruments includes diverse instruments such as thermometers, barometers, phantoms, additional ionization chambers and electrometers, etc. These are generally used for routine measurements and various research and development activities in the laboratory. This equipment is listed in Appendix II together with a brief description of its use. 3.3. UNCERTAINTIES OF MEASUREMENTS General guidance on the basic requirements for the calibration and use of radiation protection instruments, e.g. the quantities to be measured, their overall accuracy, etc. has been given by international bodies like ICRU [3, 7], ICRP [4, 8], and IAEA [9]. The overall accuracy of any dosimetry system is determined from the combined effects of a number of uncertainties. The uncertainty of measurements pertaining to the calibration of dosimeters carried out at the IAEA Dosimetry Laboratory has been estimated following the ISO recommendation [10], Determination of uncertainties is not made for each instrument calibrated at the laboratory. Typical values of the type A and type B uncertainties for various components contributing to the overall uncertainty of 17
radiation protection calibrations have been derived, based on measurements of many types of instruments calibrated at the laboratory. They are given in Appendix III together with the combined standard uncertainty (k=l) for 137Cs / 60Co beams and for the narrow spectrum x-ray beams. A similar evaluation of uncertainties for diagnostic radiology level calibration is in preparation. 3.4. QUALITY CONTROL The general need for traceability of radiation measurements is now well established worldwide. This basic principle has become the foundation for all standards. In addition, the quality of sources of traceability has to be controlled and assured by using an appropriate quality assurance programmes. The purpose of such a programme is to ensure quality of measurements through documented policies and procedures. The IAEA Dosimetry Laboratory is operated under an established quality assurance programme [11]. The technical requirements of the programme are based on the guidelines described in the ISO 9000 series documents, specifically Guide 25 [12]. The QA programme includes a quality assurance manual that describes the reference standards available at the laboratory and procedures for their maintenance, the equipment and procedures used for the calibration services and key elements of the quality control programme. The quality control programme defines the stability checks applied to the IAEA secondary standard system, the checks and tests to be performed before the calibration of instruments and the verification of the results of the calibration. It also includes quality audits that are held at regular intervals and record keeping procedures. 4. TLD DOSIMETRY AUDITS AT RADIATION PROTECTION LEVEL A TLD system has been developed within the IAEA Dosimetry programme to verify calibrations provided by SSDLs at the quality of l37Cs y-rays. A series of experiments was conducted to optimize different components of the system and involved a total of five different types of TL materials. 18
A blind test of the system was carried out at the end of 1997. It was followed by two pilot runs in March and June 1998. Twenty five SSDLs were randomly selected to participate in this experiment, the strategy being to cover all continents in each run. During each of the two runs, selected PSDLs were also supplied with sets of dosimeters and asked to irradiate them at 1.5 and lOmGy air kerma. These dosimeters were used as an independent check of the system. The results of these two runs are shown in Figure 7, where ratios between the air kerma stated by the SSDLs and that measured at the IAEA Dosimetry Laboratory are given. Most of the results are within ±3.5% limits. This limit has been set up as an acceptable deviation and is based on the estimated uncertainty of the TL measurements, evaluated at 1.8%. Participants deviating by more than 3.5% were asked to check their calibration system and invited to participate in the next run.
1.15 1.10 g, 1.05 X 1-00 0.95 0.90 0.85
oo
10
15
20
25
Laboratory no.
