# XIDER: a novel X-ray detector for the next generation of high-energy synchrotron radiation sources

M Williams, P Busca, M Collonge, P Fajardo, P Fischer, T Martin, M

Ritzert, M Ruat, D Schimansky

► To cite this version:

M Williams, P Busca, M Collonge, P Fajardo, P Fischer, et al. XIDER: a novel X-ray detector for the next generation of high-energy synchrotron radiation sources. Journal of Physics: Conference Series, 2022, 2380 (1), pp.012091. 10.1088/1742-6596/2380/1/012091. http://dx.doi.org/10.1088/1742-6596/2380/1/012091

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# XIDER: a novel X-ray detector for the next generation of high-energy synchrotron radiation sources

To cite this article: M Williams et al 2022 *J. Phys.: Conf. Ser.* **2380** 012091

View the [article online](#) for updates and enhancements.

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# XIDER: a novel X-ray detector for the next generation of high-energy synchrotron radiation sources

M Williams¹, P Busca¹, M Collonge¹,², P Fajardo¹, P Fischer², T Martin¹, M Ritzert², M Ruat¹, and D Schimansky²

¹ European Synchrotron Radiation Facility, 71 Avenue des Martyrs, 38000 Grenoble, France

² Institut für Technische Informatik der Universität Heidelberg, 69120 Heidelberg, Germany

E-mail: morag-jean.williams@esrf.fr

**Abstract.** Next-generation sources of synchrotron radiation pose significant challenges for 2D pixelated X-ray detectors, such as at the ESRF Extremely Brilliant Source (EBS), the first fourth-generation high-energy synchrotron facility. In particular, scattering and diffraction experiments require fast detectors with a high dynamic range, from single photon sensitivity to pile-up conditions under very high photon fluxes. Furthermore, in the case of high-energy applications, the high-Z sensor materials needed for efficient photon detection introduce other difficulties. Leakage current, bias- and flux-induced polarisation, and afterglow all must be carefully managed for the detector system to reach the required specifications. The XIDER project aims to fulfil the needs of the above-mentioned applications by implementing a novel incremental digital integration readout scheme. XIDER detectors seek to operate efficiently under the high-flux EBS beam of up to 100 keV photons, with a time resolution that can cope with near-continuous and pulsed beams. Simultaneously, non-constant leakage current contributions can be removed for noise-free single photon detection, resulting in a very high dynamic range. This contribution presents the recent developments of the XIDER project, including the first characterisation measurements with cadmium telluride sensors.

## 1. Introduction

The upgraded Extremely Brilliant Source (EBS) storage ring makes the European Synchrotron Radiation Facility (ESRF) the world's first fourth-generation high-energy synchrotron facility. It provides highly brilliant and coherent beams to scientific users, opening new opportunities in many fields of research [1]. Simultaneously, the EBS also poses a challenge for X-ray detectors to efficiently detect these high-energy, high-flux X-ray beams. In particular, scattering and diffraction experiments require fast pixelated 2D detectors with a dynamic range from single photon sensitivity to high photon fluxes. Photon counting systems currently used for these experiments at storage ring facilities are limited to $\le O(Mph \text{pixel}^{-1} \text{s}^{-1})$ intrinsically due to pile-up effects [2]. Specialised charge-integrating detectors [3–6] have been developed for low-energy diffraction and scattering experiments at free-electron laser (FEL) facilities.

These detectors can cope with extreme photon fluxes, typically using programmable or automatic gain-switching where analog images are post-processed and digitised off-chip, though other readout schemes are also being investigated [7]. While these detectors are suited for FELs,

<figure><img src="image_3.png" /></figure>
<figure><img src="image_4.png" /><figcaption>**Figure 1.** (Right) A prototype XIDER detector hybridised to a 1 mm thick CdTe sensor. (Left) The sensor layout of multiple 4×4 matrices of three pixel pitches: 100 µm, 200 µm, and 300 µm.</figcaption></figure>

they are not always suited to measure the near-100% duty cycle and high-energy beam of the EBS. High-Z sensor materials, such as cadmium telluride (CdTe), are used in many detector systems for efficient detection at high energies. These sensors often have high leakage currents, bias- and flux-induced polarisation, and afterglow, which must be carefully managed so the detector system reaches the required specifications [8,9].

