Philips Technical Re-vie Bound... · PHILIPS TECHNICAL REVIEW VOL. 12, No. 9

VOL.12 No. 9, pp. 241-272 MARCH 1951

Philips Technical Re-viewDEAIJNG ~TH TEC~CAL PROBLE~

RELATING TO THE PRODUCTS, PROCESSES AND ijWESTIGATIONS OFTHE PHILIPS INDUSTRIES

EDITED BY THE RESEARCH LABORATORY OF N.V. PHILIPS' GLOEILA.MPENFABRIEKEN,EINDHOVEN,NETHERLANDS

THE SYNCHROCYCLOTRON AT AM:STERDAM

I. GENERAL DESCRIPTION OF THE INSTALLATION

Il. THE OSCILLATOR AND THE MODULATOR

hy F. A. HEYN*). 621.384.61

Reports on cyclotrons often have rather the appearance of aiming at overwhelming the publicwith amazement, with all their talk about metres thick magnet poles, hundreds of tons of iron,tens of kilowatts, velocities approaching the velocity oflight, etc. Here, in this description ofthe Philips synchrocyclotron, it will be shown - without any attempt at belittling the impres-siveness of the installation - how it comes about that in the building of a cyclotron one isin many respects obliged to adopt exceptional methods of constTltction.

J. GENERAL DESCRIPTION OF THE INSTALLATION

Philips have huilt a synchrocyclotron whichhas heen installed in the Institute for NuclearResearch at Amsterdam, where it was officiallytaken into use on 10th Novemher 1949. This appa-ratus is now working day after day and only afew interruptions have heen necessary for main-tenance. It is operated hy Philips' personnel andfor two-thirds of the time is at the disposal ofPhilips for their own research work and for theproduction of artificial radioactive suhstancesused in all sorts of medical, hiological and technicalinvestigations. For the rest of the working timethe heam of accelerated particles produced hy thisapparatus is availahle for scientific research ingeneral. With the approval of the Institute alsoresearch workers at universities in the Netherlandscan avail themselves of the opportunities offeredhy the synchrocyclotron for carrying out theirinvestigations.In this first article (I) a concise general descrip-

tion of the construction of this synchrocyclotron

..) Professor at the "Technische Hogeschool"(EngineeringUniversity) at Delft (Netherlands), formerly of PhilipsLaboratories at Eindhoven.

will he given, together with a brief account of whathas so far heen achieved with it. Further articleswill deal with the most important component partsof the installation, the first of these (II) followingimmediately upon the general survey.

The theoretical principles of the cyclotron werediscussed in an article (further referred to as A)which appeared in this journal some months ago 1).For the henefit of the reader the most fundamentalfacts and formulae will he repeated here wherenecessary. . _

We shall start hy giving in figs la and Ib aschematic drawing of the chief part of the in-stallation, the actual accelerating apparatus. Fig. 2,a photograph of a model of the apparatus in apartly dismantled state, will make it easier= tounderstand the drawing.

The drawing shows the evacuated acceleratingchamber 1, in which the particles, in our case eitherdeuterons (ions of heavy hydrogen) or alpha partic-les (doubly ionized helium atoms), emitted hy the

1) W. de Groot, Cyclotron and synchrocyclotron, PhilipsTechn. Rev. 12, 65-72, 1950 (No. 3).

242 PHILIPS TECHNICAL REVIEW VOL. 12, No. 9

Fig. 1. Greatly simplified vertical and horizontal cross sections of the Philips synchrocy-elotron at Amsterdam. (The vertical cross section, a, is really a juxtaposition of two crosssections along the centre lines of 9 and 10 in b.) 1 evacuated acceleration chamber, 2 ionsource, 3 and 3' dees, 4 magnet, 4a pole face, 5 energizing coils, 6 target, 7 valves and otherparts of the oscillator, 8 modulator, 9 and 10 coaxial transmission line, 11 and 12 vacuumpumps, 13 horon counter chamber.

