chapter-1 photon interaction with matter and production of...
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Chapter-1
Photon interaction with matter and production of fluorescent
X-rays
1.1 Introduction
The study of interaction of gamma rays with matter has attained a significant
importance in the field of science and technology. Precise knowledge of the mechanism
by which radiations interact with matter is required for understanding diffusion and
penetration of radiations in the medium. During the last few decades, an advancement
of technology, gamma ray and X-ray spectroscopic techniques find enormous
applications in various diverse fields such as in medicine (Computerized Tomography
(CT) imaging , for the treatment of cancer, sterilizing medical equipments, diagnostic
studies etc.), in industry (for pasteurizing certain foods and spices, measuring and
controlling the flow of liquids, non-destructive testing to gauge the thickness of
different materials, detection of structural defects and others heterogeneities in objects),
in agriculture (to investigate the properties of soil and irradiation of seeds, non-
destructive inspection of deformations on the structure of a soil sample. etc.), in
biotechnology etc. Therefore, accurate experimental data of various X-ray or gamma
ray related spectrometric parameters such as interaction cross-sections, photon
attenuation coefficients, X-ray fluorescence cross-sections, absorption jump factor and
jump ratio etc. are needed.
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1.2 Brief introduction to fundamental processes and parameters
governing the interaction of photons with matter
1.2.1 Mechanism of photon interaction
Photons are classified according to their mode of origin, not their energy. Thus,
gamma-rays are the electromagnetic radiations accompanying nuclear transitions.
Bremsstrahlung or continuous X-rays are the result of the acceleration of free electrons
or other charged particles. Characteristics X rays are emitted in atomic transitions of
bound electrons between the K, L, M . . . shells in atoms. Annihilation radiation is
emitted when a positron and negatron combine. The quantum energy of any of these
radiations can be expressed as E=hν, where ‘ν’ is the frequency and ‘h’ is Planck's
constant. Interactions of these photons with matter are thought to be independent of the
mode of origin of the photon and dependent only upon its quantum energy.
Unlike charged particles, a well-collimated beam of γ rays shows a truly
exponential absorption in matter. This is because photons are absorbed or scattered in a
single event. That is, those collimated photons, which penetrate the absorber, have had
no interaction, while the ones absorbed have been eliminated from the beam in a single
event. This can easily be shown to lead to a truly exponential attenuation. When the
photon interacts, it might be absorbed and disappear or it might be scattered, changing
its direction of travel, with or without loss of energy. Various possible processes by
which the electromagnetic field of the gamma-rays interact with matter are described as
follow: (Evans, 1955)
Kinds of interaction Effects of interaction
1. Interaction with atomic electrons (a) complete absorption
2. Interaction with nucleons (b) elastic scattering (coherent)
3. Interaction with electric field surrounding (c) inelastic scattering
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nuclei or (incoherent) electrons
4. Interaction with the meson field surrounding nucleons
There are 12 ways of combining columns 1 and 2; thus in theory there are 12
different processes by which γ rays can be absorbed or scattered. Many of these
processes are quite infrequent and some have not yet been observed. However, in the
energy domain met most frequently in nuclear transitions, say, 0.01 to 5 MeV, the
prominent modes of interaction are the Photoelectric absorption, Compton scattering
and Pair production. Brief description of these three processes along with some other
processes which are of small interest in this energy region is given below:
1.2.1.1 Photoelectric absorption
The process of photoelectric absorption (photoionization) is one of the principle
mode of interaction of photons with matter. In this process, an electron is ejected from
an atom as a result of the absorption of photon. The energy of the ejected photon is
equal to the binding energy of the photon minus binding energy of the electron of the
atom. The law of conservation of momentum required that in addition to the incident
photon and ejected electron, a third party (residual atom) must take part in the
interaction. Consequently the photoionization is prohibited from the free electron and is
expected to increase with the tightness of the electron and binding energy. Generally the
effect varies with atomic number Z as Z4-5
and its variation with energy E, changing
from about E-7/2
at low energies (E<moC2) to E
-1 at high energies (E>moC
2), where moC
2
is the rest mass energy of the electron. Thus photoionization dominates at low energy
and for high Z elements. Since ejected electron usually comes from tightly bounded
inner shell, therefore, the photon energy must be at least or equal to binding energy of
the electron. The ejected electrons, called photoelectrons, have some angular
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distribution and the angle of maximum shifts towards zero degree with increase in
photon energy.
