Semiconductor detectors and read-out electronics
The granularity required from particle detectors is ever increasing and hence many detectors adopt a pixel architecture. Nikhef is actively involved in the design and characterisation of pixel sensors and read-out chips. Given the complexity of these chips, the institute collaborates with other institutes and universities on the design.
Around 2005, Nikhef started developing timing circuitry for pixelated particle detectors, in order to very precisely determine when a particle reaches which pixel of the detector. The Timepix3 chip, which became available in the second half of 2013, is the latest member of a successful pixel chip family. This radiation-hard integrated circuit containing 64,000 pixels was developed together with CERN and the University of Bonn. The timing resolution (1.56 ns) has been improved compared to the previous version of the Timepix chip by almost a factor of ten, giving a large improvement in the z-resolution for the gaseous detectors discussed in the next section. Another novelty of this chip is that it offers simultaneous time-of-arrival and energy measurements in each pixel. Currently, the researchers are planning a next generation of readout chips with a timing resolution better than 100 ps.
These developments towards more intelligent read-out chips for particle-tracking detectors are also utilized in x-ray imaging applications.
The gaseous detectors developed at Nikhef are composed of a pixelated read-out chip with an integrated gas-amplification grid on top. This family of detectors allows a three-dimensional track reconstruction of ionising radiation. The x-y position is given by the pixel matrix and the z position is derived from the time of arrival of the drifting ionisation charge. The detector is based on the fine-granularity Timepix chip that is able to record the time of arrival of the incoming signal. Thanks to the small pitch of the pixel cells (55 × 55 μm2), the detector collects the individual electrons that are liberated in the gas volume by a traversing charged particle. This kind of tracking detector, collecting all information that can be deduced from the ionisation trail of a charged particle in gas, has a wide range of applications and will be further developed for the time projection chamber of the proposed experiment at the International Linear Collider (ILC).
In 2014, Nikhef researchers presented the most precise gaseous pixel detector to date for measuring the position of individual ionisation electrons. This lead to improved angular (2.5 degrees) and position (in-plane 10 μm) resolutions on fitted tracks.
Ultra-fast photon detectors
In 2013, Nikhef researcher Harry van der Graaf proposed a novel photon detector based on Micro Electro Mechanical Systems (MEMS) technology, aiming at sub-ps temporal resolution. The difference between a traditional photomultiplier tube (PMT) and this novel detector is in the nature of their dynodes. A PMT has reflective dynodes, whereas this research is aimed at developing transmission dynodes that will be stacked on top of each other. This simplifies the configuration and reduces the detector size to the scale of a pixel chip (55 μm). As a consequence, the time response is improved and the sensitivity to magnetic fields is decreased. The main challenge is to fabricate ultra-thin transmission dynodes with sufficient Secondary Electron Yield (SEY) at low primary electron energy, i.e., a yield of four electrons at 500 eV.
Over the past few years, Nikhef researchers designed, built and operated a small dual-phase xenon Time Projection Chamber (TPC). The project is called XAMS and will allow the researchers to test new detection technologies in a xenon detector and to study the general properties of xenon. This is important for analysis of dark-matter data coming from detectors such as XENON100 and XENON1T. A neutron source is currently being procured. The neutrons will allow to see nuclear recoils in XAMS, the same signal as is expected from collisions of weakly interacting massive particles (WIMPs). This source will complement Nikhef’s γ-sources.
Researchers at Nikhef – in addition to CERN – play a leading role in research into gravitational waves. These tiny ripples in space-time require advanced instruments of extreme sensitivity. Ever since the first direct detection of a gravitational wave in September 2015, physicists aim to make their detection facilities even more sensitive. To this end, the so-called gravity gradient noise, a source of noise dominant at low frequencies, needs to be measured in order to correct for it. This requires the development of very sensitive interferometers and low-power, extremely low-noise read-out electronics. These developments fit well within the Dutch High-Tech Systems & Materials (HTSM) top sector. In 2011, Nikhef initiated a research programme to design an innovative interferometer using so-called microelectromechanical (MEMS) technology. In 2014, a research proposal of Nikhef in collaboration with Twente University to read out this sensor using integrated microelectronics was granted within the framework of the HTSM top sector. Future users of the technology and partners in this project are Shell, semiconductor multinational STMicroelectronics and Nikhef start-up Innoseis.
To answer major questions in elementary particle physics, ground-breaking experiments are needed. The Scientific Instrumentation programme of Nikhef aims to come up with instrumentation concepts and test them prior to implementation in Nikhef’s scientific programmes.
