STEM Today

STEM TODAY March 2017, No.18

STEM TODAY March 2017 , No.18

CONTENTS Cancer 11: What are the most effective shielding approaches to mitigate cancer risks?

International Space Station (ISS)

Editorial Editor: Mr. Abhishek Kumar Sinha Editor / Technical Advisor: Mr. Martin Cabaniss

STEM Today, March 2017, No.18

Cover Page International Space Station Solar Transit This composite image, made from ten frames, shows the International Space Station, with a crew of six onboard, in silhouette as it transits the sun at roughly five miles per second, Saturday, Dec. 17, 2016, from Newbury Park, California. Onboard as part of Expedition 50 are: NASA astronauts Shane Kimbrough and Peggy Whitson: Russian cosmonauts Andrey Borisenko, Sergey Ryzhikov, and Oleg Novitskiy: and ESA (European Space Agency) astronaut Thomas Pesquet.

Image Credit: NASA/Joel Kowsky

Back Cover Docked Soyuz Over Gulf of Mexico and Florida iss050e038462 (02/03/2017) – A Russian Soyuz spacecraft can be seen in this image from the International Space Station as it passes over the American state of Florida surrounded by the blue waters of the Gulf of Mexico on the west side and the Atlantic Ocean on the other.

Image Credit: NASA

STEM Today , March 2017

Editorial Dear Reader

STEM Today, March 2017, No.18

All young people should be prepared to think deeply and to think well so that they have the chance to become the innovators, educators, researchers, and leaders who can solve the most pressing challenges facing our world, both today and tomorrow. But, right now, not enough of our youth have access to quality STEM learning opportunities and too few students see these disciplines as springboards for their careers.

According to Marillyn Hewson, "Our children - the elementary, middle and high school students of today - make up a generation that will change our universe forever. This is the generation that will walk on Mars, explore deep space and unlock mysteries that we can’t yet imagine". "They won’t get there alone. It is our job to prepare, inspire and equip them to build the future - and that’s exactly what Generation Beyond is designed to do."

STEM Today will inspire and educate people about Spaceflight and effects of Spaceflight on Astronauts.

Editor Mr. Abhishek Kumar Sinha

Editorial Dear Reader The Science, Technology, Engineering and Math (STEM) program is designed to inspire the next generation of innovators, explorers, inventors and pioneers to pursue STEM careers.

According to former President Barack Obama, " Science is more than a school subject, or the periodic table, or the properties of waves. It is an approach to the world, a critical way to understand and explore and engage with the world, and then have the capacity to change that world..."

STEM Today addresses the inadequate number of teachers skilled to educate in Human Spaceflight. It will prepare , inspire and educate teachers about Spaceflight. STEM Today will focus on NASA’S Human Research Roadmap. It will research on long duration spaceflight and put together latest research in Human Spaceflight in its monthly newsletter.

Editor / Technical Advisor Mr. Martin Cabaniss

STEM Today, March 2017, No.18

SPACE RADIATION Cancer 11: What are the most e ective shielding approaches to mitigate cancer risks?

This gap addresses speci c issues associated with development of shielding optimization methodologies to accurately evaluate radiation protection strategies while accounting for the inherent multiple uncertainties associated with the problem. For most shield design e orts, e ective dose for speci c GCR or SPE radiation design environments has been calculated in order to determine the e ectiveness of di erent shielding con gurations to meet speci c vehicle radiation requirements. However, additional methodologies incorporating probabilistic models are needed to reduce uncertainty in optimization calculations to properly assess the cost (mass savings) vs. bene t of further reducing exposures as required by ALARA. Such integrated optimization methodologies for ALARA do not currently exist.

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Radiation Shielding effect of water filled protective curtain in ISS crew cabin

The ISS Russian crew quarters are known to be less shielded from space radiation as compared with the neighbouring compartments. The outer wall of the cabin is only 1.5 g/cm2 aluminium shielding. Based on Pille-ISS Thermoluminescent Dosimeter System (Pille) measurements in 2007 and 2009 the dose rates at the sleeping place of the ISS crewmembers are 35% higher than the average dose rate in the Service Module (SM), Zvezda of the station (Fig. 1).

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To increase the crew cabin shielding a special protective curtain was designed and then delivered to ISS in 2010. The hygienic wipes and towels containing water are stored in the protective curtain in 3 segments and 4 layers thus providing an additional shielding thickness of about 8 g/cm2 water-equivalent matter.

