Chapter 1

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295


C h a p t e r 1 0

Ionizing Radiation



radioactivity measurements

In general, radiation refers to travel of photons
in space, but the term is also used to mean
subatomic particles emitted by radioactive
nuclides or generated by machines. These

particles and photons usually ionize the
molecules or atoms along their tracks, and they
are called
ionizing radiation
. Radiation such as
infrared, microwave, long radio wave, and
visible light that do not ionize atoms or
molecules on its path are called
non
-
ionizing
radiation
.

Ionizing radiation includes

,

, protons, X
-
rays, cosmic rays, and gamma rays. In this
Chapter, we shall discuss their interactions with materials. Their interactions allow us to built
detectors and counters for their measurements
. Details of interaction at the atomic or
subatomic level are particularly interesting.

Strictly speaking, neutrons do not cause ionization. However, they induce radioactivity, and
eventually lead to ionization. Nevertheless, they deserve particular attent
ion.

Heisenberg pointed out that observations disturb the system being observed, during the
pronouncement of his uncertainty principle. Properties not being observed are affected during
a measurement. Actually, interactions link the observer to the observ
ed.

Ionizing radiation is everywhere. We encounter it in our daily lives. Ionizing radiation has been
blamed for aging, illness, environmental damage, skin cancer, and the destruction of immune
responses. No doubt, ionizing radiation causes special chemica
l phenomena, one of which is
the generation of radicals. Initiatiating polymerization reactions is one of the important
industrial applications.

Radiation effects in biology are studied due to health and safety concerns. From a medical
point of view, radi
ation may induce certain type of disease such as cancer. On the other hand,
they may have special effect on diseased cells. Radiation induces chromosome and DNA
sequence changes that affect future generations.

The perturbation of a system to be observed caused by
the observation is also an important factor in
determining the limits within which a visual
description of atomic phenomena is possi
ble.



Werner Heisenberg


an opening statement of the uncertainty principle





296


High
-
Energy Radiation

Ionizing radiation consi
sts of high
-
energy particles and high
-
energy photons. High
-
energy
radiation may be emitted by atoms (X
-
rays), nuclei (protons,

,

, and


rays), or accelerators
(atomic nuclei and others).

High
-
energy particles are moving at very high speed. When passing

by atoms or molecules,
they knock out one or more electrons from them, producing positive ions and electrons. A
positive ion and an electron make an
ion pair
, for example, O
+

and e


or O
+
2

and e

.

In addition to ionization, there are other mechanisms by w
hich energetic particles loose
energy. For the consideration of interactions between ionizing radiation and material, the
particles are divided in the following categories.



heavy charged particles

such as protons,


particles, energetic nuclei, mesons, and

hadrons.



light charged particles

such as electrons, positrons and other leptons.



electromagnetic radiation

in the forms of X
-
rays and


rays, and



neutral particles

such as neutrons.

Discovery of Ionization by Radiation

X
-
rays and radioactivity discharged

a charged electroscope, observed Curie, Rutherford and
others when they worked with radioactivity.



What causes the discharge of electroscope?

How does X
-
rays and radioactivity discharge a charged electroscope?

Can electroscopes be used as ionizing radiat
ion detectors?

An electroscope consists of two gold leaves
suspended from a metallic conductor in a glass
jar. Glass and air are insulators. Touching the
conductor with electric charges makes the
leaves stay apart in a charged
state because like
charges repel each other. Conducting the
charges away or neutralizing them with
opposite charges causes the leaves to collapse
into a discharged state.

When a charge electroscope is exposed to X
-
rays or radioactivity, the electroscope b
ecomes
discharged. Early researchers such as Curie and Rutherford interpreted the discharge as due to
the ionization of radiation. Ion pairs produced by radiation make air a conductor. Opposite
charges attract to the leaves neutralized the charge.

Electroscopes
Charged
Discharged




297


Energies

associated with molecules can be divided into kinetic energies of translation, vibration
and rotation and energies associated with electronic energy states. Small amounts of energies
excite

molecules by changing their electronic
states
, or
kinetic energie
s

of rotation,
vibration, and translation (temperature change). Large amounts of energies break up chemical
bonds and electrons. When one or more electrons break away, molecules or atoms are ionized.
The ions so formed carry a single or multiple units (cha
rge of electron) of positive charges.

The minimum energy requi
red to remove an
outer electron from atoms or molecules is called
ionization energy
. Ionization energies of some
common substances and the ionization processes
are shown in a box here. The equations in the box
indicate the amounts of energy required for th
e
ionization process. For example, 14 eV will strip
an electron off the oxygen diatomic molecule.
Electrons stripped off an atom or molecule with
minimum energy have no kinetic energy. If more
energy is transferred to the electron, it will leave
an atom o
r a molecule with a kinetic energy.
Ionizing radiation removes electrons and usually leaves them with some kinetic energy.
Recombination of electron with its ion does not occur, unless the kinetic energy of the
electron is dissipated. Collisions cause the
kinetic energy of electrons to be dissipated to other
molecules. Low
-
energy electrons are picked up by atoms and ions.

More energy is required to remove an inner
-
shell electron or from an ion. Ionization by
radiation removes electrons of inner and outer sh
ells, and sometimes bonding electrons. The
average energy

required to generate an ion pair by radiation is higher than the first ionization
energy. For example, some average energies for ion pair production in some familiar media are
listed:

Average Ioniza
tion Energy (IE eV) per Pair of Some Common Substances


Material

Air

Xe

He

NH
3

Ge
-
crystal


Average IE

35

22

43

39

2.9

In the table above, all substances are gases except for Ge crystals. Thus, for the same ionizing
radiation, the number of ion pairs genera
ted depends on the material. Note that very low
energy is required to produce an ion pair in a Ge crystal, which is a semiconductor.
Germanium (Ge) and other semiconductor crystals are very sensitive ionizing radiation
detectors because they give large sig
nals due to the low ionization energy.


