Zeplin III

Zeplin III was previously used in Boulby Underground Laboratory as a dark matter detector.

It was donated to Whitby Museum in 2018 by Imperial College London. The text below appeared in the ‘Whitby and the Cosmos’ exhibition back in 2019.

Dark Matter

Nobody has seen dark matter. So why do scientist believe it exists? The answers lie in the effect that the unseen force of gravity has on objects that can be seen. Those objects are stars, and galaxy clusters. Most of them move according to laws of motion and gravity that are constant throughout the universe.

However, some stellar objects behave in ways that seem to disobey these basic laws. Scientists believe that this can only be explained by the presence of another form of matter. This ‘dark matter’ cannot be seen, does not interact readily with ‘ordinary matter’ nor with electromagnetic radiation, such as light or x-rays. But it has mass and, through the force of gravity, influences the way ordinary matter acts.

It is thought that dark matter comprises around 80% of the matter in the universe.

Detecting Dark Matter

Dark matter is made up of Weakly Interactive Massive Particles (WIMPs). Of the four forces known in nature, WIMPs exert the weakest two: gravity and the weak nuclear force. The aim of the ZEPLIN III is to detect the weak nuclear force by converting it into both electrical charge and light.

When a WIMP collides with a nucleus of Xenon it exerts a weak nuclear force. This collision produces a tiny electrical charge (in a process known as ionisation) and a package of light or photo (in a process known as scintillation).

Other Particles apart from WIMPs also cause these effects. But measuring the ratio between the ionisation and the scintillation gives a definite indication of whether the article is a WIMP – thereby showing the presence of dark matter.

In order to detect and WIMP interactions ZEPLIN III has certain key elements in its design and construction.

• Low levels of radiation in the background (hence the use of Boulby Underground Lab)
• Tight control of radiation in the materials used in the instrument.
• Minimal chemical contamination inside the instrument.
• Highly efficient method for collecting and detecting light scintillation.

The anode plate and cathode grid at the top of the machine measure any ionisation from WIMP collisions in the xenon.

The array of Photo Multiplier Tubes (PMTs) gathers and detects light from any collision episodes. It also allows researchers to pinpoint the location of any collision with the xenon tank.

To keep xenon liquefied the machine has to run at around -150 centigrade. This is attained by circulating liquid nitrogen around the machine.

Building ZEPLIN III

The design and construction of experimental equipment is a vital part of scientific work. This takes expertise in a huge range of skills. As well as imagination and innovation, designers of ZEPLIN III employed theoretical physics, cosmology, metallurgy, electrical and mechanical engineering and radiation technology among many other skills.

The photographs show different elements of the ZEPLIN III being assembled at Imperial College London.