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Improvement in scintillation counter energy resolution with the silicon drift detector*Shuji Maeo1,2, Takayuki Yanagida1, Yuui Yokota1 and Akira
Yoshikawa1,3
1Division of Physical Process Design, Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University2Japan Association for the Advancement of Medical Equipment3New Industry Creation Hatchery Center(NICHe), Tohoku University
IntroductionHigh energy radiation measurements
Scintillation counter is useful.
Radiation
Scintillator Light detector
To processor…
Typical detector is Photo Multiplier Tube(PMT).
Problems Energy resolution is not enough. It can not use in magnetic fields.
Going to semiconduct
or
Light
Electrical signal
・ In Conventional photo diode(PD), signal intensity is low, noise level is high(low S/N ratio).
・ In Avalanche PD(APD), S/N ratio is not so high.
Scope of research
Improvement of S/N ratio in Scintillation counter Using an Silicon drift detector (SDD) as light detector
Estimations using the SDDProblems and Next plan
High energy resolution↓
Applying PD array etc.↓
High contrast imagingIn this report
Comparison with each light detector
PMT Si-pin PD APD SDD
Structure Electron from Photocathode is amplified by dynode.
Semiconductor witn pn junction.
Similar to pin PD. Electron is amplified by avalanche effect for high reverse bias.
Small anode at center of the detector can collect charge efficiently by electric field.
Count rate >107 cps >104 cps >106 cps >106 cps
Energy resolution
× △ △ ○
Characteristic - High voltage operation.- Susceptible by ambient electric field and magnetic field.
- Cheapness.- Easy operation.
- Large signal- High voltage operation.- Large leakage current.
- Expensive.- High energy resolution.
Characteristics of the SDD
Electron can be collected efficiently for electric field formed by ring like cathode.
Anode capacitance is low for small anode size.. Field Effect Transistor(FET) can be constructed at SDD directly. Thermal noise can be reduced for cooling by the peltier cooler. High energy resolution and high count rate are available.
Structure of the SDD
P. Lechner et al., Nucl. Instrum. Methods, vol. A-377, 346 (1996).
Affects of energy resolution
0
0.2
0.4
0.6
0.8
1
1.2
0 50 100 150 200
Inte
nsity
(a.u
.)
/ch
1 ch2 ch5 ch10 ch20 ch25 chreference
s=
Simulation results of peak shape depended on each resolution.
Comparison of X-ray response
0 20 40 600.0
0.4
0.8
1.2
1.6
2.0
2.4
2.8
Inte
nsity
/ cp
s
Energy / keV
0 20 40 600
1
2
3
4
Inte
nsit
y / c
ps
Energy / keV
FWHM : 1.7 keV(2.9%)
Np-Lα13.9 keV
Np-Lβ17.7 keV
Np-Lγ20.8 keV
γ26.4 keV
γ59.5 keV
Spectrum of 241Am measured by conventional Si-pin PD
Spectrum of 241Am measured by the SDD
Np-Lα13.9 keV
Np-Lβ17.7 keV
Np-Lγ20.8 keV
γ26.4 keV
γ59.5 keV
FWHM : 0.8 keV(1.3%)
Hamamatsu (S1722-02)
KETEK
Linearity of the pulse heightfor X-rays
Calibration curve
0 20 40 600
1
2
3
4
Inte
nsit
y / c
ps
Energy / keV
Np-Lα13.9 keV
Np-Lβ17.7 keV
Np-Lγ20.8 keV
γ26.4 keV
γ59.5 keV
It can be estimated that linearity between pulse height and energy.
01020304050607080
0 200 400 600 800E
ner
gy
/ k
eVChannel number
Linearity of the pulse heightfor visible light
0 200 400 6000.0
0.2
0.4
0.6
0.8
1.0
Inte
nsit
y (a
.u.)
Pulse height / ch
0 100 200 300 400 500 600 7000
100
200
300
400
500
600
Pul
se h
eigh
t / c
hLED voltage / mV
Red LED voltage / mV
100 200 300 400 500 600 700
Out put signal distribution ofLED light (red) applied each bias
Relation between pulse height
and LED bias
It can be estimated that linearity between pulse height and luminosity.
Advantage of using the SDDHigh energy resolution
High energy radiation
spectrometry
Imaging
Qualitative analysisAccurately
Quantitative analysisSensitive
ImageHigh Contrast
Meas. timebecome Short
Assemble the scintillator
ScintillatorEffective area of
SDD (7 mm2)
Peltier cooler
Coupling by optical grease
Pre AMP(OursTex Co. Ltd.)
Shaping AMP(CleaPulse; 4417)
Multi channel analyzer(Amptek; MCA8000A)
Scheme of output signal
SDD(KETEK GmbH)
Measurement of scintillation light
0 500 1000 15000
2
4
Inte
nsit
y / c
ps
Energy / keV
0 500 1000 15000
1
2
3
Inte
nsit
y / c
ps
Energy / keV661.7 keV
Back scatter peak
661.7 keV
FWHM : 66.5 keV(10%)
FWHM : 79.5 keV(12%)
g-ray response of the CsI(Tl) with the conventional Si-pin PD
g-ray response of the CsI(Tl) with the SDD
Scintillator : CsI(Tl)RI source : 137CsShaping time : 0.5 µsec.Meas. time : 1000 sec.
Short wavelength measurement
0 100 200 3000.00
0.25
0.50
0.75
1.00
Inte
nsit
y (
a.u
.)
Pulse height / ch
0 100 200 3000.00
0.25
0.50
0.75
1.00
Inte
nsit
y (
a.u
.)
Pulse height / ch
g-ray response of the CsI(Tl) with the SDD.
g-ray response of the GSO(Gd2SiO5:Ce) with the SDD.
CsI:Tl GSO(Gd2SiO5:Ce)
wavelength(nm) 約 530 約 430
luminosity(ph/MeV)
約 60,000
約 10,000
Quantum efficiency of each semiconductor detector.
RI source:137CsShaping time:0.5 msec.Meas.time:1000 sec.
RI source:137CsShaping time:0.5 msec.Meas.time:1000 sec.
Cause of high noise level
1.0E-12
1.0E-11
1.0E-10
1.0E-09
1.0E-08
1.0E-07
1.0E-06
0 20 40 60 80 100
I / A
V / V
packagedopengrease
Condition Normal (packaged)
Open(without grease)
Open(with grease)
Leakage current / nA
0.1 10 1
From these results, the humidity and air dusts had a big impact for large leakage current.
photodiode
package
Normal Si-PD
Package opened
Grease coated
Comparison with capacitance of Si-PD under some conditions
7.5
7.7
7.9
8.1
8.3
8.5
8.7
8.9
9.1
9.3
9.5
0 20 40 60 80 100
C /
pF
V / V
packagedopengrease
From these results, the capacitance was not effected in surface condition of the detector.
Signal processingAnalog signal processing
Digital signal processing
High count rateLow noise level
Pre AMP
AD converter
Pulse height
Pulse height
Scheme of output signal
Scheme of output signal
Shaping AMP
Multi channel analyzer
Pulse height
event
No.
Pre AMP Pulse height
event
No.
Summary The SDD system was applied to scintillation light
detector. High energy resolution (compared with conventional
PD) and good linearity were demonstrated. Scintillation light response was achieved.
However noise level should be decrease.
Next plan Reduction of the large noise level.
Carefully handling after open package Digital Signal Processor (DSP) will be applied for signal
collection.
Energy resolution → under 5 %(@661.7 keV) → imaging device
Recommended