Total mass of the curtain with wipes and towels is 67 kg with dimensions of 10.0 × 43.6 × 50.5 cm3 , 10.0 × 65.8 × 63.2 cm3 , and 10.0 × 71.9 × 60.0 cm3 , from upper to bottom panel. The protective curtain was installed along the wall of the starboard crew cabin. To study the radiation shielding effect 12 passive detector packages with thermoluminescent detectors (TLD) and solid state track detectors (SSTD) are used regularly; their positions are indicated with #1e#12(Fig. 2).

Pille dosimeters were exposed at Locations #6, #7, #9, #10, #11, and #12. The detector IDs of the Pille dosimeters exposed are shown in Table 1.

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Results 16 measurement periods of approximately 1 month of duration were performed with Pille dosimeters to determine the shielding effect of the protective curtain between September 2010 and December 2011. The duration of the measurement periods and the location of the dosimeters and the measured dose rates are shown in Table 1 and Fig. 2. Uncertainty of the results presented is 5%.

The ratios of the doses measured by the unshielded and shielded dosimeters were calculated and are presented in Table 5. The unshielded/shielded ratios are lower for #7/#6 than for #10/#9 and #12/#11 due to the different shielding configuration: location #7 is in front of a glass window, whereas locations #10 and #12 are in front of the wall of the module. This might be also the reason for the differences in the absorbed doses obtained during the same period for unshielded positions #10 and #7.

The maximum measured absorbed dose rate for unshielded location #7 could not be explained by changes in the attitude and the orbit of the ISS or by changes in solar activity, the occurrence of Earth-directed coronal mass ejections.

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The results for the shielded dosimeters are always lower than the corresponding unshielded ones. The shielded absorbed dose rates were in the range of 141 µGy/day to 239 µGy/day, while the unshielded dose rates were between 186 µGy/day and 413 µGy/day. The ratio of the unshieldede - shielded dose rate pairs was in the range of 1.12 - 1.78, the average ratio was 1.34 ± 0.18. The results show that the average shielding effect of the water filled protective curtain is approximately 24 ± 9%, based on the measurements carried out with the Pille ISS dosimeter system.

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Measurements on radiation shielding efficacy of Polyethylene and Kevlar in the ISS (Columbus)

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Materials are usually tested for their radiation shielding effectiveness first with Monte Carlo simulations, then on ground, using particle accelerators and a number of specific ions known to be abundant in space, and finally in space.

Highly hydrogenated materials perform best as radiation shields. Polyethylene is right now seen as the material that merges a high level of hydrogenation, an easiness of handling and machining as well as an affordable cost, and it is often referred as a sort of ’standard’ to which compare other materials’ effectiveness.

Kevlar has recently shown very interesting radiation shielding properties, and it is also known to have important characteristics toward debris shielding, and can be used, for example, in space suits.

Authors have measured the effectiveness of polyethylene and kevlar in the ISS using three detectors of the

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ALTEA system from 8 June 2012 to 13 November 2012, in Express Rack 3 in Columbus. These active detectors are able to provide the radiation quality parameters in any orbital region; being identical, they are also suitable to be used in parallel (one for the unshielded baseline, two measuring radiation with two different amounts of the same material: 5 and 10 g/cm2 ).

A strong similarity of the shielding behavior between polyethylene and kevlar is documented. Authors measured shielding providing as much as ∼40% reduction for high Z ions. In Fig.1 , the integrated behavior (3≤LET ≤350 keV/µm) is shown (ratios with the baseline measurements with no shield) both for polyethylene and kevlar, in flux, dose and dose equivalent. The measured reductions in dose for the 10 g/cm2 shields for high LET (>50 keV/µm, not shown in the figure) are in agreement with what found in accelerator measurements (Fe, 1 GeV). The thinner shielding (5 g/cm2 ) in our measurements performs ∼2% better (in unit areal density).