1.2.1.2 Compton scattering
In Compton scattering (Compton, 1923), the incoming photon interacts with the
atomic electron and degraded photon along with the electron is emitted. The energy and
momentum of the incident photon is conversed between the scattered photon and struck
electron which is assumed to be free and at rest. In practice, above assumptions simply
limits the theory to those cases for which the binding energy of the struck electron is
small as compared to the energy of the incident photon.
For an incident photon of energy ‘ h ’, scattered at an angle by a free and stationary
electron which recoils at an angle , the conservation of energy and momentum yields:
cos112
0
'
Cm
h
hh (1.1)
Where ‘ 'h ’ is the energy of the scattered photon and 2
0Cm is the rest mass energy of
the electron.
The probability of Compton scattering per atom )( of the absorber varies
linearly with atomic number Z and inversely with energy E as:
1* EZ (1.2)
1.2.1.3 Pair production
Above the incident photon energies of 1.02 MeV, a third type of interaction
becomes increasingly important. In this interaction, known as pair production, the
photon is completely absorbed and in its place appears a positron-electron pair whose
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total energy is just equal to ‘hυ’, which is given as;
h ν = (Te- + moc2) + (Te+ + moc
2) (1.3)
where Te- and Te+ are the kinetic energy of the electron and positron, respectively, and
moc2
= 0.51 MeV is the electronic rest mass energy. The process occurs only in the field
of charged particles, mainly in the nuclear field but also to some degree in the field of
an electron. The presence of this particle is necessary for momentum conservation.
The probability of pair production ‘κ’ per atom increases with increasing photon
energy and it usually increases more significantly with atomic number approximately
as:
(1.4)
1.2.1.4 Rayleigh scattering
It is the case of elastic scattering of gamma-ray from bound electrons. In this
case the electrons do not receive sufficient energy to eject themselves from the atom i.e.
bound electrons revert to their initial state after scattering. For large ‘hυ’ and small Z,
Rayleigh scattering is negligible in comparison with Compton scattering.
1.2.1.5 Thomson scattering by the nucleus
This process includes coherent scattering of gamma rays by (a) free electrons
and (b) nucleus as a whole (nuclear Thomson scattering).
1.2.1.6 Delbruck scattering
Delbruck scattering, or elastic "nuclear potential scattering", is due to virtual
electron pair formation in the coulomb field of the nucleus. It is also called elastic
nuclear potential scattering. The effect, if present, is extremely small and does not show
EZ ln*2
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up clearly in experiments designed to detect it.
1.2.1.7 Nuclear resonance scattering
This type of scattering involves the excitation of a nuclear level by an incident
photon, with subsequent re-emission of the excitation energy.
1.2.1.8 Photodisintegration of nuclei
Photodisintegration, or the "nuclear photo effect," is energetically possible
whenever the photon energy exceeds the separation energy of a neutron or proton.
Except for Be9 (γ, n) and H
2 (γ, n), these effects are generally confined to the high-
energy region above about 8 MeV. Even when photodisintegration is energetically
allowed, the cross sections are negligible compared with those of the Compton Effect
and of absorption by nuclear pair production.
1.2.1.9 Meson production
Mesons are produced if the γ rays energy is above 150 MeV. But cross-section is
very small (10-3
barn/atom)
1.2.2 Attenuation of gamma ray photons
As the radiation interacts with matter, its intensity will decrease. It is important
to know, how radiation intensity decreases as it passes through a substance. The degree
of attenuation is dependent on the absorber material and the energy of the radiation. For
all the absorbing materials, the attenuation of gamma radiations is exponential in
character. Two important physical spectroscopic parameters used for measuring the
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extent of attenuation of gamma ray as it passes through a given absorber are linear
attenuation coefficient and mass attenuation coefficient.
1.2.2.1 Linear attenuation coefficient
The probability of a photon traversing a given amount of absorber without any
kind of interaction is just the product of the probabilities of survival for each particular
type of interaction. The probability of traversing a thickness ‘x’ of absorber without a
Compton collision is just xe , where ‘ ’ is the total linear attenuation coefficient
for the Compton process. Similarly, the probability of no Photoelectric interaction is
xe , where ‘’ is the total linear attenuation coefficient for the Photoelectric process
and of no pair-production collision is xe
, where ‘’ is the total linear attenuation
coefficient for the pair-production process. Thus a collimated gamma-ray beam of
initial intensity ‘Io’ after traversing a thickness ‘x’ of absorber will have a residual
intensity ‘I’ of unaffected primary photons equal to
(oII xe xe xe )
x
II eo
)(
x
II eo
(1.5)
where the quantity )( is the total linear attenuation coefficient.
