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Astromag: Particle astrophysics magnet facility for space station freedom
Acta Astronautica Vol. 21, No. 6/7, pp. 505-512, 1990 Printed in Great Britain
0094-5765/90 $3.00+ 0.00 Pergamon Press pie
ASTROMAG: PARTICLE ASTROPHYSICS MAGNET FACILITY FOR SPACE STATION FREEDOMS" W. VERNON JONES'[
NASA, Space Physics Division, Code SS, Washington, DC 20546, U.S.A. (Received 14 February 1990)
Abstract--In June 1989 three particle astrophysics investigations were selected for the Astromag superconducting magnet facility to be flown as a U.S.-Italy project on Space Station Freedom in the late 1990s. The science goals include investigating the origin and evolution of matter in the galaxy by direct sampling of galactic material; examining cosmological models by searching for antimatter and evidence of the nature of dark matter; and studying the origin of extremely energetic particles and their effects on the dynamics and evolution of the galaxy. By augmenting Astromag's strong magnetic field with a variety of specialized detectors, state-of-the-art spectometers will be employed to address the experimental objectives. Measurements of high energy cosmic ray nuclei and electrons will be made with precision and sensitivity I0-1000 times that of previous experiments. The Space Station is an ideal spacecraft for Astromag, because it provides both the assembly and servicing capabilities needed for change-out of experiments and replenishment of liquid helium to extend the life of the facility.
I. BACKGROUND
President Reagan's January 1984 announcement of the intention to build a permanently manned Space Station led to an international workshop on "Cosmic Ray and High Energy Gamma Experiments for the Space Station Era" in October 198411]. Several attendees of that workshop came with the suggestion that the Space Station would finally permit the accommodation of a large superconducting magnet in space, and this idea envolved into the workshop consensus. Shortly thereafter, the Cosmic Ray Program Working Group chartered by NASA's High Energy Astrophysics Branch gave its top priority to this magnet facility concept. Subsequently, membership was solicited for the Superconducting Magnet Facility Definition Study Team to explore the possibility of accomplishing a broad range of particle astrophysics objectives with a super-conducting magnet facility. This team, which met over a period of more than 3 years beginning in late 1985, issued an interim report in August 1986 and a final report in May 1988 [2]. These reports detailed the scientific rationale for the facility, outlined the approach to be followed, and emphasized the Space Station Freedom as the preferred space platform for the cosmic ray observations. In mid-1988 NASA issued an Announcement of Opportunity (AO) for Attached Payloads to conduct scientific investigations during the assembly phase of ?Paper IAF-89-466 presented at the 40th Congress of the International Astronautical Federation, Malaga, Spain, 7-13 October 1989. :[:Chief Scientist for Cosmic and Heliospheric Physics and Astromag Program Scientist.
Space Station Freedom. Astromag and the Cosmic Dust Collection Facility (CDCF) were identified as two facilities that could be accommodated during the assembly, and potential experimenters were invited to propose investigations that could be carried out with them. It was the stated intention to proceed with these facilities if, and only if, the quality of the investigations proposed to use them were competitive with the other investigations proposed in response to the AO. The flight investigations proposed for both Astromag and the CDCF were indeed competitiye, and three investigations for each facility were selected in June 1989. The Astromag investigations are the focus of this paper. In late 1988 the Astromag Technical Advisory Team was formed for the purpose of further evaluating and defining the facility and its accommodation on Space Station Freedom. The report of this team was issued in May 1989 [3]. It includes discussions of: the magnet and dewar configurations; the support structures and mechanical systems; facility assembly and servicing; the impact on the Space Station and nearby equipment; and the facility operations. It also suggests an international approach for implementing Astromag in a timely and cost effective manner. From the beginning, foreign scientists have actively participated in the Astromag study activities. It is universally agreed that the project would be substantially strengthened by international involvement. The Italian Space Agency offered to make a major commitment to the Astromag project, and this offer has been tentatively accepted by NASA. Negotiations are underway for developing the core facility as a more-or-less 50-50 U.S.-Italy joint project. Furthermore, there are international co505
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W. VEgNONJONES This paper discusses the scientific objectives of Astromag and describes the core facility with the three instruments selected for flight in response to the Space Station Attached Payloads AO (NASA OSSA3-88).
investigations from other countries on all three of the investigators selected for Astromag in June 1989. The Astromag facility is the flagship of the high energy cosmic ray community. It is designed to accommodate at least two experiments operating simultaneously, and the experiments can be changedout and/or serviced as required. The first-generation investigations will provide unprecedented information about nucleosynthesis, cosmic ray origin, acceleration regions, modes of propagation, and the possible existence of antimatter, specifically:
2. NUCLEOSYNTHESIS
The standards for comparing theories of nucleosynthesis are the elemental and isotopic composition of the solar system, which have traditionally been derived from meteorites and spectroscopic measurements in the solar photosphere. More recently, solar energetic particles are providing a direct sample of the sun, and solar flare elemental abundances are being corrected for the effects of ionization potential fractionation to get an excellent set of photospheric abundances. Figure 1 summarizes the present knowledge of the isotopic composition of the cosmic ray source. It is significant that for all the isotopes for which the source composition has been determined to within about 30% (neon, magnesium, and silicon) there is in each case evidence for a difference between cosmic ray and solar system material. The most quantitative theoretical suggestions to explain such differences are the so-called supermetallicity model [4] and the Wolf-Rayet model [5]. The former proposes that the excess of neutron-rich isotopes might result if cosmic rays originate in regions of the galaxy that are metal-rich compared to the solar system. The latter proposes that a fraction of the heavy cosmic rays originate from material expelled by Wolf-Rayet stars in high velocity stellar winds. The isotopic composition of the solar system is clearly different from that of galactic cosmic rays, which are probably a sample of the interstellar
