CHAPTER 1
Radioelements, Isotopes
& Radionuclides
This chapter gives an overview needed to better understand ionizing radiation, i.e.,
radiation that has sufficient energy to remove electrons from atoms.
The Atom
Matter has mass and takes up space. Atoms are the basic building blocks of matter.
Everything is made of atoms.
The ancient Greeks once thought that atoms were the smallest pieces of matter, and
that they were indivisible. We now know that even atoms are made up of smaller
Engineering Aspects of Food Irradiation
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Radioelements, Isotopes & Radionuclides
pieces. In these activities, we will learn how to build atoms from these parts. Atoms
have a nucleus and electrons. Only protons and neutrons are in the nucleus.
Nucleus - the core of the atom, containing protons and neutrons is the nucleus.
protons (carry positive charge)
neutrons (carry no charge)
electrons are small (carry a negative
charge and circle the nucleus)
Electrons cannot live in the nucleus. ELECTRONS SPIN IN SHELLS around the
nucleus. As you know, ELECTRONS are always moving, spinning very quickly
around the NUCLEUS. As the electrons spin they can move in any direction, as
long as they stay in their shell. Any direction you can imagine; upwards, downwards, sidewards, electrons can move that way. Scientists use letters to name the
orbitals/shells around a nucleus. They use the letters “k, l, m, n, o, p, and q”. The
“k”shell is the one closest to the nucleus and “q” is the furthest away.
You know that the nucleus is positive and that electrons are negative. This means
that the electrons and the nucleus are attracted to each other. This is how an atom is
held together.
Ions: Ions are charged particles, produced when an atom gains or loses one or more
electrons. Ionization is likely to occur when an atom has a partially occupied outer
electron energy level. Ionization is especially likely if the complete atom has only 1
or 2 electrons in its outermost energy level, or if it is only 1 or 2 electrons away
from completing the full occupation of the energy level.
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Radioelements, Isotopes & Radionuclides
For example, both hydrogen and sodium have only one electron in their outermost
electron level. They are both likely to release the single electron in that ring. This
will give the atom an imbalance between the number of electrons and the number of
protons. Since they have more protons than electrons, they now have a net positive
charge and are considered to be positive ions. [This is shown by placing a plus sign
+ next to the symbol for the element.]
Chlorine, on the other hand, has 7 electrons in its outermost electron level. Chlorine
is likely to 'grab' an extra electron -- assuming one is available-- to become a negatively charged ion [symbolized by a negative sign - next to the symbol for the element.]
The diagram below shows the ionization of a hydrogen atom. In the space below
the diagram, show the ionization of sodium and chlorine.
Glossary:
Alpha decay: Alpha decay is one process that unstable atoms can use to try to
become more stable. During alpha decay, an atom's nucleus sheds two protons and
two neutrons in a little packet that scientists call an alpha particle.
Since an atom loses two protons during alpha decay, it changes from one element to
another. For example, after undergoing alpha decay, an atom of Uranium (with 92
protons) becomes an atom of Thorium (with 90 protons).
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Radioelements, Isotopes & Radionuclides
Alpha particle: An alpha particle is a fast moving packet containing two protons
and two neutrons (a helium nucleus). Alpha particles carry a charge of +2 and
strongly interact with matter. Produced during alpha decay, alpha particles can
travel only a few inches through air and can be easily stopped with a sheet of paper.
Atomic number: The atomic number is equal to the number of protons in an atom's
nucleus. The atomic number determines which element an atom is. For example,
any atom that contains exactly 47 protons in its nucleus is an atom of silver.
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Radioelements, Isotopes & Radionuclides
Beta decay: Beta decay is one process that unstable atoms can use to become more
stable. There are two types of beta decay, beta-minus and beta-plus.
During beta-minus decay, a neutron in an atom's nucleus turns into a proton, an
electron and an antineutrino. The electron and antineutrino fly away from the
nucleus, which now has one more proton than it started with. Since an atom gains a
proton during beta-minus decay, it changes from one element to another. For example, after undergoing beta-minus decay, an atom of carbon (with 6 protons)
becomes an atom of nitrogen (with 7 protons).
During beta-plus decay, a proton in an atom's nucleus turns into a neutron, a
positron and a neutrino. The positron and neutrino fly away from the nucleus,
which now has one less proton than it started with. Since an atom loses a proton
during beta-plus decay, it changes from one element to another. For example, after
undergoing beta-plus decay, an atom of carbon (with 6 protons) becomes an atom
of boron (with 5 protons).
Although the numbers of protons and neutrons in an atom's nucleus change during
beta decay, the total number of particles (protons + neutrons) remains the same.
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Radioelements, Isotopes & Radionuclides
Beta particles: Ejected from the nucleus during beta decay, a beta particle is a fast
moving electron or positron, depending on the type on beta decay involved. Beta
particles can travel a few feet through air and can be stopped with a few sheets of
aluminum foil.
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Radioelements, Isotopes & Radionuclides
Bohr Radius: The size of a ground state hydrogen atom as calculated by Niels Bohr
using a mix of classical physics and quantum mechanics. The Bohr Radius is given
by the following formula
2
– 10
κ
a o = -----------2- = 0.529 × 10 meters
mke
(EQ 1)
where κ = Plank’s constant/2 π = 1.055x10-34 Joule-seconds; m = mass of electron
= 9.109x10-31 kg; k = Coulomb’s constant = 8.988x109 J-m/C2; and e = electron
charge = 1.602x10-19
Cyclotron: A cyclotron is a machine used to accelerate charged particles to high
energies. The first cyclotron was built by Ernest Orlando Lawrence and his graduate student, M. Stanley Livingston, at the University of California, Berkley, in the
early 1930's.
A cyclotron consists of two D-shaped cavities sandwiched between two electromagnets. A radioactive source is placed in the center of the cyclotron and the electromagnets are turned on. The radioactive source emits charged particles. It just so
happens that a magnetic field can bend the path of a charged particle so, if everything is just right, the charged particle will circle around inside the D-shaped cavities. However, this doesn't accelerate the particle. In order to do that, the two Dshaped cavities have to be hooked up to a radio wave generator. This generator
gives one cavity a positive charge and the other cavity a negative charge. After a
moment, the radio wave generator switches the charges on the cavities. The charges
keep switching back and forth as long as the radio wave generator is on. It is this
switching of charges that accelerates the particle.
Let's say that we have an alpha particle inside our cyclotron. Alpha particles have a
charge of +2, so their paths can bent by magnetic fields. As an alpha particle goes
around the cyclotron, it crosses the gap between the two D-shaped cavities. If the
charge on the cavity in front of the alpha particle is negative and the charge on the
cavity in back of it is positive, the alpha particle is pulled forward (remember that
opposite charges attract while like charges repel). This just accelerated the alpha
particle! The particle travels through one cavity and again comes to the gap. With
luck, the radio wave generator has changed the charges on the cavities in time, so
the alpha particle once again sees a negative charge in front of it and a positive
charge in back of it and is again pulled forward. As long as the timing is right, the
alpha particle will always see a negative charge in front of it and a positive charge
Engineering Aspects of Food Irradiation
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Radioelements, Isotopes & Radionuclides
in back of it when it crosses the gap between cavities. This is how a cyclotron
accelerates particles!
Unfortunately, there's one more thing to worry about. The faster a charged particle
moves, the less it is affected by a magnetic field. So, as particles speed up in a
cyclotron, they spiral outwards. This makes it easy to get the particles out of the
cyclotron, but also puts a limit on the amount of acceleration they can undergo.
Deuterium: Discovered in 1932 by Harold C. Urey, deuterium is a stable isotope of
the element hydrogen. An atom of deuterium consists of one proton, one neutron
and one electron. About.015% of natural hydrogen is composed of deuterium.
Deuteron: The nucleus of a deuterium atom. A deuteron consists of one proton and
one neutron.
