Critical Analysis of Active Shielding Methods for Space Radiation Protection

Transcription

1 Critical Analysis of Active Shielding Methods for Space Radiation Protection Lawrence W. Townsend Department of Nuclear Engineering The University of Tennessee Knoxville, TN Abstract From time to time over the past several decades, designs utilizing active methods involving electromagnetic field configurations have been proposed for the purpose of protecting spacecraft crews from harmful space radiations. Designs affording protection from either solar energetic particle event protons or galactic cosmic rays or both have been proposed. Often these analyses are predicated upon simplified or even incorrect assumptions regarding the incident radiation spectra or their associated radiation risks and limits. At times the proponents of these designs make very optimistic assumptions about the abilities of the scientific and engineering communities to overcome existing technology gaps needed to move their designs from paper to practice. In this paper various active shield methods proposed over the past several decades are reviewed and critiqued. Advantages and disadvantages of the proposed methods will be presented. Shortcomings in the analyses of their shielding efficacies, where they exist, are described. 1,2 TABLE OF CONTENTS 1. INTRODUCTION 1 2. ACTIVE SHIELDING METHODS 2 3. SHIELDING EFFECTIVENESS CRITERIA 5 4. CONCLUSIONS 5 REFERENCES 6 BIOGRAPHY 7 1. INTRODUCTION Future, extended, crewed space missions, such as a human mission to Mars or the construction of a Lunar base, will expose crews to levels of penetrating space radiations far exceeding any previously encountered during the Apollo era, missions in low-earth orbit (LEO), or on the International Space Station (ISS). The two radiation sources of primary concern for deep space missions are sporadic solar energetic particle events (SPEs) and the ever-present Galactic Cosmic Rays. In the case of SPEs, the major /05/$ IEEE 2 IEEAC paper #1094, Version 6, Updated December 1, 2004 health concern is acute effects, such as acute radiation syndrome (i.e. radiation sickness), although these exposures also contribute to long term, stochastic effects such as cancer induction and mortality. For exposure of crews to the GCR environment, the main concern appears to be stochastic effects, such as cancer induction and mortality, or possible unique effects resulting from exposures to the high-energy, heavy ion (the so-called HZE particle) component of the GCR spectrum. For over four decades, investigations into the feasibility of using active shielding methods, such as electromagnetic fields or plasmas to shield spacecraft and their onboard equipment and crews from hazardous space radiation have been carried out. The lure of active shielding methods is the possibility of providing reduced health risks from exposures by eliminating parts of or all of the incident radiation environment, and doing it with less mass and cost than with passive shielding comprised of bulk matter. Designs affording protection from either solar energetic particle event protons or galactic cosmic rays or both have been proposed. Often these analyses are predicated upon simplified or even incorrect assumptions regarding the incident radiation spectra or their associated radiation risks and limits. At times the proponents of these designs make very optimistic assumptions about the abilities of the scientific and engineering communities to overcome existing technology gaps needed to move their designs from paper to practice. On the other hand, since the alternative method to active shielding involves placing bulk material, i.e. passive shielding, between the radiation sources and the crew, proponents of active methods often make very negative or even incorrect assumptions and assertions about the effectiveness of such passive shielding. Common practices include arguing that a portion of the incident spectrum, up to some particular cutoff energy, is completely deflected by the active shield while the passive shield merely reduces the energies of the incident spectrum, thereby implying that completely eliminating part of the spectrum is a superior technique. Another technique is to select for comparison a shield material such as aluminum or some heavier metal that is a poor shield material choice and 1

