Extended lifetime MCP-PMTs: Characterisation and lifetime measurements of ALD coated microchannel plates, in a sealed photomultiplier tube
Introduction
With an increasing trend towards higher luminosity and bunch crossing rates for particle physics, the lifetime limitations of MCP based photon detectors have become of particular interest. For example the proposed LHCb upgrade TORCH will require an accumulated charge of 5 C cm−2 extracted from the MCP detectors during the detector life time [1], while a standard MCP detector does well to reach 0.1 C cm−2.
The dominant ageing mechanism of MCP-PMTs is bombardment of the photocathode by ions released during the electron multiplication process inside the MCP pore. These ions are produced by secondary electrons colliding with the MCP pore walls, desorbing gas from the MCP pore surface or ionising residual gas within the detector vacuum. The ions are then accelerated towards the photocathode by the electric field, where they are either adsorbed into the photocathode or damage the molecular structure. The net effect of the ion bombardment is to reduce the photocathode's quantum efficiency (QE).
In a single photon counting MCP-PMT, with two stacked MCPs, the ion production predominantly occurs towards the end of the pores of the second MCP in the stack where the secondary electron flux is highest. As such, in a two plate MCP-PMT detector each MCP is aligned such that the pore bias angles point in opposite directions, so there is no direct path from the ion production site to the photocathode [2], [3]. A more recent development is to deposit a thin barrier film on the MCP input surface, either for the first or second MCP in the stack [4], [5], [6]. The film prevents ions from exiting the MCP pore and hence damaging the photocathode. However, the film – when placed on the first MCP – also blocks some photoelectrons from the photocathode, reducing MCP collection efficiency.
This paper characterises the lifetime improvements offered by an alternative approach, atomic layer deposition (ALD). ALD is a chemical process used to deposit atomic mono-layers on a substrate, with a highly tunable chemistry [7]. , Inc., has developed an ALD process to grow a coating with a higher secondary emission yield (SEY) compared to a typical MCP pore surface. The process also allows a number of different substrates to be used in place of the glass used in MCP fabrication [8]. The principal advantage of a higher SEY is the ability to achieve significantly higher gain at the same operating voltage across a single MCP, reducing secondary electron energy and hence the probability of an ion being produced due to electron collisions. Further to this, it is suspected that the atomic mono-layers deposited by ALD coating could prevent desorption of gaseous contaminants on the MCP surface or in the substrate, by effectively completely sealing the surface [9], [10], [11]. Hence, ALD provides a promising technique for improving MCP-PMT lifetime by reducing ion bombardment of the photocathode.
Two MCP-PMTs were manufactured each with a 10 mm active diameter, two chevron stacked pore MCPs and a Low Noise S20 photocathode [12]. For one tube the MCPs were coated at , using a ALD tool, whilst the other tube was built as standard to provide a control sample for the characterisation. The characterisation performed included detector gain as a function of MCP bias voltage (Section 2), timing performance as a function of photocathode to MCP voltage (Section 3) and finally accelerated ageing with photocathode quantum efficiency monitoring (Section 4).
Section snippets
Gain improvement
To characterise the total detector gain for each MCP-PMT a 500 nm DC light source, calibrated using a silicon photocell to have a power of 7.1×10−12 W cm−2 at a fixed distance from the light source, was used to illuminate the detector with a flat light distribution. The detector output current was then measured using a nano-amperemeter allowing the detector gain (G) to be calculated using where is the photocurrent during illumination in mA, is the current without illumination
Timing
Each MCP-PMTs timing performance was measured with a pulsed 650 nm laser diode with a 40 ps FWHM pulse duration (Photek LPG-650) and 3 ps RMS jitter attenuated to single photon levels. The delay of the detector signal from the laser trigger was then measured using a LeCroy 20 GSamples/s, 5 GHz oscilloscope. To account for the broad pulse height distribution (PHD) of MCP detectors, which would result in significant time walk if a fixed threshold was used to measure the detector's leading edge, a
Lifetime results
Following characterisation, each MCP-PMT underwent accelerated ageing, where a 0.25 cm2 area was illuminated using a DC broadband light source. During the ageing each tube's output photocurrent was recorded, in addition to a photodiode used to independently monitor the input light intensity. The initial gain of the MCP-PMT detectors was set to be 1×106, and the operating voltage was fixed for the duration of the life test. By integrating the collected current over time the total charge extracted
Conclusions
In conclusion ALD coated MCPs offer two distinct advantages for the production of MCP-PMTs; improved gain performance and a major improvement in tube life time due to a higher secondary electron yield and monoatomic layers effectively sealing the MCP surface. Timing performance was found to be unaffected, with a random single photon timing jitter equivalent to an uncoated MCP-PMT, despite the slightly longer transit time for ALD coated channels plates.
Further work is planned to investigate the
Acknowledgements
The authors would like to thank Arradiance for their support and advice.
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