Context: Researchers in the RIKEN Nishina Center for Accelerator-Based Science, in Japan have used Self-Confining Radioactive Isotope Target (SCRIT) to find the internal Structure of unstable Casesium-137 Nuclei.

Historical Context 

• Henri Becquerel discovered that uranium gives off invisible rays that affected photographic film in 1896. 
• Madam Curies built upon his work, named the phenomenon radioactivity and was also able to discover other radioactive elements.
• Rutherford’s work
  • By studying the absorption of radioactivity by thin sheets of metal foil, Rutherford was able to find at least two components of the radiation, which he named α (alpha) and β (beta).
  • Rutherford was also able to prove that radioactivity involved the transmutation of one element to another and discovered the concept of radioactive half-life.

Experiment of Ernest Rutherford, Hans Geiger, and Ernest Marsden

• In this experiment, radioactive decay of radium generated a beam of alpha particles which were directed normally onto a very thin sheet of gold foil placed in an evacuated chamber.

• Using a zinc sulfide screen at the focus of a microscope, particles deflected at any given angle could be detected.
• The actual results of the experiment showed that even though many alpha particles passed through as expected, many others were deflected and very few through angles larger than 90 degrees.
  • Rutherford said about the result that “it was almost incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.”
• Conclusion drawn from the experiment:
  • Rutherford rejected J. J. Thomson’s plum pudding model and instead proposed his own planetary model.
  • This model comprised of a high central charge concentrated into a very small volume when compared to the atom, with this central volume also accounting for the bulk of the atomic mass.

Experimental Setup to Probe a Normal Nucleus: Robert Hofstadter’s Apparatus

Robert Hofstadter developed an apparatus for studying nuclei’s internal structure. 

• Matter is composed of atoms with small nuclei surrounded by electrons.
• A high-energy electron beam from an accelerator was directed towards nuclei and by examining the scattering of the electrons, he could investigate how charges were distributed. 
• He also investigated how the magnetic moment within the nuclei’s protons and neutrons was distributed. 
• Physicists were able to ‘see’ inside stable atoms, and then inside their nuclei, by using other particles. 
• Nuclei were thereby proven not to be homogeneous, but to have internal structures.
• Based on these interactions, understanding of how charges and magnetic fields are arranged inside a nucleus was developed.

Electron Scattering for Short-lived Unstable Nuclei

The setup for probing internal structure used thin foils which contained numerous nuclei 

• The new setup uses an apparatus to hold the nuclei of caesium-137 atoms as well as making sure electrons can interact with them, using a system called SCRIT (Self Confining Radioactive Isotope Target).

Method 

Production of caesium-137 ions (atoms stripped of electrons)

• At first the researchers accelerated electrons in a particle accelerator to energise them.
• After that they smashed them into a block of uranium carbide.
• This produced a stream of caesium-137 ions (atoms stripped of electrons). 
• This isotope of caesium produced has a half-life of around 30 years.
• The target Cs (137) ions were trapped with a new target-forming technique that makes a high-density stationary target from a small number of ions by confining them in an electron storage ring.

Transporation to SCRIT 

• The ions were then transported to the SCRIT system.
• This system enables trapping the target ions in three dimensions along the electron beam using the electric attractive force between the ions and the electrons.
• Using ion beam generation and beam-stacking system enables us to extract the caesium-137 unstable nuclei as a pulsed beam immediately after the photofission of uranium.
• This results in an increasing chance of overlapping between the target caseium ions and the electron beam.
• SCRIT allowed the researchers to achieve this with as few as 10⁸ caesium-137 ions.
  • Without SCRIT, they would have required a trillion-times more.

Studying of Electron-ion Interaction 

• When an electron is scattered by an atom’s nucleus, it behaves like a wave during the interaction. Once scattered off, the electron-waves interfere with each other. 
• The physicists used a device called a magnetic spectrometer to record the resulting interference pattern.
• Information about a nucleus can be more readily obtained from the electrons’ interference patterns.

Significance:

• The development of this setup is significant because it allows scientists to probe the nuclear structure of short-lived atomic nuclei using electron scattering, leading to a better understanding of nuclear physics.
• The obtained angular distribution of elastically scattered electrons is consistent with a calculation. 
• This success marks the realization of the anticipated femtoscope which clarifies the structures of exotic and short-lived unstable nuclei.
  • “femtoscope,” a tool that can probe the femtometer scale of atomic nuclei (10^-15 meters). 
  • Using it we can probe the structures of some unstable nuclei have been hypothesised to have a non-uniform density of protons and neutrons.
  • With it the hope is that a unifying theory of nuclear structure will be found somewhere in the gaps between expected and unexpected shapes.
• This tool can help tackle the longstanding problem of explaining the structure of atomic nuclei and potentially shed light on phenomena like the “island of stability.”
• Future Implications the potential for the femtoscope to reveal new insights into the structures of nuclei that are expected to have unique shapes, including non-uniform density and unusual behavior.

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