2 compare its shielding efficacy to that of the proposed active shield design. Often these comparisons incorrectly invoke assumptions about radiation risk or apply limits that are outdated or incorrectly used. They also often neglect the shielding inherently provided by the structural materials used to construct spacecraft and the proposed active shield. Active shielding concepts fall into four distinct categories: (1) electrostatic fields [1-10]; (2) plasma shields [9, 11-15]; (3) confined magnetic fields [9, 16-19]; and (4) unconfined magnetic fields [9, 20-25]. A comprehensive listing of nearly all publications, through 1998, related to active shielding of spacecraft can be found in the review paper by Sussingham, Watkins and Cocks [26]. In this paper various active shield methods proposed over the past several decades are reviewed and critiqued. Advantages and disadvantages of the proposed methods will be presented. Shortcomings in the analyses of their shielding efficacies, where they exist, are described. Whether or not an effective active shield can be designed is not the real issue. The major question to be addressed in comparing active methods to passive methods: Is the total mass of the active shield configuration (coils, power sources, refrigeration, support structure, etc.) needed to provide some specified level of protection (as measured by radiation dose, risk, etc.) actually less than the mass of bulk material shielding needed to afford the same level of protection?. 2. ACTIVE SHIELDING METHODS Electrostatic Fields Except for a study by Townsend [7] that investigated shielding of iron nuclei from the GCR spectrum, and a study by Metzger and collaborators [10] that did include GCR particles of all types, previous electrostatic shielding studies were limited to environments consisting of protons from solar and galactic cosmic rays and protons and electrons in the Van Allen belts. Although several of the studies conducted before 1982 were favorable toward the use of electrostatic fields, most were not. The major shortcomings from the latter included: (10 extremely high voltages required; (2) vacuum and insulation breakdown characteristics, which limit minimum structural dimensions; (3) inherent instability of a concentric sphere arrangement, which was thought to be required for shielding against particles of opposite charge; and (4) production of bremsstrahlung by deflected charged particles. In ref. [7], it was demonstrated that concentric conducting sphere electrostatic shields are unsuitable for GCR shielding since the required electrostatic potentials exceed the state of the art by over an order of magnitude and require conducting sphere radii of several hundred meters in order 2 to preclude vacuum breakdown. In a concentric spheres arrangement, as presented in [7], the electrostatic field is confined between the spheres and points outward from the positively-charged inner sphere to the negatively-charged outer sphere. The relevant equations are listed in Eq. (1) where Q is the charge on the sphere, a is the radius of the inner, positively-charged sphere, b is the radius of the outer, negatively-charged sphere, and ε 0 is the permittivity of free space (vacuum). Qa Qb + ( r a) 4πε 0a 4πε 0b Qa Qb V() r = + ( a< r < b) 4πε 0r 4πε 0b Qa + Qb ( r b) 4πε 0r Denoting the potential at r = a as V a and at r = b as V b enables Eq. (1) to be rewritten as Va ( r a) bvb ava ab Vb Va V() r = ( a< r < b) b a b a r 0 ( r b) (2) Electrons of energies ~ 1 MeV are repelled by the negatively-charged outer sphere, which is at a low enough potential to have only a minor effect on the incoming positively-charged ions, since the gain in ion kinetic energy is only ~ Ze V b. For iron this is ~26 MeV which is negligible compared with their mean kinetic energies in the GCR spectrum of ~ 1 GeV per nucleon (~56 GeV). To prevent vacuum breakdown, the magnitude of the electric field intensity at the surface of either shell is limited to V/m. Hence, the minimum radii of the shells must be on the order of several hundred meters to prevent vacuum breakdown, which would conduct charge between the shells, thereby equalizing their potentials and eliminating the protective deflecting field. Hence, concentric shells with radii of several hundred meters are clearly unreasonable for use with interplanetary spacecraft. To alleviate the shortcomings of the concentric shells electrostatic shield, an asymmetric electrostatic radiation shield configuration has recently been proposed and is under current investigation [10]. The overall structure is to construct a shield using a linear quadrupole arrangement with a negative pole at each end and a positive pole at the (1)

