Applied Physics

Mokhtar Chmeissani & Thorsten Lux

The focus of the applied physics research at IFAE is to develop sensor technologies with applications in medical imaging, high-energy physics and other scientific or industrial fields by exploting the valuable knowledge available at IFAE and fostering collaborations with other research centres in Catalonia like the Centro Nacional de Microelectronica (CNM), medical centers like Hospital Parc Taulí, or companies like Multiscan Technologies.


The medical imaging group focused its activities in 2017 on 3 topics: Commissioning and testing the VIP PET ring, the construction of the 6cm x 6cm CdTe pixel camera for the 3D-biopsy project (RIS3CAT), and the THEIA project for retinal prosthesis. Beside of these 3 projects, several other instrumentation projects are ongoing among them are a project related to the MediPix chip and the development of Pevroskites for X-ray detection.

VIP: Vortex imaging PET

The VIP team has managed to optimize the cooling system of the VIP PET ring, which has 72000 voxel channels of CdTe, and achieved a working temperature at 20 oC. Using 22Na source the measured energy resolution for events undergone photoelectric absorption is 2.2% FWHM at 511keV. For events that undergone Compton scatter, the measured energy resolution is 3.2% FWHM at 511 keV. This measurement was carried out at bias -250 V/mm. Future measurement will be carried out at bias of -500 V/mm and we expect to reach the target energy resolution of 1.6% FWHM at 511 keV. This excellent energy resolution combined with high granularity detector open the way for the development of large field of view Compton Gamma Camera
Figure 1: shows one VIP module detector which has in total the equivalent of 2cm x 2cm x 2cm of pixelated CdTe detector mounted on a stack of 10 layers, equivalent to 4000 channels has the dimension of 1mm x 1mm x 2mm.

3D Tomosynthesis breast biopsy system

This RIS3CAT project aims to develop a novel 3D Tomosynthesis breast biopsy system in collaboration with IFAE, Centro Nacional de Microelectronica (CNM-IMB), Hospital Parc Tauli, IDNEO Technologies S.L., and VENTURA Medical Technologies. In 2017 IFAE team concluded the readout system at the levels of hardware, firmware, and Graphic User Interface (GUI), for single pixel sensor as one can see in figure 2. IFAE team has also studied different schemes of cooling the sensors to control the temperature of the temperature for the whole camera and hence to stabilize its performance. Peltier cooling seems the best option for being compact and with sufficient cooling capacity. In 2018 IFAE will develop the full camera with 16 sensors, the firmware and the associated GUI.
Figure 2: shows the hardware part of the DAQ system for single pixel CdTe sensor, the kernel element for the breast biopys camera, which consists of and array of 4 x 4 single pixel CdTe sensors.

THEIA project

THEIA (Toward the implementation of a multi-electrodes array for retinal prosthesis ) is an Ignite BIST (Barcelona Institute for Science and Technology) project, started in March 2017 and it is carried out by IFAE, ICFO, ICN2, and Barraquer Ophthalmology Center. It is a multidisciplinary R&D project (Instrumentation, Graphene, Optics, and Ophthalmology) aims to develop a novel retinal implant based on graphene electrodes for better stimulus of the retinal ganglion cells. IFAE has developed a current pulse source with two polarities and the amplitude and the width of the pulse can be programmed by the user. In figure 3 one can see the electrical scheme of a Howland current generator, the fabricated printed circuit board and the measured output on the oscilloscope matches the design specifications.
Figure 3: shows electrical scheme of a Howland current generator, the fabricated printed circuit board and the measured output on the oscilloscope matches the design specifications.

The ZPRO project

Perovskites have appeared recently as a promising material in the field of solar-cell sensors due to its large light-absorption capability, its potentially inexpensive production cost, its semiconductor-like band-gap and its wide coating possibilities through techniques as solution processing. In the field of solar cells the efficiency has increased enormously in only two years showing the outstanding possibilities of this material. Even though, the efficiency measurements have been probed to be non-trivial due to hysteresis processes, revealing the complex behavior of this new material. As a consequence of its impressive performance as visible-light harvesters, people have started to investigate the possibility of using perovskite materials for X-ray and gamma ray detection. Improving the properties of the current X-ray detectors may allow to reduce the radiation risks during routine medical inspections as well as to decrease production costs for a large variety of industrial technologies in the fields of security, defense, radio astronomy, spectroscopy and research among many others.

The devices were fabricated at ICIQ using FTO (Fluorine doped Tin Oxide) coated glasses (8Ω/square). Dense titanium dioxide layer was deposited by spin coating over the previously cleaned FTO. Perovskite precursor solution was filtered and deposited by spin coating obtaining a 350 nm layer over the dense TiO2 layer. The HTM layers (spiro-OMeTAD or TAE-1) were filtered and deposited again in inert atmosphere by spin-coating methods onto the perovskite layer and similar HTM thickness was obtained (~100 nm). Finally, 80 nm of gold was deposited by thermal evaporation in an ultra-high vacuum chamber. Figure 4 shows the cross-section image of a diode using the perovskite material. The quantum efficiency was measured after the showing constant performance between 350nm and 770nm (Fig.5).

Figure 4: Cross-section images of a diode using the perovskite material sandwiched between a layer of dense TiO2 and the organic semiconductor spiro-OmeTAD.
Figure 5: The EQE spectrum (red and filled symbols) of a perovskite device and the integrated photocurrent vs the solar spectrum (black line) to estimate the photocurrent expected from the diode.
Figure 6: (Left) Perovskite sensor response under pulsed X-ray illumination. (Right) Response vs X-ray tube intensity.

The diodes in Figure 4 where tested under the X-ray illumination measuring the voltage drop in a RLOAD=100MΩ resistance using a NI-USB 6289 device for the data acquisition (see Fig 6). Figure 6a illustrates the photo-response of the perovskite under an on and off illumination while Figure 6b depicts the correlation between the photo-current generated in the material and the intensity of the X-ray source. Figure 3 proves the detection of X-Rays with an extra-thin layer -500nm- of perovskite. This is the first time X-Rays response was stablished in Spain opening the road for new research.

The IGINITE program from the Barcelona Institute of Science and Technology (BIST) funded this research.

MediPix 4 project

The Medipix collaboration, born at CERN in the 1990s, created an ASIC that reads out in a virtually noise-free manner the signals of X-rays, gammas and other particles, detected in a pixelated semiconductor sensor. Medipix accumulates hit counts in each pixel; in parallel, the collaboration created another family of devices, called TimePix, to record particle arrival time and/or the energy deposited per pixel. Over the years, both Medipix and Timepix chips evolved into more advanced versions that incorporated new features in order to fulfil demands from laboratories and commercial partners. IFAE played a significant role in developing readout circuitry for these chips. Medipix and TimePix devices are being used in many of applications such as space dosimetry, synchrotron light imaging, material analysis, and spectroscopic X-ray imaging, to mention just a few. In order to satisfy the requirements of newly foreseen applications, the Medipix collaboration started in 2016 on the next family of chips, Medipix 4 and Timepix 4. These devices, in 65 nm technology and using TSVs, will include the novelties of the third chip generation while offering higher count rates and finer time resolution, with more than one possible pixel format. IFAE joined the Medipix4 collaboration in 2017 and is participating in the design of the Timepix4 chip.

[1] Marin-Beloqui, Jose Manuel; Lanzetta, Luis; Palomares, Emilio; ,Decreasing charge losses in perovskite solar cells through mp-TiO2/MAPI interface engineering,Chemistry of Materials,28,1,207-213,2016 [2] Ei, Haotong et al., Nature Photonics, 10, 333-339, 2016