- Email: [email protected]
Diaphragm correction factors for the FAC-IR-300 free-air ionization chamber
Author’s Accepted Manuscript Diaphragm correction factors for the FAC-IR-300 free-air ionization chamber Seyed Mostafa Mohammadi, Hossein TavakoliAnbaran www.elsevier.com/locate/apradiso
PII: DOI: Reference:
S0969-8043(17)30890-4 https://doi.org/10.1016/j.apradiso.2017.11.030 ARI8179
To appear in: Applied Radiation and Isotopes Received date: 24 July 2017 Revised date: 27 November 2017 Accepted date: 27 November 2017 Cite this article as: Seyed Mostafa Mohammadi and Hossein Tavakoli-Anbaran, Diaphragm correction factors for the FAC-IR-300 free-air ionization chamber, Applied Radiation and Isotopes, https://doi.org/10.1016/j.apradiso.2017.11.030 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Highlights
Diaphragm scattering correction factor varies between 0.9997 and 0.9948 in the 20 - 300 keV range. Diaphragm transmission correction factor decreases from 1.0000 to 0.9965 in the same energy range.
Diaphragm correction factors for the FAC-IR-300 free-air ionization chamber Seyed Mostafa Mohammadi 1,2, Hossein Tavakoli-Anbaran 1 1
2
Faculty of Physics, Shahrood University of Technology, P.O. Box 3619995161, Shahrood, Iran
Department of Radiation Dosimetry, Secondary Standard Dosimetry Laboratory, Alborz Research Institute, Karaj, Iran
Abstract A free-air ionization chamber FAC-IR-300, designed by the Atomic Energy Organization of Iran, is used as the primary Iranian national standard for the photon air kerma. For accurate air kerma measurements, the contribution from the scattered photons to the total energy released in the collecting volume must be eliminated. One of the sources of scattered photons is the chamber's diaphragm. In this paper, the diaphragm scattering correction factor, kdia, and the diaphragm transmission correction factor, ktr, were introduced. These factors represent corrections to the measured charge
Corresponding author: [email protected]
(or current) for the photons scattered from the diaphragm surface and the photons penetrated through the diaphragm volume, respectively. The kdia and ktr values were estimated by Monte Carlo simulations. The simulations were performed for the monoenergetic photons in the energy range of 20 - 300 keV. According to the simulation results, in this energy range, the kdia values vary between 0.9997 and 0.9948, and ktr values decrease from 1.0000 to 0.9965. The corrections grow in significance with increasing energy of the primary photons.
Keywords: Free-air ionization chamber; diaphragm scatter correction factors, Monte Carlo simulation. 1. Introduction Free-air ionization chambers (FAC) are currently used in the primary standard dosimetry laboratories (PSDLs) to establish primary medium- and low-energy X-rays air-kerma standards [1]. The free-air ionization chamber FAC-IR-300, designed by the Atomic Energy Organization of Iran (AEOI), is used as the Iranian national primary Xray air kerma standard [2]. The charge produced by scattered photons does not play a role in the definition of the air-kerma [3]. The most important sources of the scattered photons are the air inside the ion chamber volume and the diaphragm. In this study, we focused on the role of the diaphragm in the photon scattering. The diaphragm disperses the photons in two ways, namely, as a result of scattering from the diaphragm surface and due to transmission through the diaphragm body. To estimate the effect of the photon scatter from the diaphragm surface on the measured charge, the diaphragm scatter correction factor, kdia, was introduced. Moreover, to estimate the contribution of the photon transmission through the diaphragm body, the diaphragm transmission correction factor, ktr, was introduced. In this paper, Monte Carlo simulations were used to estimate these correction factors. 2
2. Materials and methods 2.1.Characteristics of the free-air ionization chamber X-ray dosimetry is based on air-kerma (Kinetic Energy Released in Matter). For freeair ionization chamber measurements, air-kerma is determined by [4]: -
∏
(1)
where m is the mass of the air inside a cylindrical volume determined by the diaphragm aperture area and the collecting volume effective length, leff, Q is the collected charge, Wair/e is the mean energy required to generate an ion pair in dry air, and g is the fraction of electron energy lost due to the bremsstrahlung effect in air (negligible for the 8 - 300 keV photons [5]). The effective length leff comprises the collector length lc and the gap length lg. The ∏
factor in Eq. (1) is
∏
(2)
where kat, kTP, kh, ks, kp, kfield, ke, ksc, kdia and ktr are the corrections for the air attenuation, air temperature, air pressure, air humidity, ion recombination, polarity effect, electric field distortion, electron loss, photon scattering in the air, diaphragm scatter and diaphragm transmission, respectively. The quantities kat, kTP, kh, ks and kp are the correction factors introduced in Ref. [2]. In Refs. [6] and [7], the values of the kfield, ke and ksc correction factors were calculated by the authors of this paper. In the present work, the kdia and ktr correction factors were estimated by a Monte Carlo simulation for the medium energy X-rays in the range of 20 - 300 keV.