Ionization Energy and Ionization


O
2

+ 14 eV



2
+

+ e
-

H
2

+ 15 eV


H
2
+

+ e
-

He + 25 eV



+

+ e
-

He
+

+ 54 eV



2+

+ e
-

H
2
O + 13 eV


H
2
O
+

+ e
-

CO
2

+ 14 eV



2
+

+ e
-

N
2

+16 eV


N
2
+

+ e
-




298


The pro
duction of ion pairs by a high
-
energy
particle on its path is depicted in a diagram
here. Electrons removed from atoms and
molecules by radiation are called
primary
electrons
. Some of these electrons carry a
very high kinetic energy, and they, like the


rays, cause further ionization. Electrons
knocked out by primary electrons are
secondary electrons
.

Ionizing radiation interacts with many atoms
per unit length on its path. A gas at 273 K
and 1.0 atm has 2.7

10
22

molecules per liter,
or 2.7

10
19

molecules

per ml (=cm
3
). Gas
molecules on average travel a short distance
(2

10
-
7

m or 0.2 micrometer) between
collisions. This distance is called
mean free
path
. A liter of water contains 3.3

x

10
25

molecules, 1,200 times denser than that of a gas.
Atomic densitie
s in solids are similar. Densities affect the movements of electrons, ions, and
molecules. Due to the excess energy, however, recombination of ion pairs reaches an
equilibrium and the net number of ion pairs remain constant for a period.

Review Questions

1.

On the path of an alpha particle, nearly all molecules are ionized. If the average energy required to produce
an ion pair is 35 eV, how many pairs of ions are produced by a 1.0
-
MeV alpha particles?

What is the total amount of charge (both positive and neg
ative) produced?

2.

At standard temperature and pressure (STP), 1 mole of gas occupies 22.4 L of volume. Calculate the
molecular density per cm
3

of gas at STP.

Heavy Charged Particles

Alpha (

) particles, protons, atomic nuclei from particle accelerators, and baryons are
heavy
charged particles
. They lose energy in the medium mostly by ionization
*
.



How fast do high
-
energy heavy particles move?

What are the factors affecting the energy loss of

particles in a medium?

How far do heavy particles travel in a medium?

Alpha (

) particles and heavy ions interact with electrons via Coulomb force of attraction.
However, these particles are moving at high speed through their media. A classical approach t
o
calculate the velocity shown below gives the approximate speed. For example, if the kinetic



*

See also properties of mesons and baryons (collectively called hadrons). They decay into other high energy particles.

Ion Pairs on a Radiation Path





ooooooooooooooooo





oooooooooo

oooooo





oooooooo+
-
ooooooo




|oooooo
-
+ooooooooo





oooo+
-
ooooooooooo





oo
-
+ooooooooooooo





+
-
ooooooooooooooo




|ooooooooooooooooo





ooooooooooooooooo



o
Atoms and molecules in a medium.




299


energy of an


particle is 1 MeV, its velocity can be calculated, by using a mass of 4

1.66

10
-
27

kg,


1 MeV = 1.60 x 10
-
13

J



=

(½) (4

1.66

10
-
27

kg)
v
2


v
2
= 4.82

10
13

(m/s)
2



v

= 6.9

10
6

m/s

Since this speed is only a fraction of the speed
of light (3

10
8

m/s), the result is reasonably
correct.

Electrons are pulled away by positive particles
despite their high speed of motion. B
ecause
the radius of an atom is 100000 times larger than that of a nucleus, the heavy


particles only
occasionally colliding with a nucleus and they travel in almost straight lines, interacting with
mostly electrons. On their path,


particles and ions knock electrons out of their atomic or
molecular orbitals, often leading to multiple ion
ization. The
ionization

process generates free
electrons and positive ions or
ion pairs

on their path. The ionization process can be
represented by




A


A
n
+

+
n
e

,


where
n

is an integer indicating the number of ion pairs. Again, energetic primary elect
rons
cause further ionization in a cascade to produce secondary ion pairs.

There may be cases in which the electrons are not removed from the atoms, but the electrons
acquire some energy from the particle. Such a process
ex
cites

a molecule. Ionization and
excitation of the electrons rupture chemical bonds and generate free radicals. Free radicals are
reactive species and they cause further chemical
reactions.

The
stopping power

of a medium is the rate of
energy loss per unit

distance along the path. Born
and Bethe have shown that the stopping power
of a medium is proportional to the mass
M
, and
to the square of atomic number,
Z
2
, of the atoms
in the medium. Thus, a medium consisting of
heavy atoms have high stopping power.
Ho
wever, the stopping power is inversely
proportional to the energy of the particle. A fast
moving particle deposit less energy per unit
length on its track. High stopping power results
in generating high ion pair density.

Sketch of Alpha Particle Paths in a Medium


source
Shield

The Born
-
Bethe Formula for Energy
Loss of Charged Particles.


-

dE
dx

=

KM
z
E
2


Energy loss per distance (
-
dE/dx
) is
proportional to the mass,
M
, and square of
charge,
z
2
, but
inversely proportional to its
energy
E
. The distance of the track is
represented by
x
, and
K

is a constant.




