Advances in Space Research 37 (2006) 1770–1776 www.elsevier.com/locate/asr Simulation of the ALTEA experiment on the International Space Station with the Geant 3.21 program R. Scrimaglio a, G. Nurzia a,*, E. Rantucci a, E. Segreto a, N. Finetti a, A. Di Gaetano a, A. Tassoni a, P. Picozza b, L. Narici b, M. Casolino b, L. Di Fino b, A. Rinaldi b, V. Zaconte b b a Department of Physics, University of LÕAquila, Via Vetoio (Coppito 1), 67010 Coppito, LÕAquila, Italy Department of Physics, University ‘‘Tor Vergata’’ and INFN Sez, Roma2, Via della Ricerca Scientifica 1, 00133 Roma, Italy Received 30 September 2004; received in revised form 25 November 2004; accepted 29 November 2004 Abstract The ALTEA (Anomalous Long Term Effects in Astronauts) project is aimed at the study of the transient and long term effects of cosmic particles on the astronautsÕ cerebral functions. The detector will fly on the International Space Station in 2005. Due to the complexity of the detector (12 double silicon detector boxes arranged around the head of the astronaut) it is necessary to have a detailed simulation of the apparatus response to cosmic ray nuclei in order to assess the detector response and its observational capabilities. The ALTEA detector was therefore simulated by using the Geant 3.21 program, by the astronautsÕ head described by 64 cubes of water. By using Creme96 program to evaluate the cosmic rays fluxes within the International Space Station we obtained the events distribution and the energy lost in the cubes of water, as well as the expected interaction rates. Furthermore, we calculated the triggered events number per unit of time in the detector. The simulation was also used to develop a technique to recognize the cosmic rays nuclei.  2004 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Light flash; Cosmic rays; Biological effects 1. Introduction In the next few years the average time stay on the International Space Station of human beings will strongly increase. For this reason the safety issue in space environment is crucial. There is a need to study the effects of particles on the human body and particularly on the functionality of the CNS. The visual system has been chosen to ‘‘probe’’ the CNS because it is particularly sensitive to space environment. In missions Apollo missions 11 through 17, Skylab 4, Apollo-Soyuz, Mir, Iss, the astronauts, after some minutes of dark adaptation, observed brief flashes * Corresponding author. E-mail address: giampietro.nurzia@aquila.infn.it (G. Nurzia). of white light with the shape of thin or thick streaks, single or multiple dots, clouds, etc. (Osborne et al., 1975; Pinsky et al., 1974, 1975). The experiments hint linking these perceptions to the passage of heavy ionizing nuclei through the retina. The specific mechanism of the interaction, and its site, remain uncertain. In order to evaluate the LF phenomenon it is necessary the simultaneous determination of time, nature, energy and trajectory of the particle passing through the cosmonautÕs eyes, as well as the cosmonautÕs LF perception time. Some previous experiments are described in (Casolino et al., 2002, 2003a,b). The ALTEA project is aimed at the study of the transient and long term effects of cosmic particles on the astronautsÕ cerebral functions. It has been funded by the Italian Space Agency (ASI) and by the National 0273-1177/$30  2004 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2004.11.029 R. Scrimaglio et al. / Advances in Space Research 37 (2006) 1770–1776 Institute for Nuclear Physics (INFN) and ‘‘Highly recommended’’ by the European Space Agency (ESA). The experiment is an international and multidisciplinary collaboration. The basic instrumentation is composed by a series of active particles telescopes, an ElectroEncephaloGrapher (EEG) and a visual stimulator, arranged in a helmet shaped device. This instrumentation is able to measure simultaneously the dynamics of the functional status of the visual system, the cortical electrophysiological activity, and the passage through the brain of those particles whose energy is included in a predetermined window. The three basic instruments can be used separately or in any combination, permitting several different experiments. 1771 arranged perpendicular each other. The distance between two sensors of each detector is 3.75 cm. Each SDU is placed inside an aluminium box 1.3 mm thick. The set-up of the detector is shown in Fig. 1. The astronauts will used two push buttons to mark the perception of a light flash. All information will be stored via an integrated data handling system which will also allow the transmission of the data to ground. In our simulation particles are generated in random way and isotropically on a big spherical surface with the ALTEA detector in the centre. The energy of a particle is chosen in random way within the cosmic ray spectrum obtained by using the Creme96 program (Fig. 2), so that events are distributed according to this spectrum. The direction of a particle is isotropically generated in random way. 2. Description of