(1) The so-called WiZard investigation (R. L. Golden, New Mexico State University, Principal Investigator) will measure the energy spectra of light (Z < 4 ) particles, including antiprotons, positrons, and electrons, and search for anti-helium at unprecedented levels of sensitivity. (2) The Large Isoptoe Spectrometer for Astromag (LISA) investigation (J. F. Ormes, Goddard Space Flight Center, Principal Investigator) will measure the spectra of elements and isotopes of heavy (Z > 3) particles, and search with unprecedented levels of sensitivity for their corresponding anti-particles. (3) The Spectra, Composition, and Interaction Study using a Magnet-Interaction Calorimeter (SCIN/MAGIC; T. A. Parnell, Marshall Space Flight Center, Principal Investigator) will use the Astromag facility to search for a new state of matter (quark-gluon plasma) in ultra-relativistic nuclear interactions, as well as for measurements of cosmic ray energy spectra up to 1015eV, which approaches the region where dramatic changes in nuclear composition are expected. 10
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medium. On the other hand, the solar system itself is a sample of the interstellar medium that happened to condense about 4.6 billion years ago. This implies that nucleosynthesis of cosmic-ray and solar-system material has differed, probably due to one, or more, of the following reasons:
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(1) The cosmic rays sample a different part of the galaxy, closer to the galactic center where there are more Wolf-Rayet stars. (2) The cosmic rays are a recent (20 million year old) sample, while the solar system is a sample from 4.6 billion years ago, i.e. the galaxy's composition is evolving. (3) The solar system is not typical of the interstellar medium, i.e it may have been "contaminated" by a nearby supernova shortly before its condensation. It is likely that this issue can be resolved only by comparing models of nucleosynthesis and chemical evolution of the galaxy with observations of isotopes of many more cosmic ray elements than just neon, magnesium, and silicon (in addition to observations of the rare ultraheavy elements). The LISA instrument on Astromag will have sufficient isotopic resolution and large enough collecting area to give definitive tests of competing models, all of which can fit the limited cosmic ray data now available. 3. P A R T I C L E
ACCELERATION
Supernova explosions in the galaxy are believed to provide the energy responsible for accelerating cosmic ray nuclei to high energy. However, it is not known whether this acceleration occurs immediately during the birth and infancy of the supernova or much later as shock waves from older supernova make multiple encounters with material in the interstellar medium. In the former case, cosmic rays would consist of freshly synthesized material ejected from recent supernova. In the latter case, cosmic rays would represent element-synthesis integrated over the age of the galaxy. Since galactic cosmic rays are fully ionized, the time scale between nucleosynthesis and acceleration to high energy can be revealed by measurements of isotopes that decay only by electron capture [6], e.g. 56Ni, 57Co, 55Fe, and 59Ni. Once accelerated and stripped of their electrons these nuclei can no longer decay, so their abundances preserve a record of the time-delay, if any, between their production and acceleration. Therefore, these isotopes clearly address the question of whether cosmic rays represent a sample of the interstellar medium, or whether they represent newly synthesized material, possibly from supernova. Figure 2 shows the expected yields of Fe, Co, and Ni isotopes as a function of the time delay. If this delay is, for example, more than a few days most of the 56Ni will have decayed to 56Co, which then decays AA 21-6/7--K
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Fig. 2. Isotope yields, e.g. production-acceleration delays. to 56Fe, thereby producing the 0.847 MeV gamma rays so diligently searched for in the recent supernova SN1987A. If the time delay is much more than 105 years, the 59Ni will have decayed to 59Co. Therefore, measurement of the isotopic composition of cobalt may provide critical information about supernovae and their role in accelerating cosmic rays. Further evidence of explosive nucleosynthesis in supernova can be obtained from observations of the abundances of a variety of other isotopes in cosmic rays, including neutron-rich isotopes of the elements Ne, Mg, Si, Fe, and Ni. Particle acceleration to extremely relativistic energies is a widespread astrophysical phenomenon, from the Earth's bow shock, to the sun, to galactic objects (SS433), to the galaxy (cosmic rays), to centers of active galaxies (radio jets). There are now specific models of shock acceleration for cosmic rays that predict cutoffs in the spectra of individual elements somewhere between 101° and 1015eV, i.e. at energies below the "knee" in the all-particle spectrum. Figure 3 illustrates the existing experimental situation [7]. This high energy data has been collected only by extensive air shower experiments above the knee and primarily by balloon flight experiments below the knee. The Cosmic Ray Nuclei Experiment (CRNE) on Spacelab-2 [8], the only space experiment approaching such high energies, focused on the spectra of nuclei over the range Z = 3-26; the SL2 data are shown separately in Fig. 3. The JACEE balloon flights over the past decade [9] have provided data on protons and helium that extended the energy range of the earlier "Ryan Proton" spectrum. Neither CRNE on Spacelab-2 for a week nor the JACEE balloon flights over the past decade have yet
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Fig. 3. The "knee" in the cosmic ray energy specturm [7]. reached the region of the knee, but the S C I N / M A G I C instrument selected for Astromag will come close enough to begin testing the theories. As a minimum, this investigation should provide definitive measurements up to several times I0 ~4eV/nucleus for the major elements, which will permit direct tests of the possibly dramatic changes in composition around the "knee" in the overall cosmic ray spectrum.
4. GALACTICCONTAINMENT The question of how cosmic rays are stored in the interstellar medium is but one example of the more general problem of their interactions with the galactic interstellar plasma. We can measure the containment time-scales and infer the interaction mechanism of particles with magnetic fields and vice versa. This helps explain the role of energetic particles in creating hydromagnetic waves, turbulence, etc. and, in turn, the effect of the latter on the motions of the particles. Since these interplays between the source and the observer change the cosmic ray spectra, they must be understood before we can know the source spectra, which is in turn essential to understanding the dominant mechanisms in the cosmic-ray accelerator. Meaurements on purely secondary particles in cosmic rays are the key to cosmic ray interactions with the interstellar medium. These include antiprotons and positrons, as well as the L-Nuclei (Li, Be, and B) and sub-Fe groups. For example, if the antiproton data really indicate that the confinement models derived from heavier secondaries are not correct for protons, then at least cosmic ray protons are accelerated in different sources. The much better positron, antiproton, dueteron, and helium-3 measurements over a wide energy interval with the Astromag WiZard instrument will directly address these issues.
