Astroparticle physics with highenergy photons II Techniques

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Astroparticle physicswith high-energy
photonsII Techniques Instruments

• Alessandro de Angelis
• Lisboa 2003

http//wwwinfo.cern.ch/deangeli
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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
• Point directly to the source
• Nonthermal above 30 keV
• What are X and gamma rays ? Arbitrary ! (Weekles
  1988)
• X 1 keV-1 MeV
• X/low E g 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)

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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

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The problem - I
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The problem - II
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3) Detection of a high E photon

• Above the UV and below 50 GeV, shielding from
  the atmosphere
• Below the ee- threshold some phase space (10
  MeV), Compton/scintillation
• Above 10 MeV, pair production
• Above 50 GeV, atmospheric showers
• Pair lt-gt Brem

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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 gt a 1m2 detector would
  detect only 1 photon/2h above 10 GeV
• gt with the present space technology, VHE and
  UHE gammas can be detected only from atmospheric
  showers
• Earth-based detectors, atmospheric shower
  satellites
• The flux from high energy cosmic rays is much
  larger

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Satellite-based and atmospheric complementary,
w/ moving boundaries
Atmospheric

• Flux of diffuse extra-galactic photons

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Satellite-based detectorsfigures of merit

• 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

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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

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X detection (direction-sensitive)
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X detection (direction-sensitive)
Unfolding is a nice mathematical problem !
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g satellite-based detectors engineering

• Techniques taken from particle physics
• g direction is mostly determined by ee-
• 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)
• gt large number of thin converters, but the of
  channel increases
• (power consumption ltlt 1 kW)
• If possible, a calorimeter in the bottom to get E
  resolution, but watch the weight (leakage gt
  deteriorated resolution)
• Smart techniques to measure E w/o calorimeters
  (AGILE)

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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

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EGRET

• High Energy g detector
• 20 MeV-10 GeV
• on the CGRO (1991-2000)
• thin tantalium plates with wire chambers
• Scintillators for trigger

• Energy measured by a NaI (Tl) calorimeter 8 X0
  thick
• Effective area 0.15 m2 _at_ 1 GeV
• Angular resolution 1.2 deg _at_ 1 GeV
• Energy resolution 20 _at_ 1 GeV
• Scientific success
• Increased number of identified sources, AGN, GRB,
  sun flares...

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g detectors on satellitecomparison with X-ray
detectors

X-ray Telescope Gamma-ray
(EGRET) Detection technology
CCD, Ge ee- pair creation
tracking Sensitivity
a few micro-Crab ten
milli-Crab Angular resolution
lt 1 arc-second
lt1 degree No. of Sources detected
gtgt106
300
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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

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Earth-based detectorsProperties of Extensive Air
Showers

• We believe we know well the g physics up to EHE
• Predominant interactions e.m.
• ee- 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

gt hadronic showers will look equal to e.m.,
apart from having 20x more muons and being less
regular
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(No Transcript)
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Hadron rejection Small field-of-view makes
protons look like gammas.
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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)

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Cherenkov (C) detectorsCherenkov light from g
showers

• C 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 _at_ 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

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Cherenkov detectorsPrinciples 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
• gt S/B1/2 ? (A/tW)1/2

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C detectorsAnalysis features

• Rejection of cosmic ray background from shape or
  associated muon detectors
• Wavefront timing allows rejection and fitting
  the primary direction as well

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Whipple-10m since 1969 ?100 PMTs 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

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Extensive Air Shower Particle Detector Arrays

• Built to detect UHE gammas
• small flux gt need for large surfaces, 104 m2
• But 100 TeV gt 50,000 electrons 250,000
  photons at mountain altitudes, and sampling is
  possible
• Typical detectors are arrays of 50-1000
  scintillators of 1m2/each (fraction of sensitive
  area lt 1)
• Possibly a m detector for hadron rejection
• Direction from the arrival times, dq can be 1
  deg
• calibrated from the shadow from the Moon
• Thresholds rather large, and dependent on the
  point of first interaction

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EAS Particle Detector ArraysPrinciple

• Each module reports
• Time of hit (10 ns accuracy)
• Number of particles crossing detector module
• Time sequence of hit detectors
• -gt shower direction
• Radial distribution of particles
• -gt distance L
• Total number of particles -gt energy

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EAS Particle Detector ArraysAn example CASA-MIA
(lt 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 gt 90 of the
  events as hadronic background

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EAS Particle Detector ArraysAnother (less
standard) example