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Shielding Effect of polyethylene (PE) pads inside the ISS Russian segment

PADLES (PAssive Dosimeters for Lifescience Experiments in Space) is a passive dosimeter package developed by JAXA. Each package consists of three CR-39 PNTD plates (HARTZLAS TD-1 PNTD: Fukuvi Chemical Industry, 0.9 mm thick) and seven TLDs. The outer dimensions of the package are 2.5 X 2.5 X 0.6 cm. Figure 1 shows a photo (a) and the configuration (b) of PADLES. PADLES provides the absorbed dose in the lower LET region (LET ≤10 keV/µm) from the TLDs and, in the higher LET region (LET >10 keV/µm), the LET spectrum from the CR-39 PNTDs. The sets of TLD elements

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selected for each experiment ensured a response deviation of less than 5.7% for 160 MeV/n protons. Using the information from PADLES, authors can determine the total absorbed dose (DT OT AL ), the total dose equivalent (HT OT AL ), and the mean quality factor (QM E AN ).

Four sets of PADLES were used in each experimental phase; one set was kept at the ground as the "ground control," and the others were put into thin storage bags and attached to the wall of the ISS Russian segment (PIRS and Zvezda). Two sets of PADLES (PADLES #1 and #2) in the ISS were installed into PE tiles with a thickness of approximately 5 g/cm2 , while the other (PADLES #3) was left unshielded as the "space control," as shown in Figure 2. Data from both shielded and unshielded PADLES were corrected by taking the difference from the ground control data. Detector location was frequently changed, as shown in Figure 3 and Table 1.

The average altitude of the ISS during the experimental span was approximately 350 km. Table 2 shows the absorbed doses and the dose equivalents for all the experimental phases (total absorbed dose (DT OT AL ), dose absorbed by TLD (DT LD ≤10 keV/µm), dose absorbed by CR-39 PNTD DC R−39 >10 keV/µm, and dose equivalent (HC R−39 >10 keV/µm)). For each experimental phase, #1 and #2 are the shielded PADLES, and #3 is the unshielded one.

Certain differences in the shielding effect of doses were observed throughout experimental phases 1-4. The shielding effect due to PE was within 0.6-7.8% for DT LD ≤10 keV/µm, 9.5-24.4% for DC R−39 >10 keV/µm, and 0.1-19.6% for HC R−39 >10 keV/µm. The shielding effect of space radiation for DT OT AL and total dose equivalent (HT OT AL ) in phases 1-4 is shown in Figure 4.

The results using the PADLES in phase 1-4 experiments as part of the ALTCRISS project were obtained on the surface of thick inside walls in the ISS Russian segment. When we added the PE shielding in this environ-

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ment, absorbed doses ≤ 10 keV/¾m showed a slight change. The shielding effect of 5 g/cm2 PE was estimated to be 6.1% for total absorbed doses and 10.6% for total dose equivalents over the four experiments.

From measurements with PADLES dosimeters, authors concluded that adding PE shielding inside the wall in the ISS Russian segment changed the total absorbed doses only slightly (an average change of 6.1% for total absorbed dose and 10.6% for dose equivalent).

In contrast, a similar shielding experiment in the ISS Russian segment in 2010 (Kodaira et al., 2014) concluded that water towels could drastically reduce doses, by up to 37% on the surface of the outside wall and window (Fig. 3). Note that the shielding thicknesses of the PE (5 g/cm2 ) in ALTCRSISS and the water towel (6.3 g/cm2 ) in the past experiment are similar, but only one side of the detector was covered in the case of the water towel experiment.

In order to clarify the PE shielding mechanism, a detailed analysis based on Monte Carlo simulation was performed using PHITS. Figure 7 shows the simulation setup for the analysis. In the simulation, the Zvezda module

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was represented as a simple spherical shell made of aluminum. The inner radius of the shell was fixed at 220 cm, and the wall thickness was changed from 1 cm to 10 cm, which corresponds to a change of shielding thickness from 2.7 to 27 g/cm2 . Three PADLES were placed 10 cm away from the wall, and two of them were covered by 5 g/cm2 PE blocks on one or both (two) sides of the detectors. Depending on the type (or lack) of PE covering, the detectors are called "bare PADLES," "one-sided PE covering PADLES," and "two-sided PE covering PADLES" hereinafter. The shielding thickness averaged over 12 typical locations for area monitoring experiments with SPD boxes in the ISS Zvezda module was estimated to be approximately 40 g/cm2 (private communication with IBMP-JAXA, 2008 and NIRS-R-62, 2009). This value is equal to the mean shielding thickness of the detector location in our simulation for a 23 g/cm2 wall thickness. This is much thicker than the corresponding data for the water towel experiments, which is 1.5 g/cm2 (Kodaira et al., 2014) on the surface of the window wall in the crew quarters (Fig. 3 (d)).

