The Liulin-5 experiment is a part of the international project MATROSHKA-R on the Russian segment of the ISS, which uses a tissue-equivalent spherical phantom equipped with a set of radiation detectors. The objective of the MATROSHKA-R project is to provide depth dose distribution of the radiation field inside the sphere in order to get more information on the distribution of dose in a human body. Liulin-5 is a charged particle telescope using three silicon detectors. It measures time resolved energy deposition spectra, linear energy transfer (LET) spectra, particle flux, and absorbed doses of electrons, protons and heavy ions, simultaneously at three depths along the radius of the phantom. Measurements during the minimum of the solar activity in cycle 23 show that the average absorbed daily doses at 40 mm depth in the phantom are between 180 μGy/day and 220 μGy/day. The absorbed doses at 165 mm depth in the phantom decrease by a factor of 1.6–1.8 compared to the doses at 40 mm depth due to the self-shielding of the phantom from trapped protons. The average dose equivalent at 40 mm depth is 590 ± 32 μSV/day and the galactic cosmic rays (GCR) contribute at least 70% of the total dose equivalent at that depth. Shown is that due to the South Atlantic Anomaly (SAA) trapped protons asymmetry and the direction of Liulin-5 lowest shielding zone the dose rates on ascending and descending nodes in SAA are different. The data obtained are compared to data from other radiation detectors on ISS.
Research Containing: Phantom
Study of radiation conditions onboard the International space station by means of the Liulin-5 dosimeter
For estimating radiation risk in space flights it is necessary to determine radiation dose obtained by critical organs of a human body. For this purpose the experiments with human body models are carried out onboard spacecraft. These models represent phantoms equipped with passive and active radiation detectors which measure dose distributions at places of location of critical organs. The dosimetric Liulin-5 telescope is manufactured with using three silicon detectors for studying radiation conditions in the spherical tissue-equivalent phantom on the Russian segment of the International space station (ISS). The purpose of the experiment with Liulin-5 instrument is to study dynamics of the dose rate and particle flux in the phantom, as well as variations of radiation conditions on the ISS over long time intervals depending on a phase of the solar activity cycle, orbital parameters, and presence of solar energetic particles. The Liulin-5 dosimeter measures simultaneously the dose rate and fluxes of charged particles at three depths in the radial channel of the phantom, as well as the linear energy transfer. The paper presents the results of measurements of dose rate and particle fluxes caused by various radiation field components on the ISS during the period from June 2007 till December 2009.
Astronaut's organ doses inferred from measurements in a human phantom outside the international space station
Space radiation hazards are recognized as a key concern for human space flight. For long-term interplanetary missions, they constitute a potentially limiting factor since current protection limits for low-Earth orbit missions may be approached or even exceeded. In such a situation, an accurate risk assessment requires knowledge of equivalent doses in critical radiosensitive organs rather than only skin doses or ambient doses from area monitoring. To achieve this, the MATROSHKA experiment uses a human phantom torso equipped with dedicated detector systems. We measured for the first time the doses from the diverse components of ionizing space radiation at the surface and at different locations inside the phantom positioned outside the International Space Station, thereby simulating an extravehicular activity of an astronaut. The relationships between the skin and organ absorbed doses obtained in such an exposure show a steep gradient between the doses in the uppermost layer of the skin and the deep organs with a ratio close to 20. This decrease due to the body self-shielding and a concomitant increase of the radiation quality factor by 1.7 highlight the complexities of an adequate dosimetry of space radiation. The depth-dose distributions established by MATROSHKA serve as benchmarks for space radiation models and radiation transport calculations that are needed for mission planning.
The dose values in body's critical organs are necessary for estimating the effective dose. The tissue-equivalent phantom is used for such assessment as a rule. The spherical phantom is best fit for this goal. Therefore, the method developed on the basis of such phantom application becomes a good mean of effective dose estimating onboard the International Space Station. The main problems connected with developing a method of assessing an effective dose in the human's body organs with usage of a spherical phantom are presented in the paper. Proposed method can be used for monitoring the daily effective dose of crewmembers exposure for undisturbed radiation conditions of the flight.
