Transcript Interaction of Radiation with Matter

Radiation
Radiation: The process of emitting
energy in the form of waves or
particles.
Where does radiation come from?
Radiation is generally produced
when particles interact or decay.
A large contribution of the radiation
on earth is from the sun (solar) or
from radioactive isotopes of the
elements .
http://www.atral.com/U238.html
Radioactivity
By the end
isotopes emit
were known:
1)
2)
3)
of the 1800s, it was
penetrating rays. Three
Alpha particles
Beta particles
Gamma-rays
(α)
(β)
(γ)
known
types
that certain
of radiation
Note: This is the
mass number, which
is the number of
protons plus neutrons
Alpha Particles (a)
Radium
Radon
R226
Rn222
88 protons
138 neutrons
86 protons
136 neutrons
+
n p
p n
a (4He)
2 protons
2 neutrons
alpha-particle (a) is a Helium nucleus.
It’s the same as the element Helium, with the
electrons stripped off !
Beta Particles (b)
Carbon
C14
6 protons
8 neutrons
Nitrogen
N14
7 protons
7 neutrons
+
eelectron
(beta-particle)
We see that one of the neutrons from the C14 nucleus
“converted” into a proton, and an electron was ejected.
The remaining nucleus contains 7p and 7n, which is a nitrogen
nucleus. In symbolic notation, the following process occurred:
n  p + e
( + n)
Yes, the same
neutrino we saw
previously
Gamma particles (g)
In much the same way that electrons in atoms can be in an
excited state, so can a nucleus.
Neon
Ne20
Neon
Ne20
10 protons
10 neutrons
(in excited state)
A gamma
+
10 protons
10 neutrons
(lowest energy state)
gamma
is a high energy light particle.
It is NOT visible by your naked eye because it is not in
the visible part of the EM spectrum.
How do these particles differ ?
Particle
Mass*
(MeV/c2)
Charge
Gamma (g)
0
0
Beta (b)
~0.5
-1
Alpha (a)
~3752
+2
* m = E / c2
INTERACTION OF RADIATION WITH
MATTER
The interaction of radiation with matter depends on:
i) The type and energy of the incident radiation.
ii) The chemical and physical properties of the target material.
iii) The manner in which the incident radiation interacts with the
material.
This section contains the mechanisms by which ionizing
radiation interacts and loses energy as it moves through
matter. This subject is extremely important for radiation
measurements because the detection of radiation is based on
its interactions and the energy deposited in the material of
which the detector is made. Therefore, to be able to build
detectors and interpret the results of the measurement, we
need to know how radiation interacts and what the
consequences are of the various interactions.
• For the discussion that follows, ionizing radiation
is divided into three groups:
• Charged particles: electrons (e-), positrons (e+),
protons (p), deuterons (d) , alpha particles (α),
heavy ions (A > 4).
• Photons: gammas (γ) or X-rays.
• Non-charged particles: Neutrons (n).
• This classification is convenient because each
group has its own characteristic properties and
can be studied separately.
INTERACTIONS OF CHARGED
PARTICLE WITH MATTER
• Charged particles traveling through matter
lose energy in the following ways:
• In Coulomb interactions with electrons and
nuclei.
• By emission of electromagnetic radiation.
• By emission of Cerenkov radiation. Cerenkov
radiation is visible electromagnetic radiation
emitted by particles traveling in a medium,
with speed greater than the speed of light in
that medium. It constitutes a very small
fraction of energy loss.
INTERACTIONS OF PHOTONS WITH MATTER
• Photons, also called X-rays or g rays are
electromagnetic radiation, are considered as
particles that travel with the speed of light c and
they have zero rest mass and charge.
• There is no clear destination between X-rays and γ–
rays. The term X-rays is applied generally to photons
with E < 1 MeV. Gammas are the photons with E > 1
MeV. X-rays are generally produced by atomic
transitions such as excitation and ionizations.
Gamma rays are emitted in nuclear transitions.
