Physics And Chemistry Basis Of Biotechnology - De Cuyper - tiera.ru

Wim Mondelaers and Philippe Lahorte
Because the common unit for stopping powers is MeV cm 2 g -1 , the range is expressed in
g cm- 2 . To obtain the range in cm R has to be divided by the density p of the absorbing
material.
The range of 1 MeV α-particles, protons and electrons in water are respectively 4.6
pm, 39 µ m and 4.37 mm. For 10 MeV these values are 0.1 mm, 1.2 mm and 49.8 mm.
For other materials the range is roughly inversely proportional with the density.
3.2. INDIRECT IONIZING RADIATIONS
Unlike charged particles, which generally lose energy through a large number of small
energy transfers, photons (X-rays and g-rays) tend to lose a large amount of energy
when they interact with matter. However, the interaction probability of photons is
rather low, so that many photons will pass through a finite thickness of material without
change in energy or direction. A simple exponential law (3) can describe the reduction
of the number of photons transmitted through a sample with thickness d:
I = 1 0 e -µd
I, and I are the radiation intensities before and after the sample and µ is the absorption
coefficient taking into account the several processes that contribute to the attenuation
and scattering of a photon beam (Genvard 1993; Henke 1993; Hubbell 1999). Their
relative importance depends on the photon energy and on the nature of the irradiated
material. There are three important photon interactions, all three producing fast
electrons causing subsequently many excitations and ionisations: photoelectric effect,
Compton scattering and pair-production.
When the photon energy is below 0,5 MeV, the photoelectric effect is predominant.
The total energy, i.e. the entire photon, is used up in the ejection of an electron from an
atom shell. Subsequently this fast electron causes many excitations and ionisations.
Compton scattering arises predominantly when photons in the energy range 0.5 – 5
MeV collide with free or loosely bound electrons in the absorber. Part of the photon
energy is transferred to the electron as kinetic energy, and the photon is deflected from
its initial direction (Cooper 1997; Harding 1997).
When a photon has an energy of 1.02 MeV or higher it may extinct in the proximity
of an atomic nucleus of the absorber, giving rise to an electron-positron pair. This
process is called "pair-production" .
Neutrons, being particles without charge, gradually lose energy by direct collisions
with nuclei of matter. Ion pairs are produced by these collisions, the hit nucleus losing
one or more of its orbital electrons (ICRU 46, 1992). The energy transfer in these
elastic (billiard-ball like) collisions are most effective when the mass of the nucleus is
comparable with the mass of the neutrons, as is the case for light elements, especially in
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(2)
(3)