300


The st
opping power of the medium is relatively
small at the time when an


particle enters the
medium because its energy is high. As it travels
through the medium, it loses energy and the
stopping power increases. Thus, the ion pairs
density generated along the
paths of a


particle
is low at the time when it enters the medium, and
increases to a maximum called
Bragg peak

just
before it stops. A plot of ion density as a function
of the distance in the medium is called a
Bragg
curve
. Such a curve has a general sha
pe as shown
here.

Heavy particles lose energy in a medium at a
faster rate than light particles. Thus, they generate higher ion
-
pair densities on their tracks.
Alpha particles generate denser ion
-
pair densities than protons of the same kinetic energy in
t
he same type of media.

Heavy particles such as protons and


particles of certain energy will lose
all their energy in a definite distance
in a medium, and this distance is
called the
range
, which depends on
charge, mass, and th
e kinetic energy
of the incident particles. One
-
MeV


particles have a shorter range than 5
-
MeV ones, and protons have longer
range than


particles if they have the
same kinetic energy. The range also
depends on the nature of the
medium. Five
-
MeV


parti
cles have a
range of several centimeters in air, but less than a millimeter in water or a solid material. The
range

is determined by measuring the intensity with an absorber between the detector and the
source. The thickness at which the intensity drops to

half is the
range
. The experiment and
graphic determination of range measurement is depicted here.

The stopping power of the medium depends on the atomic weights and densities. A medium
consisting of heavy elements such as lead have a larger stopping powe
r than one consisting of
light elements. A gas has low stopping power due to low density. Since the interaction of heavy
particles with the atoms in the medium is a matter of chance, there is a scattering of ranges for
particles of the same energy. This sc
attering is referred to as the
range straggling
.

Ion Pair Density Along the Path of Heavy
Charged Particles in a Medium
Distance along the path
Ion pairs
density

particle
proton

Variation of

Intensity as a Function of Thickness
Detector
Absorber
Intensity
thickness

source
straggling
Range




301


Ranges in pure aluminum as functions of
energies have been carefully measured for alpha
particles and protons. The low atomic number
(
Z

= 13) of aluminum makes ranges in it long.
Measuring long ranges is easier and more
precise. Thus, ranges in aluminum have been
the standards and ranges are usually expressed
in mass per unit area, (mg cm

2
, converted by
using length

density). By measuring the range
of a


particle source, its ener
gy can be
determined by comparing with a known
standard. Decay energies of many


emitters
have been tabulated, and range measurements
can be used to identify


sources. Knowing the
range, a shield thicker than the range of the
particles offers radiation p
rotection, because it
will stop all the radiation from passing through.

Heavy charged particles usually have short
ranges. The range of alpha particles from
radium in air at 298 K and 1 atm is 7.1 cm, and
the Bragg peak appears 6.3 cm from the source. The
re are about 143,000 ion pairs on its track,
giving an average 20,000 ion pair per cm along the track. These alpha particles cannot
penetrate a sheet of paper, and they may not be able to penetrate the membrane that separates
the gas in the detector from t
he environment. Thus, commercial radioactivity detectors may
not be able to detect the alpha particles.

Generally, when the velocities of protons and alpha particles are high, they do not pick up
electrons in the medium. They begin to pick up electrons onl
y when their kinetic energies are
comparable to those of ions or atoms in the medium. After they pick up required electrons,
they become part of the medium. Elastic and inelastic collisions become the dominant
interactions thereafter.

Review Questions

1.

Est
imate the velocities of protons whose kinetic energies are 1.0 and 0.1 MeV respectively using the
method of Newtonian physics. Discuss the result.

2.

How is the range of


particles measured? What purposes do the range measurement serve?

3.

How many ion pairs a
re formed in the path of a 1.0 MeV proton, if the average energy required to produce
an ion pair is 35 eV? If the range of the proton is 10 cm, calculate the number of ion pairs per cm on its
path. For a 5.0
-
MeV


particle, the range is to 7.2 cm. Evaluate

the number of ions and ion pair density
per cm on its path.

Ranges as Functions of Energy
0.1
1
10/MeV
0.1
1
10
100
Range
of

Range
of
protons
mg/cm
2




302


Light Charged Particles

Particles with mass comparable to those of electrons are
light charged particles
. Essentially,
they are high
-
speed positron and electrons. Their rest mass is only
1
/
1850

a
mu., much lighter
than those of protons. They interact with electrons of the media.



How do light charged particles lose energy in a medium?


Why and how they are different from the heavy charged particles?

Let us use the Newtonian physics to estimate of th
e speed of a


particle whose kinetic energy
is 1.0 MeV. Assume the velocity to be
v
, then the kinetic energy is (
1
/
2
)

m

v
2
. Thus,


1 MeV = 1.6

10
-
13

J



= (
1
/
2
) m
v
2



= (
1
/
2
) (9.1

10
-
31

kg
v
2

Solving for
v
,



v
2

= 3.30

10
17

or



v

= 5.7

10
8

m/sec.


This

speed exceeds the speed of light, (3.0

10
8

m/sec), and it violates principle of limiting
speed. The theory of relativity must be considered for a proper calculation of the velocity or
speed of a high
-
energy electron. The mass,
m
, of a 1
-
MeV beta particle
is actually 1.51 MeV,
rather than 0.51 MeV. In general, for a


particle of kinetic energy
E
k
, its mass in amu is,





E
k

MeV + 0.51 MeV (rest mass of electron)



m

=
───────────────────────────────

amu






931.5 MeV


The velocity,
v
, is then calculated by the equation


m/s
10
8
.
2
10
3
2
51
.
1
51
.
0
1
2
1
8
8
)
(
)
(







c
m
m
v
e

where
c

is the velocity of light, and
m
o

is the rest mass
of the electron. The velocity is 81% the

speed of light,
still a very high value.

