COSMIC RAYS, SPACE INVESTIGATIONS JONATHAN F. ORMES In 1948 it was discovered that the cosmic rays contained highly energetic (relativistic) ions that include all the elements up through iron in the periodic table, and the study of the astrophysics of galactic cosmic rays (GCR) was born. These balloon-borne studies showed the general similarity between the material in the galactic cosmic rays and solar system material. Inferences about the general nature of the spectra and composition came from ground-based neutron monitors, balloon-borne ion chambers, and air-shower arrays. These studies showed that the particles were coming beyond the solar system. The first detailed studies of these particles were made on small satellites and balloons. The direct observations from space of GCR began in the 1960s with Interplanetary Monitoring Platform (IMP) and Orbiting Geophysical Observatory (OGO) experiments. These experiments were capable of studying particles with kinetic energies up to approximately 1 GeV ***(here GeV is giga-electronvolt and *** means per nucleon). In this entry the instrumentation and methods used to observe galactic cosmic rays above 1 GeV **** will be described. Balloon-borne experiments have played an important role in this discipline and are responsible for many of the pioneering observations. In recent years satellite observations have become increasingly important. Satellites offer two very significant advantages. First of all, they carry instrumentation outside the atmosphere, where properties of the particles can be measured without the need to compensate for the effects of the overlying atmosphere. Second, and most important, the missions can be of extended duration (a year or more), whereas balloon payloads can be maintained at high altitudes for typically one day. The added observing time makes possible studies of the rarer species and more subtle signatures of astrophysical effects. Observations of GCR from space now include high-precision measurements of the energy spectra of the more abundant species, element identification of nuclei throughout most of the periodic table, and exploratory observations of the isotopic composition of the more abundant nuclei. This entry will describe the following experiments: 1. Measurements of the proton and helium spectra and the all-particle spectrum (calorimetry): Proton satellites (USSR). 2. Measurements of ultraheavy (*****) abundances in cosmic rays: Skylab, Ariel VI (UK), and HEAO-3. 3. Measurements of cosmic ray isotopes: ISEE-3 isotope experiments and the HEAO-3 Danish-French experiment. 4. Measurements of high-energy spectra using transition radiation detectors: Spacelab 2 cosmic ray nuclei experiment. Papers on these kinds of experiments appear regularly in the journal Nuclear Instruments and Methods and in books constituting the Proceedings of the International Cosmic Ray Conferences that are held biannually. CALORIMETRY The proton and helium spectra and the all-particle spectrum were the first to be measured by direct observations from space. These experiments were done on the Proton satellites flown by the Soviet Union in the early 1960s. The first major space experiment to explore the cosmic ray composition and spectra directly at high energy was an instrument known as SEZ-14 which was flown on the Proton satellites. A cross section of the instrumentation is shown in Fig.1. The observations were based on a technique in which the total energy of a highly relativistic particle is measured by counting the secondary particles produced in a cascade of nuclear and electromagnetic interactions. The results from this series of satellites were quite surprising. They indicated that while the spectrum of all primary cosmic rays could be fit with a single power law between 60 and *** GeV, the proton spectrum showed pronounced steepening at ******** GeV. Later, it was demonstrated that the number of backscattered particles from the calorimeter increased with the log(E). The presence of these backscattered particles was sufficient to confuse the identity of single charged particles, artificially steepening the slope of the proton spectrum, but not the all-particle spectrum. Notwithstanding these difficulties, these pioneering measurements were the first to observe the high-energy component of cosmic rays with direct observations from space. ULTRAHEAVY NUCLEI EXPERIMENTS The next major thrust in measurements from space focused on the measurement of elements in the transition part of the periodic table. Early results from an exposure on Skylab gave indications that actinides were present in a high proportion. The balloon and Skylab experiments used large area arrays of thin plastic sheets as a detecting medium. The very highly ionizing particles pass through the plastic sheet, leaving behind a trail of damaged polymer bonds. When the sheets are placed in a solution saturated with hydroxyl radicals, conical pits are etched out of these damaged sites. The etch rate and hence the size and shape of the conical pits had been discovered to be a strong function of the ionization loss of the incident particles. The