Life and Work of Rolf Wideröe by © Pedro Waloschek, => Contents
10 Baden - Betatrons for BBC
In the spring of 1946, after my position in Norway had been sufficiently cleared up, I went to Switzerland and started on some preliminary work for a betatron. The construction drawings produced at that time were already pretty detailed. This was a machine with which electrons were going to reach energies of up to 31MeV, which corresponds to an acceleration by 31 million volts. We had opted for 31 MeV because we wanted to extract the electrons out of the vacuum chamber, as we later in fact did. Electrons of 31 MeV penetrate about 10 cm of water - or the equivalent body tissue - and could therefore eventually exert the appropriate therapeutic effect. The machine was conceived first and foremost for such medical purposes.
The iron yoke consisted of six return sections arranged in the shape of a star, as is shown in Fig. 10.1. This was a construction which I knew well from the manufacture of transformers. The six sections were made of iron plates which had been soldered together. Mr.Hartmann and a few other members of the BBC's staff then went on to build the machine according to my instructions.
My wife, our three children and I left Oslo for good on August 19, 1946. We went with our car, first taking a ship to Amsterdam and then driving to Zurich via Luxembourg. In Zurich everything seemed to happen very casually. I somehow obtained a work permit - I don't know how myself. Apparently it was a case of `established facts' which had been taken care of by BBC.
As Ragnhild remembers very well, I had to return to Oslo in October to clear up the case about my work in Germany during the War. I stayed with my parents while I was in Oslo. Because I accepted the confiscation of the last money earned in Germany, no trial was required and I was subsequently given another passport. In November I was permitted to return to Zurich.
The betatron started to take shape at the beginning of 1947, and the manager of BBC's department in Baden allocated us a `working place'. It was a section of a tunnel beneath a large hall used for testing generators. It was one of the tunnels through which the warmed cooling air and innumerable other vapours were extracted and was also used to inspect the big machines from below.
Such was the tunnel in which we were supposed to construct the betatron. Working conditions were very bad. Above our heads were the big machines and, when they were running, it was impossible to hear anyone speak; we would be forced to flee. Now and again the generators' coils were impregnated with various insulating substances and then it would become impossible to breath as the vapours were extracted through our tunnel. However, we managed to do some work regardless.
But we had difficulties getting the betatron to work. This was because the yoke's six iron return sections were not exactly identical. This meant that the magnetic fluxes for each section went through zero at slightly different times and provided different steering fields for small currents. And this happened very close to the moment in which the electrons would have to be injected. A successful injection of electrons was thus very rare indeed.
The Hamburg machine only had two returns and was therefore easier to adjust. Kerst's second machine (in the USA) for 20MeV also only had two return sections. However, it didn't take us terribly long to find a solution (it was sometime around January 1948). On each of the six yoke returns we fitted ten copper windings which were short circuited via an adjustable resistance. By doing this we were able to optimise precisely the steering fields at the moment of injection. The fields were accurately measured by means of small permalloy strips fitted above the air gap. Incidentally, the six yoke returns proved to be rather a boon, because they screened off a large proportion of the high energy X-rays produced when the machine was running.
The hazardous spatial conditions soon made us subject to
high doses of radiation as we didn't have enough space for
shielding. We therefore had to drive to the Kantonsspital in
Zurich once a week to have our white blood cell levels checked. If we had
less than 3,000 per cubic millimetre we would have to take some
time off. After we increased the power of the machine even further,
the radiation became too high even for the workers on the level
above. This factor effected an important improvement: we were
finally provided with a proper `radiation laboratory' in which we
were able to protect ourselves from the radiation.
See also Fig.: 10.2:
The first BBC-betatron in construction
and Fig. 10.4: Betatron used for medical purposes,
BBC gave me a free hand and practically all decisions were left to me - except, of course, with regard to our place of work. This was because I was the only one who had any understanding of betatrons. Initially I was just told to build a betatron, and this was mainly thanks to Professor Scherrer who had been an ardent campaigner for the construction of such machines. His interest was probably decisive. Furthermore, BBC wanted to be `on the scene' of nuclear and particle physics; the betatron was going in that direction although, at the beginning, its only purpose was medical. Perhaps those 31 million volts had a certain hypnotic effect. And the atomic bombs which had exploded over Japan had raised the industry's awareness of nuclear physics.
I would like to mention again the support we had from Walter Boveri. He was a good friend of Professor Scherrer. Later, although not very much later, Dr.Hans Rudolf Schinz of the University of Zurich also entered the scene and he turned out to be a great advocate for the construction of betatrons. He ran the Radiotherapy Department at the Kantonsspital in Zurich.
Apart from their medical uses, the betatrons also became important for the non-destructive testing of materials. Even the 15 MeV betatron from Hamburg was used for this purpose after it was shipped to England.
When we had made ourselves comfortable in the new radiation laboratory, we progressed quickly, and in autumn 1949 we took the machine to the Kantonsspital in Zurich where a specially equipped room was ready and waiting for it. There was still much to do, especially with regard to the radiation shielding. Many measurements were taken and a lot of shielding had to be fitted to protect against unwelcome X-rays and even against neutrons, which this type of machine also produced. Lead plates served as shields and later on substances containing boron were also used for these purposes.