This attenuation coefficient is a measure of the number of primary photons
which have interactions. It is to be distinguished sharply from the absorption
coefficient, which is always a smaller quantity, and which measures the energy
absorbed by the medium (Evans, 1955).
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1.2.2.2 Mass attenuation coefficient
The mass attenuation coefficient, m (cm2/g) of the material can be calculated
with a simple relation:
m
(1.6)
where ‘ ’ is the density of the material in g/cm3.
Expression (1.5) can be expressed as:
x
II moe
(1.7)
where the product of ‘’ and ‘x’ is called mass thickness, defined as the mass per unit
area.
The linear attenuation coefficient of the material depends upon the energy of the
incident photons and nature of the material. Since the attenuation produced by a
medium depends upon the distribution of atoms present in that medium, also depends
upon the density of the medium. The mass attenuation coefficient is of more
fundamental importance than the linear attenuation coefficient because m is
independent of the density and physical state of the absorber as the density has been
factored out.
It is convenient to measure the thickness of the absorber in g/cm2, while dealing
with mass attenuation coefficient. The advantage in using units of grams per centimeter
square to measure absorber thicknesses is that equal amounts of various absorbers
measured in these units give roughly the attenuation.
The mass attenuation coefficient of a compound or a homogeneous mixture can
be obtained form the weighted sum of the coefficients for the elements using the simple
additive rule as:
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( )
m i i
i
w (1.8)
where i)/( is the mass attenuation coefficient for the ith
element and wi is its weight
fraction.
For a chemical compound with chemical Formula )......( 321 xnxxx DCBA , the weight
fraction for the ith
element is given by:
n
i
ii
iii
Ax
Axw
1
(1.9)
where Ai is the atomic weight of the ith
element.
1.2.3 Brief discussion of experimental techniques used for
measuring linear attenuation coefficient of irregular
shaped samples
1.2.3.1 ‘Two media’ and ‘Simplified Two media’ method
Gamma ray transmission geometry has been considered to be the most accurate
experimental technique for measuring linear/mass attenuation coefficients of elements,
chemical compounds and composite materials. Various workers have measured X-ray
and gamma ray attenuation coefficients for several elements, composite materials such
as glasses, biological compounds, building materials and solutions etc using this
geometry. Detail literature survey of the experimental work done on the measurement of
attenuation coefficient has been given in Chapter-2.
However, the measurement of attenuation coefficient by standard gamma ray
transmission technique depends mainly on two factors; thickness of sample under
investigation and the sample must be of regular shape. Therefore for odd shaped
samples of unknown thickness such as (such as rock fragments or construction
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materials) this method fails to delivers accurate results. To overcome this problem Silva
and Appoloni (2000) proposed a new method named ‘Two media’ method for
measuring linear attenuation coefficient of such irregular shaped sample. This method is
based upon the application of standard Lambert-Beer law for obtaining linear/mass
attenuation coefficient of odd shaped sample using transmission geometry. In this
method thickness of sample under study is not required.
Silva and Appoloni (2000) in their conclusion also suggested that if two media
are the same, the method does not work at all. Thus, larger the difference between
attenuation coefficient values of two media used; greater would be the accuracy of
method. This condition could be best met if air is chosen as one medium because
attenuation of air is usually assumed as zero while performing experiment with standard
gamma ray transmission geometry (Elias, 2003).
According to another suggestion proposed by Silva and Appoloni (2000), the
medium under consideration should be very homogenous. This condition could be best
met if media under consideration would be in liquid form, since liquid medium is in
general more homogenous than medium in powder form.
By incorporating these suggestions, Elias (2003) proposed modifications in ‘two
media method’ used for measuring linear attenuation coefficient of odd shaped samples
by choosing air as one of the medium. The modified ‘Two media’ method is called
‘Simplified Two media’ method. He theoretically demonstrates that this choice
simplifies the equation used, as well as the laboratory work. At the same time, it also
allows a greater number of repetitions as well as introduces larger difference in the
values of attenuation coefficient of the pair of media used. Detail theoretical
formulation of ‘Simplified Two media’ method is given Chapter-5.
In present study ‘Simplified Two media’ method has been used for measuring
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linear attenuation coefficient of irregular shaped FaL-G (flyash-lime-gypsum) samples.
