General radiation detection
concepts
Myroslav Kavatsyuk
KVI – Center for Advanced Radiation Technology
m.kavatsyuk@rug.nl
A generic radiation detector
detector sensitive volume
quantum of
radiation
assumptions:
• individual radiation quanta are detected
• all interactions per quantum are so fast they can be considered instantaneous
• some or all energy is deposited, resulting in ± charge carrier pairs or light
• + and/or – charge carrier, or light is collected
• per quantum, a pulse is generated: pulse mode operation
pulse shape depends on detector type
and eventual electronics
2
Information contained in detector pulses
energy related to:
amplitude
integral
time pick-off level
t0
“arrival time”
count rate (pulses/second) radiation intensity
3
Energy spectrum (pulse height spectrum)
pulses are digitized (ADC) energy calibration:
intensity vs. channel number energy vs. channel number
most often very linear
intensity vs. energy
4
Energy resolution: definition
Full width at half maximum: FWHM
G.F. Knoll, Radiation Detection
and Measurement, 3 rd Edition
• FWHM can be determined experimentally for any peak
• sometimes FWTM (FW tenth M) provides useful information
5
Energy resolution: origin
1. detector performance
– electronic noise
– drift, stability
2. statistical variation in number of primary “excitations”
E e: energy required for a primary excitation
– # of primary excitations N = E: energy deposited by radiation quantum
e
– if excitations are independent: Poisson statistics
Δ N =√ N
Δ E FWHM ∝µ √ E
Δ E FWHM 1
∝µ
E √E
1% FWHM requires N = 55 000
6
Intensity
energy spectrum results from a “counting” experiment, not a “scanning” experiment
intensity from energy spectrum is: NOT height, amplitude
BUT integral (sum over channels)
e.g. peak intensity
K.S. Krane, Introductory
Nuclear Physics, 1 st Edition
7
Detector efficiency
source detector
G.F. Knoll, Radiation Detection
and Measurement, 3 rd Edition geometry
dependence
• absolute efficiency ( abs): number of pulses detected
strong
number of quanta emitted
• intrinsic efficiency ( intr): number of pulses detected
weak
number of quanta incident on detector
• solid angle ( )
isotropic sources: e abs=e int r
point source: =
1
2( ( ))
1−cos atan
a
d
8
Pick of the time information
Leading-edge discrimination
“arrival time” for a pulse with
different amplitude will have
different offset with respect to
the true start of the signal
development
True start of a
signal
development time pick-off level
t0, ”arrival time”
Simple method; result depends on pulse amplitude; can be corrected upon
measurement of the pulse amplitude 9
Pick of the time information
Constant-fraction discrimination (CFD)
Time stamp should be set when
signal rises to a certain fraction
of the pulse amplitude
True start of a
signal
development threshold
t0, ”arrival time”
10
Pick of the time information
Constant-fraction discrimination (CFD)
Time stamp should be set when
signal rises to a certain fraction
of the pulse amplitude
Specially formed signal can
guarantee constant position of
the time stamp with respect to
the true start of the pulse
CFD method yields stable results, however
might have worse performance due to the
noise influence
11
Timing
• arrival time of a quantum of radiation in the detector ?
• timing requires different pulse handling than energy determination
• different time pick-off methods
• often, optimal timing is incompatible with optimal energy resolution
ΔV ΔV • low noise
Δ t= • fast detector
dV /dt • “bright” detector
Δt
12
Timing resolution
measurement of time differences
– between detector & reference signal
– between 2 detectors
coincidence measurement
G.F. Knoll, Radiation Detection
and Measurement, 3 rd Edition
13
Timing resolution: a real-life example
• crystal: 3x3x5 mm3
• bare LaBr3:Ce(5%)
• LYSO
• Hamamatsu SiPM
S10362-33-050c
• spectralon reflective material
• Agilent DC282 digitizer
• 8 GS/s
• 700 MHz anti-aliasing filter
• extra amplification timing channel
14
World-record timing resolution
511 keV coincidence resolving time (CRT) (FWHM)
LaBr3:Ce(5%) 95 ps
LYSO:Ce 138 ps
LaBr3
spectra shift
20 mm 20 mm as Δt=2 Δx/c
D.R. Schaart et al., Phys. Med. Biol. 55 (2010) N179
R.Vinke et al., 2009 IEEE Nucl. Sci. Symp. Conf. Rec. M06-2
S.Seifert et al., 2009 IEEE Nucl. Sci. Symp. Conf. Rec. J01-4 15
Complex detection systems
Complex detection systems are required to identify detected particles
or measure all their properties.
Examples:
Tracking detectors in magnetic field EM calorimeter
Identification of charged particles
Requirements:
●
Active volume of the detector is position sensitive
→ possible to “record” particles tracks
●
Particle looses only fraction of its energy in the active material
●
Detector is placed inside of a magnetic field
Example of recorded tracks
Energy loss in the detector
Curvature of tracks
→ momentum measurement
Calorimetry
High-energy photons/electrons generate shower of EM particles:
●
In order to measure energy of initial particle all secondary
secondary particles have to be detected and their energy
added up → Calorimeter
In most experiments it is important to measure precisely direction of
the initial particle → large detector with single active volume will
have no position sensitivity
Solution: detector volume has to be segmented
Calorimetry
Solution → detector volume has to be segmented:
●
Energy information → sum of energy deposited in fired volumes
●
Position of primary interaction → weighted average of positions of fired
volumes:
weights are calculated using deposited energy : W i = W0 + ln(Ei/E)
Ei – energy deposited in a single active volume
E – energy deposited by the complete shower
Calorimetry: energy resolution
Radiation length mean distance
over which the electron energy
is reduced to 1/e of its original
value due to radiation loss only
Ec is the electron energy at
which the cross section
of bremsstrahlung becomes
equal to that of pure ionization
Calorimetry: energy resolution
The number of electrons and
positrons in a shower
produced by a photon with a
given energy fluctuates
statistically
n ~ E/Ec
Therefore, relative energy
resolution (relative width of the
distribution) will be proportional
to:
or
Calorimetry: energy resolution
The complete form of the relative
energy resolution is:
a – constant term (calibration of
detector, looses of shower
particles)
b – statistical term
c – noise of readout electronics
