How it works Diagram showing simulated light from an exoplanet and its companion star (far left) moving through the new coronagraph. (Courtesy: Nico Deshler/University of Arizona)
A new type of coronagraph that could capture images of dim exoplanets that are extremely close to bright stars has been developed by a team led by Nico Deshler at the University of Arizona in the US. As well as boosting the direct detection of exoplanets, the new instrument could support advances in areas including communications, quantum sensing, and medical imaging.
Astronomers have confirmed the existence of nearly 6000 exoplanets, which are planets that orbit stars other as the Sun. The majority of these were discovered based on their effects on their companion stars, rather than being observed directly. This is because most exoplanets are too dim and too close to their companion stars for the exoplanet light to be differentiated from starlight. That is where a coronagraph can help.
A coronagraph is an astronomical instrument that blocks light from an extremely bright source to allow the observation of dimmer objects in the nearby sky. Coronagraphs were first developed a century ago to allow astronomers to observe the outer atmosphere (corona) of the Sun , which would otherwise be drowned out by light from the much brighter photosphere.
At the heart of a coronagraph is a mask that blocks the light from a star, while allowing light from nearby objects into a telescope. However, the mask (and the telescope aperture) will cause the light to interfere and create diffraction patterns that blur tiny features. This prevents the observation of dim objects that are closer to the star than the instrument’s inherent diffraction limit.
Off limits
Most exoplanets lie within the diffraction limit of today’s coronagraphs and Deshler’s team addressed this problem using two spatial mode sorters. The first device uses a sequence of optical elements to separate starlight from light originating from the immediate vicinity of the star. The starlight is then blocked by a mask while the rest of the light is sent through a second spatial mode sorter, which reconstructs an image of the region surrounding the star.
As well as offering spatial resolution below the diffraction limit, the technique approaches the fundamental limit on resolution that is imposed by quantum mechanics.
“Our coronagraph directly captures an image of the surrounding object, as opposed to measuring only the quantity of light it emits without any spatial orientation,” Deshler describes. “Compared to other coronagraph designs, ours promises to supply more information about objects in the sub-diffraction regime – which lie below the resolution limits of the detection instrument.”
To test their approach, Deshler and colleagues simulated an exoplanet orbiting at a sub-diffraction distance from a host star some 1000 times brighter. After passing the light through the spatial mode sorters, they could resolve the exoplanet’s position – which would have been impossible with any other coronagraph.
Context and composition
The team believe that their technique will improve astronomical images. “These images can provide context and composition information that could be used to determine exoplanet orbits and identify other objects that scatter light from a star, such as exozodiacal dust clouds,” Deshler says.
The team’s coronagraph could also have applications beyond astronomy. With the ability to detect extremely faint signals close to the quantum limit, it could help to improve the resolution of quantum sensors. This could to lead to new methods for detecting tiny variations in magnetic or gravitational fields.
Elsewhere, the coronagraph could help to improve non-invasive techniques for imaging living tissue on the cellular scale – with promising implications in medical applications such as early cancer detection and the imaging of neural circuits. Another potential use could be new multiplexing techniques for optical communications. This would see the coronagraph being used to differentiate between overlapping signals. This has the potential of boosting the rate at which data could be transferred between satellites and ground-based receivers.
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Experimental setup Top: schematic and photo of the setup for measurements behind a homogeneous phantom. Bottom: IMPT treatment plan for the head phantom (left); the detector sensor position (middle, sensor thickness not to scale); and the setup for measurements behind the phantom (right). (Courtesy: Phys. Med. Biol. 10.1088/1361-6560/adcaf9)
Proton therapy is a highly effective and conformal cancer treatment. Proton beams deposit most of their energy at a specific depth – the Bragg peak – and then stop, enabling proton treatments to destroy tumour cells while sparing surrounding normal tissue. To further optimize the clinical treatment planning process, there’s recently been increased interest in considering the radiation quality, quantified by the proton linear energy transfer (LET).
LET – defined as the mean energy deposited by a charged particle over a given distance – increases towards the end of the proton range. Incorporating LET as an optimization parameter could better exploit the radiobiological properties of protons, by reducing LET in healthy tissue, while maintaining or increasing it within the target volume. This approach, however, requires a method for experimental verification of proton LET distributions and patient-specific quality assurance in terms of proton LET.