1.3 Fluorescent X-rays
X-ray fluorescence is the process in which vacancies are created in the target
atom by photon bombardment. Characteristics X-rays emitted on decay of such
vacancies are known as fluorescent X-rays. Various processes involved in decay of
inner shell vacancies by electron emission have acquired specific name.
The term Auger effect is used to describe those transitions in which the decay
of a vacancy in an atomic shell leads to two vacancies in one or two different principal
shells. In Coster-Kronig transitions (Coster and Kronig, 1935) one of the two
vacancies produced in the non-radiative decay is in a different subshell of the same
principal shell that contained the initial vacancy. In addition, there exists the possibility
in particular cases that an initial vacancy can lead to two vacancies in the subshells of
the same shell. Such transitions are called super Coster-Kronig transitions.
The ionization of K/L/M shell followed by filling of the K/L/M vacancy leads to
production of K/L/M series of X-ray. The strongest lines in the given series is called
line and the weaker lines are called , and and so on, although the relative
intensities of these lines bear a little resemblance to the sequence of labeling. The
normal transitions, also known are diagram lines, are defined by simple atomic selection
rules, i.e
1n
1l
1j or 0
Where n , l and j are the changes in the principal quantum number, the orbital
quantum number and total angular momentum of the electron undergoing transition for
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de-excitation of state.
The transitions involved in some typical K and L X-ray series has been shown in
figure 1.1 and 1.2 respectively.
1.3.1 Fundamental processes and parameters governing the
production of fluorescent X-rays
1.3.1.1 X-ray production cross sections
The fluorescent X-ray production cross-section (Krause et al.,1978)
x
ij for an X-ray ij is the product of partial or subshell photo ionization cross-section
iP , fractional radiative decay rates ijF , fluorescence yield i :
iji
P
i
x
ij F (1.10)
Where i refers to the subshell photoionized, j to the final state of an X-ray line.
Alternatively, ij could be considered the designation of characteristic X-ray, for
example, K, L etc. The partial fractional emission rate, ijF , is given by radiative rate,
ij , for the X-ray relative to the total radiative rate, iR , for a vacancy in the ith
subshell:
iR
ij
ijF
(1.11)
Now, the cross sections for the emission of X-rays under Kα and Kβ peaks can be
defined using expression by taking the fractional intensity of these X-rays into account
as:
KK
P
K
x
K F (1.12)
KK
P
K
x
K F (1.13)
In the shells having more than one subshell (i.e. L, M....) the effect of vacancy shifting
due to Coster-Kronig transitions is taken into account, while defining the total or partial
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X-ray emission cross sections.
The L X-ray emission spectrum is more complicated than K X-ray
spectrum since it contains the contribution from all the three subshells. Since all the L
X-ray emission lines are not resolved because of the limited resolution of the available
spectrometers. The L X-ray emission spectra of intermediate Z elements taken with the
currently available XPIPS Si (Li) detectors show distinct peaks denoted by Ll, Lα, Lβ
and Lγ. Each peak covers a group of lines of the L X-ray series which have very close
energies and thus cannot be resolved due to limited resolution of the detector. The
partial L X-ray emission cross-sections corresponding for ,, and γ peak is given as:
3332321323121 ])([ Fffffx
L (1.14)
3332321323121 ])([ Fffffx
L (1.15)
333232
1323121222121111
]
)([][
Ff
fffFfFx
L
(1.16)
222121111 )( FfFx
L (1.17)
Where σ1, σ2 and σ3 and ω1, ω2 and ω3 are LI, LII and LIII subshell photoionization cross-
sections and subshell fluorescence yields respectively. 3F is the fraction of intensity of
X-rays originating from LIII transitions which contribute to the L peak of L X-ray
spectrum. All other F’s can be similarly defined. 12f is the Coster-Kronig transition
probability of shifting of electron from LI subshell to LII subshell. All other f’s can be
similarly defined.
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1.3.1.2 Fluorescence yield
The fluorescent yield (Bambynek et al., 1972) of an atomic shell or subshell is
defined as the probability that a vacancy in that shell or subshell is filled through a
radiative transition. An atom with a vacancy is in excited state; let is the total width
of that state related to the mean life of the state by / . The width is the sum of
the radiative width R , the radiationless width A and the Coster-Kronig width CK . The
fluorescent yield is therefore given by
/R (1.18)
Thus the fluorescence yield of a shell is equal to the number of emitted photons
when vacancies in that shell is filled, divided by the total number of vacancies in that
shell. The application of this definition to the K shell of an atom, which is normally
contains two S1/2 electrons is straight forward. In this case, fluorescence yield K is
given as:
K
KK
n
I (1.19)
Where KI is the total number of characteristics K shell X-ray photons emitted and Kn is
the number of primary K shell vacancies.
