The sun sets on SATURNE

25 February 1999

After a 40-year career, first as a weak focusing machine and then rebuilt with strong focusing, the French SATURNE synchrotron has exited the physics stage.


The French SATURNE National Laboratory formally ceased to exist on 31 December 1997. The authorities had actually taken the decision to close it down a few weeks earlier when a large area of the roof above the experimental areas collapsed under a heavy fall of snow. This was the sad end of a forty-year-old laboratory which had lived through two clearly distinguishable eras.

The first SATURNE synchrotron built by the CEA (French Atomic Energy Authority) in 1956­-58 was a weak focusing machine, mainly supplying 3 GeV protons. Particle physicists used bubble chambers (hydrogen or propane) among other detectors. From the mid-60s higher-energy beams elsewhere attracted away an increasing number of SATURNE users.

At the same time, a core community was becoming more aware of the importance of probes close to the GeV energy range, highly suitable for nucleons. The construction of a large magnetic energy-loss spectrometer, SPES1, showed that while nuclear levels could be measured at 1 GeV, the obsolescent synchrotron had to be replaced.

The outcome was the construction in 1978 of a new strong-focussing and separated-SATURNE (focusing and bending) synchrotron for nuclear physics, SATURNE-2. Its maximum energy, limited by the size of its buildings, remained the same (2.95 GeV for protons). This machine was the result of careful consideration followed by a project undertaken jointly by CEA-IRF and CNRS-IN2P3, establishing the new SATURNE National Laboratory.

Over the years, the Laboratory acquired several particle sources making it possible to supply all light-nucleus beams (up to helium-4) at high intensities (up to 1011 per second extracted slowly and without RF). Heavy-ion beams accelerated up to 1.15 GeV per nucleon also became available with the DIONE source and the MIMAS preinjector. Finally, following the solution of the depolarization problem by slow or fast negotiation of depolarizing resonances, SATURNE became capable of supplying the world’s most intense GeV-range proton and deuteron beams. The gradual improvement in the intensities and emittances of the HYPERION polarized particle source and the MIMAS preinjector were essential for this.

A national and then an international community built a large array of detectors to exploit these beams. The first were magnetic spectrometers (SPES 1, SPES 2, SPES 3 and SPES 4) with complementary properties (high resolution to separate nuclear levels, large pulse acceptance for the study of large objects and wide excitation energy ranges, operation close to the beam direction etc). An unusual Time Projection Chamber (DIOGENE) was built to study central collisions between heavy ions, resulting in high multiplicities. A station devoted to nucleon nucleon interactions (elastic scattering) made it possible to polarize the incident beam and the proton target along different axes independently.

More specific devices were also installed: a full solid angle detector (ISIS) for multi-fragmentation studies, cylindrical wire chambers (ARCOLE) for elastic n-p examination, high-acceptance magnetic detectors (DISTO, SPES 4¼) recoil polarimeters, photon detection for eta physics (PINOT), proton radiography etc.

Hadron studies

Most of the experiments at SATURNE were devoted to hadrons ­ their interactions and their behaviour in nuclei. The beams available also made possible a large number of nuclear structure studies, the simulation of the effects of cosmic radiation in the laboratory and a series of spallation neutron measurements with an eye to transmuting nuclear waste, to mention only a few.

The Laboratory made a particularly important contribution to the understanding of the nuclear force by polarization measurements in proton­proton and neutron­proton collisions in an energy range hitherto largely unexplored. It also contributed to research into narrow dibaryonic states consisting of six quarks.

As the wavelength of the proton in the GeV range is nucleon-sized, it is possible to calculate directly the proton­- nucleus interaction from the free nucleon­nucleon interaction without going through the intermediary of an effective force. This provides direct access to the properties of the target nucleus. It thus became possible to determine matter radii (in addition to the charge radii studied by electron scattering) and transition densities for many nuclei.

Moreover, the polarization of the deuteron beams made it possible to isolate specific (spin isoscalar) response functions of the nuclei, which it had never been possible to measure before.

The study of systems with a small number of nucleons provided a better knowledge of the reaction mechanisms for a wide variety of processes and energy and momentum transfers. It particularly concerned the role of certain baryonic resonances in meson production and kinematic limits to meson exchange models. A set of mainly pseudoscalar meson production experiments in nucleon­nucleon collisions provided the basic information needed to interpret these processes in proton­nucleus and nucleus­nucleus collisions. The contribution of the polarization was found to be highly important in determining the dominant mechanisms. This applies especially to exclusive hyperon production, one of the laboratory’s final major programmes.

A remarkable effect, the highly intense production of eta mesons (over 108 a day) at threshold in proton­deuteron collisions provided a tagged eta source for precise measurements of the eta mass and its decay into muon and photon pairs. This high production can also be seen in deuteron­deuteron reactions at threshold and may be interpreted as the first evidence for eta-mesic nuclei.

A particularly important collective phenomenon, the pion mode which characterizes propagation in nuclear media, was demonstrated by charge exchange reactions using light and heavy ions.

The heavy ion beams also resulted in several programmes for the study of macroscopic properties of nuclear systems (multi-fragmentation, stopping power, compressibility).

Two applications-oriented programmes produced important results. Proton and deuteron beams made it possible to examine neutron production by spallation. These data are essential for the validation of the computation programs used in the design of new methods of transmuting radioactive waste. SATURNE’s energy range and beams also made it ideal for simulating cosmic rays and the damage caused to instruments exposed to it. With a week of irradiation simulating a million years of exposure, a series of experiments made it possible to trace stellar history.

SATURNE’s assets were: the quality of its beams; easy and fast energy changes (half an hour or less); its variety of ions and intensity; stable polarization; and mutiple ejection (two experiments supplied with different energies and intensities). The complementary detectors were another trump card. Last, but by no means least, was the competence and willingness of laboratory staff in finding the best solutions for physics.

A book describing many of these research programmes is being published by World Scientific.

bright-rec iop pub iop-science physcis connect