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Electrons: Electrons are negatively charged particles that circle the atom's nucleus.
Electrons were discovered by J. J. Thomson in 1897.
.
Particle Data
Symbol Mass
Lifetime Charge Spin
e-.511 MeV
stable
-1
1/2
Gluons: Gluons are the particles responsible for binding quarks to each other.
Particle Data
Symbol Mass
g
Lifetime Charge Spin
0
stable
0
1
Half-life: The half-life describes the amount of time needed for half of a sample of
unstable atoms or particles to undergo decay. Thallium-208, for example, decays
into lead-208 with a half-life of 3.05 minutes. This means that half of a sample of
thallium-208 will decay into lead-208 over the course of 3.05 minutes.
Scientists can not predict when a particular atom or particle will decay. They only
know that, on average, half of a sample will decay during the span of one half-life.
Engineering Aspects of Food Irradiation
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Radioelements, Isotopes & Radionuclides
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Radioelements, Isotopes & Radionuclides
Helius: In Greek mythology, Helius was god of the sun. Helius drove his chariot
across the sky each day to provide daylight and returned home each night on the
river Oceanus in an enormous golden cup to hide the light.
Isotope: Atoms that have the same number of protons but different numbers of neutrons are called isotopes. The element hydrogen, for example, has three known isotopes: protium, deuterium and tritium.
Liquid Nitrogen: The liquid state of the element nitrogen. Liquid nitrogen freezes at
63 K (-346°F) and boils at 77.2 K (-320.44°F) under standard atmospheric pressure.
The white mist seen in the photograph is fog created by cooling the water vapor
present in the air below the dew point.
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Radioelements, Isotopes & Radionuclides
Neutrons: Neutrons are uncharged particles found within atomic nuclei. Neutrons
were discovered by James Chadwick in 1932. Experiments done at the Stanford
Linear Accelerator Center in the late 1960's and early 1970's showed that neutrons
are made from other particles called quarks. Neutrons are made from one 'up' quark
and two 'down' quarks.
Particle Data
Symbol
n
Mass
Lifetime
Charge Spin
939.6 MeV in nuclei: stable
free: 15 min
0
Quark Content
1/2
udd
Positron: The antimatter counterpart of the electron, positrons were discovered in
1932 by Carl Anderson while observing cosmic ray showers.
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Radioelements, Isotopes & Radionuclides
Particle Data
Symbol Mass
Lifetime Charge Spin
e+ .511 MeV
stable
+1
1/2
Protons: Protons are positively charged particles found within atomic nuclei. Protons were discovered by Ernest Rutherford in experiments conducted between the
years 1911 and 1919. Experiments done at the Stanford Linear Accelerator Center
in the late 1960's and early 1970's showed that protons are made from other particles called quarks. Protons are made from two 'up' quarks and one 'down' quark.
Particle Data
Symbol
p
Mass
938.3 MeV
Lifetime
Charge Spin
> 1032 years
+1
Quark Content
1/2
udd
Positrom: The antimatter counterpart of the electron, positrons were discovered in
1932 by Carl Anderson while observing cosmic ray showers.
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Radioelements, Isotopes & Radionuclides
Particle Data
Symbol Mass
Lifetime Charge Spin
e+.511 MeV stable
+1
1/2
Quarks: Quarks are believed to be one of the basic building blocks of matter.
Quarks were first discovered in experiments done at the Stanford Linear Accelerator Center in the late 1960's and early 1970's.
Three families of quarks are known to exist. Each family contains two quarks. The
first family consists of Up and Down quarks, the quarks that join together to form
protons and neutrons. The second family consists of Strange and Charm quarks and
only exist at high energies. The third family consists of Top and Bottom quarks and
only exist at very high energies. The Top quark was finally discovered in 1995 at
the Fermi National Accelerator Laboratory.
TABLE 1. Particle
Data
Name
Symbol
Mass
Charge
Spin
Up
u
3 MeV
+2/3
1/2
Down
d
6 MeV
-1/3
1/2
Charm
c
1300 MeV
+2/3
1/2
Strange
s
100 MeV
-1/3
1/2
Top
t
175000 MeV
+2/3
1/2
Bottom
b
4300 MeV
-1/3
1/2
Tritium: Discovered in 1934, tritium is an unstable isotope of the element hydrogen. An atom of tritium consists of one proton, two neutrons and one electron. Tritium is radioactive and has a half-life of about 12.5 years.
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Engineering Aspects of Food Irradiation
Radioelements, Isotopes & Radionuclides
Bremsstrahlung (‘braking radiation’): continuous X-rays. X-rays are produced
when a beam of electrons strikes a target. The electrons lose most of their energy in
collisions with atomic electrons in the target, causing ionization and excitation of
atoms. In addition, they can be sharply deflected in the vicinity of the atomic
nuclei, thereby losing energy by irradiation X-ray photons. A single electron can
emit X-ray photon having any energy up to its own kinetic energy. As a result, a
monoenergetic beam of electrons produces a continuous spectrum of X-rays with
photons energies up to the value of the beam energy. The continuous X-rays are
also called Bremsstrahlung or ‘braking radiation’.
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Radioelements, Isotopes & Radionuclides
S ym bol
Ac
Al
Am
Sb
Ar
As
At
Ba
Bk
Be
Bi
B
Br
Cd
Ca
Cf
C
Ce
Cs
Cl
Cr
Co
Cu
Cm
Dy
Es
Er
Eu
Fm
F
Fr
Gd
Ga
Ge
Au
Hf
Ha
Hs
Hi
Ho
H
In
I
Ir
Fe
Kr
La
Lr
Pb
Li
Lu
Mg
Mn
Mt
16
E le m e n t
Actinium
Alum inum
Am ericium
Antim ony
Argon
Arsenic
Astatine
Barium
Berkelium
Beryllium
Bism uth
Boron 5
Brom ine
Cadm ium
Calcium
Californium
Carbon
Cerium
Cesium
Chlorine
Chrom ium
Cobalt
Copper
Curium
Dysprosium
Einsteinium
Erbium
Europium
Ferm ium
Flourine
Francium
Gadolinium
Gallium
Germ anium
Gold
Hafnium
Hahnium
Hassium
Helium
Holm ium
Hydrogen
Indium
Iodine
Iridium
Iron
Krypton
Lanthanum
Lawrencium
Lead 82
Lithium
Lutetium
Magnesium
Manganese
Meitnerium
A to m ic # S y m b o l
89 Md
13 Hg
95 Mo
51 Ns
18 Nd
33 Ne
85 Np
56 Ni
97 Nb
4N
83 No
5 Os
35 O
48 Pd
20 P
98 Pt
6 Pu
58 Po
55 K
24 Pr
17 Pm
27 Pa
29 Ra
96 Rn
66 Re
99 Rh
68 Rb
63 Ru
100 Rf
9 Sm
87 Sc
64 Sg
31 Se
32 Si
79 Ag
72 Na
105 Sr
108 S
2 Ta
67 Tc
1 Te
49 Tb
53 Tl
77 Th
26 Tm
36 Sn
57 Ti
103 W
82 U
3V
71 Xi
12 Yb
25 Yb
109 Zn
Zr
Engineering Aspects of Food Irradiation
E le m e n t
A to m ic
Mendelevium
Mercury
Molybdenum
Neilsborium
Neodym ium
Neon
Neptunium
Nickel
Niobium
Nitrogen
Nobelium
Osm ian
Oxygen
Palladium
Phosporus
Platinum
Plutonium
Polonium
Potassium
Praseodym ium
Prom ethium
Protactinium
Radium
Radon
Rhenium
Rhodium
Rubidium
Ruthenium
Rutherfordium
Sam arium
Scandium
Seaborgium
Selenium
Silicon
Silver
Sodium
Strontium
Sulfur
Tantalum
Technetium
Tellurium
Terbium
Thalium
Thorium
Thulium
Tin
Titanium
Tungsten
Uranium
Vanadium
Xenon
Ytterbium
Yttrium
Zinc
Zirconiun
#
101
80
42
107
60
10
93
28
41
7
102
76
8
46
15
78
94
84
19
59
61
91
88
86
75
45
37
44
104
62
21
106
34
14
47
11
38
16
73
43
52
65
81
90
69
50
22
74
92
23
54
70
39
30
40
Radioelements, Isotopes & Radionuclides
Engineering Aspects of Food Irradiation
17
Radioelements, Isotopes & Radionuclides
Atomic Nature of Matter
Gay-Lussac law of combining volumes of gases: the volumes of gases that enter
into chemical combination with one another are in the ratio of simple whole numbers when all volumes are measured under the same conditions of pressure and
temperature.
