The cyclotron

G. Luijckx

290

The success of the concept of using a high-voltageinstallation to accelerate particles was quickly fol-lowed by the idea of a cyclic accelerator. In thissystem particles pass through the same acceleratingfield several times, so that a high energy (e.g. 10 MeV)can be obtained with an alternating voltage of relativ-ely low amplitude (e.g. 10 kV). In about 1930 E. O.Lawrence and M. S. Livingston constructed a protonaccelerator based on this principle in Berkeley, Cali-fornia: the first cyclotron. They obtained an energyof 80 keY. A few years later Berkeley had a cyclotrondelivering deuterons at 8 MeV, whereas the Cockroft-Walton high voltage accelerator in Cambridge onlygave about 1 MeV. Shortly afterwards Berkeley evenmanaged to reach 25 MeV.

Attempts to get above this energy, however, werehampered by the relativistic increase in the mass ofthe accelerated particles. A plan to go beyond thislimit at Berkeley failed to materialize because of theoutbreak of the Second World War. The intentionwas to use a higher accelerating voltage, which wouldhave meant that the particles would need to makefewer revolutions to attain the same energy. After thewar, however, observations by V. I.Veksler (U.S.S.R.)and E. M. MacMillan (U.S.A.) showed that the par-ticles remain in phase with' the accelerating voltage ifeither the acceleration frequency or the strength of themagnetic field in which the acceleration takes placevaries slowly. Use of these methods to compensatefor the effect of the relativistic increase in mass ledto new types of accelerator - the synchrocyclotronand the synchrotron - which would eventually giveenergies in the OeV range. In 1947 Berkeley hada synchrocyclotron that delivered deuterons of195 MeV and helium nuclei of 390 MeV; in 1957this machine could accelerate protons up to an energyof 720 MeV.

Some aspects of the history of the Philips cyclotron- with which the names of C. J. Bakker and F. A.Heyn are particularly associated - are described inthe introductory article [1] of this issue. Philips'

Ir G. Luijckx is with IKO (Institutefor Nuclear Physics Research),Amsterdam.

Philips tech. Rev. 39, 290-292, 1980, No. 11

original plan for the construction of a cyclotron wasmodified to take advantage of the latest informationand the Philips synchrocyclotron [2] at IKO was com-pleted in 1949. After extensive modifications the syn-chrocyclotron was donated to IKO in 1961. One ofthe modifications made was to replace the oil-cooledmagnet coils by water-cooled aluminium coils. Variousother changes were to be made later so that the equip-ment could meet the latest requirements for nuclear-physics research. It was finally shut down in 1977so that all the Institute's manpower could beemployed on the most recent accelerator develop-ment at IKO: the construction of a linear electronaccelerator [3].

By that time more than thirty thousand targets hadbeen irradiated and about twenty new radioisotopeshad been found with the aid of the Philips synchro-cyclotron. The research had produced hundreds ofscientific publications and numerous doctorates. Thesynchrocyclotron had proved to be a valuable instru-ment.

Operating principle of a cyclotron

A cyclotron (fig. 1) contains two hollow acceler-ating electrodes, the dees. An alternating voltage isapplied between the dees, which are located betweenthe poles of a magnet. The dees are enclosed in avacuum chamber to prevent unwanted effects due tocollisions between the accelerated particles and air. Atthe centre of the vacuum chamber, a gas - for ex-ample hydrogen gas if hydrogen nuclei (protons) areto be accelerated - is ionized in the 'ion source' [4].

The ions are accelerated in the electric field betweenthe dees. Under the action of the Lorentz force theydescribe a circular orbit in the cavity formed by thedees. The electric field between the dees is reversed sorapidly that the next time the particles cross from onedee to the other the particles are subjected once againto an accelerating force. (For example, to accelerateprotons in a magnetic field of 1.4 T requires anacceleration frequency of 20 MHz.) The highervelocity, however, corresponds to a circular orbit of a

Philips tech. Rev. 39, No. 11 CYCLOTRON 291

greater radius until ultimately the particles reach theouter boundary of the magnetic field and no furtheracceleration is possible. This means that there is amaximum to the velocity - or energy - that can beachieved.

Fig. 1. Above: Prof. Bakker and Prof. Heyn - holding an instru-ment - with the brand-new synchrocyclotron. Below: HRH PrinceBernhard testing the power of the magnetic field of the synchro-cyclotron during its official commissioning in 1949. Prof. Bakkeris on the left.

Synchrocyclotron

When particles are accelerated to higher energiesthe relativistic increase in mass becomes important.This makes the angular velocity decrease gradually,until eventually the particles cross over between thedees when there is a decelerating field, and this inter-feres with the acceleration process. In a synchrocyclo-tron this effect is prevented by modulating the acceler-ation frequency so that it decreases synchronouslywith the orbital frequency. In this way it is possible toobtain a higher energy of the particles. The methodhas the disadvantage, however, that only one groupof particles can be brought from zero to maximumenergy in each modulation period. Particles are de-

livered in pulses, and the time interval between twosuccessive pulses is determined by the modulationperiod.

Since 1950 it has been possible to get over this problem. Byincreasing the strength of the magnetic field appropriately as theradius increases it is possible in principle to continue accelerating atconstant frequency in spite of the relativistic increase in mass. Insuch a magnetic field, however, the particle orbits are not stable inthe transverse direction: any small transverse path deviation con-tinues to increase. Stability can be restored by modulating the field-strength in the azimuthal direction and increasing the meanfield-strength as the radius increases. This leads to AVF cyclotrons(AVF: Azimuthal Varying Field).