2.2. The kdia and ktr correction factors Any charge originating from the secondary photons is not included in the definition of air kerma [8]. One of the sources of secondary photons is the scatter from the diaphragm surface. When photons reach the diaphragm, some of them pass through the 3
diaphragm aperture without any scattering, as shown by the blue beam in Fig. 1, while others are scattered from the diaphragm surface and continue their path into the chamber (red beam at Fig. 1). In order to correct for the contribution of these scattered photons into the measured charge, the diaphragm scattering correction factor kdia was introduced. In this work, the kdia correction factor is calculated by the Monte Carlo simulation. The kdia factor is obtained by equation [9]: (3) where Ep is the energy transmitted by the primary photons (all photons entering the chamber through the aperture without an interaction with any component of the chamber) and Edia is the energy transmitted by the photons scattered from the diaphragm surface. Ep and Edia are calculated in the collecting volume (gray area in Fig. 1). As one can see in Fig. 2, due to the limited size of the diaphragm, some photons can always pass through the diaphragm body and reach the collecting volume (green beam in Fig. 2). In order to take these photons into account, a transmission correction factor, ktr, was introduced and, like kdia, also calculated by a Monte Carlo simulation. The value of ktr was obtained from the equation ,
(4)
where Etr is the energy transmitted by the photons passed through the diaphragm body [9]. The values of Ep and Etr in the collecting volume were calculated. Fig. 1. A diagram illustrating the calculation of the ktr factor.
2.3. Simulation Fig. 3 shows a diagram of the simulation geometry. The calculation comprised 108 histories. The computential code based on the Monte Carlo method, MCNP [10], was used. The cut-off energy was 5 keV. The photo-atomic cross-section library mcplib22 and the el032 electron library were used to simulate the photon and electron transport, 4
respectively. In accordance with the AAPM TG-61 protocol for the kilovoltage X-ray beam dosimetry [11], the Monte Carlo simulations were performed for the distance between the source and chamber reference point equal to 100 cm. The FAC-IR-300 diaphragm is made of tungsten; it is 10-mm thick and has a cylindrical aperture. Other geometric parameters of the FAC-IR-300 ionization chamber are presented in Ref. [2]. To calculate kdia and ktr from Eq. 3 and 4, one must distinguish between the primary, scattered and transmitted photons. The primary photons pass through the diaphragm aperture without interaction, and their energies not change. However, the energies of the photons scattered from the diaphragm surface and of the photons transmitted through the body of the diaphragm are smaller. In determining the contribution from photons scattered from the diaphragm surface, we divided the photons that arrive at the collecting volume in two parts: those whose their energy has not changed (primary photons) and those whose energy has reduced (scattered photons). In determining the contribution from the transmitted photons, we assumed that the photons could not pass through the diaphragm aperture and only the photons trespassed the body of the diaphragm reached the collecting volume.