Avogadro hypothesis: equal volumes of any gases at the same T and P contain the
same number of molecules. The molecules of some gaseous elements could be
comprised of two or more atoms of that element.
Avogadro Number: N0 = 6.023x1023
A gram atomic weight of any element contains Avogadro’s number of atoms. A
gram molecular weight of any gas also contains N0 molecules and occupies a volume of 22.4136 L at standard T and P (0C = 273 K and 760 torr = 760 mm Hg). The
modern scale of atomic and molecular weights is set by stipulating that a gram
atomic weight of the carbon isotope, 12C, is exactly 12.000...g. A periodic chart,
showing atomic numbers, atomic weights, densities, and other information about
chemical elements, is shown on the appendix.
Example 1:
How many gram of oxygen combine with 2.3 g of carbon in the reaction:
C + O 2 → CO 2 ?
How many molecules of CO2 are thus formed? How many liters of CO2 are formed
at 20oC and 752 torr?
Answer:
In the given reaction, 1 atom of carbon combines with one molecule (2 atoms) of
oxygen. From the atomic weights given in the periodic chart, it follows that 12.011
g of carbon reacts with 2 × 15.9994 = 31.9988g of oxygen. Rounding of the 3 significant figures, letting y represent the number of grams of oxygen asked for, and
taking simple proportions, we have:
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Engineering Aspects of Food Irradiation
Radioelements, Isotopes & Radionuclides
2.3
y =  ---------- × 32.0 = 6.13g
 12.0
(EQ 2)
The number N of molecules of CO2 formed is equal to the number of atoms in 2.3 g
of C, which is 2.3/12.0 times Avogadro’s number:
2.3
23
23
N = ---------- × 6.02 × 10 = 1.15 × 10
12.0
(EQ 3)
Since Avogadro’s number of molecules occupies 22.4 L at STP, the volume of CO2
at STP is:
 1.15 × 10 23
- × 22.4 = 4.28L
V CO 2 =  -------------------------23
 6.02 × 10 
(EQ 4)
At the given higher temperature of 20oC = 293K, the volume is larger by the ratio
of the absolute temperatures, 293/273; the volume is also increased by the ratio of
the pressures, 760/752. Therefore, the volume of CO2 made from 2.3 g of C at 20oC
and 752 torr is:
293 760
V 20 ( C, 752T ) = 4.28  ---------  --------- = 4.64L
273 752
(EQ 5)
This would also be the volume of oxygen consumed in the reaction under the same
conditions of temperature and pressure, since 1 molecule of oxygen is used to form
1 molecule of carbon dioxide.
All forms of matter emit radiation. For gases and semitransparent solids, such as
glass and salt crystals at elevated temperatures, emission is a volumetric phenomenon. That is, radiation emerging from a finite volume of matter is the integrated
effect of local emission throughout the volume. In most solids and liquids, radiation
emitted from interior molecules is strongly absorbed by adjoining molecules.
Accordingly, radiation that is emitted from a solid or a liquid originates from molecules that are within a distance approximate 1 µm from the exposed surface.
We know that radiation originates due to emission by matter and that its subsequent
transport does not require the presence of any matter. But what is the nature of this
transport? One theory views radiation as the propagation of a collection of particles
termed photons or quanta. Alternatively, radiation may be viewed as the propaga-
Engineering Aspects of Food Irradiation
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Radioelements, Isotopes & Radionuclides
tion of electromagnetic waves. In any case we wish to attribute to radiation the standard wave properties of frequency ν and wave length λ. For radiation propagating
in a particular medium, the 2 properties are related by:
c
λ = --ν
(EQ 6)
where c is the speed of light in the medium. For propagation in a vacuum, co =
2.998x108 m/s. The unit of wavelength is micrometer (µm) where 1 µm = 10-6m.
The complete electromagnetic spectrum is delineated in Figure 1. The short wavelength gamma rays, X-rays, and ultraviolet (UV) radiation are primarily of interest
to the high-energy physicist and the nuclear engineer, while the long wavelength
microwaves and radio waves are of concern of electrical engineer.
Figure 1: Spectrum of electromagnetic radiation
violet
blue
green
yellow
red
visible
infrared
X-rays
microwave
ultraviolet
thermal radiation
gamma rays
10−5
10−4
10−3
10−2
10−1
1
10
102
103
104
λ (µm)
Figure 2 shows the lines in the visible and near-ultraviolet spectrum of atomic
hydrogen. The wavelength of visible light is between about 4000 C (violet) and
7500 C (red). In 1885 Balmer published an empirical formula that gives these
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Engineering Aspects of Food Irradiation
Radioelements, Isotopes & Radionuclides
observed wavelength, λ, in the hydrogen spectrum. His formula is equivalent to the
following:
1
1 1
--- = R H  ----2- – ----2-


λ
2 n
(EQ 7)
Figure 2: Balmer series of lines in the spectrum of atomic hydrogen
3647 A
series
limit
n=3
•
8000A
n=4
•
•
6000A
7000A
Red
Yellow
•
5000A
Green
5 6 7...
•
4000A
Blue
Violet
Ultraviolot
where RH = 109,678 1/cm is called the Rydberg constant for hydrogen and n =
3,4,5,... represents any integer greater than 2. When n = 3, Eq(7) gives λ = 6562 C;
when n = 4, λ =4861 C; and so on. The series of lines, which continue to get closer
together as n increases, converges to the limit λ = 3647 C in the ultraviolet as
n → ∞ . Other series exist for hydrogen that can be described by replacing the 22 in
Eq(7) by the square of other integers. These other series lie entirely in the ultraviolet or infrared portions of the electromagnetic spectrum.
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Radioelements, Isotopes & Radionuclides
The nucleus
The nucleus of an atomic number Z and mass number A (atomic weight) consists of
Z protons and N = A-Z neutrons. The atomic masses of all individual atoms are
nearly integers, and A gives the total number of nucleons (i.e. protons and neutrons)
in the nucleus. A species of atom, characterized by its nuclear constitution - its values of Z and A (or N) - is called nuclide.It is conveniently designated by writing the
appropriate chemical symbol with a subscript giving Z and superscript given A. For
example:
1
2
1H ;1H ;
and
238
92U
are nuclides. Nuclides of an element that have different A (or N) are called isotopes
(in the same place). Nuclides having the same number of neurons are called isotones, e.g.:
206
82Pb ;
;
and
204
80Hg
are isotones with N = 124.Hydrogen has three isotopes:
1
2
3
1H ; 1H; 1H
2
3
all of which occur naturally. Deuterium, 1H , is stable; tritium, 1H , is radioactive.
Fluorine has only a single naturally occurring isotope,
19
9F ;
all of its other isotopes
are man made, radioactive, and short lived. The measured atomic weights of the
elements reflect the relative abundance of isotopes found in nature, as for example.
Example 2:
Chlorine is found to have two naturally occurring isotopes,
abundant, and
37
17Cl ,
35
17Cl ,
which is 76%
24% abundant. The atomic weights of the two isotopes are
34.97 and 36.97. Show that this isotopic composition accounts for the observed
atomic weight of the element.