3 center where the protected zone is located. The potentials are limited to ~ MV, which is technically feasible. The quadrupole arrangement, however, involves tethering spheres at these potentials over distances up to ~100 meters. A precise arrangement, suitable for shielding the deep space GCR environment remains under study. Hence, comparing the mass of such a system with that needed to reduce the radiation risk to a particular level using passive bulk material shielding is not possible at present. Another problem that must be considered in this arrangement is the loss of potential on the negatively-charged spheres due to charge pickup of the low energy positive ions in the heliospheric plasma. Plasma Shielding Early studies of plasma radiation shielding focused on shielding SPE protons [13]. The concept utilized a large electrostatic field to repel positively charged particles. To prevent this positive potential from attracting and accelerating space electrons to very high energies, a lower intensity magnetic field is used to control a cloud of free electrons, which deflect the incoming space electrons. A simplified analysis suggested substantial weight savings for shielding of SPEs. There were, however, many technological challenges including achieving electrostatic potentials on the spacecraft surface exceeding 200 MV, plasma instabilities, and the large quantity of energy (~ 12 MJ) stored in the plasma. Since the mechanism for deflecting positively charged GCR ions is an electrostatic field, this version of a plasma shield is not feasible for protecting crews from the deep space radiation environment. Confined Magnetic Field Confined magnetic field arrangements usually consist of concentric spheres or a similar arrangement that limits the spatial extent of the magnetic field to some localized finite region of space around the inhabitated volume of the spacecraft. Incoming particles from the space radiation environment, of either charge, are then deflected by the Lorentz force. Since the motion of the charged particle in the magnetic field is such that only the direction of travel is altered by the magnetic field, particle trajectories that miss the inhabitated volume are considered to be shielded, and those that still intersect the inhabitated volume after deflection are not shielded. The deflected charged particles emit bremsstrahlung radiation as a result of their motions in the field. For incident heavy charged particles, such as protons and heavier ions bremsstrahlung is not a significant radiation hazard since the photon yields are inversely proportional to the ion s masses. For incident electrons, however, bremsstrahlung yields are more significant due to the small mass of the electron. 3 For a charged particle moving under the influence of a magnetic field, the particle rigidity (momentum per unit charge) is given by R pc Ze = (3) where p is the momentum of the charged particle, Ze is its charge, and c is the speed of light in vacuum. The rigidity R is also related to the radius of curvature r (Larmor radius) of the particle trajectory in the magnetic field and the magnetic field intensity as R = 0.003Br (4) where B (units of Tesla) is the component of the magnetic field perpendicular to the momentum of the particle, r is in meters and R in gigavolts (GV). Since particle motion for heavy charged particles is often described in terms of the particle kinetic energy per nucleon (rather than its total momentum), Eq. (3) can be expressed as 2 2 A T + 2mc T R = (5) Ze where T is the particle s kinetic energy per nucleon (GeV/nucleon), A is its mass number (number of nucleons), and m is the nucleon rest mass (mc 2 = GeV). Equation (5) holds for any nuclear species with charge number Z and mass number A at any incident kinetic energy per nucleon. For example, for an incident GCR iron ion with Z =26, A = 56 and a kinetic energy of 1 GeV/nucleon the particle rigidity is 3.65 GV. Figure 1 Charged particle trajectories in a uniform confined magnetic field for both normal and grazing incidence. The spatial thickness of the magnetic field is denoted by. The most limiting particle trajectory for shielding purposes

4 is grazing incidence. If the thickness,, of the confined magnetic field spatial extent is greater than the Larmor diameter ( 2r), the particle trajectory will not intersect the inhabitated volume. This is depicted in Fig. 1. Note also that a particle normally incident on the confined magnetic field will be easily deflected if its Larmor radius is less the spatial thickness of the confined magnetic field configuration ( > r). Hence, the energy cutoffs for incident charged particles for a particular confined magnetic field geometry are determined by both the magnetic field intensity and the angle of incidence that the incoming particle makes with the field. Since SPE and GCR particle distributions in space are generally isotropic, the local cutoff in the spectrum is not fixed at a single energy, but is a more complicated function of the incident particle s kinetic energy. Figure 2 Simple sketch of a toroidal shaped confined magnetic field configuration for a concentric sphere arrangement. In general the shape of the coils (usually superconducting) used to generate the magnetic field is toroidal. A simple sketch of one such configuration is depicted in Fig. 2. In this configuration, the shielded volume is inside the inner sphere. The magnetic