3. Results and discussion Values of kdia and ktr depend on the source-chamber distance very slightly. Fig. 4 shows the values of kdia resulted from the simulation. There is a valley around 70 keV, which corresponds to the tungsten K-edge absorption [12]. The scatter from the diaphragm surface increases with increasing photon energy, and kdia gets farther from unity. For FAC-IR-300 in this range of energy, the kdia factor varied from 0.9997±0.0004 to 0.9948±0.0004. Fig. 5 shows the simulation results for ktr. With increasing energy of the primary photons, the probability of the photon transmission through the diaphragm body 5
increases [13], and the ktr becomes smaller. For FAC-IR-300, in the energy range from 20 to 300 keV, ktr varies between 1.0000±0.0005 and 0.9965±0.0002.
4. Conclusions In this work, the effect of the diaphragm on the response of a free-air ionization chamber was studied. For this purpose, the diaphragm scatter correction factor, kdia, and diaphragm transmission correction factor, ktr, were introduced. Then, Monte Carlo simulations were used to obtain kdia and ktr. According to the simulation results in the energy range of 20 - 300 keV for the FAC-IR-300 ionization chamber, the kdia values vary from 0.9997 to 0.9948 and the ktr values change from 1.0000 to 0.9965. The simulation results show that kdia and ktr strongly depend on the primary photon energy and become more significant with increasing photon energy.
References 1. 2. 3. 4. 5. 6. 7. 8.
Uei-Tyng Lin and C.-H. Chu, Correction factors for the INER-improved freeair ionization chambers calculated with the Monte Carlo method. Applied Radiation and Isotopes, 2006. 64: p. 608-614. Mohammadi, S.M., H. Tavakoli-Anbaran, and H.Z. Zeinali, Free-air ionization chamber, FAC-IR-300, designed for medium energy X-ray dosimetry. Journal of Instrumentation, 2017. 12(01): p. T01008. Burns, D.T. and L. Büermann, Free-air ionization chambers. Metrologia, 2009. 46(2): p. S9-S23. Ksouri, W., et al., Dosimetric Standard for Continuous X-Rays Radiation Fields of Low and Medium-Energies (< 300 kV), in Transverse Disciplines in Metrology. 2010, French College of Metrology. p. 221-232. National Physical Laboratory, Practical Course in Reference Dosimetry for kV X-ray Dosimetry. 2010. Mohammadi, S.M., H. Tavakoli-Anbaran, and H.Z. Zeinali, Investigation of electric field distribution on FAC-IR-300 ionization chamber. Journal of Instrumentation, 2016. 11(07): p. P07017. Mohammadi, S.M., H. Tavakoli-Anbaran, and H.Z. Zeinali, Investigation of electron-loss and photon scattering correction factors for FAC-IR-300 ionization chamber. Journal of Instrumentation, 2017. 12(02): p. P02002. Burns, D.T., et al., Preliminary Characterization of the NIS free air chamber standard at the BIPM, in Rapport BIPM-09/02. 2009, BIPM. p. 12. 6
9. 10. 11. 12. 13.
Groetz, J.E., et al., Conception and realization of a parallel-plate free-air ionization chamber for the absolute dosimetry of an ultrasoft X-ray beam. Rev Sci Instrum, 2014. 85(083304): p. 1-9. Judith F Briesmeister, MCNP-A general Monte Carlo N-particle transport code. Version 4C ed. 2000. The American Association of Physicists in Medicine (AAPM), AAPM protocol for 40–300 kV x-ray beam dosimetry in radiotherapy and radiobiology. 2011. National Physical Laboratory, http://www.kayelaby.npl.co.uk/atomic_and_nuclear_physics/4_2/4_2_1.html. National Institute of Standards and Technology, http://physics.nist.gov/PhysRefData/XrayMassCoef/tab3.html.
7
Lead shield
Collecting volume
X-ray source
Diaphragm Lead shield
Fig. 1. A diagram illustrating the calculation of the kdia factor.
8
Collecting
Lead shield
volume
X-ray source
Diaphragm Lead shield
Fig. 2. A diagram illustrating the calculation of the ktr factor.
9
Diaphragm
X-ray source 100 cm
Lead box
Fig. 3. Geometry used in the Monte Carlo simulations.
10
Fig. 4. Diaphragm scatter correction factor as a function of the photon energy. The error bars correspond to 1σ.
