Neutron Scattering

2 . Properties of the neutron, elementarg
scattering processes
D. Richter
2 .1 A few remarks on history
In 1932 the neutron was discovered by Chadwick . The naine results from the observation that
the neutron apparently does not possess an electric charge : it is neutral . Today, one knows
that the neutron is an ensemble of one up quark and two down quarks . According to the
standard theory, the total charge therefore amounts to 2/3 é + 2 ( -1/3 e) = 0 . At prescrit this
theoretical statement is proven with a precision of- 10 -21 e-1
Only four years later in 1936 Hahn and Meitner obselved the ferst man-made nuclear fission .
In the saine year also the frst neutron scattering experiment was performed. Its set-up is
shown in Fig .2 .1 . Neutrons were taken from a radium beryllium source which was covered by
a paraffm moderator . From that moderator neutron beams were extracted such that they bit
magnesium oxide single crystals which were mounted on a cylindrical circumference under
the appropriate Bragg angle. After reflection they were guided to a detector which was
mounted opposite to the radium-belyllium source . In order to avoid any directly penetrating
neutrons a big piece of absorber was mounted in between the detector and the source .
Magnesium ozidc singlc
crystals mountcd around
cylindrical circumfercncc
Figure 2 .1 :
Mitchell and Powers's apparatus for demonstrating the diffraction of neutrons
(alter Mitchell and Powers 1936) .