Transcript Radiation Sensors Zachariadou K. | TEI of Piraeus Radiation Sensors Part-II General Aspects.

Radiation Sensors
Zachariadou K. | TEI of Piraeus
Radiation Sensors
Part-II
General Aspects
Part-II
Radiation Sensors
General Aspects
The course is largely based on :
 G. F. Knoll, “Radiation detection and measurement” ; 3rd ed., New
York, Wiley, 2000
 Gordon Gilmore & John D. Hemingway, “ Practical Gamma-Ray
Spectrometry”; Willey , 21008
Radiation Sensors
Modes of operation
General properties
 Sensitivity
 Pulse Counting mode
 Current mode
 Mean square voltage mode
 Efficiency
 Energy resolution
 Time resolution
 Pulse-pair resolution
 Position resolution
Modes of Detection operation
The net result of the radiation interaction in a wide category of detectors is the appearance
of a given amount of electric charge within the active volume of the detector
The charge must be collected as an electric signal.
The collection is accomplished by applying electric field within the detector
causing the positive and negative charges created by the radiation to flow in
opposite directions
tc
0
i ( t ) dt  Q
Collection time:
Ion chambers: few ms
Semiconductor detectors: few ns
Modes of Detection operation -cont
Response of typical detector:
Current that flows for a time equal
to the charge collection time (tc)
i(t)
tc
0
i ( t ) dt  Q
tc
0
tc
time -t
i ( t ) dt  Q
Modes of Detection operation
Most commonly applied
 Pulse Counting mode
(the signal from each interaction is
processed individually)
 Current mode
The detector records each
individual radiation that interacts
Used when event
rates are high
(the electrical signals from individual
interactions are averaged together,
forming a net current signal)
 The time integral of each burst of
current is recorded
 All pulses above a low-level
threshold are registered
(pulse counting)
Pulse mode is impractical for high event rates
Modes of Detection operationPulse mode
C=equivalent capacitance of
the detector +measuring
circuit
(eg cable +preamplifier)
The voltage V(t) across R is the fundamental signal voltage on which
pulse mode operation is based
Two cases:
 Small RC (τ<<RC)
 Large RC (τ>>RC) (more common)
Modes of Detection operationPulse mode
 Small RC (τ<<RC)
Used when high event rates or time information is
more important than accurate energy information
 The time constant of the
external circuit is kept small
compared with the charge
collection time
 Large RC (τ>>RC) (more
common)
Little current flows in R during the
charge collection time
The detector current is integrated
on the capacitance
If time between pulses is large
the capacitance will discharge
through R
Modes of Detection operationPulse height spectra
Radiation detector in
pulse mode:
Displaying modes:
The pulse amplitude
distribution is used to deduce
information about the incident
radiation
 Differential pulse height
distribution
 Integral pulse height
distribution
Modes of Detection operationPulse height spectra
 Differential pulse height
distribution
Ordinate: The differential (dN)
number of pulses observed
having an amplitude within dH,
divided by dH
Total number of
pulses at [H1, H2]:
H1
N 

H1
dN
dH
dH
Modes of Detection operationPulse height spectra
 Integral pulse height distribution
Ordinate: number of pulses whose
amplitude exceeds that of a given
values of the abscissa H
Modes of Detection operationan example
 The spectrum is obtained by summing
the deposited energies in the sensitive
materials a radiation sensor
 The shape of the depends strongly on the
mechanism via which the incident photon
primarily interacts with the detector:
 If the primary photon interaction is a
photoelectric effect, its energy is fully
absorbed and it contributes to the full
energy peak (photo-peak) of the energy
spectrum. In contrast, a primary Compton
interaction creates a scattered electron that
carries only a fraction of the initial photon
energy and a scattered photon that carries
the remaining energy. If the latter is
absorbed by a sensitive material of the
detector, the event contributes to the photopeak of the spectrum. Otherwise, the event
contributes to the plateau at energies below
the photo-peak (Compton plateau).
simulated energy
spectrum of 200keV
incident γ- rays
Modes of Detection operationan example
Τhe number of incompletely
absorbed events (off-peak part of
the energy spectrum) increases
compared to the photo-peak
events as the incident photon
energy increases.
Modes of Detection operationPulse mode
Large RC (τ>>RC)
General properties
 The rise time of the pulse is
determined by the charge time
collection
 The dead time of the pulse is
determined by the time constant of
the load circuit
 Vmax : the amplitude of the signal is proportional to
the charge generated within the detector :
V max 
Q
C
The proportionality holds if C is constant
General propertiesEnergy Resolution
N=charge carriers,
(large number)
Statistical fluctuations:
Energy resolution
FWHM

