Gas Filled Detectors
counting & tracking of
particles
energy loss
generation
of
electron-ion+ pairs
Gas Filled Detectors
Primary and Total Ionization
fast charged particles ionize the atoms of a gas
fraction of resulting primary electrons have enough
kinetic energy to ionize other atoms
Gas Filled Detectors
Primary and Total Ionization
total number of created electron-ion+ pairs
ΔE = total energy loss
Wi = effective <energy loss>/pair
Gas Filled Detectors
Primary and Total Ionization
Number of primary electron/ion pairs in frequently
used (detector)gases
(Lohse and Witzeling,
Instrumentation In High
Energy Physics, World
Scientific,1992)
Gas Filled Detectors
“the noble choice !”
energy dissipation mainly due to ionization
de-excitation photon energy > ionization threshold
of metalls
=> noble gases, e.g. Ar (about 11 eV)
Gas Filled Detectors
“the curse of low density”
For thin layers & low density materials:
=> few collisions, some with high energy transfer
=> energy loss distributions show large fluctuations
towards high losses: ”Landau tails”
Gas Filled Detectors
limitations
large fluctuations in primary ionization don’t allow
relation to energy loss of particle
100 e- not trivial to detect, noise in typical
amplifiers typically an order of magnitude larger
solution:
(gas) amplification
Gas Filled Detectors
amplification
let’s take a wire (anode) in a cylinder (cathode):
Gas Filled Detectors
signal formation
for several kV/cm secondary ionization sets in
close to anode wire (several radii distance)
⇒
avalanche formation (exponential increase)
formation time on ns-scale
typical drift velocities :
e-:
5·105 cm/s
Ar+:
1.51 cm2 V-1 s-1
Gas Filled Detectors
signal formation
signal induction both on anode and cathode due to
moving charges (both electrons and ions)
electrons collected by anode wire, i.e. dr is
small (few mm)
=>
electrons contribute only very
little to detected signal (few %).
Gas Filled Detectors
signal formation
t
V(t)
ions govern signal length
signal differentiation needed !
Gas Filled Detectors
operation modes
Gas Filled Detectors
operation modes
ionization mode:
full charge collection, no amplification
proportional mode:
multiplication started,
signal proportional original ionization
we need quenching
Geiger mode
“each particle” unleaches hell !
Gas Filled Detectors
spatial sensitivity
particle
In order to get spatial information about the particle
trajectory you build a wire chamber (see above).
As counting gas you intend to use Argon, the containment
is built from Aluminum or aluminized foils.
First tests show, that there is no unique relation wire
number/particle trajectory, the position of the particle
remains diffuse, distant wires fire as well as wires close to
the particle trajectory.
What might be the origin of the problem ?
How can you improve the performance of the detector ?
Gas Filled Detectors
quenching !
photons stemming from the Ar de-excitation yield “long
distance coupling”
⇒
solution: photon absorber
poly atomic gases as quencher
Gas Filled Detectors
Multi Wire Proportional Chamber
Noble Prize 1992
typicla paramters:
L= 3 - 5 mm, d= 1-3 mm, rwire= 5 - 20
µm
V0= 5 - 10 kV/cm
address of hit wire gives spatial information
Gas Filled Detectors
Multi Wire Proportional Chamber
Noble Prize 1992
typicla paramters:
L= 3 - 5 mm, d= 1-3 mm, rwire= 5 - 20
µm
V0= 5 - 10 kV/cm
address of hit wire gives spatial information
Gas Filled Detectors
drift chambers
scintillator
shown above, a typical vertical drift chamber setup,
assume the “drift times” can be measured by a time to
digital converte (TDC) with a resolution of 500 ps.
a)
b)
c)
d)
e)
discuss the role of the scintillator, which material is
preferable ?
distance wire-wire d=5 mm
distance wire-cathode l=10 mm, V0=5 kV
discuss the response time (time particle in, signal
out) of the chamber, estimate the maximum mean
counting rate the chamber can handle
what is the range of drift times you expect ?
sketch the drift time distribution
develop a formalism to determin the crossing point:
particle trajectory-wire plane
estimate the spatial resolution