Answer:
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Engineering Aspects of Food Irradiation
Radioelements, Isotopes & Radionuclides
Taking the weighted average of the atomic weights of the two isotopes, we find for
the atomic weight of Cl:
0.76 × 34.97 + 0.24 × 36.97 = 35.45
as observed.
The various kinds of atoms differing from each other by their atomic number or by
their mass are called nuclides. The correct name of unstable (radioactive) nuclides
is radionuclides, and the terms radioelements for unstable elements and radionuclides for unstable nuclides are analogous. For identification, the symbol (or the
atomic number) and the mass number are used. For example,
14
6C is
carbon with
the mass number 14 and atomic number 6. The atomic number can be omitted
14
because it is known by the symbol. C can also be written as C-14. For complete
information, the kind and the energy of transmutation and the half-life may be also
indicated:
14
14
Cβ ( 0.156MeV ) → N
(EQ 8)
About 2800 nuclides are known. About 340 of these are found in nature and may be
subdivided into four groups: (1) 258 are stable, (2) for 25 nuclides with atomic
number Z < 80 radioactive decay has been reported, but not confirmed for 7 of
these. Many exhibit extremely long half-lives (9 nuclides > 1016 years and 4
nuclides > 1020 years), and radioactivity has not been proved ambiguously. (3)
Main sources of natural radioactivity comprising 46 nuclides are U-238, U-235 and
Th-232 and their radioactive decay products. (4) Several radionuclides are continuously produced by the impact of cosmic radiation, and the main representatives of
this group are C-14, Be-10, Be-7 and H-3. Radionuclides present in nature in
extremely low concentration, such as Pu-244 and its decay products or products of
expontaneous fission of U and Th, are not considered in this list. Radionuclides
existing from the beginning, i.e., since the genesis of the elements, are called primordial radionuclides. They comprise the radionuclides of group (2) and U-238, U235, Th-232 and Pu-244.
The following groups of nuclides can be distinguished:
• Isotopes: Z = P equal
• Isotones: N = A - Z equal
Engineering Aspects of Food Irradiation
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Radioelements, Isotopes & Radionuclides
• Isobars: A = N + Z equal
• Isodiaspheres: A - 2Z = N - Z equal
For certain nuclides, different physical properties (half-lives, mode of decay) are
observed. They are due to different energetic states, the ground state and one or
more metastable excited states of the same nuclide. These different states are called
isomers or nuclear isomers. Because of the transition from the metastable excited
states to the ground states is “forbidden”, they have their own half-lives, which vary
between some milliseconds and many years. The excited states (isomers) either
change in the ground state by emission of a γ-ray photon (isometric transition; IT)
or transmutation to other nuclides by emission of α or β particles. Metastable
excited states (isomers) are characterized by the suffix m behind the mass number
A, for instance Co-60m and Co-60 or Ru-103m and Ru-103. Sometimes the ground
state is indicated by the suffix g. About 400 nuclides are known to exist in metastable states.
By comparison of the number of protons P and the number of neutrons N in stable
nuclei, it is found that for light elements (small Z) N = P. With increasing atomic
number Z, however, an increasing excess of neutrons is necessary to give stable
nuclei. A - 2Z is a measure of the neutron excess. For He-4 the neutrons excess is
zero. It is 3 for Sc-45, 11 for Y-89, 25 for La-139, and 43 for Bi-209. Thus, if in the
chart of the nuclides the stable nuclides are connected by a mean line, this line
starts from the origin with a slope of 1 and is bent smoothly towards the abscissa.
This mean line is called the line of β stability.
Nuclides Stability and Transmutation
On the basis of the proton-neutron model of atomic nuclei the following combinations amy be distinguished:
•
•
•
•
P even, N eve (even-even nuclei) - very common, 158 nuclei
P even, N odd (eve-odd nuclei) - common, 53 nuclei
P odd, N even (odd-even nuclei) - common, 50 nuclei
P odd, N odd (odd-odd nuclei) - rare, only 6 nuclei (H-2, Li-6, B-10, N-14, V50, Ta-180)
This unequal distribution does not correspond to statistics. The high abundance of
even-even nuclei indicates the high stability of this combination. On the other hand,
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Radioelements, Isotopes & Radionuclides
odd-odd nuclei seem to be exceptions. Four of the stable odd-odd nuclei are very
light.
Alpha activity is preferably found for heavier elements, Z = P > 83 (Bi). Elements
with even atomic numbers exhibit mainly β activity or electron capture. In the case
of β decay or electron capture, the mass number A remains constant. Either a neutron is changed into a proton or a proton into a neutron. Thus, odd-odd nuclei are
transformed into even-even nuclei - for instance, K-40 into Ar-40 or into Ca-40.
In finding nuclides of natural radioactivity, the Mattach rule proved to be very helpful. It states that stable neighboring isobars do not exist (exceptions: A = 50, 180).
In the following sequences of isobars, the middle one is radioactive:
• Ar-40 K-40 Ca-40
• Ba-138 La-138 Ce-138
• Y-176 Lu-176 Hf-176
Detailed study of the chart of nuclides makes evident that for certain values of P
and N a relatively large number of stable nuclides exist. These numbers are 2, 8, 20,
28, 50, 82 (126, only for N). The preference of these “magic numbers” is explained
by the shell structure of the atomic nuclei (shell mode). It is assumed that in the
nuclei the energy levels of protons and neutrons are arranged into shells, similar to
the energy levels of electrons in the atoms. Magic proton numbers correspond to
filled proton shells and magic neutron numbers to filled neutron shells. Because in
the shell model each nucleon is considered to be an independent particle, this model
is often called the independent particle model.
Nuclei Binding Energies
The high stability of closed shells (magic numbers) is also evident from the binding
energies of the nucleons. Just below each magic number the binding energy of an
additional proton or neutron is exceptionally high, and just above each magic number it is exceptionally low, similar to the binding energies of an additional electron
by a halogen atom or a noble gas atom, respectively.
Not all properties of the nuclei can be explained by the shell model. For calculation
of binding energies and the description of nuclear reaction, in particular nuclear fission, the drop model of the nucleus has been used successful. The model assumes
that the nucleus behaves like a drop of a liquid, in which the nucleons correspond to
Engineering Aspects of Food Irradiation
25
Radioelements, Isotopes & Radionuclides
the molecules. Characteristic properties of such drop are cohesive forces, surface
tension, and tendency to split if the drop becomes too big.
To calculate the binding energy (EB) of the nuclei, the semi-empirical equation is
used:
EB = Ev + Ec + EF + Es + Eg
(EQ 9)
The most important contribution is the volume energy:
E v = av A
(EQ 10)
where av is a constant = 14.1 MeV and A is the mass number. The mutual repulsion
of the protons is taken into account by the Coulomb term Ec:
Z( Z – 1)
E c = – a c -------------------A
(EQ 11)
where ac is a constant = 0.585 MeV and Z the atomic number. A1/3 is a measure of
the radius of the nucleus and therefore also the distance between the protons. With
increasing surface energy a drop of water becomes more and more unstable.Accordingly, in the drop model of the nucleus a surface energy term EF is subtracted:
EF = –aF A
2⁄3
(EQ 12)
where aF is a constant = 13.1 MeV and A2/3 is a measure fro the surface. Neutrons
are necessary to build up stable nuclei. But the excess of neutrons diminishes the
total energy of the nucleus. This contribution is called the symmetric energy Es:
( A – 2Z ) 2
E s = – a s --------------------A
(EQ 13)
where as is a constant = 19.4 MeV. The relatively high stability of even-even nuclei
is taken into account by a positive contribution of the total binding energy EB of the
nuclei, and the relatively low stability of odd-odd nuclei by a negative contribution.
