Physics and Engineering of Radiation Detection

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