Figure 7. Ratios of the air kerma stated by SSDLs to the TLD measured value at the IAEA Dosimetry Laboratory during the pilot study. The acceptance limit is ±3.5%. Following the results achieved under this pilot study and the positive feedback from the SSDLs, this service is now established on a routine basis to all SSDL network members. Its first run was completed in June 1999. The results show that about 30% of the SSDLs were outside the acceptance limit of ±3.5%. Those SSDLs were immediately contacted and supported to resolve the discrepancies. The second run is scheduled
for the autumn 1999. A complete report on the results of the 1 st and 2nd run will be published in this SSDL Newsletter. 5. NEW DEVELOPMENTS The ISO document [4] states that "For the low air kerma rate, the narrow-spectrum series and the wide-spectrum series, a reference laboratory shall verify, by a spectrometric study, that the value of the mean energy produced is within ±3%, and the resolution of the spectra is within ±10% of the value listed in the document. " The measurement of x-ray spectra is not a simple task and it requires special equipment. As mentioned above, the IAEA Dosimetry Laboratory has recently acquired a spectrometry system for this task. It consists of a HPGe detector and a multichannel analyzer. At present, the response matrix of the detector is being evaluated and the software for spectra deconvolution prepared. In May 1999, the Dosimetry and Medical Radiation Physics Section organized a consultants meeting whose purpose was to overview the field of dosimetry in diagnostic radiology and advice the IAEA on needs for further developments. The meeting resulted in a number of recommendations; its full report is published in this issue of the Newsletter. As a first step, a secondary standard ionization chamber, to be used for calibrations in 5-150 kV diagnostic x-ray beams, has been purchased. Actions to develop a range of diagnostic x-ray qualities, based on the IEC recommendation [13], have also been initiated. 6. CONCLUSIONS The need for traceability of radiation protection measurements is now well established worldwide. Many Primary Standard Dosimetry Laboratories and the BIPM are offering a calibration service for measurement standards at radiation protection and/or diagnostic radiology level. This basic principle of traceability should not be interpreted as a requirement for an accuracy level comparable to that needed in radiotherapy. Instead, it should imply that radiation protection and diagnostic radiology measurements have to be linked to a primary standard through an unbroken metrology chain. It is only under this conditions that comparisons
of radiation measurements made with different instruments and under different conditions can be made. In the past, the IAEA has recommended that the "traceability principle" be followed for the calibration of radiation protection instruments [14]. More recently, a similar recommendation was emphasized in the International Basic safety standards [9]. In support of the IAEA/WHO Network of SSDLs, the Agency has set up a central Dosimetry Laboratory. The IAEA maintains measurement standards at radiation protection level for the calibration of national standards through the Network of SSDLs. Photon beams at energies ranging from the ISO 4037 narrow spectrum series to 60Co gamma ray beam quality are available for the calibration of instruments. That range of beam qualities is now in the process of being extended to cover mammography beam qualities. In addition, audit programmes, using postal TLDs, are now offered to SSDLs to verify the quality of their calibration services. 19
REFERENCES
[1] ANDREO, P., IZEWSKA, J., MEGHZIFENE, A., MEHTA, K., PERNICKA, F., TOLLI, H., BERA, P., CZAP, L., GIRZIKOWSKY, R., IAEA Dosimetry Programme Report on Activities in 1997-98, CCRI(I)/99-5, The Consultative Committee for Ionizing Radiation (Section I), 14th Meeting, BIPM, Paris (1999).
[2] INTERNATIONAL COMMISSION ON RADIATION UNITS AND MEASUREMENTS, Determination of Dose Equivalents Resulting from External Radiation Sources, ICRU Report 39, ICRU Publication, Bethesda, MD (1985).
[3] INTERNATIONAL COMMISSION ON
RADIATION
UNITS
AND
MEASUREMENTS, Determination of Dose
Equivalents Resulting from External
Radiation Sources - Part 3, ICRU Report 47,
ICRU Publication, Bethesda, MD (1992).
[4] INTERNATIONAL COMMISSION ON
RADIOLOGICAL
PROTECTION,
Recommendations of the International
Commission on Radiological Protection,
ICRP Publication 60, Ann. ICRP, 21(1-3),
Pergamon Press, Oxford (1991).
[5] International Organization for Standardization, X and y
Reference Radiations for Calibrating
Dosimeters and Dose Ratemeters and for
Determining their Response as a Function of Photon Energy, ISO 4037-1979, ISO, Geneva (1979).
[6] COLETTI, J.G., PEARSON, D.W., DeWERD, L.A., O'BRIEN, CM., LAMPERTI, P.J., Comparison of Exposure Standards in the Mammography X-Ray Region, Med. Phys. 24 (1997) 1263.
[7] INTERNATIONAL COMMISSION ON RADIATION UNITS AND MEASUREMENTS, Determination of Dose Equivalents Resulting from External Radiation Sources - Part 2, ICRU Report 43, ICRU Publication, Bethesda, MD (1988).
[8] INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION, General principles of Monitoring for radiation Protection of Workers, ICRP Publication 35, Ann. ICRP, 9(4), Pergamon Press, Oxford (1985). [9] International Atomic Energy Agency, International Basic Safety Standards for Protection Against Ionizing Radiation and for the Safety of Radiation Sources: A Safety Standard, Safety Series No.115 [Jointly sponsored by FAO, IAEA, ILO, OECD/NEA, PAHO, WHO], IAEA, Vienna (1991).