5. COSMOLOGICALANTIMATTER One of the fundamental, unanswered questions of cosmology is the degree to which the universe contains antimatter. In 1928 the Dirac equation accorded equal status to matter and antimatter, and until the late 1950s it was generally believed on grounds of particle-antiparticle symmetry that equal amounts of each existed in the universe as a whole. It was recognized, however, that without some mechanism for matter-antimatter separation in the early universe, most of the produced matter and antimatter would subsequently annihilate. This led to expectations that the ratio of baryons to photons would be on the order of 10 -19. Evidence for the lack of antimatter in our galaxy and nearby galaxies evolved from the new field of gamma ray astronomy, which led to an upper limit of 10 -15 for the antimatter/matter ratio in our galaxy[10], although antimatter on the scale of clusters of galaxies was not ruled out. The possibility of matter-antimatter asymmetry is dependent on [11]: (1) baryon number not being strictly conserved and (2) charge-parity symmetry not being exact. The latter, CP violation, has been observed, and baryon non-conservation is incorporated into the current Grand Unified Theories. Although proton decay has not yet been observed, it is the view of most cosmologists that the universe is baryon asymmetric, i.e. consists almost entirely of matter. Current upper limits on the antinuclei/nuclei ratio in cosmic rays exclude the possibility for a substantial fraction of antimatter in our galaxy, but those limits are inadequate to address the possible existence of all anti-galaxies. Both the WiZard and LISA investigations are capable of extending antinuclei searches by nearly four orders of magnitude, which offers the opportunity to address the question of whether the universe does indeed display a global symmetry between matter and antimatter. Either a positive or
Astromag negative result from these investigations will have a profound effect on the way we view the universe. If antinuclei are identified, thereby establishing the symmetry between matter and matter, serious reevaluations of the various cosmological models now in vogue would be necessary. On the other hand, the failure to detect antinuclei at their level of sensitivity would provide support for new theories being formulated to account for the evolution of an initially baryon-symmetric universe into one containing little antimatter.
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6. ASTROMAG FACILITY
Astromag is basically a magnetic spectrometer capable of determining the momentum per unit charge (rigidity) and sign of the charge of highly relativistic (fully ionized) cosmic rays. The approach is to have a long-lived facility on Space Station Freedom based on a set of coils that produce an intense magnetic field. When cooled by liquid helium, the niobium-titanium coils become superconducting (i.e. zero electrical resistance) so the current and associated magnetic field persists indefinitely. Track detectors in the magnetic field measure the curvature of particle trajectories; the amount of bending is inversely proportional to a particle's rigidity. The tracking detectors and other particle sensors needed for specific investigations are provided by the experimenters. Two experiments can operate simultaneously on the two-ended facility. The heart of Astromag is the superconducting magnet and its supporting cryogenic system, which is comprised of two magnet coils, a persistent switch, gas-cooled detachable current leads, and a superfluid helium circulation system, all housed inside a 3500 liters superfluid helium dewar. The identical coils have opposite polarity, so the dipole moments effectively cancel and, therefore, induce no torque on
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the Station. The net dipole moment is determined by the manufacturing differences between the coils, which are connected in series to ensure that the same current exists in both coils at all times. Since the coils are superconducting, the circulating current is persistent (i.e. d e c a y s < 1% per year) and the magnetic field is maintained without continuous application of power. In the event of a spontaneous or induced quench (in which the coil wire undergoes a conversion from the superconducting state to a normal resistive state) the current would decay in 2 s because of resistive losses and the 11 MJ of magnetic energy would dissipate in heating the coils. Figure 4 shows schematically how Astromag with two instruments will appear on the Space Station truss structure. The facility will be located on an outrigger (truss extension) to isolate the magnetic field from the main truss of the Station. The use of a two-bay (10m) extension will ensure that the magnetic field at the main truss is essentially the same as the Earth's field. The facility will utilize the standard Attached Payload Accommodation Equipment (APAE) for interfacing with the Station, including the mechanical, thermal, electrical power, and signal connections. 7. ASTROMAG INSTRUMENTS
7.1. WiZard
Fig. 4. Astromag on SSF truss extension.
As illustrated schematically in Fig. 5, the WiZard instrument consists of an array of particle detectors mounted on one end of the Astromag magnet system. From the top, it includes a pair of time-of-flight (TOF) counters, a tracking system, another TOF pair, a transition radiation detector (TRD), and a calorimeter. The detectors are held together by a spinal structure that also serves as the payload carrier for a Shuttle launch. The individual detectors are mounted within a segmented, pressurized container which is part of the backbone. When assembled, the individual modules form a single pressurized volume
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with a low mass entrance window for cosmic rays. The system design has minimized the mass in the particle paths to avoid particle interactions that could complicate event interpretation. This detector assembly is capable of identifying electrons, positrons, antiprotons, nuclei, and antinuclei over a wide range of energies. The TOF sub-system measures the transit time between the top and bottom planes, provides the direction of the particle, and the magnitude of its charge. The tracking subsystem provides almost continuous measurements of particle trajectories through Astromag's magnetic field, which allows determination of the particle's rigidity and the sign of the charge. These TOF and tracking sub-system data are adequate to search for primordial antimatter, but measurement of positron and antiproton fluxes requires additional detectors, e.g. an electron and an antiproton would be indistinguishable with just these sub-systems. The TRD and the calorimeter provide the means for separately identifying light particles (electrons and positrons) from heavier particles (protons and antiprotons). The WiZard geometry factor for measuring antiprotons and positrons is 0.1 m2sr, but for the primordial antimatter search it is 0.4 m2sr. Magnetic rigidities can be measured up to its maximum detectable ridigity (Rmax) in excess of 1 TV. At least 2 years observing time is needed for the antiprotons, positrons, and light secondary (Li, Be, B) nuclei. 7.2. LISA
