R&D Proposal submitted to INFN - CSN V - July 2013 Ionization micro-channel plates (i-mcp) for fast timing of showers in high rate environments F.Cavallari (2), D.Del Re (2), A.Ghezzi (1), P.Govoni (1), A.Martelli (1), P.Meridiani (2), S.Rahatlou (2), C.Rovelli (2), T.Tabarelli de Fatis (1) (1) INFN and Università di Milano Bicocca (2) INFN Sezione di Roma 1 and Università di Roma La Sapienza Abstract: The evolution of the research at particle colliders calls for the development of very fast and highly radiation resistant calorimeters. We propose an R&D program of a detector element based on micro-channel plates (MCP) to sample the ionizing component of electromagnetic showers. Relativistic particle detection by means of secondary emission of electrons at the MCP surface has long been proposed and is used extensively in ion time-of-flight mass spectrometers. What has not been investigated in depth is their use to detect the ionizing component of showers. The fast time resolution of MCPs exceeds anything that has been previously used in calorimeters, and, if exploited effectively, could aid in the event reconstruction at high luminosities. Recent technological developments make this solution attractive to cope, for example, with the instrumental challenges opened by the upgrade plan of the Large Hadron Collider (LHC). The expected outcome of this R&D program is the design of a radiation-hard module to be embedded in a sampling calorimeter (e.g. as a preshower). This solution would factorize the quest for precision timing from the technological choice of the full calorimeter. 1. Identification of the problem and technical approach The evolution of the research at particle colliders calls for the development of very fast and highly radiation resistant calorimeters. We propose the study of microchannel plate detectors (MCP) as active layer(s) of a sampling calorimeter, to provide extremely fast and granular information on the shower deposits. Recent technological developments make this solution attractive to cope, for example, with the instrumental challenges opened by the upgrade plan of the Large Hadron Collider (LHC). The recent discovery of a Higgs-boson particle drives the LHC program towards precision measurements of its couplings and properties, and the search for processes to clarify the origin of the EWK symmetry breaking mechanism. To achieve these goals, an upgrade of the accelerator is foreseen, leading, in 2022, to peak luminosities of 10 35 cm -2 s -1 and, in ten years, to an integrated luminosity of about 3000 fb -1. To exploit the potential offered by the High-Luminosity LHC (HL-LHC), it will be necessary to replace some components of the current detectors, which, especially in the forward region, are not suitable to operate in radiation and rate conditions 5-10 times harsher than those foreseen at the time of design of the current detectors. The HL-LHC environment is further complicated by the increase of the multiplicity of the events: about 140 interactions per beam crossing, with a spread of approximately 10 cm and 300 ps along the beam axis, are anticipated. The pileup of energy deposits coming from different interactions deteriorates the calorimeter performances in terms of energy measurement and particle identification, as particles are less isolated. In order not to compromise the sensitivity to exclusive channels and the reconstruction of the missing transverse energy (MET), a crucial variable in searches for signatures of processes beyond the standard model, innovative solutions should be identified to mitigate pileup effects. A possible strategy consists in
complementing the transversal segmentation of the calorimeters with an extreme time resolution, which would enable the energy deposits coming from different interaction vertices to be resolved in time. For time resolutions below 50 ps, the effective occupancy of the calorimeter could be similar to the occupancy at the current LHC. In order to meet these goals, we propose the development of a detector element based on MCPs [1] to sample the ionizing component of electromagnetic showers. In an ionization-mcp (imcp) embedded in a sampling calorimeter structure, the avalanche formation would be triggered by secondary emission on the MCP surface hit by charged particles (see Fig.1). Prototypes of imcp, developed in the '90s for time-of-flight (TOF) measurement of minimum ionizing particles (MIP), reached a timing resolution of about 70 ps and efficiency for MIP detection higher than 70% [2]: definitely interesting figures at shower depths where the track multiplicity is high. An