The Bristol & Oxford Chemical Dynamics Group

Development of next generation ultrafast time- and position-sensitive detectors based on CMOS architectures

The Pixel Imaging Mass Spectrometry (PImMS) sensors, developed by the University of Oxford and the Rutherford Appleton Laboratory (RAL), allow the positions and arrival times of ions or other particles to be recorded with 12.5 ns resolution in a spatial-map or velocity-map imaging experiment. A prototype 72x72 pixel PImMS1 sensor is now in widespread use for a variety of applications, including multimass VMI and SMI experiments and neutron imaging (with Dan Pooley at RAL), while a larger 324x324 pixel sensor is now at the testing and debugging stage. In VMI and SMI experiments, the PImMS sensors are currently used to detect the light emitted from the phosphor screen of a conventional MCP/phosphor ion imaging detector. This allows images to be acquired for every ion mass on each time-of-flight cycle, in contrast to the case when using conventional cameras, when images must be acquired in separate experiments for each mass. In addition to speeding up data acquisition rates and eliminating experimental complications arising from experimental drift, the sensors also enable fundamentally new types of experiment to be carried out. For example, we have already shown that the PImMS1 sensor can be used for coincidence imaging in a high-count-rate regime, something that is not possible with the delay-line detectors traditionally used in coincidence measurements. Applying this capability to the detection of multiple fragments from a molecular Coulomb explosion event offers an entirely new approach to determining gas-phase molecular structures and probing gas-phase molecular dynamics.

Subject to securing sufficient supplementary funding, a number of future developments are planned for the PImMS sensors. The detection sensitivity may be improved by coupling the PImMS chip with a microlens array in order to focus incoming light directly onto the collection diodes within the pixel, or by backthinning the sensor chips to improve the area of each pixel over which incident photons may be collected. Backthinning will also allow direct detection of electrons, thereby eliminating the need for a phosphor screen when the sensor is placed directly behind a set of MCPs in vacuum. For experiments in which a phosphor screen and a camera positioned external to the vacuum are desirable, we are also developing ultra-fast phosphors with decay lifetimes much better matched to the timing characteristics of the PImMS sensors than traditional slower P47 phosphors. More ambitiously, we hope to develop a next-generation version of the PImMS sensor chip which incorporates a single-photon avalanche diode (SPAD) into each pixel, yielding single-photon detection sensitivities and sub-nanosecond timing resolution. We have already performed proof-of-concept studies on a single-pixel SPAD-based sensor, demonstrating that by coupling such a sensor with a scintillator or fast phosphor, we can create a direct ion detector suitable for reaction dynamics and mass spectrometry experiments that eliminates the need for MCPs, and therefore the need for high-vacuum conditions. The new 'SPImMS' sensors will surpass all existing cameras in terms of sensitivity and time resolution, and will undoubtedly enable a host of new experimental studies in areas requiring time resolved particle and/or photon imaging.

From left to right: the PImMS1 sensor; multimass VMI data set for the photofragments of N,N-dimethylformamide, a model for the peptide bond; neutron tomography with the PImMS2 sensor (work carried out in collaboration with Daniel Pooley, ISIS).

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