Astroparticle physicswith high-energy photons

II – Techniques & Instruments

Alessandro de AngelisLisboa 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: 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 MeVX/low E 1 MeV-10 Me

medium 10-30 MeVHE 30 MeV-30 GeVVHE 30 GeV-30 TeVUHE 30 TeV-30 PeVEHE above 30 PeVNo upper limit, apart from low flux (at 30 PeV, we expect ~ 1 /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/ 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 e+e- threshold + some phase space (“10 MeV”), Compton/scintillation

Above “10 MeV”, pair production

Above “50 GeV”, atmospheric showers

Pair <-> Brem

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Consequences on the techniques

The fluxes of h.e. are low and decrease rapidly with energy

Vela, the strongest 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

The earth atmosphere (28 X0

at sea level) is opaque to X/Thus only a satellite-based detector can detect primary X/

8Satellite-based and atmospheric: complementary, w/ moving boundaries

Flux of diffuse extra-galactic photons

Atmospheric

Sat

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Satellite-based detectors:figures of merit

Effective area, or equivalent area for the detection of Aeff(E) = A x eff.

Angular resolution is important for identifying the 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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satellite-based detectors: engineering

Techniques taken from particle physics direction is mostly determined by e+e-

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/p) (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)

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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 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 @ 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...

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X-ray Telescope Gamma-ray (EGRET)

Detection technology CCD, Ge e+e- pair creation tracking

Sensitivity a few micro-Crab ~ ten milli-Crab

Angular resolution < 1 arc-second <1 degree

No. of Sources detected >>106 ~300

detectors on satellite:comparison with X-ray detectors

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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 (E/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

18Earth-based detectorsProperties of Extensive Air Showers

We believe we know well the 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/0); 0 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: had ~ 400 m for air=> hadronic showers will look ~ equal to e.m., apart from having 20x more muons and being less regular

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 (Č) detectorsCherenkov light from showers

Č light is produced by particles faster than light in air Limiting angle cos c ~ 1/n

c ~ 1º at sea level, 1.3º at 8 Km asl

Threshold @ sea level : 21 MeV for e, 44 GeV for

Maximum of a 1 TeV 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 backgroundOn a moonless night

~ 0.1 photons/(m2 ns deg)

Signal Afluctuations ~ (A)1/2

=> S/B1/2 (A/)1/2

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Č 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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CAT Thémis (French Pyrénées)

• first light summer 1996,• fine camera : 600 pixels

Whipple-10msince 1969

100 PMT’s by 1990

HEGRA1994-2002

5 telescopes / stereoscopyLa-Palma Canaries

CANGAROOsince 1994Australia

STACEESince 2000

Albuquerque

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Extensive Air Shower Particle Detector Arrays

Built to detect UHE gammas

small flux => need for large surfaces, ~ 104 m2

But: 100 TeV => 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 < 1%)

Possibly a detector for hadron rejection

Direction from the arrival times, 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 -> shower direction

Radial distribution of particles -> distance L

Total number of particles -> energy

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EAS Particle Detector ArraysAn 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

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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) > 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 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

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

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 (->2015)

Wide range of physics objectives:

Gamma astrophysics Fundamental physics

A HEP / astrophysics partnership

Tra

cker

Calorimeter

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GLAST: the instrument

TrackerSi 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 trackerSi 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

Geminga Radio-Quiet Pulsars

Crab

PKS 0528+134

Geminga

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AGILE (the GLAST precursor)

To be launched in 2005Lifetime of 3 years

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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…

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Ground-based detectorsImprovements in atmospheric Č

Improving flux sensitivity Detect weaker sources, study larger sky regions S/B1/2 (A/)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

40Major projects in atmospheric ČAiming 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

41Major projects in atmospheric ČAiming 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, > 2003

VERITAS (US, Arizona) 7 x Whipple-like 100 m2

telescopes in Arizona, > 2005

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Č detectorsOverview of next detectors

CANGAROO III(Australia & Japan)

Spring 20044 telescopes 10 meters

ØWoomera, Australia

Windhoek, NamibiaHESS

(Germany & France)Summer 2002

4 (16) telescopes10 meters Ø

Roque delos Muchachos, Canary Islands

MAGICMAGIC(Germany, Italy & Spain)(Germany, Italy & Spain)

Winter 2003Winter 20031 telescope 17 meters Ø1 telescope 17 meters Ø

Montosa Canyon,Arizona

WHIPPLE/VERITAS

(USA & England)

now/2005?7 telescopes10 meters Ø

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Ground-based detectorsImprovements in EAS PDAs

Higher altitude Tibet (past and future)

=> 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 Č 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/ 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)

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http://imagine.gsfc.nasa.gov/docs/science/know_l1/

history_gamma.html

http://imagine.gsfc.nasa.gov/docs/introduction/bursts.html

GLAST and satellite physics, http://glast.gsfc.nasa.gov/

INTEGRAL and CHANDRA homepages

J. Paul’s talk in Moriond 2002