Solid-state X-ray detection technology
Hybrid pixel detectors
Hybrid pixel detectors were developed at facilities such as CERN where they are required for very demanding high- energy particle detection and discrimination applications. The technical spin-offs from this state-of-the-art research have been picked up for X-ray applications such as XRD. PANalytical, having pioneered solid-state X-ray detection technology since the introduction of the X’Celerator, has chosen the most advanced technologies for incorporation in X-ray diffractometers.
One of these technologies is Medipix3, originally developed at CERN for the Large Hadron Collider. Medipix3-based detectors have a pixel size of only 55 micrometers, the smallest pixel size of any commercially available hybrid pixel detector. Only with this small pixel size, the same detector can be used for highest-resolution powder diffraction, ultra-fast reciprocal space mapping and small-angle X-ray scattering, within the confined space of a multipurpose diffractometer. The name ‘PIXcel’ was chosen to honor the next step in technology after the successful solid-state strip detector X’Celerator: an ‘X’Celerator’ with even better performance, built from pixels, allowing REAL 2D measurements.
The other technology that was recently picked up by PANalytical is the PIXIE technology, originally developed for astronomy applications. With a pixel size of 60 microns, the resolution is comparable to the Medipix technology. What makes the PIXIE technology unique, is the combination with CdTe sensor chips, making the resulting device unique and superior for detecting radiation from Mo and Ag sources. The commercial name ‘GaliPIX’ was chosen to honor its origin in Pisa, Italy, home of the famous astronomer Galileo Galilei.
Common characteristics of PANalytical’s 2D hybrid pixel detectors are the following:
- A point spread function (PSF) of only one pixel, resulting in sharp images
- Two-level (window) energy discrimination, allowing the rejection of both low- and high-energy radiation at the same time
- Virtually noise-free images, combined with the ability to handle very high count rates, giving flexibility for both high- and low-intensity applications.
The technologies involved in hybrid detector development (sensor materials, bump bonding, analog charge detection and digital processing circuitry) are all continuously advancing. Detectors can be fabricated with different sensor materials. The sensors are high-quality semiconductor crystals such as for example PIXcel1D, PIXcel3D and X’Celerator detectors have Si sensors and GaliPIX3D has a CdTe sensor. Different sensor materials have different absorption efficiencies and can be optimized for each application.
Modern hybrid detectors have a unique one-to-one relationship between the entry point of the X-ray photon into thin semiconductor sensor where the photon energy is converted into charge and the bump bond which connects the sensor to a local circuit where the charge is converted into a voltage. Each voltage signal is processed by its own integrated circuit below the bump bond. A hybrid pixel detector is therefore an array of individual pixel detectors.
By having in-house detector development, PANalytical can keep pace with detector technology as it evolves. We control and optimize the detector performance for our XRD applications.
Read on to learn about the different processes in a hybrid detector!
Counting and identifying X-ray photons
Every X-ray photon that interacts with the detector has an energy (e.g. for Cuα1 this is ~8 KeV). The energy is transferred to the sensor and creates a recognizable cascade of electron-hole pairs that are converted to a voltage in the analog circuitry. This voltage is measured in time and a voltage versus time profile is created for each photon.
In older technologies, e.g. using gas detection, the large thickness of the sensor meant that X-rays entering at a point on the surface of the sensor may eventually be measured at a neighboring pixel rather than the one that is directly below the entry point. This uncertainty in the X-ray photon position is called ‘spread’. Hybrid detectors have a single pixel spread function because the sensors that capture the X-rays are relatively thin. Also the field maintained in the sensor drives the charge cloud generated by the photon to the nearest bump bond. This results in precise pixel position measurement for the X-ray photon.
Each photon measurement is a fingerprint in the form of the shape of the voltage versus time profile. In this way the photons are individually counted and sorted according to their energies. This is the analog ‘front end’ of the detector. By careful measurement and high-speed processing photon counting can be extremely precise.
Energy discrimination and signal to noise
The digital logic circuit behind the sensor decides how the analog voltage versus time profile is to be used. Recognizing meaningful signals in a detector is about the logical choices employed in assigning meaning to photons of a particular energy. Choices are made as to whether the total information provided for a photon (energy, time and (x,y) position) is useful data. Non-useful data is discarded. Sophisticated electronics are devised by PANalytical in order to optimize the signal processing to best suit the application. This combined with the appropriate optics and the intrinsic zero noise of the detector can provide extremely high signal-to-noise even for low scattering intensities.
The increase in processing speed of hybrid detectors is responsible for the ability of the detector to achieve true photon counting even at high intensities. This speed also gives versatility to frame-based collection systems opening up the opportunity for high-speed scanning measurements.