The interactions of beta particles with matter is similar
to those of other charged particles such as protons and
alpha particles, but a high speed electron may lose half
of its kinetic energy in a single encounter

with an
electron in the stopping medium. Thus, it may suffer a
considerable deflection causing it to travel in a zigzag
path. An imaginary path is depicted here to show the
scattering of electron. Electrons with energy less than
An Imaginary Path of a

particle in
a Medium




303


1.02 MeV lose energy by el
astic and inelastic scattering. Scattering causes ionization and
excitation of electrons in the medium. Of course, the electron leaving a medium may not be
the same electron that enters the medium. Electrons have no individual identity.

The range of monoenergetic electrons
(all have the same kinetic energy) is not a
well defined quantity compared to those
of heavy charged particles. There is more
range straggling
. In spite of the severe
range straggling, kinetic energies of
elec
trons can be estimated approximately
by means of extrapolation. The
extrapolation and the range straggling as
seen from the plot of their intensity (
I
)
after they pass an absorber of thickness
x

is shown in a sketch here. The
measurement of intensity is do
ne in a
similar way as that used for heavy
charged particles discussed in the previous section. Electrons have a greater range than
protons if they all have the same kinetic energy, but the intensity drops gradually as the
thickness increases. In contrast,

the intensity of heavy particles remains essentially constant as
the thickness increases until the thickness is approximately equal to the range.

Collision between beta particles and electrons
causes ionization and excitation
of the electrons
in the medium. In addition, high
-
speed electrons
passing by atomic nuclei experience attraction
and repulsion. They are accelerated or
decelerated in a medium. Acceleration and
deceleration of charges cause them to emit
photons at the expe
nse of their kinetic energy.
Photons so emitted are known as
bremsstrahlung radiation

(braking radiation).
Their properties are similar to those of X
-
rays.
Beta particles and positrons of very high energy
(100 MeV) lose their energy almost exclusively
by b
remsstrahlung radiation from their
interaction with fields of nuclei in the medium.

Positrons,

+
, combine with electrons to form short
-
lived systems called
positroniums,

which
decay with a half life of 0.1 microseconds into two photons each. Such a process is called
annihilation
, in which both particles disappear and their masses convert to the ene
rgy of two
photons (gamma rays). Annihilation is the main mode of interaction between the short
-
lived
positron and mater. Recall that positrons from cosmic rays were discovered not from
annihilation with electron, but from their ionization track showing th
em being deflected in the
Intensity (
I
) of Electrons with the Same Kinetic Energy
as a Function of Thickness (
x
) of Absorber
.
I
x
Extrapolated
range
Range
straggling
absorber
I
0
I
detector
I
0
x

Bremsstrahlung Radiation and its
Feynmann Diagram
E =
h v
e

.
h v
Feynmann
diagram




304


opposite direction from that of electrons. Thus, positrons also produce ion pairs before they
were annihilated by electrons.

Bremsstrahlung radiations, annihilation and
ionization are the major modes of interactions
of high
-
speed

electrons with the medium. A
diagram indicating the three interaction
modes by which the


particles lose energy in
a matter is given here.

Review Questions

1.

Estimate the velocities of electrons whose kinetic
energies are 10.0 and 0.01 MeV respectively.
Discuss the results.

2.

What are the three major mechanisms by which electrons lose their energies in a medium.

3.

Draw a Feynman diagram for the annihilation of positrons and electrons.

Electromagnetic Radiation

The energies of a photon in the deep
-
red region
and in the blue region of the visible light are
1.5 and 3.0 eV respectively. The UV radiation covers a wide energy range from a few eV to
hundreds eV. X
-
ray photons have energies in the order of keV The boundary between X
-
rays
and

-
rays is a blur one, and but in general

-
ray photons have energies in the order of MeVs.
Photons have a very wide range of energies and they interact with matter in many different
ways. Photons with energies higher than a few keV are ionizing radiation.



What are the properties of high
-
energy electromagnetic radiation?

How do they lose energy in a medium?

What are the processes by which X
-
rays and

-
ray loose energy?

The X
-
ray and

-
ray photons lose their energy in a stopping medium mainly by these major
p
rocesses:
photoelectric effect, Compton effect
, and
pair production
. Photoelectric and
Compton effects produce ion pairs, and pair production produces a pair of electron and
positron from a photon.

When all the energy (
E

=
h


) of a X
-
ray and

-
ray photon
is used to release the electron from
an atom or molecule, the process is called
photoelectric effect
. From the principle of
conservation of energy, the kinetic energy of the electron so released is equal to the energy of
the photon (
h


) minus the
binding
energy

of the electron to the atom. Binding energies of
electrons in various shells are different and X
-
ray photons can ionize inner
-
shell electrons.
Absorption of photons by photoelectric effect is the most important mode for low energy
(long wavelength)
photons, especially when the energy is just sufficient to eject an electron

Ionization
Ionization
izati
on

Braking radiation

Annihilation




305


from a particular shell of the atoms in the medium. This mode of interaction is shared by
photons of low energy, including those in the UV region.

Regarding the
Compton effect
, we
need to go back a few decade to review the study of light
scattering. Drude, Lord Rayleigh, Raman, Thomson, Debye, and others, have studied the light
scattering. They found that

1.

the scattered radiation had the same
wavelength a
s the primary rays,

1.

90
-
degree scattered rays were polarized.