In the PHITS simulation, the virtual spacecraft was irradiated by trapped protons and GCRs with charges up to +28 in the isotropic irradiation geometry. The procedure for determining the energy spectra of these incident particles is described elsewhere (Sato et al., 2011). The details of ISS altitudes over the ALTCRISS phase 1-4 experiments were taken into account in this source term calculation. Doses and dose equivalents in the three PADLES were deduced from the PHITS simulation. The neutron fluxes in the bare PADLES were also estimated in order to utilize them in the PHITS simulation for investigating the radiator effects as described in the previous section. Figure 8 shows the ratios of absorbed dose and dose equivalents in the PE covered PADLES relative to

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those in the bare PADLES as a function of wall thickness. The statistical uncertainties are roughly 20%. As can be seen from Figure 8, the additional PE shielding dramatically reduces the doses and dose equivalents for thin walls, whereas the shielding effect becomes less significant with an increase of wall thickness. The reduction of dose equivalents measured by the water towel experiment were 37.7% for 1.5 g/cm2 wall thicknesses, which agrees with the calculated data for the one-sided PE covering PADLES fairly well, even though the thicknesses of the water towel and the PE shielding were slightly different.

However, the decrements due to two-sided PE shielding for 23 g/cm2 wall thickness are approximately 21% and 29% for absorbed dose and dose equivalent, respectively; these are higher than those obtained from the ALTCRISS experiment. This discrepancy may be attributable to the difference between the shielding distributions at the locations of the bare and PE-covered PADLES in the experiments: the bare PADLES was directly attached to the wall, while the PE-covered PADLES were located away from the wall due to the PE shielding.

This difference in position is important because the mean shielding distribution decreases by approximately 15% when a detector is placed 5 cm away from the wall rather than being directly attached. Based on both actual measurement and PHITS simulation, we have shown that a remarkable shielding effect for space radiation in LEO can be found only under very limited conditions with thin shielding thickness. Basically, authors have to pay attention not only to the shielding effect of the material but also location and wall thickness effects. In addition, any shielding materials are effective under thin shielding conditions of up to a wall thickness of a few tens g/cm2 (Cucinotta et al., 2006, 2013).

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TeSS Radiation Shield of International Space Station Crew Quarters

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The International Space Station (ISS) Node 2 United States On-orbit Segment (USOS) is the home of four Crew Quarters (CQs) designed as the sleeping quarters for crew members during the duration in orbit. Each CQ provides a personal, private location for crew members to sleep, relax, and call home during their stay on the ISS. The CQ was designed with an inividual ventilation system, acoustical mitigation materials, laptop connections, and internet connection to allow crew members personal communication with family and friends. Since their deployment in 2008, the CQ performance has been closely monitored to validate that the design continues to meet requirements. Throughout the last 4 years, minor issues were discovered due to on-orbit environments, and modifications were made to the existing CQ outfitting to provide additional crew safety and comfort.

The CQ provides 2.1 m3 of interior volume equipped with radiation protection, acoustic absorbing materials, light, ventilation, laptop power, and internet connections( Fig. 1). Designed to accommodate crew members for long-duration spaceflight, the CQ has a large amount of attachment points to allow crew members to personalize their sleeping quarters during their stay on the ISS. The structure can be divided into three main areas: bump-out, rack, and pop-up. To maximize the amount of interior volume, the bump-out and pop-up were designed to contain key features for operation as well as provide additional headroom. The bump-out houses the ventilation system and is comprised of aluminum panels covered in acoustic absorption blankets. The ventilation system provides airflow at three different speeds, allowing crew members to adjust airflow to their preferred settings. The rack structure is comprised of carbon fiber composite panels on the sides and bottom of the CQ. The back and pop-up of the structure were built with ultra-high molecular weight polyethelyne to provide radiation protection.

The interior and exterior structure is covered with acoustic blankets that mitigate sound absorpotion into the CQ volume, allowing crew members a quiet environment for personal use. The interior blankets consist of a quilted configuration of Gore-Texr , kevlar felt, and Nomexr . The exterior blankets consist of a quilted configuration of Gore-Texr , BISCOr , durette felt, and Nomexr . Adjustable lighting is provided for the crew member by the General Luminaire Assembly light, which uses fabric shades to allow for additional adjustability.

