Measurements of Particle Fluxes in Space

Measurements of Particle Fluxes in Space Daniel Schick October 11, 2006 Space Physics by Kjell Rönnmark

Contents 1 Introduction 1 2 Cosmic Rays 1 3 Measurement Methods 3 3.1 Indirect Methods................................ 3 3.2 Direct Methods................................. 4 3.2.1 Method 1................................ 4 3.2.2 Method 2................................ 6 4 Summary 8 5 Appendix 9 A List of Experiments 9 B References 10 i

1. Introduction In 1912 the Austrian physicist Victor Franz Hess discovered that the ionisation grade of the air increases in higher layers of the atmosphere. He explained his discovery with an external radiation that comes from space which he called cosmic rays 1. In the year 1923 the two German physicists Walther Bothe and Werner Kohlhörster wanted to show that cosmic rays were caused by high energetic γ-rays. But to their surprise it turned out that the radiation also consists of electric charged particles. Then until the 1950 s comic rays were the only source of elementary particles for physicists because there were no large particle accelerators available. And until today many different experiments have been dealing with cosmic rays. 2. Cosmic Rays What are cosmic rays? Generally we mean charged particles when we talk about it. These particles 2 are 98% atomic nuclei and 2% electrons. The atomic nuclei consists of 87% protons, 12% Helium nuclei and 1% heavy atomic nuclei. Until today all natural nuclei from hydrogen to actinides were detected. As one can see in figure 1 the energy spectrum of cosmic rays show a huge bandwidth from a few ev to the highest ever measured energies of 10 20 ev. These max. energies are many magnitudes higher than the energies ever produced by human beings and this is why cosmic rays are still very important for elementary particle physics. Cosmic rays have different origins, see figure 2. But regardless of which origin they come from they are always deflected by the interstellar magnetic fields during their journey to the earth because they are charged. This leads to an isotropic distribution when cosmic rays hit the earth and that is why we have no information on their incoming direction at all. Although we have been studying cosmic rays for over 90 years now there still are many not completely answered questions. What are the sources of cosmic rays and how are the particles accelerated to such high energies? How do cosmic rays propagate through the interstellar medium and do they change their properties during the propagation? What are the highest possible energies of cosmic rays? 1 Victor Franz Hess received the Noble Prize in Physics for the discovery of cosmic rays in 1936. 2 The chemical composition is very similar to our solar system. When measuring the ratio of stable and unstable nuclei of e.g. carbon one can also determine the time these particles were travelling through space. Also the flux of antimatter in cosmic radiation is very low. This is an indication that there are no bigger accumulations of antimatter in space. 1

Figure 1: For low particle energies the earth s magnetic field deflects nearly all particles. But for energies higher than 30 GeV the magnetic field has no influence on the particles anymore. Up to energies of approx. 10 14 ev, the so-called knee, the flux continues along an exponential law like φ(e) = φ 0 E γ with γ = 2.7. From 10 15 ev to energies of approx. 10 19 ev, the so-called ankle, the decrease is higher with γ = 3.0. Behind the ankle the decreasing of the flux is slower than before. Behind energies of 10 20 ev there should be no flux any more because of the GZK- Cutoff, but experimental results are contradictory to each other and it is really hard to do measurements in this region of very low particle flux. 2

Figure 2: Cosmic rays have a solar, galactic or extragalactic origin. Of course our sun, left picture, is the source of solar cosmic rays. During sun eruptions particles can be accelerated up to the GeV region. But the main part of cosmic rays of energies up to 1018 ev seems to have a galactic origin. Most likely cosmic particles are produced and accelerated during supernova explosions, see the picture in the middle. Because there are no sources of particles with energies of up to 1020 ev in our galaxy these high energy particles must have an extragalactic origin. But the sources for these high energies have not been discovered yet. The candidates are probably so-called cosmic jets, see right picture, of black holes or of pulsars or shock fronts of supernova explosions. 3. Measurement Methods 3.1. Indirect Methods Particles with an energy of over approx. 30 GeV are not affected by the earth s magnetic field and interact with the atoms and molecules of the atmosphere. As a result these so-called primary cosmic rays cause huge air showers3, so-called secondary cosmic rays. The first discovery of cosmic rays by Victor Franz Hess was, of course, the discovery of secondary cosmic rays which cause the high ionisation rate in the upper layers of our atmosphere. The flux of high energy particles4 is so low that it is nearly impossible to detect them directly in space. To detect these particles it is necessary to detect their secondary cosmic rays5. It is then enough to measure just a small amount of these secondary particles 3 EAS - Extended Air Shower 5 A high energy particle like a single proton with an energy of 1015 ev produces on average 106 secondary particles (80% photons, 18% electrons and positrons, 1.7% myons und 0.3% hadrons) along its flight line. They can be detected on the ground. 4 Particles with energies of more than 1015 ev are detected less than once per m2 and year. 3

