-- NielsVanBakel - 2015-10-05

MEMBrane

Introduction

We propose a new family of detectors consisting of a stacked set of curved miniature dynodes in vacuum, created through Micro Mechanical Electronic Systems (MEMS) fabrication techniques, placed on top of a CMOS pixel chip [1]. This combination in itself is an efficient single free electron detector with a good 2D spatial resolution (determined by the pixel pitch), and superb time resolution. By capping the system with a traditional photocathode, a highly sensitive Timed Photon Counter TiPC or ‘Tipsy’ can be realised. By capping it with an electron emission membrane ‘e-brane’ a charged particle tracking and timing detector ‘Trixy’ can be realised with a time resolution much better than current particle detectors.

Tipsy will have ps time resolution, excellent spatial resolution, excellent rate capabilities and very low noise. It will have impact on the field of medical imaging (notably PET scanning), optical communication, night-vision equipment and even true 3D image recording. The time resolution of Trixy is orders of magnitude better than planar Si avalanche detectors presently applied in particle physics experiments, opening new horizons for (vertex) tracking, time-of-flight spectrometers, track pattern recognition and trigger detectors. The core innovation of Tipsy and Trixy (i.e. the stacked curved dynodes on top of a CMOS pixel chip) will revolutionise electron detection in solid-state, atomic and molecular physics experiments.

1. The Timed Photon Counter ‘Tipsy’ single soft photon detector

Principle: A soft photon (0.1 Ám < λ < 2 Ám), entering through Tipsy's window, interacts with the photocathode. Due to the photo effect, an electron is emitted into the evacuated space under the window. This electron is accelerated towards the first dynode, put at a positive potential of 150 – 400 V with respect to the photocathode. This dynode is an ultra-thin e.g. silicon-nitride layer with cone-shaped protrusions, shown in fig 1. These cone shapes create an electric field between successive dynodes, which focuses the electrons towards the top of the cones, away from the non-active pillar support areas.

The electrons arrive at the first dynode with an energy of 150 – 400 eV. At the point of impact, secondary electrons are emitted not only from the top of the layer, but also from the bottom, due to the thickness of this _transmission dynode of only ~15 nm. Secondary electrons emitted from the top will fall back; those emitted from the bottom will accelerate towards the next dynode. With a secondary electron yield Y and a number of sequential dynodes N, a gain G = YN is reached. With Y = 4, typical for dynodes in a classical photomultiplier (PMT), and five dynodes, an average charge of 1 k electrons is collected onto the pixels’ input pad, which is sufficient for detection [2]. With seven dynodes, a charge of 16 k electrons causes a potential change of the input pad of one volt and can be considered as a digital signal.

Performance: The response time of the Tipsy detector, determined by the time it takes for the electrons to pass through the gaps between the photocathode, dynodes and pixel chip, is of the order of 50 ps. The _time resolution to detect a single soft photon is mainly determined by the time the electrons take to cross the last gap between the last dynode and the pixel anode, and can be of the order of a few ps. The _spatial resolution of ~10 μm, in both planar directions, is determined by the pixel pitch. The high granularity of the independent pixels allows a _high multiplicity of coincident multi-photon detection while keeping the occupancy low. The maximum event rate of the device can be extremely high because of the short response time of 50 ps. The operation of Tipsy detectors, as opposed to classical PMs, is little affected by a magnetic field, since in Tipsy detectors, Lorentz forces acting on the free moving electrons are orders of magnitude smaller than the electrostatic forces. In addition, the continuously focusing throughout the dynode stack positively contributes to the magnification of the electron signal.

In Tipsy's discrete multi-dynode system the energy of an electron-to-be-multiplied (order of 200 eV) is much higher than the binding energy of electrons in the dynode material (order of 8 eV). In Si-based avalanche photon detectors (SiPMs), this energy is of the order of the band gap energy, and multiplication generates bias current and, consequently, noise. Another drawback of SiPMs is the electron/hole mobility, limiting the effective speed of charge displacement, and therefore the charge signal speed.

The separation of the functionalities of photon absorption/conversion and electron multiplication of Tipsy, and the passive electron multiplication of the thin membranes makes it intrinsically superior in terms of noise, speed (time resolution, signal duration and detector response time), in photon spatial resolution, in detector occupancy, and in radiation hardness. The fundamental advantage of Tipsy over current photon detectors is the fact that its fast (charge) signal is generated by a small displacement of free, accelerated electrons in vacuum. The vacuum electron multiplier is free of bias current and noise. Furthermore, the fine granularity allows a good spatial resolution and a low occupancy even at high counting rates with multiple hits.

The first dynode: The focusing of the electric field above the cone array is of special interest for the first dynode: the efficiency of the Tipsy and Trixy detectors is proportional to the _acceptance of the photoelectrons and emission electrons, respectively. Electrons arriving at the non-active area between two adjacent cones will not be detected. Focusing can be achieved by curving the dynodes. The effect of this is depicted in Figure 1. The focusing is not relevant for the next dynodes where a limited efficiency results in a reduction of the effective secondary electron yield, which can simply be compensated for by a higher secondary electron yield (SEY). This can be achieved by increasing the potential difference between the dynodes, or increasing the number of dynodes.

Tipsy's efficiency to detect photons is determined by the Quantum Efficiency (QE) of the state-of-the-art photocathode (20 - 40 %) while future Si-PMs may achieve higher efficiencies. In this project we will develop theories (i.e. secondary electron generation theories) and practical methods (i.e. surface processing) that may result in the development of novel high QE photocathodes.


Fig 1. The core of the newly proposed detectors: a stack of transmission dynodes in vacuum placed on top of a CMOS pixel chip. By capping the assembly with a classical photocathode or e-brane, a photon detector “Tipsy” or charged particle tracking detector “Trixy” can be realised, respectively. The Tipsy detector, for example, is sensitive for individual (soft) photons, which are converted into photoelectrons in the photocathode and multiplied in the stack of dynodes. The resulting electron avalanche is detected by the circuitry in the individual pixels of the CMOS chip.

Tipsy

Details about samples and chips on the old Tipsy page

Measurements of SEY

Description of samples and data collected can be found HERE

Topic attachments
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PNGpng TIPSY.png manage 13591.3 K 2015-10-12 - 08:30 ConnyHansson  
Topic revision: r6 - 2016-01-11 - VioletaProdanovic
 
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