field intensity is nearly uniform in the region between the two concentric spheres. The magnetic field lines are everywhere parallel to each other (analogous to lines of equal latitude around the Earth). Such a configuration, using superconducting coils, was proposed some years ago by Bernert and Stekly [17]. The proposed configuration was developed for the purpose of protecting crewmembers from SPE protons. GCR particles were not considered. The inner sphere was assumed to be 2 m in radius with an enclosed shielded volume of 33 m 3. The radius of the outer sphere was 3 m. The thickness of each shell was 1.5 cm. The magnetic field, generated using superconducting coils that were cryogenically cooled, had a strength of 4T. The magnetic field shielded the enclosed volume where the crew was to be located from all protons 4 with energies less than 170 MeV. The analysis by Bernert and Stekly indicated that this configuration, including the superconducting coils, power supply, structural supports and refrigeration equipment, would result in a 20% mass savings over a comparable passive shield using aluminum. Townsend [18] evaluated the effectiveness of this shield configuration for GCR ions and pointed out that, as designed, it would be totally ineffective against the more energetic GCR spectrum, especially the HZE particle components. In order to use this configuration to shield from GCR particles, it was determined that the magnetic field intensity would need to be increased by a factor of ~ 600. Producing such magnetic field intensities are far beyond even current capabilities. Noting that the masses of the superconducting coils, refrigeration equipment, structural supports, and power supply will provide some shielding, even if the magnetic field is not turned on, Townsend and collaborators [19] carried out a comparative analysis of the shielding effectiveness of this confined magnetic field arrangement versus comparable bulk material shielding. In order to evaluate the effectiveness of the magnetic field, energy cutoffs for the magnetic field were applied to an incident SPE spectrum similar to the very large event of August 1972 and to an incident GCR spectrum similar to that of the 1977 solar minimum, the most intense GCR spectrum of the space era (up to 1990). These modified spectra were then transported through 11 g cm -2 aluminum, which was the equivalent bulk shielding provided by the magnetic shield component masses. The results indicated that, as expected, the magnetic field (when turned ON) was very effective against the SPE, reducing the skin and bone marrow doses equivalents to ~ 1 csv (1 rem). With the magnetic field turned OFF, the skin and bone marrow dose equivalents increased to 172 and 49 csv respectively. For the incident GCR spectrum turning the magnetic field ON or OFF made no difference in the skin and bone marrow annual dose equivalent values (skin: 53 csv; bone marrow: 39 csv). Clearly, the magnetic shield was effective for the SPE spectrum that it was designed to protect against, but was useless against a GCR spectrum. As pointed out in the comparative analysis, GCR protection would require significantly stronger magnetic fields or the use of supplemental bulk material shielding to reduce the risk from the GCR particles. Either solution was likely to substantially increase the mass required, thereby eliminating any mass savings. As an alternative, Townsend and collaborators [19] also investigated other bulk shielding materials. Previous studies [27] had shown that substantial reductions in radiation exposures behind equivalent shield thicknesses were possible through proper materials selection. In particular, significant reductions would occur whenever

5 hydrogenous materials such as water or polyethylene were substituted for aluminum in the shield. Hence, the calculations for the August 1972 SPE spectrum and the 1977 solar minimum GCR spectrum were repeated using 22 g cm -2 water shielding, which is the same areal density as that provided by the combined masses of the concentric shells and magnetic field components. No magnetic field was assumed to be available. The results were that the skin and bone marrow dose equivalents for the incident SPE spectrum were 9 and 5 csv respectively. These values are slightly higher than the values obtained with the magnetic shielding, but well below any level of concern. For the incident GCR spectrum the skin and bone marrow dose equivalent values were 27 and 24 csv, which are much lower than the values obtained with the magnetic shield configuration. The bottom line is that this magnetic shield would work for SPE but not for GCR and, the combined SPE + GCR dose equivalents are reduced by over a factor of 3 more if the magnetic field generating components are replaced by a passive shield composed of hydrogenous