the detector system Due to the complexity of the detector it is necessary to have a detailed simulation of its response to cosmic rays particles in order to evaluate its observational capabilities. The ALTEA detector was therefore simulated using the Geant 3.21 program. The Geant program simulates the passage of elementary particles through the matter. Originally designed for the High Energy Physics experiments, it has today found applications also outside this domain in areas such as medical and biological sciences, radio-protection and astronautics. The principal applications of Geant in HEP are: (1) the transport of particles through an experimental setup for the simulation of detector response; (2) the graphical representation of the setup and of the particle trajectories. The two functions are combined in the interactive version of Geant. This in very useful, since the direct observation of what happens to a particle inside the detector makes the debugging easier and may reveal possible weakness of the setup. The Geant system can be obtained from CERN (http://cernlib.web.cern.ch/cernlib/version.html) and the program runs everywhere the CERN Program Library has been installed. The detector system consists of an helmet holding 12 active silicon telescopes (Narici et al., 2001a,b,c, 2003), assembled in six independent units named Silicon Detector Unit or SDU. Each detector is made of three silicon strip sensors. The basic sensor is obtained by assembling back to back two chips each with 32 ion implanted resistive strips, with a sensitive area of 2 · (8 · 8) cm2 and a thickness of 380 lm. In order to allow both x and y coordinates measurement the strips of the two detectors are 3. Cosmic rays fluxes evaluation We used the Creme96 program to evaluate the cosmic rays fluxes within the International Space Station. Creme96 is a suite of programs for creating numerical models of the ionizing radiation environment in near Earth orbits and evaluating the resulting radiation effects on electronic systems in spacecraft and in high altitude aircraft (Tylka et al., 1996, 1997a,b). We used the Creme96 program in four steps:: (1) Because the International Space Station is inside of geosynchronous orbit, we used a first routine in order to evaluated the geomagnetic shielding effect of Earth on some particles found in interplanetary space. This routine averages the geomagnetic transmission function over the entire orbit of ISS: an orbit-generator routine is used to step along the specified orbit, which is tracked for seven days. Geomagnetic access is evaluated at each point on the orbit, and the results are averaged. The calculation is averaged over all directions, without any preferred ISS orientation or lookout direction (due to thin shielding, for example) from the ISS. The calculation does take into account, however, that some lookout directions are blocked by the solid Earth. Flux calculations using Creme96 assume that the incoming primary source flux is isotropic. (Anisotropies in the incoming primary energetic particle fluxes are generally small, except in the very early stages of solar particle events). The orbit-averaged flux calculation further assumes that the incoming primary flux varies slowly compared with the orbital period. (2) A routine of the Creme96 program evaluates the geomagnetically trapped proton fluxes. The calculation is orbit-averaged. The program provides 1772 R. Scrimaglio et al. / Advances in Space Research 37 (2006) 1770–1776 z y x Fig. 1. 3D view of the ALTEA detector. Fig. 2. Cosmic rays differential fluxes vs. kinetic energy. omnidirectional trapped proton fluxes, which means that the fluxes are averaged over the entire range of arrival directions, covering 4p Sr. Omni- directional does not imply isotropic, that is, having the same flux arriving from all directions. In fact, the low-altitude trapped proton flux is highly R. Scrimaglio et al. / Advances in Space Research 37 (2006) 1770–1776 anisotropic. (The anisotropies may be important for orientation-stabilized spacecraft and for some applications). We evaluated trapped proton fluxes near solar minimum condition. (3) The results from previous two routines enter in the third routine providing a numerical model of the space ionizing-radiation environment at the surface of the ISS, before transport through shielding. We chose the Solar Quiet (‘‘no flare’’) Model. This model represents the ambient environment which prevails in the absence of solar energetic particle (‘‘flare’’) events. This environment, which varies slowly in intensity over the 22-year solar cycle, is the basic environment in which all space systems must operate. The model is appropriate for evaluating typical and long-term average particle fluxes. (4) The last routine of the Creme96 transports the input particle fluxes through a thickness of shielding. This transport code takes into account both energy loss and nuclear fragmentation. In Creme96 the aluminum is the shielding material. We used a greater value for specifying the shielding thickness. The next step will