Pulse height at center of peak
Energy resolution

2.35  
H0
2.35  k
N
 100%
kN

2.35
N
 100%
 100% 
N
 100%
General propertiesEnergy Resolution
 Scintillators for gamma
spectroscopy: ~5-10%
 Semiconductors: ~1%
Larger number of carriers
(Semiconductors ) better
resolution
Any other fluctuations will combine with the statistical fluctuations
(FWHM)
2
all
 (FWHM)
2
statistica l
 (FWHM)
2
noise
 (FWHM)
2
drift
 ...
General propertiesDetection Efficiency
The efficiency (sensitivity) of a radiation sensor is a measure of its ability to
detect radiation
Absolute Efficiency
Intrinsic Efficiency
E abs

E abs 
E intr
Number
E abs
For isotropic sources:
E int r
detected
Number
emitted
Number
E intr 
Number incident
Number
Number incident
detected
on the detector
on the detector
emitted


4
Solid Angle of the
detector
General propertiesDetection Efficiency
E abs

Number incident
E intr
Number
on the detector
emitted
E abs
E int r


4
Ω=Solid Angle of the sensor


A
cos α
r
2
dA
r= distance of the sensor’s
surface element dA from a
radiation source
a= angle between the normal
to the sensor’s surface and
the direction of the source
As the distance from a radiation
source increases the absolute
efficiency of a radiation sensor
decreases
General propertiesDetection Efficiency
E abs

Number incident
E intr
Number
E abs
on the detector
E int r
emitted


4
Ω=Solid Angle of the sensor


A
cos α
r
2
For the case of point-source located along
the axis of a cylindrical radiation sensor (of
radius a) ,close to the source:
dA
r

  2   1 


d
d
2
a
In the far field (d>>a)
2




a
d
 
A
d
2


d
2
2
General propertiesDetection Efficiency
Use the detection efficiency to measure the absolute activity of a radiation source
Given: N recorded events
Assume isotropic
emission
Detector intrinsic peak efficiency Eins
The number of events (Io) emitted by the source over the
measurement period:
E abs 
Number
detected
Number
emitted
E abs  
4


I
I0
I0 
I
E abs

I
E int r

4

Ω: solid angle (in steradians) subtented by
the detector in a given source position
General propertiesDetection Efficiency
For the case of a parallel beam of mono-energetic gamma-rays incident on a
detector of uniform thickness:
E intr 
Number
Number incident
detected
on the detector
I  I 0e
E intr  1 - e
 x
 x
Absorption law
General propertiesDetection Efficiency
For NaI(Tl)
sensors:
For semiconductor
detectors:
the intrinsic
efficiency depends
also on the energy
of the incident
gamma
the intrinsic efficiency
increases with the increase of thickness x
decreases with the increase of the photon energy
Intrinsic efficiency of a
CdTe semiconductor
gamma radiation sensor
General propertiesDetection Efficiency
Peak efficiency
Only full energy deposition
interactions are counted
 Photopeak area
total efficiency
All interactions are counted
Entire area under the
spectrum
Most common for Gamma ray
detectors : Intrinsic peak efficiency
General propertiesDead Time
Dead time:
Minimum amount of time between
two events in order that they be
recorded as two separate pulses
Severe for high
counting rates
Main problem for
detectors in pulse
mode
 time for a detector to recover before being sensitive to another
radiation interaction (e.g. Geiger counter)
 pile-up: some detectors are forming an electrical pulse with a long tail
when a new radiation interaction takes place distorts the pulse shape
and possibly the energy measurement (based upon pulse amplitude)
 dead time of the ADC used for data acquisition
General propertiesDead Time
 Paralyzable system, an interaction that occurs during the
dead time after a previous interaction extends the dead time
 Non-paralyzable system, does not extend the dead time
At very high interaction rates, a paralyzable system will be
unable to detect any interactions after the first, causing the
detector to indicate a count rate of zero
General propertiesDead Time
Recorded count rate vs
true interaction rate for
an ideal (no dead time)
paralyzable and nonparalyzable sensor
Types of detectors
Gas detectors
Gas-filled detectors consist of a volume of gas between two electrodes
Scintillators
the interaction of ionizing radiation produces UV and/or visible light
Solid state detectors
crystals of silicon, germanium, or other materials to which trace amounts
of impurity atoms have been added so that they act as diodes
Other , Cerenkov etc…
Types of detectors (cont.)
Detectors may also be classified by the type of information produced:

Counters:
Detectors, such as Geiger-Mueller (GM), that indicate the number
of interactions occurring in the detector

spectrometers
Detectors that yield information about the energy distribution of
the incident radiation, such as NaI scintillation detectors

dosimeters
Detectors that indicate the net amount of energy deposited in the
detector by multiple interactions