To meet this need, researchers at the Institute of Nuclear Physics, Polish Academy of Sciences have used the miniaturized semiconductor pixel detector Timepix3 to perform LET characterization of intensity-modulated proton therapy (IMPT) plans in homogeneous and heterogeneous phantoms. They report their findings in Physics in Medicine & Biology.
Experimental validation
First author Paulina Stasica-Dudek and colleagues performed a series of experiments in a gantry treatment room at the Cyclotron Centre Bronowice (CCB), a proton therapy facility equipped with a proton cyclotron accelerator and pencil-beam scanning system that provides IMPT for up to 50 cancer patients per day.
The MiniPIX Timepix3 is a radiation imaging pixel detector based on the Timepix3 chip developed at CERN within the Medipix collaboration (provided commercially by Advacam). It provides quasi-continuous single particle tracking, allowing particle type recognition and spectral information in a wide range of radiation environments.
For this study, the team used a Timepix3 detector with a 300 µm-thick silicon sensor operated as a miniaturized online radiation camera. To overcome the problem of detector saturation in the relatively high clinical beam currents, the team developed a pencil-beam scanning method with the beam current reduced to the picoampere (pA) level.
The researchers used Timepix3 to measure the deposited energy and LET spectra for spread-out Bragg peak (SOBP) and IMPT plans delivered to a homogeneous water-equivalent slab phantom, with each plan energy layer irradiated and measured separately. They also performed measurements on an IMPT plan delivered to a heterogeneous head phantom. For each scenario, they used a Monte Carlo (MC) code to simulate the corresponding spectra of deposited energy and LET for comparison.
The team first performed a series of experiments using a homogeneous phantom irradiated with various fields, mimicking patient-specific quality assurance procedures. The measured and simulated dose-averaged LET (LETd) and LET spectra agreed to within a few percent, demonstrating proper calibration of the measurement methodology.
The researchers also performed an end-to-end test in a heterogeneous CIRS head phantom, delivering a single field of an IMPT plan to a central 4 cm-diameter target volume in 13 energy layers (96.57–140.31 MeV) and 315 spots.
End-to-end testing Energy deposition (left) and LET in water (right) spectra for an IMPT plan measured in the CIRS head phantom obtained based on measurements (blue) and MC simulations (orange). The vertical lines indicate LETd values. (Courtesy: Phys. Med. Biol. 10.1088/1361-6560/adcaf9)
For head phantom measurements, the peak positions for deposited energy and LET spectra obtained based on experiment and simulation agreed within the error bars, with LETd values of about 1.47 and 1.46 keV/µm, respectively. The mean LETd values derived from MC simulation and measurement differed on average by 5.1% for individual energy layers.
Clinical translation
The researchers report that implementing the proposed LET measurement scheme using Timepix3 in a clinical setting requires irradiating IMPT plans with a reduced beam current (at the pA level). While they successfully conducted LET measurements at low beam currents in the accelerator’s research mode, pencil-beam scanning at pA-level currents is not currently available in the commercial clinical or quality assurance modes. Therefore, they note that translating the proposed approach into clinical practice would require vendors to upgrade the beam delivery system to enable beam monitoring at low beam currents.
“The presented results demonstrate the feasibility of the Timepix3 detector to validate LET computations in IMPT fields and perform patient-specific quality assurance in terms of LET. This will support the implementation of LET in treatment planning, which will ultimately increase the effectiveness of the treatment,” Stasica-Dudek and colleagues write. “Given the compact design and commercial availability of the Timepix3 detector, it holds promise for broad application across proton therapy centres.”
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Physicists at CERN have completed a “test run” for taking antimatter out of the laboratory and transporting it across the site of the European particle-physics facility. Although the test was carried out with ordinary protons, the team that performed it says that antiprotons could soon get the same treatment. The goal, they add, is to study antimatter in places other than the labs that create it, as this would enable more precise measurements of the differences between matter and antimatter. It could even help solve one of the biggest mysteries in physics: why does our universe appear to be made up almost entirely of matter, with only tiny amounts of antimatter?