However, when A.H. Compton (1926) and his
collaborators studied X
-
rays scattering, they
found the wavelength for the scattered rays a
little longer than the original X
-
rays. The
amount of length
ening depended on the
wavelength and the angle of scattering. They
did not find any polarization. When a photon
transfer part of its energy to an electron, it is scattered off from a different direction. Its
wavelength becomes longer. The process is equiva
lent to inelastic collision between photons
and electrons. Compton concluded that inelastic scattering begin to appear for photons with
energy greater than 0.51 MeV. This process is now known as the
Compton effect
, by which a
photon transfers part of its e
nergy to an electron, and the photon becomes less energetic,
resulting in a longer wavelength or lower frequency.

Suppose the spectrum of a X
-
ray beam
consists of a single peak. The spectrum of the
scattered X
-
rays at a particul
ar angle consists
of two peaks, one with frequencies of the
scattered original photons, and one with
longer wavelengths. The relative intensities of
the two peaks depend on energies of the
photon, and the material used. When photons
are scattered through a
n angle

, the
wavelength increased by an amount


,
which depends on







=

o
(1
-

cos

)


where

o

is the original wavelength. The
amount



is now called the
Compton wavelength
.

Dirac postulated the existence of antiparticles, and sort after Anderson
(1932) discovered the
antiparticle of electron called it positron. A positron and an electron annihilate each other
converting to two photons. Not exactly the reverse of annihilation, but at the vicinity of an
Feynman Diagram for
the Compton Effect

Spectra of an Original and Scattered X-rays
at a Particular Fixed Angle.
Intensity
arbitrary
scale
Original
spectrum
scattered
spectrum







306


atom, a photon creates a positron and an elect
ron at the same time from a common center.
This production of a particle
-
antiparticle pair is known as
pair production
. Pair productions
happen for photons with energies greater than (2 x 0.51 MeV =) 1.02

MeV. A single photon
disappears, converting to a pa
ir of particle and antiparticle. Due to the law of conservation of
momentum, a third body must be present for the pair production. The threshold energy (1.02
MeV) corresponds to the rest mass of an electron
-
positron pair. The residual energy (
h


-

2
m
e

c
2
)

is distributed between the kinetic energies of the pair with only a small fraction going to the
nuclear recoil.

The pair production can also occur in the field of
an atomic electron, to which considerable recoil
energy is there
by imparted. Applying the Born's
first approximation, it has been shown that
photons with 2.04 MeV or more will undergo
such a transformation. In the pair production
process a pair of particles are produced from a
bundle of light energy (one photon). This
is not
the reverse of the annihilation mechanism
between a positron and a beta particle, in which
two photons

are produced.

Photons with less than 1 MeV energies lose
their energy mostly by photoelectric process.
Photons with en
ergies between 1 and 5 MeV
lose their energies mainly by Compton
scattering. Photons with energies higher than 5
MeV lose their energy by pair production. Of
course, the three processes compete with one
another. Photons with energy with 1 MeV have
higher p
robability of losing energy due to
inelastic scattering than photoelectric effect,
and the photoelectric probability increases as
their energies decrease. The domains of the
major processes are displayed in the diagram
here. As the photon energy increases,

the dominant process shift from photoelectric, to
Compton, and to pair productions. The photoelectric effect never competes with pair
production.

Pair production is now routinely used to produce positrons and electrons for synchrotrons.
Using the same pr
ocess, protons and antiprotons are also produced.

Feynman Diagram for Pair Pro
duc
tion
A negative charge in reverse is
equivalent to a plus charge.
A nucleus or field.

Interaction of Photons with Matter
1
5/MeV
Pair
production
Photo-
electric
Compton scattering




307


A beam of


rays passing a medium loses
energy by all three mechanisms: photoelectric,
Compton and pair productions. Regardless of
the mode of interaction, the absorption of

-
ra
ys is by chance. The rate of absorption per
unit length, (
-
dI/dt
) is proportional to the
intensity,
I
, itself. The reduction of

-
ray
intensity follows the equation

where
a

is the absorption coefficient.
Expressed in another form, the intensity
I

is
reduced exponentially as a function of the thickness
x

of the medium:


I

=
I
0

e


a x

where
I
0

is the initial incident intensity. The variations of monoenergetic gamma ray (all
photons have the same energy) int
ensity as a function of the thickness
x

is shown here. The
larger the
a
, the faster the decline of the intensity. Substances containing heavy elements such
as lead and lead glass having high absorption coefficient
a

are excellent absorbers of X
-
rays
and ga
mma rays.


Review Questions

1.

Evaluate the wavelengths of photons whose energies are 1 meV, 1 eV, 1 keV, 1 MeV, and 10.0 MeV
respectively. What regions do these photons belong in the electromagnetic radiation spectrum?

2.

Describe the photoelectric effect.

3.

Desc
ribe the pair production process.

4.

Describe the Compton scattering of


rays.

5.

Why lead sheets and lead glass are used as shield for gamma ray radiation?

Interaction of Neutrons with Matter

Neutrons are heavy, uncharged particles, and they interact with elec
trons weakly due to the
magnetic moment present in both electrons and neutrons. Collisions between neutrons and
atomic nuclei are rare events, because both are tiny compared to the atoms. In elastic
collisions, the neutrons do not lose any energy. In inel
astic collisions, kinetic energy is
Intensity of Parallel Gamma Rays as a
Functi
on of
Abs
orbe
r
T
hickness.
Thickness
x
Intensity,
I


aI

=

dx
dI

-





308


transferred between neutrons and the atomic nuclei. Neutrons lose their energies in
inelastic
collisions

with the atomic nuclei, not with the electrons. In cases where the neutrons transfer
energy to the atomic nuclei, o
ne or more of the nucleons are excited to a higher level. The
excitation energy may be emitted as gamma rays.



