Astroparticle physics
with high-energy photons
II – Techniques & Instruments
Alessandro de Angelis
Lisboa 2006
http://www.fisica.uniud.it/~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 for observations
3) X/g detection and some of the present & past detectors
4) Future detectors
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
4
5
The problem - I
The problem – II : the opacity of the
atmosphere
6
Consequences on
the techniques


The earth atmosphere (28 X0 at sea
level) is opaque to X/g => only satbased detectors 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


Earth-based detectors, atmospheric shower satellites
The flux from high energy cosmic rays is much larger
7
Satellite-based and atmospheric:
complementary, w/ moving boundaries
Atmospheric

Flux of diffuse
extra-galactic
photons
Sat
8
Ground-based vs Satellite

Satellite:


primary detection
small effective area ~1m2





lower sensitivity
large duty-cycle
large cost
lower energy
low bkg

Ground based


secondary detection
huge effective area ~104 m2





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Higher sensitivity
small angular opening
small duty-cycle
low cost
high energy
high bkg
9
Satellite-based detectors:
figures of merit


Effective area, or equivalent area for the detection of g
Aeff(E) = A x eff.
Angular resolution, important for identifying the g sources and for
reducing the diffuse background

Energy resolution

Time resolution
10
11
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
12
X detection (direction-sensitive)
X detection
(direction-sensitive)
13
Unfolding is a nice mathematical problem !
14
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
15
SWIFT (launched Dec 2004)
Instruments

Burst Alert Telescope (BAT)



X-Ray Telescope (XRT)



New CdZnTe detectors
Most sensitive imager <~ 150 keV
ever
Arcsecond GRB positions
CCD spectroscopy
UV/Optical Telescope (UVOT)




Sub-arcsec positions
Grism spectroscopy
24th mag sensitivity (1000 sec)
Finding chart for other observers
UVOT
BAT
BAT
XRT
UVOT
XRT
Spacecraft
g satellite-based detectors:
engineering

Techniques taken from particle physics

g direction is mostly determined by e+e-
16
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)
17
g detectors on satellite:
comparison with X-ray detectors
X-ray Telescope
Detection technology
Sensitivity
Angular resolution
No. of Sources detected
Gamma-ray
CCD, Ge
e+e- pair creation tracking
a few micro-Crab
~ ten milli-Crab
< 1 arc-second
<1 degree
>>106
~300
18
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...
19
Near future: the GLAST
observatory and the LAT
Gamma Ray Burst Monitor (GBM)
Spacecraft
Rocket
Launch base
Launch date
Orbit
Delta II
Kennedy Space Center
August 2007
575 km (T ~ 95 min)
Large Area Telescope (LAT)
Heart of the instrument is the LAT,
detecting camma conversions
g
ACD
TRACKER
LAT Mass
Power
3000 Kg
650 W
International collaboration USA-Italy-France-Japan-Sweden
e+ e–
CAL
20
LAT overview
Si-strip Tracker (TKR)
18 planes XY ~ 1.7 x 1.7 m2 w/ converter
Single-sided Si strips 228 mm pitch, ~106
channels
Measurement of the gamma direction
AntiCoincidence Detector (ACD)
89 scintillator tiles around the TKR
Reduction of the background from charged
particles
g
Astroparticle groups
INFN/University Bari,
Padova, Perugia, Pisa,
Roma2, Udine/Trieste
The Silicon tracker is mainly built
in Italy
Italy is also responsible for the
detector simulation, event display
and GRB physics
e+
e-
Calorimeter (CAL)
Array of 1536 CsI(Tl) crystals in 8 layers
Measurement of the electron energy
21
The GLAST LAT outperforms EGRET by two orders of
magnitude
22
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
23
AGILE: the GLAST precursor
L’uso della tecnologia del silicio per rivelare i gamma nasce
con un rivelatore spaziale tutto italiano, AGILE
Prima piccola missione scientifica dell’ASI
Lancio previsto per la fine 2006 (vettore indiano)
Anticoincidenza di scintillatore
plastico
• 12 piani di tracciatore di silicio,
10 con convertitore di tungsteno
(0.07 X0)
IASF-INAF Milano
• Mini-Calorimetro (15+15 barre di
CsI ~1.5 X0)
IASF-INAF Roma
• Rivelatore X (15-40 keV) a
maschera codificata
IASF-INAF Milano
INFN Roma2
IASF-INAF Bologna
Gruppi INFN di Trieste, Roma1, Roma2 e istituti IASF-INAF di Milano, Roma, Bologna
24
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…
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
25
26
Ground detectors: EAS vs. IACT
•
EAS (Extensive Air
Shower): detection of
the charged particles in
the shower
•
Cherenkov detectors:
(IACT): detection of the
Cherenkov light from
charged particles in the
atmospheric showers
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
27
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
28
29
Milagro
Tibet III
First Generation EAS Arrays
30
The Tibet Air Shower Array



