DEEP-SPACE ASTRONOMY SENSOR PEERS INTO THE HEART OF AN ATOM

Initially developed for space observations, the Compton camera has now proven its worth as a tool for addressing complex scientific questions in other domains as well

Scientists have taken an instrument originally designed to study huge celestial objects in the cosmos and repurposed it to investigate the world on an infinitely smaller scale. With this instrument, they managed to probe the heart of an atom. The team wanted to understand quantum-scale changes that occur within unstable atoms and realised there’s a state-of-the-art gamma-ray polarimeter they could tap into.

This device, known as a Compton camera, can measure the polarisation of highenergy light waves. In other words, it can dissect what direction such high-energy light is oriented towards.

This instrument was built for deep-space astronomy, not atomic investigations. In fact, scientists constructed it because they wanted to place it on the Hitomi satellite to make observations of high-energy cosmic processes. Yet the camera has now proven its versatility. By capturing the polarisation of gamma rays emitted from atomic nuclei rather than faraway galactic objects, it managed to reveal the internal structure of the atomic nucleus, as well as any changes such nuclei may be undergoing.

Compton cameras are used to determine the direction and energy of gamma rays using a phenomenon called Compton scattering. This happens when a highenergy particle of light, or photon, bounces off a charged particle, usually an electron. This forces the photons hitting the electrons to scatter, meaning they transfer some of their energy and momentum to the particles they’ve just hit. In turn, those electrons can recoil and essentially pop off the atom they were previously attached to. This process can help reveal something about the atom that’s involved. “The research team demonstrated that this Compton camera serves as an effective polarimeter for nuclear spectroscopy, revealing insights into the nuclear structure,” Tadayuki Takahashi, researcher leader and Kavli Institute scientist, said. “Developed initially for space observations, this instrument has now proven its worth as a tool for addressing complex scientific questions in other domains as well.”

Atoms are made up of smaller particles like protons and neutrons

You can think of atoms as being composed of ‘shells’. Each shell is filled with varying portions of negatively charged electrons ‘buzzing’ around. The outermost shell is known as the valence shell and the electrons within the valence shell are called valence electrons. These atomic shells surround a central nucleus composed of positively charged protons and electrically neutral neutrons. The number of protons in an atomic nucleus defines what element that atom represents.

Gamma rays are made up of particles of light

An unstable atomic nucleus will attempt to reach stability by ejecting a proton or a neutron. This is known as radioactive decay, and it’s a process that carries energy away from the atom in the form of photons.

Gamma rays are a kind of photon, and the Compton camera can detect those gamma rays. The polarisation of photons from charged particles turns unpolarised light into polarised light, with the orientation of polarisation arising as a result of the scattering angle. The Compton camera can precisely measure this angle and the polarisation of gamma rays, indicating properties of particles within the atom, such as the value of quantum mechanical characteristics called ‘spin’ and ‘parity’.

The scientists used accelerator experiments at the RIKEN research institute in Japan to perform a series of nuclear spectroscopy tests that involved blasting a film of iron nuclei with a beam of protons. This caused the electrons in the thin iron film to reach an excited state and emit gamma rays as they returned to their ground state. The team controlled both the position and intensity of these emissions artificially. This allowed for a detailed analysis of scattering events and the realisation of a highly sensitive polarisation measurement to test the capabilities of the Compton camera.

The emitted gamma rays were measured, revealing a peak structure, and the team was able to determine the angle at which photons were scattered. The team expected that their results could be crucial for investigating the

structure of rare radioactive nuclei, but even the lead researcher was surprised by just how successful this test was. “The research group, composed of experts in astronomical observation and nuclear physics, anticipated to some extent that gamma-ray polarimetry would be feasible for nuclear gamma-ray spectroscopy experiments,” Takahashi said. “However, the performance and results surpassed all expectations.”

These experiments could be the tip of the iceberg when it comes to using space instruments to investigate atomic nuclei. “There are various types of Compton cameras in astronomical observation, and they could be used similarly to measure the linear polarisation of photons,” Takahashi concluded. © Getty Images