MYTHEN: Microstrip sYstem for Time rEsolved experimeNts

MYTHEN is a 1-D detector working in single photon counting mode. This allows to satisfy the stringent requirements of synchrotron radiation experiments in terms of low noise, high dynamic range and spatial resolution and additionally it allows to perform time resolve measurements thanks to its large number of independent channels that provide a parallel acquisition and can be read out at high frame rates rate (up to 1 kHz).

The detector has a modular stucture such that even large systems can be built to increase the area coverage. A detector module is shown in the figure. It is composed of 1280 independent channels acquiring in parallel in single photon counting mode, each connected to a strip of the silicon sensor.

The X-rays are absorbed in the silicon sensor, and the charge generated is collected at the electrodes and read out by a custom developed frontend electronics. The strip pitch, giving the spatial resolution of the detector, is 50 μm, the sensor thickness is 320 μm and the strip length is 8 mm. Custom sensors with different geometrical characteristics or materials different than sislicon can also be used to match the application requirements.

Thanks to the single photon counting capability the detector is virtually noiseless and has a dynamic range of up to 24 bits. The fluctuation on the number of detected photons is purely Poisson-like and thus the data quality is maximized also with low statistics. The low noise of the front-end electronics allows the detection of photons of energy down to 5 keV, while the short shaping time of the analog signal permits counting rates of up to 1 MHz/channel. The channels are read out in parallel with an inter-frame dead time of 0.07-0.25 ms. The maximum frame rate of the whole detector is limited by the data transfer rate which can be up to 1 kHz for a single module. Acquisition times down to 100 ns are possible and can be synchronized to users’ experiments using external signals. A small on-board memory can store 4 to 32 frames (depending on the dynamic range) in real time.

After 12 year of operation at the Material Science beamline, MYTHEN is going to be upgraded with a new version.

The new MYTHEN III readout chip features lower noise and threshold dispersion than its predecessor which will allow operation at lower energies and facilitate the selection of the energy also in presence of fluorescence. the shaping time is also much shorter, allowing a count rate capability higher than 1 MHz per channel, which can also be improved by pileup-tracking.

Moreover, each channel contains three independent trimmable comparators with gateable counters. This allows pump-probe experiments with multiple time slots, energy binning or pileup-tracking to increase the count rate capability.

Together with the new chip, the readout system will be upgraded, providing a frame rate which can be higher than 1 MHz when using a 1 bit for a single counter (and is still several kHz wit all 3 counters, 24 bit dynamic range). The readout is fully parallel therefore the frame rate does not slow down for increasing detector size.

The detector will also have a new mechanics which will allow the acquisition of a diffraction pattern in a single shot, without the need to move the detector to cover the gaps between the modules.

The new MYTHEN III detector is under commissioning and will be installed at the MS beamline in the first half of 2020.

MYTHEN was originally designed for powder diffraction measurments at the Material Science Beamline of the SLS. The system installed at the beamline is capable of acquiring 120° (in 2θ) diffraction patterns with sub-sec time resolution. Overall it consists in more than 40'000 channel acquiring independently in parallel and is optimal for time-resolved and dose-critical measurements.

Thanks to its outstanding performance and the calibration procedure developed at the SLS, the data quality is now comparable to that of traditional high-resolution detectors, with the further advantage of very fast data acquisition or, equivalently, very high counting statistics in acquisition times of the order of tens of seconds. MYTHEN is therefore also ideal for analyses of pair distribution functions (PDFs).

In 2012 the MYTHEN Detector has been upgraded by adding a second detector capable of detecting the hard X-rays transmitted by the first one. The use of thicker silicon sensors further increases the efficiency of the system, as shown in the pictures below.

MYTHEN modules are used also for several other type of experiments including energy dispersive spectrometers, beam position monitors and medical imaging.