High Resolution CdTe detectors
and Applications to Gamma-ray Imaging
Tadayuki Takahashi
Shin Watanabe, Yuto Ichinohe, Hirokazu Odaka,
Goro Sato, Shin’ichiro Takeda,
Institute of Space and Astronautical Science (ISAS),JAXA&Department of Physics, Univ. of Tokyo
Acknowledgement
ACRORADMHI
IDEAS
Nagoya U.
Gunma U.
JAEA

1. Introduction

Need Good Detectors
Good Energy Resolution similar to Ge (0.2%@662keV)
High Efficiency, above 10 keV upto 1 MeV
Position resolution a few hundred micron
A detector which can be operated at “room temperature”

Situation is same in the fields ofAstronomical Observation(gamma-ray astronomy)
Medical Application
Homeland Security
Monitoring (Radioisotopes)
IEEE NSS-MIC 20112

2. CdTe (CdZnTe) Semiconductor in Space
2002
2003

X-ray Observatory
ASTRO-H (JAXA)
CdTe pad/strip detectors
total area 3457 cm2\readout channels 31,232
2014

Nuclear Astrophysics INTEGRAL (ESA)•16,384 CdTe detectors
in the IBIS instrument•total area 2620 cm2

coded mask

IEEE NSS-MIC 2011

Study of γ-ray Bursts: Swift (NASA)•32,768 CdZnTe detectors
in the BAT instrument•total area 5240 cm23

3. Schottky CdTe diode1. Thin device (0.5 – 2.0 mm)
2. Schottky diode3. Guard ring

Low work function
metal (In or Al)+Schottky barrier

p-CdTeTakahashi et al. 1998
with ACRORAD–• Extremely low leakage current• High bias voltage

Full charge collection(NO TAIL)Best spectra we presented 10 years ago.

0.26 keV(FWHM)@6.4keV(3x3 mm, 1 mmt)
IEEE NSS-MIC 2011

2.1keV (FWHM)
0.32%@662 keV(2x2 mm, 0.5 mmt)
4

4. Even better performance - the recent production-20 ℃Typical Spectra we can obtain, today(2 x 2 mm, 0,5 mm)
Histo500

HistoIn/CdTe/Pt

Temp. 5 ℃
HV 800 V

0.85%400740 eV@14 keV
490 eV-40 ℃

10000204060

Al contact
900

In contact

800
700

1.04 keV@122 keV

300

200

1 pA

600
500

2.0keV (FWHM)
0.3%@662 keV

400780 eV-40 ℃

In/CdTe/Pt

Temp. -40 degree
HV 1400 V

300
20010080100120 140
Energy [keV]

00100

200

300

400500600700Energy [keV]

CdTe manufactured by ACRORAD, 2011
IEEE NSS-MIC 201152.56 x 2.56 cm, 0.75 mm

5. Application to Large Area Pixel Detectors
In/CdTe/Pt Diode2.56 x 2.56 cm, 0.75 mmt
64 pixels (8x8)pixel size 3.2mm x 3.2 mm
Read out by a 1-dim ASIC
5.12 x 5.12 cm detector array

55Na350300

511 keVFWHM
6.6 keV

250
200
150
100

Energy Resolution (FWHM)√ (1.5-2)2 +( E (keV) * 0.0125)2IEEE NSS-MIC 2011

5000100 200 300 400 500 600 700 800

ADCCH66. Application to Fine pitch pixel detector
CdTeASIC4 mm

Good performance
A hybrid
both for imaging & spectroscopy
CdTe/ASIC
270 µm pixels in self-trigger mode
Sato et al. 2010
144 chIEEE TNSNutΦ 2 mm

Composite spectrum of the 144 ch (single hit)
– 20 deg., 500 V
241AmIEEE NSS-MIC 201159.5 keVshadow image

3.24 mm

870 eV
FWHM

3.24mm7

7. CdTe double side strip (3.2cm x 3.2 cm)
3.2 cm

250 micron
pitch strips
for both side

Al-strip (+)(Schottky barrier)2 x 128 = 256 channels(<<128*128=16,384)

Al-electrode side (+HV)
Pt-electrode side (GND)

bad strips(open)[mm]

φ3 mm hole
100 µm width slit
800 µm pitch
φ2 mm hole-20deg

Applied bias: 250 V
ΔE: 1.7-1.9 keV at 60 keV (FWHM)
Low threshold: 5 keV[mm]IEEE NSS-MIC 2011(a)Watanabe et al., 20118(b)8. Now, we have a full set of technologies
Hybridization

Good Material

In/Gold Stud Bump
(Takahashi et al. 2000)

Single Crystal CdTe(Cl) (1,1,1)
CdTe

InGoldACRORAD

Blocking Electrode
In or Alforp-type CdTe

50μm

ASIC

JAXA&MHI

ASICs (1 dim & 2 dim)I-V Curve

TSMC 0.35 µm process

Digital circuit270 µm
Pad for bumpbonding
60 µm

Analog circuit

Takahashi et al. (2000)
IEEE NSS-MIC 2011

with ACRORAD

Sato et al. (2011)99. Efficiencies?
A Solution : Stack
Easy. If we have corresponding
technologies.
In 2000
only withPlanardetectors

133Ba

40 CdTeLayers(total 2 cmt)
22Na

In 2007
0.9 %@511 keV

withImaging
Capability
Four CdTe
Image Layers(total 2mmt)
IEEE NSS-MIC 201110

t al.

