Aaldert Hendrik Wapstra, a distinguished Dutch physicist, died on December 4th in his home in Naarden, The Netherlands at an age of 83 years. After finishing his High School in 1939 Aaldert studied physics at the University of Utrecht, where he graduated cum laude for his Masters degree in 1947. He earned his PhD in 1953 with a thesis "The decay schemes of 209Pb, 207Bi and 214Bi and the binding energies of the heavy nuclei" under C.J. Bakker, who later became the first director of CERN in Geneva. Aaldert was employed by the Instituut voor Kernphysisch Onderzoek (IKO) in Amsterdam in 1948. Twice, in 1951 and 1954 he worked at the Nobel Institute in Stockholm. In these years his interest was born for Atomic Masses and became an international authority on beta-decay and nuclear spectroscopy of radioactive nuclei. For the production of these isotopes Aaldert used the IKO synchro-cyclotron, which came into operation in 1946. In those days beta- and gamma ray spectroscopy was his favourite subject and he frequently visited the U.S. for longer periods. With the results of these measurements he could construct complicated decay schemes. He could use the obtained information in his atomic mass calculations. These made him internationally famous in the circles of nuclear physics. During many years he headed the citation index in the Netherlands. Another keystone was his Nuclear Spectroscopy Tables, produced together with Bob van Lieshout and Gerard Nijgh. This book was for a long time the standard book of reference in nuclear physics.
In those days one of his most outstanding results was his discovery of the difference between the measured and theoretically calculated values of the internal conversion coefficients. This anomaly could be explained by the finite size of the nucleus, which totally had been neglected in the calculations.
In the book Surely You're Joking Mr. Feynman! by Richard Feynman in 1985 he mentioned Aaldert twice in his recount of Feynman's visit to Professor Chien-Shiung Wu's laboratory at the University of Columbia. The fact that the electrons in the experiment came out spinning to the right in stead of to the left in beta decay made Feynman to ask Hans Jensen, Aaldert Wapstra and Felix Böhm about how the situation was in beta decay. This led Feynman to believe the beta decay was a matter of V-A interaction instead of the old idea of S-T interaction.
In 1961 Aaldert was appointed as one of IKO's directors and from 1965 to 1983 he became the managing director of the Institute. In 1956 he was appointed as a Professor of nuclear physics at the Technological University of Delft, where he lectured until 1987 when he retired. He was head of various work groups dealing with cosmic radiation and nuclear physics within FOM in The Netherlands.
Aaldert was part of many international committees among others he was president of the IUPAP committees Atomic Weights, IUPAP (Chemistry), Atomic Masses and SUNAMCO (Symbols, Units, Nomenclature, Atomic Masses, Fundamental Physical Constants), The International Atomic Energy Commission (IAEA) in Vienna, Member of the Council European Physical Society (EPS), and member of the Nederlandse Academie van Wetenschappen. At his retirement in 1987 he was honoured by the Queen of The Netherlands as Ridder in de Orde van de Nederlandse Leeuw.
A large number of young students earned their PhD under his supervision. He was valued everywhere for his fast en always-critical reactions on research results en texts. His fast conception of how to work was proverbial and the many stories and anecdotes demonstrate this in an obvious way.
Another of his favourites was playing the piano and his pronounced enjoying of modern classic music.
With Aaldert we loose an outstanding man in physics with international fame. It is ironical to note that only two weeks after his death there appeared an article in Physical Review Letters 97, 15 December 2006 about the double magic nucleus Hassium-270, a combination of 108 protons and 162 neutrons with the extremely long half life of 22 seconds. The prediction of these islands of stability had been one of his ultimate goals with the atomic mass studies. The name of Hassium was officially given during Aaldert's period as president of SUNAMCO.