be the use of more sophisticate Low Earth Orbit (LEO) particle environmental models in order to obtained more realistic results. The differential fluxes, in minimum solar condition, are shown in Fig. 2. There is a strong predominance 1773 of protons and alpha particles with respect to heavier nuclei and a great predominance of protons with respect to alpha particles when kinetic energy is below 1 GeV/ nucl. The maximum value for protons flux is near 102 MeV/nucl. The remaining particles have maximum values near 103 MeV/nucl. 4. Trigger rate We define ‘‘trigger signal’’ an event which hits a single strip along y direction of three silicon plane of a SDU and release in each plane an energy above the detection value of 10 mip  1.09 MeV. In fact, 1 mip in 380 lm of silicon is equal to 109 keV. In Fig. 3 the energy spectrum of some particle that yield trigger signals in any SDU is shown. We can see that the ALTEA detector does not reveal relativistic protons and alpha nuclei. We define ‘‘single trigger events’’ a particle that yield a trigger signal in one SDU and ‘‘double trigger events’’ a particle that yield a trigger signal in two SDU. The ‘‘double trigger events’’ are very important because they will permit a better reconstruction of the experimental tracks. Remembering that the cosmic particle flux is assumed isotropic, we calculated the mean number of events per second that yield a single or a double trigger in the ALTEA detector with and without the astronautsÕ head. The results are shown in Tables 1 and 2. The results Fig. 3. Energy spectra of ions that yielding trigger signals in any SDU. 1774 R. Scrimaglio et al. / Advances in Space Research 37 (2006) 1770–1776 Table 1 Mean number of ‘‘single trigger events’’ per second in the SDU p 20.3 27.5 10 7 Li3 a 3 0.0249 0.0279 12 B5 3 1.3 · 10 1.3 · 103 14 C6 3 3 · 10 3.2 · 103 10.6 · 10 10.8 · 103 16 N7 O8 3 2.8 · 10 2.9 · 103 9.4 · 103 10.2 · 103 Table 2 Mean number of ‘‘double trigger events’’ per second in the SDU p 7 a 0 2.45 10 Li3 4 0.0023 0.0054 12 B5 2 · 10 3 · 104 14 C6 3 0.9 · 10 1.1 · 103 shown in the first row are obtained by including the astronautsÕ head in the computations, whereas the results in the second row do not consider the astronautsÕ head. From Table 2, we notice that protons cannot yield events of double trigger when the astronautsÕ head is present. In this case, in fact, the protons energy spectra relative to the first and to the second trigger signal have not common parts. However, in (Casolino et al., 2003b) are assumed at least two causes of Light Flash, one due to protons and one due to heavy nuclei. The number of events triggered per second in the SDU is very important because it can have a relation with the experimentally observed frequency of Light Flashes. Considering that ALTEA does not reveal relativistic protons and that protons can not yield events of double trigger when the astronautsÕ head is present, we can say that our experiment allows us to study properly only the heavy nuclei (see Section 7). We changed the trigger configuration in order to calculate the mean number of events per second that yield signals above the detection energy in the outermost silicon plane of the SDU. The results are shown in Table 3. As before, the results shown in the first row are obtained by including the astronautsÕ head in the computations, whereas the results in the second row do not consider the astronautsÕ head. These values are about 10 times greater than those of Table 1. 16 N7 3 3.5 · 10 3.6 · 103 O8 3 2.6 · 103 3.4 · 103 0.9 · 10 1 · 103 5. Energy lost in the astronautsÕ head The astronautsÕ head is simulated with 64 cubes of water (Fig. 1) because the Geant program does not simulate the human tissues. The approximation is acceptable at this level of calculation. More advanced simulation programs (Geant 4) will permit to obtain more realistic results. The length of the side of the cubes is 4.2 cm. In Table 4, the mean energy lost in one cube of water during the passage of a particle for different species of cosmic rays is shown. The first row includes all events generated by Geant ð wÞ; the second row includes only the events triggered by any SDU ð wt Þ. In the second case there is a smaller energy release, except for alpha and lithium. Obviously, the values increase with the atomic number of the ionizing nuclei. We calculated the mean energy lost because, when there is a Light Flash, a cosmic particle loses energy in a certain zone of the astronautsÕ head. 6. Rate of events occurring in water cubes In Table 5, the mean number of interactions occurring in one cube of water per second for different species of particles is shown. The first row includes all events generated by Geant ðqÞ; the second row includes only the events triggered by any SDU ðqt Þ. Table 3 Mean number of triggered events per second in the outermost silicon plane of the SDU p a 7 394.22 412.63 1.26 1.26 1.12 · 102 1.13 · 102 Li3 10 12 14 1.9 · 102 1.92 · 102 6.46 · 102 6.47 · 102 1.72 · 102 1.73 · 102 B5 C6 N7 16 O8 6.05 · 102 6.16 · 102 Table 4 Mean energy