Radiation exposure ranks among the foremost dangers to human enterprise into space. Being able to determine the accumulated dose and dose rates for organs and other body tissues is paramount to further exploration and a permanent human presence in space. The Matroshka experiment aimed at gaining deeper understanding of the exposure to ionizing radiation as posed for a stay on the International Space Station (ISS) in Low Earth Orbit (LEO).
The mixed radiation field onboard a spacecraft (intravehicular activity- IVA) as well as the exposure during space walks (extravehicular activity- EVA) differs significantly from that found on earth.
Dose rates are increased by at least a factor of one hundred and further complications arise through the build- up of secondary particles in the shielding structure and the human body.
The presence of heavy charged particles (HCP) that form one major constituent of space radiation in addition to electrons and protons from the trapped radiation belts (van Allen belts), which are absent on earth, make special radiation protection considerations for crew members necessary due to the particles' high relative biological effectiveness (RBE).
The Matroshka phantom was designed to give a deep insight into the various processes at hand. Its anthropomorphic torso consists of 33 polyurethane slices. The density was chosen according to organ tissue and donor bones are embedded. Channels for passive radiation detectors namely thermoluminescence dosemeters (TLDs) and cut-outs for active instruments are embedded within the structure. Six additional organ dose boxes for passive dosemeters were installed at the sites of radiosensitive organs and the head, a poncho and hood including polyethylene stripes with sewed in TLDs and six dosemeter packages simulated the skin. A containment was used to cover the structure. For an EVA a multilayer insulation was added.
The Experiment was conducted in three main phases: an outside exposure (EVA) - Matroshka I (active and passive instruments), an inside exposure (IVA) onboard the Pirs module (passive instruments only) - Matroshka II-A - which is the focus of this work- and an IVA onboard the Zarya module (active and passive instruments) - Matroshka II-B.
In the frame of Matroshka Phase II-A the Institute of Atomic and Subatomic Physics provided 996 TLDs for dose measurements with high spatial resolution. Three types of TLDs were used in order to account for thermal and intermediate neutron components and HCPs. From January to December 2006, 337 days of exposure inside the Russian segment of the ISS were recorded. In contrast to the results of Matroshka I (EVA), a much flatter dose gradient can be seen and doses especially to the skin and head are roughly two times lower than for the outside exposure. On average, the skin received a dose rate of 0.25 mGy/day. This dose rate represents a conservative estimate for the whole body exposure. Build up and thermalisation of neutrons is significant for the organs, foremost the intestines, where they make up roughly a third of the 60Co- equivalent neutron absorbed dose. Dose related hot spots in the monitored organs were not found.
In the event of further prolonged missions in LEO and beyond, these findings represent a valuable source for mission planners and policy makers in the space sector as well as terrestrial applications i.e.
radiotherapy or radiation monitoring onboard aircraft. Scenarios for missions to Mars or a permanent lunar habitat have to rely on accurate risk estimates in order to safeguard human lives and to ensure minimizing long term stochastic radiation effects such as cancerogenesis and hereditary effects.