Nikhef collaborates with industry in various ways. In order to improve the instrumentation of existing experiments or to create new experimental setups, new technologies are continuously being developed in the areas of detection techniques, electronics and mechanics. To realize a new system, somewhere along the development funnel industry is called upon. The most frequent and simple way is to buy certain components. However, for the highly specialised systems used at Nikhef, standard solutions often don’t suffice. In this case, Nikhef researchers build the required components themselves, provided only small quantities are involved. If it is clear that many components will need to be produced, collaboration with industry is obvious as they are better equipped to produce large quantities. In developing a new component, designers at Nikhef ideally work together with their industry counterparts from the get-go. In this way, everyone is on the same learning curve and the industry partner is fully aware of what is required from the component. This eventually results in the industrial production of the component.
Another important aspect is fitting techniques that were developed for various experiments by or in collaboration with Nikhef into commercial equipment. An example hereof is the Medipix2 chip that is being sold in X-ray diffraction equipment of PANalytical for ten years now. The Nikhef spin-off company Amsterdam Scientific Instruments sells products in various other application areas based on the same Medipix2 chip systems. Often they are developed in a collaboration of the Nikhef Instrumentation group with other institutes. Examples hereof include the use of mass spectrometry to detect various proteins found in cancer tissue. Studying fragile proteins and crystals using electron microscopy is another example.
The collaboration with industry stretches even further whenever Nikhef and companies have a shared interest in the development of detection systems. In such cases, Nikhef and the companies submit joint research proposals. The aim here is to jointly establish the specifications that are essential to future experiments at Nikhef and can also be used by the companies to open new opportunities for their product portfolio.
Medical imaging systems
The number of people that survive up to five years after the diagnosis of cancer increases rapidly. However, many of these survivors suffer from (long-term) side effects of cancer treatment in general and more specifically from treatment with (conventional) radiotherapy. Radiotherapy using proton beams can make a difference here. This form of radiotherapy is about to be introduced in the Netherlands. Compared to conventional radiotherapy, proton therapy allows the radiation dose release to be targeted to a volume as small as a green pea, leading to dose reduction in healthy organs, glands or nerves surrounding the tumour. By preventing such collateral damage, the quality of life can be improved considerably. For head and neck cancer, this may be the difference between losing or maintaining vision, speech or the ability to swallow. In some cases of breast cancer, the risk of damage to the heart can be decreased significantly.
Nikhef researcher Els Koffeman develops techniques, experimental methods and instruments that exploit and enhance the intrinsic benefits of the proton beam precision. Her work focusses on novel and more accurate imaging techniques: spectral CT scanning and proton radiography. Present imaging techniques are adequate but not dedicated for proton therapy; this opens the opportunity to conceive a genuinely new experimental approach with ‘smart’ pixel detectors. The goal is to extract data on the tissue composition of a patient with a fast scan that can be used during the course of proton therapy treatment.
Given its expertise, the Scientific Instrumentation group at Nikhef aims to become a serious partner on different medical imaging techniques. For mammography, the group works with Andre Mischke (Utrecht University) and the University Medical Centre Utrecht (UMCU), for proton radiography with Sytze Brandenburg (Groningen University) and others.
Imaging with muons
High up in the Earth’s atmosphere, cosmic radiation creates an avalanche of secondary particles, bombarding the Earth’s surface in high numbers with nearly the speed of light. Within such an avalanche, C.D. Anderson discovered the muon in 1936. The muon is basically a heavy cousin of the electron. High-energetic muons can penetrate far more deeply into matter than electrons and photons. The exact penetration depth depends on the energy of each individual muon and the material properties. A cosmic muon has an average energy of 4 GeV (in physics units of energy) and will be stopped by 4 meter of iron or 12 meter of rock. Muons with less energy will be stopped earlier, muons with more energy penetrate deeper.
Since the energy distribution and flux of the cosmic muons reaching the Earth are known with high precision, one can determine the amount of material above a muon detector by measuring the decrease in flux. The idea is to use cosmic muons in combination with a smart detector topology to image the interior of a steel furnace during production. In practice, this means that the muon flux is measured along several lines of sight through the furnace, using pairs of muon detectors. This way, the penetrated and the expected flux can be compared in order to quantify the amount of iron along a line of sight. By measuring with high statistics, a detailed image emerges of the iron-coke ratio inside the steel furnace – a quantity that is not known very accurately today.