Vacuum-based Photon Detectors (VPD) are still the primary choice of technology for many applications even 90 years after their invention. Their relatively high gain, large area and excellent intrinsic time resolution make PMT, MCP-PMT and HPD the most suitable detectors to equip most Cherenkov-based and Time-of-Flight detectors used in particle identification. These same features are also behind the use of VPD in crystal-based calorimeters like the CMS electromagnetic calorimeter allowing the performance of the PbWO4 crystals to be fully exploited.
The continuous progress in nano-technology and solid-state physics to produce regular microand nano-structured plates using electrochemical, laser and plasma-based techniques and the big progress achieved by chemists in coating such structures with different kinds of material allow one to envisage improvements in the performance of the VPD even more, and at the same time to extend their lifetime and their rate capabilities even in harsh conditions. This will possibly lead to a new generation of VPDs for which a time resolution of a few picoseconds with submicron intrinsic spatial resolution as well as robustness are achievable.
VPDs with such extraordinary intrinsic resolutions are useless unless associated to electronics read-out systems that allows the exploitation of their time and spatial performances simultaneously, and in addition can cope with the high detection rates required in some of the future experiments.
This WP is structured in three tasks, with the goal of developing a new generation of VPDs in partnership with European and international companies, including the development of read-out electronics to fully exploit their performance.
Task 2.1 - New materials, new coatings, longevity and rate capability studies
A microchannel plate (MCP) is an array of miniature electron multipliers oriented parallel to each other. An MCP plate contains millions of hollow tubes of 2 to 25 µm diameter and several hundreds of microns in length. A typical MCP-based photodetector is a vacuum tube equipped with a photocathode, providing the desired sensitivity for a given range of wavelengths, and “collecting” the signals with anodes ranging from simple to complicated geometries.
MCP-based detectors feature excellent timing resolution (<20 ps), excellent granularity, negligible dark-count rate at room temperature, and low sensitivity to magnetic fields. However, current MCP-PMTs suffer from limitations that prevent them from being used in very high photon rate experiments, like for example the long microchannel recharge times, the saturation current, and ageing effects which lead to a strong QE reduction.
Long-term goals:
Develop a new generation of MCPs with high-rate capabilities and long lifetime for use in future HEP experiments.
New techniques aiming to simplify the production process and, therefore, the cost while allowing the realization of large-area MCPs have been initiated by the LAPPD collaboration a few years ago. Plates made of float glass are perforated to create micrometer holes, and Atomic Layer Deposition (ALD) techniques are then used to coat the hole walls with resistive and emissive material to transform the holes into amplification cavities like the standard MCP tubes. ALD techniques were also used by international commercial companies to increase the longevity of the standard MCP providing an important asset for the use of MCP in future experiments.
Reducing the diameter d of the MCP tubes/holes while keeping the same Open Area Ratio (OAR) and the same aspect ratio (L/d) results in reduced Time Transit Spread (TTS) which improves the intrinsic timing of the MCP and provides naturally a better spatial resolution. New technologies using chemical processes like the Aluminium Anodization Oxide (AAO) applied on thin aluminum plates or electrochemical etching techniques applied on Silicon or GaAs or other wafers can produce regular nanometric structures. Applying ALD techniques in a similar way to the one proposed by LAPPD can result in what one may call NanoChannel Plates (NCP).
Objectives:
1. Develop new materials and techniques to prolong the lifetime of a MCP-PMT tube, improving at the same its time rate capabilities
2. Use new techniques with new materials to achieve high aspect ratio with small diameter, to have better gain, time, and spatial resolution.
Description of work:
In collaboration with VPD producers, review state-of-the-art techniques and aim at the production of new materials with the desired performance. The produced samples will be evaluated and characterized in detail by the groups involved in the WP, and further design steps for future improvements will be defined.
Milestones:
- M2.1.1 Report on state-of-the-art technologies to produce electron multipliers with excellent timing and spatial resolutions (M18)
- M2.1.2 Report on state-of-the-art long lifetime and high-rate capability VPDs (M24)
Deliverable:
- D2.1 Prototype production of a new generation of MCP-PMT using innovative techniques (M36)
Task 2.2 - New photocathode materials, structure and high quantum efficiency VPD
The development of photocathode materials has never stopped, with new materials still being discovered and studied. Searching for a combination of several photocathode materials providing high photoelectron yield for a large range of wavelengths is one of the important topics in the photoelectron detection field. Another important topic is the robustness of the photocathodes against returning ions, particularly in the case of MCP-PMT where the photocathode is separated from the MCP. New photocathode structures featuring increased surface area can increase the photoelectron yield but also the addition of new materials with appropriate negative affinity can help reduce the energy gap of the photocathode allowing the photoelectrons to exit more easily.
Long-term goals:
Develop different types of photocathodes (transmissive and reflective) with increased QE and extended spectral sensitivity with long lifetime.
Objectives:
1. Search for new materials with the required characteristics to be used as photocathodes
2. Develop a photocathode on new structures with high granularity to improve QE and position resolution
Description of work:
We propose to study in collaboration with industrial partners the performance of new photocathodes in terms of quantum efficiency as well as their stability under intense radiation environments. We also propose to study new topological configurations of the photocathodes, either in transmission or reflective modes, to assess the impact of these configurations on the VPD performance.
Milestones:
- M2.2 Realization of a reflective photocathode with granular structure of a few microns pitch (M24)
Deliverable:
- D2.2 Production of photocathodes made of different materials using either granular structure or different structure (M36)
Task 2.3 VPD time and spatial resolution performance
Readout electronics developed for HL-LHC has achieved significant progress in the time measurement. ASICs developed for HGCAL of CMS, HGTD for ATLAS, for instance, have reached a precision of a few tens of picoseconds. New TDCs like the picoTDC recently developed at CERN has reached a precision of a few picoseconds, a performance similar to that obtained with the waveform TDC like SAMPIC. Exploiting the existing electronics and adapting it to fully exploit the precision that present and future VPDs can provide is now at reach.
VPDs can also provide a spatial measurement with excellent precision. Combining VPDs with the new Timepix4 chip, for instance, will allow the exploitation of this important VPD feature while still providing an excellent time measurement.
New concepts such as the PICMIC intend to fully exploit both the intrinsic spatial and temporal resolution of the present VPDs. A precision of a few tens of picoseconds and a few microns seems to be within reach thanks to this new readout scheme. The concept could be adapted to reach a time resolution of picosecond and submicron spatial resolution to cope with the intrinsic resolution that the nanostructured VPD could provide.
To summarize, we propose to adapt the existing electronics readout systems for present VDPs in order to fully exploit their intrinsic time resolution and/or their intrinsic spatial resolution. As a next step, we propose to develop new readout electronics to cope with the expected picosecond and submicron resolutions the new generation of VPDs will be able to deliver.
Several techniques are available to exploit both spatial and timing resolution.
Long-term goals:
Development of large area photodetector with combined excellent timing and position resolution
Objectives:
1. Development of a photodetector with full 4D-capability
Description of work:
We will develop read-out systems fulfilling the requirements of excellent time and spatial resolution, and we will work on the integration aspects with dedicated companies to improve compactness.
Milestones:
- M2.3 Design of read-out electronics capable to reach O(10 ps) timing resolution (M24)
Deliverable:
- D2.3 Production of a read-out system demonstrator able to fully exploit timing and spatial resolution of MCPs (M36)