Radiation Assessment The TeSS was a protoflight unit developed in 2001 and launched on STS-105/7A.1. TeSS was located in the US Laboratory Module (LAB), Destiny, and provided a short-term solution for sleeping quarters that allowed the

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ISS crew member size to increase from two to three. TeSS provided a private sleeping volume with limited functionality as compared to the current ISS CQ in Node 2. The operational life for TeSS was extended beyond the original 2 years, and TeSS was operational until 2010.

In March 2010, TeSS was scheduled to be demanifested from the ISS. At that time, the CQ project decided to assess the benefits and feasibility of integrating the TeSS radiation bricks into the CQ on the ISS for additional radiation protection.

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The radiation reduction benefit was characterized by the Space Radiation Analysis Group (SRAG) at Johnson Space Center (JSC) and the CQ project studied the integration feasibility of deploying and stowing the radiation bricks inside the CQ volume.

The TeSS radiation shield (Fig. 9) is assembled from eight custom brick assemblies and 20 flat brick assemblies (Fig. 10). Each brick assembly is composed of 5.08 cm of High-Density Polyethylene.

Two 2.54-cm blocks are pinned together to make the 5.08-cm brick. Each block is wrapped in a Nomexr sleeve and then assembled into the 5.08cm brick that is then wrapped in aluminum tape.

The goal of the radiation analysis completed by SRAG was to provide input on the most effective placement of the TeSS radiation bricks in the CQ to reduce crew exposure during a contingency radiation event.

Repurposing the TeSS radiation bricks provides a benefit to crew health by supplementing the already existing radiation protection in the four ISS CQ located in Node 2 and adheres to the As Low As Reasonably Achievable principle, which guides NASA Radiation Protection. The analysis evaluated eight different configurations of multiple flat TeSS bricks in the CQ volume assuming that all four CQ are located in Node 2.

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The reduction of effective dose results are based on ISS Incement 21 environment for the low Earth orbit estimates, and the 1972 King Spectrum for worst case solar particle event estimates. The analysis concluded that addition of the TeSS radiation bricks to the upper body locations in CQ is the best use of the material and there is a clear benefit to integrating the additional radiation protection inside the CQ. Deploying six TeSS bricks on the back wall of each of the CQs can achieve approximately 16% of additional radiation reduction in nominal conditions.

Furthermore, if the bricks are placed in other configurations that were analyzed, additional radiation reduction in nominal conditions can range from approximately 4% to 16%. The integration assessment evaluated five different configurations of six flat brick assemblies inside the CQ volume. The CQ trainer that is located at JSC was used for the evaluation. The five different configurations included permutations of attaching the bricks to the CQ back wall (Fig. 11), back wall and floor, and sleep wall and floor. Additionally, the evaluation considered whether the radiation bricks could be deployed on the CQ back wall underneath the CQ blankets. Installing the bricks in this manner would keep the integrity of the crew attachment points (Velcror ) for personal items on the back wall, as well as eliminate the abrasion points from the TeSS brick pins.

In addition, the SRAG further characterized the five different configurations in terms of additional days in orbit for a crew member, where low is approximately 3 additional days, medium is approximately 6 additional days and high is approximately 12 additional days. As confirmed by the radiation analysis, the configuration with six bricks located on the CQ back wall provided the the most effective reduction. Table 1 shows the results for each configuration evaluated in terms of additional days in orbit and number of crew attachment points lost based on placement of the TeSS bricks in the CQ on top of the acoustic blankets.

Based on the integration evaluation, the CQ project team concluded that the crew has many options for deployment and/or stowage of the radiation bricks in the CQ volume. The CQ project considers the bricks as crew preference items and all configurations of the bricks are acceptable. Currently the 20 flat brick assemblies are onorbit as crew preference items and are used in the CQs at the discretion of the crew.

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Space Shuttle drops down the SAA doses on ISS

Figure 3 (upper panel) shows the results of R3DE measurements of the SAA dose rate in µGy h−1 for the time span between 1st of March 2008 and 6th of December 2008. The SAA doses are separated from all R3DE data by 2 simple requirements. The first one is that the dose rate be larger than 200 µGy h−1 , which exclude the Galactic cosmic rays (GCR) dose rates being usually below 50 µGy h−1 . The second one is that the dose to flux ratio has to be larger than 1 nGy cm2 part−1 . This requirement excludes the parts of orbits with relativistic electrons precipitations (REP), in which the dose-to-flux ratio is less than 1 nGy cm2 part−1 .

