Fast protons in lead
A part of NIKHEF's research is aimed at studying the atom's nucleus.
The atom's nucleus is stable thanks to a continuing powerplay of attracting and repelling forces between the particles in the nucleus: the protons and the neutrons. The attracting forces keeps the nucleus particles together and the repelling force keeps the nucleus from imploding. The repelling force becomes operative when particles get closer than twice their radius. (figure 1)
The particles are not in fixed positions, but move constantly throughout the whole nucleus. Particle speeds are quit different: most particles have speeds between 0 m/s and a quarter of the speed of light but some particles can have an extremely high speed, more than half the speed of light. This extremely high speed is a consequence of the repelling force which pushes particles apart when they get to close to one another. So, the number of high speed particles is a measure of the strength of the repelling force.
Experiments are carried out with the MEA/AmPS accelerator to establish these speeds and so obtain a better insight in the repelling force which works between the particles in the atom's nucleus. To this end, lead atoms with 208 particles in the nucleus are fired upon by the accelerator's high-energy electron beam. Dispersed electrons are registered by a large magnet, and the speed of the protons which are knocked out of the nucleus is measured by a second magnet. The original speed of the proton in the nucleus can be calculated from the speed at which the dispersed electrons and the knocked out proton arrive. The whole process is like a miniature pool game in which all balls move and gives a precise determination of how many protons in the nucleus have an extremely high speed.
The theory looks simple, but in real life these experiments are not so easy. Even though ten million billion electrons per second are fired at the nuclei, only a small portion comes close to a nucleus, a much smaller portion penetrates one and a minute fraction actually hits a proton in the nucleus. To give an indication of this problem: In the NIKHEF experiment only 5 protons per day were measured to have more than half the speed of light. These rare events must be separated from all other particles which reach the detectors. This makes high demands upon the detectors but also on the quality of the used electron beam. Without the specific qualities of the AmPS beam this experiment would be impossible.
The results of these experiments are surprising. In figure 2 we show the measured distribution of the speed of the proton and the distribution of the theoretically calculated speeds. If in the theory we only take the attracting force into account (blue line) the observations only agree for protons with speeds up to half the speed of light. But if we also take the repelling force into account (red line) it shows that the measured proton speed distributions still deviate from the calculated curve!
The difference between this last curve and the measurement is an indication that the repelling force between two particles in the nucleus is not the same as the force between two free particles, something that was always assumed in the calculations. In larger company particles behave differently than when they're alone!
Experiments are now conducted with the MEA/AmPS accelerator to shed new light on this problem. The most promising experiments are those in which we simultaneously try to observe both partners which have high speeds due to a head-on collision.
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