materials. Preliminary results of a new confined magnetic field design for crew quarters in an interplanetary spacecraft using superconducting coils in a Faraday cup arrangement have been recently reported by Ting and collaborators [28]. As final details of the design and underlying assumptions of this proposed configuration have not yet been published, a critique of the efficacy of it as a combined GCR and SPE shield is not possible now, but will be possible in the future. Unconfined Magnetic Field Unconfined magnetic field configurations typically mimic the dipole-like magnetic field of the Earth. Usually the spacecraft is assumed to be cylindrical or toroidal-shaped surrounded by a dipole-like magnetic field. The magnetic filed is often assumed to result from passing current through coils on the spacecraft or the skin of the spacecraft itself. This shielding method is popular as a possible method of shielding large space colonies. One such analysis [23] considered GCR protection in the analysis but assumed that the GCR exposure levels needed to be reduced to those applicable to the general population on Earth. The resulting shield had a mass in excess of 10 6 kg with a diameter over 100 meters. A more recent proposal [24, 25] provides directional protection against SPE protons, but does little to shield from GCR particles. The design is flawed, however, in that it assumes that the incoming protons in an SPE are highly directional and so really provides only a sector-type shield configuration. In fact, the energetic protons that arrive initially are highly directional, but the spectrum quickly becomes isotropic before any significant dose is received. Hence, assuming that one can point a sector shield in some 5 particular direction and effectively shield from SPE protons arriving from all directions is just not a valid assumption. 3. SHIELDING EFFECTIVENESS CRITERIA In order to determine if a proposed electromagnetic shield configuration is more effective than a passive, bulk material shield, for radiation protection purposes, the appropriate radiation environments must be considered. It is clear from the previous discussion that many of the proposed active shielding methods only considered part of the space radiation environment (e.g. SPE components) without considering other important components that are part of the complete spectrum that must be addressed by any spacecraft shield configuration. Also, the figure of merit used to evaluate effectiveness must be one that is actually relevant for radiation protection purposes. It is true that the masses, and ultimately the costs of various shielding alternatives must be compared to arrive at the engineering solution to the shielding problem. Ultimately, however, the actual engineering criteria will be derived from best estimates of the actual risk to humans of developing a fatal cancer or some other mission or life-threatening health hazards resulting from exposure to the space radiation environment. At present these health risks are highly uncertain, especially for the GCR environment, because there are few data for actual exposure of humans to these radiation fields [29]. Invoking criteria such as requiring shields to completely stop all particles below some arbitrary energy cutoff [25] in order to compare the mass of passive versus active shielding required to do so is meaningless, as is invoking some arbitrary career limit [28], not based upon any scientific rationale or recommendation from an advisory body such as the NCRP (National Council on Radiation Protection and Measurements). The appropriate environments to use are ones that include both solar particle events and the ever-present galactic cosmic ray environment. Although SPEs are a concern because of the possibility of crew impairment or death resulting from acute levels of exposure, they are not the only radiation protection concern, and may not be the primary concern. In addition, except for estimates from a hypothetical, absolute worst case event [30], estimates of SPE doses to critical organs of crewmembers indicate that adequate protection is obtained with ~ 20 g cm -2 Al shielding against even the largest events that have occurred in the space era [31, 32]. The GCR environment presents a unique environment, unlike that anywhere on Earth. While GCR exposures are not an acute health risk (their doses and dose rates are below any acute radiation syndrome response threshold), they are a major concern for cancer induction and mortality, and may be the limiting concern for long duration deep space missions since their particle energies are much higher and they are much harder to shield than the