alternative design, with a Cherenkov radiator and photocathode upstream of the MCPs, would farther enhance the efficiency to MIPs. Fig.1: Left: proposed usage of an MCP as ionization detector (if embedded in a calorimeter the track multiplicity would be higher); Right: MCP with optical window and photocathode (assembly of a PMT-MCP). The proposed detection option, pioneered in calorimetry already in 1990 [3], is now mature for a detailed study thanks to recent technological developments in the production, on large scale and at reduced cost, of large area (20x20 cm 2 ) MCPs by the LAPPD collaboration [4, 5]. The LAPPD detectors, characterized by good radiation tolerance, position resolution better than 1 mm and fast (~10 ps) intrinsic response, are photomultipliers where the amplification stage (G~10 7 ) is realized by two MCP plates placed downstream an optical window with a photocathode layer. The imcp detector proposed here does not require the photocathode and the glass window, with the advantage of a simpler and more robust structure. The fast time resolution of MCPs exceeds anything that has been previously used in calorimeters, which if exploited effectively, could aid in the event reconstruction at the high luminosities. The proposed R&D consists in the definition, optimization and characterization of a fast, granular and radiation tolerant imcp prototype, to supplement conventional calorimeters with a sampling layer with time resolution suited to resolve shower deposits in high pileup conditions. If high efficiency and resolution could be demonstrated for relatively low multiplicities, the realization of a TOF-preshower could be envisaged. Alternatively, a sampling layer to be embedded in the calorimeter structure will be designed. The first solution would entirely decouple the quest for precision timing from the technological choice of the full calorimeter.
Other areas where this detection technique could find application include beam monitoring in high intensity muon beams where time resolution is needed, and medical and nuclear applications in which a medium to high-energy electron beam is utilized with precision event timing. 2. State of the art The method of detection of particles by means of secondary emission of electrons released at the MCP surface struck by relativistic particles has been discussed as far back as in 1979 [1] and is used extensively in ion time-of-flight mass spectrometers. What has not been investigated, beyond an isolated pioneering work in 1990 [3], is their use to detect the ionizing component of showers inside calorimeters, which is the goal of the current project to time-stamp with high resolution the showers. The usage of MCP is, however, a consolidated technique for the amplification stage of photomultipliers (PMT-MCP) in applications where extreme time resolution is requested. Several configurations of PMT-MCPs are commercially available. Although excellent time resolutions and relatively wide active area are available offthe-shelf (e.g. the PLANACON device from Photonis has an area of 53x53 mm 2 with 8x8 readout pixels and different MCP sizes with pores from 10 to 25 µm diameter; the Hamamatsu R10754X has 22x22 mm 2 with 4x4 readout pixels and a declared TTS of 70 ps), the cost of these devices (unit price around $8000) is unsuited for large scale applications. The cost is mainly driven by the MCP production, based on lead glass to achieve the desired resistivity of the plates. A new generation of fast timing PMT-MCPs is being developed by the Large Area Pico-second Photo-Detectors (LAPPD) collaboration [4], with time resolutions of ~10 ps and space resolutions of ~0.5 mm. These detectors are based on new types of MCP wafers, based on an alveolar structure of borosilicate glass (commercially available) covered, via Atomic Layer Deposition (ALD), by layers with suitable resistivity and low work function. The technology for mass-production of MCP wafers has been developed in collaboration with industry, and naked MCPs, with different pore size and wafer thickness, are available on request at Incom Ltd. The complete LAPPD prototypes consist of two 20 cm by 20 cm MCP layers arranged in a chevron configuration, as shown in Fig.2, mounted in a vacuum case with the optical window, the photocathode and an anode plane segmented in readout strips delay lines. The modules include both the readout electronics and a DAQ interface [5]. The chevron configuration is the same as the one adopted in the two stage PMT-MCPs commercially available from Photonis and Hamamatsu. Fig.2: Layout of a LAPPD prototype (from [4])
