Carbon nanotube photocathodes for stationary CT scaners – Nanoscanner

Classical computed tomography scanners incorporate a rotating ring or “gantry” which holds the X-ray source and the X-ray sensors. They exhibit a high resolution but are large, heavy and are characterised by a low throughput. Using multiple X-ray sources, low cost, stationary (gantry free) and efficient scanners could be fabricated. Xintek (American company) proposes to use, for each X-ray tube, a carbon nanotube (CNT) cathode driven by a control circuit. In this case, the cathode is grounded and the anode is biased at high voltage. The anode cooling is then complex. Each electron source can be advantageously a CNT photocathode driven by a laser diode. As the circuit driving the laser power is insulated from the cathode, this cathode can be biased at high voltage and the anode can be grounded. The anode cooling is then facilitated. Moreover, for low/medium power scanners, one can use a transmitting target X-ray window (e.g. a tungsten film deposited on a beryllium window) that delivers wide angle X-ray beams. This allows to increase the analysed volume.

The long term objective of this project is to develop a new generation of optically switchable multiple X-ray source for compact, efficient and low cost CT scanners for medical applications. High in depth resolution imaging implies the use of a large number of X-ray sources (around 50). When one X-ray tube is emitting X-rays, the emission from other tubes should be low. To satisfy the requirements of high resolution CT scanners, an ON/OFF ratio of 1000 is required. Other requirements relates to the ON current (10 mA/mm2) and to the fabrication of photocathodes operating with backside illumination (to ease their integration).

CNT photocathodes based on silicon p-i-n photodiodes exhibit an ON/OFF ratio of 30. In order to meet the ON/OFF ratio requirement (1000), an alternative to p-i-n photodiodes is to use a photoconductor based on a high resistivity low-temperature-grown GaAs (LTG-GaAs) developed by IEMN. This material exhibit a resistivity of around 107 W.cm and a high breakdown voltage of 200 - 300 kV/cm.
The CNT photocathode studied in this project will be an array of individual and vertically oriented multiwall CNTs, each nanotube being associated with a LTG GaAs photoswitch.

Such arrays are already fabricated by different laboratories (including LPICM) on doped silicon substrates. The growth method is plasma enhanced chemical vapour deposition (PECVD) on Ni catalyst dots. The crystalline quality exhibited by the nanotubes is not sufficient to attain the objective of current density. To improve the CNT crystalline quality, LPICM has initiated the study of the growth of aligned CNTs on Fe (instead of Ni) catalyst using a new water vapour based PECVD process and showed that these nanotubes exhibit a significantly higher crystalline quality. Thus arrays of individual CNTs grown on Fe nanodots will be studied.

The workprogram includes one task for management, dissemination and exploitation and another one to study the safety procedures of the photocathode process. Task 2 will deliver an accurate model of the photocathode. Task 3 will define the CNT technology, the photoswitch technology and the method to fabricate/transfer a photoswitch on transparent substrates. Task 4 will demonstrate a photocathode on a GaAs substrate (front side illumination) with an ON/OFF ratio of 1000. Finally Task 5 will show a photocathode operating with back side illumination and delivering an ON current of 10 mA/mm2 and an ON/OFF ratio of 1000.

Thales has already shown that a CNT X-ray tube exhibit a good life time. This project will allow Thales Electron Devices to develop a new generation of optically switchable multiple X-ray source. The first application is the development of low cost, compact and efficient stationary (gantry free) CT scanners. The total market addressed by this new generation of multiple X-ray sources is estimated to be above $250 million per year.