4300m asl
Scintillator array
497 detectors





0.5m2 each
5mm lead on each
5.3x104 m2 (phys. area)
680 Hz trigger rate
0.9o resolution
31
Milagro







2600m asl
Water Cherenkov Detector
898 detectors

450(t)/273(b) in pond

175 water tanks
3.4x104 m2 (phys. area)
1700 Hz trigger rate
0.5o resolution
90% proton rejection
10 m
32
Gammas
Protons
Background Rejection in Milagro
Retain 50% g and 9% protons
Not angular resolution – inherent rejection
Gamma MC
Data
Proton MC
C
NBottom(  2 Pes )
PEMax ( Bottom)
33
Sky Surveys
Milagro sky map
34
Near future Instruments: ARGO-YBJ
35
ARGO-YBJ layout
time resolution ~1 ns
space resolution = strip
74 m
99 m
Detector layout
10 Pads
8 Strips
1 CLUSTER = 12 RPC (56 x 62 cm2)
(6.7 x 62 cm2)
(43 m2)
for each RPC for each Pad
78 m
111 m
Layer (92% active surface) of
Resistive Plate Chambers (RPC),
4300 m asl – high altitude
5800 m2 fully instrumented area
10,000 m2 total area
dense sampling of shower
36
ARGO: Progress & Schedule
1800 m2 now acquiring data
 Physics runs since 2005
all 154 clusters complete in 2006
With g/hadron discrimination algorithms
Without any g/hadron discrimination
37
Future:HAWC


Build pond at extreme altitude (Tibet 4300m or
Chile 5200m)
Incorporate new design

Optical isolation between PMTs

Larger PMT spacing

Deeper PMT depth (in top layer)
e
m
g
4 meters
300 meters
~$20M for complete detector
~60x sensitivity of Milagro – instantaneous sensitivity of Whipple over 2 sr
~0.02 Crab/year
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 ~ a few ns
Angular spread ~ 0.5º
38
39
Cherenkov detectors: principles of operation

Cherenkov light detected using
mirrors which concentrate the
photons into fast PMs

In the beginning, heliostats Problem:

Signal  A
fluctuations ~ (AtW)1/2
=> S/B1/2  (A/tW)1/2
Now, a 3rd generation
night sky background





Smaller integration times ~ns
Improved PMs
Large areas => Low threshold
Multi-telescope
Close the gap ~ 100 GeV between
satellite-based & ground-based
instruments
40
The “Big Four”
MAGIC
(Germany, Italy & Spain)
1: Autumn ‘03
236 m2
(2)
VERITAS
(USA,UK1…)
1: Oct ’03
4: 2005
100 m2
(7)
Montosa
Canyon,
Arizona
Roque de
los Muchachos,
Canary Islands
HESS
Windhoek,
Namibia
(Germany & France)
2: Sum ’02
4: Early ’04
100 m2
(16)
CANGAROO III
(Japan & Australia)
2: Dec. ’02, 3: Early ’04
57 m2 (4)
Woomera,
Australia
41
3rd Generation
Telescopes
Overlap of ‘Big4’
allows for
~continuous
Observations for
mid-latitude sources
[ space for another
two installations …
]
Negotiations towards MoU to balance between competition
and cooperation
In 2004:
#
CANGAROO
4x
57
HESS
4x
MAGIC
1x
(2007)
2x
VERITAS
(2007)
2x
4x
area m2
Camera
pixels
FOV
deg
arrangement
427
4
~100m
107
960
5
~120m
240
577
3.5
~80m
110
499
3.5
~80m
42
43
The MAGIC site
La Palma, IAC
28° North, 18° West
Telescopio Nazionale Galileo
Grantecan
MAGIC
MAGIC and its Control House
MAGIC
Fast alert for GRB observation
MAGIC
• Mirror: 17 m diameter
• 240 m2 Al panels + heating
• 85%-90% reflectivity
• Frame deformation
 Active Mirror Control
•
•
•
•
Camera: 3.5° FOV
577 pixels
Optical fibre readout
2 level trigger &
300 Mhz FADC system
• Light carbon
fiber tubes
• Telescope:
65 tons
• Positioning:22s
44
After upgrade of the45
optics in July 2004
the telescope is in its
final shape
Cycle1 DAQ finished
(Feb2005-Apr2006)
~300Hz shower rates
Eth ~40GeV
46
the Active Mirror Control laser beams
Photo by R. Wagner
Photograph of the 576-pixel imaging camera of MAGIC-I.
In the central part one can see the 396 high resolution pixels
of 0.1° size. Those are surrounded by 180 pixels of 0.2°.
47
Gamma / hadron separation
h (m)
Gamma shower
45 GeV gamma
( narrow, points to source )
Proton shower
( wide, points anywhere )
alpha
m
m
48
100 GeV proton
49
Timing information
hadron
gamma
candidate
muon
Three events of
different type showing
the energy (left) and
arrival times (right)
50
51
Size2energy conversion
E in
GeV
1000
100
10
2
3
4
5
log(size)
size and energy are statistically correlated, as shown above (derived from
Monte Carlo data). The relation is sensitive to the point spread function.
Ground-based detectors
Improvements in atmospheric Č