• There are several possible interactions of photons,
but the three most important ones are: the
photoelectric effect, Compton scattering and pair
production.
A) The Photoelectric Effect
• The energy of the gammaray photon is completely
transferred to an orbital
electron which is ejected
from its atom .
• The gamma-ray no longer
exists after the collision.
The ejected electron then
causes ionization until it
loses its energy, and is
captured by an atom. The
photoelectric effect is more
likely to occur when the
photon energy is low, i.e.
below 0.5 MeV and the
absorber is a heavy
material
B) Compton Scattering
• Higher energy photons may lose
only part of their energy to the
atomic electron which is again
ejected from its atom .This
electron goes on to create
ionization. The remaining energy is
taken up by another photon of
reduced energy which is scattered
in a new direction. The new
photon will either be absorbed by
a photoelectric effect, or if the
energy is still high by further
Compton scattering.
• Compton scattering occurs in all
materials and predominantly with
photons of medium energy, i.e.
about 0.5 to 3.5 MeV.
C) Pair Production
• Gamma-ray photons with energy
greater than 1.02 MeV may interact
with a nucleus to form an electronpositron pair. This amount of energy
is just sufficient to provide the rest
masses of the electron and
positron (0.51 MeV each). Excess
energy will be carried away equally
by these two particles which
produce ionization as they travel in
the material. The positron is
eventually
captured
by
an
electron and annihilation of the two
particles occurs. This results in the
release of two photons each of 0.51
MeV
known
as
annihilation
radiation. These two photons then
lose
energy
by
Compton
scattering or the photoelectric
effect.
D) Coherent scattering
• In addition to Compton scattering, another type of
scattering can occur in which the gamma-ray photon
interacts coherently with all the electrons of an
absorber atom. This coherent scattering or Raleigh
scattering process neither excites nor ionizes the
atom, and the gamma-ray photon retains its original
energy after the scattering event. Because virtually no
energy is transferred, this process is often neglected
in basic discussions of gamma-ray interactions.
However, the direction of the photon is changed in
coherent scattering. The probability of coherent
scattering is significant only for low photon energies
(typically below a few hundred keV for common
materials) and is most prominent high-Z absorbers.
NEUTRONS
(1) Slow Neutrons
• Slow neutrons are that with energies from
zero to about 1000 eV
(2) Intermediate Neutrons
• Intermediate neutrons having energies in the
range 1000 eV-0.5 MeV. We haven't enough
information about intermediate neutrons than
about slow neutrons due to the difficulty of
finding efficient detectors.
(3) Fast Neutrons
• Neutrons with energies having range between
0.5 and 10 MeV are called fast neutrons. This
energy region range gives the possibility of
many
nuclear
reactions
which
are
energetically impossible at lower ranges of
which the inelastic scattering is dominant.
(4) Very Fast Neutrons
• These are neutrons having energies in the
range 10-50 MeV and is distinguished from
the proceeding by the appearance of nuclear
reactions involving the emission of more than
one product such as the (n, 2n) reaction.
(5) Ultra Fast Neutrons
• Neutrons with energies beyond 50 MeV are
called ultra high neutrons. They are produced
by p-n interactions induced in nuclei by high
energy protons. The cosmic radiation is also a
source of neutrons with energies well above
those which are likely to be produced by
accelerations.
Interactions of Neutrons
• In common with gamma rays, neutrons carry no charge and
therefore cannot interact in matter by means of the
Coulomb force. Neutrons can also travel through many
centimeters of matter without any type of interaction and
thus can be totally invisible to a detector of common size.
As a result of the interaction of the neutron with the
nucleus of the absorbing material, it may either totally
disappear and be replaced by one or more secondary
radiations, or else the energy or direction of the neutron is
changed significantly.
• In contrast to gamma rays, the secondary radiations
resulting from neutron interaction are almost heavy
charged particles. These particles may be produced either
as a result of neutron-induced nuclear reactions or they
may be the nuclei of the absorbing material itself, which
have gained energy as a result of neutron collisions.
