Neutron matter interactions topic in Nuclear physics.ppt

US Particle Accelerator School
FERMILAB
Beam Loss & Machine Protection
Lecture – Interactions of Radiation with Matter

FERMILAB
Interactions of particles with matter
 Dominant interaction for lower energy particles used in
industrial applications (generally <10 MeV) is due to
Coulomb (electromagnetic) interactions
 Inelastic collisions between incident electrons & orbital electrons of
absorber atoms
 Elastic collisions between the incident electron & nuclei of absorber
atoms
 The ionization & excitation of atomic electrons (inelastic) in
target material are the most common processes
 X-ray emission can become important, particularly for
electrons in high Z materials
 Nuclear interactions play a less significant role

FERMILAB
Bremsstrahlung & pair production
 High-energy electrons (> “critical energy”) predominantly lose
energy in matter by Bremsstrahlung
 The energy loss by Bremsstrahlung is exponential
 High-energy photons predominantly lose energy by e+
e-
pair
production
 Xo = mean distance over which an electron’s energy is reduced by
a factor of 1/e due to radiation losses only
 Also, Xo = 7/9 of mean free path for pair production

FERMILAB
Radiation length
 The characteristic amount of matter traversed for both of these
loses is the radiation length Xo, [ g-cm−2
]
 Radiation loss is approximately independent of material when
thickness expressed in terms of X0
 Critical energy is the energy at which losses due to ionization are
equal to losses by radiation

FERMILAB
Classical energy loss (dE/dx)
 Charged particles passing through matter collide with nuclei &
electrons
 For an incident particle of mass M, charge z1e, velocity v1.
colliding with a particle of mass m, charge z2e:
(For Z electrons in
an atom with A~2Z)
If m = me and z2=1 for e,
M = Amp and z2=Z for n:

FERMILAB
 Total energy lost by incident particle per unit length:
where
 This classical form is an approximation.
 Energy loss for a minimum ionizing particles,
(Eo > 2mparticlec2
)
averaged over its entire range, is ~2 MeVcm2
/g
 ~2 MeV/cm in water & water-like tissues
Energy loss and stopping power (cont’d)
 characteristic orbital frequency for the atomic electron

FERMILAB
Range of Particles
When Coulomb scattering dominates the energy loss, a pure beam of
charged particles travel roughly the same range R in matter
Example: 1 GeV/c protons have a range of about 20 g/cm2
in lead
(17.6 cm)
The number of heavy charged particles in a beam decreases with
depth into the material
Most ionization loss occurs near the end of the path, where velocities
are small => Bragg peak: increase in energy loss at end of path
Mean Range depth at which 1/2 the particles remain.

FERMILAB
Beam interactions with absorbing medium
 Inelastic collisions with orbital electrons of target atoms
 Loss of incident electron’s kinetic energy through ionization &
excitation of target atoms
 Two types of ionization collisions:
 Hard collisions - ejected orbital electron gains enough energy to be
able to ionize atoms on its own (called delta rays)
 Soft collisions - ejected orbital electron gains an insufficient
amount of energy to be able to ionize matter on its own
 Elastic collisions between incident particles & target
nuclei
 Incident electrons lose kinetic energy through a cumulative action
of multiple scattering events
 Each event characterized by a small energy loss

FERMILAB
Interactions of photons
 For three major types of interaction play a role in photon
transport:
 Photoelectric absorption
 Compton scattering
 Pair production

FERMILAB
Photoelectric Absorption
 The photon transfers all of its energy to a bound electron
 The electron is ejected as a photoelectron
 This interaction is not possible with a free electron due to
momentum conservation.
 The photoelectron appears with an energy: Ee- = hν - Eb
 Photoelectron emission creates a vacancy in a bound shell of
electrons
 The vacancy is quickly filled by an electron from a higher shell
 As a result one or more characteristic X rays may emitted
‐ .
 These X rays are generally reabsorbed close to the original site
‐
 In some cases an Auger electron is emitted instead of the X ray
‐

FERMILAB
Compton scattering of photons
 Compton scattering is the predominant interaction for gamma rays
with energies < a few MeV
 The incident gamma scatters from a loosely bound or free electron in
the absorbing material
 The incoming photon transfers a portion of its energy to the electron depends on
the scattering angle
 The photon is deflected at an angle Θ & the electron is emitted as a recoil

FERMILAB
Pair production is possible if Eγ > 2 me-
 The gamma ray is replaced by an e+
e-
pair
 To conserve energy & momentum, pair productions must take
place in the coulomb field of a nucleus
 The photon energy in excess of 2 me- (1.02 MeV) is converted
into kinetic energy shared between the e+
& e-
 The e+
subsequently slows down in the medium & annihilates with
another electron, releasing two 511 keV photons in the process.
 The pair production probability remains very low until the gamma ray
energy approaches several MeV.
 The probability varies approximately with Z2
of the absorber
No simple expression exits for this relation.

FERMILAB
Regimes of gamma transport
Source: Knoll, G. F., Radiation Detection and Measurement, 4th Edition, John Wiley (2010)
The lines show values of Z and hν for which the two neighboring effects are just equal

FERMILAB
Interactions of neutron with matter
 Neutron beams pass through matter until each undergoes a
collision at random & is removed from the beam.
 Neutrons are scattered by nuclei not electrons
 They leave a portion of their energy until they are thermalized &
absorbed.
 Beam intensity drops continuously drop as it propagates
through the material
 mean kinetic energy of the neutrons also generally decreases
 Beam intensity follows an exponential attenuation law
 Characterized by an attenuation length

Neutron matter interactions topic in Nuclear physics.ppt

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