STEM TODAY February 2017, No.17
STEM TODAY February 2017 , No.17
CONTENTS Cancer 11: What are the most effective shielding approaches to mitigate cancer risks? MIR Space Station Part Â3 March 2017 , No.18
Editorial Editor: Mr. Abhishek Kumar Sinha Editor / Technical Advisor: Mr. Martin Cabaniss
STEM Today, February 2017, No.17
Cover Page NASA’s SDO Sees Sun Emit Mid-Level Flare Oct. 1 NASA’s Solar Dynamics Observatory captured this image of a solar flare - as seen in the bright flash on the lower right limb of the sun - at 8:12 p.m. EDT on Oct. 1, 2015. The image is a blend of three wavelengths of extreme ultraviolet light that have been colorized. Image Credit: NASA/SDO
Back Cover SDO-The Sun Now This channel (as well as AIA 211) highlights the active region of the outer atmosphere of the Sun - the corona. Active regions, solar flares, and coronal mass ejections will appear bright here. The dark areas - or coronal holes - are places where very little radiation is emitted, yet are the main source of solar wind particles. Where: Active regions of the corona Wavelength: 335 angstroms (0.0000000335 m) = Extreme Ultraviolet Primary ions seen: 15 times ionized iron (Fe XVI) Characteristic temperature: 2.8 million K (5 million F)
Image Credit: NASA/SDO
STEM Today , February 2017
Editorial Dear Reader
STEM Today, February 2017, No.17
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 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, February 2017, No.17
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.
Special Edition on Space Radiation
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Mir Space Station
Space radiation has been the subject of numerous studies from the early beginning of space flight. The launch of Skylab in a 50.2◦ inclination orbit in 1973 began a period in which astronauts spent weeks to months at a time in space. However, with the conclusion of the three manned flights during the Skylab program, the space shuttle era (starting in 1981) has had flight durations of only 7-15 days. The launch of the Mir Orbital Station in 1986 began a fresh era of long duration manned flight that will be carried forward with the International Space Station (ISS). The ISS is in the same 51.6◦ inclination orbit as the Mir Orbital Station. Thus, the radiation data acquired onboard Mir has direct applicability to the ISS.
Why is the space station in a 51.6◦ inclined orbit instead of something less or something more?
51.6◦ is the lowest inclination orbit into which the Russians can directly launch their Soyuz and Progress spacecraft. Both of these vehicles serve an important role in ISS operations.
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The Soyuz – there is always one attached to ISS – serves as an escape vehicle in the event the ISS would need to be abandoned in an emergency. The Progress spacecraft is basically a cargo version of the Soyuz and is used to bring up fresh food and supplies to the ISS. Ideally, one would want to launch due east from a launch site to maximize the cargo-to-orbit capability for a given launch vehicle. This is because the Earth, rotating from west to east, gives rockets a "free" head start in the right direction. Launching due east from Kennedy Space Center would place the shuttle in a 28.5◦ inclination orbit. Notice that the inclination is the same as the latitude of KSC. Launching due east from Russia’s main launch site, Baikonur, would place spacecraft in a 45.6◦ inclination orbit – the launch site latitude. However, doing so would also drop the lower stages of the boosters on China. To avoid this, the Russians crank up the minimum inclination to 51.6◦ . Although the shuttle does trade some payload capacity for propellant needed to make up the difference between launching at 28.5◦ vs. 51.6◦ , doing so allows the Russians to participate in the ISS program. It also has the added benefit to Earth Sciences since ISS flies over more of the Earth’s surface – about 75 percent, which covers about 95 about of the inhabited lands – at the higher inclination orbit. Jim Cooney ISS Trajectory Operations Officer (TOPO) Orbit 3 (planning shift) for STS-112
The construction of the Mir Space Station began with the launch in February 1986 of Mir, the central portion of this complex. The first module to be attached to the Mir was the Kvant-1 module on March 31, 1987, and the last was the Priroda module in May of 1997. The first crew was launched on March 13, 1986 using a Soyuz module and spent a total of 123 days in orbit. The station was left unmanned until February 1987, when another crew was launched for a 10 -month mission on the Mir. From that time until August 1999, when the last crew returned to earth, the Mir Station was continuously manned. Thus, the Mir Station has provided space radiation data spanning a complete solar cycle, which had one of the highest recorded levels of solar activity, one of the largest Solar Particle Events (SPE) observed in the last 40 years, and one of the largest geomagnetic storms (March 1991). The March 1991 magnetic storm was responsible for the creation of an additional region of trapped particles. A wide variety of detectors were used to make these observations, including blood draws from the US astronauts. Mir Orbital Station and shielding distributions Fig. 1 shows the full Mir Orbital Station with all of the modules attached. The attachment of modules results in the Core, Kvant-2, and Kristall modules at right angles to each other. This resulted in changing the shielding configuration over time, and should be kept in mind when interpreting the radiation data. The Core Assembly
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was launched in February 1986. It is the heart of the Mir complex and is made of four compartments: Transfer, Working, Intermediate, and Assembly. The assembly compartment is unpressurized. The working compartment is the main habitable volume, and is made up of two concentric cylinders connected by a tapered conical section. The maximum diameter is 4:1 m, with a total length of 13:2 m and a weight of approximately 21 metric tons. The forward and aft bulkheads of the working compartments are spherically curved with the forward bulkhead connecting to the transfer compartment. It has a 2:2 m diameter. The living area in the smaller diameter section operations zone is divided into the galley area with table, a cooking area, and a trash storage facility. The operations area contains the piloting section, medical monitoring equipment, ergometer, etc. The cosmonauts spent most of their time in this module, and it is here where most of the radiation measurements were made.