What interactions will neutron have with material?

What type of material is effective to slow neutrons?

How can neutrons be detected?

Neutrons can
not be detected directly from their interactions with matter. However, neutrons
are detected due to nuclear reactions induced by neutrons. For example, a nuclear reaction
induced by neutron first observed is the reaction with nitrogen:




14
N + n


11
B +

,


in which case, the tracks due to the


particles were observable (Chadwick, 1935). Since boron
nuclei absorb neutrons readily, a common neutron detector makes use of the reaction,



10
B + n
-
>
7
Li +

.


A detection chamber is usually fill
ed with gaseous boron trifluoride, BF
3
, with enriched
10
B.
The detector gives out signals due to


particles.

After having received the kinetic energy from a fast neutron in an inelastic collision, the nuclei
have the ability to cause ionizing of other at
oms, causing further excitation and ionization in
the media. Since such collision is rare, the density of ion pair is very small. Thermal neutrons
have energy levels similar to the kinetic energy of atoms in the medium, and they cause much
less ionization
, if any.

Neutrons lose very little energy per collision when they collide with heavy nuclei. Nuclei of
hydrogen and neutrons have approximately the same mass. In collisions with hydrogen nuclei,
neutrons can transfer almost all their kinetic energy to the

hydrogen nuclei. Thus,
hydrogen
-
containing compounds such as H
2
O, paraffin wax, and hydrocarbons (oil and
grease) slow down neutrons rapidly.

Biological systems contain a high percentage of water, and water is an effective neutron
moderator. Fast neutron
s quickly become thermal neutrons in a biological system. Thermal
neutron capturing reactions take place even when biological materials are exposed to fast
neutrons.

We have extensively discussed nuclear reactions induced by neutrons in the chapters on
Nuc
lear Reactions and on Fission Reactions. A summary is given below. The first two
reactions take place in nuclear reactors, whereas the (n, p) and (n,

) reactions produce
radioactive isotopes. Boron, B, absorbs neutrons readily, and its reaction is often used for the
detection of neutrons.




309




1
H + n


2
D +


(capture or n,

)



238
U + n


239
U +


(n,

)



14
N + n


14
C + p (n, p)



35
Cl + n


35
S + p (n, p)



35
Cl + n


32
P +


(n,

)



200
Hg + n (10 MeV)


197
Pt +





197
Pt


197
Au +




200
Hg + n (10 MeV)


198
Au + p




198
Au


198
Hg +


Web Sites About Neutron Detector

Ultra High Sensitivity Fissio
n Counter (UHSFC):
http://www.ic.ornl.gov/HTML/ic94130.html

ROMASHKA
-

multipurpose instrument for measuring neutron cross sections, neutron
resonance parameters and gamma
-
multiplicities in interactions of neutrons with nuclei (with
nice diagrams):
http://nfdfn.jinr.dubna.su/flnph/usersgui/romash.htm


Review Questions

1.

Aside from neutron induced reactions, how do neutrons interact with atoms and molecules in a medium?

2.

What neutron reaction i
s commonly used for detecting neutrons?




310


Radiation Detectors

Radioactivity was discovered from images left on silver bromide photographic plates. New
particles were discovered from the study of their tracks in hydrogen bubble chambers and
cloud chambers. I
onization of radiation was inferred from their discharging of electroscopes.
Photographic plates, bubble chambers, cloud chambers, and electroscopes are radiation
detectors. In this section, we shall discuss the principles of these and some other detectors
.
Most detection methods are based on the ionization in the medium.

Ionization Chamber

Ionizing radiation produces ion pairs in a gas, and the ion pairs do not recombine until the
energies of the electrons have dissipated. Thus, there are positive ions and

negative electrons in
a gas medium when exposed to radiation.



How can ionizing radiation be detected?

How do ionizing radiation detectors work?

In a gas, ions and electrons move freely as do the gas molecules. If we place two electrodes
connected to a ba
ttery in the medium, the electrodes afford an electric field, which causes the
electrons to drift towards positive electrode, and the positive ions towards the negative
electrode. Such an arrangement is
ionization chamber

for the detection of radioactivity

When the ionization chamber is exposed
to a source of ionization radiation as
shown here, the drifting of electrons and
ions will make the
detector chamber

a
conductor. Thus, a current is registered
on the ampere
-
meter. The ion
ization
chamber is a simple detector for
radioactivity.

An ionization chamber is a little more
sophisticated than an electroscope for the
detection of radioactivity. Rutherford
placed a thorium oxide (ThO
2
) sample
directly in the detector chamber. He
noti
ced that the radioactivity increased with time after a sample was just put in. After removing
the sample carefully without changing the air in the chamber, he found the air remained
radioactive. The radioactive air decayed with a specific half life. Other
radioactive samples did
not give the same observation. His experiment suggested the existence of a radioactive gaseous
element. The experiment was later interpreted as due to the decay of Th (,

) Ra (,

) Rn.
Several isotopes of Th produce radon isotopes
in their decays.

Key Component
s
in a Simple Io
niz
ation Ch
am
ber
–+–+–
+–+–+
+

Ampere-
meter
Detector
chamber
Ionizing
radiation
Battery
Load
resister




311


Of course, one uses the most sensitive ampere
-
meter in setting up the ionization chamber. The
current registered in the ionization chamber is proportional to the number of ion pairs
generated by radioactivity. Thus, the higher the radioact
ivity, the higher the current. Ionization
chambers quantitatively measure radioactivity. The voltage supply enables the electrodes to
collect electrons and ions, but the current is mainly determined by the number of free
electrons in the chamber, not on th
e voltage. However, depends on the electrode
arrangements and chamber geometry, the voltage must be sufficiently high for effective
collection of electrons.