The two side views of the LISA instrument are illustrated in Fig. 6. The sub-systems consist of a central region for tracking particles through Astromag's magnetic field and an outer ring (four segments of a polygon) for determining velocity, charge, and TOF. Particle trajectories are determined with five planes (H l-H5) of scintillating optical fiber detectors, each capable of providing two-dimensional coordinates with 70 #m or better resolution. Two aerogel Cherenkov counters (C1 and C2 or C3 and C4), one Pilot 425 Cherenkov counter (T1 or T2), and a set of
scintillators (S1 or $2) are combined in each of eight modules. The Cherenkov response of the Pilot 425, which is nearly saturated by the particles to be measured, provides a direct measure of a particle's atomic number Z, after pathlength and mapping corrections are applied. For particles with energy less than about 10GeV/nucleon the aerogel counters measure the particle's velocity, or equivalently its momentum per nucleon. The TOF scintillators determine the particle direction (up or down), thereby distinguishing downward moving negatively charged particles from upward moving particles or positive charge. The following physical quantities of interest can be derived from the various sub-system measurements. (1) Charge identification is obtained from the Pilot 425 and aerogel Cherenkov counters, supplemented by the TOF scintillators; rigidity measurements from the trajectory sensors help correct these pulse heights for velocity dependence. (2) Isotope identification is based on combining the rigidity obtained from magnetic deflection with the momentum per nucleon derived from the aerogel Cherenkov emission. This combination, together with the measured atomic number, provides mass resolution sufficient for separating adjacent isotopes for Z = 4-30 nuclei over the energy range 2.5-4 GeV/nucleon. (3) Matter-antimatter separation is based on the sense of particle deflection in the magnetic field. The LISA investigation of Z = 4-30 elements is based on a 2 year exposure of the 0.1 m2sr instrument. Elemental composition studies will cover the range 2-1000 GeV/nucleon, but isotopic composition will be limited to 2.5-4GeV/nucleon. The charge and energy resolutions depend on the particle species and its energy. The charge resolution is approx. 0.18 amu for C, 0.20 amu for Si, and 0.25 amu for Fe. The energy resolution is about 0.3% at 3 GeV/nucleon, 2% at 30GeV/nucleon, and 20% at 300GeV/ nucleon. The antimatter search will have a sensitivity of about 4 x 10 -7 for Z > 6. 7.3. S C I N / M A G I C
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The SCIN/MAGIC instrument, shown schematically in Fig. 8, is basically a stack of passive, high resolution track detectors in the magnetic field of Astromag. The stacks contain nuclear track emulsions, track-etch detectors, X-ray films, lead or tungsten plates, and other inert materials. From the top, the major modules are the charge identification module, three conseuctive low-density target-tracking chambers, and a calorimeter. The investigation requires two pallets of four chambers, two of which exclude the intermediate tracking detectors depicted in Fig. 7. The mission duration is limited by the
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Fig. 7. Schematic of the SCIN/MAGIC instrument. accumulation of tracks in the integrating detectors to, preferably, about 90 days. This short duration makes S C I N / M A G I C a good companion experiment for the Astromag facility, e.g. it can be run during the initial checkout of Astromag or during changeout of major experiments. The S C I N / M A G I C chambers have the dynamic range to measure all nuclei from protons to Fe, as well as the characterisics of nuclear interactions with secondary particle multiplicities up to 1000 or more. The fundamental position resolution is about 1/~m. By using delta ray range measurements instead of the conventional grain counts with fixed range, the charge resolution will vary from about 0.1e for protons to 1 e for Fe nuclei. The calorimeter provides energy resolution of about 20%, almost independent of energy. In addition to extending the proton, helium, and heavy nuclei spectra up to about 10~SeV, which approaches the region where the cosmic ray power law energy spectrum has a change in slope, this investigation performs a unique study of the characteristics of nuclear interactions at energies exceeding plausible thresholds for production of quark-gluon plasmas. The quest for the quark-gluon plasma phase of matter is driven to a large extent by the realization that understanding the origin of the majority of the mass in the universe may rest on knowledge of this phase transition [12, 13]. The rationale for building high energy heavy ion accelerators over the past decade have been strengthened by hints of these new states in cosmic ray heavy ion interactions [14, 15] but artificial accelerators have so far fallen short of the required threshold energy to produce the requisite
511
interaction density and temperature. Galactic nuclei are available at much higher energies, and they may provide the first opportunity to study the smallest dimensions of space-time required to observe this phase change.
8. CONCLUDING REMARKS
The WiZard experiment offers the first opportunity to determine the energy spectra of antiprotons and positrons up to a few hundred GeV. It is complementary to LISA, in that it fulfills the low-Z strawman objectives identified in the Astromag report, whereas LISA fulfills the high-Z strawman objectives in that report [2]. It is also worth noting that LISA will perform isotopic observations of galactic cosmic rays in an energy region complementary to the Advanced Composition Explorer (ACE), which has just completed a Phase A study. The ACE investigation will be a free-flying spacecraft in a halo-orbit at the LI Lagrangian point during the last half of 1990. Together LISA and ACE, with about the same statistics, can fulfill the long-awaited dream of the particle astrophysics community for isotopic measurements of galactic cosmic rays up to iron nuclei over decades in energy. In addition to these Astromag investigations, the Heavy Nuclei Collector (HNC) was also selected as an attached payload for the Space Station Freedom assembly phase. The HNC instrument is capable of collecting a significant sample of the actinide nuclei, which is the only practical primary clock for radioactive dating of cosmic rays. Clearly, investigations on Space Station Freedom will play a crucial role in the future of NASA's particle astrophysics program.
REFERENCES
1. Proceedings of the Workshop on Cosmic Ray and High Energy Gamma Ray Experiments for the Space Station Era (Edited by W. V. Jones and J. P. Wefel). Louisiana
State University, Division of Continuing Education (1985). 2. Report of the Astromag Definition Team--The Particle Astrophysics Magnet Facility (Edited by J. F. Ormes, M.
Israel, M. Wiedenbeck and R. Mewaldt). NASA Goddard Space Flight Center (1988). 3. Report of the Astromag Technical Advisory Team (Edited by J. F. Ormes et al.). NASA Goddard Space
Flight Center (1989). 4. S. E. Woosley and T. A. Weaver, Astrophys J. 243, 651~59 (1981). 5. M. Casse and J. A. Paul, Astrophys. J. 258, 860-863 (1982). 6. A. Soutoul, M. Casse and E. Juliusson, Astrophys. J. 219, 753-755 (1978). 7. T. Hara et al., 18th ICRC Conference Papers, Vol. 9, p. 198. Bangalore, India. Tata Institute of Fundamental Research, Bombay (1983). 8. J. M. Grunsfeld, J. L'Heureux, P. Meyer, D. Muller and S. P. Swordy, Astrophys. J. 327, L31-L34 (1988). 9. W. V. Jones, Y. Takahashi, B. Wosiek and O. Miyamura, A. Rev. Nucl. Part. Sci. 37, 71-95 (1987).