The Kvant-1 Astrophysics Module was launched on March 31, 1987, and because of technical difficulties did not dock to the Mir until April 12, 1987. It is 5:8 m in length and has a maximum diameter of 4:35 m. The Kvant-2 was launched on November 26, 1989, and docked with Mir on December 6, 1989. The module has an airlock that made Extravehicular Activity (EVA) easier to perform. It is 12:4 m long with a maximum diameter of 4:35 m. The module has two solar panels. The launch of the Kristall module occurred on June 1, 1990. It was docked
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to the Mir on June 10, 1990, opposite the Kvant-2 module. The Kristall module measured 12 m long with a maximum diameter of 4:35 m and had two solar panels that could be folded or unfolded depending upon the electric power demand. This con:guration remained until the Spektr (May 1995) and then the Priroda (May 1997) modules were attached during the joint NASA-Mir Program. The Mir complex was periodically serviced using the Soyuz and Progress crafts.
Table 2 gives the shielding distributions at seven different Mir locations. These distributions are used to compute the doses at the different Mir locations. Fig. 2 presents a comparison of four different shielding distributions corresponding to the locations of four active radiation instruments.
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Special Edition on Space Radiation The Russian space program has always considered radiation a possible hazard to the cosmonauts and undertook an aggressive effort for radiation dosimetry measurements. Therefore, a variety of operational radiation detectors have been flown on the Mir Station.
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The major active instruments are listed in Table 3. In addition, numerous results were obtained using passive systems, like plastic track detectors of different LET threshold, various kinds of thermoluminescence detectors and nuclear emulsions. Only a brief description is given of those detectors that have provided a significant amount of data. This is done in order to make a comparison of data sets and to understand the sources of the differences between the various measurements. It is by no means a comprehensive description of these instruments for which original references should be used as the primary source of description and information.
It should be clear from the variety of devices used and the length of time they have operated, that a wealth of radiation data has been accumulated.
However, because each detector has a different response that is not easily calibrated or understood, and because data were acquired under differing solar conditions, it is not straight forward to compare different data sets. An attempt has been made to understand these differences. It is, however, worth pointing out that most of the data were obtained during the EuroMir and NASA Mir missions.
A large number of measurements by a variety of groups has been made using passive detectors. In this section only the measurements of the ionizing part of the radiation field are described, the neutrons are covered in a separate section. Table 6 gives details of the measurements.
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Crew exposures
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Table 7 gives the Russian data for each of the Mir missions. Fig. 4 is a plot of the average, upper, and lower cosmonaut dose rates as a function of time. In interpreting these data, it should be noted that the variation in dose rates is due to changing solar activity, and variations in altitude, including periodic altitude boosts. The measured dose rate is from charged particles only, and does not include the contribution due to neutrons. The highest dose rate measured during the solar minimum is about 480ÂľG y −1 The dose rate is about a factor of 2 lower at the time of solar maximum. During the NASA Mir missions, the US astronauts wore dosimeters from both NASA and IBMP. Table 8 gives a comparison of these observations.
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Fig. 5 is a cross-plot of the two dose rates. The solid line is the least-square fit line, and is given by: US Dose Rate (µG yd a y −1 ) = (3.72 ± 30.55) + (1.095 ± 0.11) IBMP Dose Rate with χ2d f = 0.014. The two rates are well correlated; however, the US dose rates are systematically higher by about 10%. The US and Russian dosimeters were calibrated using a137 Cs source.
Recently, the US dosimeters were re-calibrated using protons, helium, carbon, silicon, neon, and iron ions of various energies that spanned the linear energy transfer range observed on board Mir.
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Based on heavy ion calibration and measured LET spectra on each of these missions, US crew absorbed doses are estimated to be about 20% lower than the values with 100% TLD dose efficiency. About 5% of the dose reduction can be accounted by the lower TLD dose response to high LET particles.
Crew exposures were also monitored for the German astronauts during the solar minimum time period of the Phase 1 program. Measurements were made at the wrist using TLD-700. During the EuroMir 95 mission (Mir-20), the dose rate was 245±5 µG yd a y −1 in excellent agreement with measurements using Pille´ 95 of 247 ± 3 µG yd a y −1 . These measurements are consistent with measurements made on cosmonauts around the same time.