Review Questions

1.

How does an ionization chamber work?

What are the key components in the ionizatio
n chamber?

2.

What are the isotopes of Rn from decays of
227
Th,
228
Th,
230
Th?

Which isotopes of Rn has the shortest half life?

(The merit of this exercise is to know how to find information in problem solving.)

Proportional Counter

Discovery is an importa
nt goal of scientific research, and methodologies for doing research are
constantly under development. Soon after getting ionization chambers to work, improvements
are made. The improvement in the instrumentation leads to new phenomena, making research
and

development an interesting adventure.



How can the sensitivities of ionization chambers be improved?

What happens when the voltage is increased?

At some hundreds volts, the improvement in sensitivity is more than collecting all the When
voltages applied to electrodes of ionization chambers increase, the sensitivities increase.
electrons and ions on the electrode. The currents corr
esponding to multiples of ions and
electrons produced by radioactivity. To distinct them from simple ionization chambers, these
detectors are called
proportional counters
.

In proportional counters, the high voltage applied to the
electrodes created a stron
g electric field, which not only
collect but also accelerate electrons. The energies acquired
from the electric field by electrons accumulate and they are
used to ionize other molecules, producing secondary ion
pairs, initiating an avalanche of ionization
by every a single
electron generated by radiation. Such a process is called
gas
multiplication
.

It should be noted, however, that the small mass and high
energy of electrons make them drift 100,000 times faster
Gas Multiplication



+




+

+

+




+

+

+

+

+

+

+

+

+




+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+





312


than ions. Thus, the current is mainly due to

the drifting electrons with only a small fraction
due to the drift of ions.

Despite the multiplication due to secondary ion pairs, the ampere
-
meters register currents
proportional to the numbers of primary electrons caused by radiation entering the detect
ors.
Thus, currents of proportional chambers correspond to amounts of ionization radiation
entering the proportional chamber.

When voltages applied to proportional counters get still higher, sparks jump (arcs) between the
two electrodes along the tracks of

ionizing particles. These detectors are called
spark
chambers
, which give internal amplification factors up to 1,000,000 times while still giving an
initial signal proportional to the number of primary ion pairs.

Review Questions

1.

How does an ionization ch
amber work?

What are the key components in the ionization chamber?

2.

Assume that 35 eV is required to create an ion pair.

If the applied voltage is 200 V, what is the amplification factor for the proportional counter?

Geiger
-
Muller Counters

Perhaps the mos
t widely used radioactivity detectors are Geiger
-
Muller (often called Geiger)
counters. More precisely these counters detect numbers of ionization particles entering the
detector. Unlike the ionization chambers or proportional counters, Geiger counters do
not
reveal numbers of primary ion pairs in their detecting chambers.



What is the working principle of Geiger
-
Muller Counters?

Geiger
-
Muller counters evolve from proportional counters. When the voltages applied to the
ion chamber of the ionization chamber r
each more than 1,000 V, as few as one primary ion
pair in the chamber causes a spark (arc). Whenever an ionization particle (including photons)
enters the chamber, a primary ion pair causes a single spark. The charges jumped over the
electrodes depend on t
he number of primary ion pairs, but we often are more interested in the
numbers of particles entering the chamber.

A spark causes a temporary conduction in the detector chamber. The voltage across the two
electrodes drops during the sparking period, but a
current flowing across a resister causes the
voltage between points on both sides of a resister to increases (
V

=
i R
). The sudden drops or
increases in voltage are called
pulses
. Geiger counters count the number of pulses, and this
can easily be achieved
by electronic means. The counters can also be designed to give an
audible signal for each pulse.





313


A Geiger counter usually refers to an instrument consisting of
a detector, a high voltage
supplier
, and
an electronic pulse counte
r
. Usually, audio and meter outputs are parts of an
instrument. When the audio output is switched on, the Geiger counter gives a clicking sound
whenever a pulse is registered. The frequency of the clicks is proportional to the radioactivity
(Bq) of the sou
rce. The meter output is very similar to an ampere
-
meter. The current is
proportional to the frequency of the pulses, however, not related to the energy of the particles
entering the detector.

Geiger
-
Muller counters count the
number of radiation events, n
ot
the energy of the ionizing particles.
At this level of operation, the
number of counts per unit time
from a steady source is
independent of the voltage applied
to the electrode. To insure the
stability and uniformity of the
detector, a voltage in the mi
ddle of
a range of voltages that gives a
steady number of counts from a
steady radiation source is usually
chosen.

Geiger
-
Muller counters are usually
used to detect X
-
, gamma
-

and
beta
-
rays. Alpha particles have a limited range, and they may not be able to

enter the chamber
to cause any ionization of the gas in it. Thus, alpha particles may escape detection by Geiger
-
Muller counters.

Due to their high sensitivity, Geiger
-
Miller counters are useful for geological survey, personnel
monitoring, tracking of rad
ioactivity movement, and radioactivity detection. The frequency of
clicks is proportional to the radioactivity of the source, and the audio output frees the visual
sense for other purposes. However they can not differentiate


sources from

-
ray sources,
a
nd other detectors are required for proper characterization of radioactivity.

