TU Delft will shortly have the use of a brand-new laboratory with an electron accelerator that is unique in the world. This facility, called APPEAL (Advanced Picosecond Pulsed Electron Accelerator Laboratory), will be used to study the properties of materials and it will be located at the Reactor Institute Delft (RID). The start of construction of the new facility was celebrated in the RID complex on Monday 18 December.

As the name suggests, the workhorses of an electron accelerator are electrons. In the accelerator a short ultraviolet laser pulse will release electrons from a copper plate. The electrons are then accelerated using microwaves and fired at a high speed through a sample.

This sample could be a slice of a specific material. ‘By studying how the electrons interact with a material, we can learn more about its properties,’ says Professor Laurens Siebbeles. ‘This will enable us to do in-depth material research, for example into new materials for use in solar cells, LEDs transistors or nanoelectronics.’ The device can also be used to improve scientists’ understanding of the effects of radiation on tissues, which could help to prevent cancer or improve radiotherapy techniques.

Pancake
The APPEAL electron accelerator is a unique device, of which only a few exist in the world. What makes this accelerator extra special is that it will deliver no less than 100 pulses per second, while other labs are limited to about 10 pulses per second.

More importantly, the electron pulse produced by this accelerator is only one picosecond long, i.e. one millionth of a millionth of a second. ‘This ultrashort laser pulse releases all the electrons from the copper plate almost simultaneously in a pancake-shaped cloud,’ explains Siebbeles.

The flatter the pancake, the higher the time resolution and so the more precisely scientists can monitor what the electrons do in a material. The TU Delft electron pancakes will be extremely flat indeed.

Mobility
Another innovation of this accelerator is that it is combined with what is called terahertz detection. ‘Normally, researchers measure the mobility of a charge in a material by attaching electrodes to it,’ explains Siebbeles. ‘But this only works if the electrons can move freely throughout the sample. If there is a small crack in the material, this will influence the results. Using terahertz detection, we will soon be able to look deep inside the material and see exactly how the electrons move there, without having to touch the sample.’

A bunker will be built on the RID site first to ensure the researchers who work with the accelerator are protected. This is because harmful X-rays are released when the accelerated electrons in a material are deflected by atomic nuclei. When the bunker has been completed, the accelerator will be assembled and the scientists can begin their research. The new laboratory for the electron accelerator is expected to be completed by mid-2018.