In the years immediately after the Second World War, several countries that were pushing to develop more powerful particle accelerators created an exclusive club. A recent symposium in Uppsala looked back to the first Swedish post-war accelerators.
The first cyclotron in Sweden was a small 80 cm device built in 1938 at the Nobel Institute in Stockholm and capable of accelerating deuterons to 7 MeV. After the Second World War, an ambitious new machine at Uppsala took energies into a new domain.
The 50th anniversary of accelerator-based research at Uppsala, on 8 December 1999, merited a jubilee symposium. Gunnar Tibell, Sven Kullander and Arne Johansson looked back, while Bo Höistad, Göran Possnert, Nils Olsson, Jörgen Carlsson and Curt Ekström surveyed the present scene and looked to the future.
The first two copies of a new book (in Swedish), relating half a century of accelerator-based activities in Uppsala and edited by Torsten Lindqvist, was handed to the vice-chancellor of the university, Bo Sundqvist, and to Helge Tyrén, former professor of high-energy physics.
In the early 1940s, Tyrén, a student of 1926 Nobel Chemistry Laureate Theodor Svedberg, had constructed a neutron generator for the production of radionuclides. In Svedberg’s discussions with the main customer of these radionuclides, gynaecology professor John Naeslund, plans emerged for a more powerful accelerator – a cyclotron – to increase the quantities of radioactive substances.
Naeslund’s wife knew Göteborg textile magnate Gustaf Werner, who was the richest person in Sweden at that time and known for his generosity. The firm Werner & Carlström offered to finance a cyclotron, and one of its research objectives was to see how synthetic fibres were affected by neutron irradiation.
From cyclotron to synchrocyclotron
In 1945 Tyrén visited cyclotron laboratories in the US. The original intention was to obtain drawings of a cyclotron with an energy of about 20 MeV for radioisotope production. However, during the visit he found that a new principle of accelerating particles to much higher energies had just been invented by E McMillan.
This principle of phase stability was tested and shown to work during the spring of 1946, while Tyrén was in Berkeley. The question was whether it was too early to implement this new idea in Uppsala. Tyrén managed to convince Svedberg that, instead of a cyclotron, a synchrocyclotron should be built, and Werner & Carlstroem had no objections. The instrument’s energy of 200 MeV was above the threshold energy for the production of the newly discovered pi meson. Protons of this energy also have a sufficiently short wavelength to interact with individual nucleons in the nucleus, and the range in tissue is 25 cm, which is important for proton therapy.
On 8 December 1949, Crown Prince Gustaf Adolf inaugurated the Gustaf Werner Institute for Nuclear Chemistry (GWI). Exactly two years later, on 9 December 1951, Ernest O Lawrence, the inventor of the cyclotron, pressed the button to initiate the first beam circulating in the new synchrocyclotron.
For a few years, scientists in Uppsala could access protons of the highest energy in Western Europe. Several foreign physicists visited to familiarize themselves with these high-energy projectiles.
Meanwhile, CERN was founded and construction began for its first two accelerators: the synchrocyclotron (SC) and the proton synchrotron (PS). A large percentage of the Uppsala staff were recruited to take part in the build up of CERN’s new European laboratory. The Uppsala group, under Bengt Hedin, was responsible for the construction of the SC magnet.
A spectrum of research
A milestone in the development of the GWI was the extraction of the proton beam in 1955, giving more flexibility to the arrangement of nuclear physics experiments. Nuclear spectroscopy was always an important part of the research, both off line and on line. Among the intermediate-energy physics experiments were the first studies of quasi-free proton-proton reactions, and later studies of pion production on atomic nuclei – so-called sub threshold reactions.
Biomedical research began under the leadership of Börje Larsson, who was later to become professor of radiobiology. He was one of the pioneers in the history of radiosurgery, as well as in other therapeutic and diagnostic uses of the proton beam at the GWI. On 23 November 1957, in a pioneer proton irradiation of a malignant human cancer, he and physician Stig Stensson irradiated a woman suffering from a large malignant tumour of the uterus. In 1958 Boerje Larsson also participated, together with medical doctors Lars Leksell and Bror Rexed, in the first neurosurgery on patients.
For well over two decades, a large number of physics and biomedical research projects were carried out at the Uppsala machine. In 1977 it was closed for a major overhaul and also modification into a sector-focusing synchrocyclotron. It was reopened in 1986, with much improved performance. The energy can be varied in the 20-190 MeV range for protons, and a variety of different ions can be accelerated.
From ICE to CELSIUS
As well as providing beams for physics and biomedical research, the cyclotron is an injector for the cooler storage ring CELSIUS (Cooling with Electrons and Storing of Ions from the Uppsala Synchro-cyclotron). The CELSIUS proton energy can be raised to well over 1 GeV, making a new field of research available to the physicists.
CELSIUS uses the lattice magnets of the Initial Cooling Experiment (ICE), which was built at CERN in the late 1970s. The decision to send the equipment to Sweden was taken by CERN Council in December 1982, and additional funding for the ring was included in the 1983/4 budget of the Swedish Ministry of Education.
Ongoing work to improve the machine and extend laboratory space was accelerated so that the “new” ring could be accommodated. In the summer of 1983, 20 trucks transported the 40 magnets and 4 coils from Geneva to Uppsala. The experience acquired in building the ICE ring was of value to the CELSIUS project, and in particular the contributions by CERN’s Heiner Herr and Alfredo Susini should be mentioned. The first CELSIUS proton beam circulated in 1988.
The GWI ceased to exist in 1986, when it, together with the Tandem Accelerator Laboratory, formed a new national laboratory – the The Svedberg Laboratory (“The” is an abbreviation of Theodor), and a new university department, the Department of Radiation Sciences, with units in high-energy physics, nuclear physics, ion physics and physical biology.
Today around 300 physicists and medical researchers, from Sweden and abroad, use the two The Svedberg Laboratory accelerators. The Programme Advisory Committee can only accept a fraction of the proposed experiments. Notable experiments are those performed by two large collaborations – WASA and CHICSi, both at CELSIUS, and operating with light and heavy ions, respectively. The storage ring has two internal target stations – one for a gas jet target and one for pellets of frozen hydrogen or deuterium. Among the SC projects, international teams are using a neutron beam for both fundamental and applied research.
Scientists from the GWI, and since 1986 from the Department of Radiation Sciences, have been very active in research at CERN. At the end of the 1960s, accelerator physicists contributed to the improvement of the SC, where experiments were initiated by an Uppsala group in 1967. Later SC experiments were carried out on pi mesic atoms and muon-induced fission.
At CERN’s LEAR antiproton ring, the production of heavy hypernuclei and lambda-antilambda pairs were studied. Pion-helium and proton-helium interactions were studied at the PS between 1967 and 1976. In 1977 the Uppsala group took part in one of the first experiments at the SPS proton synchrotron: hadron-hydrogen elastic scattering in the Coulomb interference region.
Since 1980 the group has collaborated in studies using CERN’s high-energy muon beam for the determination of polarized and unpolarized structure functions from nucleons and nuclei. A significant contribution to the build-up of the RICH detectors for the Delphi experiment at LEP has been made, as well as the running and analysis of Delphi. For the future LHC collider, the Uppsala group contributes to the semiconductor tracker for the ATLAS collaboration.