I remember one day Professor Schinz came along to see us with a visitor. At the time I happened to be lying underneath the machine. He pointed his walking stick in my direction and said, "there lies my greatest enemy". We weren't progressing fast enough for him. But we did complete the machine eventually, and the first patients were given X-ray treatment in April 1951.
By 1952 we were in a position to deliver a further two betatrons, one to the Inselspital in Berne and one to the Radiumspital in Oslo. With regard to the latter hospital, I would like to say a few words more. My friend Olav Netteland told me that Dr.Johan Baarli (who later became the head of the Norwegian Service for protection against radiation) measured the number of neutrons in the surroundings of the machine and found that it was far too high. He even called it `Wideröe's sterilisation machine' - if I remember rightly. However, Baarli had not taken into account the difference between fast neutrons, which are dangerous, and slow neutrons, which are relatively harmless. Nevertheless, someone at the hospital had claimed that he was suffering from headaches... It is my opinion that most of the measurements made at that time were plain and simply wrong.
We did have some protective regulations regarding radiation, but they had not yet been defined very precisely. The permitted radiation doses were about five times today's top limits, which are really quite low. The people of Kerala in southern India live under constant exposure to radiation doses which are five times higher than those permitted by our regulations. The cause is monazite sand containing radioactive thorium. Nevertheless, the local population does not appear to have suffered adverse effects.
As I can remember very well, that the instruction sent to us by Oslo's Radiumspital was the most unusual Brown Boveri ever received. The head of the hospital, Dr. Reidar Bjarne Eker, simply wrote us a letter with the words, "We order a betatron", his signature and the date. Not a word about energy specifications or any other data. We built him a 31 MeV X-ray betatron.
In 1956 we managed to extract the electrons from the glass tube of our betatron. We did this by using a process for which I had submitted a patent several years previously [Wi52]. In 1957 we converted a betatron, which we had delivered to the Inselspital in Berne in 1953, so that it would function with this supplement. Special coils were fitted in the air gap above the ring tube. They were called `pancake coils' because they were so flat.
By the way, most of our betatrons were able to deliver two beams simultaneously, emitted in opposite directions, as shown in fig. 10.3. The tube was thus exploited more efficiently as particles were accelerated during both the positive and the negative rise of the alternating current. This made it possible to treat two patients simultaneously in separate rooms. Naturally we had to provide the machine with electrons which moved in opposite directions at injection, as well as ensuring that the electrodes on which the X-ray beams were produced were shaped appropriately
An interesting variation was developed for the non-destructive investigation of large industrial components. The X-rays were produced at two opposite points in the tube so that the target object could be X-rayed simultaneously from two different directions, thus making two stereo pictures of the interior (see fig. 10.5 and 10.6). We were able to reduce the size of the `sources' of the two X-ray beams to a few tenths of a millimetre in order to achieve a better photographic resolution. We had got the hang of it and our betatrons were probably among the best industry could produce.
It may be interesting at this point to mention the development of radiation therapy at the Radiumspital in Oslo, because similar processes were also taking place in other countries. Initially, a generator for high voltages was due to be built in Bergen during the War, a `Van-de-Graaff machine'. Odd Dahl describes all this very nicely in his book published in 1981 [Da81].
First they tried to get the machine built by Philips in Holland, but this proved too expensive; they had managed to collect just about 150,000 Kroner, and that wasn't enough. We must remember that such a `Van-de-Graaff' could replace the radiation of a kilogram of radium. And in those days, as I already mentioned, one gram of radium cost around one million Kroner!
Philips had recommended that they should build the machine themselves, especially since Odd Dahl could be in charge of the technical process. He had already successfully built and operated high voltage machines in the USA.
The Van-de-Graaff machine was completed in 1941 in an extension of the Bergen Hospital. It reached 1.7 million volts. After this, Dahl supervised the construction of another machine (of the same type) at the hospital in Haukeland. This one even reached two million volts. Finally, the Radiumspital in Oslo asked for a similar machine and construction began.
However, when betatrons became available on the market in 1948, the head physician at the Radiumspital, Dr. Bull-Engelstad, ordered one from Siemens in Erlangen. It was to have an energy of 6 MeV and was scheduled for delivery in 1949. As already mentioned, Siemens had been developing this type of machine since 1941. The parts of the Van-de-Graaff machine under construction were given to the University of Bergen as a gift.
This was the situation as Olav Netteland found it when he began working at the Radiumspital in September 1949. In the autumn of the same year, Netteland went to Erlangen to take a look at the betatron. By then Siemens was already developing a 12 or perhaps even an 18 MeV betatron.
