THE STRUCTURE OF THE NUCLEUS
– THE DENSEST CLOUDS
Nuclear physics was born in 1896 in France, but is now a small activity.
ot many researchers are working on the topic now. The field produced
ot more than one daughter, experimental high energy physics, which was born
around 1930.These activities have been in strong decline since 1985, with the exception
of the latest CERN experiment, the Large Hadron Collider. Given the short time nuclear
physics has been in existence, the Ref. 140 history of the field is impressive: it discovered why
stars shine, how powerful bombs work, how cosmic evolution produced the atoms we
are made of and how medical doctors can dramatically improve their healing rate.
“Nuclear physics is just low-density astrophysics.Anonymous”
A physical wonder – magnetic resonance imaging
Arguably, the most spectacular tool that physical research produced in the twentieth century
wasmagnetic resonance imaging, or MRI for short.This technique allows us toimage
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 up to several million
euro – there is hope that they will become cheaper in the future. Such a machine,
shown in Figure 58, 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 59.
In MRI machines, a radio transmitter emits radio waves that are absorbed because
hydrogen nuclei are small spinning magnets.The magnets can be parallel or antiparallel
to the magnetic field produced by the coil. The transition energy E is 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 = ħω = ħγB (48)
Thematerial constant γ/2π has a value of 42.6MHz/T for hydrogen nuclei; it results from
the non-vanishing spin of the proton. This is a quantum effect, as shown by the appearance
of the quantum of action ħ. Using some cleverly applied magnetic fields, typically
with a strength between 0.3 and 7 T for commercial and up to 21 T for experimentalmachines,
the machines are able to measure the absorption for each volume element separately.
Interestingly, the precise absorption level depends on the chemical compound the
nucleus is built into.Thus the absorption value will depend on the chemical substance.
When the intensity of the absorption is plotted as grey scale, an image is formed that
retraces the different chemical compositions.Using additional tricks, modern machines can picture blood flow in the heart or air flow in lungs; they now routinely Ref. 141 make films of the heart beat. Other techniques show how the
location of sugar metabolism in the brain depends on what you are thinking about.