The following values are taken for this odd-even energy:
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Radioelements, Isotopes & Radionuclides
 δ ( A, Z )…even – even

E g =  0…even – odd, odd – even

 – δ ( A, Z )…ood – odd
(EQ 14)
The value of δ is equal to ag/A, where ag is a constant = 33 MeV.
EB plotted as function of Z will give parabolas, one parabola for odd mass numbers
A (Eg = 0) and two parabolas for even mass numbers A (Eg = +δ).
Binding Energy E B [MeV]
Figure 3: Binding energy with odd mass numbers.
Z
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Radioelements, Isotopes & Radionuclides
Nuclide Masses
The mass number A is equal to the number of nucleons, A = P + N, and is always
an integer. The nuclide mass M, on the other hand, is the exact mass of the nuclides
in universal atomic mass units u, and the atomic mass is the mean of the nuclide
masses of the stable nuclides in their natural abundance.
The basis of the atomic mass unit u is the mass of the carbon isotope C-12: M(C12) = 12.000000. Nuclide masses and atomic masses include the mass of the electrons of the neutral atom: M = mass of the nucleus +Zme, where me the mass of one
electron in atomic mass units u. One atomic mass unit is 1.660566 x 10-24g (1/N0 =
1/6.03x1023).
The mass m of particles traveling with very high velocities increases as the velocity
approaches the velocity of light c:
m0
m = ---------------------------2
1 – (v ⁄ c)
(EQ 15)
where m0 is the mass of the particle at rest and v its velocity. Eq(15) was derived by
Eistein in his theory of relativity. Another result of this theory is the equivalence of
mass and energy:
E = mc
2
(EQ 16)
Since 1 u = 1.660566x10-24 g and c = 2.997925x10-8 m/s, 1 u is equivalent to
1.49244x10-10 J. The energy units mainly used in nuclear science are eV, (the
energy gained by an electron passing in vacuum a potential of 1 V; 1 eV =
1.60219x10-19 J), keV and MeV. So,
1 u = 931.5 MeV
On the basis of the proton-neutron model of atomic nuclei, the following equation
can be written for the mass of a nuclide:
M = ZM H + NM n – δM
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Engineering Aspects of Food Irradiation
(EQ 17)
Radioelements, Isotopes & Radionuclides
where MH is the nuclide mass of H-1 and comprises the mass of one proton as well
as that of one electron. Mn is the mass of the neutron in atomic mass units, and δM
is the mass effect. It is due to the fact that the binding energy EB of the nucleons
according to Eq(16) results in a decrease in the mass compared with the sum of the
masses of the individual particles. The effect of the binding energy of the electrons
is very small with respect to the binding energy of the nucleons and can be
neglected.
Application of Eq(16) gives:
E
δM = -----2B- = ZM H + NM n – M
c
(EQ 18)
If EB is divided by A, the mean binding energy per nucleon is obtained, which is a
measure of the stability of the nucleus:
2
EB
c
------ = ---- ( ZM H + NM n – M )
A
A
(EQ 19)
Physical Properties of Nuclei
Diameter
The diameters of atoms vary between about 0.8x10-10 and 3.0x10-10 m and the
diameters of nuclei are in the range of about 0.3x10-14 to 1.6x10-14 m. The radius of
an atomic nucleus can be described by the equation:
r N = r0 A
1⁄3
(EQ 20)
where r0 = 1.33 fm (femtometers) (1 fm = 10-15 m) is a constant and A the mass
number.
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Radioelements, Isotopes & Radionuclides
Density
The charge distribution (distribution of the protons) is practically constant in the
interior of the nucleus and decreases near its surface, as shown in Figure 4.
Figure 4: Charge distribution in nuclei (c= half-density radius; d = skin thickness)
Relative charge density
1.0
d
c
0.5
0
0
Radius, R
The layer of decreasing density is about 2.5 fm, independently of the atomic number. The distribution of the neutrons is assumed to be approximately the same as
that of the protons. Then the mass distribution in the nucleus is also the same as the
charge distribution. The density of nuclear matter is the interior of the nuclei is
given by:
1
14 g
A
ρ = --------------------- = ----------------- ≈ 2 × 10 --------34
3
4
3
cm
--- r 0 N 0
--- πr N N 0
3
3
30
Engineering Aspects of Food Irradiation
(EQ 21)
Radioelements, Isotopes & Radionuclides
Nuclear Forces
Strong Interaction: The nucleon-nucleon interaction becomes effective only at
distance less than 2.4 fm. The interaction is very strong, resulting in a high negative
potential of about 50 MeV and a very small equilibrium distance of about 0.6 fm.
The Coulomb repulsion energy Ec between two protons is given by:
2
e
E c = -------------4πε 0 r
(EQ 22)
where e is the electric charge of a proton, ε0 the electric field constant, and r the distance apart within the nucleus. Since r = 3 fm, Ec is about 0.5 MeV. The repulsion
energy is small compared with the mean binding energy of about 8 MeV. For a
greater number of protons the total repulsion energy increases according to:
2
3
e
E c = --- Z ( Z – 1 ) -------------5
4πε 0 r
(EQ 23)
where r is the effective distance between the protons, which can be set equal to the
radius of the nucleus. Whereas the nuclear forces strive for saturation, the Coulomb
repulsion energy between protons increases continuously with the atomic number
Z, causing the instability of heavy nuclei with high atomic numbers.
Weak Interaction: Nuclear forces are due to the strong interaction between nucleons. Besides the strong interaction, weak interaction and electromagnetic interaction are important for nuclei and elementary particles. Weak interaction also has a
limited range, of the order of some femtometers. It is responsible for β-decay processes.
Electromagnetic Interaction: Electromagnetic interaction is observed for all particles carrying electromagnetic field (charged particles such as protons and neutral
particles with a magnetic momentum such as neutrons). Electromagnetic interaction is also responsible for chemical bonding. As weak and electromagnetic interactions have some common features, they are assumed to have a common origin.
Gravitation Interaction: The fourth kind of interaction is gravitation, the range of
which is extremely large. Gravitation is responsible for gravity and the motion of
the planets. The four fundamental types of interaction are summarized in Table
2.According to quantum theory, virtual mediating particles are responsible for the
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Radioelements, Isotopes & Radionuclides
interactions, e.g. exchange of gluons for strong interaction and exchange of photons
for electromagnetic interaction.
TABLE 2. The
four fundamental types of interactions
Type of Interaction
Mediating particle
Relative force constant
Strong
Gluon
1
Electromagnetic
Photon
10-2
Weak
Boson (Z, W-,W+)
10-5
Gravitation
Graviton
10-40
Nuclear Momentum
The hyperfine structure of atomic spectra that is observed under the influence of an
external magnetic field is due to the interaction of electrons and nuclei. This hyperfine structure may be caused by: (a) different masses of the atoms (if the element
contains two or more isotopes) which represents an isotope effect and/or (b) the
interaction of the magnetic momenta of the lectrons and the nuclei (if the latter have
an angular momentum) which proof the existence of a nuclear angular momentum.
The nuclear angular momentum is measure in units of κ/2π, as well as the angular
momentum of an electron, a proton or a neutron, which is 1/2 κ/2π, for each of
these particles. It is a vector of magnitude
κ
I ( I + 1 ) -----2π
where I is the quantum number of the nuclear angular momentum, called the
nuclear spin. Nuclei with even mass numbers A have integral nuclear spins, I = 0, 1,
2,., whereas nuclei with odd numbers have half-numbered nuclear spins, I = 1/2, 3/
2, 5/2,... Even-even nuclei in the ground state always have I = 0. Odd-odd nuclei
have an integral spin, in most cases I = 0.; and even-odd and odd-even nuclei have
half-numbered spins varying between I = 1/2 and I = 11/2. It is assumed that protons and neutrons compensate their spins in pairs. The main contribution to the
nuclear spins come from the last unpaired nucleon.