At that time we at BBC in Baden had progressed quite far with the 31 MeV machine for the Zurich hospital. A congress of radiologists took place in London in 1950 where Siemens exhibited their 6 MeV machine. However, it emerged later that this was a non-functional exhibit and hadn't even been fitted with a tube. After this, Olav Netteland contacted me, and in September 1951 he and head physician Dr. Steen came to Switzerland to see our 31MeV betatron which was already up and running at the Kantonsspital. I went to Erlangen in the autumn of the same year where Siemens could only show me the 6 MeV betatron. Completion of the 12 MeV machine was still a long way off. I did not have any difficulty in having the Siemens order cancelled and Prof. Eker immediately ordered `a betatron' from BBC. We delivered a 31MeV machine in the summer of 1952. Within six months it was operational. I think Siemens did provide the Radiumspital with a machine eventually, but I don't know much about that.
After that I was in frequent contact with the Radiumspital in Oslo, especially with Professor Eker. I have kept my letters of that period. The hospital did not provide radiation therapy until 1953, and in the first few years we had a few problems. The cathode of the electron source had a very short life and we frequently had to replace the tubes. The `oxide-cathodes' available at that time only run for about 500 to perhaps 1,000 hours. That was far too little. We experimented with other cathodes but our trials were not very successful. The barium aluminate contained in the cathodes attacked and dissolved the filament. Although Olav Netteland said that things were much better during the second year, the problem was not solved until I went to visit Philips in Eindhoven who suggested their own patented method. This was in the autumn of 1957.
See Box 12: Betatrons and Industry
After that, Philips supplied us with cathodes in the form of small tubes made of sintered tungsten powder impregnated with barium aluminate (which corresponded to approximately 30% in volume). We fitted these tubes with narrow cylinders made of aluminium oxide, each of which included a filament. We had to take great care to ensure that the filaments were completely protected by the aluminium oxide and that they could not come in contact with the barium aluminate, otherwise they would corrode and break very quickly. The most favourable temperature for the filaments was a little below 1,100°C. At this temperature, approximately as much barium oxide was diffused to the surface of the cathode as would be used up by ion bombardment.
These cathodes were very robust. Discharges did not destroy them, they regenerated themselves very quickly and they had an unbelievably long life, certainly well over 20,000 hours, perhaps even as much as 40,000 hours. We subsequently built betatrons which could run for more than 25 years without needing a new tube. Some of them are probably still in use today.
We had an excellent mechanic at BBC, Mr. W. Gräf, who knew how to execute the very precise work involved in building these cathodes. It is greatly thanks to him that our machines lasted so long. He also looked after the manufacture of the glass tubes. We had some very good people in our department who would help during the installation of the machine on-site. They would get it started and also assist in running it. We also undertook all repairs and supplied spare parts. From 1954 onwards I was in charge of `EA', the Electrical Accelerators Department, which was renamed `EKB' (Electrical Components for Betatrons) after 1973.
I would have to draw up a very long list if I were to mention all the colleagues who contributed to our success over the many years. I apologize for not being able to do this. However, I would like to call to mind just a few names, for example Dr.A. vonArx, Dr.M.Sempert, Dr.H.Nabholz, Mr.K.E.Drangeid (a Norwegian who later joined IBM's Research Laboratory), Mr.Gamper (he worked on materials testing), Mr.vonDechend (design engineer), Mr. E.Jonitz (head of the workshop) as well as Messrs. Vikene, Fischer and Gerber who took care of assembling and commissioning the machines.
The betatrons continued to be manufactured until 1986, by which time BBC had delivered 78 of them. I had submitted 53 patents for BBC, most of them in Germany but also quite a few in Switzerland. My time at BBC in Baden was, therefore, a very productive period. Towards the end of my career there I had put in applications for over 200 patents in all. A copy of each one is kept in the Archives of the ETH Library in Zurich [Wi70].
See Box 13: BBC-Betatrons from 1949 to 1986
In 1959, we supplied the prototype of a `mobile' betatron to the private hospital `Casa di Cura S. Ambroglio' in Milan. The Director, Professor P. L. Cova, placed the order with us and the machine was still in use a few years ago. This machine `revolved' around the patient. We christened it `Asclepitron' after the Greek god of medicine `Asclepius' or `Aesculapius'. As of 1967 we were able to increase the energy of our betatrons to 35 MeV and, in 1970, we even went as high as 45 MeV, which was of significance for particular applications in materials testing.
I had also developed a revolving `magnetic lens' which allowed one to direct the electrons which came from different directions on to the spot which required irradiation. This minimised damage to healthy tissue. The lens also became a great commercial success. Many hospitals which ordered betatrons asked for them to come fitted with the magnetic lens.
After 1970 the demand for betatrons declined. By then it had become possible to build linear accelerators which were smaller and lighter than our betatrons. But above all they were cheaper, and in the end this was decisive. Important contributions to these developments came from the Stanford Linear Accelerator Center SLAC, frequently in collaboration with the company `Varian'. This company recently took over the entire department at BBC which I directed. And in the meantime BBC had been renamed `Asea Brown Boveri' (ABB).
I think that the most important machine after the
betatrons which the BBC department under my direction developed was
a synchrotron for Turin University, although a better name for
it may be `beta-synchrotron'. I would like to describe this in a
bit more detail.