• Milagro in New Mexico

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Air fluorescence detectors

• The flux of EHE photons is very low
• 2/(Km2 week sr) gt 1 PeV

• gt need for huge effective volume
• use the atmosphere as converter
• Luckily, excited N2 emits fluorescence photons
  (5 photons/m/electron as for C, but not
  beamed)
• Flys Eye 67 x 1.5 spherical mirrors seen by
  PMs (1981-) A second detector added in 1986
• Superior in shower imaging

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4) The future

• Satellite-based EGRET had a large success
• But disposables (gas for 5 refills) gt 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
• Larger mirrors and higher quantum-efficiency
  detectors
• Improvement in EAS Particle Detector Arrays
• Higher altitude
• Increased sampling
• New concept (EUSO, OWL)

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GLAST
Tracker

• g telescope on satellite for the range 20 MeV-300
  GeV
• hybrid tracker calorimeter
• International collaboration US-France-Italy-Japan-
  Sweden
• Broad experience in high-energy astrophysics and
  particle physics (science instrumentation)
• Timescale 2006-2010 (-gt2015)
• Wide range of physics objectives
• Gamma astrophysics
• Fundamental physics

Calorimeter
A HEP / astrophysics partnership
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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

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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
• 19 tray structures
• 12 with 2.5 Pb on bottom
• 4 with 25 Pb on bottom
• 2 with no converter
• Electronics on the sides of trays
• Minimize gap between towers
• Carbon-fiber walls to provide stiffness

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GLAST performance (compared to EGRET)
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GLAST performance two examples of application

• Cosmic ray production
• Facilitate searches for pulsations from
  millisecond pulsars

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AGILE (the GLAST precursor)
To be launched in 2005 Lifetime of 3 years
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But despite the progress in satellites

• The problem of the flux (1 photon/day/km2 _at_ 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

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Ground-based detectorsImprovements in
atmospheric C

• Improving flux sensitivity
• Detect weaker sources, study larger sky regions
  S/B1/2 ? (A/tW)1/2
• Smaller integration time
• Improve photon collection, improve quantum
  efficiency of PMs
• Use several telescopes
• Lowering the energy threshold
• Close the gap 100 GeV between
• satellite-based ground-based
• instruments
• Use solar plants

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Major projects in atmospheric CAiming at lower
threshold (20 GeV)

• STACEE (past and future)
• US, heliostats in Albuquerque (NM)
• CAT/CELESTE (European, lead by France)
• 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

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Major projects in atmospheric CAiming at
improved flux sensitivity

• CANGAROO (past and future)
• Australia Japan is building new telescopes
• HESS (European, lead by Germany)
• 4 x 110 m2 telescopes in Namibia, gt 2003
• VERITAS (US, Arizona)
• 7 x Whipple-like 100 m2 telescopes in Arizona, gt
  2005

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C detectorsOverview of next detectors
MAGIC(Germany, Italy Spain)Winter 20031
telescope 17 meters Ø
WHIPPLE/ VERITAS(USA England)now/2005?7
telescopes10 meters Ø
Montosa Canyon, Arizona
Roque delos Muchachos, Canary Islands
CANGAROO III(Australia Japan)Spring 20044
telescopes 10 meters Ø
Windhoek, Namibia
HESS(Germany France)Summer 20024 (?16)
telescopes 10 meters Ø
Woomera, Australia
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Ground-based detectorsImprovements in EAS PDAs

• Higher altitude
• Tibet (past and future)
• gt TibetII
• Increased sampling
• Larger density
• Better sensitive elements (scintillators at
  present)
• ARGO in Tibet (Italy/China) full coverage
  detector of dimension 5000 m2

ARGO
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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 C radiation
• Fluorescence detectors as well

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Sky coverage in 2003
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An armada of detectorsat different energy ranges
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some are coming now
MAGIC 2003
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Sensitivity
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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

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The EeV and ZeV energies and EUSO

• EUSO can open a new energy frontier at the ZeV
  scale...

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Summary

• High energy photons (often traveling through
• large distances) are a great probe of physics
  under extreme conditions
• What better than a crash test to break a theory ?
• 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
• Do they exist or is this just a technological
  limit ?
• We are just starting
• Future detectors have observational
  capabilities to give SURPRISES !

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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/bur
  sts.html
• GLAST and g satellite physics, http//glast.gsfc.n
  asa.gov/
• INTEGRAL and CHANDRA homepages
• J. Pauls talk in Moriond 2002