Faculty of Science, University of Kragujevac R. Domanovic 12, Kragujevac 34000, Serbia 2

Transcription

1 ABSORBED FRACTIONS IN SENSITIVE REGIONS OF HUMAN RESPIRATORY TRACT CALCULATED BY MCNP5/X SOFTWARE FOR ELECTRONS AND BETA PARTICLES DUE TO RADON PROGENY * D. KRSTIC 1*, D. NIKEZIC 1, V.M. MARKOVIC 1, D. VUCIC 2 1 Faculty of Science, University of Kragujevac R. Domanovic 12, Kragujevac 34000, Serbia 2 Institute on Occupational Health Protection NIS Vojislav Ilic bb, Nis 18000, Serbia * Corresponding author. E mail: dragana@kg.ac.rs Tel ; Fax Received November 15, 2012 Radon, 222 Rn, is radioactive noble gas which decays by alpha emission with half-life of d. Its short-lived progeny, 218 Po, 214 Pb and 214 Bi ( 214 Po), are alpha and beta radioactive and they emit gamma radiation as well. Radon progeny can be inhaled by humans where they deposit on the inner layers of bronchi, and bronchioles. Particles (alpha, beta and gamma) emitted in radioactive decay damage surrounding tissue which can lead to development of lung cancer. The Absorbed Fractions (AF) of electrons and beta particles in sensitive layers of human respiratory tract were calculated in this paper. For that purpose the MCNP5/X simulation software [1], based on Monte Carlo method, was used. The human respiratory tract was modeled according to ICRP66 publication [2]. Key words: radon progeny, electrons absorbed fractions. INTRODUCTION Inhalation of the short-lived radon decay products ( 218 Po, 214 Pb, 214 Bi/ 214 Po) in homes, in the outdoor atmosphere and at work places contributes the largest fraction to the natural radiation exposure of population [3, 4]. Radon or rather its decay products, are known to represent a risk of lung cancer when inhaled. Besides alpha radiation from 218 Po and 214 Po, beta and following gamma radiation are produced during the decay of radon progeny. More specifically, 214 Pb and 214 Bi are short-lived radon progeny, which decay with subsequent emission of beta particles * Paper presented at the First East European Radon Symposium FERAS 2012, September 2 5, 2012, Cluj-Napoca, Romania. Rom. Journ. Phys., Vol. 58, Supplement, P. S164 S171, Bucharest, 2013

2 2 Absorbed fractions in sensitive regions of human respiratory tract S165 followed by intensive gamma radiation. Inhaled radionuclides deposit in various regions of Human Respiratory Tract (HRT), where are decaying, and irradiating surrounding tissue. For this reason, it is necessary to model respiratory tract and investigate the effect of radiation produced in decaying processes. The Human Respiratory Tract Model (HRTM) was described in ICRP66 publication [2]. More recently the guidance for application of HRTM was given by ICRP Annals in Vol. 32 [5]. According to these publications there are six tissues in HRT that are potentially at risk from inhaled radioactive materials. Bronchial region (denoted by BB) and the bronchiolar region (bb) of the HRTM are deposition sites of inhaled radionuclides, and in their structure ciliated epithelium is included, which is sensitive tissue. To evaluate AF s, Monte Carlo Electron Gamma Shower transport code EGS4 [6] was used in ICRP66 publication. The code models the production of both knock-on electrons and bremsstrahlung above a certain energy threshold (taken to be 1 kev). In those calculations, a practical upper limit for energy loss in each scattering event was set at 6% of the current electron energy. In this paper MCNP5/X software was used to calculate absorbed fractions of electrons in sensitive tissues. The code treats an arbitrary three-dimensional configuration of materials in geometric cells bounded by first- and second-degree surfaces and fourth-degree elliptical tori. Point wise cross-section data were used. For photons, the code accounts for incoherent and coherent scattering, the possibility of fluorescent emission after photoelectric absorption, absorption in pair production with local emission of annihilation radiation, and bremsstrahlung. A continuous-slowing-down approximation was used for electron transport that includes positrons, K, x-rays, and bremsstrahlung, but does not include external or self-induced fields. The user creates an input file that is subsequently read by MCNP. This file contains information about the problem such as: the geometry specification, the description of materials and the location and characteristics of the source, selection of cross-section evaluations, the type of answers or tallies desired, and any variance reduction techniques used to improve efficiency. Input file also contains information about type of radiation emitted by the source, which could be neutrons, photons, electrons. Newer version as MCNPX includes heavy charged particles as well. The main advantage of MCNP over EGS4 is probably its advanced geometry coding tools. One drawback of MCNP in comparison to EGS4 is that the latter often runs a similar problem faster. The main differences between the two codes are different cross sections and electron transport algorithms used. For MCNP the electron approximations are valid for an arbitrary angle while the algorithms used in EGS4 assumes the scattered angle to be small. This is the reason why AF of electrons in sensitive layers was recalculated in this work using MCNP.

