CHAPTER IX
INSIDE THE NUCLEUS
Nuclear physics is just low-density astrophysics.
26. The structure of the nucleus – the densest clouds
Nuclear physics was born in 1896 in France, but is now only a small activity. It produced
ot more than one daughter, experimental high energy physics, which was born around
1930. But since 1985, also these activities are in strong decline. Despite the shortness, the
family history is impressive; the two fields uncovered why stars shine, how powerful bombs
Ref. 743
work, how cosmic evolution produced the atoms we are made of and how medical doctors
can dramatically improve their healing rate.
A physical wonder: magnetic resonance imaging
Arguably, the most spectacular tool that physical research has produced in the twentieth
century was magnetic resonance imaging, or MRI for short. This technique allows to image
human bodies with a high resolution and with (almost) no damage, in strong contrast to
X-ray imaging. Though the machines are still expensive – costing 400 000 Euro and more –
there is hope that they will become cheaper in the future. Such a machine consists essentially
of a large magnetic coil, a radio transmitter and a computer. Some results of putting part of
a person into the coil are shown in Figure 275.
In these machines, a radio transmitter emits radio waves that are absorbed because hy-
drogen nuclei are small spinning magnets. The magnets can be parallel or antiparallel to the
magnetic field produced by the coil. The transition energy E can be absorbed from a radio
wave whose frequency ω is tuned to the magnetic field B. The energy absorbed by a single
hydrogen nucleus is given by
E =h̄ω =h̄γB
(495)
The material constant γ/2π has a value of 42.6MHz/T for hydrogen nuclei; it results from
the non-vanishing spin of the proton. Using some cleverly applied magnetic fields, typically
with a strength between 0.3 and 1.5 T, the machines are able to measure the absorption
for each volume element separately. Interestingly, the precise absorption level depends on
the chemical