The 3 patterns marked with filled but transparent rectangles and labelled as STS-123, 124 and 126 show the time when the 3 different missions of Space Shuttles were docked with the station. The labels in the upper part give the exact date, UT time of docking and undocking. One non-filled nontransparent rectangle shows the longest period with "No Data". A few others are seen in the picture. The difference with the Shuttle troughs is

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that for "No Data" occasions blanks are seen in both panels. The points in the upper panel show the measured dose rates during one crossing of the SAA region. Because of large time differences in the X axes these dose rates are seen as bars between 200 µGy h−1 and the maximum observed in the SAA dose rate level.

The lower panel of Figure 3 shows the altitude where the doses are measured (bars). The data in both panels are plotted for all parts of orbits of the station where the dose rate is higher than 200 µGy h−1 . That is why the altitudes in the lower panel are not seen as points but as bars between the beginning and end altitudes. The relatively low dose rates at the left side of the figure are connected with ISS altitudes in the range 350-365 km.

The increase of the station altitude up to 365-375 km after 21st of June 2008 is connected with an expected increase of the SAA dose rate in average with 10.4 µGy h−1 . The main feature seen in Figure 1 is that during the Space Shuttle docking time SAA doses fall below the level of more than 1000 µGy h−1 down by 600 µGy h−1 and reach an average level of 500 µGy h−1 for the STS-119 and 124 missions. For the STS-126 the drop down is also of 600 ìGy h-1 but from an average level of 1400 µGy h−1 .

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R3DE Latitudinal distribution of the ISS radiation environment components Figure 4 presents the latitudinal distribution in L-values of the doses along the ISS orbit. On the X axis the L-value is plotted. On Y axis the dose rate measured by the R3DE instrument in 3 panels is plotted. The 3 panels cover data from 5th to 31st of March 2008.

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On the bottom panel the 5-12 March data are plotted. This is a period before the docking of STS-119. Three different radiation sources are easy to distinguish from the data. The major amount of measurements is concentrated in the GCR points, which are seen as area with many points in the lower part of the panels in Lvalues range between 0.9 and 6.2. The covered dosed rate range is between 0.03 and 2025 µGy h−1 . The lowest rates are close to the magnetic equator, while the highest are at high latitudes equatorwards from both magnetic poles.

The measured by R3DE GCR dose rates in the whole invariant latitudes profile show a small decrease during the STS-119 docking period in March 2008. For example, the average dose for the period 19-24 March during the Shuttle docking at L=5 (the real diapason is 4.9<L<5.1) is 14.13 µGy h−1 . For the period 04-08 April 2008 (after the Shuttle visit) the average dose at L=5 rises up to 14.6 µGy h−1 . This tendency is realized in reverse to the GCR Oulu neutron monitor measurements http://cosmicrays.oulu.fi/, with a mean value for the period 19-24 March of 6678 corrected count rate per minute, while 6627 for the period 04-08 April 2008 is 6627. Similar peculiarities are observed during the STS-126 docking time. GCR dose rates before the docking of the Shuttle (11-17 November 2008) have an average value of 16 µGy h−1 , while with Shuttle (19-24 November 2008) the dose rates fall to 13.14 µGy h−1 . This happens also in reverse to the GCR Oulu neutron monitor measurements.

In the first period the corrected count rate per minute is 6722, while for the second it is 6772.

Another attempt to compare the GCR dose rates at L=5 for the time after the docking STS-126 shows dramatically low dose rates of 10.8 µGy h−1 in the period 2-8 December 2008 when the average Oulu neutron monitor count rate is 6772 per minute. The second source are the protons in the inner radiation belt (RB), which are situated as a large maximum in the upper-left part of the panels. They cover the range in L-values between 1.2 and 2.6. This area is usually denoted as the South Atlantic anomaly (SAA) region. The dose rates in the SAA region vary between 10-15 and 1130 µGy h−1 . The structure seen inside of the SAA maximum is connected with the different way of crossing of the anomaly by ISS along the orbit. The wide peaks, which cover almost the whole L range, are measured during the descending parts of the orbits of the station. The maximum dose rates of about 1130 µGy h−1 are observed at these descending orbits. The ascending parts of the orbits are seen as short peaks and the maximum dose rates there are about 1000 µGy h−1 .