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10. G. Steigman, A. Rev. Astron. Astrophys. 14, 339-372 (1976). 1I. A. D. Sakharov, J E T P Lett. 5, 27-30 (1967). 12. R. Pisarski and F. Wilczek, Phys. Rev. D. 29, 338-341 (1984).
13. E. Witten, Phys. Rev. Lett. 51, 2351-2354 (1983). 14. T. H. Burnett et al., The JACEE Collaboration. Phys. Rev. Lett. 50, 2062-2065 (1983). 15. T. H. Burnett et al., The JACEE Collaboration. Phys. Rev. Lett. 57, 3249-3256 (1986).
0094-5765/90 $3.00+ 0.00 Pergamon Press pie
ASTROMAG: PARTICLE ASTROPHYSICS MAGNET FACILITY FOR SPACE STATION FREEDOMS" W. VERNON JONES'[
NASA, Space Physics Division, Code SS, Washington, DC 20546, U.S.A. (Received 14 February 1990)
Abstract--In June 1989 three particle astrophysics investigations were selected for the Astromag superconducting magnet facility to be flown as a U.S.-Italy project on Space Station Freedom in the late 1990s. The science goals include investigating the origin and evolution of matter in the galaxy by direct sampling of galactic material; examining cosmological models by searching for antimatter and evidence of the nature of dark matter; and studying the origin of extremely energetic particles and their effects on the dynamics and evolution of the galaxy. By augmenting Astromag's strong magnetic field with a variety of specialized detectors, state-of-the-art spectometers will be employed to address the experimental objectives. Measurements of high energy cosmic ray nuclei and electrons will be made with precision and sensitivity I0-1000 times that of previous experiments. The Space Station is an ideal spacecraft for Astromag, because it provides both the assembly and servicing capabilities needed for change-out of experiments and replenishment of liquid helium to extend the life of the facility.
I. BACKGROUND
President Reagan's January 1984 announcement of the intention to build a permanently manned Space Station led to an international workshop on "Cosmic Ray and High Energy Gamma Experiments for the Space Station Era" in October 198411]. Several attendees of that workshop came with the suggestion that the Space Station would finally permit the accommodation of a large superconducting magnet in space, and this idea envolved into the workshop consensus. Shortly thereafter, the Cosmic Ray Program Working Group chartered by NASA's High Energy Astrophysics Branch gave its top priority to this magnet facility concept. Subsequently, membership was solicited for the Superconducting Magnet Facility Definition Study Team to explore the possibility of accomplishing a broad range of particle astrophysics objectives with a super-conducting magnet facility. This team, which met over a period of more than 3 years beginning in late 1985, issued an interim report in August 1986 and a final report in May 1988 [2]. These reports detailed the scientific rationale for the facility, outlined the approach to be followed, and emphasized the Space Station Freedom as the preferred space platform for the cosmic ray observations. In mid-1988 NASA issued an Announcement of Opportunity (AO) for Attached Payloads to conduct scientific investigations during the assembly phase of ?Paper IAF-89-466 presented at the 40th Congress of the International Astronautical Federation, Malaga, Spain, 7-13 October 1989. :[:Chief Scientist for Cosmic and Heliospheric Physics and Astromag Program Scientist.
Space Station Freedom. Astromag and the Cosmic Dust Collection Facility (CDCF) were identified as two facilities that could be accommodated during the assembly, and potential experimenters were invited to propose investigations that could be carried out with them. It was the stated intention to proceed with these facilities if, and only if, the quality of the investigations proposed to use them were competitive with the other investigations proposed in response to the AO. The flight investigations proposed for both Astromag and the CDCF were indeed competitiye, and three investigations for each facility were selected in June 1989. The Astromag investigations are the focus of this paper. In late 1988 the Astromag Technical Advisory Team was formed for the purpose of further evaluating and defining the facility and its accommodation on Space Station Freedom. The report of this team was issued in May 1989 [3]. It includes discussions of: the magnet and dewar configurations; the support structures and mechanical systems; facility assembly and servicing; the impact on the Space Station and nearby equipment; and the facility operations. It also suggests an international approach for implementing Astromag in a timely and cost effective manner. From the beginning, foreign scientists have actively participated in the Astromag study activities. It is universally agreed that the project would be substantially strengthened by international involvement. The Italian Space Agency offered to make a major commitment to the Astromag project, and this offer has been tentatively accepted by NASA. Negotiations are underway for developing the core facility as a more-or-less 50-50 U.S.-Italy joint project. Furthermore, there are international co505
506
W. VEgNONJONES This paper discusses the scientific objectives of Astromag and describes the core facility with the three instruments selected for flight in response to the Space Station Attached Payloads AO (NASA OSSA3-88).
