«1 An Anthropomorphic Tissue Equivalent Phantom for Radiation Dosimetry A. Zanini(1), E. Durisi(1,2), L. Visca(1,3), F. Fasolo(1,4), M. Perosino(1) ...»
An Anthropomorphic Tissue Equivalent Phantom for Radiation Dosimetry
A. Zanini(1), E. Durisi(1,2), L. Visca(1,3), F. Fasolo(1,4), M. Perosino(1)
(1) INFN Sez. Torino, Via P. Giuria 1 10125 Torino, Italy
(2) Dipartimento di fisica Sperimentale, Università di Torino, Via P. Giuria 1 10125 Torino, Italy
(3) ASP, Viale Sttimino Severo 65 10125 Torino, Italy
(4) FIRMS, Via Valeggio 4 10100 Torino, Italy
A new anthropomorphic phantom has been designed and realized in order to get an economic and simple tool to study the internal dose distribution during the exposition in different radiation field. The phantom, especially developed for neutron dosimetry, is designed following the International Commission on Radiation Units and Measurements (ICRU) recommendations and is calibrated against the ICRU sphere and a standard water phantom by exposure to an Am-Be neutron source (599 GBq activity) at the Joint Research Centre (JRC, Ispra, Italy). The phantom is realized in polyethylene and plexiglas and human bone is inserted in correspondence of column; it is characterized by holes placed in correspondence of critical organs, in which passive detectors can be allocated for neutron and gamma dose evaluation. A full characterization of the phantom is proposed in this paper along with its applications both in medical and environmental field. Exposures in mixed radiation field, such as high mountain laboratories, intercontinental flights and stratospheric balloons, have been carried out and comparisons with MC simulations are presented in this work.
1 Introduction The object of the present study is an anthropomorphic phantom especially developed to investigate the neutron internal dose distribution and exploited for measurements in mixed radiation field.
Any material that simulates a body tissue in its interaction with ionising radiation is termed a tissue substitute ; a structure that contains one or more tissue substitutes and is used to simulate radiation interaction in the body is called phantom. Unlike photons or electrons, neutrons lose energy through interactions with the nuclei of the constituent elements of the tissue. The absorption and scattering processes of the elemental constituents of a body tissue and its substitute are the main concern in neutron applications. Therefore in order to realize a phantom for neutron dosimetry the elemental mass fraction of H, C, N and O, that are the main elements involved in the absorption and scattering reactions, should be the same for the tissue substitute and the “real” one. The choice of a good tissue substitute depends also on the neutron energy range detected. The neutrons RBE (Radio Biological efficiency) becomes important at energy higher than 10 keV, in this range the most important contribution to the absorbed dose comes from elastic scattering with H (69% - 97%) , scattering with C, N and O gives a contribution about 10 times lower, while inelastic scattering and nuclear reactions give a contribution of few percent.
An anthropomorphic phantom for neutron dosimetry, Jimmy, is designed and realized by INFN, section of Turin, in collaboration with Joint Research Centre (JRC, Ispra – Va Italy). It is made up of polyethylene and plexiglas layers and it has human bone in correspondence of column. By the comparison between Jimmy and the ICRU sphere, that is the standard phantom for the definition of operational quantities for dose equivalent, it is evident that the hydrogen content inside both phantoms is comparable (Jimmy 10.2%, ICRU tissue 10.1%). Thus Jimmy represents a well suited tissue substitute for fast neutrons. The anthropomorphic phantom has sixteen holes in correspondence of critical organs in which it is possible to allocate passive dosimeters such as bubble detectors, TLDs, polycarbonate foils and biological samples.
Results of the phantom characterization and measurements, concerning exposures in mixed radiation field along with comparisons with MC calculations, are presented in this paper.
2 Characterization of the anthropomorphic phantom Jimmy The Jimmy backscattering properties are investigated through the comparison with the ICRU sphere and a standard water phantom exposed in front of an Am-Be source. Moreover it was demonstrated that, in despite of the simple geometrical structure, Jimmy phantom could be a reasonable approximation of the much more complicated mathematical phantoms (e.g. MIRD, ADAM, EVA). This was realized using a Monte Carlo code, MCNP4B , through the assessment of the organ absorbed dose conversion factors in an antero-posterior irradiation and through the comparison with International Commission on Radiological Protection (ICRP) data .
Figure 1: Experimental set up of measurements and Albedo response of a TL personal dosimeter on three different phantoms: ICRU sphere, Jimmy and water phantom Massimo. AP exposure in front of Am/Be source Ispra, Va (JRC) (bare source, graphite moderate spectrum and graphite+ Cd moderated spectrum).
The good agreement between the three responses indicates that Jimmy has the same backscattering properties than the standard phantom and that it may be used instead of ICRU sphere during exposition to neutrons.
2.2 Comparison between Jimmy and mathematical phantom The mathematical model of Jimmy phantom was realized using MCNP4B code and it was used to calculated the neutron conversion factors DT/Φ, where DT is the dose in the organ T and Φ is the neutron fluence. Then these factors were compared to the ICRP data achieved with different mathematical phantoms, such as MIRD, EVA and ADAM and different Monte Carlo codes.