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Special Edition on Space Radiation Normally, the EVA doses of crewmembers are not recorded. NASA Mir-4 was an exception in which the system was used to monitor the EVA exposures of two crewmembers. These data are summarized in Table 9 and show that EVA dose rates through the South Atlantic Anomaly are nearly 3 times greater than doses inside the Core module.
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Mir-18 (NASA Mir-1) was the first flight with a US astronaut. Table 10 gives a comparison of NASA and IBMP measured dose rates at locations in the Core module. Care was taken to assure that the two sets of dosimeters were next to each other so that the shielding distributions were identical.
A plot of the IBMP dose rate versus the NASA dose rate is given in Fig. 6. Typically, the errors on IBMP dose rate are about twice that of NASA measured dose rates. The dose rates are well correlated and given by the regression equation: IBMP Dose rate (µG yd a y −1 )=145.5 + 0.75 NASA Dose Rate. There are some IBMP measurements that are higher than NASA measurements. In addition to area dosimetric measurements, time resolved measurements were made using the R-16 and TEPC detectors, and these are given in the Table 10 also. In addition, an area passive dosimeter (APD) containing TLD-700 and CR39 was flown adjacent to the NASA TEPC. Correcting the APD measurements for the TLD-700 efficiency with LET using the CR39 measured LET spectrum, the APD dose rate increased by 31 µG yd a y −1 , which is in excellent agreement with TEPC measurements. Using the AP-8MIN model and the shielding distributions given in Table 2 and Fig. 2, the calculated dose rates are given in Table 10. The calculations use the Jensen and Cain (1962) 1960 field. The proton flux is about 30% less than obtained using the International Geomagnetic Reference Field 1964/epoch 1965.
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Lemaire and Daly (1996) have successfully argued that the correct geomagnetic field model to use with the AP-8MIN trapped proton model is the Jensen and Cain field model (epoch 1960), since it was the model used to construct the AP-8MIN model. It is also noted the differences between the R-16 and TEPC dose rates are likely to be due to shielding differences. The Mir-19 mission lasted 76 days. Every attempt was made to co-locate the NASA and IBMP dosimeter next to each year and thus these measurements should be directly comparable.
However, for about 30 days, both sets of dosimeters were stored together near the Engineer’s cabin, and thus do not truly represent the dose rate at the six locations (Table 11). Fig. 6 is a plot of IBMP dose rate versus the NASA dose rate. The two dose rates are highly correlated by the IBMP are systematically lower with the regression equation given by: IBMP Dose rate (µG yd a y −1 )=-4.72+0.87 NASA Dose rate. The ISDA dosimeter was launched separately on a Progress, and thus has a somewhat diLerent exposure history than the NASA/IBMP set. The ISDA dose rates are higher than NASA dose rates (Table 11).
Nominally, the location of the ISDA dosimeter is supposed to be the same as the IBMP or NASA dosimeters. However, these results show that although the ISDA dosimeters are in the general area identified, the location is unlikely to be the same as the NASA or IBMP dosimeters. Thus, even in a fairly small area there are variations in dose rates of 25-30%. Thus, intercomparisons of different experiments are possible if they are well-planned and coordinated. It is also worth noting that TLD-600 dose rates are always higher than the TLD-700 dose rates, indicating the presence of the thermal neutron component. It also points to the fact that if purely charged particle doses are to be measured, TLD-700, and not TLD-600 or TLD-100 dosimeters, should be used. Benton, University of San Francisco (USF), flew a set of six area passive detectors containing both TLDs and CR39 during NASA-Mir 2, 3 and 4. Table 12 gives these data together with a comparison of dose rates with measurements made by IBMP during NASA-MIR 2 at the same locations. The TLD-600 measurements are again higher than TLD-700 measurements, indicating the presence of low energy neutrons. It is clear that IBMP dose rates using TLD-700 are systematically higher than the rates measured by USF using TLD-700 at the same location. It is more useful to compare dose rate by various investigators at the same location. This is possible at some of the locations in the Core module, with the caveat that the location may still not represent an identical shielding distribution. Table 11 summarizes the measurements at fixed locations, although it is not complete. The largest number of observations was made in the area of Flight engineer’s cabin and span the time period from May 1991 to August 1998 thus covering the solar minimum. The altitude range was small varying from 378 to 408 km or a variation of about 30 km. There is a variation factor of about 2 in dose rates due to solar modulation, which is similar to that seen for the cosmonauts. It is worth noting that individual investigators had their own calibra-
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tions and no inter-calibrations were done (Table 13).
