by Scientists have taken an instrument originally designed to study massive celestial objects in the cosmos and repurposed it to investigate the world on an infinitely smaller scale. With this instrument, they succeeded in exploring the core of the atom.
The team wanted to understand quantum-scale changes that occur within unstable atoms, and realized that there was a state-of-the-art gamma-ray polarimeter that they could use. This device, called a Compton camera, can measure the polarization of high-energy light waves. In other words, it can break down the direction in which such high-energy light is focused.
The only thing, though, is that this instrument was technically built for deep-space astronomy, not atomic investigations. In fact, the scientists built it because they wanted to put it on the Hitomi satellite to observe high-energy cosmic processes. However, the camera has now proven its versatility. By capturing the polarization of gamma rays emitted from atomic nuclei rather than from distant galactic objects, he was able to reveal the internal structure of the atomic nucleus as well as any changes these nuclei may be undergoing.
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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 particle of light, or “photon,” bounces off a charged particle, usually an electron. This interaction forces the photons to “scatter” the electrons, meaning they transfer some of their energy and momentum to the particles they just hit. After that, those electrons can recoil and essentially pop off the atom they were previously attached to. This process can reveal something about the atom in question.
“The research team showed that this Compton camera is an effective polarimeter for nuclear spectroscopy, revealing insights into the nuclear structure,” Tadayuki Takahashi, head researcher and scientist of the Kavli Institute for the Physics and Mathematics of the Universe, told Space. com. “Originally developed for space observations, this instrument has now proven its value as a tool for addressing complex scientific questions in other areas as well.”
Heart of an atom
You can think of atoms as being made up of “shells.” Each shell is filled with different parts of negatively charged electrons “buzzing” around; the outermost shell is called 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 which element that atom represents.
For example, hydrogen is the lightest element in the universe, and it always has one proton in its 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. Number of neutron in a nucleus it does not define what element is in an atom, so it can change. For example, hydrogen can have no neutrons, one neutron for deuterium, or two neutrons for tritium. The atoms are different in weight, however, called “isotopes.” Some isotopes are stable – others are not.
Although there are 270 known stable atomic nuclei in nature, the number of known isotopes of an element jumps to 3,000 when unstable atomic nuclei are included.
Interestingly, scientists have recently noticed phenomena associated with unstable atomic nuclei that are not seen around the stable ones. These include anomalies in electron energy levels as well as the disappearance and emergence of so-called “magic numbers”. Magic numbers refer to the amount of electrons it would take to fill those energy shells around an atomic nucleus. Traditionally, these numbers are 2, 8, 20, 28, 50, 82 and 126.
Until now, however, conventional methods have not been sufficient to investigate changes in the 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 made by atoms.
Herein lies the important part of the team’s investigation.
An unstable atomic nucleus will attempt to achieve stability by ejecting a proton or neutron. This is called radioactive decay, and is a process that carries energy away from the atom in the form of photons. Gamma rays are a type of photon – and Compton’s camera can detect those gamma rays! An understanding of the transition between instability and stability might help to decode some of those strange atomic phenomena that have been observed by scientists.
Therefore, these researchers believed that a Compton camera, including something called a Cadmium Telluride (CdTe) semiconductor imaging sensor, might be suitable for measuring the polarization of gamma rays from unstable nuclei. Again, this is because such a sensor offers high detection efficiency and precise accuracy when determining the location of gamma rays (although it was originally intended for deep space gamma ray signals).
Polarization of photons from charged particles converts unpolarized light into polarized light, and the orientation of the resulting polarization is the result of the scattering angle. The Compton camera can precisely measure this scattering angle and the polarization of these gamma rays, which reveals the properties of the particles inside the atom, such as the value of the quantum mechanical characteristics called “spin” and “parity.”
The scientists used accelerator experiments at the RIKEN research institute to carry out a series of nuclear spectroscopy tests involving blasting a film of iron nuclei with a proton beam. The reason for this was that the electrons in the thin iron film reached an excited state and emitted gamma rays as they returned to their ground state. The team artificially controlled the location and intensity of these emissions. This allowed a detailed analysis of scattering events and the realization of highly sensitive polarization measurements to test the capabilities of the Compton camera.
“The multilayer CdTe Compton camera has several characteristics that make it well suited for this research. The first is the CdTe detection efficiency,” said Takahashi. “Typically, gamma rays emitted from nuclei have energies in the order of Mega-electron Volts (MeV), where the detection efficiency of a gamma-ray polarimeter is low. However, the 20 layers of CdTe greatly increase efficiency these sensors. gamma rays.”
The Kavli Institute for Physics and Mathematics of the Universe scientist also said that the CdTe sensor developed by his group is a high-energy solution for sub-MeV gamma rays.
“Finally, it achieves a few millimeters of positional resolution within the detector’s effective distance, enabling it to ‘see’ detailed Compton scattering patterns,” Takahashi said. “These patterns show the characteristics of linear polarization of light, including gamma rays.”
The gamma rays emitted were measured, revealing a peak structure, and the team was able to determine the angle at which the photons were scattered. The team hoped that their results could be crucial for investigating the structure of rare radioactive nuclei, but even the lead researcher was surprised at how successful this test was.
“The research group, composed of experts in astronomical observation and nuclear physics, expected to some extent that gamma-ray polarization would be possible for nuclear gamma-ray spectroscopy experiments,” Takahashi said. “However, the performance and results exceeded expectations.”
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These experiments could be the tip of the iceberg when space instruments are used to probe atomic nuclei.
“There are different types of Compton cameras in astronomical observation, and they could also be used to measure the linear polarization of photons,” concluded Takahashi.
The team’s research is published in the journal Scientific Reports.
