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Edited by Vincenzo Buttaro

Introduction

A detailed study and understanding of the radiation environment in space and its effects on human physiology has a growing importance with current work on the International Space Station (ISS) and of a future mission to Mars. Radiation in orbit comes from cosmic rays of different energies and origins. In addition to the galactic component - which is modulated by the solar activity at low energies - there are also solar energetic particles associated to transient phenomena such as Solar Flares or Coronal Mass Ejections. Inside Earth's magnetosphere there is also the significant contribution of trapped particles: to the well-known proton and electron belts, recent studies have shown a more complex nuclear composition, for instance, with trapped components of anomalous cosmic rays.
For Low Earth Orbits (LEO) such as those of MIR, ISS or Shuttle (altitude of 300-400 km, inclination of 51.6^°) the effect of trapped radiation is mostly evident in the South Atlantic Anomaly (SAA). This is a region located between South America and Africa were the orbit of LEO spacecraft crosses the inner radiation. It is also important to study the cosmic ray nuclear component for its high quality factor which - even with a low flux - can give a non-negligible contribution to the dose absorbed by astronauts.

Cosmic ray environment in space

Cosmic rays provide a unique probe of the most energetic processes in the Universe. Cosmic rays are electrons, positrons, protons, antiprotons and atomic nuclei, light as deuterons and alpha, produced in the primordial nucleosyntesis, or heavier, coming from the explosion of supernova stars, from which all surrounding electrons have been stripped away during their high speed passage through the galaxies. Cosmic rays come mostly from outside the solar system but generally from within our galaxy (Galactic Cosmic Rays). They have been accelerated to nearly the speed of light, within the last ten million years, and have travelled many times across the galaxy, trapped by the galactic magnetic field. While astronomical observations of light from distant objects yield clues to the state of matter in our galaxy and beyond, cosmic rays bring us a small but valuable sample of that matter itself. Through studies of the composition and energy spectra of cosmic rays, we are able to learn about the origin and evolution of material in our galaxy and about fundamental physical processes that govern its dynamics.

In addition to the interest related to fundamental physics, cosmic rays are the source of radiation absorbed by astronauts and therefore involve many health physics and medical issues. These not only include the measurement of the radiation dose but also other effects such as the Light Flash phenomenon. Even though the doses absorbed are below safety margins in solar quiet conditions, they could increase to high values in case of a big solar particle event, especially during a mission outside the Earth's magnetosphere. Up to now only Apollo missions have carried man outside the protective shielding of the geomagnetic field and only for a relatively short time (10 days). In case of a longer (3 years) mission to Mars, the crew would be exposed not only to the small (but continuous) component of Galactic cosmic Rays but to the sporadic but potentially very intense component of Solar Cosmic rays generated by solar flares and Corona Mass Ejections.

Even for shorter and "safer" missions in Low Earth Orbit a continuous monitoring of the dose absorbed by the astronauts and its relation with their health is necessary for a complete assessment of the risks and the effects encountered during space flight. In addition to passive and active dosimetric measurements it is also required a detailed study of the different nuclear component of cosmic rays.

Cosmic rays in the magnetosphere are deflected from the geomagnetic field according to their energy, charge and angle of incidence. The shielding is higher at the magnetic equator and lower at the poles: this means that at the equator only higher energy particles can reach and penetrate the spacecraft, resulting in a higher flux and dose at the poles. A notable exception to this rule of thumb is the South Atlantic Anomaly (SAA), where the magnetic field is lower (due to multipole terms) and thus particle flux is higher. Particles in the SAA are mostly protons trapped in the inner Van Allen Belts: therefore flux can increase up to three orders of magnitude in this region.

Figure 1: A sketch of the magnetosphere and the relative position of the ISS (The inclination of the ISS is 51^°, so its orbit does not cross the poles as shown in the picture). Note how cosmic rays are deflected more strongly at the geomagnetic equator and are more easily channelled at the poles.

Figure 2: All particle flux as function of position measured with Sileye-2. It is possible to see the increase at the poles and mostly in the South Atlantic Region (SAA) due to trapped protons.

Figure 3: Particle flux for nuclei from Helium and above as function of position measured with Sileye-2. It is possible to see the rate increase at the poles but the absence of any structure in the South Atlantic Region (SAA).

The Light Flash phenomenon
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Figure 4: Some of the most probable processes by which cosmic rays can cause Light Flash: Ionization by nuclear reaction, Retinal hit or Scintillation in Vitreous.

Besides to the effects due to radiation, there are also other processes that have to be studied in order to have a more complete knowledge of the human response to space environment. One of these phenomena is the "Light Flash" (LF) effect, originally predicted by Tobias in 1952 and reported for the first time in 1969 by the Apollo-11 mission to the Moon.

Subsequently, LF were observed by astronauts in the Apollo, Skylab, Shuttle and MIR missions. LF consist of unexpected visual phenomena caused by the interaction of cosmic rays with the eyes of the astronaut. They occur in space with frequency and type which can vary considerably from subject to subject. In addition, geomagnetic and spacecraft shielding play a role in this effect since they can alter the cosmic ray nuclear composition. Accelerator beam tests with human subjects have shown evidence of a ionisation caused by heavy nuclei which interact with the back part of the eye, although a clear understanding of the causes and mechanisms involved is still lacking. Other hypothesis include Cherenkov radiation in the eye, direct stimuli to the brain or knock on electrons in the vitreous.

Figure 5: Various Light Flashes typologies observed during accelerator tests.

Light Flash Observation in Space

A detailed study of the frequency and type of Light Flash occurrence is of particular interest to understand the interaction between cosmic rays and astronauts’ visual apparatus (eye or Central Nervous System) to reduce risk in manned missions.