Monodirectional and monoenergetic neutron beams, from 10 keV to 20 MeV, were simulated with MCNP4B code and the phantom was irradiated in antero-posterior (AP) condition, in which the ionising irradiation is incident on the front of the body in a direction orthogonal to the long axis of the body. The kerma approximation was used in the calculation since it is considered a good assessment of the dose for neutron energies up to 20 MeV.
Within the body neutrons undergo many interaction by which they lose energy until they are finally absorbed in or escape from the body. In the neutron energy range of interest the production of secondary photons by neutron interactions, (e.g. 1H(n,γ)2H Eγ=2.2 MeV), is of particular importance because of the considerable penetration of this photons in the tissue, for these reason both neutron and photon energy depositions in the
organ T were taken into account:
The comparisons between simulation results and ICRP data are displayed in figure 2 for two organs: gonads and colon.
Figure 2: Organ absorbed dose conversion factors for gonads and colon in AP irradiation, as a function of neutron energy. Comparison between MCNP4B results and ICRP data.
The calculations and the ICRP conversion coefficients are in agreement within 20% for neutron energies higher than 1 MeV, while the differences are within 30% for neutrons below 1 MeV. This represents a good results as indicated in ICRP recommendation.
Therefore it was demonstrated that the mathematical model of Jimmy phantom could be a reasonable approximation of the much more complicated mathematical phantoms.
3 Neutron spectra in depth The phantom was also tested during a preliminary exposure in front of an Am-Be neutron source, in order to compare neutron spectra measured in depth with Monte Carlo calculation.
The experimental set up consist of a Bubble Detector Spectrometer (BDS, manufactured by BTI Bubble Technology Industries) associated with the unfolding code BUNTO , especially developed to analyse the experimental data. The BDS was placed inside the phantom in a cavity at two cm depth in correspondence of gonads to get neutron spectra, while integral dosemeters (BD-100R), calibrated in terms of H according to NCRP definition, were located in all the cavities to measure neutron dose equivalent. In figure 3 the comparison between experimental measurements and simulation, carried out with MCNP4B, shows good agreement from a dosimetric point of view .
Figure 3: Preliminary exposure of the phantom in front of Am/Be source Ispra, Va (JRC): measure of neutron spectra (BDS) in depth and integral dose (BD-100R); comparison with simulation results (MCNP-4B code).
4 Exposure in mixed field
4.1 Enviromental application The recommendations of the ICRP issued in 1991  first included exposure of workers to environmental radiation as occupational exposure. In Europe, attention has been focused by the Council Directive (96/29 Euratom, 1996), which included particular protection (Article 42) to cosmic rays radiation. The complexity of the radiation environment at high altitudes require a diverse array of both passive and active instruments to study the exposure, however the limited room, the limited power and other stringent requirements make the passive detectors a convenient choice for example for on-board measurements on airplanes and stratospheric balloon.
The phantom Jimmy, the BDS spectrometer and the BD-100R integral dosemeters were exposed at different altitudes and latitudes  (table 2) in such a way to assess the neutron doses, both in air and in depth, related to cosmic ray radiation.
The dose equivalent values, measured at organs, (obtained using the BD-100 dosemeters) were compared with the simulation of the transport inside the phantom of the measured in air neutron spectrum. The data concerning the experiment at Testa Grigia Research Station are displayed in figure 4 and it is evident a good agreement between measurements and simulation.
Figure 4: Testa Grigia Research Station comparison between experimental BD100R H rates at organ position and H rates calculated with MCNP4B transport of the measured (BDS) spectrum in the phantom simulation.
The experimental doses equivalent at organs, H, were also compared with the equivalent doses at organs HT, calculated using the measured in air spectrum and the ICRP dose per unit neutron fluence conversion coefficients. The agreement between measured and calculated data, displayed in figure 5, points out that Jimmy (together with the BD100 detector) can be used to give a good evaluation of the neutron equivalent dose in most organs.
Figure 5: Testa Grigia Research Station, comparison between experimental BD100R H rates at organ position and HT rates obtained folding the measured spectrum with ICRP74 conv. coeff. (DT/Φ and wr).
In table 3 an estimation of the effective neutron dose rates, evaluated from measured organ dose data for the different exposures, are shown and are compared with
The measured data pointed out the effect of altitude and latitude on the cosmic rays doses. The maximum neutron dose is achieved in correspondence of the intercontinental flights altitudes that are the closest ones to the Pfotzer maximum (around 16 km). On the other hand, at quite the same altitude (Alitalia flights) the shielding effect of the Earth magnetic field is evident, in fact polar paths like Rome-Tokyo, receive radiation in higher quantities than the equatorial ones.
5. Conclusion A new complete apparatus for the evaluation of neutron doses at organs inside an anthropomorphic tissue equivalent phantom has been developed and fully characterized. In this paper results concerning the expositions of the phantom in environmental field are presented through the assessment of the neutron dose during intercontinental flights and in two other experimental set-up at high altitudes. Measurements pointed out that H values at critical organs seem to give a good assessment of HT from which a first approximation of the effective dose rate can be done. Therefore Jimmy can be used to map the environment neutron dose at various positions inside the aircraft (cockpit, at wing seats, etc.). The phantom is also employed in medical field , for the evaluation of the undesired neutron dose during radiotherapy treatment with Linacs.
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