MARCH 1951 SYNCHROCYCLOTRON (DESCRIPTION) 243

ion source 2 are to he accelerated, by means of analternating voltage of the order of 15 kVpeakbetween the dees (3 and 3'). The magnetic fieldof the large magnet 4, energized with the aid ofthe coils 5, causes the particles to describe a cir-cular path, at such a rotational speed that theyreturn each time in the right phase to the gapbetween the dees to undergo again an acceleratingaction from the alternating voltage, but with the

for the magnet, whilst the energizing coils contain32 tons of copper and are fed with 180 amperes.The electric power required to compensate the[2R-losses is 80 kW and oil cooling is provided todissipate this power. The great distance between thepole pieces (great also in comparison with similarinstallations) was chosen so as to he able to producea very intensive beam of particles. This wasparticularly important for the object aimed at

Fig. 2. View of the synchrocyclotron, with the magnet partly cut away to show theacceleration chamber with the two dees.

radius of the orbit increasing with the increasingenergy of the particles. The ultimate energy tobe reached is determined entirely by the flux den-sity B of the magnetic field and the ultimate radiusr of the orbit 2). In our case B = 1.38 Wbjm2

(13,800 gauss) and r = 78 cm, so that deuteronscan be accelerated to a final energy of 28 millionelectron volts and alpha particles to 56 MeV.To generate the powerful magnetic field mention-

ed above in the large gap between the pole pieces(diameter of the pole pieces 1.80m, distance betweenpole pieces 36 cm) 200 tons of iron was required

2) See, e.g., Philips Techn. Rev. 11, 70, 1949 (No. 3).

in the designing of this synchrocyclotron. The rela-tion between pole distance and beam intensity ismade clear when bearing in mind that, althoughby a suitable field variation the ideal path of theparticles is stabilized in the centre plane of the gap,the particles may still oscillate vertically (and radi-ally) about that ideal path. The more space isleft for the vertical oscillations, i.e. the higher theaccelerating chamber, the more particles can escapecollision with the walls of the dees and thus parti-cipate in the acceleration right to the end.The beam of accelerated particles is made to

strike the target 6, which can be insertedmore or less deep into the accelerating chamber

244 PHILlPS TECHNICAL REVIEW VOL. 12, No. 9

to adjust the energy of the particles collected.The materials to be transmuted can be put on thetarget. Due in part also to a suitable design of theion source, it has already been possible to increasethe beam intensity of our synchrocyclotron sothat the average intensity of current carried bythe charged particles (deuterons) to the targetamounts to 20 micro-amperes. This is to be regardedas a high value. With this current on the targetan energy of 28 X 106 • 20 X 10-6 = 560 watts isconverted into heat, which has to be dissipatedby water cooling (it is by measuring this heat thatwe get to know the current strength). Also thedees and many other parts are cooled with water.The alternating voltage required between the

dees is obtained with the aid of an oscillator (7 infig. I). For the acceleration of particles with massm and a charge q the alternating voltage must havean angular frequency of WE = qB/m (see article A).For deuterons as well as for alpha particles thefrequency required, with the aforementioned valueof B, is therefore 10.7Mc/s. For accelerating protonstwice this frequency would be needed.

Since it seemed impracticable at the time to makethe oscillator suitable tor both these frequencies,one of the first steps in the designing of our syn-chrocyclotron was to decide with what particles itwas to be worked. Fortunately tbis choice is notvery critical: both with deuterons and with protonsa large number of nuclear reactions can be broughtabout which yield a great· variety of radioactivesubstances, among which are materials that cannotbe produced from a uranium pile. Weighing theusefulness of protons against that of deuterons,it may be said that with an energy of some tensof MeV deuterons are certainly morvésuitable forproducing neutrons and that in general they mayperhaps offer more universal possibilities of appli-cation. Anyhow there was no doubt that if the(lower) deuteron frequency were chosen therewould be fewer difficulties to overcome· in theconstruction of !he oscillator and, particularly, of,the modulator, so that in our case it was decidedin favour of the deuterons.

The modulator is indicated in fig. 1 by 8. Let usrecall briefly the function of this important part(a detailed explanation is given in A). As soon asthe particles reach a high energy in the acceleratingchamber they have a tendency to "get out of step":firstly because they get closer to the edge of thepole pieces, where the flux density B must neces-sarily be smaller than in the middle in order tostabilize the path of the particles, and secondlybecause the relativistic increase of their mass then

becomes noticeable. Owing to these two causesthe condition for synchronism WE = qB/m is thenno longer fulfilled, the angular frequency WE being.too high for the particles at the edge. This limitsthe final energy attainable with the classical cyclo-tron. When, however, WE is gradually reduced - theprinciple of the synchrocyclotron - the particlescan be accelerated to greater final energies. Thedesired frequency variation is brought about withthe modulator. This apparatus contains a capacitorthe value of which varies periodically with timeand acts on the tuning of the oscillator. In thatpart of each modulating cycle in which the oscillatorfrequency dec rea ses, a stream ofparticles is acceler-ated. An important feature is the phase stabili-zation that takes place, in consequence of whichit is ~ot necessary that the reduced frequency shouldat every instant be exactly in step with thechanging situation as the radius of the orbit of theindividual particles increases. It is due to thisthat with. the synchrocyclotron powerful inter-mittent beams of accelerated particles can beobtained.The modulation frequency in our apparatus is

