Cyclotron

The cyclotron (Greek kyklos, lat heterocycle "circle ", " circular " ) is a particle accelerator, namely a circular accelerator. A magnetic field brings to accelerating particles in a spiral-like path, on which the acceleration sections are run through again and again. Due to the multiple use of the acceleration sections of circular accelerators are generally more economical than linear accelerator.

Ion cyclotrons accelerate Today at energies in the range of about 10 to 500 MeV. For energies that are large compared to the rest mass of the particles, cyclotrons are less suitable, so they are no longer used for electrons today.

History

Ernest Lawrence suggested in 1930 the concept of the cyclotron, the first circular accelerator before. Realized it was first used by MS Livingston. Together, the two reported in 1932 through commissioning of the first cyclotron in Berkeley, USA, the protons accelerated up to 1.2 MeV.

In Paris, Frédéric Joliot- Curie began with the construction of a cyclotron, but was delayed by the Second World War. It was only after the armistice between Germany and France in 1942, it was completed in collaboration with Walther Bothe and Wolfgang Gentner. 1943, a cyclotron in Heidelberg was built in Bothe's Institut and put into operation; the construction of which was supported by the Army Ordnance Department, although Bothe had declared that the device will be useful only for medical and biological research.

Classic cyclotron

The classical cyclotron consists of a large electromagnet with a homogeneous and temporally constant field and a flat round vacuum chamber between the poles. Inside the chamber, the Duanten, two hollow semicircular metal electrodes are (due to their D -shape in English Dees called ), and in the center of an ion source. The Duanten are part of a high-frequency oscillating circuit. The alternating electric field in the gap between the Duanten decomposes the ion " cloud " the source "packages" and this accelerates alternately in one of the Duanten into it. Inside the Duanten there is no electric field; the particles describe here under the Lorentz force of the magnetic field an arc. At the outer edge of the chamber there are usually a deflection electrode, a so-called septum. Your field against a ground electrode counteracts the magnetic deflection and thus deflects the particle beam on a target lying outside, the target.

The ions with the mass and the charge move in the magnetic field of flux density with the orbital frequency ( " cyclotron frequency " )

In the practically achievable with iron magnet magnetic flux densities - order of 1 Tesla - is this frequency for light ions about 5 to 20 megahertz. The frequency of the accelerating voltage is now chosen equal to the cyclotron frequency of each type of ion. Thus, the ions impinge at each reaching the gap again at an accelerating electric field. They thereby gain energy, so that the radius of the following circle arc is greater. The web in the nip itself is one piece each of a spiral path.

The same extent as the track radius increases, the velocity of the ions. Their orbital period thus remains constant. The frequency of the acceleration voltage can thus also be kept constant.

In classical cyclotron 20 MeV ( deuterons ) and 40 MeV ( 4He ions) were at accelerating voltages of several hundred volts in approximately 50 rounds final energies of around 10 MeV (protons), is reached.

Higher particle velocities

The classical cyclotron works only with non- relativistic particle velocities; at higher speeds, the orbital period of the ions no longer remains constant but increases markedly, so they get over the acceleration frequency " out of step ".

This corresponds to the fact that the above equation holds only approximately for the cyclotron frequency. The exact, valid for all particle velocities equation is

This is the rest mass of the particle and the Lorentz factor,

Is the speed of light. For apparently and it results in the simplified equation.

Synchrocyclotron

So that the cyclotron for higher particle velocities can be used, one can modulate the high frequency, that is lower during acceleration in accordance with the gradually decreasing the cyclotron frequency of the particles, for example by means of a rotating capacitor in the resonant circuit. Such Synchrozyklotrone were built in the 1950s and reached with light ions up to 800 MeV. Their disadvantage is that only a narrow group of bunches can be accelerated simultaneously. Only when they have passed over and the high frequency is returned to the initial value, can " start " the next group. The beam is thus inevitably pulsed with a low duty cycle of the order of 1 %. This is for physical experiments usually detrimental, however, irrelevant for many applied purposes.

Isochronzyklotron

The synchrocyclotron was technically obsolete by the Isochronzyklotron. In this, the rotational frequency, instead of modulating the high frequency is kept constant for relativistic ions by an inhomogeneous, is namely used outwardly increasing magnetic field. However, such a field acts on the beam defocusing, ie scattering. Isochronzyklotrone therefore could only be built after it was discovered by Livingston and Other strong focusing. For this purpose, the magnet is in sectors designed to be field in the radial direction has alternating positive and negative gradients. This results in a focusing; it clearly corresponds to the serial arrangement of converging and diverging lenses for light, with a focus as a net effect. If the magnet is accordingly divided into individual sectors, so pie-shaped individual magnets each with its own winding, one speaks of a Sektorzyklotron. When the compact cyclotron sectors, however, are realized by the pole shoe on a common yoke.

Some newer Isochronzyklotrone have superconducting magnetic coils for energy savings. Also often not two, but three or more acceleration electrodes are used; they are also called Duanten or laboratory jargon Dees, although they are not D-shaped.

The current strength of a Isochronzyklotron beam is typically between about 10 and 100 micro amperes.

H - cyclotron

Cyclotrons for proton, the most commonly used ions work in some cases as H - cyclotron. Negative hydrogen ions ( H, "H minus " ) accelerated in him. This happen after the acceleration a mounted in the gap of graphite foil ( " Stripper " ), the " stripping " the two electrons. The ion is now a proton and is deflected due to its reverse charge in the magnetic field of the cyclotron to the other side, ie from the cyclotron addition. This type of beam extraction allows opposite the baffle method leads to larger currents of the beam.

Applications

Cyclotrons are used for example in physical research to trigger nuclear reactions. They are also used for medicinal radiation therapy, as well as for the production of radionuclides for diagnostic purposes, e.g. for positron emission tomography ( PET). Many of the medically used radionuclides have very short half-lives of minutes to a few hours; therefore they can not be transported over long distances and must be produced close to the point of use. Is suitable for this Protonenzyklotron a typical 15 to 30 MeV. In Germany there are about 25 cyclotrons that produce these radionuclides.