Isotope beams at RIKEN

Last month there was RIKEN Open Day, where RIKEN opens its doors to the general public. They prepared various talks on some of the research that is done here and let you have a look at a lot of the fun labs with fancy equipment. So I took the chance to see the RI-beam factory deep in the underground of RIKEN.

What is the RI-beam factory?1

RI stands for radioactive isotopes. Basically it is a large experimental facility where various (especially heavy) isotopes are accelerated to high velocities and then shot into a target made of stable nuclei. These collisions have a chance to produce very short lived, but new isotopes. In general an isotope is just a variant of a known chemical element with different neutron numbers. However, under the right conditions an entirely new element could be found. This is exactly what happened in 2004 when element 113 was created at the RIBF at RIKEN. The element is also called “Nihonium”. It is the only element that has been found outside of Europe or the USA.

Another important research field of the RIBF is the mutation of plants to create new species. This includes a new strain of yeast which is used to produce sake.2

How does acceleration work?

There are a few different kinds of accelerators used in sequence at RIBF. The beam usually starts in a linear particle accelerator and then moves through a series of ring cyclotrons to increase the velocities of the particles in the beam even more, the biggest of which is called SRC. If you are curious about the energies of the beam, here is a chart of the different isotopes used and what energies they can achieve. 3 At the end of the beam there is a beam separator called BigRIPS which can “filter out” different kinds of isotopes. The RIBF page has a nice plan of their facility.

What does a cyclotron look like up close?

The cyclotron I got to see is the SRC, a superconducting ring cyclotron weighing 8300 tons!

Photo of the SRC by Hideto Enyo

Two principles are important for its function:

  • Having a magnetic field that forces the charged particles into a circular orbit as a result of the Lorentz force.
  • Having some oscillating electric fields which accelerate the particles on their way through the magnetic field.

The first one is achieved here by superconducting magnets which need to be kept at low temperatures.

Cooling system using liquid Helium.

The particle beam is injected in the centre of the cyclotron, which in a simple model has two halves which are isolated from each other. The voltage then gets applied between these two halves such that the charged particles get accelerated in the direction they are flying when crossing between them. To guarantee continued acceleration the frequency of the alternating voltage has to match the travel time of the particles. The SRC works on the same principle but with a little more complex geometry. More or less, the green parts hide the superconducting magnets (coils) and the violet parts contain the acceleration chambers.

The yellow pillar controls the frequency of the voltage in the acceleration chamber.

As the particles are accelerated they emit radiation. To shield the outside of the SRC from some of the radiation as well as from the strong magnetic fields, the whole thing is wrapped in thick iron walls:

Heavy metal (doors)! 🙂

The beam travels for 14,000m inside the SRC, increasing its radius from 3.56m to 5.36m before it gets extracted and sent on its merry way.

Beam extraction line. The green “boxes” are quadrupole magnets which help focus the beam.
  3. To get a rough estimate of the velocity, you can use the formula v^2 = \frac{2E}{m}. It’s easiest to use the unified atomic mass unit as 931.4940954(57) MeV/c². This gives you the velocity as a fraction of the speed of light. Notice that energies given in the chart are per nucleon! (You might also want to compare it with the relativistic kinetic energy equation.)

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