National Physical Laboratory

Electron Trees

What is an Electron Tree?

Electron tree - blue

Electron tree - red-green

An Electron Tree is a small Perspex block which has been internally fractured by electrons escaping through a tiny break in the Perspex's base.

How are they made?

NPL made* Electron Trees on-site at NPL using a machine called an Electron Linear Accelerator or 'linac' for short. Firstly, a normal block of Perspex is placed in the accelerator beam, which fires an intense beam of electrons at the Perspex. The electrons, being particles (travelling at enormous speeds) penetrate the Perspex and come to a stop in the centre of the block (this stopping distance was initially calculated) - the electrons are now trapped inside the block. This can change the block's appearance through a process called solarisation. Next, the block is removed from the accelerator, and an operator makes a very small shock to the base of the Perspex using a centre punch that is earthed. This gives all the extra electrons inside the block a way out - so they all rush out of the block through the dent in an instant, fracturing the inside of the Perspex as they go.

Watch an Electron Tree being made.

Why are they called Electron Trees?

They are known as Electron Trees as the pattern the electrons make in the Perspex as they rush out of the block looks like a tree, or fern-like structure. They are also known as Lichtenberg figures, Beam Trees, or Lightning Trees - you can see the flashes of 'lightning' in our atom smasher video.

What does NPL have to do with Electron Trees?

We made them on-site using our linac* and, as they are such beautiful objects, we have one on display in our Reception area. The linac's 'day job' was to deliver beams of electrons to a fixed point, which is currently one of the most effective treatments used to kill cancerous cells in the human body.

Part of NPL's role is to ensure that hospitals' linacs deliver an accurate and consistent radiation dose for all patients. The accuracy of the beam is critical to ensure that the cancerous cells are uniformly hit during the treatment and do not damage the healthy surrounding cells. Hospitals using linacs ensure their machines are accurate and consistent by measuring their output with a small measurement device called an ionisation chamber and that is traceable back to our national standards.

* NPL is in the process of upgrading its facilities in this area and the new machines are no longer capable of making Electron Trees.

Find out more about NPL's work in Radiation Dosimetry