Using Hybrid Pixel Detectors in Macromolecular Crystallography

Macromolecular crystallography has benefited from several methodological developments such as the adoption of cryo-crystallography and improved X-ray sources. Simultaneously, advancements in detector technology from film via image plates and CCD detectors to state-of-the-art hybrid pixel detectors facilitated research in macromolecular crystallography. DECTRIS pioneered the application of photon counting pixel detectors in crystallography. Now, numerous PILATUS detectors are installed at high-profile MX beamlines around the world with hundreds of structures determined and deposited in the PDB from their data. DECTRIS hybrid pixel detectors are the instrument of choice for beamlines concerned about data quality and efficiency.

The success of DECTRIS hybrid pixel detectors in macromolecular crystallography is based on their unique characteristics:

• No readout noise and dark current
• Sharp point spread function smaller than one pixel
• High dynamic range of 20 bits
• Short readout times in the range of few milliseconds
• High frame rates up to 250Hz (PILATUS3 2M)

The advantages of DECTRIS hybrid pixel detectors over previous detector technologies in numerous ways are:

• The sharp point spread function ensures excellent resolution of closely spaced reflections over the entire dynamic range of the detector
• It minimizes overlap of diffraction intensities with scattering background and maximizes the signal-to-noise ratio
• The high dynamic range of 20 bits virtually abolishes overloaded low-resolution reflections
• The short readout time and high frame rates enable high throughput data collection and optimize beamline efficiency
• Furthermore, it allows collection of diffraction data with continuous rotation and eliminates the shutter as a source of error. Dark current and readout noise are completely absent, leading to better signal-to-noise ratio particularly for weak high-resolution reflections

Unique Characteristics: Hybrid Pixel and Microstrip Technology

DECTRIS X-ray detectors enable direct detection of X-rays with optimized solid-state sensors and CMOS readout ASICs in hybrid pixel technology. These detectors include single dimension microstrip detectors with a one-dimensional (linear) array of stripes and 2D pixel array detectors (area detectors) with a two-dimensional array of pixels.

Key Advantages

• Direct detection of X-rays
• Single-photon counting
• Excellent signal-to-noise ratio and very high dynamic range (zero dark signal, zero noise)
• Low-energy X-ray suppression (energy resolution by single energy threshold)
• Short readout time and high frame rates
• Modular detectors enabling multi-module detectors with large active area

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Direct Detection

As shown in Figure 1, in direct detection sensor technology, X-ray photons are directly converted into electric charge in the solid-state sensor. Direct detection sensors are fully depleted monolithic sensors and are typically fabricated from high-resistivity solid-state sensor materials such as silicon. Ohmic and diode type pixelated sensors with various sensor thicknesses are available with ohmic contacts or diode structures in each single pixel.

Figure 1. Principle of direct detection of X-ray photons in a solid-state sensor. Image credit: Dectris Ltd

Hybrid Pixel

In hybrid pixel technology, a pixelated solid-state sensor is connected to a CMOS readout ASIC by bump or wire bonding technology as shown in Figure 2. The sensor, which is typically a one- or two-dimensional array of PN-diodes processed in high-resistivity silicon, is connected to an array of readout channels designed in CMOS technology.

Figure 2. Principle of a hybrid pixel with a sensor bump bonded to a CMOS readout ASIC. Image credit: Dectris Ltd

Single-Photon Counting

Every photon is individually detected and counted in the discriminating detector in single-photon counting technology. The readout channel includes an amplifier, which amplifies the electric charge pulse generated in the sensor by an incoming X-ray photon, a discriminator, which generates a digital pulse signal if the incoming charge pulse exceeds an adjustable predefined threshold, and a digital counter, which counts the number of generated digital pulses.

In Figure 3, the first diagram shows the sequence of incoming X-ray photons with their corresponding energy. The second diagram shows the analog voltage pulses at the output of the amplifier and the predefined threshold voltage of the successive comparator that acts as the discriminator.

The fourth diagram shows the digital voltage pulse sequence that appears at the output of the comparator and that feeds the counter in order to determine the number of incoming X-ray photons with energy higher than the predefined threshold energy. This represents a completely digital detection and storage scheme and achieves a noiseless determination and readout of the number of detected X-ray photons per pixel.

Figure 3. Principle of single-photon counting detectors vs. integrating detectors. Image credit: Dectris Ltd

The single-photon counting principle features zero dark signal and zero readout noise and achieves excellent signal-to-noise ratio. Furthermore, the energy threshold offers energy resolution and a single energy threshold can be used for low-energy suppression. Single-photon counting achieves short readout time and high frame rates due to the completely digital detection and storage scheme.

High-performance single-photon counting detectors typically include counter sizes in the range of 20 bits, dynamic ranges of up to 1,000,000, readout times of less than 1 ms and frame rates of several 100 Hz. The maximum count rates, typically about 106 photons per second in a single pixel, allow the handling of the high flux of modern synchrotron light sources.

Modular Detector

The complete active area and pixel array of a large-area detector consists of multiple identical modules of a predetermined size in modular detector technology. The detector module is the fundamental unit of the multi-module detector and includes a multi-chip module with a single monolithic sensor and a number of CMOS readout ASICs, a mounting bracket and control electronics. The detector modules are mounted onto a high-precision mechanical frame to create multi-module detectors with up to 60 modules or more. As seen in Figure 4, PILATUS detector modules include a multi-chip module with a single sensor and an 8 x 2 array of CMOS readout ASICs assembled by bump-bonding technology.

Figure 4. Multi-chip module with 16 CMOS readout ASICs bump-bonded to a single sensor. Image credit: Dectris Ltd

Each sensor is a continuous 487 x 195 array of 94,965 pixels without dead areas and covers an active area of 83.8 mm x 33.5 mm. The multi-chip module is wire-bonded to the mounting bracket with the control electronics and forms the PILATUS detector module as seen in Figure 5. Multiple PILATUS detector modules are assembled in a multi-module setup to form large-area PILATUS detectors as shown in Figure 6.

Figure 5. PILATUS detector module. Image credit: Dectris Ltd

Figure 6. PILATUS detector modules assembled in a multi-module setup in a large- area PILATUS detector. Image credit: Dectris Ltd

MYTHEN detector modules include a single sensor and a linear array of 8 CMOS readout ASICs assembled by wire bonding technology. The sensor is a linear array of 1280 microstrips on a 50µm pitch with a total active area of 8x64mm. Multiple MYTHEN detector modules can be combined and operated with a single detector control system to form a large multi-detector system as seen in Figure 7.

Figure 7. MYTHEN detector system consisting of a detector control system and one multi-chip detector module (without housing). Image credit: Dectris Ltd

This information has been sourced, reviewed and adapted from materials provided by Dectris Ltd.

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