Moon's orbit proposed as a gravitational wave detector

Researchers from the UAB, IFAE and University College London propose using the variations in distance between the Earth and the Moon, which can be measured with a precision of less than a centimeter, as a new gravitational wave detector within a frequency range that current devices cannot detect. The research, which could pave the way for the detection of signals from the early universe, was published recently in Physical Review Letters.

Gravitational waves, predicted by Albert Einstein at the start of the 20th century and detected for the first time in 2015, are the new messengers of the most violent processes taking place in the universe. The gravitational wave detectors scan different frequency ranges, similar to moving a dial when tuning into a radio station. Nevertheless, there are frequencies that are impossible to cover with current devices and which may harbour signals that are fundamental to understanding the cosmos. One particular example can be seen in microhertz waves, which could have been produced at the dawn of our universe, and are practically invisible to even the most advanced technology available today.

In an article recently published in the journal Physical Review Letters, researchers Diego Blas from the Department of Physics at the Universitat Autònoma de Barcelona (UAB) and the Institut de Física d'Altes Energies (IFAE), and Alexander Jenkins from the University College London (UCL), point out that a natural gravitational wave detector exists in our immediate environment: the Earth-Moon System. The gravitational waves constantly hitting this system generate tiny deviations in the Moon's orbit. Although these deviations are minute, Blas and Jenkins plan on taking advantage of the fact that the Moon's exact position is known with an error of at most one centimeter, thanks to the use of lasers sent from different observatories which are continuously reflected upon mirrors left on the surface of the Moon by the Apollo space mission and others. This incredible precision, with an error of one billionth of a part at most, is what may allow a small disturbance caused by ancient gravitational waves to be detected. The Moon's orbit lasts approximately 28 days, which translates into a particularly relevant sensitivity when it comes to microhertz, the frequency range researchers are interested in.

Similarly, they also propose using the information other binary systems in the universe may provide as gravitational wave detectors. This is the case of pulsar binary systems distributed throughout the galaxy, systems in which the pulsar's radiation beam allows obtaining the orbit of these stars with incredible precision (with a precision of one millionth). Given that these orbits last approximately 20 days, the passing of gravitational waves in the microhertz frequency range affect them particularly. Blas and Jenkins concluded that these systems could also be potential detectors of these types of gravitational waves.

With these "natural detectors" in the microhertz frequency range, Blas and Jenkins were able to propose a new form of studying gravitational waves emitted by the distant universe. Specifically, those produced by the possible presence of transitions in highly energetic phases of the early universe, commonly seen in many models.

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Thinnest X-ray detector ever created

Scientists in Australia have used tin mono-sulfide (SnS) nanosheets to create the thinnest X-ray detector ever made, potentially enabling real-time imaging of cellular biology.

X-ray detectors are tools that allow energy transported by radiation to be recognised visually or electronically, like medical imaging or Geiger counters.

SnS has already shown great promise as a material for use in photovoltaics, field effect transistors and catalysis.

Now, members of the ARC Centre of Excellence in Exciton Science, based at Monash Universityand RMIT University, have shown that SnS nanosheets are also excellent candidates for use as soft X-ray detectors.

Their research, published in the journal Advanced Functional Materials, indicates that SnS nanosheets possess high photon absorption coefficients, allowing them to be used in making ultrathin soft X-ray detectors with high sensitivity and a rapid response time.

These materials were found to be even more sensitive than another emerging candidate (metal halide perovskites), boasting a faster response time than established detectors and are tuneable for sensitivity across the soft X-ray region.

The SnS X-ray detectors created by the team are less than 10 nanometres thick. To put things in perspective, a sheet of paper is about 100,000 nanometres thick, and your fingernails grow about one nanometre every second. Previously, the thinnest X-ray detectors created were between 20 and 50 nanometres.

Considerable work remains to explore the full potential of the SnS X-ray detectors, but Professor Jacek Jasieniak of Monash's Department of Materials Science and Engineering, the senior author of the paper, believes it's possible this could one day lead to real-time imaging of cellular processes.

"The SnS nanosheets respond very quickly, within milliseconds," he said.

"You can scan something and get an image almost instantaneously. The sensing time dictates the time resolution. In principle, given the high sensitivity and high time resolution, you could be able to see things in real time.

"You might be able to use this to see cells as they interact. You're not just producing a static image, you could see proteins and cells evolving and moving using X-rays."

Why are such sensitive and responsive detectors important? X-rays can be broadly divided into two types: 'Hard' X-rays are the kind used by hospitals to scan the body for broken bones and other illnesses.

Perhaps less well known but just as important are 'soft' X-rays, which have a lower photon energy and can be used to study wet proteins and living cells, a crucial component of cellular biology.

Some of these measurements take place in the 'water window', a region of the electromagnetic spectrum in which water is transparent to soft X-rays.

Soft X-ray detection can be conducted using a Synchrotron, a particle accelerator like the Large Hadron Collider in Switzerland, but access to this type of hugely expensive infrastructure is difficult to secure.

Recent advances in non-synchrotron soft X-ray laser sources may allow lower cost, portable detection systems to be designed, providing an accessible alternative to Synchrotrons for researchers around the world.

But for this approach to work, we will need soft X-ray detector materials that are highly sensitive to low energy X-rays, provide excellent spatial resolution, and are cost effective.

Some existing soft X-ray detectors use an indirect mechanism, in which ionizing radiation is converted into visible photons. This approach allows for multiple energy ranges and frame rates to be studied, but is difficult to prepare and offers limited resolutions.

Direct detection methods are easier to prepare and offer better resolutions, because the detector material can be thinner than indirect approaches.

Good candidate materials need a high X-ray absorption coefficient, which is calculated using the atomic number of the absorbing atoms, X-ray incident energy, density and atomic mass of an atom.

High atomic mass and low energy X-rays favour high absorption, and soft X-rays are more strongly absorbed in thin materials compared to hard X-rays.

Nanocrystal films and ferromagnetic flakes have shown promise as certain types of soft X-ray detectors, but they are not well equipped to handle the water region.

That's where the SnS nanosheets come in.

One of the lead authors, Dr Nasir Mahmood of RMIT University, said the sensitivity and efficiency of SnS nanosheets depends greatly on their thickness and lateral dimensions, which are not possible to control through traditional fabrication methods.

Using a liquid metal-based exfoliation method allowed the researchers to produce high quality, large area sheets with controlled thickness, which can efficiently detect soft X-ray photons in the water region. Their sensitivity can be further enhanced by a process of stacking the ultrathin layers.

They represent major improvements in sensitivity and response time compared to existing direct soft X-ray detectors.

The researchers hope their findings will open new avenues for the development of next-generation, highly sensitive X-ray detectors based on ultrathin materials.

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