Transcript High Energy Detection

High Energy Detection
High Energy Spectrum
• High energy EM radiation:
l (nm)
E (eV)
Soft x-rays
10
100
X-rays
0.1
10 K
Soft gamma rays
0.001
1M
Hard gamma rays
(highest energy)
• Nuclear particles (cosmic rays)
E = 106 eV to 1020 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
I
E
+
• A beam of charged particles
will ionize gas.
– Particle energy E
– Chamber area A
A
-
V
• An applied field will cause ions
and electrons to separate and
move to charged plates.
– Applied voltage V
– Measured current I
Saturation
• Ion – electron pairs created will
recombine to form neutral atoms.
– High field needed to collect
all pairs
– V > V0
I
I0
• Uniform particle beam creates
constant current.
– Saturation current I0
V0
Ion
Saturation
recombination
V
Avalanche
• Many electrons reach the anode
for each initial pair.
– Typically 104 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
I
+
• Cylindrical geometry is
common for proportional
counters.
– Grounded outer cathode
– High voltage anode
-
• The avalanche is limited to a
region near the wire.
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 lightsensitive 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 lmax.
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

N
1 1
 2L1 - 2 2  2
– CsI scintillator
l
  n l
– Silicon strip readout
• The GLAST Burst Monitor detects soft gammas
– Few keV to 30 MeV
– NaI and BGO scintillator
Photoelectron Emission
e-
vacuum
energy
EA
conduction band
Fermi energy
hn
y
•
Counting photons requires
conversion to electrons.
•
The photoelectric effect can eject
electrons from a material into a
vacuum.
– Exceed gap energy EG and
electron affinity energy EA
•
The probability that a photon will
produce a free electron is expressed
as the quantum efficiency.
EG
valence band
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
e
emissive surface
substrate electrode
• This electrode is called a
dynode.
Amplifier
• A single photon can produce a
measurable charge.
– Single photoelectron
– Qpe ~ 10-12 C
• Each dynode typically
multiplies by a factor of 2 to 6
• Photomultiplier tubes often
have 10 to 14 stages.
– Gain in excess of 107
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
– A0, l0 medium dependent
• The index can be viewed as a
result of scattering.
– Scattering amplitude A(0)
n  c/v
n  1+
2

A0
1 - l20 / l2

2N ( Z / A)
n -1 
A(0)
3
k
Frequency Dependence
• The index varies with wavelength.
water
glass
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.

d
• First observed by Pavel
Cherenkov in 1934.
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
cosd  v / v  c / vn
cosd  1 / n

uˆ - nvˆ
p
 2n2 -1
Emission Spectrum
• The number of photons from
Cherenkov radiation is fixed for
a given wavelength by the angle
of the radiation.

2 N
1  1
 2 1 - 2 2  2
xl
  n l
  1 / 137
• This can be integrated within a
range of wavelengths.
– Detector sensitivity

N
1  1
1 

 2 1 - 2 2  x
  n  lL lH 
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.
Electrons
• Beam magnets can select a
fixed momentum.
• Cherenkov counters can
identify particles by mass.
Pions
Mazziotta, GLAST (2005)
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 m-neutrinos make
a clean ring.
• Electrons from e-neutrinos
make a diffuse ring.
– Electrons interact and
shower
Neutrino Observation