Chapter 6.2: space based cosmic ray experiments 1
A bit of history... space based experiments 1912-1950: first observations of the cosmic ray flux with detectors onboard balloons and air-planes. 1950s/60s: observations using detectors onboard spacecraft and rockets. 1970s: observations using spacecraft and satellites. 1992: launch of SAMPEX (Solar Anomalous and Magnetospheric Particle Explorer), a satellite for flux and composition studies. launch of IMAX (Isotope Matter Antimatter Experiment), a balloon-borne superconducting magnet spectrometer. It measured galactic cosmic ray abundances of protons, anti-protons, hydrogen and helium isotopes. 1990s: JACEE balloon-borne lead emulsion chambers measure composition up to 1000 TeV BESS (balloon borne experiment with superconducting spectrometer) other balloon-borne experiments include: MASS, Runjob, TRACER... 1997: launch of ACE (Advanced Composition Explorer). 1998: test-flight of the Alpha-Magnetic Spectrometer (AMS 01) onboard the space shuttle Discovery. 2003: launch of TIGER (Trans-Iron Galactic Element Recorder), balloon borne. 2008: After major upgrades, AMS 02 is waiting to be deployed on board the International Space Station. 2
Space-based Experiments: AMS The Alpha Magnetic Spectrometer (AMS) is designed to provide high accuracy measurements of cosmic ray energies and composition up to TeV energies. AMS01 was a prototype detector that was tested on a flight on the Space Shuttle Discovery in 1998. The upgraded detector AMS02 will be deployed on the International Space Station (ISS) for a 3 year long mission. Its goals are: accurate measurement of the cosmic ray composition (including separation of isotopes) search for dark matter antimatter search 3
A Model of AMS 4
(c) 1998-2002 by the AMS Collaboration 5
Instrumentation Very compact design combines several detectors for accurate energy measurement and particle identification. The main components are: Transition Radiation Detector (TRD) Time Of Flight hodoscopes (TOF) Silicon Tracker Ring Imaging Cherenkov Counter (RICH) 3-D Sampling Calorimeter (ECAL) Two star trackers allow precise reconstruction of the origin of high energy γ-rays. A superconducting magnet surrounds the Tracker and is cooled by a cryogenic vessel filled with superfluid Helium. Magnetic flux density at the center: 0.86 T. Anticoincidence Counters ensure that only particles that pass through the ring of the magnet will be accepted for further analysis. 6
Transition Radiation Detectors (TRD) The TRD consists of 20 layers of TRD modules (in total 328 modules). It is used to distinguish between particles of different masses and to identify e- and e+ against hadrons. Each module consists of: a thin radiator, a 23 mm thick fleece of polypropylene/ polyethylene. Photons are generated by traversing particles. (Reminder: Photon flux from TRD is proportional to the particle's energy!) a detector made of 16 straws, i.e. wire-chambers that are filled with a Xe/CO2 gas mixture. The X-ray photons from the radiator are detected by photo-ionisation. Since the particle momentum is determined in the Silicon Tracker, the measurement of its energy in the TRD permits a determination of its mass. 7
Time Of Flight Hodoscopes (TOF) hodoscope - (physics) scientific instrument that traces the path of a charged particle Four layers of TOFs provide precision measurements on time of flight (resolution of ~ 120 picoseconds) and de/dx. Each layer consists of several scintillator strips, with PMTs at their ends. When the particle traverses the scintillator, its energy loss de/dx can be measured, which is proportional to its charge squared. The TOF is thus used to determine the particle's charge. The velocity of the particle can be measured by comparing the trigger times of the upper and lower TOF. This also provides a separation of downward- and upward-going particles. 8
Silicon Tracker The Silicon Tracker consists of ~2500 silicon micro-strip detectors arranged in 8 layers. It measures the track of the particle in the magnetic field and provides information on its rigidity (=momentum/charge) and charge : The track of the particle is determined by high resolution position measurements in the 8 planes. One can thus 2 determine its rigidity. mv mv qvb= r q = Br The track measurement provides also the sign of the particle's charge. The silicon strip detectors allow also a measurement of de/dx, and thus of the charge of the particle. The silicon tracker of AMS01 (design very similar to AMS02!) With known charge and rigidity, the momentum of the particle can be derived. 9
Computer model of the RICH detector (c) 1998-2002 by the AMS Collaboration 10
Ring Imaging Cherenkov Detector (RICH) The RICH measures the velocity and charge of the particle with high precision. The particle traverses a radiator made of silica Aerogels. A cone of Cherenkov light forms in its direction of motion. The Cherenkov photons are detected by an arrray of 680 PMTs. A reflector surrounding the RICH chamber collects photons that do not hit the PMT array directly. The Cherenkov cone is reconstructed from the spatial coordinates of the triggered PMTs and the known direction of the incoming particle. The velocity of the particle is determined from the opening angle of the cone. The charge can be determined from the total light flux. Response to Z=3 (left) and Z=16 (right) The velocity measurement together with the momentum measurement from the silicon tracker is used to calculate the particle's mass with high accuracy. 11
3-D Sampling Calorimeter (ECAL) The 3-D Sampling Calorimeter is made out of 16.7 radiation lengths (X0) of lead and scintillating fibers. It measures the energy of γ-rays, e+, e- and distinguishes them from hadrons. ECAL is made of 9 "super-layers", each of which consists of layers of lead foils and scintillating fibers, alternatively parallel to the x- and the y-axis. The fibers are connected to PMTs (36 per super-layer). An incoming particle or γ-ray will induce an electromagnetic or hadronic cascade when traversing the lead foils. The longitudinal and lateral shower development is measured with use of the scintillating fibers. The threedimensional image of the cascade can be compared to simulations. One can then estimate the total energy of the primary particle. An energy resolution of <6 % is attained for e+ and e 12
Detection of γ-rays with ECAL γ-rays can be detected either directly or by their pair production products. the electromagnetic (and hadronic) showers generated in ECAL are compared with simulations to derive the characteristics of the particle that caused them. figure taken from Loïc Girard 13
Some Scientific Results from AMS01 14