INTERACTIONS OF RADIATION WITH MATTER

twCshttp:// twCs query=radiation+effects&sm=1https:// query=radiation+effects&sm=1

Radiation Radiation, in our context, is energy in the form of high speed particles and electromagnetic waves. Radiation is further defined into ionizing and non- ionizing radiation.

Ionizing radiation Radiation with enough energy so that during an interaction with an atom, it can remove bound electrons, i.e., it can ionize atoms. Examples are X-rays and electrons.

Ionizing radiation consists of photons and/or moving particles that have sufficient energy to knock an electron out of an atom or molecule → an ion. The photons usually lie in the ultraviolet, X-ray, or γ -ray regions of the electromagnetic spectrum The moving particles can be the α and β particles emitted during radioactive decay.

Radiation types: Alpha particle (2 Ps + 2 Ns: 4 He nucleus) Beta particle (- or + electron) Gamma ray (photon energy packet) Neutrons Cosmic Rays

An energy of roughly 1 to 35 eV is needed to ionize an atom or molecule The particles and γ -rays emitted during nuclear disintegration often have energies of several million eV. Therefore, a single α- particle, β- particle, or γ -ray can ionize thousands of molecules

Radioactivity Spontaneous emission of particles/energy from unstable nuclei 235 U alpha particle 231 Th

A heavy charged particles moving through a solid, liquid or a gas travels a well-defined distance or range before coming to rest. As the highly energetic particles passes through the medium, it loses energy continuously before coming to rest, some lost through elastic collision with nuclei the medium atoms exited and ionized

Interaction of Charged Particles with Matter: Ionization

The range depends on the charge, mass, energy of the particles the density of the medium, the ionizing potential and atomic number of the atoms in the medium

Mass It is convenient to use unified mass units, u, to express masses –1 u = x kg –Based on definition that the mass of one atom of C-12 is exactly 12 u Mass can also be expressed in MeV/c 2 –From E R = m c 2 –1 u = MeV/c 2

Summary of Masses Masses ParticlekguMeV/c 2 Proton x Neutron x Electron9.109 x x

a The range of particles and protons in air under standard conditions For a given energy Proton has a range ~10X that of the particle proton ~ less charge interact weakly with the medium α particle ~ more massive, it travels at low speed loss energy more readily, it has more time to interact

AAA The energy loss vs the energy of a charged particle of mass moving through a medium At low energies The energy loss rate /stopping power is proportional to kinetic energy At high energies The energy loss rate /stopping power is approximately energy independent

The energy loss rate of the charged particles is approximately proportional to the density of the medium through which it travel. The higher medium density means more excitation of electrons involve during the ionization process in the medium. For proton with energy 1 to 10 The range in aluminum of the range in air

Electronvolt Energy that one electron gains when being accelerated over 1V potential difference is called one electronvolt (1eV) 1eV=1.6x C 1V= 1.6x J Another unit to measure energy, Commonly used in atomic and particle physics.

Electrons Energy << 1 MeV ~ same process Range not well defined ~ smaller than proton ~ large statistical variations of the path ~ takes a few deflection collision before coming to stop ~ easy to scatter ~ easy to accelerate ~ decelerated electrons by atom emits EM radiation (photon)

Photon ~ uncharged not effective to produce ionization and excitation ~ the direction interaction of photon with matter does not cause the bulk of ionization directly. ~ Some photons interaction result in the ejection of orbital electrons and these in turn cause the bulk of ionization ~ High energy photons are classified as secondary ionization radiation or indirect ionization radiation.

~ can be removed from a beam by scattering or absorption in the medium ~ photoelectric effect ~ compton effect ~ pair production

Energy of the photons,.

Interaction of x or  rays (photons) with matter

AAA

Non-ionizing radiation Radiation without enough energy to remove bound electrons from their orbits around atoms. Examples are microwaves and visible light.

INTERACTION OF PHOTON WITH MATTER

ABSORPTION OF PHOTON IN MEDIUM

Incident 1MeV photons

SCATTERED PHOTONS

For monoenergetic beam of photons incident on either thick or thin slabs of material, an exponential relationship exists between number of incident photons (I o ) and those transmitted (I) through thickness x without interaction

Intensity of the photon beam inside the matter

Half-value thickness The thickness of the absorber that will reduce the intensity of a beam of particles by factor of 2.

Example: Calculate HVT for lead, assuming an x-ray beam of wavelength is 20 picometer