1.6. General Properties and Sources of Particles and Waves 43
saturation
Filament Current
Electron Beam Current
Figure 1.6.8: Dependence of
electron beam current on fila-
ment current.
Example:
Compute the number of electrons carried in a second by an electron beam of
1.6nA.
Solution:
Electrical current is defined as
I=
Q
t
,
whereQis the total charge passing in timet. Using this equation we can
compute the total charge carried in a second by the beam.
Q = It=(1. 6 × 10 −^9 )(1)
=1. 6 × 10 −^9 C.
Since each electron carries a unit charge of 1. 6 × 10 −^19 coulombs, the total
number of electrons carried by the beam is
N =
Q
1. 6 × 10 −^19
=
1. 6 × 10 −^9
1. 6 × 10 −^19
=10^10 s−^1.
Radioactive Sources of Electrons
We saw earlier that cobalt-60 emitsβ-particles together withγ-rays. Although it
can, in principle, be used as a source of electrons but because of the associated high
γ-ray background flux, it is not generally used for this purpose. There are a number
of other elements as well whose unstable isotopes emitβ-particles (see Table.1.6.2)
with very lowγ-ray backgrounds. Most of these radionuclides are extracted from the