Transcript Solid and Liquid Radiation Detectors

dispersive property of a G-M tube
1010
C
pulse size
108
- HV +
106
104
102
1
0
250
500
tube voltage
750
In the proportional region a G-M
tube has dispersive properties
Solid Radiation Detectors
Advantage:
• High density improves the efficiency of radiation absorption.
• Recombination is not a serious problem.
Types:
• Scintillation detectors.
• Semiconductors detectors.
Excitation and Recombination
in Solids
electron
hole
energy
Conduction Band
D
A
Valence Band
defect
Electronic Band
Structure
Crystal structure
Scintillation
1) Deliberately added impurities
enhance trapping of charge
carriers and recombination.
energy
Conduction Band
D
A
Valence Band
2) Excess energy is released in a
form of visible radiation.
3) Due to the size of the band
gap, the light is not absorbed
in the crystal.
Properties of Selected Scintillators
scintillator
Density
(g/cm3)
Emission
(Å)
Decay time
(ns)
Anthacene
1.24
4400
26
p-Terphenyl
1.17
4100
10
Stilbene
1.19
4100
8
NaI (Tl)
3.67
4100
0.25
ZnS (Ag)
4.1
4500
9
Sodium iodide activated with thallium may produce one
electron-hole pair for every 50 eV of radiation energy.
The intensity of the emitted light is proportional to the energy
of the radiation.
Scintillation Detector
scintillator
photocathod
e
dynodes
_
900 – 1500 V
anode
+
output
signal
Scintillation detectors are suitable for radiation spectroscopy.
Scintillation Statistics
counts
Monoenergetic radiation results in a response
distributed about the most probable value.
FWHM
The primary reasons are:
• random nature of electron-hole pair formation
• variations in energy distribution between light
and other recombination mechanisms
• uneven efficiency of reaching the PM tube
• nonuniform distribution of sensitivity at the
photocathode and dynodes.
channel
The overall response of a scintillator detector obeys Poisson
statistics, convergent to normal distribution for large numbers.
charge collection in semiconductor detectors
Preferably
each
generated
carrier should be collected by
its attracting electrode.
The signal (fraction of collected
carriers) depends on both the
lifetime and mobility of the
carriers.
The
equilibrium
carrier
concentration should be small
(low conductivity).
-
i
+
V
Detectors of this type must be cooled (with LN) for proper operation.
depletion layer semiconductor detectors
depletion
layer
-
+
p
n
V
Low intrinsic concentration
is achieved in the depletion
region.
The diode conducts only
after electron-hole pairs are
generated by the radiation in
the depletion region.
surface-barrier semiconductor detectors
Small combined thickness
of the n-layer and the gold
contact results in very low
radiation absorption in these
two regions. (Front surface
detector.)
p
n
output
signal
V
-
+
Multi Channel Analyzer (MCA)
number of pulses
Multi channel analyzer counts separately (in a channel) pulses
with amplitudes in a specific range.
channel
When connected to a dispersive detector, each channel resisters
radiation particles with energy in a corresponding energy range.
 - spectroscopy
In a magnetic field, charged
particle moves along a
circular path determined by
the magnetic field, mass and
charge of the particle, and
its kinetic energy (speed)
r
mB
qv
+
G-M
scaler