However, the situation becomes complicated for higher atomic shells for the two
reasons.
(i) Shells above K shell consist of more than one subshell because electrons have
different angular momentum quantum numbers. Moreover, it is very difficult to
ionize only one out of all subshell. All the subshells are ionized in specific ratios
depending upon the subshell cross sections of ionizing process. The average
fluorescence yield, thus, depends in general on how the shells are ionized since
different ionization methods give rise to different primary vacancy distributions.
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Fig. 1.1: Typical K X-ray emission Spectrum.
K
LIII
LII
LI
MV
MIV
MIII
MII
MI
K
K
K
K
K
30
Fig.1.2: Typical L X-ray emission Spectrum.
MV
MIV
MIII
MII
MI
NV
NIV
NIII
NII
NI
OIII
OII
NVII
NVI
OI
L
L
L
OIV
L
L
L L
L
L L
L
L
L’
LIII
LII
LI
L
L
L
L
L
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(ii) Coster-Kronig transitions which are radiation less transitions among the subshell
of an atomic shell having the same principal quantum number make it possible
for a primary vacancy created in one of the subshells to shift to a higher subshell
before the vacancy is filled by another transition. Because Coster-Kronig
transitions change the primary vacancy distribution, great care is to be taken in
formulating the definitions of the quantities.
In the absence of Coster-Kronig transitions, the vacancy distribution in the
subshell of a shell is proportional to their ionization cross sections only. Thus for higher
shells, in the absence of Coster-Kronig transitions, the average fluorescence yield
X for the shell ‘X’ can be defined as
X
i
n
i
X
iX N
1
(1.20)
where n=1 for K shell
n=2 for L shell
n=3 for M shell and so on
where X
iN refers to the relative number of primary vacancies in the ith
subshell of Xth
shell and is given as:
n
i
X
i
X
iX
i
N
NN
1
(1.21)
and also 11
n
i
X
iN (1.22)
The definition of the average fluorescence yields in the presence of Coster-
Kronig transitions are rather complicated (Fink et al., 1966 and Bambynek et al., 1972)
and involve Coster-Kronig yields ijf .
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1.3.1.3 Auger effect and yield
The ejection of an electron from given shell through photoelectric interaction,
creates a vacancy in that shell and leaves the atom in an excited state. The atom reverts
to the lower state when an electron from the higher shells fills this vacancy. As a
consequence of this transition, energy is released in the form of X-ray photon. If the
energy of emitted X-ray is greater that the binding energy of the higher shell, then the
electron can be ejected from the higher shell along with photoelectron. This extra
electron is called Auger electron and the effect is called Auger effect. The process is
schematically shown in figure 1.3.
Auger effect is more common for low Z (i.e. upto Z=20) element, because their
inner shell electrons are comparatively loosely bound. For the same reason, this effect is
more prominent for L and higher shells than K-shell. Thus the term Auger effect is used
to describe those transitions in which the decay of a vacancy in an atomic shell leads to
two vacancies in one or two different principal shells.
The Auger yield X
ia is the probability that a vacancy in the ith
subshell of Xth
shell is filled through a non-radiative transition by an electron from a higher shell. The
Auger transitions are radiation less transitions and differ from Coster-Kronig transitions
because the later occur among various subshells of the major shell only whereas the
former may occur from the higher shell also.
The average Auger yield is defined in analogy to the definition of average fluorescence
yield Xa as:
X
i
X
i
X
iX aVa
1
(1.23)
where the coefficient k
iV is the modified vacancy numbers in ith
subshell of the Xth
shell
due to Coster-Kronig transitions. The sum of the average fluorescence yield and average
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Auger yield of a shell for the same initial vacancy distribution is unity, i.e.
1 kk a (1.24)
1.3.1.4 Coster-Kronig transitions and yield
These are the non-radiative transitions which occur in shells having more than
one subshell (i.e L, M, N and so on). In such shells, the vacancies are shifted to higher
subshells from lower subshells as non-radiative transitions occurring within a short time
of about 10-17
second. These transitions are called Coster-Kronig transitions, which were
first studied by Coster and Kronig (1935). For example, the vacancies from low lying LI
subshell may be shifted to LII or LIII subshell and similarly vacancies from LII subshell
may be transferred to LIII subshell. For L shell these transition probabilities are defined
as; Lf12 , Lf23 and Lf13 ; where Lf12 is the probability of shifting vacancy from LI subshell to
LII subshell and similarly other’s can be defined.