10. The next-generation gamma-ray Imager
Stacked Si/CdTe Compton Camera Compton Cameragamma-ray E : Energy deposited
iSiγxi : Position of an interaction

EinComptonscattering(E1 , x1 )

E1E2
CdTe
Imaging 137Cs

IEEE NSS-MIC 2011

2001, 2002
Takahashi et al.
2005
Watanabe et al.
2008, 2011
Takeda et al.

(E2 , x2 )E = E1 + E2 ,scatterer detector
absorber detector
photoelectric
absorptioncos = 1me c2

1E2

1
E1 + E2

By a simple back projection method11⇥11. Si/CdTe Compton Camera - Spectrum Semiconductor is best suited for Compton Camera :High Energy/Position Resolution
1st Prototype★ Si (Z=14): Small momentum of target electrons
Small Doppler effects★ CdTe (Z=48,52) works better as an absorber

Spectrum (511 keV)
511 keV
FWHM 8 keVDoppler
400 micron pitch

Takeda et al. 2008
IEEE NSS-MIC 201112

11. Compton Camera - Advantage Large FOV and good energy resolution
Imaging a moving 137Cs source

Takeda et al. 2008,2011

Large FOV > 160deg (80% at ±80 deg)

Takeda et al. 2008

Good performance for a diffuse source.

Image taken
at 3 cm131I

Gap ~3mm

364 keV, 1.6 MBq
JAXA/Gunma U./JAEA(2008)

IEEE NSS-MIC 201113

13. Si/CdTe Compton Camera - Searching for Hot Spots -133BaIEEE NSS-MIC 2011

137Cs

22Na

14

13. Si/CdTe Compton Camera - Searching for Hot Spots Hide 137Cs

ConcreteBlockIEEE NSS-MIC 2011

Found out 137Cs

PaperCylinder
15

14. Our technology goes to space - ASTRO-H •2014

International X-ray Observatory

ASTRO-H

Hard X-ray (< 80 keV)

XRTTotal Length: 14 m
Mass: ~ 2.6 metric ton

12 m (Focal Length)

Si and CdTe Hybrid Imager ( 5 - 80 keV):
Hard X-ray

Soft X-ray

Size 3cm x 3cmResolution
250 µm pitch
Double Side
Si & CdTe-39d30m

Takahashi et al. 2010-40d00m

17h16m17h14m3 cm17h12m

3220 ×10

Si DSSD

200180FWHM 0.9 keV@14 keV

160Count140120

22000~1.6 keV FWHM
14000
@ 60 keV
16000

241Am

12000
10000

80

8000

60

6000

40

4000

2020000CdTe DSD

20000
18000

241Am

100

histe2_000

10203040
Energy (keV)

50

60001020304050

60

70

Kokubun et al. 2010
IEEE NSS-MIC 20111616. ASTRO-H - Soft Gamma-ray Detector (SGD) Concept:

Narrow FOV Si/CdTe
Compton Camera
to achieve
extremely low
background32 layers of Si (2 cm thick)
8 layers of CdTe (Bottom)
2 layers of CdTe (Side)
CdTe : pixel size 3.2x3.2 mm: thickness 0.75 mm

50 keV - 600 keVCompton Efficiency 10-20 %(more than 100 times
efficient than our prototypes
shown before)
IEEE NSS-MIC 2011

Tajima et al. 201017

17. Summary
1) Space science mission requires cutting edge technology due to its highScientific Requirements
2) We are now able to access all the technologies which are necessary to makethe next-generation CdTe-based gamma-ray imager (ASTRO-H HXI&SGD)
3) Our Si/CdTe Compton Camera can be applied to various fields.
One example is...
4) Still needfurther efforts
to reach the final
goal.

thyroidskull

spineadrenallumbarfemur

genitals

See presentation by Takeda, today (MIC 5-7)
IEEE NSS-MIC 2011

Takeda et al. 201118

CdTe/CdZnTe seem to be the only candidate
at least, at this moment• High Z semiconductor

NDIP08100 keV(ZCd = 48, ZTe = 52), ρ =5.9 g/cm3• Room Temperature Operation

or Cool Environment

511 keVP. Luke (2006)

Takahashi and Watanabe (2000)

T. Takahashi IEEE N.S. Lyon 2001

Solution 1 : High Resolution CdTe Diode
Schottky diode (In/CdTe/Pt) (e.g. Takahashi et al.)p-i-n diode (e.g. Khusainov et al. , Niraula et al.)!
all based on high quality THM-CdTe wafer

Low leakage current

Schottky CdTe diode 2x2mmx0.5mmt
at 5 ℃
Bias 800V (16 kV/cm)

57Co

122 keVHigh bias voltage
With Bias >800 V, charge collection
is complete for a detector with athickness of 0.5 mm
If this thickness (90 % at 40 keV, 30% at 100 keV) is sufficient for thepurpose of the detector, best energy
resolution is obtained from thisapproach.

No TailTakahashi et al. 1999 NIM A 436