Accelerators and Experiments
Particle accelerators are an essential part of the physicists toolbox. There are two different types of accelerators: linear accelerators --like the MEA accelerator at NIKHEF-- where particles are sent through a straight vacuum tube and accelerated up to even higher energies in accelerator sections. In the synchrotrons at CERN and DESY particles fly through almost circular vacuum tubes. They are accelerated in short straight sections, while powerful dipole magnets steer them through the ring and quadrupole magnetic lenses keep them together.
When the energy of particles is increased, stronger and stronger magnetic fields are needed to keep the particles in the ring. At the HERA accelerator in Hamburg superconducting magnets are used for this purpose. A number of these magnets were build by Dutch industries in collaboration with NIKHEF. At the LHC accelerator which will work with protons with an energy up to 8000 GeV, 1250 superconducting magnets of each 14 m long will be used. Dutch industry in collaboration with NIKHEF is working to develop these magnets.
The LEP accelerator --with a circumference of 27 km-- is the largest accelerator on Earth. Packets of electrons and their antiparticles (positrons) orbit the ring in opposite directions at almost the speed of light. NIKHEF participates in two of the four experiments: the L-3 and the DELPHI experiment. A large amount of experimental data is gathered with the enormous LEP-detectors to verify the theory of electro-weak interaction.
The L-3 detector is the largest of all, and contains a magnet with a length of 14 m, a diameter of 16 m and 6500 tons of steel. Inside this magnet are different modules such as wire chambers to detect trails of particles and calorimeters to establish their energy. Built-in electronics select only the most significant events from millions of collisions: within a thousandth of a second a decision is made whether to store the data for further analysis.
The ATLAS detector at the LHC accelerator will be even bigger and more complicated than the LEP-detectors. This is where physicists will hunt the Higgs-particle. The whole process is like seeking a needle in a haystack; even though a billion events per second happen, only a few Higgs-particles per year will be produced. So, extremely high standards have to be met by the detector which will be a great challenge for the physicists and technicians who design and build it.
Apart from collision experiments, CERN also has experiments in which a particle beam is shot at a target. One such experiment is CHORUS, in which neutrinos are studied. Neutrinos are of frequent occurence in the universe, but are rarely detected since they fly through almost everything. By using an intense neutrino beam in combination with a robust detector (800 kg of photo-emulsion, 110 tons of lead and 200 tons of iron), detection is possible. In this way researchers can determine the characteristics of a cosmologically important particle.
In other experiments the protons and neutrons in the nucleus of the atom are shattered. Information about their building blocks, the quarks, are deduced from the detected fragments. This way the theory of strong interaction, which keeps the quarks together, can be verified. Research into quarks and strong interaction is done at the HERA accelerator in Hamburg.
The subterranean tunnel of the HERA accelerator consists of two overhead rings with a diameter of 6.3 km. In one ring electrons are accelerated up to 30 GeV, in the other the much heavier protons reach energies up to 820 GeV. There are four points where electrons meet protons in a head-on collision; the protons are smashed apart in this process. The HERA accelerator can be compared to a gigantic 'electron microscope' to investigate the interior of protons. In the HERA experiments enormous amounts of data are stored: in one experiment the particle beams cross 100.000 times per second and each time there are hundreds of collisions.
The ZEUS detector at HERA detects the proton fragments and measures energy and impulse of the electron after the collision. The quark structure of the proton can be determined with great precision from the data. Indirectly the researchers at HERA also measure the gluons, the carrier particles of the strong force. The spin or polarisation of quarks in protons is investigated by the HERMES experiment.
In a completely different HERA-experiment, and in the future also at LHC, the detectors will look for particles containing a 'beauty' quark. Through these particles scientists wish to learn more about CP-symmetry, a symmetry in which charge and parity play a role. Violation of this symmetry is responsible for only having matter and no antimatter in the universe.
In the AmPS accelerator in Amsterdam properties and movements of protons and neutrons in the atomic nucleus are studied. Packets of electrons are accelerated in a 180 m long vacuum tube to an energy of approximately 600 MeV. The electron packets are then injected into an storage ring with a circumference of 212 m. Once the ring is full, the electrons are gradually and continuously extracted and led to the experiments. An alternative mode of operation is to keep the electrons orbiting in the ring; collisions then occur by placing a very thin or gaseous target in the ring. For these experiments the Internal Target Facility was build in collaboration with some foreign institutes.
By using a continuous beam of electrons, the sensitivity of the detectors at AmPS is vastly increased thereby enabling unique experiments. The atomic nucleus is often described as a shell model: protons are in shells with a specific binding energy and speed. By knocking protons from the nucleus, scientists can get precise information about the shells. Experiments like this show that protons in heavy nuclei have a surprisingly high speed. Until now measured speeds were several decimal fractions of light speed. AmPS experiments now show that speeds in the extremely small nucleus can go up to more than half the speed of light.
In the advanced experiments in the Internal Target Facility rarefied gas jets are sent perpendicular through the electron beam. In these gas jets atomic nuclei all spin in the same direction; they are polarised. By simultaneously using a polarised electron beam, the distribution of (charged) quarks in the neutral neutron can be studied. The production of polarised electrons is possible by using semiconductor crystals made by the Institute for Semiconductor Physics in Novosibirsk. The 'Siberian Snake', a superconducting coil made by the Budker Institute in Novosibirsk keeps the electrons polarised in the accelerator. The measurement is finally done by the 'Big Bite' detector, weighing in at twenty tons and will, for the first time, give a good insight in the spatial distribution of the electromagnetic field caused by quarks in the neutron.
Back to the index.