3 S166 D. Krstic et al. 3 METHODOLOGY To calculate absorbed fractions in sensitive layers of HRT regions, bronchies and bronchioles were modeled as proposed in ICRP66 [2]. Simplified geometrical model of cross section is presented on Fig. 1. Dimensions are given in ICRP66 [2] but not includes in Fig 1. Sources of electrons are mucus and cilia layers where deposition of radionuclides occurs. In principle mucus and cilia layers present sources of beta particles since these regions are deposition sites of radon progeny. Emission energy of beta particle is according to beta spectra of corresponding radionuclide. In order to determine energy absorbed in target layers it is necessary to know absorbed fraction for initial energy of beta particle. In this way absorbed fraction is function of initial beta particle energy. To determine function AF(E) simulation was performed for large number of monoenergetic electrons. This simulation was performed for different initial electrons energy in range from 10 kev to 10 MeV. Starting points of electrons were randomly chosen within source layers, as well as direction of electron movement. Transport of electrons through tissue was completely followed including secondary particles that were created along the path. Energy deposited in sensitive layers from primary electrons and secondary particles were scored. Fig. 1 Simplified geometrical model of various sources and targets involved in dosimetry of bronchial and bronchiolar regions, [5], (bb doesn t contain Basal cells). The desired information from a Monte Carlo simulation was obtained by a tally, i.e. predefined algorithms that score the contribution for each particle. MCNP tallies were normalized to be per starting particle and are printed in the output. The pulse height tally F8 with asterisk (*F8 tally), was used to provide the energy deposition in a cell.

4 4 Absorbed fractions in sensitive regions of human respiratory tract S167 Summing up deposited energy from N simulated electrons, total absorbed energy was obtained. For initial energy, E, of electron the absorbed fraction, AF, is given as to: AF E Dep = = N E * F8 E (1) RESULTS AND DISCUSSION The simulations were performed by MCNP code, for sources placed in various parts of BB and bb region. Calculated AF for monoenergetic electrons are shown in Figs. 2 to 7 as an open circle. Obtained data are compared with ICRP66 publication (full circles). AF s in BB region, where source is in fast clearance mucus and targets are secretory and basal cells are presented in Figs. 2 and 3. AF s shown in Figs. 4 and 5 are given for sources in slow clearing mucus for BB region. In addition, AF s in bb region, where source is in fast and slow clearance mucus, are presented in Figs. 6 and 7. Fig. 2 Absorbed fractions in function of initial electron energy for source in mucus layer where target is secretory cells of bronchial - BB region.

5 S168 D. Krstic et al. 5 Fig. 3 Absorbed fractions in function of initial electron energy for source in mucus layer where target is basal cells of bronchial BB region. Fig. 4 Absorbed fractions in function of initial electron energy for source in cilia layer where target is secretory cells of bronchial BB region.

6 6 Absorbed fractions in sensitive regions of human respiratory tract S169 Fig. 5 Absorbed fractions in function of initial electron energy for source in cilia layer where target is basal cells of bronchial BB region. Fig. 6 Absorbed fractions in function of initial electron energy for source in mucus layer where target is secretory cells of bronchiolar bb region.