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The nuclear angular momentum originates from the individual angular momenta of
the nucleons, which have two contributions, spin angular momenta and orbital
angular momenta, which are due to the spin and orbital motions, respectively, of the
nuclei. The spin angular momenta s i of the nucleons as well as their orbital angular
momenta I i are vectors. With respect to the interaction of particles in a system, two
cases may be distinguished:
• The interaction of the individual si and I i of each particle is strong compared
with the interaction between the particles, i.e., the spin-orbital coupling is
strong. The resulting angular momentum of each particle is calculated according
to the rules of vector addition:
ji = si + Ii
(EQ 24)
and the angular momenta of the system is given by:
I i = Σj i
(EQ 25)
where j i is the angular momentum. This kind of coupling is called jj coupling.
• The interaction of the individuals s i and I i of each particle is weak compared
with the interaction between the particles; i.e., the spin-orbital coupling is weak.
Then the resultant spin angular momentum and the resultant orbital angular
momentum are calculated first:
S = Σs i …and…L = Σl i
(EQ 26)
and the angular momentum of the system is given by:
I = S+L
(EQ 27)
This kind of coupling is called LS or Russel-Saunders coupling.
The jj coupling holds for the nucleons in nuclei and for electrons of heavy atoms,
LS coupling for the lectrons of light and medium-heavy atoms. The term “nuclear
spin” is correct for the spin momentum of a single nucleon, but is commonly used
Engineering Aspects of Food Irradiation
33
Radioelements, Isotopes & Radionuclides
for the quantum number for the resultant angular momentum of a nucleus consisting of two or more nucleons.
The law of conservation of momentum is also valid for nuclear angular momenta.
Magnetic momentum: Rotation of a charged particle causes a magnetic momentum (dipole momentum). The magnetic momentum of an electron is:
µ 0 eκ
– 29
µ B = ------------ = 1.1653 × 10 Vsm
4πm e
(EQ 28)
where µ0 is the magnetic field constant, e the electrical elementary unit, and me the
mass of an electron µB is called the Bohr magneton. The magnetic momentum of
the nucleus is much smaller:
µ 0 eκ
– 33
µ N = ------------ = 6.3466 × 10 Vsm
4πm p
(EQ 29)
where mp is the mass of protons and µN is called the nuclear magneton. The magnetic momentum of the proton is much greater than the calculated value (+2.7926
µB, parallel to the spin). The neutron has also a magnetic momentum (-1.9135 µN,
antiparallel to the spin). the se values are explained by the inner structure of the
proton and neutron. the magnetic momentum of a nucleus is also a vector:
µ I = g I Iµ N
(EQ 30)
where gI is the nuclear g factor. From Eq(30) you can see that all nuclei with
nuclear spin I = 0 (even-even nuclei) have no magnetic momentum.
If the magnetic momentum of a nucleus is not zero, the nucleus performs a precession with frequency ν0 (Larmor frequency) under the influence of an outer magnetic field:
gI µN
ν 0 = ----------- B 0
κ
(EQ 31)
where B0 is the magnetic flux density. For B0 = 1 tesla, ν0 is 42.6x10-6 1/s, which is
in the region of radiofrequencies. the nucleus may adopt 2I + 1 energy levels from
each other by:
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Radioelements, Isotopes & Radionuclides
∆E = κν = g I µ N B 0
(EQ 32)
By absorption or emission of photons of frequency:
gI µN
ν = ----------- B 0
κ
(EQ 33)
which is identical to the Larmor frequency, the nucleus can pass from a certain
energy level to a neighboring level. This process is known as nuclear magnetic resonance (NMR).
Quadrupole momentum: many nuclei also have an electrical quadrupole momentum, which is a measure of the deviation of charge distribution from spherical symmetry. the electrical quadrupole momentum is:
2
4 2 ∆r
2
2
Q = --- Z ( a – b ) = --- Zr -----r
5
5
(EQ 34)
where a and b are the radii of an ellipsoid of revolution along the axis of symmetry
and perpendicular to it, respectively, r is the mean radius, ∆r = a - b, and ∆r/r is a
measure of the deformation. Q may be positive (a>b) or negative (a<b). Nuclides
with I = 0 or 1/2 do not have an electrical quadrupole momentum; that means their
nuclei have spherical symmetry.
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35
Radioelements, Isotopes & Radionuclides
APPENDIX
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Radioelements, Isotopes & Radionuclides
RADIATION AND RADIOACTIVITY
Radiation is energy traveling in the form of particles or waves in bundles of energy
called photons. Some everyday examples are microwaves used to cook food, radio
waves for radio and television, light, and x-rays used in medicine.
Demonstration with Chart of Electromagnetic Spectrum
Radioactivity is a natural and spontaneous process by which the unstable atoms of
an element emit or radiate excess energy in the form of particles or waves. These
emissions are collectively called ionizing radiations. Depending on how the
nucleus loses this excess energy either a lower energy atom of the same form will
result, or a completely different nucleus and atom can be formed.
Ionization is a particular characteristic of the radiation produced when radioactive
elements decay. These radiations are of such high energy that when they interact
with materials, they can remove electrons from the atoms in the material. This
effect is the reason why ionizing radiation is hazardous to health, and provides the
means by which radiation can be detected.
THE ATOM
A typical model of the atom is called the Bohr Model, in honor of Niels Bohr who
proposed the structure in 1913. The Bohr atom consists of a central nucleus composed of neutrons and protons, which is surrounded by electrons which “orbit”
around the nucleus.
Protons carry a positive charge of one and have a mass of about 1 atomic mass unit
or amu (1 amu =1.7x10-27 kg, a very, very small number). Neutrons are electrically
“neutral” and also have a mass of about 1 amu. In contrast electron carry a negative
charge and have mass of only 0.00055 amu. The number of protons in a nucleus
determines the element of the atom. For example, the number of protons in uranium
is 92 and the number in neon is 10. The proton number is often referred to as Z.
Atoms with different numbers of protons are called elements, and are arranged in
the periodic table with increasing Z.
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Atoms in nature are electrically neutral so the number of electrons orbiting the
nucleus equals the number of protons in the nucleus.
Neutrons make up the remaining mass of the nucleus and provide a means to “glue”
the protons in place. Without neutrons, the nucleus would split apart because the
positive protons would repel each other. Elements can have nucleii with different
numbers of neutrons in them. For example hydrogen, which normally only has one
proton in the nucleus, can have a neutron added to its nucleus to from deuterium, ir
have two neutrons added to create tritium, which is radioactive. Atoms of the same
element which vary in neutron number are called isotopes. Some elements have
many stable isotopes (tin has 10) while others have only one or two. We express
isotopes with the nomenclature Neon-20 or 20Ne10, with twenty representing the
total number of neutrons and protons in the atom, often referred to as A, and 10 representing the number of protons (Z).
Radionuclides can be arranged by A and Z in the chart of the nuclides (http://
www2.bnl.gov/CoN/).
This is sort of like the periodic table of elements. A very good Web Periodic Table
can be found at this site.
Alpha decay is a radioactive process in which a particle with two neutrons and two
protons is ejected from the nucleus of a radioactive atom. The particle is identical to
the nucleus of a helium atom.
ALPHA PARTICLES
Alpha decay only occurs in very heavy elements such as uranium, thorium and
radium. The nuclei of these atoms are very “neutron rich” (i.e. have a lot more neutrons in their nucleus than they do protons) which makes emission of the alpha particle possible.
After an atom ejects an alpha particle, a new parent atom is formed which has two
less neutrons and two less protons. Thus, when uranium-238 (which has a Z of 92)
decays by alpha emission, thorium-234 is created (which has a Z of 90).
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Because alpha particles contain two protons, they have a positive charge of two.
Further, alpha particles are very heavy and very energetic compared to other common types of radiation. These characteristics allow alpha particles to interact
readily with materials they encounter, including air, causing many ionizations in a
very short distance. Typical alpha particles will travel no more than a few centimeters in air and are stopped by a sheet of paper.