2000 cis, which means to say that 2000 times persecond a group of accelerated particles reachesthe target. This relatively high modulation frequen-cy, which is favourable for a high average intensityof the beam current - the number of acceleratedparticles per group being in the first instanceindependent of the modulating frequency -,involved no great technical difficulties on accountof the fact that in our case the frequency sweepneed only be 4%. In fact, from the middle to theedge the flux density drops in our apparatus byabout 2.5%, and the mass increase of a deuteronhaving received a kinetic energy of about 30 MeVat the edge is about 1.6% (the rest energy of thedeuteron is 1840 MeV), so that with a frequencydrop of about 4% the condition for synchronismWE = qB/m is again fulfilled at the edge 3).

For the sake of comparison some figures are given re-lating to the synchrocyclotron at Harwell in England, whichwas put into service some time ago 4). This accelerates protonsto an energy of about 180MeV. The relativistic mass increaseis then already about 20% and the frequency sweep has tobe about 30%. The modulation frequency has for the timebeing not been made greater than 80 cIs.

3) This is likewise the case for alpha particles, since the restenergy and the maximum kinetic energy obtained arein this case both twice these energies of the deuteron, sothat the mass increase is again about 1.6%.

4) T. G. Pickavance, J. B. Adams and M. Snowdon,The HarweIl cyclotron, Nature 165, 90-91, 21st January1950.

MARCH 1951 SYNCHROCYCLOTRON (DESCRIPTION) 245

Fig. 3. Part of the control and operating room of the synchrocyclotron at Amsterdam. Thepersonnel in this room are protected against the radiation from the accelerating apparatusby a layer of water 3.5 m thick.

It is well to emphasize once more (cf. A) that thehigher final energy attainable is not the only advan-tage of the synchrocyclotron compared with theclassical cyclotron. In our case the final energy ofthe deuterons is 28 MeV, whilst with a classicalcyclotron deuterons can also be accelerated upto 25 MeV with a not unreasonably small efficiency.The synchrocyclotron, however, requires a muchlower oscillator power for such a final energy andalso in other respects the construction is muchsimpler than that of a classical cyclotron, as willbe seen in the next article in this series.

Let us turn back to fig. 1 again. The high-fre-quency oscillation is applied to the dees and themodulator via a coaxial transmission line, 9 and 10;the fact is that at the frequency used (wavelengthabout 30 m) and with the large dimensions of theparts - to begin with the accelerating chamber -,the system can no longer be regarded as a lumpedcircuit. The whole system is set up with an anglein the middle instead of in one straight line; the onlyreason for this is that, for the purpose ofmaintenance,part 9 of the transmission line with the dee 3 shouldbe easily removable. To that end the part 9 is moun-

ted on an undercarriage running on rails laid radiallyto the magnet. After the part 7 of the oscillatorhas been removed parts 9 and 3 can be run outwithout having to shift part 10 of the transmissionline and the modulator.

Further it is to be seen in fig. 1 that there aretwo vacuum pumps with their backing pumps,one (11) connected to the accelerating chamber,the other (12) to the modulator. The reason whyone pump alone was not sufficient will be seen later.Finally the synchrocyclotron installation com-

prises all sorts of parts, some of them very bulky,

corresponding to what is needed for a radio trans-mitter, these parts not being visible in figs 1 and 2.For instance, there is a rectifying installation sup-plying the direct voltage of 12 kV for the oscillatorvalves; a drum with 2 X 10 metres of rubber hosefor taking up the potential difference of the coolingwater, coming from the water mains, and thecooled anode at a high positive potential; a controland operating room, with a large number of controlsand measuring instruments (fig. 3). This room isseparated from the cyclotron by a water tank 3.5 ID

thick protecting the personnel against the neutron

246 PHILIPS TECHNICAL REVIEW VOL. 12, No. 9

and gamma rays generated in the accelerating cham-ber (in the target and the walls). On the other sides. the cyclotron is enclosed by concrete walls 1 mthick. When entering the cyclotron compartment,after the apparatus has been switched off, to collect

the products of the transmutations by withdrawingthe target from the accelerating chamber, the neces-sary precautions are of course also taken. Thetarget is fixed to a holder 1.25 m long. Each holder,after having been used for an irradiation, is first

Fig. 4. The synchrocyclotron viewed in the direction of the arrow in fig. lb. In the fore-ground on the right is the modulator.

