Synchrotron studies of carrier physics in a CZT ring detector

Transcription

1 Synchrotron studies of carrier physics in a CZT ring detector VG 1-5 th NDIP conference - Aix-les-Bains June Roland den Hartog R. den Hartog, A. Owens, European Space Agency, ESTEC / SCI-PAT, Noordwijk, The Netherlands A.G. Kozorezov, J.K. Wigmore Department of Physics, Lancaster University, United Kingdom LANCASTER U N I V E R S I T Y Department of Physics V. Gostilo, V. Kondratjev Bruker Baltic, Riga, Latvia A. Webb, E. Welter HASYLAB at DESY, Hamburg, Germany

2 Ring detector VG 2-5 th NDIP conference - Aix-les-Bains June Roland den Hartog small anode: low capacitive noise by Apotovsky et al. for Digirad, 1997 X-rays 1 mm V cathode 5 mm max. ΔV tested: ~150 V V GR «V cat : drift mode V cat «V GR : hemispherical mode

3 VG 3-5 th NDIP conference - Aix-les-Bains June Roland den Hartog Interesting features: Low noise, no hole collection Wide dynamic range from at least 6 to 662 kev Excellent energy resolution up to 0.73% FWHM at 662 kev Charge collection adaptable to detector shape and material Performance

4 Synchrotron characterization VG 4-5 th NDIP conference - Aix-les-Bains June Roland den Hartog Spatial response strongly non-uniform HASYLAB X-1 synchrotron beamline at DESY, Hamburg: kev monochromatic light MOSTAT intensity stabilizer ΔE < 1 ev at 10 kev ΔE < 20 ev at 100 kev Beamsize < 50 x 50 μm 2

5 Model for charge transport VG 5-5 th NDIP conference - Aix-les-Bains June Roland den Hartog a. Solve electrostatic problem: Dirichlet problem for electrostatic potential Φ(r,L) in cylindrical coordinates - renormalize to V cat = 0, so that potential differences are limited to top surface - potential only specified on surface electrodes, not in between 2 2 Φ Φ 1 Φ z r r r Φ( r, z = 0) = 0 Φ( r, z = 0 V V = L) = V ( r ) = V V Φ( r, z) = zv GR anode R1 R 2 GR : r : r : r : r t sinh zt + dt J0( r t ) dx xj 0( xt ) sinh t 0 [ V ( x) V ] b. Integrate electron and hole trajectories in potential, until top or bottom surface is reached r r r a r GR c. Electrons are detected once they traverse near field of anode r r rgr 0 Owens et al., JAP 102 (2007) Kozorezov et al., IEEE NSS/MIC 2007 GR

6 Model for charge transport VG 6-5 th NDIP conference - Aix-les-Bains June Roland den Hartog hemispherical mode holes electrons drift mode dead undetected zone detected detection only in near field undetected

7 Model vs synchrotron scans VG 7-5 th NDIP conference - Aix-les-Bains June Roland den Hartog Model provides at least a qualitative explanation for complex patterns in drift mode E = 20 kev V cat = 46.2 V V cat = 89 V V cat = 100 V V cat = 119 V nice!

8 Model vs synchrotron scans VG 8-5 th NDIP conference - Aix-les-Bains June Roland den Hartog Model correctly predicts: widening of central spot with E widening of ring with E shrinking of ring at highest E absense of central response at lowest E (near field of anode not shown) Extent and width of ring structure not fully explained 10 kev λ = 14 μm 20 kev λ = 84 μm V cat = 100 V 50 kev λ = 164 μm 100 kev λ = 1.05 mm

9 Model vs synchrotron scans VG 9-5 th NDIP conference - Aix-les-Bains June Roland den Hartog 50 μm Ø beam 0.1 mm step size 0.5 mm Ø Collection of e - seems to take place across R1: ring sizes are confirmed by bondwire shadows anode near field extends out to first ring extension into first ring conflicts with data for V cat = 119 V r a = 0.14 μm r a = 0.24 μm ring structure to scale V cat = 100 V Bond wires V cat = 119 V

10 Advantage of monochromatic light VG 10-5 th NDIP conference - Aix-les-Bains June Roland den Hartog Secondary ring + sausage feature show up above low rejection threshold: ring forbidden by topology of field lines in cylindrical model sausage breaks cylindrical symmetry causes ring feature? E = 10 kev E = 20 kev E = 50 kev E = 100 kev low detection threshold high detection threshold

11 Differential spectrometry VG 11-5 th NDIP conference - Aix-les-Bains June Roland den Hartog Dislocation Bolotnikov et al. (2007): Te inclusions found along grain boundary and dislocations no effect on charge transport for Te inclusions 3 μm and densities 3x10 5 cm 3, i.e. fractional enhancement of 3.4x10 5 Hypothesis: features due to Te inclusions along grain boundary Te density enhancement affects charge transport Sub-grain boundary Is Te excess density at level where it can be detected by cross-edge differential spectrometry? Dislocation Te inclusions identified in CZT sample using IR microscopy by Bolotnikov et al. IEEE NSS/MIC 2007

12 VG 12-5 th NDIP conference - Aix-les-Bains June Roland den Hartog A Differential spectrometry B C Method: 8 scans, at two E on either side of Cd and Te edge (resp and ev) Use ratios r meas (E) = ( A - B ) / C to avoid normalization and systematic effects Make a simultaneous fit with a model with 4 parm.: x 1 : depth under surface of feature x 2 : bottom of feature δ : enhancement of Te fraction over 45% η : differential detection efficiency for photons landing in A or B, compared to photons landing in C r fit x = 1 x2 x1 ( E) η exp 1 exp λ ( ) (, ) 0 E λ1 E δ x 1 Cd 0.45 Zn 0.1 Te 0.45 λ 0 (E) x 2 Cd 0.45-δ Zn 0.1 Te 0.45+δ λ 1 (E,δ)

13 Differential spectrometry VG 13-5 th NDIP conference - Aix-les-Bains June Roland den Hartog # free parms.: x 1 [μm] x 2 [μm] η δ χ = Fit with Te enhancement better than without, while other parms are consistent Fitted enhancement level ~30x higher than levels found by Bolotnikov et al. Method appears at least sensitive to Te enhancements at 0.1% level

14 Conclusions Application of synchrotron radiation to detector characterisation extends well beyond FWHM measurements High-resolution surface scans appear a powerful tool for charge transport diagnostics, in particular in combination with model Cross-edge differential spectrometry is capable of in-situ characterization of defects in detector: 3-D shape, composition 0.1% enhancement of Te demonstrated Next steps: Application to TlBr, optimization for effective volume Simulation of line shapes, optimization of resolution VG 14-5 th NDIP conference - Aix-les-Bains June Roland den Hartog