1. DESCRIPTION OF PIXIRAD  The PIXIRAD Imaging Counter is an INFN-Pisa Spin-off  It works in photon counting  Pulse discrimination with two thresholds  Active area: 30.7×24.8 mm 2  Pixels matrix: 512×476, each one of 55×55 μm 2  CdTe sensor: 650 μm thick  Energy range: 2-100 keV  Count rate > 30 GHz  Frame rate ~ 100 frame/s  Noise free  Acquisition: 2 color reading (2 thresholds, 2 counters) or counting in one counter while reading the other one Characterization of the C-MOS Cd-Te Imager Pixirad for energy discriminated X-ray imaging Characterization of the C-MOS Cd-Te Imager Pixirad for energy discriminated X-ray imaging D. Pacella 1, A. Romano 1, G. Claps 1, F. Causa 1, L. Gabellieri 1 1 Association EURATOM-ENEA, C.R. Frascati, via E. Fermi, 45 - 00044 Frascati, Rome, Italy email: danilo.pacella@enea.it 16 th International Workshop on Radiation Imaging Detectors - Trieste, Italy 22-26 June 2014  The goal of this work is the characterization of the PIXIRAD Imaging Counter to assess it as candidate for high definition energy resolved X-ray imaging for slow control in magnetic fusion plasma experiments  In magnetic plasma experiments, the energy resolved X-ray imaging is appealing because it allows to investigate different regions of the plasma, which emit in different X-ray spectral bands depending on electron temperature and impurity content  X-ray energy resolved imaging could allow better performances in radiography and tomography in many domains: bio-science, medical imaging, material scienceINTRODUCTION [1] http://pixirad.pi.infn.it [2] D. Pacella, et al., Self Consistent Calibration of Detector and Sources for Hard and Soft X-Ray Diagnostics, Modern Instrumentation (2014). [3] D. Pacella, et al., Polycapillary optics for soft X-ray imaging and tomography, Il Nuovo Cimento, Vol. 34 C, N. 4, 513-520 (Luglio-Agosto 2011). [4] A. Romano, et al., Characterization of a 2D soft X-ray tomography camera with discrimination in energy band, Rev. Sci. Instrum, Vol. 81, 10E523 (2010).REFERENCES CONCLUSIONS  Energy resolution is good up to 30 keV and then progressively degrades due to the cluster size higher than one  Different thresholds have been defined to describe the effects in term of energy discrimination  A “smooth energy discrimination” in bands has been demonstrated  Energy discriminated imaging has been shown, revealing as an example “hidden” structures buried under a background  This technique could improve significantly the imaging capabilities for radiography and tomography, in many fields, from bio-sciences to imaging and material science SMOOTH ENERGY BAND DISCRIMINATION ENERGY RESOLUTION  Since the pulse amplitude distribution is pretty wide, there is no correspondence one to one between threshold and energy  Th0(E) is defined as the threshold where the pulse amplitude distribution, corresponding to an energy E, is completely cut off  Th0(E) is defined as the threshold where the pulse amplitude distribution, corresponding to an energy E, is cut of 50% ENERGY DISCRIMINATED IMAGING Fig. 1 SETUP  Two absolutely calibrated X-ray sources (Moxtek 50 kV Bullet and Oxford Instruments SB-80-1M) were utilized  SXR line transitions were generated by fluorescence on different samples (Ca, Fe, Cu, Br, Mo, Ag, I, Ta) and by means of BaCs radioactive source  The spectra were measured and optimized with an SDD (XGLab) up to 30 keV or with a CdTe spectroscopic detector (Ampetek X-123CdTe ) MF 35 kV – 100 μA, THR = 350  The energy resolution is coupled to the imaging properties  Images at different energy bands can be retrieved thanks to the energy discrimination MF 35 kV – 100 μA and Moxtek 15 kV – 100 μA THR = 350 MF 35 kV – 100 µA and Moxtek 15 kV - 100µA THR = 2600 corresponding to TH0 (12 keV)  When an X-ray uniform background (100 times more intense), produced by another tube at 15 kV (see spectrum in the Fig. 9), is superimposed to the previous one, it covers completely the image produced by the lens  Once we put the threshold at 2600, corresponding to an energy of 12 keV (TH50), the background is completely cut off and the image of the lens appear again (Fig. 12), only slightly less intense (counts from 20 to 4) but with the same contrast (=5)  This is possible thanks to the energy discrimination of the detector  An image of a Microfocus X-ray tube, in the range 12 -35 keV (see spectrum in Fig. 8) is produced on the detector by means of a polycapillary lens (Fig.10). On the right of the Fig. 10 is the intensity along the line shown in the X-ray picture  Counts across the modulations go from 34 to 7 (contrast =5) MF spectrum 35 kV-100 μA Moxtek spectrum 15 kV-100 μA  The threshold is defined as the number of electrons collected on the pixel (and then transformed in a pulse); it goes from 50 to 5000 electrons, while the dynamic range arrives up to about 10000  Our scans start from 350 electrons in order to cut completely the noise off (Fig. 3)  The scan of Ta starts from a 2800 threshold because the low energy side of the spectrum could not be cut off  Scans are normalized to the value corresponding to the minimum threshold (350) Fluorescence lines:  Ca 3.7 keV  Fe 6.4 keV  Cu 8.0 keV  Br 11.9 keV  Mo 17.4 keV  Ag 22.1 keV  I 28.6 keV  Ta 57.5 keV Radioactive source:  BaCs 81 keV Fig. 3 Fig. 4 Fig. 5 Microfocus X-ray source Polycapillary lens PIXIRAD Moxtek X-ray source Fig. 7 Fig. 8 Fig. 9 Fig. 11 Fig. 12 Fig. 2 PIXIRAD SAMPLE X-ray source  Thresholds vs Energy are linear up to about 30 keV, revealing a good energy resolution  Beyond this value non linearity and then saturation are due to the cluster size that becomes larger than one because the energy is no more released in one single pixel  Discrimination of Fe (6.4 keV) and Cu (8 keV) lines (red curve) is smoother than discrimination of Cu (8 keV) and Mo (17.3 keV) lines (blue curve)  Energy discrimination in bands is possible, in a “smooth way”  With TH0(E) we can cut all the energies below E off, but the counts corresponding to energies higher than E are reduced, less and less as E increases  With TH50(E) we can cut the counts, corresponding to the energy E of 50%  As an example, pulse amplitude distributions are simulated as gaussians  Dotted line represents the threshold at 8 keV  Different lines are cut off:  TH0(E)= -2.77×10 3 + 5.23×10 3 log (E)  TH50(E)= -0.64E 2 + 1.14×10 2 E+1.03×10 2 3.7 keV 10.5 % 6 keV 27.6 % 8 keV 50.8 % 12 kev 89.6 % 17 keV 99.9 % Fig. 6 Fig. 10