The XIDER project is a collaboration between the ESRF and Heidelberg University, performing detector R&D targeting 2D X-ray scattering and diffraction experiments at the EBS. XIDER detectors seek to fulfil the needs of these high-energy applications by implementing a novel incremental digital integration readout scheme, described in [10] and simulated in [11]. The exposure time is partitioned into sub-frames and the XIDER front-end contains a charge sensitive amplifier that integrates the input signal during a sub-frame. Each signal is then discriminated with an in-built half-photon energy discrimination and digitized, all on the per-pixel level. Below-threshold sub-frames are discarded, allowing for noise-free single-photon sensitivity under low flux illumination, even in the presence of a non-constant leakage current. At fluxes higher than 3 eq. 30 keV ph pixel⁻¹ subframe⁻¹, rounding errors that occur from the digitisation are cancelled out by averaging over a large number of subframes. Under high flux, they are therefore negligible and the readout scheme is functionally equivalent to a conventional charge integrating system.

This implementation of a digital integration readout is suited for detectors requiring a high dynamic range, one of the three main goals of the XIDER project. XIDER detectors aim to have single-photon sensitivity, high dynamic range under pile-up conditions $\le 10^{11}$ ph mm⁻² s⁻¹, high absorption efficiency to detect photons of energy $\le 100$ keV, and have a flexible and fast front-end that can detect both near-continuous 100 kframes s⁻¹ beams and pulsed $\le 5.7$ MHz beams using in-pixel memory.

Prototype hybrid XIDER assemblies have been produced [12] and tested at the EBS. These characterisation measurements aim to assess how close the current prototypes are to the project aims, testing both the front-end behaviour and sensor dynamics.

## 2. Prototype assemblies

Prototype XIDER ASICs have been designed in the 65 nm TSMC technology. A 16:1 multiplexer connects the desired pixel within a 4×4 matrix to the readout channel. Two readout stages operate in a pipeline mode to reduce the ADC conversion time, each stage consisting of three main steps: a charge-sensitive amplifier, a comparator, and a charge-pump [12].

XIDER ASICs have been bump-bonded by Polymer Assembly Technology (PAT) to 1 mm thick 4×4mm² planar CdTe sensors from Acrorad Co., Ltd. produced with both ohmic and Schottky contact variants. The sensor design, shown in Figure 1, includes 4×4 pixel matrices of different pixel pitch. One matrix is bonded to the readout ASIC for each assembly, and each pixel matrix is surrounded by a grounded guard ring. Each assembly was assessed using internal
<figure><img src="image_5.png" /><figcaption>**Figure 2.** Diagram of the characterisation set-up placed in the BM05 hutch at the ESRF.</figcaption></figure>

<table><caption>**Table 1.** Summary of beam parameters and XIDER assemblies used during data taking.</caption><thead><tr><th>beamtime</th><th>EBS beam mode</th><th>Beam area</th><th>Beam type</th><th>Beam energy</th><th>XIDER assembly</th></tr></thead><tbody><tr><td>1st</td><td>4-bunch</td><td>1×1mm<sup>2</sup></td><td>Monochromatic</td><td>30keV</td><td>PAT04</td></tr><tr><td>2nd</td><td>16-bunch</td><td>0.55×0.59mm<sup>2</sup></td><td>Monochromatic</td><td>30keV</td><td>PAT08</td></tr></tbody></table>

charge injection and LED light exposure to ensure the readout channel functionality and identify any potential bump-bonding or sensor issues. In addition, the threshold and charge-pump packet (CP) size for each readout stage was calibrated using external charge injection. In the first stage, the CP size and threshold are equivalent to eight 30 keV photons; in the second stage, the CP size and threshold are equivalent to 100% and 90% of a 30 keV photon respectively.

### 3. Beamtime set-up

To determine the performance of the prototype assemblies with respect the project aims, XIDER assemblies were characterised using X-rays. The set-up placed in the BM05 beamline of the ESRF, illustrated in Figure 2, was used during two separate beamtimes. An automated filter system in front of the experimental hutch beam opening attenuates the beam with thicknesses of Al up to 7.9 mm. In order to have a proportional measurement of the beam flux, a 500 µm thick silicon diode is placed approximately 50 cm upstream of the assembly and its current recorded. The DMAK DAQ system, developed in-house, acquires data from the XIDER assembly.