investigations from other countries on all three of the investigators selected for Astromag in June 1989. The Astromag facility is the flagship of the high energy cosmic ray community. It is designed to accommodate at least two experiments operating simultaneously, and the experiments can be changedout and/or serviced as required. The first-generation investigations will provide unprecedented information about nucleosynthesis, cosmic ray origin, acceleration regions, modes of propagation, and the possible existence of antimatter, specifically:
2. NUCLEOSYNTHESIS
The standards for comparing theories of nucleosynthesis are the elemental and isotopic composition of the solar system, which have traditionally been derived from meteorites and spectroscopic measurements in the solar photosphere. More recently, solar energetic particles are providing a direct sample of the sun, and solar flare elemental abundances are being corrected for the effects of ionization potential fractionation to get an excellent set of photospheric abundances. Figure 1 summarizes the present knowledge of the isotopic composition of the cosmic ray source. It is significant that for all the isotopes for which the source composition has been determined to within about 30% (neon, magnesium, and silicon) there is in each case evidence for a difference between cosmic ray and solar system material. The most quantitative theoretical suggestions to explain such differences are the so-called supermetallicity model [4] and the Wolf-Rayet model [5]. The former proposes that the excess of neutron-rich isotopes might result if cosmic rays originate in regions of the galaxy that are metal-rich compared to the solar system. The latter proposes that a fraction of the heavy cosmic rays originate from material expelled by Wolf-Rayet stars in high velocity stellar winds. The isotopic composition of the solar system is clearly different from that of galactic cosmic rays, which are probably a sample of the interstellar
(1) The so-called WiZard investigation (R. L. Golden, New Mexico State University, Principal Investigator) will measure the energy spectra of light (Z < 4 ) particles, including antiprotons, positrons, and electrons, and search for anti-helium at unprecedented levels of sensitivity. (2) The Large Isoptoe Spectrometer for Astromag (LISA) investigation (J. F. Ormes, Goddard Space Flight Center, Principal Investigator) will measure the spectra of elements and isotopes of heavy (Z > 3) particles, and search with unprecedented levels of sensitivity for their corresponding anti-particles. (3) The Spectra, Composition, and Interaction Study using a Magnet-Interaction Calorimeter (SCIN/MAGIC; T. A. Parnell, Marshall Space Flight Center, Principal Investigator) will use the Astromag facility to search for a new state of matter (quark-gluon plasma) in ultra-relativistic nuclear interactions, as well as for measurements of cosmic ray energy spectra up to 1015eV, which approaches the region where dramatic changes in nuclear composition are expected. 10
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medium. On the other hand, the solar system itself is a sample of the interstellar medium that happened to condense about 4.6 billion years ago. This implies that nucleosynthesis of cosmic-ray and solar-system material has differed, probably due to one, or more, of the following reasons:
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(1) The cosmic rays sample a different part of the galaxy, closer to the galactic center where there are more Wolf-Rayet stars. (2) The cosmic rays are a recent (20 million year old) sample, while the solar system is a sample from 4.6 billion years ago, i.e. the galaxy's composition is evolving. (3) The solar system is not typical of the interstellar medium, i.e it may have been "contaminated" by a nearby supernova shortly before its condensation. It is likely that this issue can be resolved only by comparing models of nucleosynthesis and chemical evolution of the galaxy with observations of isotopes of many more cosmic ray elements than just neon, magnesium, and silicon (in addition to observations of the rare ultraheavy elements). The LISA instrument on Astromag will have sufficient isotopic resolution and large enough collecting area to give definitive tests of competing models, all of which can fit the limited cosmic ray data now available. 3. P A R T I C L E
ACCELERATION
Supernova explosions in the galaxy are believed to provide the energy responsible for accelerating cosmic ray nuclei to high energy. However, it is not known whether this acceleration occurs immediately during the birth and infancy of the supernova or much later as shock waves from older supernova make multiple encounters with material in the interstellar medium. In the former case, cosmic rays would consist of freshly synthesized material ejected from recent supernova. In the latter case, cosmic rays would represent element-synthesis integrated over the age of the galaxy. Since galactic cosmic rays are fully ionized, the time scale between nucleosynthesis and acceleration to high energy can be revealed by measurements of isotopes that decay only by electron capture [6], e.g. 56Ni, 57Co, 55Fe, and 59Ni. Once accelerated and stripped of their electrons these nuclei can no longer decay, so their abundances preserve a record of the time-delay, if any, between their production and acceleration. Therefore, these isotopes clearly address the question of whether cosmic rays represent a sample of the interstellar medium, or whether they represent newly synthesized material, possibly from supernova. Figure 2 shows the expected yields of Fe, Co, and Ni isotopes as a function of the time delay. If this delay is, for example, more than a few days most of the 56Ni will have decayed to 56Co, which then decays AA 21-6/7--K
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Fig. 2. Isotope yields, e.g. production-acceleration delays. to 56Fe, thereby producing the 0.847 MeV gamma rays so diligently searched for in the recent supernova SN1987A. If the time delay is much more than 105 years, the 59Ni will have decayed to 59Co. Therefore, measurement of the isotopic composition of cobalt may provide critical information about supernovae and their role in accelerating cosmic rays. Further evidence of explosive nucleosynthesis in supernova can be obtained from observations of the abundances of a variety of other isotopes in cosmic rays, including neutron-rich isotopes of the elements Ne, Mg, Si, Fe, and Ni. Particle acceleration to extremely relativistic energies is a widespread astrophysical phenomenon, from the Earth's bow shock, to the sun, to galactic objects (SS433), to the galaxy (cosmic rays), to centers of active galaxies (radio jets). There are now specific models of shock acceleration for cosmic rays that predict cutoffs in the spectra of individual elements somewhere between 101° and 1015eV, i.e. at energies below the "knee" in the all-particle spectrum. Figure 3 illustrates the existing experimental situation [7]. This high energy data has been collected only by extensive air shower experiments above the knee and primarily by balloon flight experiments below the knee. The Cosmic Ray Nuclei Experiment (CRNE) on Spacelab-2 [8], the only space experiment approaching such high energies, focused on the spectra of nuclei over the range Z = 3-26; the SL2 data are shown separately in Fig. 3. The JACEE balloon flights over the past decade [9] have provided data on protons and helium that extended the energy range of the earlier "Ryan Proton" spectrum. Neither CRNE on Spacelab-2 for a week nor the JACEE balloon flights over the past decade have yet
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4. GALACTICCONTAINMENT The question of how cosmic rays are stored in the interstellar medium is but one example of the more general problem of their interactions with the galactic interstellar plasma. We can measure the containment time-scales and infer the interaction mechanism of particles with magnetic fields and vice versa. This helps explain the role of energetic particles in creating hydromagnetic waves, turbulence, etc. and, in turn, the effect of the latter on the motions of the particles. Since these interplays between the source and the observer change the cosmic ray spectra, they must be understood before we can know the source spectra, which is in turn essential to understanding the dominant mechanisms in the cosmic-ray accelerator. Meaurements on purely secondary particles in cosmic rays are the key to cosmic ray interactions with the interstellar medium. These include antiprotons and positrons, as well as the L-Nuclei (Li, Be, and B) and sub-Fe groups. For example, if the antiproton data really indicate that the confinement models derived from heavier secondaries are not correct for protons, then at least cosmic ray protons are accelerated in different sources. The much better positron, antiproton, dueteron, and helium-3 measurements over a wide energy interval with the Astromag WiZard instrument will directly address these issues.
