Positron Imaging Centre

The MC40 Cyclotron

   Images of the MC40 Cyclotron

The MC40 Cyclotron is the third cyclotron to be operated in the School of Physics and Astronomy at Birmingham. The Nuffield Cyclotron operated from 1948 until 1999, and the Radial Ridge Cyclotron from 1961 until 2003. The MC40 Cyclotron is one of several such cyclotrons manufactured by Scanditronix, and was originally installed in the Veterans Affairs Medical Center, Minneapolis, where it operated from 1993 to 2001. The cyclotron was purchased by the University of Birmingham early in 2002, was moved to Birmingham during 2002 and after a period of commissioning has been operational since early 2004. It is used primarily for the production of radio-isotopes for a range of users for medical imaging, as well as activating particles/beads for use in PEPT imaging.

How a Cyclotron Works

Proton-rich positrons emitting radionuclides are generally produced by nuclear reactions induced by accelerated ions from a cyclotron. The simplest such reaction is the (p,n) reaction, in which a proton is captured by the target nucleus and a neutron is emitted. In order for such reactions to occur, the incident ion (e.g. the proton) must be accelerated to sufficient energy to overcome the Coulomb repulsion between it and the target nucleus.

A cyclotron is a particle accelerator in which ions are accelerated as they orbit in a magnetic field. In the simplest form it consists of a uniform magnetic field, B. The circular area is divided into two D-shaped regions ("dees"), one of which is held at earth potential while an alternating potential is applied to the other. Ions produced in an ion source near the centre of the area travel in circular orbits, whose radius r is related to their speed v through the equation:

where m and q are the mass and charge of the particle). As they cross the gap between the two dees they gain energy from the potential difference and so the radius of their orbit increases. Ions spiral outwards, acquiring energy on each orbit, until at the outside of the cyclotron they reach their full energy.

In order for the ions to be accelerated every time they cross the gap between the dees, the potential between the dees must alternate with a frequency which is an exact multiple of the orbital frequency of the particles. Since r is proportional to v, the orbital period of the particles is independent of their energy neglecting relativistic effects).

What the MC40 Cyclotron Can Do

In the Birmingham MC40 cyclotron the orbit is divided into four quadrants. Radiofrequency alternating voltage is applied to two 90 degree cavities, which are still referred to as "dees" even though they are actually shaped in the form of a half-D. The dees are mounted diametrically opposite each other, and the intervening 90 degree sections ("dummy dees") are at ground potential. Ions are accelerated four times on each orbit as they cross the gaps between dees and dummy-dees. There are two modes of operation, as can be seen in the figure below.

In the fundamental (N=1) mode, the RF performs one cycle for every orbit of the beam, and the potentials on the two dees are in antiphase. In the first harmonic (N=2) mode, the RF performs two cycles during every orbit, and the potentials are in phase. Note that in both cases, the positive ions experience a negative potential as they enter the dee (pulling them in) and a positive potential as they leave (pushing them out). Using these two modes, a wider range of ions and energies can be accelerated using a relatively limited range of RF frequencies.

These figures show the variation in potential on dee 1 (upper) and dee 2 (lower) during the course of one orbit. The shaded region denotes the period when the beam is inside this dee.

The RF power supply to the dees is tuned to resonate at the required frequency, primarily by adjusting the length of a hollow cavity to match 1/8 wavelength. The system can be tuned to any frequency between about 14.5 and 26 MHz.

Ions of hydrogen (protons or deuterons) or helium (3He or 4He) are produced in an ion source near the centre of the accelerator. The ion source is divided into two sections, one supplying N=1 beams and the other for N=2, with the appropriate geometry to direct the beam on its first orbit in each case. The ions perform approximately 500 orbits before reaching the outside of the cyclotron. In order to focus the beam during acceleration, the pole faces of the magnet are shaped into three spiral ridges so that the strength of the magnetic field varies azimuthally along each orbit, and as a result the orbits are not circular but more like curved triangles.

The magnetic field is adjustable to match the required final energy. The maximum field achievable is approximately 1.8T. The cyclotron can accelerate ions within the following energy ranges

  • Protons: 38-11MeV (N=1) and 9-3MeV (N=2)
  • Deuterons: 19-5.5MeV (N=2)
  • 3He: 53-35MeV (N=1) and 27-9MeV N=2)
  • 4He: 37-11MeV (N=2)

On its final orbit, the beam is extracted by diverting it through an electrostatic deflector consisting or a pair of curved electrodes to which a potential difference of up to 50kV is applied. Under favourable circumstances around 60% of the beam can be extracted.

A switching magnet directs the beam along one of 12 possible directions. Eleven of these correspond to target positions within the cyclotron vault. One beam line conveys the beam into an adjoining room where scattering chambers and other research equipment is installed.