Stacks of CR-39 detectors were exposed during the EUROMIR-94 mission inside the Mir Station. The evaluation yields LET spectra separately for long-range cosmic ray heavy ions and for short-range target fragments. The short-range spectrum is normalized to 4π sr, and the GCR spectrum is normalized to 0.67 X 4π sr to consider the effect of the earth shadow. The GCR spectrum in Fig. 7 shows the change in the slope at the LET of minimum ionizing iron. For LETs larger than 60 keV µm −1 there is an excellent agreement between the measured spectra and the Siegen model calculation for a shielding of 20 g cm−2 . The LET spectrum was determined using only particles within a restricted angular dependence for which the measured distribution agrees with the expected one. This angular acceptance, for which the detection efficiency is one, shrinks with falling LET-values. The result is that the measured cosmic ray flux is lower than predicted as shown in Fig. 7. The "all track" curve in Fig. 7 contains all tracks observed on the detector foil. The LET-spectrum of the secondary particles (difference between all particles and GCR spectrum) is given together with the GCR spectrum in Fig. 8. For lower LETs the secondary particles dominate, whereas for higher LETs, the GCRs dominate. Due to the etching process (50 µm of the detector surface is removed) most of the target fragments with charges equal or greater than 3 are not included in the measurements. It is clear from these measurements that secondary particles contribute significantly to the absorbed dose, their part possibly equals that of the GCR contribution.
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NASA TEPC measurements The NASA TEPC arrived at the Mir Space Station in late 1994 on a Progress supply spacecraft. Fig. 9 shows the first 3575 min of TEPC dose rate data after it was activated. The "spikes" are passes through the SAA and the lower values are background GCR dose rates. TEPC dose and dose equivalent measurements were taken at several locations in the Mir during the NASA-Mir Program; some of these are shown in Table 14.
Fig. 10 shows the integral LET spectrum for GCR particles compared with the Badhwar-O’Neill GCR model and also shows the characteristic change in slope around 138 keVµm due to relativistic iron nuclei. The calculated absorbed dose rate was 133.6 µG yd a y −1 with quality factors Q(ICRP-26) = 3.35 and Q(ICRP-60) = 3.60. Fig. 11 shows the combined integral trapped and GCR LET spectrum and is compared with the calculated NAUSICAA spectrum based on the shielding distribution shown in Fig. 2. Fig. 12 is a plot of the integral LET spectrum for trapped protons and is compared with the calculated NAUSICAA and Liulin spectra, again, based on the shielding distributions shown in Fig. 2. Fig. 13 shows the measured LET spectra for trapped protons for various Mir modules. These measurements were taken near solar minimum, and the observed spectral differences arise due to shielding differences and to the east-west asymmetry of trapped particle fluxes. The effect of atmospheric density on trapped proton dose rate was also observed. Fig. 14 shows a plot of
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Special Edition on Space Radiation REM, R-16, and TEPC absorbed dose rates as a function of atmospheric density. Thus, based on the instrument and module location (shielding), one can "predict" the trapped proton absorbed dose rate as a function of atmospheric density. TEPC GCR dose rates have also been observed as a function of the deceleration potential from measurements taken on both the Space Shuttle and Mir.
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TEPC Mir data acquired inside the Core and Kvant-1 modules showed insensitivity of the GCR dose rate to its location. The Mir GCR dose rate data are also plotted in Fig. 15 and are in complete agreement with the Shuttle observations. Thus, as a practical matter, the quadratic fit line can be used to predict GCR dose rate at locations inside the habitable Space Station modules to better than Âą10%, which is the largest deviation of points from the fitted line.
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Special Edition on Space Radiation
DOSTEL measurements The Dosimetry Telescope DOSTEL was flown as part of the Active Dosimetry of Charged Particle (ADCP) experiment in the Kristall module of the Mir Orbital Station during the NASA-Mir-6 mission. Fig. 16 (top panel) shows the measured DOSTEL count rate on November 4-7, 1997, in a single detector. The lower panel of Fig. 15 gives the particle count rates of the GOES satellite, which is located outside the magnetosphere indicating a strong solar particle event (SPE) on November 6, 1997.
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Special Edition on Space Radiation The variation of the DOSTEL count rates in the absence of an SPE is due to the spacecraft traveling through different magnetic cutoff conditions during the orbit and to crossings of the radiation belt in the South Atlantic Anomaly (SAA) which produce pronounced peaks in count and dose rate. Outside the SAA the measured rate drops to the minimum at the equator (high magnetic cutoff) and reaches maximum values every 45 min at highest latitudes (low magnetic cutoff) on the Northern and the Southern Hemisphere, respectively.
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Connected to the Earth’s rotation the latitude regions of the orbit are reached at different longitudes, which cause an additional daily variation of the fluence at the highest latitudes. Given the time of activation, the DOSTEL internal time can be converted to mission elapsed time, which allows the determination of the orbital position for each measurement. The first phase of the SPE, as measured by GOES (bottom panel) is well detected inside the MIR Station by DOSTEL. The count rates are lower by three decades reflecting the shielding of the earth’s magnetic field. The SPE particles have only access to the MIR orbit in higher latitudes.