5. COSMOLOGICALANTIMATTER One of the fundamental, unanswered questions of cosmology is the degree to which the universe contains antimatter. In 1928 the Dirac equation accorded equal status to matter and antimatter, and until the late 1950s it was generally believed on grounds of particle-antiparticle symmetry that equal amounts of each existed in the universe as a whole. It was recognized, however, that without some mechanism for matter-antimatter separation in the early universe, most of the produced matter and antimatter would subsequently annihilate. This led to expectations that the ratio of baryons to photons would be on the order of 10 -19. Evidence for the lack of antimatter in our galaxy and nearby galaxies evolved from the new field of gamma ray astronomy, which led to an upper limit of 10 -15 for the antimatter/matter ratio in our galaxy[10], although antimatter on the scale of clusters of galaxies was not ruled out. The possibility of matter-antimatter asymmetry is dependent on [11]: (1) baryon number not being strictly conserved and (2) charge-parity symmetry not being exact. The latter, CP violation, has been observed, and baryon non-conservation is incorporated into the current Grand Unified Theories. Although proton decay has not yet been observed, it is the view of most cosmologists that the universe is baryon asymmetric, i.e. consists almost entirely of matter. Current upper limits on the antinuclei/nuclei ratio in cosmic rays exclude the possibility for a substantial fraction of antimatter in our galaxy, but those limits are inadequate to address the possible existence of all anti-galaxies. Both the WiZard and LISA investigations are capable of extending antinuclei searches by nearly four orders of magnitude, which offers the opportunity to address the question of whether the universe does indeed display a global symmetry between matter and antimatter. Either a positive or
Astromag negative result from these investigations will have a profound effect on the way we view the universe. If antinuclei are identified, thereby establishing the symmetry between matter and matter, serious reevaluations of the various cosmological models now in vogue would be necessary. On the other hand, the failure to detect antinuclei at their level of sensitivity would provide support for new theories being formulated to account for the evolution of an initially baryon-symmetric universe into one containing little antimatter.
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6. ASTROMAG FACILITY
Astromag is basically a magnetic spectrometer capable of determining the momentum per unit charge (rigidity) and sign of the charge of highly relativistic (fully ionized) cosmic rays. The approach is to have a long-lived facility on Space Station Freedom based on a set of coils that produce an intense magnetic field. When cooled by liquid helium, the niobium-titanium coils become superconducting (i.e. zero electrical resistance) so the current and associated magnetic field persists indefinitely. Track detectors in the magnetic field measure the curvature of particle trajectories; the amount of bending is inversely proportional to a particle's rigidity. The tracking detectors and other particle sensors needed for specific investigations are provided by the experimenters. Two experiments can operate simultaneously on the two-ended facility. The heart of Astromag is the superconducting magnet and its supporting cryogenic system, which is comprised of two magnet coils, a persistent switch, gas-cooled detachable current leads, and a superfluid helium circulation system, all housed inside a 3500 liters superfluid helium dewar. The identical coils have opposite polarity, so the dipole moments effectively cancel and, therefore, induce no torque on
Measurement
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the Station. The net dipole moment is determined by the manufacturing differences between the coils, which are connected in series to ensure that the same current exists in both coils at all times. Since the coils are superconducting, the circulating current is persistent (i.e. d e c a y s < 1% per year) and the magnetic field is maintained without continuous application of power. In the event of a spontaneous or induced quench (in which the coil wire undergoes a conversion from the superconducting state to a normal resistive state) the current would decay in 2 s because of resistive losses and the 11 MJ of magnetic energy would dissipate in heating the coils. Figure 4 shows schematically how Astromag with two instruments will appear on the Space Station truss structure. The facility will be located on an outrigger (truss extension) to isolate the magnetic field from the main truss of the Station. The use of a two-bay (10m) extension will ensure that the magnetic field at the main truss is essentially the same as the Earth's field. The facility will utilize the standard Attached Payload Accommodation Equipment (APAE) for interfacing with the Station, including the mechanical, thermal, electrical power, and signal connections. 7. ASTROMAG INSTRUMENTS
7.1. WiZard
Fig. 4. Astromag on SSF truss extension.