11
Fig. 5. Diaphragm transmission correction factor as a function of the photon energy. The error bars correspond to 1σ.
12
PII: DOI: Reference:
S0969-8043(17)30890-4 https://doi.org/10.1016/j.apradiso.2017.11.030 ARI8179
To appear in: Applied Radiation and Isotopes Received date: 24 July 2017 Revised date: 27 November 2017 Accepted date: 27 November 2017 Cite this article as: Seyed Mostafa Mohammadi and Hossein Tavakoli-Anbaran, Diaphragm correction factors for the FAC-IR-300 free-air ionization chamber, Applied Radiation and Isotopes, https://doi.org/10.1016/j.apradiso.2017.11.030 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Highlights
Diaphragm scattering correction factor varies between 0.9997 and 0.9948 in the 20 - 300 keV range. Diaphragm transmission correction factor decreases from 1.0000 to 0.9965 in the same energy range.
Diaphragm correction factors for the FAC-IR-300 free-air ionization chamber Seyed Mostafa Mohammadi 1,2, Hossein Tavakoli-Anbaran 1 1
2
Faculty of Physics, Shahrood University of Technology, P.O. Box 3619995161, Shahrood, Iran
Department of Radiation Dosimetry, Secondary Standard Dosimetry Laboratory, Alborz Research Institute, Karaj, Iran
Abstract A free-air ionization chamber FAC-IR-300, designed by the Atomic Energy Organization of Iran, is used as the primary Iranian national standard for the photon air kerma. For accurate air kerma measurements, the contribution from the scattered photons to the total energy released in the collecting volume must be eliminated. One of the sources of scattered photons is the chamber's diaphragm. In this paper, the diaphragm scattering correction factor, kdia, and the diaphragm transmission correction factor, ktr, were introduced. These factors represent corrections to the measured charge
Corresponding author: [email protected]
(or current) for the photons scattered from the diaphragm surface and the photons penetrated through the diaphragm volume, respectively. The kdia and ktr values were estimated by Monte Carlo simulations. The simulations were performed for the monoenergetic photons in the energy range of 20 - 300 keV. According to the simulation results, in this energy range, the kdia values vary between 0.9997 and 0.9948, and ktr values decrease from 1.0000 to 0.9965. The corrections grow in significance with increasing energy of the primary photons.
Keywords: Free-air ionization chamber; diaphragm scatter correction factors, Monte Carlo simulation. 1. Introduction Free-air ionization chambers (FAC) are currently used in the primary standard dosimetry laboratories (PSDLs) to establish primary medium- and low-energy X-rays air-kerma standards [1]. The free-air ionization chamber FAC-IR-300, designed by the Atomic Energy Organization of Iran (AEOI), is used as the Iranian national primary Xray air kerma standard [2]. The charge produced by scattered photons does not play a role in the definition of the air-kerma [3]. The most important sources of the scattered photons are the air inside the ion chamber volume and the diaphragm. In this study, we focused on the role of the diaphragm in the photon scattering. The diaphragm disperses the photons in two ways, namely, as a result of scattering from the diaphragm surface and due to transmission through the diaphragm body. To estimate the effect of the photon scatter from the diaphragm surface on the measured charge, the diaphragm scatter correction factor, kdia, was introduced. Moreover, to estimate the contribution of the photon transmission through the diaphragm body, the diaphragm transmission correction factor, ktr, was introduced. In this paper, Monte Carlo simulations were used to estimate these correction factors. 2
2. Materials and methods 2.1.Characteristics of the free-air ionization chamber X-ray dosimetry is based on air-kerma (Kinetic Energy Released in Matter). For freeair ionization chamber measurements, air-kerma is determined by [4]: -
∏
(1)
where m is the mass of the air inside a cylindrical volume determined by the diaphragm aperture area and the collecting volume effective length, leff, Q is the collected charge, Wair/e is the mean energy required to generate an ion pair in dry air, and g is the fraction of electron energy lost due to the bremsstrahlung effect in air (negligible for the 8 - 300 keV photons [5]). The effective length leff comprises the collector length lc and the gap length lg. The ∏
factor in Eq. (1) is
∏
(2)
where kat, kTP, kh, ks, kp, kfield, ke, ksc, kdia and ktr are the corrections for the air attenuation, air temperature, air pressure, air humidity, ion recombination, polarity effect, electric field distortion, electron loss, photon scattering in the air, diaphragm scatter and diaphragm transmission, respectively. The quantities kat, kTP, kh, ks and kp are the correction factors introduced in Ref. [2]. In Refs. [6] and [7], the values of the kfield, ke and ksc correction factors were calculated by the authors of this paper. In the present work, the kdia and ktr correction factors were estimated by a Monte Carlo simulation for the medium energy X-rays in the range of 20 - 300 keV.