THE BETA PARTICLE
Beta decay is a radioactive process in which an electron is emitted from the nucleus
of a radioactive atom, along with an unusual particle called an antineutrino. The
neutrino is an almost massless particle that carries away some of the energy from
the decay process. Because this electron is from the nucleus of the atom, it is called
a beta particle to distinguish it from the electrons which orbit the atom.
Like alpha decay, beta decay occurs in isotopes which are “neutron rich” (i.e. have
a lot more neutrons in their nucleus than they do protons). Atoms which undergo
beta decay are located below the line of stable elements on the chart of the nuclides,
and are typically produced in nuclear reactors. When a nucleus ejects a beta particle, one of the neutrons in the nucleus is transformed into a proton. Since the number of protons in the nucleus has changed, a new daughter atom is formed which
has one less neutron but one more proton than the parent. For example, when rhenium-187 decays (which has a Z of 75) by beta decay, osmium-187 is created
(which has a Z of 76). Beta particles have a single negative charge and weigh only
a small fraction of a neutron or proton. As a result, beta particles interact less
readily with material than alpha particles. Depending on the beta particles energy
(which depends on the radioactive atom), beta particles will travel up to several
meters in air, and are stopped by thin layers of metal or plastic.
High energy betas that travel through water sometimes produce Cerenkov Radiation, which in turn produces the blue glow seen around fuel and reactors.
GAMMA RADIATION
After a decay reaction, the nucleus is often in an “excited” state. This means that
the decay has resulted in producing a nucleus which still has excess energy to get
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39
Radioelements, Isotopes & Radionuclides
rid of. Rather than emitting another beta or alpha particle, this energy is lost by
emitting a pulse of electromagnetic radiation called a gamma ray. The gamma ray is
identical in nature to light or microwaves, but of very high energy.
Like all forms of electromagnetic radiation, the gamma ray has no mass and no
charge. Gamma rays interact with material by colliding with the electrons in the
shells of atoms. They lose their energy slowly in material, being able to travel significant distances before stopping. Depending on their initial energy, gamma rays
can travel from 1 to hundreds of meters in air and can easily go right through people.
It is important to note that most alpha and beta emitters also emit gamma rays as
part of their decay process. However, their is no such thing as a “pure” gamma
emitter. Important gamma emitters including technetium-99m which is used in
nuclear medicine, and cesium-137 which is used for calibration of nuclear instruments.
X-RAYS
Over a century ago in 1895, Roentgen discovered the first example of ionizing radiation, x-rays. The key to Roentgens discovery was a device called a Crooke’s tube,
which was a glass envelope under high vacuum, with a wire element at one end
forming the cathode, and a heavy copper target at the other end forming the anode.
When a high voltage was applied to the electrodes, electrons formed at the cathode
would be pulled towards the anode and strike the copper with very high energy.
Roentgen discovered that very penetrating radiations were produced from the
anode, which he called x-rays.
X-ray production whenever electrons of high energy strike a heavy metal target,
like tungsten or copper. When electrons hit this material, some of the electrons will
approach the nucleus of the metal atoms where they are deflected because of there
opposite charges (electrons are negative and the nucleus is positive, so the electrons
are attracted to the nucleus). This deflection causes the energy of the electron to
decrease, and this decrease in energy then results in forming an x-ray.
Medical x-ray machines in hospitals use the same principle as the Crooke’s Tube to
produce x-rays. The most common x-ray machines use tungsten as there cathode,
and have very precise electronics so the amount and energy of the x-ray produced is
optimum for making images of bones and tissues in the body.
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Radioelements, Isotopes & Radionuclides
PROPERY OF RADIATION
Different radiations have different properties, as summarized below:
Radiation
Type of
Radiation
Mass
(AMU)
Charge
Shielding material
Alpha
Particle
4
+2
Paper, skin, clothes
Beta
Particle
1/1836
±1
Plastic, glass, light metals
Gamma
Electromagnetic Wave
0
0
Dense metal, concrete, Earth
Neutrons
Particle
1
0
Water, concrete, polyethylene,
oil
In summary, the most common types of radiation include alpha particles, beta and
positron particles, gamma and x-rays, and neutrons. Alpha particles are heavy and
doubly charged which cause them to lose their energy very quickly in matter. They
can be shielded by a sheet of paper or the surface layer of our skin. Alpha particles
are only considered hazardous to a persons health if an alpha emitting material is
ingested or inhaled. Beta and positron particles are much smaller and only have one
charge, which cause them to interact more slowly with material. They are effectively shielded by thin layers of metal or plastic and are again only considered hazardous if a beta emitter is ingested or inhaled.
Gamma emitters are associated with alpha, beta, and positron decay. X-Rays are
produced either when electrons change orbits within an atom, or electrons from an
external source are deflected around the nucleus of an atom. Both are forms of high
energy electromagnetic radiation which interact lightly with matter. X-rays and
gamma rays are best shielded by thick layers of lead or other dense material and are
hazardous to people when they are external to the body.
Neutrons are neutral particles with approximately the same mass as a proton.
Because they are neutral they react only weakly with material. They are an external
hazard best shielded by thick layers of concrete. Neutron radiation will be discussed in more detail in the discussion of nuclear power.
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Radioelements, Isotopes & Radionuclides
HALF-LIFE
Half-life is the time required for the quantity of a radioactive material to be reduced
to one-half its original value.
All radionuclides have a particular half-life, some of which a very long, while other
are extremely short. For example, uranium-238 has such a long half life, 4.5x109
years, that only a small fraction has decayed since the earth was formed. In contrast, carbon-11 has a half-life of only 20 minutes. Since this nuclide has medical
applications, it has to be created where it is being used so that enough will be
present to conduct medical studies.
Here is a on-line calculator that will calculate the activity of some radionuclides at
some time after it is formed.
RADIATION MEASUREMENT
When given a certain amount of radioactive material, it is customary to refer to the
quantity based on its activity rather than its mass. The activity is simply the number
of disintegrations or transformations the quantity of material undergoes in a given
period of time.
The two most common units of activity are the Curie and the Becquerel. The Curie
is named after Pierre Curie for his and his wife Marie's discovery of radium. One
Curie is equal to 3.7x1010 disintegrations per second. A newer unit of activity if the
Becquerel named for Henry Becquerel who is credited with the discovery of radioactivity. One Becquerel is equal to one disintegration per second.
It is obvious that the Curie is a very large amount of activity and the Becquerel is a
very small amount. To make discussion of common amounts of radioactivity more
convenient, we often talk in terms of milli and microCuries or kilo and MegaBecquerels.
Radiation is often measured in one of these three units, depending on what is being
measured and why. In international units, these would be Coulombs/kg for roentgen, Grays for rads and Seiverts for rem.
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GAS FILLED DETECTOR
Since we cannot see, smell or taste radiation, we are dependent on instruments to
indicate the presence of ionizing radiation.
The most common type of instrument is a gas filled radiation detector. This instrument works on the principle that as radiation passes through air or a specific gas,
ionization of the molecules in the air occur. When a high voltage is placed between
two areas of the gas filled space, the positive ions will be attracted to the negative
side of the detector (the cathode) and the free electrons will travel to the positive
side (the anode). These charges are collected by the anode and cathode which then
form a very small current in the wires going to the detector. By placing a very sensitive current measuring device between the wires from the cathode and anode, the
small current measured and displayed as a signal. The more radiation which enters
the chamber, the more current displayed by the instrument.
Many types of gas-filled detectors exist, but the two most common are the ion
chamber used for measuring large amounts of radiation and the Geiger-Muller or
GM detector used to measure very small amounts of radiation.
The second most common type of radiation detecting instrument is the scintillation
detector. The basic principle behind this instrument is the use of a special material
which glows or “scintillates” when radiation interacts with it. The most common
type of material is a type of salt called sodium-iodide. The light produced from the
scintillation process is reflected through a clear window where it interacts with
device called a photomultiplier tube.
