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Astroparticle physics with high-energy photons II – Techniques & Instruments Alessandro de Angelis Lisboa 2003 http://wwwinfo.cern.ch/~deangeli The subject of these lectures… (definition of terms) Detection of high-energy photons from space High-E X/g: probably the most interesting part of the spectrum for astroparticle 2 Point directly to the source Nonthermal above 30 keV What are X and gamma rays ? Arbitrary ! (Weekles 1988) X X/low E g 1 keV-1 MeV 1 MeV-10 Me medium 10-30 MeV HE 30 MeV-30 GeV VHE 30 GeV-30 TeV UHE 30 TeV-30 PeV EHE above 30 PeV No upper limit, apart from low flux (at 30 PeV, we expect ~ 1 g/km2/day) 3 Outline of these lectures 0) Introduction & definition of terms 1) Motivations for the study high-energy photons 2) Historical milestones 3) X/g detection and some of the present & past detectors 4) Future detectors 4 The problem - I 5 The problem - II 3) Detection of a high E photon Above the UV and below “50 GeV”, shielding from the atmosphere Below the e+e- threshold + some phase space (“10 MeV”), Compton/scintillation Above “10 MeV”, pair production Above “50 GeV”, atmospheric showers Pair <-> Brem 6 Consequences on the techniques The earth atmosphere (28 X0 at sea level) is opaque to X/g Thus only a satellite-based detector can detect primary X/g The fluxes of h.e. g are low and decrease rapidly with energy Vela, the strongest g source in the sky, has a flux above 100 MeV of 1.3 10-5 photons/(cm2s), falling with E-1.89 => a 1m2 detector would detect only 1 photon/2h above 10 GeV => with the present space technology, VHE and UHE gammas can be detected only from atmospheric showers 7 Earth-based detectors, atmospheric shower satellites The flux from high energy cosmic rays is much larger Satellite-based and atmospheric: complementary, w/ moving boundaries Atmospheric Flux of diffuse extra-galactic photons Sat 8 Satellite-based detectors: figures of merit 9 Effective area, or equivalent area for the detection of g Aeff(E) = A x eff. Angular resolution is important for identifying the g sources and for reducing the diffuse background Energy resolution Time resolution 10 X detectors The electrons ejected or created by the incident gamma rays lose energy mainly in ionizing the surrounding atoms; secondary electrons may in turn ionize the material, producing an amplification effect Most space X- ray telescopes consist of detection materials which take advantage of ionization process but the way to measure the total ionization loss differ with the nature of the material Commonly used detection devices are... gas detectors scintillation counters semiconductor detectors 11 X detection (direction-sensitive) X detection (direction-sensitive) 12 Unfolding is a nice mathematical problem ! g satellite-based detectors: engineering Techniques taken from particle physics g direction is mostly determined by e+e- 13 conversion Veto against charged particles by an ACD Angular resolution given by Opening angle of the pair m/E ln(E/m) Multiple scattering (20/pb) (L/X0)1/2 (dominant) => large number of thin converters, but the # of channel increases (power consumption << 1 kW) If possible, a calorimeter in the bottom to get E resolution, but watch the weight (leakage => deteriorated resolution) Smart techniques to measure E w/o calorimeters (AGILE) 14 Satellite-based detectors in the ‘70s Two satellites in the ‘70s : SAS-2 in 1972, COS-B in 1975 SAS-2 (Derdeyn et al. 1972) Prototype COS-B (Bignami et al. 1975) thin W plates with wire chambers range 50 MeV - 2 GeV Scintillators for trigger Energy measured by a CsI calorimeter 4.7 X0 thick Effective area ~ 0.05 m2 Angular resolution ~ 3 deg Energy resolution ~50% 15 EGRET High Energy g detector 20 MeV-10 GeV on the CGRO (19912000) thin tantalium plates with wire chambers Scintillators for trigger Energy measured by a NaI (Tl) calorimeter 8 X0 thick Effective area ~ 0.15 m2 @ 1 GeV Angular resolution ~ 1.2 deg @ 1 GeV Energy resolution ~20% @ 1 GeV Scientific success Increased number of identified sources, AGN, GRB, sun flares... 