2.2. The kdia and ktr correction factors Any charge originating from the secondary photons is not included in the definition of air kerma [8]. One of the sources of secondary photons is the scatter from the diaphragm surface. When photons reach the diaphragm, some of them pass through the 3
diaphragm aperture without any scattering, as shown by the blue beam in Fig. 1, while others are scattered from the diaphragm surface and continue their path into the chamber (red beam at Fig. 1). In order to correct for the contribution of these scattered photons into the measured charge, the diaphragm scattering correction factor kdia was introduced. In this work, the kdia correction factor is calculated by the Monte Carlo simulation. The kdia factor is obtained by equation [9]: (3) where Ep is the energy transmitted by the primary photons (all photons entering the chamber through the aperture without an interaction with any component of the chamber) and Edia is the energy transmitted by the photons scattered from the diaphragm surface. Ep and Edia are calculated in the collecting volume (gray area in Fig. 1). As one can see in Fig. 2, due to the limited size of the diaphragm, some photons can always pass through the diaphragm body and reach the collecting volume (green beam in Fig. 2). In order to take these photons into account, a transmission correction factor, ktr, was introduced and, like kdia, also calculated by a Monte Carlo simulation. The value of ktr was obtained from the equation ,
(4)
where Etr is the energy transmitted by the photons passed through the diaphragm body [9]. The values of Ep and Etr in the collecting volume were calculated. Fig. 1. A diagram illustrating the calculation of the ktr factor.
2.3. Simulation Fig. 3 shows a diagram of the simulation geometry. The calculation comprised 108 histories. The computential code based on the Monte Carlo method, MCNP [10], was used. The cut-off energy was 5 keV. The photo-atomic cross-section library mcplib22 and the el032 electron library were used to simulate the photon and electron transport, 4
respectively. In accordance with the AAPM TG-61 protocol for the kilovoltage X-ray beam dosimetry [11], the Monte Carlo simulations were performed for the distance between the source and chamber reference point equal to 100 cm. The FAC-IR-300 diaphragm is made of tungsten; it is 10-mm thick and has a cylindrical aperture. Other geometric parameters of the FAC-IR-300 ionization chamber are presented in Ref. [2]. To calculate kdia and ktr from Eq. 3 and 4, one must distinguish between the primary, scattered and transmitted photons. The primary photons pass through the diaphragm aperture without interaction, and their energies not change. However, the energies of the photons scattered from the diaphragm surface and of the photons transmitted through the body of the diaphragm are smaller. In determining the contribution from photons scattered from the diaphragm surface, we divided the photons that arrive at the collecting volume in two parts: those whose their energy has not changed (primary photons) and those whose energy has reduced (scattered photons). In determining the contribution from the transmitted photons, we assumed that the photons could not pass through the diaphragm aperture and only the photons trespassed the body of the diaphragm reached the collecting volume.
3. Results and discussion Values of kdia and ktr depend on the source-chamber distance very slightly. Fig. 4 shows the values of kdia resulted from the simulation. There is a valley around 70 keV, which corresponds to the tungsten K-edge absorption [12]. The scatter from the diaphragm surface increases with increasing photon energy, and kdia gets farther from unity. For FAC-IR-300 in this range of energy, the kdia factor varied from 0.9997±0.0004 to 0.9948±0.0004. Fig. 5 shows the simulation results for ktr. With increasing energy of the primary photons, the probability of the photon transmission through the diaphragm body 5
increases [13], and the ktr becomes smaller. For FAC-IR-300, in the energy range from 20 to 300 keV, ktr varies between 1.0000±0.0005 and 0.9965±0.0002.