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The wide maximum in L-values between 3.5 and 6.2 is connected with the observations of rare sporadic Relativistic electrons precipitations (REP) generated in the outer RB . Here the absolute maximum of dose rate value of 19194.6 ÂľGy hâ&#x2C6;&#x2019;1 is reached. This large dose is deposited in 10 seconds by 167264 electrons with energies above 0.8 MeV. The average maximum of the REP tends to be close to the Lvalue of 4.6. The higher density of the points for L-value below 4.6 is explained with the asymmetries in the magnetic field in the Northern and Southern hemisphere. Because the larger distance of the magnetic pole in the Southern hemisphere than in the Northern the observed L-values in the Northern hemisphere are with a maximum L value of 4.7. In the Southern hemisphere the maximum L-value of 6.2 is reached.

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The middle panel (dose rate distribution) shows very similar features as the lower one with the exception that the SAA maximum is reaching maximum values of 530 µGy h−1 again at the descending orbits. The effect is explained by the additional shielding against the SAA 30 to 150 MeV protons, provided by the 78-tons Shuttle, which during the period 13-25 March is docked with the ISS. The upper panel distribution is accumulated after the visit of the Space shuttle and is very similar to the lower panel.

Liulin-5 data Figure 5 presents the dose rates measured by the 3 detectors of the Liulin-5 instrument from 7t h to 31st of March 2008. The lower panel presents the measured doses in the first detector of the telescope, which is closer to the Phantom surface and respectively at lower shielding. The middle panel presents the data from the second detector, while in the upper panel are the data from the third (most shielded) detector.

As is expected, the maximum dose rates are seen in the bottom panel, while the minima are in the top panel.

Even in the less shielded first detector the dose rates are about twice lower than the ones observed at the same time by the R3DE instrument and presented at Figure 3. This is because even the less shielded direction of this detector is behind about 5 gcm−2 , which is about 12 times heavier shielded than the R3DE detector (0.4 gcm−2 ).

Another important feature seen in the three panels is the decrease of the doses after 13th of March 2008 and increase again after 25th of March. The exact moments are 03:49:00 UT on 13th of March and 00:25:00 on 25th of March. These moments are connected with the docking and undocking, respectively, of STS-119 to the ISS. The decrease has different values in the three detectors. In the first detector (bottom panel) the dose rate decreased from about 500 µGy h−1 to 300 µGy h−1 , while in the third detector the doses fall from 150 to 100 µGy h−1 .

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Because of very low time resolution in Figure 5 each crossings of the SAA anomaly is presented by a pair of 2 bars. The first one corresponds to the ascending orbit, while the second one to the descending. The differences are produced by the east-west asymmetries of the proton fluxes in the region of the SAA. Looking more precisely on the data in the bottom panel we see that after 13th of March the first (ascending) maximum starts to be larger than the second- descending. This is not connected with the arrival of the Shuttle but with a change of the orientation of ISS. the station was rotated at 180â&#x2014;Ś from its usual orientation when the US laboratory module is against the vector of the station velocity toward the opposite one. This maneuver rotates the ascending and descending maxima in the first and second detectors of Liulin-5 but not in the third (most shielded) detector.

As is seen from the top panel of Figure 5 the ascending maximum is always smaller than the descending one.

R3DE data show a similar behavior as is seen in Figure 4.

The decreases of the dose rates, observed by the R3DE and Liulin-5 instruments, when the Space Shuttle is docked with the station can be explained by the additional shielding against the SAA 30 to 150 MeV proton fluxes, provided by the 78-tons Shuttle to the instruments inside and outside of the ISS.

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Qualitatively this is shown on Figure 6. The large and heavy body of the Shuttle covers a wide angle of view (shown with light and heavy and dashed lines) of the R3DE and Liulin-5 instruments.

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Shielding effect of the ISS truss

Shielding produces modification of the quality of radiation. Slow ions may be stopped in the shield, while more

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energetic larger Z ions tend to be fragmented into lighter ions that will deposit lower energy (lower LET). These will provide lower dose and even lower dose equivalent (that depends on the LET through the weighting factor , but will also increase the flux, with the fragmented lighter ions. This is noticeable by observing the larger differences in dose rates and dose equivalent rates when changing detector position and across directions(La Tessa et al. 2009; Narici et al. 2012).

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To interpret the radiation behavior during detector position changes, or across different directions, we observe that for each position there are two contributions to the shielding: the parts of the ISS, "external" to the modulus where the measurement takes place, and the "internal" contribution by racks and experimental apparatuses.

