Radiation Protection Dosimetry (2006), Vol. 118, No. 3, pp Advance Access publication 6 October 2005

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1 Radiation Protection Dosimetry (2006), Vol. 118, No. 3, pp Advance Access publication 6 October 2005 doi: /rpd/nci353 DOSE BUILD UP CORRECTION FOR RADIATION MONITORS IN HIGH-ENERGY BREMSSTRAHLUNG PHOTON RADIATION FIELDS Haridas G. Nair 1,, M. K. Nayak 1, Vipin Dev 1, K. K. Thakkar 1, P. K. Sarkar 2 and D. N. Sharma 2 1 Health Physics Unit, Centre for Advanced Technology, Indore , India 2 Radiation Safety Systems Division, Bhabha Atomic Research Centre, Mumbai , India Received June , amended August , accepted September Conventional radiation monitors have been found to underestimate the personal dose equivalent in the high-energy bremsstrahlung photon radiation fields encountered near electron storage rings. Depth-dose measurements in a water phantom were carried out with a radiation survey meter in the bremsstrahlung photon radiation fields from a 450 MeV electron storage ring to find out the magnitude of the underestimation. Dose equivalent indicated by the survey meter was found to build up with increase in thickness of water placed in front of the meter up to certain depth and then reduce with further increase in thickness. A dose equivalent build up factor was estimated from the measurements. An absorbed dose build up factor in a water phantom was also estimated from calculations performed using the Monte Carlo codes, EGS-4 and EGSnrc. The calculations are found to be in very good agreement with the measurements. The studies indicate inadequacy of commercially available radiation monitors for radiation monitoring within shielded enclosures and in streaming high-energy photon radiation fields from electron storage rings, and the need for proper correction for use in such radiation fields. INTRODUCTION Bremsstrahlung X-ray photons form the major radiation hazard at electron storage rings. These photons have a continuous energy spectrum, whose maximum energy extends up to the energy of electrons in the ring. Bremsstrahlung X rays in electron storage rings are produced as a result of electron beam losses and their interaction with accelerator structures, mainly vacuum envelopes and residual gas molecules (1,2). Within shielded enclosures of the storage ring, the radiation field is mainly composed of these high-energy photons, whereas outside the radiation shield, photons are of relatively low energy owing to spectral degradation, due to the shield. There are locations like shield joints, holes in the shield structure for accommodating synchrotron radiation beam lines, front ends, etc. through which the high-energy bremsstrahlung photons can stream out to accessible areas. Also, there are occasions during which the accelerator operation crew has to go near the storage ring, within the shielded enclosure, at low electron beam currents for minor adjustments. In all these circumstances, workers are exposed to high-energy photon radiation. For monitoring of these photons no proper radiation survey instruments are commercially available since nearly all conventional radiation monitors have a useful energy response only up to a few MeV. When a high-energy photon is incident on a medium, an energetic electron and positron pair is Corresponding author: haridas@cat.ernet.in formed, as pair production is the dominant interaction mechanism at high photon energies. After the pair formation, the energetic electron and positron radiate out photons by radiative loss and these photons in turn produce further pairs, giving rise to a cascade of electrons, positrons and photons (3) in the medium. As a result, the particle fluence increases with depth of the medium and their mean energy decreases. Therefore, energy deposition in the medium increases (4) with depth giving rise to build up in absorbed dose. Thus, absorbed dose and, hence, the dose equivalent would be higher, deep within the body than the dose equivalent indicated by a conventional radiation survey meter. Conventional radiation monitors, usually calibrated in terms of personal dose equivalent, H p (10), when used to survey such high-energy photon radiation field do not indicate the true dose received by the worker as they show only the dose equivalent at a depth of 10 mm in tissue. Though the dose build up effect is taken into account in high-energy electron accelerator shielding applications and in electron beam shutter designs (5,6) it is usually not accounted for in monitors used for radiation protection purposes. Therefore, a study was carried out in the high-energy photon radiation field at Indus-1 (a 450 MeV electron storage ring (7), operational at the Centre for Advanced Technology, Indore, India) to find out the dose equivalent build up factor using a conventional radiation survey meter and a water phantom. In the study, a Victoreen survey meter, model 450 P (which is a pressurised ion chamber based survey meter, used routinely for Ó The Author Published by Oxford University Press. All rights reserved. For Permissions, please journals.permissions@oxfordjournals.org