[10] INTERNATIONAL ORGANIZATION FOR STANDARDIZATION, Guide to the Expression of Uncertainty in Measurements, 2nd ed. [Published by ISO in the name of BIPM, IEC, IFCC, IUPAC, IUPAP and OIML], ISO, Geneva (1995).
[11] ANDREO, P., IZEWSKA, J., MEGHZIFENE, A., MEHTA, K., PERNICKA, F., TOLLI, H., QA Activities in Dosimetry and Medical Radiation Physics, in preparation
[12] INTERNATIONAL ORGANIZATION FOR STANDARDIZATION, General Requirements for the Competence of Calibration and Testing Laboratories, Guide 25, ISO, Geneva (1990).
[13] INTERNATIONAL
ELECTROTECH-
NICAL COMMISSION, Medical
Diagnostic x-ray Equipment - Radiation
Conditions for Use in the Determination of
Characteristics, IEC 61267, IEC, Geneva
(1994).
[14] INTERNATIONAL ATOMIC ENERGY
AGENCY, Handbook on Calibration of
Radiation
Protection
Monitoring
Instruments. Calibration of Radiation
Protection Monitoring Instruments,
Technical Reports Series No. 133, IAEA,
Vienna (1971).
20
APPENDIX I
REFERENCE RADIATION BEAMS FOR THE CALIBRATION OF RADIATION PROTECTION AND DIAGNOSTIC RADIOLOGY EQUIPMENT AT THE IAEA DOSIMETRY LABORATORY
Buchler OB 85 Radionuclide 60Co Activity 20.5 GBq (99-01-01) Air kerma rate at the calibration position Radionuclide l37Cs Activity 630.3 GBq (99-01-01) Air kerma rate at the calibration position Height of the source center above the floor Field size Source-to-detector distance Dose equivalent rate of leakage radiation at 100 cm
21.04 nGy/min (99-01-01) 83.66 u-Gy/min (99-01-01) 110 cm 075 cm at 300 cm 300 cm <1 uSv/h
Philips MG 164/324 x-ray unit Metal-ceramic tube MCN 165 with beryllium window, oil cooling Target material Generating potential Tube current Inherent filtration Added filters, changeable Height of x-ray focus above floor Field size Focus-to-detector distance Leakage dose equivalent rate through shutter at 100 cm distance from focus Metal-ceramic tube MCN 321 with beryllium window, oil cooling Target material Generating potential Tube current Inherent filtration Added filters, changeable Height of x-ray focus above floor Field size Focus-to-detector distance Leakage dose equivalent rate through housing at 100 cm distance from focus
tungsten continuously adjustable up to 160 kV continuously adjustable 0.1 to 18 mA at 160 kV 1 mm beryllium medium or heavy filtration 110 cm 024 cm at 300 cm 300 cm < 6 uSv/h tungsten continuously adjustable 16 to 320 kV continuously adjustable 0.1 to 10 mA at 320 kV 3 mm beryllium medium or heavy filtration 110 cm 024 cm at 300 cm 300 cm < 6 uSv/h
Senographe DMR mammography unit Dual target GS 412-49 x-ray tube Target material Target angle with respect to the reference axis Nominal focal spot values Generating potential Tube current Inherent filtration Added filters, changeable Height of x-ray focus above floor Field size Focus-to-detector distance
molybdenum (vanadium-doped), rhodium fO.l-6°, f0.3-20° 0.1 and 0.3 22 to 49 kV in steps of 1 kV 20 to 130 mA 0.8 mm Be 1 mm Al, 0.03 mm Mo, 0.025 mm Rh 120 cm 010 cm at 100 cm 100 cm
21
APPENDIX II
INSTRUMENTS USED FOR THE CALIBRATION OF RADIATION PROTECTION AND DIAGNOSTIC RADIOLOGY EQUIPMENT AT THE IAEA DOSIMETRY LABORATORY
Reference instruments Instrument 1000 cm3 ionization chamber LS-01 1000 cm3 ionization chamber HS-01 1000 cm3 ionization chamber LS-01 6 cm3 ionization chamber Radcal 10X5-6M electrometer Keithley 6517 electrometer Keithley 617 electrometer Keithley 617 capacitor General Radio 1404A voltage cell Eppley Laboratory No.121 mercury barometer Lambrecht 604 thermometer Keithley 8696
Ser. no. 114 102 130 8362 0599918 435176 511853 1202 3267 944016 0707095
Measured quantity K^ for x-rays (40-300 kV), 137Cs,S0Co H* for x-rays (40-300 kV), 137Cs, 60Co Kail for 137Cs and 60Co Kair for x-rays (23-40 kV) produced on Mo or Rh target current, charge current, charge current, charge collected charge voltage standard air pressure air temperature
Traceability BIPM, PTB BIPM, PTB PTB NIST, PTB BEV BEV BEV BEV BEV PTB BEV