In order to demonstrate the dosimetric characteristics of the three main radiation contributions including secondary particles, Fig. 17 shows the energy deposition distribution of GCR and SAA particles averaged during five quiet days and of the SPE averaged over the available SPE data for orbit segments above 45◦ latitude on both hemispheres. The peak in the GCR distribution belongs to high-energy particles with charge Z = 1. Obviously, the SAA and SPE contributions show a different slope compared to GCR due to the different energy spectra of the detected particles. It can be deduced that the SPE contains more high-energy particles than the proton radiation belt (harder energy spectrum). The mission averaged absorbed dose with 247 ± 14 µG yd a y −1 is in good agreement with the doses given in Table 10 for the heavier shielded locations. GCR contribute 126 ± 4 µG yd a y −1 and the SAA components 121 ± 13 µG yd a y −1 to the total absorbed dose. The mean quality factors deduced from the LET spectra are 3.5 ± 0.2 (GCR) and 1.3 ±0.1 (SAA) and 1.55 ± 0.1(SPE). Quality factors reported from the TEPC are always higher for the SAA contribution due to the reduced sensitivity of the TEPC in the very low LET range, and the same holds true for the SPE contribution. However, just the opposite situation exists for the TEPC GCR quality factors, i.e., they are too low.
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CIRCE measurements Mir doses with the low pressure, tissue equivalent proportional counter, CIRCE, during the FrenchSoviet mission, Aragatz. The measurements were taken in December 1988 near solar maximum. The mean absorbed dose rate and dose equivalent rate were 0.32 µG yd a y −1 and 0.62 µG yd a y −1 , respectively. The neutron dose component of 0.023 µG yd a y −1 was recorded using a bubble detector. Measurements taken during the MarchApril 1989 timeframe showed an increase and were 0.45 µG yd a y −1 and 0.80 µG yd a y −1 with a mean quality factor, <Q>, of 1.9 ± 0.3 calculated from the LET spectra. The maximum absorbed dose and dose equivalent rates measured on the Mir during passes through the SAA were 0.76 µG yh −1 and 1.05 µG yh −1 , respectively. A mean <Q> of 1.4 was calculated during these passes.
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R-16 measurements
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Special Edition on Space Radiation
The R-16 instrument provided a continuous monitoring of the space radiation environment and absorbed doses in the Mir Core module as shown in Fig. 18 for the period of late 1989 to early 1995. This period coincided with near solar maximum to near solar minimum. The D1 (depth dose) detector shows a gradual dose rate increase with approaching solar minimum. The D2 (skin dose) detector, which is more sensitive to altitude and spacecraft attitude, also shows a gradual dose rate increase. The dose rate increases due to the September-October
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Special Edition on Space Radiation 1989 and March 1991 SPEs are apparent. Fig. 19 shows a color world plot (Mercator) with the corresponding absorbed doses for the CIRCE taken during Mir-4, the Liulin taken during Mir-11, and the TEPC taken during Mir-18. Note the changes of the SAA that occurred during the time period. Of particular interest are the Liulin measurements that show the effects of the major solar and geomagnetic activity that occurred during March 1991. Additional regions east of the SAA show transient particle L-shell trapping and the corresponding absorbed dose contours that were created as the result of this activity.
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Neutron measurements In the few experiments where both TLD-600 and TLD-700 dosimeters have been flown, the dose measured by TLD-600 is always higher than the dose measured by TLD-700, and since their efficiencies are nearly the same, the difference has been interpreted as arising from the presence of thermal neutrons. However, in estimating the radiation dose to the crewmembers, the contribution of neutrons is mostly neglected.
Although the absorbed dose due to neutrons relative to the absorbed dose from charged particles may be small, this is not necessarily true of the dose equivalent due to the high radiation quality factor of neutrons. Many earlier experiments performed on the Space Shuttle using metal foils (neutron energy, En <1 MeV) indicated a small dose contribution, and led to reinforcing the feeling that the neutron contribution can be ignored. Over the last several years, measurements made both on the Shuttle and onboard Mir Station clearly show that the
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Special Edition on Space Radiation neutron contribution to dose equivalent is comparable to the dose equivalent from galactic cosmic radiation and can no longer be ignored. These results are based on a wide variety of measurements made mostly using passive detectors. Two active detectors, the Rubina and the GRANAT-S(externally mounted on the Kristall module), can measure neutrons. The Rubina instrument has an integral neutron counting rate in the approximate 1-10 MeV energy range. Although these active devices show a significant presence of neutrons onboard the Mir Station, they cannot provide the dose equivalent. Thus, all of the dose equivalent data are obtained from passive detectors. Fig. 20 shows the differential neutron energy spectra measured by fission foil and emulsion experiments covering the energy range from 1 keV to about 50 MeV. Since the measurements were made at different locations, the average shielding depth is indicated and shows the increase in flux due to increased depth Table 4 shows the dose equivalent rates as a function of energy and shielding depth. These measurements, which were made two years past the solar maximum, show the total dose equivalent rate is comparable to the dose equivalent rate from galactic cosmic rays. Since the TLD measurements clearly indicate that the charged particle dose is about a factor of two larger at solar minimum than at solar maximum and the neutrons are produced by the collisions of charged particles, the neutron dose equivalent would also go up by roughly a factor of two.
