15

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.

16

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

17

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

18

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

19

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

20

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

21

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).

22

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:

23

( )

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

24

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

25

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

26

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

27

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.

28

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.

29

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

31

(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 .

32

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

33

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).