Performance limits of a 55  m pixel CdTe detector G.Pellegrini, M. Lozano, R. Martinez, M. Ullan Centro Nacional de Microelectronica, Barcelona, 08193, Spain M. Chmeissani, M. Maiorino, G. Blanchot, J. Garcia C. Puigdengoles Institut de Física d'Altes Energies, UAB Campus, Bellaterra, Spain

2004 IEEE NSSGiulio Pellegrini Objectives of the research CdTe is one of the most widely studied material in the field of X-ray detection for energies above 10keV, due to its high absorption efficiency. CdTe detectors for imaging applications are usually fabricated in thick substrates and with pixel size smaller than 100μm. However, reducing the pixel size might lead to an increase in the charge shared among neighboring pixels due to the low mobility of the major carriers. The operation of a pixel detector is strongly influenced by the ratio of the pixel size to the thickness of the detector. We present charge sharing results from simulation and experimental data obtained with a specific purpose circuitry using Medipix-II with CdTe detectors.

2004 IEEE NSSGiulio Pellegrini X-ray absorption MammographyGeneral radiography

2004 IEEE NSSGiulio Pellegrini Charge sharing problem Charge sharing depends strongly on the position of the interaction of the incident X-ray. Since detectors are illuminated from the back contact, low energy x-rays will interact “far” from the pixel side. Charges generated in point 1 will be shared between different pixels. Charges generated in point 2 will be collected only in the central pixel. Mean free path* = 80  m for a 20 KeV X-ray in CdTe. 80% of the X-rays interact in the first 120  m *Average distance traveled before the interaction take place

2004 IEEE NSSGiulio Pellegrini CdTe detector 55  m 45  m 15  m Solder bumps CdTe pixel electrodes from ACRORAD

2004 IEEE NSSGiulio Pellegrini MEDIPIX-II chip Chip Specifications 55 µm pixel pitch (256×256) pixels (14.1×14.1 mm active area) 14 DAC settings Very good energy resolution (0.75keV) 1 Mbit data Serial & Parallel read-out bus (input bus is Serial only) Maximum estimated communication clock: 80 MHz Pixel Specifications 55×55 µm area Dynamic range: -1 ÷ bits (8 levels) threshold adjustment Schematic view of a detector pixel bump bonded to Medipix II chip Single photon counting mode

2004 IEEE NSSGiulio Pellegrini X-ray Imaging Image taken with Medipix2 chip with CdTe detector with low temperature bump bonding. Microfocus X-ray tube: 40 keV, 10 µA, 1 sec. Pixel size 55 µm Detector bias –100 V (electron collection)

2004 IEEE NSSGiulio Pellegrini Experimental setup 3×3 pixels matrix at the bottom of MPX2 chip (pixels 120,0 to 122,2) Spectrum 241 Am source Bias voltage :100V Chip analog outputs, got via DEar-MaMa read-out system, were digitised and read through GPIB bus Collected 500 events per bias value Events analyzed: values 3  above noise peak in the central pixel NOTE: the chip threshold level is NOT involved in this operation M. Chmeissani et al., [1] ”, Proc. of the 21st IEEE Instr. and Meas. Tech. Conf., Vol. 1, pp , Como, May 2004, Italy, ISBN

2004 IEEE NSSGiulio Pellegrini Charge sharing measurements The maximum charge collected in the central pixel is 60% at 400V.

2004 IEEE NSSGiulio Pellegrini Software package Electrical simulators: 3D simulator : ISE TCAD software Monte Carlo simulator: Geant4 Parameters : Carrier carrier scattering Recombination High field saturation No k-shell effect No traps added to the model.

2004 IEEE NSSGiulio Pellegrini Simulation results Charge collection efficiency is calculated for pixel 5 generating the charge in different points and at various depths.

2004 IEEE NSSGiulio Pellegrini Integrated charge Minuit fit Charge cloud profileCurrent density profile Bias =100 V x-ray generated in point 1 and at a depth of 100um from the back contact

2004 IEEE NSSGiulio Pellegrini Charge sharing comparison  Simulation results combining DESSIS and Geant4 Monte Carlo simulation.  Experimental result using an Americium source. The detector was biased at –100V. Average relative charge sharing for the 3x3 matrix simulated and measured experimentally.

2004 IEEE NSSGiulio Pellegrini Conclusions The data collected with the pixilated CdTe coupled to Medipix-II chip and the simulated results by DESSIS are in good agreement, thus one can use such a model to optimize the design of the pixilated CdTe detector for photon counting readout ASICS. A 1mm thick CdTe with small pixel pitch will have poor performance when coupled to Photon Counting (PC) ASIC. Either it does not trigger, if the threshold is relatively too high, or many pixels count the same photon if the threshold is very low.

2004 IEEE NSSGiulio Pellegrini CdTe spectroscopy