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A comparison of dose equivalent derived from emulsions and fission foils with those made using bubble detectors or those derived by differences in TLD-600 and TLD-700 responses is difficult. Ing (2001), from a Mir 1993 flight, reported a dose equivalent rate of 92 µG yd a y −1 with 51% contribution due to neutrons above 10 MeV. Measurements taken on Shuttle flights that docked with the Mir Station (STS-81 to STS-89) gave dose equivalent rates varying from 96 to 138 µG yd a y −1 which are consistent with measurements taken on Mir. Luszik-Bhadra et al. (1999) flew a package of CR-39, TLD-600, TLD-700, and Makrofol track detector, flown during EuroMir 95 and EuroMir 97 missions. The response of TLDs was calibrated at the CERN neutron field. Both track detectors (CR-39 and Makrofol) have no response to thermal and intermediate energy neutrons. Both of these detectors are sensitive to α-particles and heavier nuclei, but the CR-39 is sensitive to protons as well. However, an electrochemical-etching procedure used for processing CR-39 allows a small energy window in which such protons can be detected (50 keV to 2 MeV). Thus, the contribution of primary protons and their secondaries is small. In the longer EuroMir 95 flight (178 days) the track density was large leading to a voltage breakdown and as such the data are not reliable. The astronaut measurement data and measurements at the bottom of core module gave a dose equivalent rate of 114 ± 6 µG yd a y −1 , whereas during the EuroMir 97 mission, the dose equivalent rate was 245±25 µG yd a y −1 . The rate determined from CR-39 during this mission, 235 ± 14 µG yd a y −1 , when there was no voltage breakdown, is in complete agreement with Makrofol measurements. Although the 10.7 cm solar radio flux and flight altitude were the same for the two missions, the GCR dose rate was higher during the EuroMir 97 mission, but cannot account for doubling of the dose equivalent. It is worth mentioning, again, that these dose equivalent rates are for fast neutrons only. These results show that: The secondary neutron dose equivalent rate varies by about a factor of two due to solar modulation, ranging from about 130-250 µG yd a y −1 . There is a factor of two increase in dose equivalent as the average shielding thickness increases from 20 to 40 gcm −2 of aluminum equivalent. The dose equivalent rates are comparable to the dose equivalent rates due to GCR. There are measurement difficulties with passive detectors, leading to uncertainties of up to a factor of two. Model calculated rates are consistent with measurements only if these uncertainties are taken into account. The high-energy neutrons (>1 MeV) provide most of the dose equivalent. The propagation of neutrons in the body is different than protons; the relative neutron contribution to organs will be diLerent than the skin dose equivalent measurements.
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Measurements on solar particle events
Several instruments flown on the Mir Space Station observed SPEs over the course of time. Shurshakov et al. (1999) reported measurements of 48 SPEs with free-space fluences (E<30 MeV) of 105 protons cm−2 or greater between November 8, 1987, and March 12, 1993. Because of geomagnetic shielding by the Earth’s magnetic field, fluences observed in the Mir were much smaller. Free-space fluxes were estimated from measurements made on the METEOR satellite. Table 15 lists the SPEs observed with the R-16 instrument on the Mir when free-space fluences were greater than 107 protons cm −2 .
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Note that the ratio of absorbed doses measured in the Mir to that in free-space ranged from 0.001 to 0.006. Thus, the geomagnetic shielding plays a major role in reducing the exposures in the Mir Station orbit. In the 3-month period between September and November 1989, the free-space dose accumulated behind 2 gcm −2 was 1.9 Gy. However, the accumulated dose in the Mir was only 0.036 Gy, 0.003 times less. For comparison, the absorbed dose measured in the Mir during this time from other sources (trapped protons and GCR particles) was about 240µG yd a y −1 . Thus, the exposure in the Mir during this period was 22 mGy. During this very active period for SPEs, the exposure in the Mir was twice that from the trapped protons and GCR particles. The largest SPE of this period was the October 19, 1989 event with an accumulated dose in the Mir of 27 mGy, which equals an extra 3-month stay on board. Fig. 21 is a plot of the cumulative R-16 dose as a function of time for this period. As can be clearly seen, there was a slight increase in dose during the September 1989 event.
This can be understood as due to geomagnetic shielding that did not change during the time and was nearly the same as that for a quiescent field. The situation was nearly the same for the October 19, 1989 event, when bow shockaccelerated particles arrived and the proton flux (E>100 MeV) at the Geostationary Operational Environmental Satellite (GOES) increased by an order of magnitude. The dose rate increased significantly.
If one looks at the Kp or Dst indices during this time, it is seen that the magnetic field changed significantly, with the geomagnetic cutoff dropping as low as 10 MeV soon after the passage of these particles. Calculations (Smart et al., 1994) have shown that the Mir Station was in the high latitude region during this time and, as such, the crew received the full impact of these events. The measurements reported above were made in the Mir core module where the shielding thickness prevents
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protons below â&#x2C6;ź 70 MeV from being observed. This, however, is not the case during an EVA where a crewmember is protected only by his spacesuit. As shown in Table 9, the crew receives nearly three times the dose outside the Core module than inside during SAA passes. Thus, if the SPE spectrum is similar to that in the SAA, the EVA doses would be 3-4 times larger that predicted in the Core module.