As illustrated schematically in Fig. 5, the WiZard instrument consists of an array of particle detectors mounted on one end of the Astromag magnet system. From the top, it includes a pair of time-of-flight (TOF) counters, a tracking system, another TOF pair, a transition radiation detector (TRD), and a calorimeter. The detectors are held together by a spinal structure that also serves as the payload carrier for a Shuttle launch. The individual detectors are mounted within a segmented, pressurized container which is part of the backbone. When assembled, the individual modules form a single pressurized volume
510
W. VERNONJONES
with a low mass entrance window for cosmic rays. The system design has minimized the mass in the particle paths to avoid particle interactions that could complicate event interpretation. This detector assembly is capable of identifying electrons, positrons, antiprotons, nuclei, and antinuclei over a wide range of energies. The TOF sub-system measures the transit time between the top and bottom planes, provides the direction of the particle, and the magnitude of its charge. The tracking subsystem provides almost continuous measurements of particle trajectories through Astromag's magnetic field, which allows determination of the particle's rigidity and the sign of the charge. These TOF and tracking sub-system data are adequate to search for primordial antimatter, but measurement of positron and antiproton fluxes requires additional detectors, e.g. an electron and an antiproton would be indistinguishable with just these sub-systems. The TRD and the calorimeter provide the means for separately identifying light particles (electrons and positrons) from heavier particles (protons and antiprotons). The WiZard geometry factor for measuring antiprotons and positrons is 0.1 m2sr, but for the primordial antimatter search it is 0.4 m2sr. Magnetic rigidities can be measured up to its maximum detectable ridigity (Rmax) in excess of 1 TV. At least 2 years observing time is needed for the antiprotons, positrons, and light secondary (Li, Be, B) nuclei. 7.2. LISA
The two side views of the LISA instrument are illustrated in Fig. 6. The sub-systems consist of a central region for tracking particles through Astromag's magnetic field and an outer ring (four segments of a polygon) for determining velocity, charge, and TOF. Particle trajectories are determined with five planes (H l-H5) of scintillating optical fiber detectors, each capable of providing two-dimensional coordinates with 70 #m or better resolution. Two aerogel Cherenkov counters (C1 and C2 or C3 and C4), one Pilot 425 Cherenkov counter (T1 or T2), and a set of
scintillators (S1 or $2) are combined in each of eight modules. The Cherenkov response of the Pilot 425, which is nearly saturated by the particles to be measured, provides a direct measure of a particle's atomic number Z, after pathlength and mapping corrections are applied. For particles with energy less than about 10GeV/nucleon the aerogel counters measure the particle's velocity, or equivalently its momentum per nucleon. The TOF scintillators determine the particle direction (up or down), thereby distinguishing downward moving negatively charged particles from upward moving particles or positive charge. The following physical quantities of interest can be derived from the various sub-system measurements. (1) Charge identification is obtained from the Pilot 425 and aerogel Cherenkov counters, supplemented by the TOF scintillators; rigidity measurements from the trajectory sensors help correct these pulse heights for velocity dependence. (2) Isotope identification is based on combining the rigidity obtained from magnetic deflection with the momentum per nucleon derived from the aerogel Cherenkov emission. This combination, together with the measured atomic number, provides mass resolution sufficient for separating adjacent isotopes for Z = 4-30 nuclei over the energy range 2.5-4 GeV/nucleon. (3) Matter-antimatter separation is based on the sense of particle deflection in the magnetic field. The LISA investigation of Z = 4-30 elements is based on a 2 year exposure of the 0.1 m2sr instrument. Elemental composition studies will cover the range 2-1000 GeV/nucleon, but isotopic composition will be limited to 2.5-4GeV/nucleon. The charge and energy resolutions depend on the particle species and its energy. The charge resolution is approx. 0.18 amu for C, 0.20 amu for Si, and 0.25 amu for Fe. The energy resolution is about 0.3% at 3 GeV/nucleon, 2% at 30GeV/nucleon, and 20% at 300GeV/ nucleon. The antimatter search will have a sensitivity of about 4 x 10 -7 for Z > 6. 7.3. S C I N / M A G I C
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The SCIN/MAGIC instrument, shown schematically in Fig. 8, is basically a stack of passive, high resolution track detectors in the magnetic field of Astromag. The stacks contain nuclear track emulsions, track-etch detectors, X-ray films, lead or tungsten plates, and other inert materials. From the top, the major modules are the charge identification module, three conseuctive low-density target-tracking chambers, and a calorimeter. The investigation requires two pallets of four chambers, two of which exclude the intermediate tracking detectors depicted in Fig. 7. The mission duration is limited by the
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Fig. 7. Schematic of the SCIN/MAGIC instrument. accumulation of tracks in the integrating detectors to, preferably, about 90 days. This short duration makes S C I N / M A G I C a good companion experiment for the Astromag facility, e.g. it can be run during the initial checkout of Astromag or during changeout of major experiments. The S C I N / M A G I C chambers have the dynamic range to measure all nuclei from protons to Fe, as well as the characterisics of nuclear interactions with secondary particle multiplicities up to 1000 or more. The fundamental position resolution is about 1/~m. By using delta ray range measurements instead of the conventional grain counts with fixed range, the charge resolution will vary from about 0.1e for protons to 1 e for Fe nuclei. The calorimeter provides energy resolution of about 20%, almost independent of energy. In addition to extending the proton, helium, and heavy nuclei spectra up to about 10~SeV, which approaches the region where the cosmic ray power law energy spectrum has a change in slope, this investigation performs a unique study of the characteristics of nuclear interactions at energies exceeding plausible thresholds for production of quark-gluon plasmas. The quest for the quark-gluon plasma phase of matter is driven to a large extent by the realization that understanding the origin of the majority of the mass in the universe may rest on knowledge of this phase transition [12, 13]. The rationale for building high energy heavy ion accelerators over the past decade have been strengthened by hints of these new states in cosmic ray heavy ion interactions [14, 15] but artificial accelerators have so far fallen short of the required threshold energy to produce the requisite
511
interaction density and temperature. Galactic nuclei are available at much higher energies, and they may provide the first opportunity to study the smallest dimensions of space-time required to observe this phase change.
8. CONCLUDING REMARKS
The WiZard experiment offers the first opportunity to determine the energy spectra of antiprotons and positrons up to a few hundred GeV. It is complementary to LISA, in that it fulfills the low-Z strawman objectives identified in the Astromag report, whereas LISA fulfills the high-Z strawman objectives in that report [2]. It is also worth noting that LISA will perform isotopic observations of galactic cosmic rays in an energy region complementary to the Advanced Composition Explorer (ACE), which has just completed a Phase A study. The ACE investigation will be a free-flying spacecraft in a halo-orbit at the LI Lagrangian point during the last half of 1990. Together LISA and ACE, with about the same statistics, can fulfill the long-awaited dream of the particle astrophysics community for isotopic measurements of galactic cosmic rays up to iron nuclei over decades in energy. In addition to these Astromag investigations, the Heavy Nuclei Collector (HNC) was also selected as an attached payload for the Space Station Freedom assembly phase. The HNC instrument is capable of collecting a significant sample of the actinide nuclei, which is the only practical primary clock for radioactive dating of cosmic rays. Clearly, investigations on Space Station Freedom will play a crucial role in the future of NASA's particle astrophysics program.
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10. G. Steigman, A. Rev. Astron. Astrophys. 14, 339-372 (1976). 1I. A. D. Sakharov, J E T P Lett. 5, 27-30 (1967). 12. R. Pisarski and F. Wilczek, Phys. Rev. D. 29, 338-341 (1984).
13. E. Witten, Phys. Rev. Lett. 51, 2351-2354 (1983). 14. T. H. Burnett et al., The JACEE Collaboration. Phys. Rev. Lett. 50, 2062-2065 (1983). 15. T. H. Burnett et al., The JACEE Collaboration. Phys. Rev. Lett. 57, 3249-3256 (1986).