‘Gamma spectroscopy shows a fingerprint of the bronze, as it were, from which its origin can be deduced’
Schematic representation of the measurement set-up.
© Jos Wassink
Adjacent to the nuclear reactor of the TU Delft Reactor Institute (RID), there is a labyrinth of corridors and laboratories. Neutron specialist Dr Lambert van Eijck knows this place like the back of his hand. Having passed through several swinging doors and sharp turns, he opens a steel door. There are labels warning of radioactivity, and fluorescent tubes flash on. Clasped in a large purple steel frame, we see a large sword. Weighing in at three kilos and seventy centimetres long, the sword is here to “cool down,” Van Eijck (faculty of Applied Sciences, AS) tells us. Not literally, of course – the sword is not warmer than its surroundings – but it is emitting gamma radiation. Neutron irradiation from the nuclear reactor has made the sword radioactive. Neutron irradiation of historic metal objects has become one of RID’s new specialities. For example, Van Eijck and his team recently radiographed a Van Leeuwenhoek microscope from the Rijksmuseum Boerhaave in Leiden. Over the course of several weeks, the gamma radiation will slowly decrease, after which the sword can be returned to the National Museum of Antiquities (RMO) in Leiden.
Ceremony
“The sword is an important object from the Bronze Age, the period between 2000 and 800 BC,” says Prof. Luc Amkreutz, curator of prehistory at the RMO and professor of Public Archaeology at Leiden University. “It is a very valuable European object, of which only six have been found throughout Europe. Two in the Netherlands, two in France and two in England. This sword was found in 1896 at the edge of a peat bog near Ommerschans, a former Dutch bulwark, on a scaffold of birch logs, as a gift to the gods. So, we are dealing with a symbolic ritual object, an enlargement of a functional item – the sword. It probably symbolised the importance of warriors to society and served as a means of communication with a world of gods.” In the reactor hall, Van Eijck shows the set-up used to radiograph the sword. It is a steel scaffold that sandwiched the sword between a pipe to the reactor core and a neutron camera. During radiography, the sword was rotated around its vertical axis in five hundred increments, to arrive at a three-dimensional reconstruction (tomography). Since the maximum height of the beam was about ten centimetres, capturing the entire sword required eight takes.
“The castes of warriors and of leaders emerged during the Bronze Age,” Amkreutz says. “Swords played an important part in this process. The sword was conceived and created in the Bronze Age, specifically for one-to-one combat. With a spear or bow and arrow, you can also hunt, but not with a sword. Being a warrior was given a place in society, and belonged to the elite. Nevertheless, we hardly ever see swords in tombs. It is as if being a warrior did not remain tied to a specific person. Rather, it appears to have been temporary and acquired, and a status to be laid down over time as well.”
Tomography
At the time of writing this article, the eight sets of five hundred shots had not yet been fully processed into a three-dimensional tomography of the sword of Ommerschans. It became evident, however, that very few air bubbles are trapped in the bronze. Earlier, a tomography was made of a replica that was cast using state-of-the-art means. That sword has plenty of bubbles, especially at the tip. The sword of Ommerschans has far fewer bubbles.
“This is our first clue that the casting process was tremendously meticulous,” Amkreutz says. “You want to avoid air bubbles in bronze, because they weaken the sword and produce holes on the surface. It should be noted that both modern replicas are clearly inferior to what people produced three thousand years ago. We have lost that level of craftsmanship.”
Fingerprint
The neutron beam makes objects radioactive. Stricken atomic nuclei jump back into their ground state, emitting gamma radiation in the process. That process is called ‘cooling’ here. The good news is that different elements and isotopes (same element, different atomic mass) have individual characteristic gamma wavelengths that become visible as ‘peaks’ in the spectrum, as well as individual half-lives. As a result, the radiation is spread out over time.
“Initially, you mostly see tin-125 and copper-66,” Van Eijck explains. “That is gone after half an hour. Next up is copper-64, followed by trace elements like arsenic, zinc, antimony or indium.” Thus, gamma spectroscopy shows a fingerprint of the bronze, as it were, from which its origin can be deduced. Amkreutz explains: “Spectroscopy gives us information about the material composition of the sword and the composition of the bronze (an alloy of copper and tin-ed.). For example, about the ratio of copper to tin. We know that these six swords contained a lot of tin, lending them a bright silvery gold shine. They also contain trace elements that may indicate where the bronze may have come from. It appears that the same stock of bronze was used for all six swords, supporting the idea that they were made in a single place at a single time.” Amkreutz paints a picture of a society 3500 years ago, with a lively barter economy between different regions of central and western Europe and Scandinavia. Amber was exchanged for copper, tin, or bronze, for example. Trade routes were crucial for prosperity. As an archaeologist, Amkreutz suspects that the ceremonial swords played a role between tribes, gods and nature. “We note that (the sword of) Ommerschans appears to have been sacrificed at a time when the landscape was changing tremendously. The climate became wetter and substantial peat growth occurred that made access to the area difficult. Over time, these developments may have threatened trade routes. We suspect that the offering of the sword was related to that. It may well have been sacrificed to the gods in hopes of counteracting or controlling natural changes.”
© RID TU Delft
Van Leeuwenhoek’s microscope radiographed
Five years ago, Dr Lambert van Eijck placed one of Antoni van Leeuwenhoek’s microscopes from Rijksmuseum Boerhaave in a neutron beam of the reactor core. The superior quality of these microscopes had been a mystery for more than 350 years. “Neutron radiation is ideally suited to this task,” Van Eijck said of this project at the time. “Unlike X-rays, neutrons can easily pass through metals while also recording glass.” For the research, the museum object was rotated 180 degrees for 24 hours. The computer then made a 3D reconstruction of approximately 500 transmission images. The neutron image of the instrument revealed a thin lens sandwiched between two brass plates at the intersection of yellow hairlines. Curator Tiemen Cocquyt (Boerhaave Museum) concluded: “It was not a matter of an exotic production method. Van Leeuwenhoek was simply highly skilled at polishing these minuscule lenses.”