In addition there is some evidence that Light Flash occurrece decreases with permanence of cosmonauts in space. For instance LF frequency during Apollo missions were systematically lower on the transearth coast (returning to Earth) as compared to the translunar coast (going toward the Moon). Although a number of different causes has been investigated (e.g. relative posiiton of the Apollo capsule to Earth’s magnetosphere) no plausible explanation has been found. Experiments on Mir have shown a decrease in the LF rate as observed by one of the cosmonauts who performed observations during two different long duration permanence on Mir.

In both cases a decrease of LF occurrence with time was reported, although time scales (respectively days and months), orbit (resp. Lunar and Low Earth Orbit), shielding (higher for Apollo), permanence (r. week and months) were significantly different. Up to now no clear answer has been given to the presence and even less to the mechanisms involved in this phenomenon: however a confirmation or denial of this effect would have significant impact in planning for future long term missions in Earth orbit or outside the magnetosphere (Moon or Mars).

We therefore propose to to repeat and expand the observations performed during Soyuz-34 flight by cosmonaut Roberto Vittori to address this issue and continue investigation of this phenomenon. Soyuz-34 flight resulted in the first observation of LF on board the ISS in a controlled environment and with a continuous monitoring of cosmic ray flux and nuclear component as a function of time.

To improve LF observation technique minimizing dead time and at the same time recording LF perception, time and type and time the Dictaphone present on board the ISS will have to be used. Time of start of the session will be recorded on paper and on the voice recorder: the occurrence of LF and its typology and shape will be recorded on the voice recorder and by a pushbutton.

A scheme of the instrument is shown in the following Figure:

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Scheme of Light Flash / voice recorder: A joystick is attached to a dictaphone to record time of LF percption with LF typologies.

The Sileye project and its experiments

Sileye-1 and Sileye-2

The SilEye project aims to study the composition of cosmic rays inside manned spacecraft in relation to its radiation and non-radiation effects. Two devices (Sileye-1 and Sileye-2) worked on MIR between 1995 and 2000, performing the only LF observations on MIR and monitoring nuclear component in solar quiet and active periods (during several Solar Particle Events).

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Figure 6: Sileye-2 helmet and silicon detector box

Figure 7: Cosmonaut Avdeev using Sileye-2 experiment on MIR space station

Figure 8: Nuclear abundances measured inside MIR space station with Sileye-2 detector

Sileye-3/Alteino

The experience acquired in the realisation of the first two detectors was used in the construction of the Alteino experiment. The goal is to continue and expand the research performed up to now, studying the radiation environment on board ISS and its relations with the Light Flash phenomenon and investigating at the same time brain activity in space.

The experimental device is composed of two detectors: the cosmic ray detector (AST/Sileye-3) and the Elecroencephalograph (EEG). The EEG is used to monitor brain activity of the astronaut during work in order to study long and short term effects due to work, fatigue and stress in space. It is a compact and light device, to be worn by the astronaut during Light Flash observation sessions.

AST represents the evolution of the previous cosmic ray detectors and is specially optimised to study the component above 50 MeV/nucleon for nuclei from Hydrogen to above Iron. A detailed study of the relative and absolute abundance in different points of the geomagnetic field and in different solar activity conditions will be carried out. The detector array is composed by two scintillators and a series of 8 silicon strip detector planes. The two scintillators are located on top and bottom of the device and are used in coincidence to trigger the acquisition when the device is crossed by a cosmic ray.

Each silicon detector plane is divided in 32 strips with a pitch of 0.25 mm to detect nature, energy, and angle of incidence of incoming particles. There are 4 planes with strips oriented in the X direction and 4 planes oriented in the Y direction to provide a stereoscopic view of the track. This independent data flow is analysed off line to monitor correlation between cerebral dynamics, particle flux and LF perception.

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Figure 9: Picture of Alteino detectors. To the left it is possible to see the EEG cup and detector, to the right Sileye-3/AST cosmic ray detector.

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Figure 10: Left: AST detector tower open (without readout electronics): it is possible to see the stack of silicon detectors and the top scintillator (the detector is upside down). The bottom scintillator has been removed for clarity. Rigth: One of the 8 silicon detector boards (X view). It is possible to see the segmentation of the 32 strips of the detector. (Photos taken during assembly in the clean room facilities of Tor Vergata.)

Soyuz-34 (Marco Polo) mission with astronaut Roberto Vittori

The last but under many aspects most important cosmic ray detector is represented by the astronaut itself. Indeed the Light Flash phenomenon is probably the only case by which astronauts can directly detect cosmic rays through their interaction with the eye. The astronaut signals the observation of a LF by pressing a joystick button and recording his voice observations via a dictaphone.

ast_det_iss
Location of Sileye3-AST detector (top left) in Pirs module of the ISS. To the right is possible to see the airlock to Soyuz spacecraft.

Alteino was placed (see Figure) on board the International Space Station in spring 2002 with theSoyuz-34 flight of the Italian astronaut Roberto Vittori who has placed the detector in the Pirs module and performed severeal light flash observation sessions. Data cards have been sent to the ground after the flight and have been analyzed. They have provided new information on the cosmic ray composition on board the ISS and the LF phenomenon.

Some results of the Soyuz-34 mission on board ISS. To the top left it is possible to see the acquisition rate of cosmic rays as a function of time. The highest peaks correspond to passage in the South Atlantic anomaly. In the bottom left panel is shown a part of the acquisition, where it is possbile to see the dependence of particle flux from latitude. When the station is in the northenmost or southernmost parts the rate is higher, at the equator the rate is lower. The right panels show some cosmic ray events crossing the silicon detectror.

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Cosmic ray nuclei measured inside the ISS with AST/Sileye-3 detector