CHAPTER
RADIOACTIVE NUCLEI
1.1: INTRODUCTION
The natural environment involves nuclear radiation. Mankind has added
to this by production of power and artificial radiation to medical and
other uses. Detection provides information about the radiation
environment. Shielding involves protection by confinement of radiation.
Nature provides essential shielding. The atmosphere and earth dipole
magnetic field protect us from external solar and galactic cosmic rays.
The earth matter contains most of the natural radiations which provides
significant heating of the interior.
Knowledge of gamma ray interaction with matter is important to the nondestructive analysis in order tounderstand gamma ray detection and
attenuation.
Mass attenuation coefficient represents a subject of great interest and
importance to the scientist due to its utility in solving various problems
related to radiation physics and radiation doisimetry.
The study of mass attenuation coefficient is of interest due to its
applications in the industrial, biological, medicine and agriculture studies.
1
Mass attenuation coefficient measures the probability of all interaction
between gamma ray and atomic nuclei. It depends on the energy of
incident photon and energy and nature of the absorbing material [1].
The linear attenuation coefficient, mass attenuation coefficient, the
effective atomic number and effective electron density are the basic
quantities useful in determining the penetration, penetration depth of
gamma ray photon in matter [2]. These quantities can be evaluated
theoretically and experimentally when a beam of gamma rays fall on a
material, it is absorbed scattered and are transmitted by interaction of its
photon with atoms of the matter. Gamma ray and X-ray radiation are
widely used in medical imaging and radiation therapy. The use of
radioisotope must have knowledge about how radiation interacts with
matter, particularly with human body.
Early measurements of the absorption coefficient of the gamma rays were
hampered by the difficulty of producing sources of monochromatic
radiation. This difficulty has recently been overcome by the use of
radioactive isotopes, and several workers have made accurate
measurement in this way. Cohen (1948) and Davisson and Evans have
published comprehensive measurements which covers a wide range of
quantum energy and of atomic number with an accuracy to within 2 or
3%. Most of the measure values of absorption coefficients agree with
theoretical values within the limit of experimental errors, but Davisson
2
and Evans reported difference of about 5 % in the case of Tantalum both
at 1.1 and at 1.25 MeV.
The mass attenuation coefficient of elements, molecules and materials are
widely used in space physics, dosimetry, plasma physics and many other
applications in radiation studies. Hubbell has obtained mass attenuation
coefficient of various elements and mixtures. [3]
With the increasing use of gamma active isotopes in various fields, the
study of the absorption of gamma radiation in material become an
important subject.
The set up geometry plays an important role in measurement of mass
attenuation coefficient and other parameters.
There are significant number of reports in the literature on the
experimental determination of mass attenuation coefficient of various
elements compounds and mixtures [4-9].
Historically radioactivity and hence the existence of the nucleus, were
discovered
as
a
consequence
of
apparently
incomprehensible
experimental observations. Becquerel placed a piece of mineral
containing uranium on an unexposed photographic plate wrapped in black
paper. Some days later he found that the pale was clouded as if it had
been exposed to light, in spite of the fact that the black paper was
completely opaque. Becquerel was at the time looking for a phenomenon
of remission of light called fluorescence. He had discovered something
very different, and his genius lay in the fact that he realized this. He
showed subsequently that the radiation which was affecting the
3
photographic plate depended neither on the particular chemical
compound used(provided it contained uranium)nor on its temperature or
other external circumstances. Then Pierre and Marie Curi isolated from
uranium ore two elements either to unknown-polonium and radium which
were far more active than uranium. They showed that the radiation was a
property of the atoms themselves, and that it was capable of ionizing
gases, that is, of creating electric charges in an enclosed volume of gas, so
that the latter become a conductor of electricity-a phenomenon previously
observed in connection with the x-rays discovered by Roentgen. More
surprisingly, this radiation carried with it a great deal of energy, since in
spite of the very small quantities of emitting matter; it produced an
observable heating effect in anything that absorbed it. Next, Rutherford
demonstrated that there were three types of “rays” emitted by radium and
its derivatives. These are alpha, beta, and gamma rays.
1.2: RADIOACTIVE DECAY
The radioactive decays of naturally occurring minerals contemning
uranium and thorium are in large part responsible for the study of nuclear
physics. In radioactive decay, an unstable nuclei or ‘parent’ is
transformed into a stable nuclide called the ‘daughter’. If the daughter
product is also radioactive, process continuous in decay chain until a
stable product is reach. Radioactivity is a random process, it is not
predicted to know exactly when a unstable nucleus will decay and can
only specify a probability per unit time that it will do so. This is normally
4
described by half-life (t half), which the time taken for half nucleus in a
sample to decay.
