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