Scientists have taken an instrument originally designed to study enormous celestial bodies in the cosmos and repurposed it to explore the world on an infinitely smaller scale. With this instrument they managed to explore the heart of the atom.
The team wanted to understand the quantum-scale changes that occur in unstable atoms, and realized there was a state-of-the-art gamma-ray polarimeter they could take advantage of. This device, known as a Compton camera, can measure the polarization of high-energy light waves. In other words, it can parse which direction such high-energy light is directed.
The only thing is that this instrument is technically built for deep space astronomy, and not for nuclear research. In fact, scientists built 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 polarization of gamma rays emitted from atomic nuclei rather than from distant galactic objects, it has managed to reveal the internal structure of the atomic nucleus, as well as any changes such nuclei may be undergoing.
Related: Atomic clocks on Earth could reveal secrets about dark matter throughout the universe
Compton Chemistry 101
Compton cameras are used to determine the direction and energy of gamma rays using a phenomenon called ‘Compton scattering’.
Compton scattering occurs when a high-energy light particle, or ‘photon’, bounces off a charged particle, usually an electron. This interaction forces the photons that hit the electrons to “scatter,” meaning they transfer some of their energy and momentum to the particles they just hit. In turn, those electrons can recoil and essentially break free from the atom they were previously attached to. This process can help reveal something about the atom involved.
“The research team has shown that this Compton camera serves as an effective polarimeter for nuclear spectroscopy, revealing insights into nuclear structure,” Tadayuki Takahashi, research leader and Kavli Institute for the Physics and Mathematics of the Universe scientist, told Space .com. “Initially developed for space observations, this instrument has now proven its value as a tool for tackling complex scientific questions in other domains.”
The heart of an atom
You can think of atoms as composed of ‘shells’. Each shell is filled with varying proportions of negatively charged electrons that “buzz” around; the outer 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 consisting of positively charged protons and electrically neutral neutrons.
The number of protons in an atomic nucleus determines which element that atom represents.
For example, hydrogen is the lightest element in the universe, and there is always one proton in the atomic nucleus. At the other end of the periodic table is uranium, one of the heaviest natural elements, which always has 92 protons in its nucleus. The number of neutrons in a nucleus does not define which element an atom is, so it can vary. For example, hydrogen can have no neutrons, one neutron in the case of deuterium, or two neutrons in the case of tritium. However, these atoms that vary in weight are called ‘isotopes’. Some isotopes are stable, others are not.
Although 270 stable atomic nuclei are known to exist in nature, the number of known isotopes of elements rises to 3,000 when unstable atomic nuclei are taken into account.
Interestingly, scientists have also recently observed phenomena associated with unstable atomic nuclei that do not occur near stable atomic nuclei. These include deviations in the energy levels of the electrons, as well as the disappearance and creation of so-called ‘magic numbers’. Magic numbers refer to the amount of electrons needed to fill the energy-level shells around an atomic nucleus. Conventionally these numbers are 2, 8, 20, 28, 50, 82 and 126.
However, to date, conventional methods have proven insufficient in investigating changes in nuclear structure associated with these phenomena. This is due to the difficulty of balancing sensitivity and detection efficiency for instruments that analyze the characteristics of transitions undertaken by atoms.
This is the most important part of the team’s research.
An unstable atomic nucleus will attempt to achieve stability by ejecting a proton or a neutron. This is known as radioactive decay, and it is a process that carries energy away from the atom in the form of photons. Gamma rays are a type of photon – and the Compton camera can detect those gamma rays! Perhaps understanding the transition between instability and stability can help decipher some of those strange atomic phenomena that scientists have observed.
These researchers thus believed that the Compton camera, which contains a so-called Cadmium Telluride (CdTe) semiconductor image sensor, could be ideal for measuring the polarization of gamma rays from unstable nuclei. Again, this is because such a sensor offers high detection efficiency and pinpoint accuracy in determining the position of gamma rays (even though it was initially intended for gamma ray signals in deep space).
The polarization of photons from charged particles changes unpolarized light into polarized light, with the orientation of the polarization resulting from the scattering angle. The Compton camera can accurately measure this scattering angle and the polarization of these gamma rays, which indicates the properties of particles in the atom, such as the value of quantum mechanical features called ‘spin’ and ‘parity’.
The scientists used accelerator experiments at the RIKEN research institute to perform a series of nuclear spectroscopy tests that involved inflating 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 artificially controlled both the position and intensity of these emissions. This allowed detailed analysis of scattering events and the realization of a highly sensitive polarization measurement to test the capabilities of the Compton camera.
“The multilayer CdTe Compton camera possesses several characteristics that make it well suited for this study. First is the detection efficiency of CdTe,” Takahashi said. “Normally, gamma rays emitted from nuclei have energies on the order of mega-electron volts (MeV), where the detection efficiency for gamma-ray polarimeters is typically low. However, the 20 layers of CdTe significantly improve the efficiency of detecting them.” gamma rays.”
The Kavli Institute for the Physics and Mathematics of the Universe scientist added that the CdTe sensor developed by his group also achieves high energy resolution for sub-MeV gamma rays.
“Finally, it achieves a positional resolution of several millimeters within the effective area of the detector, allowing it to ‘see’ detailed Compton scattering patterns,” Takahashi added. “These patterns reflect the characteristics of the linear polarization of light, including gamma rays.”
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 how successful this test was.
“The research group, composed of experts in astronomical observation and nuclear physics, expected to some extent that gamma-ray polarimetry would be feasible for nuclear gamma-ray spectroscopy experiments,” Takahashi said. “However, performance and results exceeded expectations.”
RELATED STORIES:
– Albert Einstein: biography, theories and quotes
– 10 mind-blowing things you need to know about quantum physics
— Einstein’s general theory of relativity
These experiments could be the tip of the iceberg when it comes to using space instruments to examine atomic nuclei.
“There are several types of Compton cameras for astronomical observation, and they could be used in the same way to measure the linear polarization of photons,” Takahashi concluded.
The team’s research has been published in the journal Scientific Reports.