The LAPPD detectors are intended to operate as MCP PMTs multiplying the photoelectrons from the photocathode by as much as 10 7 in a chevron (two-plate) structure. In our application we would dispense with the photocathode and the window, and use only the charge multiplication properties of the MCP. This would also relax the vacuum requirements, which in PMT-MCP are driven by the need to preserve the photocathode from ion-feedback. The insertion of foils with high emissivity of secondary electrons will be considered to enhance the efficiency to MIPs if needed. 3. Detailed research plan and goals The expected outcome of the R&D is the design and verification of radiation hard detector based on MCPs as active layer(s) of a sampling calorimeter, to provide extremely fast and granular information on the shower deposits in high rate environments. The project is structured with some intermediate goals needed to achieve the final goal. 1) Preliminary definition of the detector configuration and proof of principle: According to the results reported in [2], MCP configurations suitable to the development of detectors with the performance needed by this project exist. The MIP detection efficiency and the time resolution depend both on the geometry, which affects the probability for a track to hit the MCP surface and the signal amplitude, and on the working point (gain). In the preliminary phase of the R&D, we intend to reproduce those results and to identify suitable configurations by means of exposures of imcp prototypes to cosmic rays, exploiting cosmic ray stands at Milano Bicocca and Roma1. Results will be interpreted with the help of simulations. A full characterization will be carried out upon exposure to particle beams (see below). Prototypes of imcps will be assembled following the schematic of Fig.3, and arranging two (or more) MCP layers inside an optically and vacuum sealed module, with feed-through for the power and read-out of the anodic signal. Appropriate vacuum tight boxes will be prepared to this purpose in the workshop of INFN at Milano Bicocca. Commercially available MCP wafers will be procured from Incom Ltd, and for comparison from other vendors of traditional lead glass MCPs. Prototypes of different pore sizes and aspect ratios (defined as the ratio of the pore diameter and the MCP thickness) will be tested. We expect to achieve a proof of principle of the imcp concept, verify their potential for application in calorimetry and select the best configurations. Fig.3: Schematic of test device with two MCPs (from [3]) In this phase, we also envisage exploiting commercially available PMT-MCPs (Hamamatsu and Photonis). Contacts with the LAPPD collaboration have started to be also able to test some of their prototypes. With these devices, the impc concept can be tested by making the photocathode inactive, upon the application of a small reverse bias compared to the MCP plane. The MIP detection efficiency, and time resolution through the Cherenkov emission in the optical window could also be measured (we anticipate here a MIP efficiency close to 100%). These data will enable us to set a reference with well-established detectors, albeit used in a non-conventional mode.
2) Optimization and characterization: Prototypes of imcps with different thickness and pore dimensions will be exposed to electron beams, in order to define the optimal geometry and to fully characterize the following parameters: 1. Time resolution, efficiency and spread of the signal amplitude in response to single electrons using different geometrical configurations, and for different angle of incidence and bias supply 2. Time resolution, efficiency and stability of the signal amplitude as a function of the sampling depth (i.e. track multiplicity). 3. Rate capability (efficiency and resolution dependence of the local flux) To this purpose, we plan to rely primarily on the Beam Test Facility at Laboratori Nazionali di Frascati, which provides electron beams with energies between 25 MeV and 750 MeV, and intensity variable on 10 orders of magnitude (1 10 10 particles/pulse) with a repetition rate up to 50 Hz. Exposure to high energy electron beams at CERN will be considered for the final prototypes. The study of the response as a function of the sampling depth, through the addition in front of the detector of some radiation lengths of passive material, will measure the evolution of the efficiency and time resolution as a function of the increase of secondary particles multiplicity along the shower development. These data, compared to simulations, will be crucial to define the minimum sampling depth necessary to achieve the desired efficiency and time resolution with high-energy electrons and photons. These measurements are crucial for the design of a TOF-preshower configuration. PMT-MCPs used as MIP detectors through the Cherenkov emission in the optical window, will also be tested for comparison. 