External beam

In the early days the Philips synchrocyclotron atIKO was used for making radioisotopes by intro-ducing a target into the vacuum chamber (using aspecial vacuum lock) and irradiating it with particles.The energy of the particles was varied by selecting theorbital radius where the target was located.

Later there arose a keen interest in the study ofnuclear reactions. These take place so quickly, how-ever (10-22 s), that the decay products can only beobserved during the irradiation. For this kind ofnuclear experiment the accelerated particles had to bebrought outside the synchrocyclotron ('beam extrac-tion'). It was not easy to produce an external beamwith a relatively small energy spread, however, be-cause the particle orbits corresponding to differentenergy values are less than a millimetre apart. Afteran attempt at electrostatic deflection had failed, it wasdecided to use a 'regenerator', a highly suitable ironconstruction based on K. J. Le Couteur's machine inLiverpool. In this arrangement the magnetic field isperturbed at its outer edge in such a way that the par-ticles begin to vibrate strongly in the radial direction.After the particles have passed through the regenera-tor several times the distance between two adjacentorbits is large enough to allow only the particles in theoutermost orbit to leave the accelerator in a path de-termined by control magnets. Five to ten per cent ofthe internal particle current could be extracted in thisway, a percentage quite normal for that time. (In latercyclotrons almost 100070has been achieved.)

[IJ See J. M. Waalwijk and N. Wiedenhof, The Institute forNuclear Physics Research 'has finished its work', this issue,page 286.

[2J F. A. Heyn and J. J. Burgerjon, The synchrocyclotron at Am-sterdam, I, 1I, Ill, IV, Philips tech. Rev. 12,241-247,247-256and 349-364, 1950/51, and 14, 263-279,1952/53.

[3J See for example P. Bakker, Linear electron accelerators, thisissue, page 325.

[4J See pages 267-270 of F. A. Heyn and J. J. Burgerjon, Thesynchrocyclotron at Amsterdam, IV, Philips tech. Rev. 14,263-279, 1952/53.

292

Stochastic extraction

Between 1965 and 1970 the BOL [5] nuclear-re-search system was developed at lKO. It was speciallydesigned to observe two or more reaction products or'coincidences' from a single nuclear reaction. Tominimize the problems that might be caused byrandom coincidences - the simultaneous observationof two or more reaction products of different nuclearreactions - the particle current should have as fewpeaks as possible, since the number of random coinci-dences is proportional to the square of the current,whereas the number of real coincidences is propor-tional to the current. Because of the modulation,however, a particle current leaves a synchrocyclotronfor only ten per cent of a modulation period: a moreuniform distribution was therefore desirable. Thismodification was achieved by giving the acceleratoran extra accelerating electrode, the 'cee', with itsassociated frequency generator and a high-speed high-voltage switch (pulser) to switch off the dee oscillatorwhen the particles had almost reached the maximumenergy. The acceleration was then taken over by thecee, with a modulated acceleration frequency, so thatthe particle orbits were spread out. The accelerationfrequency of the cee then slowly decreased, enablingthe particles to be guided ou t of the accelerator insuccession. This stochastic extraction continued, eventhough the high voltage at the dees had already beenswitched on again and a new acceleration cycle started(fig. 2).

Shutdown

After the synchrocyclotron was transferred to IKOin 1961 a number of other improvements and modifi-cations were made. The angular spread of the externalbeam was guided by magnets towards screened-off

CYCLOTRON Philips tech. Rev. 39, No. 11

experimental areas and, finally, a newly developedradio- frequency system [6] was installed for genera-ting the accelerating voltage.

From 1973 onwards the synchrocyclotron graduallycame to be used less and less and on the 1st April 1977the accelerator was finally shut down. After all themodifications the only component remaining from thesynchrocyclotron of 1946 was the iron of the magnet.

1

2

3

4

Fig. 2. Stochast ic extraction. Oscillogram of the particle current Ifrom the synchrocyclotron, the pulser voltage 2, the frequencymodulation 3 of the cee oscillator and the voltage 4 at the dee elec-trode. The time I is plotted on the horizontal axis; one scale divisioncorresponds to 100 >LS.

[5] See for example R. van Dantzig, BOL, this issue, page 302.[6] The radio-frequency system consists of a resonant cavity,

formed by the dee electrodes, a coaxial transmission line and avariable capacitor, the 'modulator'. This circuit can be con-sidered as a resonant line with a shortening capacitor al eachend. Altering the characteristic impedance of the transmissionline changes the electrical length of the line and hence theresonant frequency. The circuit forms part of a power oscil-lator (Colpitts circuit). The r. f. system had a frequency rangeof 10 to 22 MHz, so that it was possible to accelerate four typesof particle: protons, deuterons, helium-3 and helium-4 nuclei.Feedback at the various frequencies was obtained with the aidof large variable capacitors. Further details are given in w. vanGenderen. Multi-particle rf-systern for the IKO synchrocyclo-tron, Nuc!. Instr. Meth. 54, 288-292, 1967.

Philips tech. Rev. 39, No. 11 293

Casting one of the inner copper hemispheres for the BOL nuclear measurement project.