2 H. G. NAIR ET AL. Figure 1. Schematic diagram of the experimental set-up used for the measurement of photon depth-dose profiles in water. radiation surveying in the facility) was used to measure the depth-dose profile in a water phantom. From the measurements, the dose equivalent build up factor was estimated. The build up factor was also calculated using the Monte Carlo codes, EGS-4 (8) and EGSnrc (9). From the dose build up studies, a conservative high-energy correction factor for the survey meter was deduced. DEPTH-DOSE MEASUREMENTS During the operation of the Indus-1 storage ring, depth-dose measurements in water phantom were carried out at (i) the bremsstrahlung photon radiation field near a bending dipole magnet within the shielded enclosure and (ii) the streaming radiation field at a front end, in an accessible area. For the measurements, the Victoreen survey meter was positioned at an appropriate distance from the bending dipole magnet at the median plane (1.25 m from floor level). The schematic diagram of the experimental set-up used for the measurements is shown in Figure 1. Initially the reading of the meter was noted without any water phantom. Thereafter, cylindrical water slabs made out of plastic containers of radius 150 mm and width of 50 mm were added in front of the survey meter and readings were taken at each depth. A similar experiment was carried out at the front end location. The measurement were carried out at a low stored electron beam current of 10 ma. It was essential to carry out the measurement at low beam current as it gave a steady radiation field due to higher beam lifetime in the ring. The depth-dose profile obtained is presented in Dose equivalent rate (msv/h) At front end At bending magnet Depthinwater(cm) Figure 2. Measured photon depth-dose profile at the front end and near the bending magnet of the 450 MeV storage ring, Indus-1. Figure 2. In both cases the dose equivalent rate was found to increase until a certain depth in water and then decrease. The dose equivalent build up factor was calculated as the ratio of the maximum dose equivalent rate with water phantom, recorded by the survey meter to that recorded without water phantom. The maximum dose was found to occur at different depths at each location and the build up factor was also found to be different. 234

3 MONTE CARLO CALCULATIONS Monte Carlo calculations were performed to estimate the dose build up factors. The different build up factors observed during measurement indicates difference in bremsstrahlung spectra at the measurement locations. Therefore to simulate different geometries, the calculation was done in two stages. In stage I, a set of bremsstrahlung spectra was generated by impinging a pencil beam of 450 MeV electrons on different thicknesses of semi-infinite copper Photon fluence (photon / cm 2 /e) DOSE BUILD UP CORRECTION FOR RADIATION MONITORS 3mm 50 mm 100 mm Photon energy (MeV) Figure 3. Calculated bremsstrahlung photon spectra from semi-infinite copper slabs on 450 MeV electron bombardment using EGS-4. plates and scoring the emergent bremsstrahlung spectra in the forward direction with a central axis scoring radius of 5 mm, using the EGS-4 code. Spectra from 3, 6, 15, 33, 50 and 100 mm thick copper targets were calculated. For radiation transport an electron cut-off (ECUT) of 521 kev and a photon cut-off (PCUT) of 10 kev was used in the calculations. Ten million histories were used for the depthdose calculation to have better statistical accuracy. The statistical accuracy obtained in the calculation was within 1%. Figure 3 shows the spectra obtained from 3, 50 and 100 mm thick copper targets. Copper was selected as the target material because materials in and around the storage ring are mainly of medium atomic number. In stage II, the photon spectra calculated in stage I were allowed to be incident as a broad parallel beam on cylindrical water slabs of radius 0.15 m, and the depth dose was scored using the DOSRZ user code from EGSnrc. A schematic diagram of the geometry used for the calculation is shown in Figure 4. Depth-dose curves obtained are presented in Figure 5. The maximum absorbed dose (D max ) and the depth where it occurs, hereafter called the shower maximum (X max ), was observed for each incident spectrum. From the depth-dose curves, dose build up factor was calculated as the ratio of the maximum absorbed dose (D max ) to the absorbed dose at a depth of 1 cm in water. The use of absorbed dose at a depth of 1 cm in water for the calculation of build up factor is justified since, for routine radiation monitoring, dosemeters are usually calibrated in terms of personal dose equivalent or ambient dose equivalent, at a depth of 1 cm in tissue. Therefore, absorbed dose at a depth of 1 cm in water Figure 4. Schematic diagram of the geometry used for Monte Carlo calculations of the depth-dose curves in water. 235

4 H. G. NAIR ET AL. Absorbed dose per incident photon fluence (Gy.cm 2 ) 4.50E E E E E E E E E-12 3mm 6mm 15 mm 33 mm 50 mm 100 mm 0.00E Depth in water (cm) Figure 5. Calculated photon depth-dose curves in water due to various incident photon spectra. corresponds to the dose rate the survey meter indicates without the water phantom. The calculated build up factor plotted as a function of the shower maximum is presented in Figure 6. RESULTS AND ANALYSIS The depth-dose profiles measured in the bremsstrahlung photon radiation field near the bending dipole magnet of the storage ring and in the streaming photon radiation field through the front end (Figure 2) indicate that the dose equivalent rate increases up to a certain depth in water and declines thereafter. The profile near the bending magnet showed that the maximum dose equivalent rate occurred at a depth of 15 cm in water and the build up factor obtained was 2.5. The respective values obtained at the front end were 20 cm and 3.3. The shower maximum and the build up factor obtained at the front end were found to be higher than those at the bending magnet. The different shower maximum and build up factor at each place may be attributed to the difference in bremsstrahlung spectra at these locations. Photon spectra calculated for different thickness of copper target (Figure 3) indicate that the photon spectra emerging from thin targets are more penetrating (harder) than those from thick targets. Since the structural materials in the storage ring vary in thickness from a few millimetres to tens of millimetres, the spectra generated from different thickness of copper targets were used to generate depth-dose curves in water phantom (Figure 5). Dose equivalent build up factor Calculated build up factor Fit Shower maximum in water, X max (cm) Figure 6. Dose equivalent build up factor for various incident bremsstrahlung photon spectra as a function of shower maximum. Table 1. Comparison of dose build up factor obtained from measurements and Monte Carlo calculations. Location Shower maximum, X max (cm) (measured) Build up factor Measured Simulated a Bending magnet Front end a From Equation 1 in the text. The build up factor, B, obtained from the calculated depth-dose curves in water was found to increase with the shower maximum (Figure 6). An exponential fit to the build up factor as a function of the shower maximum, X max assumes the following form B ¼ 1:08 exp½x max =16:9Š ð1þ where X max is in centimetres. The dose build up factors obtained from the exponential fit as a function of the shower maximum were found to be in very good agreement with measured values (Table 1). As the vacuum chamber is generally of only few millimetres thick, harder bremsstrahlung spectra are possible around the ring, and hence the shower maximum shifts to higher values. Therefore, evaluating the fit function for X max ¼ 30 cm, the size of human body, the build up factor works out to be 6.4. This build up factor can be used as a 236

