Transcript Gamma-Ray Astroparticle Physics

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)
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Detection of high-energy photons from space
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High-E X/g: probably the most interesting part of the spectrum for
astroparticle
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Point directly to the source
Nonthermal above 30 keV
What are X and gamma rays ? Arbitrary ! (Weekles 1988)
X
X/low E g
medium
HE
VHE
UHE
EHE
1 keV-1 MeV
1 MeV-10 Me
10-30 MeV
30 MeV-30 GeV
30 GeV-30 TeV
30 TeV-30 PeV
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
3) Detection of a
high E photon
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Above the UV and below
“50 GeV”, shielding from
the atmosphere
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Below the e+e- threshold +
some phase space (“10 MeV”),
Compton/scintillation
Above “10 MeV”, pair
production
Above “50 GeV”,
atmospheric showers
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Pair <-> Brem
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Consequences on
the techniques
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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
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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
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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
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Flux of diffuse
extra-galactic
photons
Sat
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Satellite-based detectors:
figures of merit
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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
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Energy resolution
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Time resolution
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X detectors
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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...
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gas detectors
scintillation counters
semiconductor detectors
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X detection (direction-sensitive)
X detection
(direction-sensitive)
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Unfolding is a nice mathematical problem !
g satellite-based detectors:
engineering
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Techniques taken from particle physics
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g direction is mostly determined by e+e-
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conversion
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Veto against charged particles by an ACD
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Angular resolution given by
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Opening angle of the pair m/E ln(E/m)
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Multiple scattering (20/pb) (L/X0)1/2 (dominant)
=> large number of thin converters, but the # of channel increases
(power consumption << 1 kW)
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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)
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Satellite-based detectors in the ‘70s
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Two satellites in the ‘70s :
SAS-2 in 1972, COS-B in 1975
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SAS-2 (Derdeyn et al. 1972)
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Prototype
COS-B (Bignami et al. 1975)
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thin W plates with wire chambers
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range 50 MeV - 2 GeV
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Scintillators for trigger
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Energy measured by a CsI
calorimeter 4.7 X0 thick
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Effective area ~ 0.05 m2
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Angular resolution ~ 3 deg
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Energy resolution ~50%
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EGRET
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High Energy g
detector
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20 MeV-10 GeV
on the CGRO (19912000)
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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
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Increased number of identified sources, AGN, GRB, sun flares...
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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
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INTEGRAL/CHANDRA
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INTEGRAL, the International Gamma-Ray
Astrophysics Laboratory is an ESA
medium-size (M2) science mission
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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
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We believe we know well the g
physics up to EHE…
Predominant interactions e.m.
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e+e- pair production dominates
electrons loose energy via brem
Rossi approximation B is valid
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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
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Hadron rejection :
Small field-of-view makes
protons look like gammas.
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Earth-based detectors
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An Extensive Air Shower can be detected
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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
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Č light is produced by particles faster than light in air
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Limiting angle cos qc ~ 1/n
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qc ~ 1º at sea level, 1.3º at 8 Km asl
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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
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Cherenkov detectors
Principles of operation
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Cherenkov light is detected
by means of mirrors which
concentrate the photons into
fast optical detectors
Often heliostats operated
during night
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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
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Č detectors
Analysis features
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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 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
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Built to detect UHE gammas
small flux => need for large surfaces, ~ 104 m2
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Typical detectors are arrays of 50-1000 scintillators of
~1m2/each (fraction of sensitive area < 1%)
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Possibly a m detector for hadron rejection
Direction from the arrival times, dq can be ~ 1 deg
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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
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EAS Particle Detector Arrays
Principle
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Each module reports:
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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
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EAS Particle Detector Arrays
An example: CASA-MIA (< 1996)
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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
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EAS Particle Detector Arrays
Another (less standard) example
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Milagro in New Mexico
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Air fluorescence
detectors
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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
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4) The future
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Satellite-based: EGRET had a large success
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But: disposables (gas for 5 refills) => Room for improvement
Higher sensitivity would be very useful...