Time-resolved dose rates from the Liulin (Dachev, 1996) instrument have been obtained for the initial portion of the October 19, 1989 SPE, and SPE dose rates have been measured with the TEPC and DOSTEL instruments for the November 6-8, 1997 SPE. These events help to understand the dynamics of such events, since all of the input parameters are known. Fig. 22 shows the Liulin data as a function of time for the six passes when the dose rate was above background. Fig. 23 shows the TEPC and DOSTEL data as a function of time along with the >100 MeV GOES proton data. Even though the DOSTEL and TEPC were not co-located in the Kristall module (unknown shielding distribution), there is good agreement between each of the dose-time profiles (DOSTEL was turned-off after the last profile shown and thus does not cover the full time history of the event).
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The astronaut doses vary from 138 to 484 µ Gy d a y −1 over one solar cycle. The October 1989 SPEs cause an additional exposure of 9 mGy to the astronaut in orbit during this time. In total 23 SPEs are observed, but did not contribute significantly to the daily exposures, except the October 89 series. It is clear that 51.6◦ inclination still provides an effective shielding for SPE exposures.
R
Drift rate of the South Atlantic anomaly
The dose rate measured by Skylab and Mir, two manned space programs that were 21.2 years apart, was used to measure the drift rate (Badhwar, 1997). The data were acquired using tissue equivalent ion chambers from December 7, 1973, to January 8, 1974, at an average altitude of 438 km onboard Skylab, and from March 2, 1995, to March 11, 1995, at an average altitude of 400 km. The solar radio 10.7 cm flux was nearly identical during these two time periods. Fig. 24 gives the dose rates, averaged over latitude, as a function of longitude and clearly shows that the SAA has shifted in longitude by 6.0 ± 0.5◦ westward. Fig. 25 is a plot of the dose rates, averaged over longitude, as a function of latitude.
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The two dose rate profiles match with a shift of 1.6 ± 0.5◦ North. Thus, the SAA has been moving northwest at rates of 0.28 ± 0.03◦ West yr−1 and 0.08 ± 0.03◦ North yr−1 in the time period from 1973 to 1995. Although the westward rate is normally taken into account in modeling the external trapped environment, the northward component has always been neglected. These rates are consistent with other observations from TOPEX (Lauriente et al., 1995) and Space Shuttle measurements. Attempts to measure the drift rate using data from the Nausicca tissue equivalent proportional counter were unsuccessful due to a paucity of data. It should also be mentioned that the magnetic field has been decaying at the rate of 0.05% yr−1 and must also be taken into account for correct prediction of the current and future trapped particle populations.
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Effect of geomagnetic activity factors on the cardiac rhythm regulation and arterial pressure of cosmonauts
Based on the data of medical control of cosmonauts’ cardiac rhythm changes at the 32-nd orbit of the flight were analyzed for all crews of the Soyuz transporting vehicles (TV) for the period of 1986-1995. In total, 49 records were selected for analysis (for 49 crew members including the members of both main and visitation crews). A group of 8 cosmonauts was selected, who were in flight during all days of geomagnetic disturbances for the analyzed period. In addition, two control groups were selected: 9 cosmonauts whose flight has fallen on the time 3-7 days apart from magnetic storms, and the second group included the remaining 32 cosmonauts. The obtained data testify that during the magnetic storm the following effects took place simultaneously: reduction of heart rate (HR) and displacement of the vegetative balance to the side of sympathetic link of regulation (decrease of HF %, CV, MxDMn, pNN50, and RMSSD, as well as increase of stress index SI and sympathetic link activity VLFs). The increase of indicators SNCA and LFs/HFs testifies that there were specific changes of vascular regulation. This is confirmed by growing LFt indicator. The mentioned changes can be interpreted physiologically as activation of a vasomotor (vascular) center and slowing of the time of reception and processing of information in it. Some authors note that the most prominent deviations of physiological functions come in 24-48 hours after a magnetic storm and become apparent, most frequently, as increasing arterial pressure and arising vegetovascular distony. In this study authors have also observed more significant changes of cardiac rhythm variability indicators on the 1st-2nd day after the magnetic storm. These changes manifested themselves in essential growth of the values of indicators 1C, VLF %, VLFs. LF decreases to a lower degree, but HF drops considerably. Attention is drawn to the fact of considerable increase of the indicator of activity of regulatory systems (IARS) and significant growth of the number of arrhythmias (Narr). Physiological interpretation of the effects, revealed during magnetic storms and, especially, in the next 24-48 hours, indicated that such effects took place for heart (appearance of arrhythmias) and vascular system (lengthening of vascular tone regulation time and a functional stress of the vasomotor center).
