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 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  b Energetic particle Electron in medium

Bethe-Bloch Formula Need to integrate over impact parameter b and ionization potential of atoms I, find 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 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 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 610-10 photon cm-2 s-1. Rate in 1 m2 detector is 610-6 photon/second = 0.5 photon/day. Need detector with much larger effective area.

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

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

Air Cherenkov Telescope Effective area is given by size of light pool, about 120 meters in radius with an area of 5104 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.