All naturally occurring and artificially produce nuclei are eitherα−active,
β− active or both and emits a combination of α, β and γ radiation.
Following three aspects of radioactivity are extraordinary.
1) When a nucleus undergoesα, β- decay in atomic number Z changes
and it becomes the nucleus of different atoms.
2) The energy liberated during radioactive decay comes from within
individual nuclei without external excitation.
3) The radioactive decay is a statistical process.
Activity of a radioisotope measures the disintegration rate, which is not
same with the emission rate of radiation produced in its decay. The
activity of a radioisotope source is defined as its rate at which the
constituent atoms disintegrate. Thus, if dN be the number of atom which
disintegrate during a time interval dt, we have
dN
= −λN
dt
………………….(1.1)
Where N is the number of radioactive nuclei and is defined as the decay
constant. The negative sign indicates that the number of atoms is
decreasing with time. The activity of a sample depends upon the number
of atoms in the samples that is upon the mass of sample and upon type
atom. The traditional unit of activity is Curie (Ci). The Curie was
originally defined as the activity of one gram of radium. The
5
Becquereldefined as one disintegrationper second, has become standard
unit of activity. Thus,
1bq = 2.703× 10−11Ci
1.2.1 Alpha decay
Alpha (α) radiation of natural origin is emitted from heavy, unstable
nuclei in a transmutation; with conservation of total charge (Z) and mass
number (A).While easily absorbed themselves their emission is
frequently accompanied by ore penetrating β and by still more
penetrating gammas.
Heavy nuclei are energetically unstable against the spontaneous emission
of an alphaparticle (or He4 nucleus). The half -life of useful sources varies
from days to many thousands of years. The decay process is written
schematically as
X ZA → Y ZA=−24 + α 24
………………….(1.2)
where, X and Y are the initial and final nuclear species. The most nuclei
with A>150 are actually unstable with respect to tight bound
2
4
He
.Causing a change of atomic number ∆ Z = 2. Usually the alpha particles
emitted by radioactive substances have kinetic energy range from 8.9
MeV for
212
84
Po to 4.1 MeV for thoriumcorresponding, to the velocities of
the order of 107 m/s. The q/m values for the alpha particle as obtained by
Lord Rutherford and coworkers are 4.82× 107C/kg.
1.2.2 Beta decay
The spontaneous decay process in which mass number of nucleus remain
unchanged, but the atomic number changes, is known as β-decay. The
6
change in atomic number is accomplished by the emission of electron,
emission of a positron, or by the capture of orbital electron. Depending on
the three modes of decay, the β-decay is known as β- decay, β+ decay and
electron capture (k-capture). The half-life of β−nuclei range 0.00sec to
1018 yrs. The energy of emitted particle goes up to few MeV.
The most common source of fast electrons in radiation measurement is
aradioisotopethat decays by β- emission. The process is written
schematically
Z X → Y ZA+1 _ + β − + v
………………….(1.3)
where, X and Y denotes the initial and final nuclear species and is the
antineutrino. The only significant ionizing radiation produced by beta
decay is the fast electrons or beta particle itself. Because most
radionuclides produced by neutron bombardment of stable material are
beta-active, large assortments of beta emitters are readily available
through production in a reactor flux. In most of decay process an excited
state of the product nucleus, the subsequent de-excitation gamma rays are
emitted together with beta particles in many common beta sources.
1.2.3 Gamma decay
Gammas (γ) do not involve nuclear transmutation, but a change in state
like that involving atomic photon emission, with the high energy nuclear
photon emission maintaining the total energy balance. In gamma decay,
the final nucleus is the same as the initial one. It has the same A (atomic
number), Z (proton number), and N (neutron number), but in a lower
7
energy state. A = N+Z. The γ-ray has high-energy photon, carriers of
some mass-converted energy.
Gamma radiation is the spontaneous emission of γ-quanta by the nucleus.
By emitting γ-quanta, the nucleus goes over from an excited state to a
state with a lower energy (radioactive transition). There are single
radioactive transitions when the nucleus emits a single quantum and at
once goes over to the ground state. Or cascade transitions when the
excitation is removed by a successive emission of several -quanta.
Different radiations have different properties, as summarized below:
Type of
Mass
Radiation
(amu)
Alpha
Particle
Beta
Particle
Radiation
Charge
Shielding material
4
+2
Paper, skin, clothes
1/1836
±1
Dense metal, concrete,
Earth
Water, concrete,
Neutrons
Particle
1
0
polyethylene, oil.