7 S170 D. Krstic et al. 7 Fig. 7 Absorbed fractions in function of initial electron energy for source in cilia layer where target is secretory cells of bronchiolar bb region. One can see very good agreement for all data for larger electron energies. There are discrepancies in results for electron energies below 100 kev between AF calculated with EGS4 and MCNP. EGS4 values for AF rapidly decreases and goes to null at about 40 kev, depending of position of source, and target. These values are much smaller than MCNP s. The difference in results for low energies is consequence of working methods build in EGS4 and MCNP. Multiple scattering theories used in EGS4 gives good results for higher energies (about 100 kev and above), while this method is not accurate for low energies. On the other hand, MCNP can accurately simulate electrons scattering for energies down several kevs. In bb region disagreement is found in across investigate energy range. Simulation with EGS4 code is not suitable for thin layers like those in bb region. For larger electron energies multiple elastic scattering has to be switched off when track length excides distance to nearest surface [7]. Because of this, stabilization for short step lengths does not necessarily imply that simulation results are correct. Consequently, the errors during simulation will be larger. Differences in results for low energies could be of great importance when calculate AF for beta spectrum of various radionuclides, due to the large fraction of beta particles with small energy. Application of AF for monoenergetic electrons obtained in this work for calculation of AF for spectrum might produce very different results when it is done with AF recommended by ICRP66.

8 8 Absorbed fractions in sensitive regions of human respiratory tract S171 CONCLUSION Interaction of beta radiation with tissue of HRT were simulated in this work. AFs for monoenergetic electrons in sensitive layer of HRT were calculated using MCNP5/X code. Results showed the differences in range of electron energies to up 100 kev in BB region. However, in bb region, discrepancy was found in all investigated range of energy. Results led to conclusion that AFs from betas given in ICRP66 should be corrected for several percents. Absorbed fractions in sensitive layers were calculated previously in reference [8], using PENELOPE software [9] for simulation of radiation transport. Results presented here are in very good agreement with [8], and discrepancies in comparison with ICRP66 for bb region are also evident. Because of small thickness of layers it is necessary to perform detail simulations in order to avoid large errors. According to [10] DCF from beta particles of radon progeny calculated with results from [8] is 0.21 msv/wlm. When compared to DCF from alpha particles this value comprise of about 2% of alpha dose. REFERENCES 1. X-5 Monte Carlo Team, MCNP a General Monte Carlo N-Particle Transport Code, Version 5 Vol. I: Overview and Theory, Los Alamos, NM: Los Alamos National Laboratory; LA- UR , ICRP Human respiratory model for radiological protection. A report of a task group the International Commission on radiological protection, ICRP Publication 66, Pergamon, Oxford, NCRP Report no. 93 Ionizing radiation exposure of the population of the United States. National Council on Radiation Protection and Measurements, Bethesda, Maryland, ICRP Limits for inhalation of radon daughters by workers. A Report of a Task Group the International Commission on Radiological Protection, ICRP Publication 32. Pergamon Press, Oxford, Vol 6, Issue 1, 1 24, Annals of the ICRP Guide for the Practical Application of the ICRP Human Respiratory Tract Model - ICRP Supporting Guidance 3Approved by ICRP Committee 2 in October 2000, Volume 32, Number 1, 13 14, Nelson WR, Hirayama H, Rogers DWO The EGS4 code system. Report SLAC-265, Stanford Linear Accelerator Center, Stanford, Fernández -Varea J M, Mayol R J, Baró, Salvat F, On the theory and simulation of multiple elastic scattering of electrons. Nucl. Instrum. Meth. B 73, , Markovic V, Stevanovic N, Nikezic D, Absorbed fractions for electrons and beta particles in sensitive regions of human respiratory tract. Radiat Environ Biophys 47: , Salvat F, Fernández-Varea JM, Sempau J PENELOPE 2006 a code system for Monte Carlo simulation of electron and photon transport. OECD Nuclear Energy Agency, Issy-les- Moulineaux, Markovic V M, Stevanovic N, Nikezic D, Doses from beta radiation in sensitive layers of human lung and dose conversion factors due to 222Rn/220Rn progeny, Radiat Environ Biophys, 50(3), , 2011.