3) Definition of the readout schema for a prototype embedded in a calorimeter In parallel, we will define the design and readout schema of a imcp module to be integrated in (one) layer(s) of a sampling calorimeter. To mitigate the pileup effects, a time measurements resolved in the two planar coordinates is needed for each shower hit. A baseline solution would consist in using an anode segmented in pads, as implemented in the PMT-MPCs commercially available from Photonis and Hamamatsu. The pitch of the pads will be determined via simulation, to match the shower size and the expected occupancy. The readout electronics will have to provide the desired time resolution, a baseline analog front-end chip with leading edge discrimination with the desired performance is already existing, developed by G.Pessina et al. at Milano Bicocca [6]. 4) Simulations and data analysis To complement the instrumental developments and on the basis of the data analysis results of the direct measurements campaign, we will develop the response model of the detector for the Monte Carlo (MC) simulations. This simulation will be used to interpret results, and tuned according to the measurements. In addition to be used for the final optimization of imcp configuration for fast timing of showers, the MC simulation could be exploited, for example, to evaluate the energy resolution of a sampling calorimeter fully built of imcp alternated to a passive absorber (which is the original design proposed in [3]).
5) Radiation hardness studies Once the proof of principle is achieved, wafers of MCPs will be exposed at irradiation facilities to integrated dose levels corresponding to those anticipated for the HL-LHC program. The performance of imcps prototypes assembled with these wafers will be also measured at test beams, and compared to those of new prototypes. For the neutron irradiation, we plan to rely on the facilities available both at ENEA Casaccia (TAPIRO) and Laboratori Nazionali di Frascati (Frascati Neutron Generator). For the gamma irradiation we foresee instead the use of the CALLIOPE facility at ENEA Casaccia. 4. Time profile and milestones 06/2014: Preparation work completed: starts data taking in cosmic ray stands 06/2014: Skeleton of DAQ, data format and analysis framework for test beam ready 09/2014: Proof of concept of MIP detection with first prototypes in comic rays stands 09/2014: Prototypes to be exposed and corresponding electronics ready 12/2014: First exposure to electron beam (BTF) completed, preliminary results ready 03/2015: Final results of first test beam campaign available 06/2015: Test beam results incorporated in simulation, and optimization of design parameters (definition of the segmentation and of the sampling depth) started 09/2015: Optimized prototype ready for second test beam exposure 12/2015: Second test beam completed and preliminary results ready 06/2016: Final results and summary ready for submission. 5. Dissemination of the results The results achieved in this collaboration will be made public through: i) Publications on journals: at least one paper with final results, and possibly at least one additional paper with more specific technical aspects; ii) Reports at international conferences: at least two conferences per year with progress reports on the results; iii) Master thesis works: one or two theses per participating institute are anticipated. 6. Anticipated public benefits: scientific and technological impact The development of fast, granular and radiation resistant modules based on ionization-mcp can extend the scope of detectors needed for advances in highenergy physics at colliders. The detector concept is simple and founded on promising experimental facts; the technology for MCP (for photomultipliers) mass production is close to maturity. This proposal represents, therefore, a frontier project and, at the same time, has a good chance of success. The use-case for the LHC upgrade, foreseen in 2022, is clear: the proposed detection technique can be used to reconstruct the time of electromagnetic showers with an extreme time resolution (30 ps). In high pile-up conditions, the matching of neutral deposits (photons) to the respective interaction vertices can play a crucial role for the correct energy reconstruction with particle flow techniques (see for example [7]). In this context, the proof-of-principle of the detectors proposed as TOFpreshower or TOF-layer of a calorimeter could largely decouple the requirements on