16 g detectors on satellite: comparison with X-ray detectors Detection technology Sensitivity Angular resolution No. of Sources detected X-ray Telescope Gamma-ray (EGRET) CCD, Ge e+e- pair creation tracking a few micro-Crab ~ ten milli-Crab < 1 arc-second <1 degree >>106 ~300 17 INTEGRAL/CHANDRA INTEGRAL, the International Gamma-Ray Astrophysics Laboratory is an ESA medium-size (M2) science mission Energy range 15 keV to 10 MeV plus simultaneous X-ray (3-35 keV) and optical (550 nm) monitoring Fine spectroscopy (DE/E ~ 1%) and fine imaging (angular resolution of 5') Two main -ray instruments: SPI (spectroscopy) and IBIS (imager) Chandra, from NASA, has a similar performance Earth-based detectors Properties of Extensive Air Showers We believe we know well the g physics up to EHE… Predominant interactions e.m. e+e- pair production dominates electrons loose energy via brem Rossi approximation B is valid Maximum at z/X0 ln(E/e0); e0 is the critical energy ~80 MeV in air; X0 ~ 300 m at stp Cascades ~ a few km thick Lateral width dominated by Compton scattering ~ Moliere radius (~80m for air at STP) Note: lhad ~ 400 m for air => hadronic showers will look ~ equal to e.m., apart from having 20x more muons and being less regular 18 Hadron rejection : Small field-of-view makes protons look like gammas. 21 Earth-based detectors An Extensive Air Shower can be detected From the shower particles directly (EAS Particle Detector Arrays) By the Cherenkov light emitted by the charged particles in the shower (Cherenkov detectors) Cherenkov (Č) detectors Cherenkov light from g showers Č light is produced by particles faster than light in air Limiting angle cos qc ~ 1/n qc ~ 1º at sea level, 1.3º at 8 Km asl Threshold @ sea level : 21 MeV for e, 44 GeV for m Maximum of a 1 TeV g shower ~ 8 Km asl 200 photons/m2 in the visible Duration ~ 2 ns Angular spread ~ 0.5º 22 Cherenkov detectors Principles of operation Cherenkov light is detected by means of mirrors which concentrate the photons into fast optical detectors Often heliostats operated during night Problem: night sky background On a moonless night ~ 0.1 photons/(m2 ns deg) Signal A fluctuations ~ (AtW)1/2 => S/B1/2 (A/tW)1/2 23 Č detectors Analysis features Rejection of cosmic ray background: from shape or associated muon detectors Wavefront timing: allows rejection and fitting the primary direction as well 24 25 Whipple-10m since 1969 100 PMT’s by 1990 HEGRA 1994-2002 5 telescopes / stereoscopy La-Palma Canaries CANGAROO since 1994 Australia STACEE Since 2000 Albuquerque CAT Thémis (French Pyrénées) • first light summer 1996, • fine camera : 600 pixels Extensive Air Shower Particle Detector Arrays Built to detect UHE gammas small flux => need for large surfaces, ~ 104 m2 Typical detectors are arrays of 50-1000 scintillators of ~1m2/each (fraction of sensitive area < 1%) Possibly a m detector for hadron rejection Direction from the arrival times, dq can be ~ 1 deg But: 100 TeV => 50,000 electrons & 250,000 photons at mountain altitudes, and sampling is possible calibrated from the shadow from the Moon Thresholds rather large, and dependent on the point of first interaction 26 EAS Particle Detector Arrays Principle Each module reports: Time of hit (10 ns accuracy) Number of particles crossing detector module Time sequence of hit detectors -> shower direction Radial distribution of particles -> distance L Total number of particles -> energy 27 EAS Particle Detector Arrays An example: CASA-MIA (< 1996) CASA: 0.25 km2 air array which detects the em showers produced by gamma rays and cosmic rays at 100 TeV and above; 1089 stations A second array, the Michigan Anti Mu (MIA), is made of 2500 square meters of buried counters in 16 patches. MIA measures the muon content of the showers, which allows to reject > 90% of the events as hadronic background 28 EAS Particle Detector Arrays Another (less standard) example Milagro in New Mexico 29 Air fluorescence detectors The flux of EHE photons is very low ~2/(Km2 week sr) > 1 PeV => need for huge effective volume use the atmosphere as converter Luckily, excited N2 emits fluorescence photons (~5 photons/m/electron ~ as for Č, but not beamed) Fly’s Eye : 67 x 1.5 spherical mirrors seen by PMs (1981-) A second detector added in 1986 Superior in shower imaging 30 31 4) The future Satellite-based: EGRET had a large success But: disposables (gas for 5 refills) => Room for improvement Higher sensitivity would be very useful... Very near future: Improvement in air Cherenkov telescopes Flux sensitivity Better angular & time resolutions Lower energy thresholds Improvement in EAS Particle Detector Arrays Larger mirrors and higher quantum-efficiency detectors Higher altitude Increased sampling New concept (EUSO, OWL) GLAST g telescope on satellite for the range 20 MeV-300 GeV hybrid tracker + calorimeter International collaboration USFrance-Italy-Japan-Sweden Tracker 32 Broad experience in high-energy astrophysics and particle physics (science + instrumentation) Timescale: 2006-2010 (->2015) Wide range of physics objectives: Gamma astrophysics Fundamental physics A HEP / astrophysics partnership Calorimeter 33 GLAST: the instrument Tracker Si strips + converter Calorimeter CsI with diode readout (a classic for HEP) 1.7 x 1.7 m2 x 0.8 m height/width = 0.4 large field of view 16 towers modularity 34 GLAST: the tracker Si strips + converter High signal/noise Rad-hard Low power 4x4 towers, of 37 cm 37 cm of Si 18 x,y planes per tower 12 with 2.5% Pb on bottom 4 with 25% Pb on bottom 2 with no converter Electronics on the sides of trays 19 “tray” structures Minimize gap between towers Carbon-fiber walls to provide stiffness GLAST performance (compared to EGRET) 35 36 GLAST performance two examples of application Cosmic ray production Geminga Radio-Quiet Pulsars Facilitate searches for pulsations from millisecond pulsars Geminga Crab PKS 0528 +134 37 AGILE (the GLAST precursor) To be launched in 2005 Lifetime of 3 years 38 But despite the progress in satellites… The problem of the flux (~1 photon/day/km2 @ ~30 PeV) cannot be overcomed Photon concentrators work only at low energy The key for VHE gamma astronomy and above is in earth-based detectors Also for dark matter detection… Ground-based detectors Improvements in atmospheric Č Improving flux sensitivity Detect weaker sources, study larger sky regions S/B1/2 (A/tW)1/2 Lowering the energy threshold Smaller integration time Improve photon collection, improve quantum efficiency of PMs Use several telescopes Close the gap ~ 100 GeV between satellite-based & ground-based instruments Use solar plants 39 Major projects in atmospheric Č Aiming at lower threshold (~20 GeV) STACEE (past and future…) CAT/CELESTE (European, lead by France) US, heliostats in Albuquerque (NM) Solar plant in Pyrenees MAGIC (European, lead by Germany) large parabolic dish (17m), automatic alignment control, technique at the state of the art Canary Islands, 2003 40 Major projects in atmospheric Č Aiming at improved flux sensitivity CANGAROO (past and future…) HESS (European, lead by Germany) Australia; Japan is building new telescopes 4 x 110 m2 telescopes in Namibia, > 2003 VERITAS (US, Arizona) 7 x Whipple-like 100 m2 telescopes in Arizona, > 2005 41 42 Č detectors Overview of next detectors MAGIC WHIPPLE/ VERITAS (USA & England) now/2005? 7 telescopes 10 meters Ø (Germany, Italy & Spain) Winter 2003 1 telescope 17 meters Ø Montosa Canyon, Arizona Roque de los Muchachos, Canary Islands Windhoek, Namibia HESS (Germany & France) Summer 2002 4 (16) telescopes 10 meters Ø CANGAROO III (Australia & Japan) Spring 2004 4 telescopes 10 meters Ø Woomera, Australia Ground-based detectors Improvements in EAS PDAs Higher altitude Tibet (past and future) => TibetII Increased sampling Larger density Better sensitive elements (scintillators at present) ARGO ARGO in Tibet (Italy/China): full coverage detector of dimension ~5000 m2 43 44 But also the generic CR detectors... Auger Southern Observatory in Argentina When completed, world's largest cosmic ray observatory with 1600 detectors spread over 3000 km2 - A complementary observatory is planned for the northern hemisphere The detectors are water tanks equipped with PMs, which detect Č radiation Fluorescence detectors as well 45 Sky coverage in 2003 An armada of detectors at different energy ranges 46 47 …some are coming now MAGIC 2003 48 Sensitivity 49 A new concept: EUSO (and OWL) The Earth atmosphere is the ideal detector for the Extreme Energy Cosmic Rays and the companion Cosmic Neutrinos. The new idea of EUSO (2009-) is to watch the fluorescence produced by them from the top 50 The EeV and ZeV energies and EUSO EUSO can open a new energy frontier at the ZeV scale... 51 Summary High energy photons (often traveling through large distances) are a great probe of physics under extreme conditions Observation of X/g rays gives an exciting view of the HE universe Many sources, often unknown Diffuse emission Gamma Ray Bursts No clear sources above ~ 30 TeV What better than a crash test to break a theory ? Do they exist or is this just a technological limit ? We are just starting… Future detectors: have observational capabilities to give SURPRISES ! 52 Bibliography C.M. Hoffman et al., Rev. Mod. Physics 71 (1999) 4 http://imagine.gsfc.nasa.gov/docs/science/know_l1/history_gamma.html http://imagine.gsfc.nasa.gov/docs/introduction/bursts.html GLAST and g satellite physics, http://glast.gsfc.nasa.gov/ INTEGRAL and CHANDRA homepages J. Paul’s talk in Moriond 2002