lost in one cube of water during one interaction for different species of particles  w w t p a 7 10 12 14 16 23.5 ± 2.2 21.9 ± 1.5 28.3 ± 2.9 67.1 ± 6.1 62.5 ± 5.9 66.5 ± 6.1 172.1 ± 14.9 170.0 ± 13.8 249.3 ± 22.3 243.1 ± 20.1 341.1 ± 30.0 331.8 ± 26.8 429.2 ± 37.8 422.0 ± 34.2 The values reported in this table are in MeV. Li3 B5 C6 N7 O8 1775 R. Scrimaglio et al. / Advances in Space Research 37 (2006) 1770–1776 Table 5 Mean number of interactions occurring in one cube of water per second for events which do not trigger the experiment ðqÞ and for those which give a trigger ð qt Þ  q  qt 10 p a 7 440 2.67 2.05 0.0119 0.0099 0.0007 Li3 B5 12 14 16 0.0540 0.0057 0.0144 0.0015 0.0476 0.0047 C6 0.0153 0.0015 N7 O8 Table 6 Mean energy lost per second in one cube of water for different species of particles  E t E  E t E= p a 7 10340 58.7 176 58.0 0.80 72.5 0.62 0.047 13.2 Li3 10 12 14 2.63 0.26 10.1 13.5 1.39 9.71 4.91 0.50 9.7 B5 C6 16 N7 O8 20.4 1.98 10.3 The values reported in this table are in MeV. We obtain the mean energy lost per second in one cube of water multiplying the values reported in Table 4 times the values reported in Table 5. The results are shown in Table 6. Again the first row includes all  the second row inevents generated by Geant ðEÞ;  t Þ. At cludes only the events triggered by any SDU ðE last, the third row consist of the ratio between values at the first row and the second row. The values are high for protons and alpha and nearly constant for heavier particles. In Tables 4–6, only the second rows are important because the ALTEA detector stores only the quantities triggered. The first rows are written for confrontation. Fig. 4 shows the energy lost in the 26th cube of water by a carbon nucleus, considering only the events triggered by any SDU. We can note the presence of a maximum value. In Table 7, the maximum energy that a particle can lose in one interaction in one cube of water for some particles is shown. The maximum values are calculated using a fit with a gaussian shape and a polynomial of degree 3 of the distribution of the energy in one cube of water. 7. Recognition of particles The Geant simulation was also used to develop a technique to identify cosmic particles. We considered only one detector of one SDU and studied different kinds of nuclei by a standard statistical analysis. We simulated the species of nuclei reported in Table 3 except nitrogen. The particles flux was isotropic. We considered the plot shown in Fig. 5. The abscissa variable is DEud = DE1 + DE2  DE5  DE6, where DEi is the energy P lost in the ith silicon plane. On the ordinate 6 axis DEtot ¼ i¼1 DEi , the total energy lost in six silicon planes, is reported. The statistical analysis is divided in two complementary phases: h > p/2 (downward particles) and h < p/2 (upward particles), where h is the zenit angle. Fig. 4. Energy lost in the cube of water number 26 from a carbon nucleus considering only the events triggered by any SDU. The values of energy are in MeV. Table 7 Maximum energy lost in one cube of water 7 Li3 68.7 ± 0.06 10 12 14 16 213 ± 0.06 301 ± 0.15 411 ± 0.21 532 ± 0.27 B5 C6 N7 O8 The values reported in this table are in MeV. As shown in Fig. 5, the points relative to different species of particles are arranged in different zones of the plane DEtot  DEup. It is possible to identify different species of particles in an efficient way by apply opportune cuts on the values of the variables. The confusion matrix shows the percentage of particles of a species present in a fixed plane zone as regards the total number of particles of that species. In this way we write 4 confusion matrixes. In Table 8, the conclusive results obtained combining the 4 cases are shown. When the threshold energy of the 1776 R. Scrimaglio et al. / Advances in Space Research 37 (2006) 1770–1776 about 1 MeV, we can distinguish particles with Z P 5. Further developments in the detector will improve its actual performances. The ALTEA experiment will allow to evaluate the risk on the Central Nervous System due to cosmic rays fluxes in microgravity conditions during long term space missions. It will provide information to define the causes of the anomalous phosphenes perception of the astronauts. The detector will fly on the International Space Station in 2005. References Fig. 5. Events distribution as a function of DEtot versus DEud. Table 8 Confusion matrix obtained by combined analysis p a 7 Li3 10 B5 12 C6 16 O8 p (%) a (%) 7 Li (%) 10 B (%) 12 C (%) 16 O (%) 23.3 13.5 8.2 0 0 0 10.9 11.7 11 0 0.01 0.21 45 50.6 60.5 0.01 0 0.02 20.8 18 16.2 58.8 0.9 0.01 0 5 3.3 35.7 84.5 0.54 0 1.1 0.8 5.5 14.6 99.2 silicon planes is equal about 1 MeV, we can distinguish particles with Z P 5. If board conditions will be favourable, the threshold energy could be lowered in future until about 0.5 MeV. 8. Conclusion This is a preliminary work. The following step will consist in a more realistic simulation realized with other programs, which are able to simulate also human tissues (Geant 4). 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