Ancillary equipment Instrument 1 cm3 ionization chamber Standard Imaging Ml 1 cm3 ionization chamber Exradin TW-11 0.2 cm3 ionization chamber PTW 23344 monitor chamber PTW 30 363 HV sources (IAEA made) electronic barometer/thermometer MR 5031/6100 precision mercury thermometer Pinco precision aneroid barometer Negretti & Zambra MK2 kVp meter Gammex RMS 232 attenuation filters of defined purity (material from Goodfellow) HVL filters of defined purity (material from Goodfellow) water phantom PTW 4322
Use of instrument measurement of K^for mammography x-ray beams measurement of Kair for mammography x-ray beams measurement of K^for mammography x-ray beams monitoring output of x-ray generators power sources for ionization chambers and monitors measurement of the pressure and temperature in 60Co irradiation room measurement of temperature in the x-ray room measurements of pressure kVp measurements on mammography unit filtration of x-ray beams measurements of HVLs for x-ray beams dose measurements in water
22
APPENDIX III
ESTIMATED STANDARD UNCERTAINTIES FOR THE CALIBRATION AT RADIATION PROTECTION LEVEL OF IONIZATION CHAMBERS AT THE IAEA DOSIMETRY LABORATORY
Calibration in terms of air kerma and ambient dose equivalent in 137Cs / ^Co beams
Source of uncertainty
Type A (%)
Type B (%)
1 Uncertainties related to the IAEA secondary standard 'NK from BIPM (or PSDL) 2Temperature and air pressure correction (kT p) 'Current measurements voltage capacitance time base ""Leakage current Long term stability of the secondary standard Uncertainties related to the instrument to be calibrated* 'Positioning in air at the calibration distance 'Current measurement (user's electrometer) 8Field inhomogeneity 2Temperature and air pressure correction (kTP) 4Leakage current (user's electrometer) Relative combined standard uncertainty
0.85
0.04
0.06
0.01
0.02
0.04
0.04
0.01
0.00
0.00
0.2
0.02
0.1
0.06
0.1
0.04
0.06
0.00
0.00
0.9
Calibration in terms of air kerma and ambient dose equivalent in x-ray beams
Source of uncertainty
TypeA(%)
Type B ( %)
Uncertainties related to the IAEA secondary standard 'NK from BIPM (or PSDL) 2Temperature and air pressure correction (kT P) 'Current measurements voltage capacitance time base 4Leakage current difference in x-ray spectra (BIPM/PSDL-IAEA) Long term stability of the secondary standard Uncertainties related to the instrument to be calibrated*
0.75
0.04
0.06
0.01
0.02
0.04
0.04
0.01
0.00
0.00
0.1
0.2
'Positioning in air at the calibration distance 7Current measurement (user's electrometer) 'Field inhomogeneity 2Temperature and air pressure correction (kT P) 4Leakage current (user's electrometer) Relative combined standard uncertainty
0.02
0.1
0.06
0.1
0.04
0.06
0.00
0.00
0.8
* The values are given for a typical dosimeter and may change slightly with different types of dosimeters.
'The values are given in the calibration certificates issued by BIPMVPSDL. 2The standard deviation fOT the temperature reading is assumed to be 0.1°C. It is also assumed that the real temperature inside the chamber air cavity does not deviate by more than 0.3°C of the measured temperature. 'The current, I, is integrated on an external capacitor until a specified voltage, U, is reached, during a time interval t The current is determined according to I=C U/t. 'For secondary standard class instruments, leakage is negligible. 'This corresponds to the estimated difference between the spectra of the IAEA beam and that of BIPM/PSDL. "The centers of the two chambers are assumed to be within ± 0.1mm at the same distance from the source. 'This applies only to system calibration where the ionization current is read from the user's electrometer. The uncertainty is based on a typical value of secondary standard class electrometer. "Lateral displacement of the chambers and their different sizes can give rise to a small difference in response due to field non-uniformity.
23

F Pernicka

File: standards-for-radiation-protection-and-diagnostic-radiology-at.pdf
Author: F Pernicka
Published: Mon Nov 1 11:21:35 2004
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