SODIUM IODIDE DETECTOR
The first part of the photomultiplier tube is made of another special material called
a photocathode. The photocathode has the unique characteristic of producing electrons when light strikes its surface. These electrons are then pulled towards a series
of plates called dynodes through the application of a positive high voltage. When
electrons from the photocathode hit the first dynode, several electrons are produced
for each initial electron hitting its surface. This “bunch” of electrons is then pulled
towards the next dynode, where more electron “multiplication” occurs. The
sequence continues until the last dynode is reached, where the electron pulse is now
millions of times larger then it was at the beginning of the tube. At this point the
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electrons are collected by an anode at the end of the tube forming an electronic
pulse. The pulse is then detected and displayed by a special instrument.
Scintillation detectors are very sensitive radiation instruments and are used for special environmental surveys and as laboratory instruments.
ACCELERATOR
What are accelerators used for?
Quite simply, accelerators give high energy to subatomic particles, which then collide with targets. Out of this interaction come many other subatomic particles that
pass into detectors. From the information gathered in the detectors, physicists can
determine properties of the particles and their interactions.
The higher the energy of the accelerated particles, the more closely we can probe
the structure of matter. For that reason a major goal of researchers is to produce
higher and higher particle energies.
Accelerator: A device (i.e., machine) used to produce high-energy high-speed
beams of charged particles, such as electrons, protons, or heavy ions, for research in
high-energy and nuclear physics, synchrotron radiation research, medical therapies,
and some industrial applications. The accelerator at SLAC is an electron accelerator.
El;ectron accelerator: Electrons carry electrical charge and successful manipulation
of electrons allows electronic devices to function. The picture and text on the video
terminal in front of you is caused by electrons being accelerated and focused onto
the inside of the screen, where a phosphor absorbs the electrons and light is produced. A television screen is a simple, low-energy example of an electron accelerator. A typical medical electron accelerator used in medical radiation therapy is
about 1000 times more powerful than a color television set, while the electron
accelerator at SLAC is about 2,000,000 times more powerful than a color TV. One
example of an electron accelerator used in radiotherapy is the Clinac, manufactured
by Varian Associates in Palo Alto, CA
How many kinds of accelerators are there?
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Particle accelerators come in two basic designs, linear (linac) and circular (synchrotron). The accelerator at SLAC is a linac.
The longer a linac is, the higher the energy of the particles it can produce. A synchrotron achieves high energy by circulating particles many times before they hit
their targets.
Linacs are used in medicine as well as high energy physics research. How does the
SLAC linac work? Check it out!
How do they work?
Your TV set or computer monitor contains the components of an accelerator. As
you might suspect, operating an accelerator as large as the linac at SLAC is a challenging task. To learn more about the SLAC linear accelerator structural components and experimental facilities, select a link below.
Accelerator Components
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Beam Switch Yard
Damping Rings
Electron Gun
Klystrons
Linac
Positron Production
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THE CYCLOTRON
The cyclotron is a particle accelerator conceived by Ernest O. Lawrence in 1929,
and developed, with this colleagues and students at the University of California in
the 1930s. (For a short pictorial history, see The Development of the Cyclotron at
LBNL.)
A cyclotron consisted of two large dipole magnets designed to produce a semi-circular region of uniform magnetic field, pointing uniformly downward.
These were called Ds because of their D-shape. The two D's were placed back-toback with their straight sides parallel but slightly separated.
An oscillating voltage was applied to produce an electric field across this gap. Particles injected into the magnetic field region of a D trace out a semicircular path
until they reach the gap. The electric field in the gap then accelerates the particles
as they pass across it.
The particles now have higher energy so they follow a semi-circular path in the
next D with larger radius and so reach the gap again. The electric field frequency
must be just right so that the direction of the field has reversed by their time of
arrival at the gap. The field in the gap accelerates them and they enter the first D
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again. Thus the particles gain energy as they spiral around. The trick is that as they
speed up, they trace a larger arc and so they always take the same time to reach the
gap. This way a constant frequency electric field oscillation continues to always
accelerate them across the gap. The limitation on the energy that can be reached in
such a device depends on the size of the magnets that form the D's and the strength
of their magnetic fields.
Once the synchrotron principle was developed (see below), it was found to be a
much cheaper way to achieve high energy particles than the cyclotron and so the
original cyclotron method is no longer used.
Synchrotron
A synchrotron (sometimes called a synchro-cyclotron) is a circular accelerator
which has an electromagnetic resonant cavity (or perhaps a few placed at regular
intervals around the ring) to accelerate the particles.
There are several circular accelerators at Fermi National Accelerator Laboratory.
Particles pass through each cavity many times as they circulate around the ring,
each time receiving a small acceleration, or increase in energy. When either the
energy or the field strength changes so does the radius of the path of the particles.
Thus, as the particles increase in energy the strength of the magnetic field that is
used to steer them must be changed with each turn to keep the particles moving in
the same ring. The change in magnetic field must be carefully synchronized to the
change in energy or the beam will be lost. Hence the name "synchrotron". The
range of energies over which particles can be accelerated in a single ring is determined by the range of field strength available with high precision from a particular
set of magnets. To reach high energies, physicists sometimes use a sequence of different size synchrotrons, each one feeding the next bigger one. Particles are often
pre-accelerated before entering the first ring, using a small linear accelerator or
other device.
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Synchrotron Radiation
Synchrotron radiation is the name given to the electromagnetic radiation emitted
by the charged particles circulating in a synchrotron. It occurs because the charged
particles are accelerated (deflected) by the magnetic field from the dipole magnets
to make the beam travel around the ring. Any accelerated charged particle produces
some electromagnetic radiation.
The wavelength and intensity of the synchrotron radiation depends on the energy
and type of the emitting particle. If all you are interested in is storing a high energy
beam, then synchrotron radiation is a problem. The energy lost from the beam by
this radiation effect must be restored by introducing accelerating cavities at one or
more places in the ring, to give the particles a kick in energy every time they pass.
The amount and energy of the radiation depends on the speed of the radiating particles and the magnetic field strength. As the particle approaches the speed of light,
the effect increases rapidly. The special relativity factor, gamma (, is the ratio of the
energy of the particle to its rest mass-energy, mc2. The energy loss for a given electron energy is proportional to ()3.
Dependence on Particle Type
For an 1.5 GeV electron in the SPEAR storage ring, gamma is approximately 3000.
For a 50 GeV electron in the SLC arcs, gamma is approximately 100,000. Gamma
is the ratio of the energy of the particle to its rest mass-energy, mc2. Thus, because
a proton is so much more massive than an electron, a proton with 1 TeV = 1,000
GeV energy has a gamma factor of only 1,000. (1 TeV is the energy produced by
the synchrotron at Fermilab). Thus synchrotron radiation is much greater for electrons than for equal energy protons. This is the reason why much higher energy
synchrotrons can be built for protons than for electrons.
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SSRL
At SPEAR, the synchrotron radiation has wavelengths from ultraviolet to x-ray, just
the right scale to use it as a probe of the atomic and molecular scale structure of
matter. The Stanford Synchrotron Radiation Laboratory at SLAC is devoted to
studies using this powerful tool.
Storage Ring
A storage ring is the same thing as a synchrotron, except that it is designed just to
keep the particles circulating at a constant energy for as long as possible, not to
increase their energy any further. However, the particles must still pass through at
least one accelerating cavity each time they circle the ring, just to compensate for
the energy they lose to synchrotron radiation.
Two storage rings have been built at SLAC; SPEAR, a 3 GeV ring completed in the
early 70's and PEP a 9 GeV ring completed in the early 80's. SPEAR is now used
solely by SSRL while PEP is being rebuilt as a two-ring facility known as the B
factory.
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