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The results of daily continuous monitoring of the Holter electrocardiogram were studied for two crew members of the EI-22 expedition on the 176-179 days of flight (after 6 months, on February, 9-12, 1997).
As evidenced by the experience of numerous past expeditions of duration up to 6 months, the functional state of cosmonauts in this period of flight was characterized by occurrence of the stage of relatively steady adaptation. The age of cosmonauts no. 1 and no. 2 was 44 and 37 years, respectively.
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Based on the Holter monitoring results, the time database was generated, which included the files of initial values of all cardiointervals for the day (70-100 thousand numerical values, approximately). For statistical processing, the data on 5min intervals were used, which then were averaged over all days, over the periods of a day (day, evening, night), and over hour intervals.
Figure 1 presents the diurnal dynamics of indicators of cardiac rhythm variability of cosmonauts during geomagnetic storms and under quiet conditions at the 6th month of flight.
Attention should be paid to the changes of values of basic indicators in the control group (as compared to the one-month flight data).
This is, first of all, a higher heartbeat rate and lowered cardiac rhythm variability, as well as redistribution of frequency components of the spectrum to the side of prevalence of slow second-order waves and increase of IC. All this indicates to activation and stress of regulatory mechanisms of a cardiovascular system after half-year stay in weightlessness conditions. Against this background, under the magnetic storm effect the HR becomes confidently lower, and the cardiac rhythm variability (pNN50, RMSSD) grows. In this case, the relative and absolute power of vasomotor waves (LF %, LFs) confidently grows, and the slowwave components (VLFs) become stronger. The LFt indicator is confidently lower, which means the absence of the vascular center overstress. However, in general, the indicated changes suggest increased activity of regulatory systems (IARS). After the magnetic storm the increased total power of spectrum is conserved in all ranges, and the number of arrhythmias (NArr) considerably grows. On the whole, it is seen from the presented results, that after six-month stay under weightlessness conditions the cardiac rhythm regulation system of cosmonauts under geomagnetically quiet conditions is already in the state of functional stress because of the long flight itself. The magnetic storm effect, as a new stressor, disturbs the vegetative balance, established to this time, and causes short-term overstress of regulatory mechanisms. In this case, the parasympathetic link of regulation strengthens.
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Investigations of Cosmonautsâ&#x20AC;&#x2122; Vascular Tone under Ground Conditions and during Space Flight For checking the statistical confidence of the results obtained above, the 24-hour monitoring of arterial pressure (AP) and heartbeat rate (HR) were analyzed for a rather large group of cosmonauts: 24 ground preflight records and 12 records for the 4-6th months of flight were studied.
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Indicators were measured each 15 min h before and during the space flight. tical estimation of revealed effects the package (Studentâ&#x20AC;&#x2122;s t-criterion) was used
during 24 For statisStatistica6 (Table 3).
The analysis of cosmonautsâ&#x20AC;&#x2122; records distribution over the experimental groups has shown that various records of a single person fall into some groups. Preflight daily average values of arterial pressure varied from 106/75 to 122/85 mm Hg, and HR ranged from 55 to 79 beats per minute.
The AP daily average values during stay in weightlessness conditions varied from 108/64 up to 126/83 mm Hg and the HR values from 57 to 76 beats per minute. For smoothing individual distinctions, the primary data were normalized for each person separately, so that the individual average values of indicators be equal to 0, and the individual standard deviations be equal to 1. It is seen from Table 3 that the response of the cardiovascular system to the effect of geomagnetic disturbances exists both before and during the flight. In this case, the response under ground conditions and in weightlessness conditions is opposite. This can be due to the following causes. As a result of adaptation to weightlessness conditions, the cardiovascular system of a man passes to a new level of functioning. Under ground conditions the response of the cardiovascular system to a geomagnetic disturbance is mainly characterized by changing vascular tone, namely, by its increase. This becomes apparent in reduction of the arterial pressure and heartbeat rate. Under weightlessness conditions at the 6th month of flight the state of cosmonauts is characterized by lower values of pulse and pressure as compared to the ground data on the background of stress of regulatory systems. Their increase under an effect of magnetic storms (Fig. 2) reflects strengthening of a nonspecific component of the adaptive stress-response. At the initial stage of a long-term space flight (at the second day of flight), under the magnetic storm effect, a general nonspecific stress-response of adaptive syndrome type was observed (increase of pulse rate, decrease of power of the spectrum of respiratory waves, etc.). The most prominent deviations of physiological functions appeared in 24-48 h after the magnetic storm. Immediately in the period of magnetic storm effect the specific response is observed, associated with regulation of a vascular tone (meteo-responses). At the instants of reconstruction of regulatory processes the heart work instability can arise with increasing number of arrhythmic contractions. At the end of a half-year stay in space under the magnetic storm effect, destabilization of the functional stress state that arose to this time because of additional stressor effect of a magnetic storm is observed. The revealed effects are of principal significance for the concept of helio-geomagnetic factors as the time sensors of living organisms.
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