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
52
Major projects in atmospheric Č
Aiming at improved flux sensitivity

CANGAROO (past and future…)


Australia; Japan is building new telescopes
VERITAS (US, Arizona)

4 x Whipple-like 100 m2 telescopes in Arizona, > 2007?
53
54
HESS II
55
MAGIC-II (a clone)
MAGIC-I
MAGIC-II
56
IACTs vs. EAS
High Sensitivity
HESS, MAGIC, CANGAROO, VERITAS
Large Aperture/High Duty Cycle
Milagro, Tibet, ARGO, HAWC?
Large Effective Area
Excellent Background Rejection (>99%)
Small Aperture
Moderate Area/Large Area (HAWC)
Good Background Rejection
Large Aperture
High Resolution Energy Spectra
Studies of known sources
Surveys of limited regions of sky
Unbiased Sky Survey
Extended sources
Transients (GRB’s)
Solar physics/space weather
57
The future is very near (<2008)


Satellite-based: GLAST
Improvement in air Cherenkov telescopes (2nd generation of
the big ones)



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
58
Sensitivity
59
Long-term vision in 1991
120 x 3m-5mφ telescopes system
Hamamatsu Flatpanel
1994
2004
60
Long-term vision by MAGIC 2005?
61
Long-term vision by HESS 2005?
62
Long-term vision by Milagro 2005?
63
EUSO

The Earth
atmosphere is the
ideal detector for
the Extreme Energy
Cosmic Rays and the
companion Cosmic
Neutrinos. The new
idea of EUSO is to
watch the
fluorescence
produced by them
from the top
Explore new technologies before brute64
force? Fresnel optics at ground?
>30GeV γ
1016-1020eV CR,γ
Air fluorescence
& Cherenkov
R&D Proposal by
Kifune, Shimizu & Teshima
In 2001
A new fact: the cooperation towards a
large telescope array (CTA) has started

HESS/MAGIC agreement on May 4-5




Goal: present a proposal in December 06
Foreseen budget: ~ 150 MEUR
Two installations: one in the N, the other in the S


Participation from Cangaroo
S/N ~ 2
Working groups already set up
65
66
Array layout: 2-3 Zones
High-energy section
~0.05% area coverage
Medium-energy section
~1% area coverage
FoV increasing
to 8-10 degr.
in outer sections
Low-energy section
~10% area coverage
70 m
250 m
Eth ~ 10-20 GeV
Eth ~ 50-100 GeV
Eth ~ 1-2 TeV
few 1000 m
67
Option:
Mix of telescope types
Not to scale !
68
Modes of operation
 Deep wide-band mode: all
telescopes track the same source
 Survey mode: staggered fields of
view survey sky
 Search & monitoring mode:
subclusters track different
sources
 Narrow-band mode: halo
telescopes accumulate highenergy data, core telescopes hunt
pulsars
…
Not to scale !
Telescope structure
Proven:
MAGIC rapid-slewing 17 m dish
Cost /
Dish Area
Proven:
H.E.S.S. 12 m dish
Camera
cost dominates
0
10 m
Dish cost
dominates
20 m
30 m
Dish size
Construction started:
H.E.S.S. II 28 m dish
69
70
Possible CTA sensitivity
GLAST
-11
10
Crab
2
E x F(>E) [TeV/cm s]
E.F(>E)
[TeV/cm2s]
-12
10% Crab
10
MAGIC
looks a bit
optimistic
30 m stereo
telescopes
Konopelko
Astropart.Phys.
24 (2005) 191
-13
10
20 wide-angle
10 m telescopes
de la Calle Perez,
Biller, astro-ph
0602284
H.E.S.S.
Current
Simulations
1% Crab
-14
10
10
100
1000
E [GeV]
10
4
10
5
71
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
Handles for the study of new mechanisms
No clear sources above ~ 50 TeV


What better than a crash test to break a theory ?
Do they exist or is this just a technological limit ?
We just started…
Present detectors: have observational capabilities to give SURPRISES !
New instruments in the pipeline
But… technological breakthrough needed to avoid brute force from 2010 (and
unemployement from 2015?)
72
Bibliography

polywww.in2p3.fr/actualites/congres/cherenkov2005/

F. Aharonian, “Very High Energy Cosmic Gamma Radiation”,
World Scientific 2004

C.M. Hoffman et al., Rev. Mod. Physics 71 (1999) 4