Imaging and Spectroscopy With Cd(Zn)Te Detectors H. Lambropoulos Technological Educational Institute of Sterea Ellada

Outline of the presentation  Properties of C(Z)T and how they are connected to imaging and spectroscopy figures of merit  Methods to take into account the properties of C(Z)T for the imaging and spectroscopic systems  Some comments on interconnection technologies  A brief presentation of the work done in Greece for the development of hybrid pixel detectors.  Some Final Remarks

CdTe binary compound Z ave =50 (48,52), ρ=5.85 g/cm 3 Eg= eV Zn, K shell absorption edge total photoelectric Compton pair production L shell absorption edge K shell absorption edge Cd, Te Rayleigh CdZnTe ternary compound Z ave =48.2, (48,30,52), ρ=5.78 g/cm 3 Eg= eV

Figure of merit for imaging: Detective quantum efficiency gain = light conversion × gain Direct Conversion: Indirect Conversion: (for monoenergetic X-rays) Where (for indirect conversion) (spatial frequency)

Conclusion: Cd(Zn)Te means sharp line spread function + high absorption efficiency => high detective quantum efficiency

Examples not a complete reference …

A good detector for spectroscopy + Holes ++ Cathode Electrons Holes + Anode - - HV bias X=0 X=X 0 (t=0) X=L Electric Field (Homogeneous) d1 d2 Where mean free path λ e/h : Should have: for CCE=1 and low leakage current (either bulk or surface)

Resistivity due to the band room temperature(RT) CdTe bandgap Cd 0.9 Zn 0.1 Te Bandgap high purity single crystals resistivity: Is an upper limit Electrical compensation is needed

τ : carrier lifetime v thermal : thermal velocity N + D : electrically active defect concentration σ : capture cross section Δμ : Fermi level E d : Defect energy level mobility – lifetime product electrons (M. Amman et al., IEEE Trans. Nucl. Sci. 56(2009) 795) : Holes (T. Takahashi and S. Watanabe, IEEE Trans. Nucl. Sci. 48(2001) 950) : Am-241 spectrum low energy tail 3-D map of tellurium inclusions in a cut CdTe wafer: A. Zapettini, (IMEM Parma)

CdTe In doped 75 mm diameter ingot grown at Freiburg M. Fiederle (FMF-Freiburg) CdZnTe growth furnace at Madrid E. Dieguez (UAM-Madrid) Polizon facility for growth in Space (FOTON- Russia) But we all know that the winners in Cd(Zn)Te material are not in this Continent

How to deal with insufficient charge collection COPLANAR GRID FRISCH GRID

Hemispherical detectors Bi-parametric spectra Montemont G. et al.,

The small pixel effect and some complications R. Ballabriga et al., 2016 JINST 11 P01007 Barrett H.H., Eskin J.D., Barber H.B., Phys. Rev. Lett. 1995, 75,

Bumping and interconnect technologies The present Cd(Zn)Te detector Readout circuit Sensor hybrid

Bumping and interconnect technologies the (very near) future

WORK IN GREECE ON C(Z)T HYBRID PIXEL DETECTORS Acknowledgments to: D. Loukas, J. Kazas (NCSR D) K. Potiriadis, I. Kaissas (GAEC) C. Papadimitropoulos (TEISTE), D. Hatzistratis, G. Theodoratos, M. Zervakis, V. Zografos (formerly TEISTE)

P4DI=Photon 4-dimensional Digital Information= hybrid of the COCAE Compton Camera concept real life

Number of channels P4DI_v2F:50 rows x25 col=1250 pixels P4DI_v1 and v28 rows x 8 col =64 pixels Pixel pitch400μm Charge measurement capability P4DI_v1 Up to electrons (36fCb) Up to electrons (7Cb) Time measurement capability0 to 4.5 ms Power Detector leakage current P4DI_v1: +100pA P4DI_v2, v2_F: + 900pA capacitance fF

Images

Gain measurements with pulser and external ADC Gain measurement with pulser and in-pixel ADC Time tag linearity measurement with pulser and external ADC

Am 241 spectra

A high dynamic range x/γ ray hybrid pixel detector But power consumption : XPAD3: 40μW/pixel Medipix 3: 10μW/pixel Photon counting Impressive results Charge integration power consumption < 1μW/pixel Cmos image sensor 60db dynamic range Cmos image sensor 154db dynamic range Medipix 3

Folded multiple captures

Specification Threshold value Target value Number of pixels(128 x 8 )(128 x 256 ) Pixel pitch 100 x 100um frames/sec100 Integration time10ms Readout modesnapshot Num. of captures in 1frame 4 Min. time between captures 1.5ms column clock10MHz row clock10MHz/(8xN) 10MHz/(128x N) Video data rate10MHz Analog output voltage range 0.5V-1.5V Integrating capacitance1pF Dynamic range enhancement method folded multiple captures Dynamic range>100db Power Supply1.8V Power consumption 1.7uW/pixel +34.2uW/col HV bias-500V for Al/CdTe/Pt +500V for In/CdTe/Pt The HDRAPS CMOS ASIC

some thoughts… C(Z)T material needs to be improved further New detector concepts will help but for the moment the winner in molecular imaging and security seems to be the the Scintillator + SiPm For hybridization and CMOS electronics the next step is 3-d integration