MARCH 1951 SYNCHROCYCLOTRON (DESCRIPTION) 247

n. THE OSCILLATOR AND THE MODULATOR

left to "cool down" between blocks of concretefor a certain length of time (depending on the "life"and the concentration of the radioactive isotypesformed) before being used again.

The materials are usually irradiated until aradioactivity of some tens of millicuries 5) has beengenerated in them. The neutron radiation emanating

5) With phosphorus,for instance, an activity of 150 micro-curiesper (LAh is obtained, thus in this caseour beam of20 IJÀ yields3 millicuriesper hour. The unavoidablelossesof radioactive phosphorus in the chemical processingrequiredbeforethe productcanbe suppliedto the "custom-er" are already discounted in this figure for the yield.(The loss of activity during the chemical processingisnegligible consideringthat the half-value time of thisatom is about 14 days.)

Principles underlying the construction of the oscil-lator

Owing to the phase stabilization of the particlestravelling round in the synchrocyclotron these canbe allowed to describe a large number of revolutionswithout risk of too many being lost. Consequentlyfor a certain final energy a small gain per "loop"suffices. This is the reason why the alternatingvoltage between the dees in a synchrocyclotron ofthe dimensions chosen by us need not be greaterthan, say, 15 kVpeak, whereas for a classical cyclo-tron of the same dimensions a voltage of certainly100 kVpeak would be required between the dees.

Much smallervoltages than 15 kVpeakare also possible,but then there is the objectionthat the permissiblemodulatingfrequencyand thus the strength of the beam current likewisebecomemuch smaller;this willbe reverted to later.

This so much lower voltage has two very import-ant advantages. In the first place there is muchless risk of a breakdown of the insulators anddisruptive discharges through the gas in theaccelerating chamber, and this is an advantagethat can be turned to good account in various ways.A higher gas pressure can be permitted in theaccelerating chamber, thus making it possible to getgreater ion concentrations and therefore morepowerful beam currents. Further, one of the twodees can be earthed (thus the full dee voltage, in-stead of half of it, comes to lie between the otherdee and the earthed chamber walls), thereby makingit much easier to mount the ion source, the target,etc.

from the target (and other parts) is being con-tinuously measured with a boron counter chamber 6)mounted in a fixed position close to the acceleratingchamber (13 in fig. I).The photograph in fig. 4 finally gives a view of

the cyclotron as seen in the direction of the arrowin fig. lb. This and the photograph in fig. 3 alsogive some idea of the extensive accessory apparatusthat is needed. In the foreground on the right offig. 4 is the modulator, which will be discussed indetail in the article now following.

6) This contains a solid or gaseousboron compound fromwhichalphaparticlesare releasedbyneutrons. Theaverageionization current produced by the alpha particles ismeasured.

The second advantage of the lower dee voltageis that a much smaller high-frequency poweris required than is the case with the classical cyclo-tron. If the dee circuit hás a quality factor Q anda capacitance C (it is imagined for a moment asbeing replaced by a lumped L-C circuit), then apower

wepp = ___::__-

2Q(1)

is needed to maintain, in the circuit, an oscillationwith peak voltage V and angular frequency w.(The index E used with w in article A will henceforthbe omitted.) The capacitance C is determinedmainly by constructional requirements; in our caseit could not be kept below 400 pF. Careful construc-tion allowed of the quality factor Q being raisedto 1500. With w = 2:n;X 10.7 X 106 and V= 14,kVwe therefore get P ~ 1.6 kW for the dee circuit.The modulator circuit takes up an equal amount ofpower, so that the high-frequency power requiredfor our synchrocyclotron totals somewhat over3 kW, as compared with about 100 kW for a classic-al cyclotron of the same dimensions.

Infig. 1 a very schematic representation is givenof the oscillatory circuit. It may be regarded as atransmission line with two, large, lumped eapaci-tances at the ends: the (non-earthed) dee in theaccelerating chamber at one end, with the capaci-tance Cl = 400 pF, and at the other end an equallylarge capacitance C2 (the modulator M). Withthe aid of an oscillator the transmission line iscaused to oscillate in such a way that a standing