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"External" shielding are the other modules, and the truss. The other modules (and the nodes) strongly increase the shielding in the X direction when measuring in the USLab (X is looking through several modules and nodes), the truss is increasing the shielding in positions 3 and 4, direction Z (looking straight toward the truss), and, to a lesser extent, in direction Y (looking obliquely to the truss). The dose equivalent rate in all US Lab positions is indeed much lower for the X direction, by as much as a factor of 3, confirming previous measurements (Di Fino et al. 2011; Larsson et al. 2015).

"Internal" shielding is due to the presence of all racks and experiments. In this case it is important to consider the exact location of the detector system (see Figs. 1 and 3). In position 4 all three directions are similarly positioned externally to the racks (Fig. 3 bottom right). In the other three US Lab sites one telescope pair (one direction) is inserted in an empty drawer (or in a missing rack, for position 1) and the other two pairs are flush on racks. When inserted in a drawer the shielding is reduced with respect to the one in position 4 (due to the empty drawer), even more so if inserted where a rack is missing (in this case the telescope is facing only the hull of the module).

When the telescopes are flush to the racks the shielding is higher, because part of the field of view of the telescopes looks inside the racks. In the case of position 1 this effect is reduced as the closest rack is missing. Combining all the above effects it is possible to interpret the radiation changes when moving the detector system from site to site.

Figure 6 shows a comprehensive view of the HL radiation levels. The results are time averaged over the same position and shown independently for each direction, providing important inputs for model validations. The X direction featuring the highest shielding, similar in the four US Lab sites, provides the least differences in dose and dose equivalent rates in the four measured sites. The Y radiation values appear similar in positions P2, P3, and P4. In these measurements the detectors were looking transversely to the module and racks, and the total amount of shielding was comparable. Measurements in P1,which correspond to the mentioned missing rack, show values higher than in the other positions. Finally, the Z radiation values show the largest differences across the five measured sites, and this is due to the position relative to the truss and to deployment of the detectors, either flush to the rack or facing the center part of the US Lab. The dose rates to an astronaut (within the ALTEA acceptance) would be described by the average of the levels shown in Figure 6 across directions for each position. This description of the radiation environment variations

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(from one detector position to the next) linked to the shielding distribution is qualitative. A quantification of the shielding influence on the radiation environment needs a detailed CAD model that will have to be used in conjunction with these data (and with all the similar data that will become available) for a complete and detailed validation of space habitat radiation models.

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Habitats for deep space exploration (vessels as well as bases) will feature a similar degree of complexity that we observed in the ISS, therefore models should be able to manage this complexity. The ISS appears to be a good test-bed for this endeavor. One of the important issues to face would be the definition of the degree of details a model (including CAD) must feature to properly describe these radiation differences to the precision needed for risk assessment.

The radiation during the passes through the SAA is strongly variable as shown in Figure 13. The flux measured during these passes depends on altitude and, being mostly composed of low energy protons, it is heavily influenced by the shielding. The larger amount of SAA flux measured at P5 (Columbus) is partly due to the altitude and partly due to the lower shielding in the Z direction (as mentioned the USLab is to some extent Z-shielded by the truss, vice versa Columbus is out from its protective shadow).

In order to compare our results with several other previous works, authors also provided integrated, whole orbit values. These are significantly lower than what was found by other groups. As an example, in dose ALTEA measures only about 20% of the values reported by Semkova et al. (2014) (silicon detector Liulin).

This is mostly due to the reduced proton sensitivity and the large contribution of the SAA (about two third of the total dose according to Semkova et al. 2014). An interesting comparison is with data from CR39, a passive detector with sensitivity only above 10 keV/lm, so with almost no proton sensitivity. In this case (see, for example, Nagamatsu et al. 2013, measurements in the ELM-PS Kibo module, just a few months before the ALTEA ones) the dose measured by ALTEA is slightly larger than what was reported for the 2009-10 CR39 Kibo measurements.

Dose equivalent in Kibo, however, is about 80% larger than what is reported by ALTEA. This could be accounted for by the different shielding (Kibo is less protected by the other modules and by the truss).

The truss is confirmed to be an effective shield, while just staying close to a rack is shown to lower the quality of the radiation, therefore to decrease biological risk (these could be useful information for the best planning for sleep quarters).

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