Three aspects differed between the beamtimes, summarised in Table 1: the XIDER assembly used, the beam area, and the beam filling mode. Both assemblies used in each beamtime have XIDER ASICs bump-bonded to the 200 µm pitch matrix of an ohmic-CdTe planar sensor, biased with −500 V during the data taking period. The first beamtime used a beam with a 4-bunched filling mode: 4 bunches within the storage ring, each filled with 16 mA and equally spaced by 705 ns. In the second beamtime, the 16-bunch filling mode meant there were 16 32 mA filled bunches within the storage ring, equally spaced by 176 ns. During both periods, the 30 keV X-ray beam was measured between measurements to be approximately uniform by substituting the diode with a camera-based beam viewer of the beamline.

### 4. Beamtime data analysis

Each set of data recorded at BM05 corresponds to one of the three main project objectives.

#### 4.1. *Time-resolved experiments*

During the first beamtime, the flexible front-end of the ASIC was investigated. In the final ASIC version, storage cells will be implemented to achieve the goals for time-resolved experiments, however they are not implemented in the current prototype ASIC. This early ASIC version can still distinguish isolated bunches, another goal of the project.
<figure><img src="image_6.png" /><figcaption>**Figure 3.** Pixel 8 front-end potential of assembly PAT04 and 4-bunch trigger signal of ESRF storage ring over time, recorded using an oscilloscope.</figcaption></figure>

Bunch discrimination was observed by highly attenuating the beam to achieve a flux of 0 to 1 ph pixel⁻¹ bunch⁻¹ and disabling the charge pumps of both stages, allowing any voltage rises in the front-end potential to be measured using an oscilloscope and compared easily to the theoretically expected value for one 30 keV photon.

Figure 3 presents the oscilloscope trace obtained, showing both the 2nd stage front-end potential of pixel 8 of PAT04 and the 4-bunch beam trigger. Two rises in the potential occur simultaneously with the arrival of two bunches, spaced 705 ns apart. Theoretically, a 30 keV photon in a CdTe sensor should produce 6772 e from ionisation, resulting in a potential difference of 21.5±2.2mV over the 50±5fF capacitor quoted in the front-end design [12]. The voltage step at 1.34 µs has an approximate size of 27 mV with an uncertainty of ±4mV, while the voltage step at 1.98 µs is 22±6mV. These values agree with the theoretical expectation and therefore it is concluded that the voltage steps seen in Figure 3 are the integration of single 30 keV photons.

## 4.2. *Flux dynamic range*

A flexible pixel front-end is the second project goal. In Section 4.1, single photon sensitivity has been demonstrated, and the dynamic range was tested during the second beamtime period. The beam flux was adapted by inserting 35 different filter configurations into the beam with the following data taking pattern: the sensor was first stabilised for 50 min under dark conditions with the sensor biased, then a 2 min illumination was performed per filter setting, closing the beam shutter in between for approximately 3 s of darkness while the next filter configuration was inserted, reducing the filter thickness each step. Data set 'ramp-up' in Figure 4 presents the average XIDER signal measured per increasing flux as a function of the measured diode current, a proportional measurement of the beam flux. A linear trend is observed, with sub-linear data points at low fluxes due to net charge trapping over the first approximately 4 min of illumination. Polarisation is a well documented issue with CdTe sensors and is discussed further in Section 4.3. Due to polarisation effects, the first 3 data points are excluded from the applied uncertainty-weighted linear fit of function y=3.86x+14.4.

In the final XIDER ASIC version, the aim is to achieve a dynamic range of approximately 200 ph per 170 ns subframe with a 100 µm pixel pitch, corresponding to 10¹¹ph mm⁻²s⁻¹. The
<figure><img src="image_7.png" /><figcaption>**Figure 4.** Average number of eq. 30 keV photons recorded per 4 µs subframe by the 14th pixel of PAT08, as a function of the current measured by a silicon diode within the beam path. Each data point corresponds to a different filter configuration, either with increasing (ramp-up) or decreasing (ramp-down) flux. The uncertainty of each data point is ±0.5× standard deviation.</figcaption></figure>

described measurement, taken to see how close the current prototype ASIC is to this goal, is limited in ways pertinent to the evaluation of Figure 4. Firstly, the 16-bunch beam mode used in this case limits the flux range achievable at an energy of 30 keV; secondly the dynamic range could be increased significantly by shortening the subframe duration from the 4 µs used during the measurement. The use of a longer subframe duration is currently necessary due to a rate limitation of the DAQ system, which will be addressed by using a new system in the future. Under these measurement conditions, a dynamic range of 300 ph per 200 µm-pitch pixel per 4 µs subframe is achieved, equivalent to $2 \times 10^9$ ph mm$^{-2}$ s$^{-1}$.