The following general relation exists between the Auger yield, the fluorescence
yield and the Coster-Kronig yields as:
11
k
ij
X
ij
X
i
X
i fa (1.25)
For L shell the above relation gives following three relations
1
1
1
131211
2322
33
LLLL
LLL
LL
ffa
fa
a
(1.26)
Then non-radiative Coster-Kronig transitions occur within subshell is of great
importance in the measurement of fluorescence yield and Auger yields. These
transitions alter the initial vacancy distribution of primary vacancies and must be taken
34
Fig.1.3: The schematic diagram showing principle of characteristics X-ray
and Auger emission.
Atomic shells
M
L
K
Vacancy
Auger electron
Characteristics
X-rays
Nucleus Incident
radiation
35
into account.
1.4 Absorption edge jump ratio and jump factor
A plot of the total atomic cross section versus incident photon energy is found to
exhibit a characteristic saw tooth structure in which the sharp discontinuities, known as
absorption edges, arise whenever the incident energy coincides with the ionization
energy of electrons in the K, L, M shells. These sharp discontinuities are due to the fact
that photoelectric interaction becomes energetically possible in the shell considered.
Ratio of values of photoelectric cross-section on the higher energy to that of lower
energy side of edge of a particular shell/ sub-shell is called jump ratio of that shell/ sub
shell. Whereas, absorption jump factor is defined as the fraction of the total absorption
that is associated with a given shell rather than for any other shell. Typical plot showing
abrupt jumps at absorption edges has been given in figure 1.4.
1.4.1 Brief description of various experimental techniques used for
measuring absorption edge jump ratio and jump factor
Absorption edge jump ratios and jump factors can be measured experimentally
using four different methods given as under:
1.4.1.1 Gamma-ray or X-ray attenuation method
In this method, mass attenuation coefficient of a given target element are
measured employing transmission geometry at different energies which covers the
energy region lying both above and below the particular shell/ subshell absorption edge
of the target element under consideration. Mass attenuation coefficients so obtained are
then plotted against photon energies and the resultant plot gives a saw tooth structure
around edge of that particular shell/ subshell of given target element. By calculating the
36
ratio of mass attenuation coefficient on upper and lower energy branch of absorption
edge, its corresponding jump ratio is then determined.
1.4.1.2 Compton scattered photon method
In this method, the Compton scattered photons are made to fall on the absorber
whose absorption edge is to be studied. As Compton scattered photons are function of
scattering angle, so by adjusting this angle, the energy of the Compton scattered photon
is varied. Mass attenuation coefficients are then measured as a function of Compton
scattered photon energy around that particular absorption edge of given target. By
plotting mass attenuation coefficients against photon energy, its corresponding
absorption jump ratio is determined.
1.4.1.3 Bremsstrahlung method
In bremsstrahlung method, continuous bremsstrahlung radiation from a weak
beta source is allowed to fall on a thin target. Then the transmitted bremsstrahlung
spectrum, so produced, shows a sudden drop in intensity at the absorption edge of the
target element studied. From this sudden drop, the absorption edge jump factor can be
measured for high Z elements.
1.4.1.4 Energy Dispersive X-ray Fluorescence (EDXRF) method
In EDXRF method, strong radioactive source (100 mCi) is used to generate K
and Li X-ray photons in a given target element. Then by noting the net counts falling
under Ki (i= and and Lii=l, and X-ray peaks, its corresponding Ki and Li
X-ray production cross-sections has been determined. Similarly, by knowing the
incident and transmitted X-ray photon intensities, photoionization cross-section of a
37
given target of interest has been measured. By making use of these fluorescent
parameters, jump ratios and jump factors of given target element has been determined.
In the present study, EDXRF technique has been used for measuring K shell and
LIII subshell absorption edge jump ratio and jump factor of some low and high Z
elements.
38
0.01 0.1 1100
1000
10000
100000
Tot
al a
tom
ic c
ross
-sec
tion(
b/at
om)
Energy(MeV)
K absorption edge
L3 absorption edge
0.015 0.02 0.025 0.03 0.035 0.04 0.0455000
10000
15000
20000
25000
30000
35000
40000
45000
50000
Total
atom
ic cros
s-sec
tion(b
/atom
)
Energy(MeV)
L2 absorption edge
L1 absorption edge
Fig. 1.4: Typical plot of Th showing variation of atomic cross-section (b/atom)
with energy (MeV).