the time resolution from those on the calorimetric performance, providing larger design flexibility. A TOF-preshower would moreover offer the possibility to supplement, where convenient, the performance of existing calorimeters, without modifying the calorimeter structure. Beyond the immediate scientific impact which motivates the proposal, the development of this detection technique can open the way to more extreme and alternative solutions such as a sampling, electromagnetic or hadronic, calorimeter with alternating layers of imcp and passive absorber. The high granularity and potential robustness make these detectors, for example, possible candidates for the hadronic digital calorimetry discussed for experiments at a future Linear Collider (CLIC/ILC). Other areas where this detection technique could find application include beam monitoring in high intensity muon beams where time resolution is needed, and medical and nuclear applications in which a medium to high-energy electron beam is utilized with precision event timing. Eventually, although this project is focused on applications in high-energy physics, it will lead to the acquisition of an advanced technology with application prospects also in other fields. For example, important applications of large area MCP photomultipliers are envisaged in medical imaging, for the realization of RX panels or TOF-PET systems (PET scanners with time of flight resolution to suppress accidental coincidences). 7. Existing infrastructures, required materials and costs The inclusive costs of the project are summarized below. - Total costs: 137 k - MCPs: 65 k (MiB) - Front-end electronics: 10 k (MiB) - Mechanics (sealed boxes + TB): 10 k (MiB/Rm1) - Radiation hardness studies: 20 k (Rm1) - Consumables: 10 k (MiB/Rm1) - TDC (shared with Electronic Pool) 2 k (Rm1) - Travel expenses (beam tests): 20 k (MiB/Rm1) We anticipate that about 2/3 of the total expenditures of the project will occur in the first year, to procure the materials to setup the detectors and the test benches. The driving costs of the project are for the procurement of the MCPs wavers and PMT-MCPs to test (1-2 k per wafer for small quantities and ~6 k per PMT-MCP). To achieve the desired performance suitable readout electronics is necessary. For the front-end, we plan to rely on the CLARO-CMOS chip developed at Milano Biocca by G.Pessina and collaborators [6]. It provides an analog output and a logic gate from leading edge discrimination with time precision matched to the goals of this R&D. We envisage the need to procure a few tens of channels, with discrete elements components. As back-end for digitization, both a VME based and CAMAC based DAQ are available in a cosmic ray stand in Milano Bicocca developed for a previous project. TDCs with 8 channels input with time digitization of 25 ps and 100 ps respectively are available, as well as one digital scope with 1 GHz bandwidth. Similar instrumentation is available at Rome1, where a fast multichannel TDC is necessary.
Digitization on the front-end cards will not be pursued within this project. Costs are envisaged for mechanical structures, including the vacuum tight boxes housing the MCP wafers, and the mechanical supports for the cosmic rays and beam test stands. Supplementary equipment for HV distribution, electrodes, connectors and cabling and will also be necessary as consumables. Personnel costs are limited to travel expenses mainly for beam tests at LNF (2014) and CERN (2015). References [1] J.Wiza, Microchannel plate detectors, Nucl. Instr. and Meth. 162 (1979) 587 [2] M.Bondila et al., Nucl. Instr. and Meth. A 478 (2002) 220-224 (and ref. therein) [3] A. Derevshchikov et al., On possibility to make a new type of calorimeter: radiation resistant and fast, IHEP Report, 90-99, Protvino 1990 [4] K. Byrum, Development of large area fast microchannel plate photo-detectors http://psec.uchicago.edu/library/doclib/documents/190 [5] G.S. Varner et al., LAPPD Electronics https://twindico.hep.anl.gov/indico/getfile.py/access?contribid=6&resid=0&materiali d=slides&confid=1201 [6] P. Carniti et al., CLARO-CMOS, a very low power ASIC for fast photon counting with pixellated photodetectors, JINST 7 P11026 (2012); P. Carniti et al., CLARO-CMOS, an ASIC for single photon counting with Ma-PMTs, MCPs and SiPMs, JINST 8 C01029 (2013) [7] CMS Collaboration, Missing Transverse Energy performance of the CMS detector, JINST 6 901001 (2011).