Radiation measurements of surface and deep organ doses were performed aboard the International Space Station, for the period of January 2006 to April 2007, using a MOSFET dosimetry system combined with the Matroshka-R spherical phantom. The averaged internal and surface dose rates are found to be 0.19 and 0.29 mGy d(-1), respectively. The levels of radiation dose to blood-forming organs (BFO) and to surface organs are compared with recommended safe limits. The maximum measured BFO dose has an average dose rate of 0.23 mGy d(-1) (84 mGy y(-1)), corresponding to 44 % of the recommended annual limit of 0.5 Sv, for a space radiation quality factor of 2.6. The annual surface dose is found to be higher at 126 mGy, corresponding to 16 % of the eye dose limit and to 11 % of the skin dose limit. Doses calculated using the Spenvis software showed deviations of up to 37 % from measurements.
To estimate the radiation risk of spacecraft crew during the mission, it is necessary to measure dose distribution at various compartments, on and inside the human body that can be simulated using various phantoms. Due to some convenient characteristics (especially small weight and dimensions), passive detectors are used to measure dosimetric quantities onboard spacecraft. This contribution deals with the measurement of dosimetric characteristics and spectra of linear energy transfer (LET) onboard the International Space Station (ISS) during two experiments with tissue-equivalent spherical Russian phantom MATROSHKA-R realized in years 2006 and 2008. To obtain LET spectra, total absorbed doses, and dose equivalents, we used combination of plastic nuclear track detectors and thermoluminescence detectors. The detectors were placed at various locations on the surface of the MATROSHKA-R phantom; some detectors were also inserted inside this phantom. The variation of dosimetric quantities obtained during both missions is discussed. The dose characteristics vary with the position of the detectors on or inside the phantom; the absorbed dose and dose equivalent can differ almost twice.
Results of measuring neutron dose inside the Russian segment of the International Space Station using bubble detectors in experiment Matreshka-R
Distribution of neutron equivalent dose both inside and outside the spherical phantom (experiment Matryeshka-R) was determined with the help of dedicated research equipment "Bubble-dosimeter". Equipment is built up from an automatic bubbles counter and 8 bubble detectors of neutrons with energy ranging from approximately 200 keV to 15 MeV. Measurements inside the ISS were made in several 7-day sessions in the period from April 2006 till October 2007 (ISS increments 13-15). According to the bubble detectors on the outside of the phantom, ambient neutron dose H*(10) was equal to 0.1 mSv/d or approximately 20% of the dose from charged particles inside the ISS. In the tissue-equivalent phantom, neutron dose was 1.2 +/- 0.2 times less as compared with the phantom surface which characterized the degree of dose attenuation in cosmonaut's body.
Austrian dose measurements onboard space station MIR and the International Space Station – overview and comparison
The Atominstitute of the Austrian Universities has conducted various space research missions in the last 12 years in cooperation with the Institute for Biomedical Problems in Moscow. They dealt with the exact determination of the radiation hazards for cosmonauts and the development of precise measurement devices. Special emphasis will be laid on the last experiment on space station MIR the goal of which was the determination of the depth distribution of absorbed dose and dose equivalent in a water filled Phantom. The first results from dose measurements onboard the International Space Station (ISS) will also be discussed. The spherical Phantom with a diameter of 35 cm was developed at the Institute for Biomedical Problems and had 4 channels where dosemeters can be exposed in different depths. The exposure period covered the timeframe from May 1997 to February 1999. Thermoluminescent dosemeters (TLDs) were exposed inside the Phantom, either parallel or perpendicular to the hull of the spacecraft. For the evaluation of the linear energy transfer (LET), the high temperature ratio (HTR) method was applied. Based on this method a mean quality factor and, subsequently, the dose equivalent is calculated according to the Q(LET∞) relationship proposed in ICRP 26. An increased contribution of neutrons could be detected inside the Phantom. However the total dose equivalent did not increase over the depth of the Phantom. As the first Austrian measurements on the ISS dosemeter packages were exposed for 248 days, starting in February 2001 at six different locations onboard the ISS. The Austrian dosemeter sets for this first exposure on the ISS contained five different kinds of passive thermoluminescent dosemeters. First results showed a position dependent absorbed dose rate at the ISS.