Patterned anodes with sub-millimeter spatial resolution for large-area MCP-based photodetector systems

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Abstract

Micro-channel-plate-based photo-detectors are unique in being capable of covering very large areas such as those required in elementary particle and nuclear physics, while providing sub-millimeter space resolution, time resolutions of less than 10 picoseconds for charged particles, and time resolutions of 30 psec–50 psec for single photons. In such systems the electronic channel count is a major cost driver. Incorporating a capacitively-coupled anode allows the use of external pickup electrodes with patterns of individual channels optimized for occupancy, rate, and time/space resolution. The signal pickup antenna can be economically implemented as a printed circuit card with a 2-dimensional array of pads for high-occupancy/high-rate applications such as in particle colliders and medical imaging, or a 1-dimensional array of strips for a lower channel count in low-occupancy/low-rate applications such as large neutrino detectors. Here we present pad patterns that enhance signal-sharing between pads to lower the channel count per unit area in large-area systems by factors up to 4, while maintaining spatial resolutions of approximately 100 to 200μm for charged particles and 400 to 1000μm for single photons. Patterns that use multiple signal layers in the signal-pickup board can lower the channel count even further, moving the scaling behavior in the number of pads versus total area from quadratic to linear.

Introduction

The development of large-area micro-channel-plate-based photodetectors (MCP-PMTs) such as the LAPPDTM  [1], [2] with sub-mm space resolution and time resolutions of <10 psec for charged particles [2], [3], [4], [5] and <3050 psec for single photons [6] enables 3-dimensional imaging of charged particle tracks in transparent media [7], [8].

In particle and nuclear physics there are large experiments that require many square meters of photosensitive coverage. Examples include the current JUNO neutrino experiment with 20,000 50.8 cm-diameter phototubes [9], the 2009 commercial proposal for a 3-year production of 100,000 MCP-based LAPPDTM photodetectors for the proposed DUSEL neutrino detector [2], and the current Theia proposal employing 10,000 LAPPDs [10]. In medical imaging, a single low-dose whole body TOF-PET scanner based on LAPPDs would require 5m2–10 m2 [11], [12], [13], [14].

For such large detector systems and high-volume medical facilities, economies in the number of photodetectors required and the electronic channel count are essential. The photodetector count can be reduced by factors greater than two by reconstruction of reflected photons [7], [8]. Here we address the reduction of the electronic channel count per individual photodetector by similar factors using enhanced charge sharing among anode readout pads.

The planar geometry of MCP-PMTs allows capacitive coupling of the signal induced on the anode plane to a plane of signal-pickup electrodes external to the detector vacuum package. This has been extensively explored using a dielectric anode substrate [15], [16], [17], [18], [19], [20], [21], [22], [23]. The recently-developed fast rise times and higher gains inherent in ALD-coated MCP-PMT signals [1], [24], [25], [26], [27], [28] alternatively allow the use of a metal internal anode, with the resistance of the thin metal layer acting as a high-pass RC filter for signals transmitted through the metal layer and the vacuum package base [29]. The external signal-pickup geometry, which is the focus of this paper, can be implemented as printed circuit boards, a widely-available and economical technology.

The geometry of anode readout patterns has also been well-explored in the context of other large-area technologies [30], [31]. Here we focus on MCP-PMT anode patterns for the charge clouds produced by single photons and by charged particles traversing the photodetector entrance window.

The organization of the paper is as follows. Section 2 discusses the shape of the image of the charge produced by single photons and Cherenkov emission in the entrance window. The calculation of spatial resolution is presented in Section 3 as a function of the ratio of pad pitch to charge cloud diameter. Results from the simulation of a high-sharing pattern are compared to those from a pattern of square pads in Section 4. Section 5 introduces a method for lower-occupancy applications that uses connections on multiple layers of the signal pickup board to change the scaling behavior of channel count versus area from quadratic to linear. The details of the discrete simulation used in calculating charge sharing among pads are presented in Appendix.

Section snippets

The image of the charge distribution from signals in MCP-based photomultipliers

Signals induced on a segmented anode, or equivalently a capacitively coupled signal pickup board [29], can be analyzed to constrain the position of the incident particle. Here we consider the case of an array of pads with a regular pitch. Knowledge of the image shape at the anode plane and modeling of the sharing between pads yield a measurement of the position of the image at much higher resolution than the pad size.

We consider two cases, the image generated by a single photon incident on the

Sub-millimeter position reconstruction using patterned anodes

Capacitive coupling of the anode plane enables the use of printed circuit boards as the external signal-pickup component [29]. These may be designed in complex patterns optimized for specific applications using widely-available computer programs. Because the pickup boards are external to and electrically isolated from the photomultiplier, they can be replaced without changes to the detector module. Printed circuit boards are inexpensive and widely available with fast turnaround which enables

Anode pattern simulation results

We apply the algorithm of Appendix to two patterns: a regular pattern of square pads and a pattern of sinusoidal pads which has reduced channel count per unit area and enhanced signal sharing. To allow the results to be used independent of the details of the image formation (Section 2.2), for a given pattern the spatial resolutions are calculated as a function of the size of the pads via a scale factor, L, defined as the ratio of the pad-to-pad pitch to the diameter of the image of the charge

Distributed pads using pickup internal layers

There are applications that require large-area photo-coverage but have low occupancies, and for which time resolutions less than 100 psec are adequate [41]. These applications are natural for RF strip-line readouts [6], [33], [42], which are 1-dimensional and for which the channel count scales linearly with area rather than quadratically.

However, the signal pickup board can be economically and quickly implemented as a multi-layer printed-circuit (PC) card, allowing multiple internal signal and

Summary

The development of large-area MCP-PMT photodetectors has opened the possibility of applications with photocoverage measured in tens or hundreds of square meters with sub-millimeter spatial resolutions and time resolutions measured in tens of picoseconds. For high-rate applications, such as medical imaging and high-energy particle colliders, a highly-segmented readout is required. Thus an anode geometry consisting of pads is preferred over a strip geometry with a lower channel count.

CRediT authorship contribution statement

Jinseo Park: Conceptualization, Methodology, Software, Validation, Formal analysis, Data curation, Writing - original draft, Writing - review & editing, Visualization. Fangjian Wu: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Data curation, Writing - original draft, Writing - review & editing, Visualization. Evan Angelico: Conceptualization, Methodology, Data curation, Writing - original draft, Writing - review & editing, Supervision. Henry J. Frisch:

Declaration of Competing Interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Authors Angelico, Spieglan, and Frisch are Inventors of intellectual property on LAPPD technology held or applied for through the University of Chicago.

Acknowledgments

This work was supported by the High Energy Division of the Department of Energy, United States of America through awards DE-SC-0008172 and DE-SC-0020078. E. Angelico gratefully acknowledges funding by the DOE Office of Graduate Student Research (SCGSR) program, United States of America , managed by ORAU under contract number DE-SC0014664. All opinions expressed in this paper are the authors’ and do not necessarily reflect the policies and view of DOE, ORAU, or ORISE. J. Park and F. Wu thank

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