1.3: SHIELDING OF GAMMA RAYS
The goal of shielding is confinement, with eventual conversion of
radioactive energy to heat which can be dissipated by cooling (air, water,
etc.). The primary radioactivity particle may sometimes induce secondary
energetic particles by basic interaction processes, and they in turn
produce tertiary particles etc. It is necessary to study the dependence of
various fundamental interaction processes on particle energy and on
8
absorber atomic number Z, to understand these cascade processes in a
quantitative way. Depending on primary energy and type, various
shielding methods illustrated schematically below will be appropriate.
Sometimes a combination is employed, layered to deal with the
successive cascade particles.
The attenuation of a gamma ray photon beam is governed by the decay
law given below [10-13].
ܰ = ܰ0݁−ߤ‫ݔ‬
………………….(1.4)
Where µ represents the linear attenuation coefficient and x gives the
thickness of the attenuator. However, practically, more photons are
received at a reference point than what one can calculate using the decay
law. The additional dose is received due to the buildup factor B of the
attenuator and the simple mathematical form of the decay law is modified
to a new form given in following equation.
ܰ = ‫ܰ·ܤ‬0݁−ߤ‫ݔ‬
9
………………….(1.5)
Equation (1.4) is valid only in narrow beam geometry when a collimated
beam of radiation is used. However, in a broad beam geometry, we must
take into account the buildup factor and hence Eq.1.5 will give us a more
correct and precise value.
1.4: PHOTO DISINTEGRATION
Photo Disintegration occurs in high energy γ-rays at energies over 10
MeV. The γ-ray interacts with the nucleus of an atom, therefore, exciting
it. The excited nucleus immediately decays into two or more daughter
nuclei. In nuclear fission, or the breakdown of an atomic nucleus neutron
or protons are produced. The neutrons produced in this reaction can cause
radiation protection problems if they are not accounted for. This type of
reaction does not occur in this experiment because the energy levels used
are much lower than the threshold for this type of reaction. [14].
1.5 LITERATURE SURVEY
The study of linear attenuation coefficient, mass attenuation coefficient,
total photon cross section is important in solving various problems in
radiation physics and radiation dosimetry. These parameters are also
useful in industrial, biological, agricultural and medical studies. The mass
attenuation coefficient measures the probability of all interactions
between gamma rays and atomic nuclei. The interaction depends on the
incident photon energy and the nature of the absorbing material.
Theoretical calculations of mass attenuation coefficient of various
elements were carried out by Scofilled [15], Soloman, Hubbel and
10
Scofiled [16] Henke et. al. [17,18], Creagh et. al. [19] and Chantler [20,
21]. The data is presented in the literature and most of the scientific
communities are using the same data for comparison with the
experimentally obtained mass attenuation coefficient. The discrepancy
between the theoretically tabulated values of mass attenuation coefficient
and experimentally values is found for certain elements. There are many
reports in the literature on experimental determination of mass
attenuation coefficient of various elements, compound and mixtures [2224]. The experimental data are compared with theoretical tabulations
available in the literature, mostly with Hubble X-COM program [25]. It is
fact that significant systematic discrepancy between theoretical and
experimental values of mass attenuation coefficient even after taking all
the precautions in the experiment. The factors that give rise discrepancy
are 1) lack of proper collimation of transmuted beam, 2) the method used
(Narrow beam, broad beam) 3) Scattering effect. The discrepancy can be
minimized by controlling the above mention parameters.Kulwant Singh
et. al. [2] have studied shielding properties of CaO-SrO-B2O3 glasses by
measuring the mass attenuation coefficient using a narrow beam
geometry with NaI (Tl) detector crystal and using
137
Cs Gamma source.
The experimental data on mass attenuation coefficient is helpful in
potential applications in gamma ray shielding. The obtained experimental
data compared with theoretical data.
11
G Apaydinet.al [26]has carried out studies on mass attenuation
coefficient, effective atomic number and electron densities for CoCuAg
alloy thin film.
1.6: AIM OF THE PRESENT WORK
The study of attenuation coefficient of various elements by using suitable
method is essential to understand there possible use in shielding purpose
and other biological industrial and medical applications. Usually narrow
beam geometry technique is helpful in obtaining accurate values of mass
attenuation coefficient. It is necessary to compare the experimental data
with the theoretical data published in the literature.
Aim of the present work is to measure the mass attenuation coefficient of
Lead (Pb), Gold (Au), Silver (Ag), Zinc (Zn), Nickel (Ni) and
Magnesium (Mg) elements as function of energy. The effect of collimator
size (diameter) is also studied in the present work. The experimentally
obtained various parameters (Linear attenuation coefficient, mass
attenuation coefficient, total photon cross-section) were compared with
Hubbell XCOM program. The measurements on attenuation coefficient
were carried out using narrow beam geometry. The purpose of this study
is to check their applicability for shielding purpose.
12
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