RADIATION UNITS
• Shortly after the discovery of x-rays in 1885 by
Professor Roentgen scientists began to notice the
harmful effects of exposure to this radiation. There
were many years before people realized how
dangerous x-rays and other radiation could be. Quite
number of pioneer radiologists suffered severe
injuries, and some even died, as a result of prolonged
exposure to dangerously high intensities of X-rays.
These early workers in the field had no means of
measuring the harm caused by radiation accurately
and depended on unreliable effects such as the
degree of skin reddening caused by the exposure, or
on timing the exposure from a certain type of X-ray
machine to establish quantity.
• In order to evaluate the hazards of radiation it is
necessary to have a measure of the harm which is the
radiation cause. The radiation effect is measured in
terms of exposure or dose
A) Exposure
• One of the earliest observed properties of X-rays
was their ability to ionize air. In 1928 the
International Congress of Radiology specified this
property as means of measuring the amount of Xradiation, and defined a unit which was named
the roentgen, (R).
• 1 R = exposure to X-rays or gamma-rays of such
intensity that the electrons produced by this
radiation in 1cm3 of dry air, at standard
temperature and pressure, generate along their
tracks electron - ion pairs carrying a total charge of
1esu of either sign
• The SI unit of exposure is defined as 1 C / Kg air,
without any new name proposed for it.
Numerically
• 1R = 2.58 x 10-4 C/Kg air
• The roentgen suffers from two limitations, first
it was defined in terms of electromagnetic
radiation only, and secondly it was defined in
terms of air only.
B) Absorbed Dose
• Because of the limitation of roentgen (R), another unit
called the radiation absorbed does or rad was defined as
energy absorbed per unit mass of material. So,
de
D
dm
Where D is the absorbed dose and de is the mean energy
imparted by ionizing radiation to matter in a volume
element and dm is the mass of matter in the volume
element. The unit of the absorbed dose is rad defined as,
• 1 rad = 100 erg/g
• The SI unit of absorbed dose is the Gray (Gy), defined as
• 1 Gy = 1 J/Kg =100 rad
C) The Dose Equivalent
• The units of absorbed dose defined in the pervious
section are quite adequate for the quantitative
assessment of the effects of radiation to inanimate
objects, like irradiated transistors or reactor fuel.
For protection of people, however, the important
thing is not the measurement of energy deposited
(the absorbed dose), but the biological effects due
to radiation exposure. Unfortunately, biological
effects and absorbed dose do not always have oneto-one correspondence, and for this reason a new
unit had to be defined, a unit that takes into
accounts the biological effects of radiation.
• The first step toward that task was the
introduction of a factor called the relative
biological effectiveness (RBE), defined as
RBEi 
[Absorbed dose from X - ray or gamma radiation(200– 300 KeV) producinga certainbiologicaleffect]
[Absorbed dose from radiationtypei producingthesame biologicaleffect]
In the meaning of RBE, there are the following notes
:
• RBE is defined in terms of photons; therefore, it follows
that
RBE=1 for electromagnetic radiation. Also,
although the definition of RBE specifies the energy of
the photons to be 200–300 KeV, RBE is taken as equal
to 1 for photons of all energy.
• A given type of radiation does not have a single RBE,
because RBE values depend on the energy of radiation,
the cell, the biological effect being studied, the total
does, dose rate and other factors.
• It is a well-known that the biological damage increase as
the energy deposited per unit distance, the linear
energy transfer (LET), increases. Thus, heavier particles
(alphas, heavy ions, fission fragments) are, for the same
absorbed dose, more biologically damaging than
photons, electrons and positrons.
In 1963, the International Commission on Radiological Units and
Measurements (ICRU) proposed the replacement of RBE by a new factor
named the quality factor (QF). In 1973 the ICRU (82) recommended
dropping "F" from QF, a suggestion that has now become practice. In
1977 the International Commission on Radiological Protection (ICRP)
recommended that the dose equivalent (H) at a point in tissue be written
as:
H = NQD
Where Q = Quality factor its values are given in Table 1.1.