4. Conclusions In this work, the effect of the diaphragm on the response of a free-air ionization chamber was studied. For this purpose, the diaphragm scatter correction factor, kdia, and diaphragm transmission correction factor, ktr, were introduced. Then, Monte Carlo simulations were used to obtain kdia and ktr. According to the simulation results in the energy range of 20 - 300 keV for the FAC-IR-300 ionization chamber, the kdia values vary from 0.9997 to 0.9948 and the ktr values change from 1.0000 to 0.9965. The simulation results show that kdia and ktr strongly depend on the primary photon energy and become more significant with increasing photon energy.
References 1. 2. 3. 4. 5. 6. 7. 8.
Uei-Tyng Lin and C.-H. Chu, Correction factors for the INER-improved freeair ionization chambers calculated with the Monte Carlo method. Applied Radiation and Isotopes, 2006. 64: p. 608-614. Mohammadi, S.M., H. Tavakoli-Anbaran, and H.Z. Zeinali, Free-air ionization chamber, FAC-IR-300, designed for medium energy X-ray dosimetry. Journal of Instrumentation, 2017. 12(01): p. T01008. Burns, D.T. and L. Büermann, Free-air ionization chambers. Metrologia, 2009. 46(2): p. S9-S23. Ksouri, W., et al., Dosimetric Standard for Continuous X-Rays Radiation Fields of Low and Medium-Energies (< 300 kV), in Transverse Disciplines in Metrology. 2010, French College of Metrology. p. 221-232. National Physical Laboratory, Practical Course in Reference Dosimetry for kV X-ray Dosimetry. 2010. Mohammadi, S.M., H. Tavakoli-Anbaran, and H.Z. Zeinali, Investigation of electric field distribution on FAC-IR-300 ionization chamber. Journal of Instrumentation, 2016. 11(07): p. P07017. Mohammadi, S.M., H. Tavakoli-Anbaran, and H.Z. Zeinali, Investigation of electron-loss and photon scattering correction factors for FAC-IR-300 ionization chamber. Journal of Instrumentation, 2017. 12(02): p. P02002. Burns, D.T., et al., Preliminary Characterization of the NIS free air chamber standard at the BIPM, in Rapport BIPM-09/02. 2009, BIPM. p. 12. 6
9. 10. 11. 12. 13.
Groetz, J.E., et al., Conception and realization of a parallel-plate free-air ionization chamber for the absolute dosimetry of an ultrasoft X-ray beam. Rev Sci Instrum, 2014. 85(083304): p. 1-9. Judith F Briesmeister, MCNP-A general Monte Carlo N-particle transport code. Version 4C ed. 2000. The American Association of Physicists in Medicine (AAPM), AAPM protocol for 40–300 kV x-ray beam dosimetry in radiotherapy and radiobiology. 2011. National Physical Laboratory, http://www.kayelaby.npl.co.uk/atomic_and_nuclear_physics/4_2/4_2_1.html. National Institute of Standards and Technology, http://physics.nist.gov/PhysRefData/XrayMassCoef/tab3.html.
7
Lead shield
Collecting volume
X-ray source
Diaphragm Lead shield
Fig. 1. A diagram illustrating the calculation of the kdia factor.
8
Collecting
Lead shield
volume
X-ray source
Diaphragm Lead shield
Fig. 2. A diagram illustrating the calculation of the ktr factor.
9
Diaphragm
X-ray source 100 cm
Lead box
Fig. 3. Geometry used in the Monte Carlo simulations.
10
Fig. 4. Diaphragm scatter correction factor as a function of the photon energy. The error bars correspond to 1σ.
11
Fig. 5. Diaphragm transmission correction factor as a function of the photon energy. The error bars correspond to 1σ.
12