to get information on the primary particle. For example the incoming direction of the primary particle can be determined by the highest intensity of its secondary rays. But to determine the energy and mass of the primary particle difficult simulations of the EAS are needed. 3.2. Direct Methods In order to detect primary cosmic rays the measurements should take place in heights where interactions with the atmosphere can be neglected. The different possibilities to do measurements of primary cosmic rays from a height of approx. 40 km up to solar orbits are shown in figure 3. But what are the physical values of cosmic particles we want to measure? At least we want to know the type, this means the mass and charge, of the particle and of course its energy. Therefore different measurement methods for different types and energies are needed. Figure 3: A cheap and simple way to make experiments in 40 km is a balloon flight, see left picture. Balloons can have a volume of about 106 m3 and can carry a weight of up to 3000 kg. But the disadvantage is their relatively short flight duration of typically just a few days. That makes it quite hard to get some good statistics of particles with a low flux. The better, but of course, much more expensive way is to attach the experiments to a satellite, space probe or even the ISS, see pictures in the middle and on the right side. The orbits of satellites are usually placed over the Antarctic because of the weaker magnetic field in the polar region. Experiments in space make it possible to measure the particle flux of cosmic rays over years and to extend the particle s energy spectrum to higher energies because of the longer measuring time and also to lower energies for space probes with their solar orbits because of the missing magnetic field of the earth. 3.2.1. Method 1 A quite simple method to detect electric charged particles is a channel electron multiplier. Today Micro Channel Plates (MCP) consist of thousands of channel electron multipliers 4

where each channel has a diameter of 10 µm and the whole plate is not thicker than 1 mm. That makes it possible to have two-dimensional detector arrays for charged particles. To select the incoming charged particles by their energy an electrostatic analyser (ESA) can be used. It will bend the particle s flight line until its centrifugal force equals the electrostatic force. m v 2 r = q E (1) We can now select particles by their kinetic energy mv 2 /2 per charge q by only varying the electric field or switching its polarity to select negative or positive charged particles. But only information on the energy per charge of a particle is not enough to determine its mass and velocity. Although it is enough for electrons ions can not be distinguished this way. As a second selector we use a magnetic spectrometer that will bend the particle s flight line again because of a strong magnetic field perpendicular to the particle s velocity. Once again this happens until the particle s centrifugal force equals the Lorentz force. F L = q v B (2) Now we know the energy per charge and also the mass and velocity of the incoming particles. A combination of all three devices is shown in figure 4 and is called Threedimensional Ion Composition Spectrometer. Figure 4: Incoming particles are selected by their energies in the ESA. Particles with another energy than the selected one will hit the ESA s walls and can not be detected. Afterwards the particles trajectory will bend again in the magnetic spectrometer because of the Lorentz force. Of course, the orbits of lighter ions will bend more than the ones of heavier ions. A MCP at the end of the spectrometer can measure the bending radius of the particles. The device is fairly simple but its disadvantages are the limited resolution of the MCP and also the energy range of measurable particles depends on the ability to vary the electric field in the ESA. 5

3.2.2. Method 2 A different way of measuring particle fluxes in space is the combination of the following devices. Cherenkov Detector To determine the velocity of a very fast, charged particle one just has to measure the aperture angle θ of the light cone this particle produces because of the Cherenkov effect 6. The velocity then can be calculated by the simple equation cos θ = 1 β n where n is the index of refraction of the detector medium and β = v/c. The measurement of the light cone and its aperture angle can be done e.g. by a photosensitive gas where photons produce free photo electrons which can be detected and located by cathodes. Time of Flight (ToF) Scintillators are quite old instruments to detect charged particles or radiation. They have an excellent time resolution and the measured signal strength is proportional to the energy loss of particles in the scintillator. Using the Bethe-Bloch formula one can also calculate the absolute value and sign of the charge of the incoming particles. Two scintillators in a row can measure the particle s time of flight. The velocity of a particle can then be determined by just knowing the distance between the scintillators. Another important application of the scintillators is to start and stop the trigger signal for all instruments between them. That is why they also act like a counter for the measurements. A last but not unimportant usage of the ToF is to detect particles that enter the experiment from the wrong side, so-called Albedo particles. They can be easily separated because of their negative velocity. ToF measurements are simple and easy to handle and proper for low energies. High energy particles are maybe so fast that the time resolution of the scintillators is not good enough. Magnetic Rigidity When a fast, charged particle propagates through a magnetic field its trajectory will, of course, bend due to the Lorentz force. The magnetic rigidity R describes how far the trajectory will bend. (3) R = γ β m c2 B z e (4) 6 When a charged particle propagates through a transparent medium with a higher velocity than the velocity of light in this medium it will produce a light cone along its trajectory with an aperture angle θ. 6

