Detection of Gamma-Rays and Energetic Particles

• Interactions of high energy photons• Detectors for 100 keV – 10 MeV

– Scintillators, Solid-state detectors– Compton telescopes

• Detectors for 30 MeV – 20 GeV– Spark chambers– Silicon trackers

• Detectors for higher than 20 GeV– Air Cherenkov– Extensive air shower

Three interactions

• Photoelectric absorption (E < 10 MeV)– Photon is absorbed by atom– Electron is excited or ejected

• Compton scattering (10 keV < E < 10 MeV)– Photon scatters off an electron

• Pair production (E > 10 MeV)– Photon interacts in electric field of nucleus and

produces an e+ e– pair

Detectors for 100 keV – 10 MeV

• Can not use detectors for standard X-ray band (0.1-10 keV) because interaction cross-sections are too small – need more material.

• Thick semiconductor detectors– CdTe, CdZnTe, Ge, PbI2, HgI2, …

– Work like X-ray semiconductor detectors– Typically have pixilated readout, one channel per

pixel– Typical thickness 0.1 to several millimeters

Detectors for 100 keV – 10 MeV

• Scintillators – convert gamma-ray to optical photons then detect optical photons

Thick scintillators are cheaper than thick semiconductors.

Energy resolution is worse because only part of optical light is collected.

Photomultipliers are the traditional photo-detectors.

Phototube

Photon produces an electron by interacting with photocathode.

Electron is accelerated by E-field, produces multiple electrons upon striking dynode. Several stages of dynodes can give multiplications of 106. Response time is in nanoseconds.

Compton Telescope

iEE

EEE

mcE

)(

1)cos(212

21

1

Direction of incident photon can be measured to within a cone around the vector between the two interactions:

COMPTEL

Advanced Compton Telescope

Many layers of position sensitive silicon detectors.

Much better sensitivity than COMPTEL.

30 MeV to 10 GeV

In 30 MeV to 10 GeV range use pair conversion, then measure tracks of electron-positron pair.

Old detectors used spark chambers. New detectors (Agile, GLAST) use silicon strip detectors.

Energy is measured using a scintillator at bottom.

Ionizing Particle

Energetic particle

Electron in medium

rel)(non 1

or 12

2

2sinF

2222

422

2

0

2

2

2

pvmvb

ez

m

pE

bv

zed

bv

zedt

r

zeF

ee

e

b

Bethe-Bloch FormulaNeed to integrate over impact parameter b and ionization potential of atoms I, find

2

222

2

42 2ln

c

v

I

vm

vm

Nez

dx

dE e

e

e

Low energy dependence is ~ 1/v2, reaches minimum around mec2, increases as ln(2) at high energies.

Ionizing Particle

Radiation Length

Total radiation loss of an electron traversing a medium is

)/exp( 000

XxEEX

E

dx

dE

Where X0 is the “radiation length”.

GLAST

Interleaved Si strips and converters, strips alternate in direction.

Calorimeter for energy measurement.

Segmented anticoincidence.

Expected launch in late 2007.

Ionizing Particle

Glast strips are 400 microns thick. A minimum ionizing particle deposits 388 eV/micron in Si or about 0.16 MeV in each layer of silicon.

Electron/positron trajectory is slightly altered by multiple scatttering (same process leading to energy loss). The RMS deflection in a layer of thickness t is

0X

t

pv

Esrms Es = 21 MeV

Angular Resolution

Strip pitch is 228 microns, tracker height is 50 cm, best possible angular resolution is 0.03 degrees.

Angular resolution is worse, and depends on energy, due to multiple scattering in converter and Si.

Energies above 30 GeV

• Effective area of pair production telescope is limited to cross-section of the telescope, GLAST geometric area is 25,600 cm2, effective area is ~ 1 m2.

• Photon flux from Crab at energies above 60 GeV is 610-10 photon cm-2 s-1. Rate in 1 m2

detector is 610-6 photon/second = 0.5 photon/day.

• Need detector with much larger effective area.

Electron-Photon CascadesFor ultrarelativistic electrons (or positions), the radiation length is the same for bremsstrahlung radiation as for pair production, pair ~ bremss

Cherenkov RadiationRadiation induced by a charged particle that moves faster than the speed of light in a medium

The wavefront of the radiation propagates at a fixed angle with respect to the velocity vector of the particle.

Cherenkov ConeCherenkov radiation is contained in a cone around the direction of motion with an opening angle

v

nc /cos

where n is the index of refraction of the medium. Radiation is produced only when the particle moves at relativistic speeds v > c/n. For air n = 1.0003.The radiative power per unit frequency is given by

fvn

c

c

ve

df

fdP

22

2

2

2

12)(

Air Cherenkov Telescope

Effective area is given by size of light pool, about 120 meters in radius with an area of 5104 m2.

Extensive Air Showers

Cosmic ray rate above 1019 eV is one particle per square kilometer per year.

Auger Observatory has 1,600 detectors (water tanks) separated by 1.5 km.