### 4.3. Efficient detection of high energy photons

The final aim is to efficiently detect photons of ≤100 keV, which could be achieved by using 1 mm thick CdTe sensors. It is one of the few sensor materials that can efficiently detect photons of this energy, however CdTe itself introduces multiple issues related to its deep-level charge trapping properties [8, 9]. These charge trapping properties have been extensively studied, showing that the electric field within an ohmic-CdTe sensor can collapse under high flux X-ray illumination due to the collective charge of trapped carriers [13–15]. After X-ray illumination, the sensor releases the trapped charge carriers as a significant dark signal, commonly termed afterglow [16].

Currently the sensor material is the main limitation of the XIDER project, which can be illustrated by revisiting the flux dynamic range measurement in Figure 4. The same data-taking procedure can be repeated, but decreasing the flux with each exposure, denoted as ‘ramp-down’ in Figure 4. A different linear slope and offset are observed when decreasing the flux, thought to be due to the opposing sensor trap dynamics impacting each of the datasets.

Theoretically, the rate of charge trapping within the deep levels of CdTe is dependent on the number of free charge carriers within the sensor, and therefore on the incident photon flux. Increasing the illumination flux from an equilibrium state causes free charges to become trapped in previously empty deep levels until a new equilibrium is reached with a higher proportion of
filled traps, thus reducing the average recorded signal within this dynamic period. The opposite is also true: reducing the photon flux causes a net detrapping of charges from deep acceptors until an equilibrium state with fewer filled traps is achieved. The detrapping rate, however, is flux independent. Afterglow is only dependent on the trap energy level, typically resulting in time-scales of *O*(hour) [16,17], therefore increasing the average signal recorded during this time.

As mentioned in 4.2, the sensor was first stabilised before the ramp-up and ramp-down datasets of Figure 4. The first data points in time of each scan are sub-linear as traps are filled within the sensor over a time frame of *O*(minute), since the number of free charges within the sensor bulk has increased by starting illumination in both cases. The data points $\le 8$ nA or $\ge 56$ nA are therefore excluded from the linear fit functions of the ramp-up and ramp-down datasets respectively. In the linear region of both datasets, the ramp-down dataset systematically has a higher signal for similar beam fluxes. During this dataset, each step of flux reduction reduces the number of free charges, leading to net detrapping from deep level sensor traps, and a higher recorded signal than expected. Afterglow occurs over a much longer time-period than the 2 min illumination time, therefore it is thought that the offset between the two datasets shown in Figure 4 is due an additional afterglow signal component within the ramp-down data. Charge trapping effects make it complex to analyse the signal from CdTe-sensor XIDER assemblies.

## 5. Conclusion

XIDER assemblies have been produced by bump-bonding CdTe sensors to prototype XIDER ASICs, implementing a digital integration readout scheme with per-pixel digital data for the first time. Data was taken during beamtimes at the ESRF, focusing on measurements related to the three main project goals. A bunch discrimination measurement shows that single 30 keV photons can be integrated, and the front-end flux dynamic range is measured to be $2 \times 10^9$ ph mm$^{-2}$ s$^{-1}$ with the current prototype ASIC design. The measurements also highlight the limitations of CdTe as a sensor material, due to its deep-level charge trapping effects. In the future, XIDER assemblies are to be produced using an updated ASIC version with a 16 channel simultaneous readout, an improved front-end linearity, and a higher dynamic range.

## Acknowledgments

This work is part of the ESRF-EBS Upgrade Programme and received financial support from the ATTRACT EU project. The authors thank P. Cook and the scientists of BM05 for their assistance during data-taking and J. Clayton of PAT for bump-bonding the XIDER assemblies.

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