D = absorbed dose
N = product of all the modifying factors. The suggested value of N is 1
When the unit of absorbed dose is multiplied by the corresponding Q
value, the unit of dose equivalent (H) is obtained. The H units are
1 rem = Q x 1 rad
and the SI unit,
1 Sievert (Sv) = Q x 1 Gy
Thus,
1 Sv = 100 rem.
• In some references the H= absorbed dose× wg
where is wg is a radiation weighting factor
which reflects the ability of the particular type
of radiation to cause damage. The value of wg
is found to depend on the density of
ionization caused by the radiation .
Table 1.1 Quality Factors for Several
Types of Radiation
Q
Radiation Type
Neutrons
Q
Radiation Type
1
Gamma-rays
2
Thermal
1
X-rays
5
0.02 MeV
1
Beta particles
7.5
0.10 MeV
1
Electrons
11
0.50 MeV
1
Positrons
11
1.00 MeV
10
Protons
8
20
5.0 MeV
65
10 MeV
Alpha Particles
20
Recoil nuclei
• Example 3.1
In 1 year a worker receives a gamma dose of 0.01
Gy, a thermal neutron (Ns) dose of 0.002 Gy and
a fast neutron dose (N1) of 0.0002 Gy. What is his
total equivalent dose?
(Take the radiation weighting factor for fast
neutrons as 20.)
Equivalent dose = absorbed dose x radiation
weighting factor
Equivalent dose, gamma = 0.01 x 1 = 0.01 Sv
Equivalent dose, Ns = 0.002 x 2.5 = 0.005 Sv
Equivalent dose, N1= 0.0002 x 20 0.004 Sv
Total equivalent dose = 0.019 Sv
• EFFECTIVE DOSE
further complication is that different organs and
tissues have differing sensitivities to radiation. To
deal with the very common situation in which the
body is not uniformly exposed, other concept is
needed, which is called effective dose, E. This is
obtained by summing the
equivalent doses to all tissues and organs of the
body multiplied by a weighting factor WT for each
tissue or organ. This is written as follows:
E=∑ HT .wT
where HT is the equivalent dose in tissue T.
• SUBMULTIPLES
in terms of the levels of radiation exposure
encountered in the working environment, the
gray and the sievert are very large units. It is often
convenient to have smaller units, and this
is done by using the prefixes milli (one
thousandth), abbreviated to m, and micro (one
millionth), abbreviated to 𝛍. Thus:
lGy= l000mGy= l000000 𝛍 Gy
lSv= l000mSv= l000000 𝛍 Sy
• Example 3.2
On three successive days, a nuclear reactor
operator received the following doses of gamma
radiation:
Day 1 95 𝛍Sy
Day2 5𝛍Sv
Day3 l mSv
What was his total dose in mSv over the 3 days?
Day Dose mSv
1 95 𝛍Sv = mSv 0.095
2 5𝛍Sv = mSv 0.005
3 1 mSv 1.000
Total dose = 1.100 mSv
• DOSE RATE
Grays and Sieverts are units that express an
amount of radiation which may have been
received over any period of time. In controlling the
radiation hazard, it is usually necessary
to know the rate at which the radiation is being
received. The relationship between dose,
dose rate and time is:
Dose = dose rate X time
Thus, if someone works in an area for 2h and
receives a dose of 4 mSv, then the dose
rate in that area is 2 mSv/h. Similarly, absorbed
dose rates are expressed in Gy/h.
• Example 3.3
If a person is permitted to receive a total
dose of 200 𝛍Sy ¡n a week, for how many
hours during that week may they work in an
area in which the dose rate is lo 𝛍Sv/h?
Dose = dose rate x time
Time= 200 𝛍Sv/10 𝛍Sv
=20h