According to the Standard Model of particle physics, each of the matter particles we see around us – from baryons like protons to leptons such as electrons – should have a corresponding antiparticle that is identical in every way apart from its charge and magnetic properties (which are reversed). This might sound straightforward, but it leads to a peculiar prediction. Under the Standard Model, the Big Bang that formed our universe nearly 14 billion years ago should have generated equal amounts of antimatter and matter. But if that were the case, there shouldn’t be any matter left, because whenever pairs of antimatter and matter particles collide, they annihilate each other in a burst of energy.
Physicists therefore suspect that there are other, more subtle differences between matter particles and their antimatter counterparts – differences that could explain why the former prevailed while the latter all but disappeared. By searching for these differences, they hope to shed more light on antimatter-matter asymmetry – and perhaps even reveal physics beyond the Standard Model.
Extremely precise measurements
At CERN’s Baryon-Antibaryon Symmetry Experiment (BASE) experiment, the search for matter-antimatter differences focuses on measuring the magnetic moment (or charge-to-mass ratio) of protons and antiprotons. These measurements need to be extremely precise, but this is difficult at CERN’s “Antimatter Factory” (AMF), which manufactures the necessary low-energy antiprotons in profusion. This is because essential nearby equipment – including the Antiproton Decelerator and ELENA, which reduce the energy of incoming antiprotons from GeV to MeV – produces magnetic field fluctuations that blur the signal.
To carry out more precise measurements, the team therefore needs a way of transporting the antiprotons to other, better-shielded, laboratories. This is easier said than done, because antimatter needs to be carefully isolated from its environment to prevent it from annihilating with the walls of its container or with ambient gas molecules.
The BASE team’s solution was to develop a device that can transport trapped antiprotons on a truck for substantial distances. It is this device, known as BASE-STEP (for Symmetry Tests in Experiments with Portable Antiprotons), that has now been field-tested for the first time.
Protons on the go
During the test, the team successfully transported a cloud of about 105 trapped protons out of the AMF and across CERN’s Meyrin campus over a period of four hours. Although protons are not the same as antiprotons, BASE-STEP team leader Christian Smorra says they are just as sensitive to disturbances in their environment caused by, say, driving them around. “They are therefore ideal stand-ins for initial tests, because if we can transport protons, we should also be able to transport antiprotons,” he says.
The next step: BASE-STEP on a transfer trolley, watched over by BASE team members Fatma Abbass and Christian Smorra. (Photo: BASE/Maria Latacz)
The BASE-STEP device is mounted on an aluminium frame and measures 1.95 m x 0.85 m x 1.65 m. At 850‒900 kg, it is light enough to be transported using standard forklifts and cranes.
Like BASE, it traps particles in a Penning trap composed of gold-plated cylindrical electrode stacks made from oxygen-free copper. To further confine the protons and prevent them from colliding with the trap’s walls, this trap is surrounded by a superconducting magnet bore operated at cryogenic temperatures. The second electrode stack is also kept at ultralow pressures of 10-19 bar, which Smorra says is low enough to keep antiparticles from annihilating with residual gas molecules. To transport antiprotons instead of protons, Smorra adds, they would just need to switch the polarity of the electrodes.
The transportable trap system, which is detailed in Nature, is designed to remain operational on the road. It uses a carbon-steel vacuum chamber to shield the particles from stray magnetic fields, and its frame can handle accelerations of up to 1g (9.81 m/s2) in all directions over and above the usual (vertical) force of gravity. This means it can travel up and down slopes with a gradient of up to 10%, or approximately 6°.
Once the BASE-STEP device is re-configured to transport antiprotons, the first destination on the team’s list is a new Penning-trap system currently being constructed at the Heinrich Heine University in Düsseldorf, Germany. Here, physicists hope to search for charge-parity-time (CPT) violations in protons and antiprotons with a precision at least 100 times higher than is possible at CERN’s AMF.
“At BASE, we are currently performing measurements with a precision of 16 parts in a trillion,” explains BASE spokesperson Stefan Ulmer, an experimental physicist at Heinrich Heine and a researcher at CERN and Japan’s RIKEN laboratory. “These experiments are the most precise tests of matter/antimatter symmetry in the baryon sector to date, but to make these experiments better, we have no choice but to transport the particles out of CERN’s antimatter factory,” he tells Physics World.
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