size and geometry of the etch pits were used to determine the identity of the incident nucleus. The ionization loss rate is given by the Bethe-Block where Z is the charge of the incident nucleus, * is its velocity in units of the speed of light, and *(**) represents the relativistic increase in ionization loss at energies above several giga-electronvolts. The next generation of detectors to study the transition elements were those flown by the British on a satellite known as Ariel VI and by a Washington University (St.Louis)-California Institute of Technology (Caltech)-University of Minnesota collaboration on the third in a series of High Energy Astronomy Observatory (HEAO) satellites. These detectors were electronic counter experiments in which a combination of ionization chambers and Cerenkov detectors were used to determine velocity and charge. The functional dependence of Cerenkov light on velocity and charge is given by *********************** The index of refraction * can be chosen, within some restrictive limits, to cover a specific velocity domain of interest, *******. Just above threshold, the response is very sensitive to *. Once * is known, Z can be determined and the species identified. The Ariel VI detector had an unusual spherical geometry. It is shown in Fig.2. The detector consisted of a thin spherical shell of doped acrylic Cerenkov detector suspended concentrically within a thin aluminum shell. The complete volume was filled with a mixture of scintillating noble gases, which responded proportionally to the ionization loss. The next major satellite investigations of GCR were planned by NASA as part of the HEAO series. A very nice history of this program can be found in Tucker's The Star Splitters. Two cosmic ray investigations were part of the payload carried on the third satellite, HEAO-C, renamed HEAO-3 following the launch. The HEAO-3 heavy nuclei experiment was of a more conventional design than that of the Ariel VI experiment. It consisted of two large ionization chambers mounted back to back, with a large Cerenkov counter between. The instrument measured the elemental composition of nuclei from chlorine(*****) through at least uranium (*****). The relative abundances of the elements in the cosmic rays observed by the Ariel VI and HEAO-3 experiments are similar to the relative abundances of the elements in solar system material (SSM), but they are not identical. These experiments did not find the excess of actinides initially reported by the Skylab investigators. The differences in abundances between GCR and SSM may contain important clues as to how elements originate. It was observed on HEAO-3 that the elements that are more difficult to ionize are less abundant in the GCR than in SSM. This behavior is difficult to understand if cosmic rays are produced by violent events such as supernova explosions. Under these extremely energetic conditions, the atoms should be completely stripped of all their electrons. On the other hand, if cosmic rays originate in a comparatively low-energy environment having a temperature of about 10,000 K, the ionization potential may parametrize an important selection effect. These conditions are found on the edges of bubbles carved in interstellar space by hot young stars, around red dwarf stars known for their frequent, intense flares (flare stars), or in the wakes of shock waves plowing through the interstellar medium. If this is true, then the material sample represented by the GCR is probably part of the interstellar medium. ISOTOPE MEASUREMENTS Studying the isotopic composition of GCR is another powerful method for investigation of these ideas. Here again, low-energy cosmic ray experiments on balloons, deep space missions (Pioneer, Voyager) and near-Earth monitors of solar and galactic cosmic rays made the pioneering observations. Early balloon results indicated that the isotopic composition of GCR is different from solar system material. Subsequent studies using solid-state detectors on the ISEE-3 spacecraft, in orbit outside the Earth's magnetosphere, have confirmed the composition differences and have established their magnitude for the most abundant species (e.g., the isotopes of Ne). The first attempt to make measurements at high energies came on a HEAO-3 experiment built by the Danish Space Research Institute and the Center for Nuclear Studies, Saclay, France. This experiment utilized a set of five Cerenkov detectors, including two with very low index of refraction silica aerogel counters, in conjunction with a flash tube hodoscope????