Introduction to Health Physics: Fourth Edition - Ruang Baca FMIPA UB

60 CHAPTER 3
Figure 3-1. Diagram showing the principle of Rutherford’s
experiment with the scattering of alpha particles.
The alpha source, its collimator, and the scattering foil
are fixed; the alpha-particle detector—consisting of a
collimator, a ZnS scintilling crystal, and a microscope—
rotates around the point where the alpha beam strikes
the scattering foil.
of scattered alpha particles was observed as the alpha-particle detector traversed a
scattering angle from 0 ◦ to 150 ◦ . Similar results were obtained with other scatterers.
The observed angular distributions of the scattered alpha particles agreed with those
predicted by Rutherford’s theory, thereby providing experimental evidence for the
nuclear atom. Matter was found to consist mainly of open space. A lattice of atoms,
consisting of positively charged nuclei about 5 × 10 −15 m in diameter and separated
by distances of about 10 −10 m, was inferred from the scattering data. Detailed analyses
of many experimental data later showed the radius of the nucleus to be as follows:
r = 1.2 × 10 −15 × A 1/3 m, (3.1)
where A is the atomic mass number. The number of unit charges in the nucleus
(1 unit charge is 1.6 × 10 −19 C) was found to be approximately equal to the atomic
number of the atom and to about one-half the atomic weight.
Later work in Rutherford’s laboratory by Moseley and by Chadwick in 1920
showed the number of positive charges in the nucleus to be exactly equal to the
atomic number. These data implied that the proton, which carries one unit charge,
is a fundamental building block of nature. (Based on data from high-energy particle
experiments accumulated over a period of several decades, the American physicist
Murray Gell-Mann showed that the proton consisted of three basic particles called
quarks and was held together by a nuclear force so strong that the quarks cannot be
separated. For this discovery, Gell-Mann was awarded the Nobel Prize in Physics in
1969. Although the proton [as well as the neutron] is an assembly of three smaller
particles, for the purpose of health physics, the proton and neutron are considered
to be fundamental particles.)
The outer periphery of the atom, at a distance of about 5 × 10 −11 m from the nucleus,
was thought to be formed by electrons—equal in number to the protons within
the nucleus and distributed around the nucleus. However, no satisfactory theory to
explain this structure of the atom was postulated by Rutherford. Any acceptable
theory must answer two questions: First, how are the electrons held in place outside
the nucleus despite the attractive electrostatic forces, and, second, what holds the
positive charges in the nucleus together in the face of the repulsive electrostatic
forces?