Making the invisible visible 

A telescope consisting of a cubic kilometre of sea water is the project with which Professor Dr Maarten de Jong is involved. He has devised a method of using this telescope even more efficiently. On Friday 4 April he will deliver his inaugural lecture.

Maarten de Jong: 'I am especially fascinated by the experiments.'

Elementary particles
From the very start of his studies, De Jong was fascinated by the elementary particles which constitute matter. 'The more you think about these particles, the more elementary it becomes,' he explains. 'From atoms to atom nuclei to protons, and finally to quarks. I was particularly fascinated by the experiments: the research on elementary particles and related to this the  fundamental forces which play a role in nature.' He was therefore delighted to be able to spend three years at CERN  (European Organisation for Nuclear Research) in Geneva, working on his PhD. He has now been attached to the Nikhef (National Institute for subatomic physics) in Amsterdam for the past  ten years.

Neutrinos
Later, De Jong focused more on neutrinos and their interaction with matter. Neutrino's are special, almost invisible particles. De Jong: 'Their weak interaction with matter makes them difficult to detect. We now use neutrinos as a means to study phenomena in the universe. Neutrinos can literally penetrate anything and you need enormous telescopes to detect them.

KM3NeT
The telescope which is being built now near the French Mediterranean sea coast is a totally new way of looking at the cosmos, wholly different from the traditional method. This large telescope consists of a cubic kilometre of sea water and is called KM3 Net (pronounced  Kay-Em-three-Net), in which 'NeT' stands for 'neutrino telescope'. 'Building such a telescope is a technological challenge which we are today able to meet.'

Sea water
Neutrinos are not immediately detectable. They first have to undergo material interaction and then they will be transformed into another particle, a muon - a heavy electron - which can be observed. The muon can be made visible by means of sea water. De Jong: 'A muon moves through sea water at the speed of light. It actually travels faster than light, as the speed of light is reduced in water.  An electro-magnetic shock wave, comparable to a sound barrier, occurs, the light of which we are able to detect.' Sea water has a couple of excellent advantages: it is free and it is transparent. This makes it possible to detect the light with very sensitive cameras at relatively large distances from the trail that muon leaves behind.

Daylight
The area of the cubic kilometre of sea water is located two and a half to five kilometres under the surface. At such depth there is no longer any pollution from daylight. 'The whole area has been demarcated with a grid of very sensitive cameras placed at a distance of every hundred metres,' says De Jong.  We can register the arrival of the light on the sensors with nano-second precision. 'Because the sea water moves, the cameras also move. But these movements are compensated by keeping track very precisely with acoustic signals of where each detector is a a given moment. A true technological feat.

The grid of very sensitive cameras of the sea water telescope Antares. Antares is the precursor of KM3Net, but works according to the same principle.

Three dimensional
As the telescope is three dimensional, it does not have to be directed towards a particular place in the sky. Nevertheless, De Jong's innovation is based on 'directing' the telescope. 'It may sound paradoxical, but the method have devised is to look at only one side specifically rather than at all sides at once. This has to do with how you process the data. In this way, I can increase the telescope's sensitivity from a factor of two to five.'

Animation of muon detection in the sea water telescope, Antares. More animations on the Nikhef website More animations on the website of the Nikhef.

Cosmic radiation
Apart from daylight, there is even more noise pollution which De Jong would rather not encounter: the cosmic radiation which is present everywhere causes the rise of muons in the atmosphere. These muons also leave a trail behind in the telescope. But muons cannot travel limitlessly through matter as neutrinos can. Hence, the earth itself functions as an effective filter for cosmic radiation. Light traces of muons which have risen from the sea bed must derive from neutrinos which have interacted with sea water after having travelled through the earth. Muons coming from above may derive from neutrinos but also from cosmic radiation.

Other signals
What does De Jong expect to see with the telescope? 'I don't only want to look at electro-magnetic radiation - visible light but also ultra violet light and x-rays - such as occurs in traditional astronomy. But that is limited. You can only see part of what takes place, which is only the electromagnetic force. A way to study the processes which occur in cosmic sources is by also examining other signals than just electro-magnetic radiation. By tracing the direction of  the neutrinos, you can find their precise source. Under the influence of magnetic fields, electromagnetic radiation is deflected between the source and the earth, but neutrinos always go straight ahead and are not affected by magnetic forces.'

Inaugural lecture  4 April, 16.00 hrs
Professor Dr  Maarten de Jong
Title: The standard model and the knowledge-based economy