**** used to measure the trajectory of the particles through the instrument. The two outermost Cerenkov detectors were made of glass (* ***) and were used for charge determination. The requirement of a consistent response in these two detectors was used to eliminate all those particles which had interacted in the material in the detector. The three inner Cerenkov detectors were chosen to match the spectrum observable in the satellite's orbit and were for velocity determination. They were made of teflon (n=1.33), aerogel blocks (n=1.053), and an aerogel sand (n=1.012). These three indices of refraction were chosen to cover a reasonable energy range, as shown in Fig. 3. This experiment was able to obtain excellent charge and velocity (and hence momentum) resolution. High precision spectra were obtained covering the nuclei ******** and over the energy range from 0.5 to 20 GeV **. Using the Earth's magnetic field as a filter, isotopic composition information was also obtained. At each point on the satellite's orbit there is a range of energies for which particles of a given A/Z ratio are allowed to reach the spacecraft, whereas particles of a different A/Z ratio are not allowed (e.g., their trajectories, when traced back, hit the Earth). This experiment indicated that the isotopic composition anomalies found at lower energies persist into the trans-giga-electronvolt energy range. The excellent charge resolution and energy spectra from this experiment have firmly established the spectral difference between the secondary and primary cosmic rays hinted at by prior balloon experiments. TRANSITIONS RADIATION DETECTORS The study of the spectra of individual elements at still higher energies required the introduction of a large-area, lightweight detector to obtain the necessary collection power. A new detector, a transition radiation detector, was incorporated into an experiment designed and built at the University of Chicago and flown on the Spacelab-2 mission. known as the cosmic ray nuclei (CRN) experiment, its purpose was to extend knowledge of the charge composition of cosmic rays to energies in the range 50-5000 GeV ***, well beyond the range of the Danish-French experiment. The experiment orbited for the eight days of a space shuttle mission in 1985. CRN employed scintillation detectors for charge measurement and proportional counters for measuring particle trajectories. It used a gas Cerenkov detector for energy measurements in the range from 40-150 GeV ***. The transition radiation detector was used for energy measurements above 500 GeV *****. It utilized the radiation of x-rays produced at the boundary of two media with different indices of refraction by the passage of a charged particle. The radiation is proportional to the Lorentz factor * of the particle and the square of the charge of the particle. The production of x-rays is improbable at any single interface, so the detector is made up of several layers, each consisting of thousands of interfaces. The detector is made of thin polyolefin fiber threads of 4.5*** and 21*** diameter, which are also used for the insulation of ski jackets. The x-rays generated in the fiber bundles are detected in the multiwire proportional counters filled with high Z gas. The response for singly charged particles as a function of Lorentz factor is shown in Fig. 4. This new detector system has extended knowledge of the spectra of individual nuclear components of the cosmic rays to higher energies. CRN has shown that the energy dependence of the secondary nuclei to primary nuclei ratio extends to the highest energies yet measured. THE FUTURE The experiments described have traced 25 years of space-based investigations of the galactic cosmic rays. A few basic detectors have been used in a variety of configurations for different objectives. Great progress has been made, but much more remains to be done. Determination of the isotopic composition of galactic cosmic rays for rarer isotopes will be the focus of the next generation of instruments. Studies of the spectra of antiprotons and searches for other heavier antinuclei will be important in defining the limitations on matter-antimatter symmetry in the Universe. The positrons and antiprotons, secondaries from the collisions of cosmic ray protons and interstellar matter, will be compared to the spallation products of heavier nuclei to establish the relationship between the origin of cosmic ray protons and heavier nuclei. These investigations will require magnetic spectrometers for measurements above 1 GeV *** and new large arrays of high precision solid-state detectors for measurements at lower energies. The 1990s should see some of these new investigations, currently on the drawing boards, come to fruition. Additional Reading Engelmann, J.J., Goret, P., Juliusson, E., Koch-Miramond L. Lund, N., Masse, P., Rasmussen, I.L., and Soutoul, A.(1985) Source energy spectra of heavy cosmic ray nuclei as derived from the French-Danish experiment on HEAO-3. Astron. Ap.148 12. Fleisher, R.L., Price, P.B., and Walker, R.M.(1975). Nuclear Tracks in Solids: Principles and Applications. University of California Press, Berkeley. Grigorov, N.L., Murzin, V.S., and Rapoport, I.D.(1958). Method of measuring particle energies above **** eV. Zh. Eksper. Teor. Fiz.(Sov. Phys.-JETP)34 506. Jelley, J.V.(1958). Cerenkov Radiation and its Applications. Pergamon Press, London. Rossi, B.(1952). High Energy Particles. Prentice-Hall, Englewood Cliffs, NJ. Tucker, Wallace H.(1984). The star splitters. NASA Report No.SP-466 See also Antimatter in Astrophysics; Cosmic Rays, Observations and Experiments; Supernova Remnants, Evolution and Interaction with the Interstellar Medium.