Speaker
Description
Since the recent success in solving the long-standing aging issues of Microchannel Plate Photomultiplier Tubes (MCP-PMTs) by applying atomic layer deposition (ALD) technology to the MCP pores, these fast and B-field-tolerant devices have become very attractive sensors for future experiments. Given the harsh radiation environment and the placement of the photosensors in magnetic fields of $\sim$1 Tesla, MCP-PMTs were selected as sensors for the DIRC detectors of the PANDA experiment at FAIR. The chosen XP85112-S-BA sensors from PHOTONIS have an active area of 2$\times$2 inches, a grid of 8$\times$8 anode pixels, and MCPs with a pore diameter of 10 $\mu$m. A minimum performance of 16% detective quantum efficiency and 500 kHz/cm$^{2}$ rate capability is requested. To meet the experiment’s lifetime requirement of $\sim$5 C/cm$^{2}$ integrated anode charge over a period of ten years, all MCPs were treated with two ALD coatings of Al$_{2}$O$_{3}$ and MgO to improve their lifetime.
A comprehensive and systematic quality control program was carried out at the University of Erlangen. It includes a wavelength scan of the quantum efficiency (QE) and measurement of the gain curve, as well as scans of the spatial homogeneity of QE and gain. In addition, collection efficiency, time resolution, and rate capability are measured. Using GSI's DiRICH/TRB DAQ system, additional parameters such as dark count rate (DCR), afterpulse probability (AP) and its time-of-flight (TOF) distribution, and crosstalk as a function of the active area are assessed. The large number of MCP-PMTs tested provides insights into production quality and issues and produces a high statistics sample of the various performance parameters.
Although the complex process of ALD coating extends the lifetime of MCP-PMTs, it also introduces undesirable side effects with regard to some key parameters in some of the sensors. A few MCP-PMTs exhibit characteristic peaks in AP-TOF spectra, most likely corresponding to Mg and Al ions, suggesting that many AP ions originate from the ALD layers. Furthermore, a subset of tubes shows a phenomenon referred to as “escalation”, in which massive photon rates are emitted from inside the MCP. In some cases, escalation occurs only in local areas of the sensor. Tubes with higher DCR and AP values tend to enter escalation at lower gains, suggesting a correlation between ALD-induced impurities and this behavior.
Despite the issues observed, the large data set of more than 100 tested sensors allows a detailed analysis of the correlations between some sensor parameters. For example, tubes with lower MCP resistance generally perform better under high-rate conditions and maintain stable gain at photon rates of up to $>$10$^{6}$ photoelectrons/s/cm$^{2}$. However, no direct correlation was found between MCP resistance and the occurrence of escalation.
In this talk, we will present the setups and analysis techniques used to measure the various performance parameters. The results of a systematic evaluation of $>$100 series-produced MCP-PMTs will be discussed and compared, and any problems encountered will be highlighted.
- Supported by BMBF and GSI -
| Speaker Confirmation | Yes |
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