6 less energetic SPE protons. In general, any shield that is capable of reducing GCR exposures to acceptable levels will adequately protect crews from SPE protons. At present no career limits exist for deep space missions. The existing guidance provided by the NCRP pertains only to missions in low Earth orbit (LEO) [33]. Hence, invoking those limits for purposes of comparing shielding efficacies may be illustrative, but they should not be taken to be definitive since they are unlikely to be the actual limits that will be adopted for future deep space missions. Note also that those limits are given in terms of effective dose. Comparing shielding efficacy for different proposed methods requires that the shielding comparisons be made for effective dose, which is not the same as estimating dose equivalent, and does require the use of actual human geometries in order to be properly estimated. Summary of Criteria for Comparing Shielding Methods The space radiation environment used must include contributions from both solar particle events and galactic cosmic rays The appropriate target radiation levels for determining estimated shielding requirements for comparing different shielding methods must be realistic ones that are based upon risk to humans. Estimates of masses must include not only the components of the shield, but must also include the inherent shielding provided by the spacecraft structure and internal components. Crews are not going to venture into space with only a magnetic field and no spacecraft structure to support it and to house the crew. 4. CONCLUSIONS In this paper the various types of active shield methods proposed over the past several decades have been reviewed and critiqued. Advantages and disadvantages of the proposed methods have been described. Shortcomings in the analyses of the shielding efficacies of these methods, where they exist, have been pointed out described. Whether or not an effective active shield can be designed is not a real issue. The major question to be addressed in comparing active methods to passive methods: Is the total mass of the active shield configuration (coils, power sources, refrigeration, support structure, etc.) needed to provide some specified level of protection (as measured by radiation dose, risk, etc.) actually less than the mass of bulk material shielding needed to afford the same level of protection? Answering this question requires that the complete space radiation environment, relevant to the deep space mission, be considered. It also requires that the criteria used to determine the shielding effectiveness be one that is recognized as valid by the radiation protection community, i.e. one based upon risk to the crew. Finally, the shielding provided by the inherent mass of the spacecraft structure, internal components, along with any additional structure needed to support the active shielding components must be included in any intercomparisons of the effectiveness of active versus passive methods. REFERENCES [1] R. F. Tooper, Electrostatic Shielding Feasibility Study, ASD-TDR , U. S. Air Force, May [2] F. H. Vogler, Analysis of an Electrostatic Shield for Space Vehicles, AIAA Journal 2, , [3] J. E. Felten, Feasibility of Electrostatic Systems for Space Vehicle Radiation Shielding, Journal of the Astronautical Sciences XI, 16-22, [4] E. E. Kovalev, E. D. Molchanov, R. U. Nazirov, T. Y. Riabova, Y. G. Schneider, Electrostatic Shielding Against Cosmic Radiation and its Earth Applications, 24 th International Astronautical Congress,Baku, Azerbaidzhan SSR, October 7-13, [5] E. E. Kovalev, E. D. Molchanov, Yu. G. Pekhterev,, T. Y. Riabova, B. I. Tikhomirov, A. I. Khovanskaya, An Investigation of the Basic Characteristics of Electrostatic Shielding from Cosmic Radiations on the Artificial Earth Satellite Kosmos 605. I. Measurement Procedure and the Complex Scientific Aparatus, Cosmic Research 13(5), , 1976 (translated into English from Kosmicheskie Issledovaniya 13(5), ). [6] E. E. Kovalev, E. D. Molchanov, Yu. G. Pekhterev,, T. Y. Riabova, B. I. Tikhomirov, An Investigation of the Basic Characteristics of Electrostatic Shielding from Cosmic Radiations on the Artificial Earth Satellite Kosmos 605. II. Results of Measurements, Cosmic Research 14(1), , 1976 (translated into English from Kosmicheskie Issledovaniya 14(1), ). [7] L.W. Townsend, Galactic Hevy Ion Shielding Using Electrostatic Fields, NASA Technical Memorandum 86265, [8] W. Frisina, Optimizing Electrostatic Radiation Shileidng for Manned Space Vehicles, Acta Astronautica 12, , [9] L. W. Townsend, Overview of Active Methods for Shielding Spacecraft from Energetic Radiation, Physica Medica XVII, Supplement 1, 84-85, [10] P. T. Metzger, J. E. Lane, R. C. Youngquist, Asymmetric Electrostatic Radiation Shielding for Spacecraft, 2004 IEEE Aerospace Conference, Big Sky, MT, March 6-13, [11] R. H. Levy, G. S. Janes, Plasma Radiation Shielding, AIAA Journal 2, ,

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