## 3 Energy-Sensitive X-ray and Gamma-ray Imaging in Wide FoV
### Spectral-Sensitive Imaging, Energy Resolution  
The energy-sensitive imaging response was tested in terms of energy range and collimator transmission efficiency. Tests were performed with laboratory X-ray RTG units including measurements in vacuum chambers. The experimental set-up in the air for detection and imaging of collimated X-ray and field-of-view (FoV) measurement is show in Figure 6. A micro-focus X-ray tube with wolfram anode was used as the X-ray source, which provides a broad spectra of tunable ranges (set by the X-ray unit voltage).  
Prior to operation and radiation imaging tests, the pixel detector was equalized including per-pixel response threshold and energy calibration. Timepix is energy calibrated at the per-pixel level with the use of discrete-energy X rays and low-energy gamma rays [31]. We investigated the dependence on voltage of the Wolfram X-ray source on the images produced by the detector coupled to the optical collimator in front. Results for the examined range from 20 kV to 120 kV are given in Figure 7. The specific collimator segments described above are resolved and were characterized. Moreover, the influence of radiation energy change was tested. Verification of transmission suppression of selected metal optical element, of customized design, was also performed for the X-ray source of high voltage (120 kV).  
The energy of the X rays and gamma rays detected is measured by the spectral response of the Timepix detector. Energy spectra can be produced for the entire detector region and for the separate collimator/optics regions. The spectral sensitivity and energy resolution of the device are given by the spectral response of the pixel detector. For X rays and gamma rays the resolution for the Timepix detector with a CdTe sensor is at the level between 4% and 15% for the high (>300 keV) and low (<100 keV) energy range, respectively. The energy resolution depends on the energy as shown in Figure 8. Experimental values of the full width at half maximum were measured for selected gamma rays of discrete energy from calibrating sources (\({}^{241}Am\), \({}^{152}Eu\), \({}^{133}Ba\), \({}^{137}Cs\), \({}^{60}Co\)).  
Figure 6: Experimental set up for detection and imaging of collimated X-ray and FOV measurement. The X-ray source is a micro-focus unit at Advacam. The detector and optical-collimated device are mounted on movable stepper motor.  
Figure 8: Energy resolution for spectrometry of X rays and gamma rays with a Timepix detector with a 2000 \(\upmu\)m thick CdTe sensor. Absolute values of the full width at half maximum (in black—see left vertical axis) are fitted by polynom function (given in inset). The FWHM is included in relative values (in blue)—see right vertical axis.  
Figure 7: Laboratory test of detection and imaging response of collimated X-rays by the developed payload. X-ray images were acquired in counting mode by the Timepix detector with a 2000 \(\upmu\)m CdTe sensor and correspond to the entire pixel detector matrix (14 mm \(\times\) 14 mm). The X-ray source consisted of an X-ray RTG unit with wolfram anode operated in the range 20 kV to 120 kV. The X-ray images reveal the response to the specific optical-collimating regions. The per-pixel intensity (event count rate) is displayed in color.