5 DOSE BUILD UP CORRECTION FOR RADIATION MONITORS conservative correction factor for the survey meter reading for radiation protection in the high-energy photon radiation field emerging from the storage ring. Since the corrected dose equivalent is nearly equal to the maximum dose equivalent in a 30 cm diameter sphere consisting of material equivalent to soft tissue with unit density the corrected result equals the dose equivalent index, which is suggested for radiation protection at high-energy accelerator facilities (10). storage ring to carry out experiments. We are highly indebted to Dr R. V. Nandedkar, Head Synchrotron Utilisation Division for technical discussions on the experimental results. We also thank S. K. Gupta for helping us in the use of EGS-4 code for the work. The authors are indebted to Dr M. V. Dingankar and D. S. Joshi, Accelerator Radiation Safety Section, for reviewing the manuscript and giving useful suggestions. We would also like to thank S. Jafar Ali for preparing the drawings. CONCLUSION Experimentally measured dose build up factors around the storage ring, Indus-1, were found to be in very good agreement with those obtained from Monte Carlo simulations. It is concluded from the studies that the radiation field around the storage ring and at streaming locations is comprised of high-energy photons and with spectra varying at different locations. Conventional radiation survey meters when used for monitoring in such radiation fields show only the dose equivalent at 1 cm, which a worker receives, whereas deep within the body the dose received might be much higher than that indicated by the radiation survey meter. Therefore, survey meters, area monitors, personnel dosemeters, etc. when used for measurements in high-energy photon radiation fields have to be properly corrected for high-energy response by applying correction factors deduced from dose equivalent measurements using a water phantom. It is planned to study the dose build up effects in radiation monitors for still higher electron energy electron storage rings. ACKNOWLEDGEMENTS We are highly thankful to Dr H. S. Kushwaha, Director, Health, Safety & Environment Group, Bhabha Atomic Research Centre, for providing all the necessary support and motivation in carrying out the present investigations. The authors acknowledge with gratitude Gurnam Singh, Head, Beam Dynamics Section and his entire team including the acceleration operation crew for providing the REFERENCES 1. Rindi, A. Gas bremsstrahlung from electron storage rings. Health Phys. 42(2), (1982). 2. Ipe, N. E. and Fasso, A. Gas bremsstrahlung considerations in the shielding design of the Advanced Photon Source synchrotron radiation beam lines. Nucl. Instrum. Meth. Phys. Res. A 351, (1994). 3. International Commission on Radiation Units and Measurements. Basic aspects of high energy particle interactions and radiation dosimetry. ICRU Report 28 (Bethesda, MD: ICRU) (1978). 4. Hubbel, J. H. Photon cross-sections, attenuation coefficients and energy absorption coefficients from 10 kev to 100 GeV. NSRDS-NBS Report No. 29 US Department of Commerce, US Government Printing Office (1969). 5. Dinter, H. and Tesch, K. Dose and shielding parameters of electron photon stray radiation from a high-energy electron Beam. Nucl. Instrum. Meth. Phys. Res. 143, (1977). 6. Dinter, H., Pang, J. and Tesch, K. Calculation of electron and photon doses behind beam absorbers at high-energy electron accelerators. Radiat. Prot. Dosim. 28(3), (1989). 7. Angal-Kalinin, D. et al. Synchrotron Radiation source Indus-1. Curr. Sci. 82(3), (2002). 8. Nelson, W. R., Hirayama, H. and Rogers, D. W. O. The EGS-4 code system. Report SLAC-265 (Stanford Linear Accelerator Center, Stanford, CA) (1985). 9. Rogers, D. W. O., Kawrakow, I., Seuntjens, J. P., Walters, B. R. B. and Mainegra Hing, E. NRC user codes for EGSnrc. NRCC Report PIRS-702 National Research Council of Canada. (Rev. B) (2003). Available on EGSnrc.html 10. International Commission on Radiation Units and Measurements. Radiation quantities and units. ICRU Report 19 (Bethesda, MD: ICRU) (1971). 237