Neutron beams reveal Van Leeuwenhoek’s secret

Van Leeuwenhoek microscope up close. (Photo: TU Delft)

The superior quality of Van Leeuwenhoek’s microscopes, of which eleven specimens remain, has been a mystery for over 350 years. Whereas other microscopes from that period magnified about thirty times, Van Leeuwenhoek’s microscopes performed up to ten times better. This allowed him to discover blood cells, bacteria, and sperm cells. Did Van Leeuwenhoek (1632–1723) develop a superior procedure to produce tiny glass balls, as he seems to have said to a group of German visitors? Or did he transform glass balls into thin lenses in a long and careful process of grinding with ever finer polish?

Neutron image reveals lens within the microscope. (Photo: TU Delft)

The neutron image of the instrument shows a thin lens wedged between two brass plates at the intersection of yellow hairlines. Curator Tiemen Cocquyt (Boerhaave Museum) concludes: “There was no exotic production method involved. Van Leeuwenhoek was simply very proficient in polishing these minuscule lenses.”

Neutron radiation is uniquely suited for the task, said TU Delft researcher Dr Lambert van Eijck. In contrast to X-rays, neutrons pass easily through metals, while also recording glass.

Placed in one of the neutron bundles of the TU Delft reactor core, the museum object was rotated over 180 degrees over twenty-four hours. The computer made a 3D reconstruction from about 500 transmission images.

Neutron tomography, as the imaging technique is called, will be applied in a NICAS (Netherlands Institute for Conservation, Art, and Science) project called Beeldvorming. Van Eijck mentions the study of bronze statues and Roman swords from the Rijksmuseum and the Dutch National Museum of Antiquities (RMO). A number of statues have embedded structures within them.. Neutron tomography is uniquely suited to bring these into the open.

Neutron radiation has a side-effect called neutron activation. Objects that have been irradiated by neutrons become temporarily radioactive and have to be stored for some time to ‘cool down’. By performing spectrographic analysis of the gamma rays emitted during this period, one can deduce the material composition of the object.

The NICAS project aims to fuse the neutron and gamma data sets into one 3D virtual object, enabling curators can tell, for example, if there are, for instance, clay or iron embedded support structures inside a statue. Such information is crucial for conservation and preservation strategies of cultural heritage or for authenticity research.

Neutron tomography is an imaging technology for the sub-millimetre to metre scale and complements the current instrument suit at the research reactor. On the opposite side of the spectrum stands the neutron diffractometer PEARL, which was opened in 2015. PEARL measures distances between atoms in crystals.