R is more or less the radius of the particle s curve in the magnetic field, m is the particle s rest mass, q = z e its charge and B the absolute value of the magnetic field 7. It is quite simple to form (4) to an equation for the mass: m = R B z e 1 β 2 β c 2 (5) Out of the Cherenkov detector and time of flight measurement we already know the velocity and exact charge of the particle. Of course, the magnetic field is artificial and we know its absolute value. The only thing we need to know is the bending radius R of the particle s trajectory. To do this we use so-called silicon microstrip detectors. Their basic idea is described in figure 5. p side Al strip p + implant n-substrate ionisation alsong the trajectory n + implant n side charged particle Al layer Figure 5: A silicon microstrip is produced of n- and p-semiconductors. An applied voltage at the top and bottom of the strip will leave neither free electrons nor holes in the strip. But when a charged particle will propagate through this strip it produces many free electrons and holes. These charges will immediately flow to the top or bottom of the strip and they can be measured as a current. The advantage of the silicon microstrip detector is that the propagating particles do not need that high energies to be detected than e.g. in a normal drift chamber you perhaps know from elementary particle physics. Arranging the silicon microstrips in many layers with an alternating alignment in the x- or y-direction you can easily determine the trajectory of the particle through the array. A strong electromagnet around this array will cause a bending of the trajectory and the magnetic rigidity can be measured. The whole device of the electromagnet and the array of silicon microstrip detectors is also called a magnetic spectrometer. 7 We assume that the magnetic field is perpendicular to the particle s trajectory. 7

Overview Figure 6 shows the basic design of the three combined measurement devices. Cherenkov detector upper time of flight detector electromagnet array of silicon microstrip detectors lower time of flight detector Figure 6: The particles will enter the whole device through the Cherenkov detector that measures their velocity. Afterwards the upper ToF device starts the trigger of the magnetic spectrometer that consists of a strong electromagnet and an array of silicon microstrip detectors. When the particles propagate through the lower ToF the trigger of the magnetic spectrometer stops and it is also possible to calculate the velocity of the particles with the ToF. Using (5) we can determine the mass of the particle. The scintillators included in the ToF can measure the sign and absolute value of the particle s charge. In reality such a device is much more complex than this simple description. First of all more measurement methods are included, e.g. an anticoincidence-system, a calorimeter or a transition radiation detector. And also the combination of all the instruments is a complex topic. But the advantage of this method is its wide measurable energy spectrum. But one always has to compare the advantages with the costs of a detector. 4. Summary Although measurement methods of cosmic rays are quite well developed we still do not know much about these particles. But many different experiments on the ground, in the atmosphere and in space were realised or will be realised soon, see appendix A. Maybe we will then get more information on the formation of the universe, the origins of high energy particles with energies of up to 10 20 ev and the composition of other galaxies. 8

5. Appendix A. List of Experiments Name of Experiment Scientific Goals Energy Spectrum Akeno Giant Air Shower Array (AGASA) ground based studies of the origin of extremely high energy cosmic rays up to 10 20 ev Pierre Auger Observatory (AUGER) studies of the universe s highest energy particles up to 10 20 ev Cosmic AntiParticle Ring Imaging Cherenkov Experiment (CAPRICE) balloons measurements of e +, e, antiprotons and the atmospherically myon spectrum 0.5 50 GeV Isotope Magnet Experiment (ISOMAX) measurements of beryllium 10, isotopes with 2 Z 8 0, 2 3 GeV /nuc Russian-Nippon Joint Balloon Experiment (RUNJOB) spectra of Z 26 < 100 TeV /nuc Alpha Magnetic Spectrometer Experiment (AMS) space experiments experiment to search in space for dark matter, missing matter & antimatter on the international space station - Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA) energy spectra of cosmic electrons, positrons, antiprotons and light nuclei 0.05 10 2 GeV Ulysses Ulysses spacecraft s unique orbit over the solar poles gives scientists otherwise unattainable insight into the driving forces behind space weather - 9

B. References References [1] Kolanoski, Hermann: Einführung in die Astroteilchenphysik Institut für Physik, Humboldt-Universität zu Berlin [2] Göbel, Holger: Analyse des CAPRICE97 Flugzeitzählers und Bestimmung des Proton/Myon Verhältnisses für den Impulsbereich von 0.4-1.1 GeV /c in einer Höhe von 1270 m ü. N. N. Universität-Gesamthochschule Siegen, Fachbereich Physik, 1997 [3] Rönnmark, Kjell: Lecture Notes on SPACE PHYSICS - From the Sun to the Aurora Umeå Universitet, Institutionen för fysik, 2003 [4] Schaller, Sven: Leistungsmerkmale der HERA-B Vertexdetektors und Suche nach semileptonischen Charm-Zerfällen Ruprecht-Karls-Universität Heidelberg, Naturwissenschaftliche- Mathematische Gesamtfakultät, 2001 [5] http://www.astroteilchenphysik.de [6] http://www.wikipedia.org 10