This JRA aims to advance in the R&D studies of novel photon detectors based on Multi-Wall Carbon Nano Tubes (MWCNT) photocathodes coupled to Micro Pattern Gaseous Detectors (MPGD) devices by entirely new approaches for MWCNT fabrication and surface treatment, likely leading to a modulable photodetection bandwidth, and innovative solutions in the MPGD design.

Description of work and role of partners

The final goal of the R&D programme is an innovative gaseous photon detector characterized by both (i) an exotic photocathode and (ii) a novel detector architecture. The development of items (i) and (ii) progress in parallel, while merging them in a single, revolutionary photon counter will be the final step of the whole activity, not included in this JRA programme because a longer time scale is required. The developments (i) and (ii) represent not only important steps towards the final goal: also at intermediate steps, they represent important achievements that can determine progresses in the field of the photon detectors. In fact: (i) CNT- ased photocathode can be coupled to the existing gaseous photon detectors; (ii) novel, better performing gaseous photon detectors can operate with the available CsI photocathode. The R&D programme makes large use of the results being reached within HadronPhysics2 WP17 and represents a step forward thanks to new approaches suggested by the experience gained so far. Moreover, the detectors we are developing, exhibit the interesting feature of achieving the detection of both ionizing particle and photons possible, as required by
specific rare-event experiments.
The response of arrays of densely packed multi-wall nanotubes to single UV photons, both in vacuum and in various gases, as a function of their orientation relative to the substrate as well as the feasibility of fabricating hybrid photocathodes by depositing thin films of caesium, alkali antimony or zinc oxide compounds on a carbon
nanotube substrate are being explored within HadronPhysics2 WP17. Based on these starting results we propose the following innovative steps:

  1. The hydrogenation of the CNTs surface, to be obtained in the same film growing plant by means of a hydrogen plasma. Recently, this technique has been successfully applied on diamond films to obtain a surface with negative electron affinity, showing, in parallel, that hydrogenation have no effects on the graphite component normally present in a diamond film. However, while the photoemission is a well known mechanism in materials with macroscopic size, in nanomaterials, like CNTs, the problem is still open because the mechanism of photoelectron emission is poorly known.
  2. Growing nanotubes on conductive materials. Recently, some tests have been performed using as substrate for the photocathode fabrication, an aluminum film on a silicon base, obtaining good results. Tests have to be continued on this material and also on substrates coated with a titanium nitride film. In particular, the last one is interesting because of its electric conductivity, because it does not present an oxidation phases and it is stable at very high temperatures, like those needed for CNTs fabrication.
  3. A fabrication procedure starting from CNT powder. The high temperature (700-900°C) necessary to grow the CNTs limits the substrate materials that can be used. To overcome this limitation, fabrication tests of photocathodes obtained spanning CNT powder diluted in organic solvents on different substrates will be tested.
  4. The characterization of a gaseous photon detector based on a multilayer structure of Thick GEMs is being performed in the context of the WP17 work package of HadronPhysics2, making use of standard photocathodes.

Effective photon detection, high gain operation and detector robustness are emerging as positive features of this photon detector architecture. Two relevant questions are still open and we propose to overcome them by then following novel approaches in the WP18 FPD work package of HadronPhysics3:

  1. A principle question is related to the level of ion backflow suppression reachable. The studies performed so far indicate ion backflow at the 20% level, while a more effective suppression able to provide one order of magnitude more is required. Electrostatic calculations indicate that, introducing between the Thick GEM layers a dedicated electrode, the suppression can become much more effective. Different electrode implementations will be tested: wires arranged in a plane, metallic grids and double layer PCBs.
  2. The large electric capacitance of a THGEM plane represents an engineering problem when progressing towards large size detectors: the energy stored can damage the front-end components of the readout electronics in case of occasional detector discharges. Due to the robustness of the discharges, the standard schemes often implemented to protect the front-end electronics require discrete electronic components of corresponding characteristics, that can compromise the quality of the pre-amplification stage. These components are large size ones and they make the front-end elements of the electronic readout system growing in size. A scheme including resistive layers at the electrodes where the signals are collected have been recently proposed for the INGRID counter, a compact MPGD. Similar schemes can be implemented in a Thick GEM-based gaseous detector, resulting in a detector self-protected against even extremely violent discharges.

The CNT photocathode studies are performed by the group from INFN-BA, where specific expertise in solid state physics and nanotechnology is present.
The development of the architecture of the Thick GEM-based gaseous photon detector is pursued by the groups from INFN-TS and TUL, where large experience in the field of gaseous photon detectors is available. The groups from ALU-FR and INFN-TO are responsible for the electronics developments; they profit from specific competence in the electronics sector. The validation and characterization of the prototypes is in charge to these four groups.
The coherence of the project towards the final goal, namely a detector making use of CNT photocathode and Thick GEM-based detector, is responsibility of the whole collaboration.

The HadronPhysics3 project is supported by the European Union
under the 7th Framework Capacities Programme in the area of Research Infrastructures (RI).