Biomedical Engineering Reference
In-Depth Information
Fig. 3.9
Sources of electrons from
137
55
Cs and their energy
-
decay, with maximum
energies of 0.512 MeV (95%) and 1.174 MeV (5%).
Internal-conversion electrons also occur, with discrete energies
of 0.624 MeV (from the K shell) and 0.656 MeV (L shell) with a
total frequency of 10%. See decay scheme in Fig. 3.8. The total
spectrum of emitted electrons is the sum of the curves shown
here.
spectra. There are two modes of
β
transition is responsible for the gamma photon listed in Appendix D, 0.140 (89%). By
implication, internal conversion must occur the other 11% of the time, and one finds
two electron energies (e
-
), one a little less than the photon energy (by an amount that
equals the L-shell electron binding energy in the Tc atom). Because internal conver-
sion leaves inner-shell vacancies, a
99
43
Tc source also emits characteristic Tc X rays,
as listed. Since
9
43
Tc decays by
-
emission into stable
9
44
Ru, this daughter radiation
β
also occurs.
The way in which gamma rays penetrate matter is fundamentally different from
that of alpha and beta particles. Because of their charge, the latter lose energy al-
most continually as a result of electromagnetic forces that the electrons in matter
exert on them. A shield of sufficient thickness can be used to absorb a beam of
charged particles completely. Photons, on the other hand, are electrically neutral.
They can therefore travel some depth in matter without being affected. As dis-
cussed in Section 8.5, monoenergetic photons, entering a uniform medium, have
an exponential distribution of flight distances before they experience their first in-
teraction. Although the intensity of a beam of gamma rays is steadily attenuated
by passage through matter, some photons can traverse even thick shields with no
interaction. Protection from gamma and X radiation is the subject of Chapter 15.
