In retrospect: electron scattering in Amsterdam

29 January 1999

The recent closure of the Amsterdam Pulse Stretcher marks the end of 30 years of electron accelerators in the Netherlands.

The turn of the year was significant for the Dutch NIKHEF laboratory: 1998 was the last year in which data were taken at the institute’s Amsterdam Pulse Stretcher (AmPS). Even before the AmPS was built, funding organizations had decided that it would only be exploited from 1992 to 1998 because it was a heavy load on the Dutch science budget.


The first steps towards electron scattering experiments in Amsterdam were taken at the beginning of the 60s when institute director Prof. Gugelot sent his former PhD student Conrad de Vries to Stanford. There, at the cradle of electron scattering, de Vries worked in Robert Hofstadter’s group. The 3 km linear electron accelerator was being designed next door at SLAC. On his return to Amsterdam, de Vries convinced the institute (then called IKO, the Instituut voor Kernfysisch Onderzoek) that electron scattering provided the best possibilities for future nuclear physics experiments.

While de Vries formed a research group and designed experiments, a linear accelerator was constructed in a joint effort by IKO and the Philips company. In 1946 Philips had built the synchrocyclotron at IKO ­ the first in Europe. Now it wanted to gain experience in constructing linear accelerators, thereby using superconductivity. It was an ambitious goal, and when the detectors were ready there were still problems with the accelerator. Long delays were foreseen and de Vries turned to his former colleagues in Stanford. In 1966 the US Atomic Energy Commission approved a plan to send two spare SLAC sections to Amsterdam ­ officially “on permanent loan”. The two 3 metre sections formed a 90 MeV linear accelerator (with the Dutch acronym EVA), which became operational in 1968.

Since the Netherlands could not go for large accelerators or huge projects, de Vries decided to aim for precision measurements. Very precise data on the charge radius of carbon-12 are still standard today. The spatial magnetic distribution was measured for a variety of nuclei, ranging from lithium-6 to indium-115. At these low energies it is difficult to separate the small magnetic contribution from the much larger charge contribution, with one exception: at a scattering angle of 180° only the magnetic component contributes. In a specially built 180° arrangement ­ comparable to the one built by Barber and Peterson at SLAC ­ magnets were used to separate back-scattered electrons from the incoming electron beam. The resulting data were complementary to higher energy results at the 600 MeV electron accelerator (ALS) at Saclay.

From the start it was clear that a larger accelerator than EVA was needed, and the first plans for a medium-energy accelerator (MEA) were submitted at the end of the 60s. The cost of this 500 MeV machine ­ about 40 million guilders for construction ­ was very high by Dutch standards, and the project required much prior organization. Construction started only in 1975 and the first measurements with MEA electrons were made in 1981. In 1973 MEA could deliver up to 40 microamps at a duty factor of 1%. This was lower than the 10% originally aimed for, but much higher than the 0.02% of EVA.

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