High Energy Detection

High Energy Spectrum High energy EM radiation:  (nm)E (eV) Soft x-rays X-rays K Soft gamma rays M Hard gamma rays (highest energy) Nuclear particles (cosmic rays) E = 10 6 eV to eV Nuclei (p, He) Electrons (e , e  ) Neutrinos Secondary cosmic rays

Ionizing Radiation High energy particles have sufficient energy to ionize atoms and produce electrons. –Compton scattering, bremsstrahlung, pair production Ionized electrons can be measured through a number of processes. –Solid state: CCDs –Gas systems –Scintillators

Measuring Ions A beam of charged particles will ionize gas. –Particle energy E –Chamber area A An applied field will cause ions and electrons to separate and move to charged plates. –Applied voltage V –Measured current I I V   A E

Saturation Ion – electron pairs created will recombine to form neutral atoms. –High field needed to collect all pairs –V > V 0 Uniform particle beam creates constant current. –Saturation current I 0 I0I0 V0V0 I V Ion recombination Saturation

Avalanche Many electrons reach the anode for each initial pair. –Typically 10 4 electrons –An “avalanche”

Proportional Region Ionization chambers at increased voltage move from an ionization plateau to the proportional region. Proportional counters were common on satellites through the ’90s. –Roentgen satellite (ROSAT) –Rossi x-ray timing explorer (RXTE)

Cylindrical Chamber Cylindrical geometry is common for proportional counters. –Grounded outer cathode –High voltage anode The avalanche is limited to a region near the wire. I V  

Multiwire Chamber An array of proportional or Geiger readout wires can be placed in an array. Provides excellent position resolution for charged particle tracks. –Compton gamma ray observatory

Scintillation Detector Scintillation detectors are widely used to measure radiation. –Ionization from inner shells The detectors rely on the emission of visible light from excited states. 1. An incident photon or particle ionizes the medium. 2. Ionized electrons slow down causing excitation. 3. Excited states immediately emit light. 4. Emitted photons strike a light- sensitive surface. 5. Electrons from the surface are amplified. 6. A pulse of electric current is measured.

Jablonski Diagram Jablonski diagrams characterize the energy levels of the excited states. –Vibrational transitions are low frequency –Fluoresence and phosphoresence are visible and UV Transistions are characterized by a peak wavelength max.

Inorganic Scintillators Fluorescence is known in many natural crystals. –UV light absorbed –Visible light emitted Artificial scintillators can be made from many crystals. –Doping impurities added –Improve visible light emission

Organic Scintillators A number of organic compounds fluoresce when molecules are excited. –Compare to % anthracene light output Organic scintillators can be mixed with polystyrene to form a rigid plastic. –Easy to mold –Cheaper than crystals –Used as slabs or fibers

Fermi Space Telescope In 2008 the Fermi Gamma-ray Space Telescope was launched. –Formerly Gamma-ray Large Area Space Telescope (GLAST) –Improvement on EGRET aboard Compton A large area telescope (LAT) detects hard gammas –20 MeV to 300 GeV –CsI scintillator –Silicon strip readout The GLAST Burst Monitor detects soft gammas –Few keV to 30 MeV –NaI and BGO scintillator

Photoelectron Emission Counting photons requires conversion to electrons. The photoelectric effect can eject electrons from a material into a vacuum. –Exceed gap energy E G and electron affinity energy E A The probability that a photon will produce a free electron is expressed as the quantum efficiency. valence band conduction band Fermi energy vacuum energy EGEG EAEA  h ee

Electron Multiplier Single photoelectrons would produce little current. Electrons can be multiplied by interaction with a surface. –Emitter: BeO, GaP –Metal substrate: Ni, Fe, Cu This electrode is called a dynode. substrate electrode e emissive surface

Amplifier A single photon can produce a measurable charge. –Single photoelectron –Q pe ~ C Each dynode typically multiplies by a factor of 2 to 6 Photomultiplier tubes often have 10 to 14 stages. –Gain in excess of 10 7

Photomultiplier Tube A photomultiplier tube (phototube, PMT) combines a photocathode and series of dynodes. The high voltage is divided between the dynodes. Output current is measured at the anode. –Sometimes at the last dynode

Index of Refraction When light passes through matter its velocity decreases. –Index of refraction n. The index depends on the medium. –Wavelength dependence –A 0, 0 medium dependent The index can be viewed as a result of scattering. –Scattering amplitude A(0)

Frequency Dependence The index varies with wavelength. waterglass

Faster than Light A charged particles passing through matter will polarize some atomic electrons. If the particle exceeds the speed of light c/n then an electromagnetic shock wave will be formed. First observed by Pavel Cherenkov in  

Cherenkov Radiation The Cherenkov radiation has a characteristic angle compared to the particle. –No radiation below  = 1/n The Cherenkov light is linearly polarized in the plane of the particle. –u, v unit vectors along photon and particle directions

Emission Spectrum The number of photons from Cherenkov radiation is fixed for a given wavelength by the angle of the radiation. This can be integrated within a range of wavelengths. –Detector sensitivity

Threshold Detector If any light is emitted, then the particle  exceeds 1/n. Varying the pressure of a gas in a detector can allow the identification of particles that exceed a desired speed. Mazziotta, GLAST (2005)

Particle ID Momentum and speed differ based on the mass of the particle. Beam magnets can select a fixed momentum. Cherenkov counters can identify particles by mass. Mazziotta, GLAST (2005) Pions Electrons

Ring Imaging A particle with velocity v creates light at a fixed angle. A spherical mirror will focus the light into a ring of fixed radius. –Center sets the particle position –Radius sets the speed These are called RICH detectors. LHCb

Neutrino Telescopes Cherenkov imaging is used in neutrino detectors. –Underground observatories Muons from  -neutrinos make a clean ring. Electrons from e-neutrinos make a diffuse ring. –Electrons interact and shower

Neutrino Observation