Very near future: Improvement in air Cherenkov telescopes
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Flux sensitivity
Better angular & time resolutions
Lower energy thresholds
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Improvement in EAS Particle Detector Arrays
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Larger mirrors and higher quantum-efficiency detectors
Higher altitude
Increased sampling
New concept (EUSO, OWL)
GLAST
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g telescope on satellite for the
range 20 MeV-300 GeV
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hybrid tracker + calorimeter
International collaboration USFrance-Italy-Japan-Sweden
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Tracker
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Broad experience in high-energy
astrophysics and particle physics
(science + instrumentation)
Timescale: 2006-2010 (->2015)
Wide range of physics
objectives:
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Gamma astrophysics
Fundamental physics
A HEP / astrophysics partnership
Calorimeter
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GLAST: the instrument
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Tracker
Si strips + converter
Calorimeter
CsI with diode readout
(a classic for HEP)
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1.7 x 1.7 m2 x 0.8 m
height/width = 0.4 
large field of view
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16 towers 
modularity
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GLAST: the tracker
Si strips + converter
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High signal/noise
Rad-hard
Low power
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4x4 towers, of 37 cm  37 cm of Si
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18 x,y planes per tower
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12 with 2.5% Pb on bottom
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4 with 25% Pb on bottom
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2 with no converter
Electronics on the sides of trays
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19 “tray” structures
Minimize gap between towers
Carbon-fiber walls to provide stiffness
GLAST performance
(compared to EGRET)
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GLAST performance
two examples of application
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Cosmic ray production
Geminga Radio-Quiet Pulsars
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Facilitate searches for
pulsations from
millisecond pulsars
Geminga
Crab
PKS 0528
+134
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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…
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The problem of the flux (~1
photon/day/km2 @ ~30 PeV)
cannot be overcomed
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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
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Improving flux sensitivity
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Detect weaker sources, study larger sky regions S/B1/2  (A/tW)1/2
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Lowering the energy threshold
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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
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Major projects in atmospheric Č
Aiming at lower threshold (~20 GeV)
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STACEE (past and future…)
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CAT/CELESTE (European, lead by
France)
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US, heliostats in Albuquerque (NM)
Solar plant in Pyrenees
MAGIC (European, lead by
Germany)
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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 Č
Aiming at improved flux sensitivity
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CANGAROO (past and
future…)
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HESS (European, lead by
Germany)
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Australia; Japan is building
new telescopes
4 x 110 m2 telescopes in
Namibia, > 2003
VERITAS (US, Arizona)
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7 x Whipple-like 100 m2
telescopes in Arizona, >
2005
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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
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Higher altitude
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Tibet (past and future)
=> TibetII
Increased sampling
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Larger density
Better sensitive elements
(scintillators at present)
ARGO
ARGO in Tibet (Italy/China):
full coverage detector of
dimension ~5000 m2
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But also the generic CR detectors...
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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
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Fluorescence detectors as well
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Sky coverage in 2003
An armada of detectors
at 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)
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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
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EUSO can open a new energy frontier at the ZeV scale...
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Summary
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High energy photons (often traveling through
large distances) are a great probe of physics under extreme conditions
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Observation of X/g rays gives an exciting view of the HE universe
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Many sources, often unknown
Diffuse emission
Gamma Ray Bursts
No clear sources above ~ 30 TeV
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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 !
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Bibliography
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C.M. Hoffman et al., Rev. Mod. Physics 71 (1999) 4
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http://imagine.gsfc.nasa.gov/docs/science/know_l1/history_gamma.html
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http://imagine.gsfc.nasa.gov/docs/introduction/bursts.html
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GLAST and g satellite physics, http://glast.gsfc.nasa.gov/
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INTEGRAL and CHANDRA homepages
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J. Paul’s talk in Moriond 2002