Geiger counters count pulses. After each pulse, the voltage
has to return to a certain level before the next pulse can be
counted. Thus, after each pu
lse, there is a period called
dead time

during which radiation can not be detected. The
length of the dead time depends on the gas mixtures used
in the detector, and on the sophistication of the electronics.
When the source has a very strong radioactivity,

the pulses
generated in the detectors are very close together. As a
result, the Geiger counter may register a zero rate. In other
words, a high radioactive source may overwhelm the
Geiger counter, causing it to fail. When you use a Geiger
Working Components of a Geiger
Muller Counter
1500 V
supplier

+
Detector
Source
Geiger-
Muller Counter:
Pulse counting electronics

Dead Time in Pulse Counting
Dead time




314


counter for a su
rvey, keep this in mind. The zero reading from a Geiger counter provides you
with a (false) sense of safety when you actually walk into an area where the radioactivity is
dangerously high.

Ionization chamber, proportional counter, spark chamber and Geiger
-
Muller counters are
similar in design and construction. Different voltages applied to the detector chambers make
them perform differently. Depending on the applied voltage, the characteristic of the detector
changes.


Review Questions

1.

How does a Geiger
-
M
uller counter work?

What are the key components in a Geiger counter?

2.

Why there is a dead time in Geiger counters?

What caution should be exercised in using Geiger counters for survey work?


Solid
-
state Detectors

Solid
-
state detectors are for accurate mea
surements of radiation energy. They are based on
ionization, but they are very different from ionization chambers, proportional counters, spark
chambers and Geiger counters.



What solids are used for solid
-
state detectors?

How do solid state detectors work?

Semiconductors such as silicon and germanium are used for solid state detectors. Every atom
in the crystal is bonded to four other atoms throughout the entire crystal. However, usually
doped semiconductors are used as detectors. Signals in solid states af
ter receiving ionization
radiation are processed by electronic means.

In a solid semiconductor, atoms are fixed in their locations. Electrons are tightly bound to
atoms or chemical bonds. Pure semiconductors have some free electrons and holes due to
therma
l motions or defects. Electrons are negative charge carriers, whereas holes are positive
charge carriers. Both electrons and holes are responsible for the small conductivity of
semiconductors, but movement of hole is much slower than that of electron in a
solid.

Energy required to free an electron from the valance band into the conduction band is called
the
band gap
, which depends on the material: diamond, 5 eV; silicon, 1.1 eV; germanium,
0.72 eV. At room temperature, the thermal energy gives rise to 10
1
0

carriers per cc. At liquid
nitrogen temperature, the number of carriers is dramatically reduced to almost zero. At low
temperature, it is easier to distinguish signals due to electrons freed by radiation from those
due to thermal carriers.




315


Doping semicon
ductors is a process by which some atoms of the crystal are replaced by other
type of the atoms. For example, atoms of a germanium crystal are replaced by atoms of
phosphorus. A phosphorus atom has one more electron than the host atoms, and the
phosphorus
doping adds negative carriers in the crystal creating a
negative (N) junction
.
Similarly, doping with impurities deficient in electron adds positive carriers to the region,
forming a
positive (P) junction
. Solids with a N or P junction is called a diode,
and those
with both a N and a P junction are called transistors. Electrons and holes in transistors do not
belong to particular atoms. They belong to either the valence band or the conduction band.
Electrons in the conduction band move easily under the inf
luence of an electric field.


The electronics used to analyze the pulses does more than counting. It separates the pulses
into hundreds of gro
ups called channels according to the pulse heights. Therefore, the
equipment is often called multi
-
channel analyzer. When intensities of these channels are
displayed according to their energies, the measurement gives a spectrum. A gamma
-
ray
spectrum of
56
C
o measured using a solid state detector is shown above. The continuous
background is due to Compton scattering. Single and double escape peaks (marked SEP and
DEP) are also shown.


Gamma ray spectrum of
207m
Pb (half
-
life 0.806 sec)


207m
Pb Decay Scheme




13
/
2
+____________1633.4 keV

-
1e5




1063

-
1e4


569



5
/
2
-
____________569.7 keV


1063

-
1e3


. .

569





1
/
2
-
____________0.0 stable

-
1e2







-
10
569 + 1063








-
1




316


The modern instruments for X
-
ray and gamma ray detection use doped solid d
etectors. A P
-
N
junction of semiconductors is placed under reverse bias, thus no current flows. Passage of
ionizing radiation through the depleted region excites electrons into the conduction band,
causing a temporary conduction which gives rise to a pulse

corresponding to the number of
excited electrons or energy entering the solid state
*
.


Review Questions

1.

What are semiconductors?

What elements are used to dope semiconductors to make N and P junctions?

2.

What advantages do solid state detectors have over
proportional and Geiger counters?


Scintillation Counters and Fluorescence Screens

Scintillation counters are commonly used for X
-
rays and gamma rays. The name suggests that
the working principle for scintillation counters is not based on ionization, but b
ased on light
emission.



What is the working principle of scintillation counters?

Photons striking a sodium iodide (NaI) crystal, which contains 0.5 mole percent of thallium
iodide (TlI) as an activator, cause the emission of a short flash of light in the
wavelength range
of 3300
-
5000 A (in the ultraviolet region). The light flashes are detected by a photomultiply
tube, which gives a pulse corresponding to the light intensity. These pulses are measured by a
multi
-
channel counter.




*

Low level radiation sensor seeking industrial collaborators to develop and commercialize futu
re low
-
level
radiation sensor systems: Germanium detector provides high
-
resolution data has been developed. See web site:
http://www.llnl.gov/sensor_technology/STR11.html




317


The Key Componen
ts o
f a Typical Scintilla
tio
n Coun
ter
High voltage
supplier and
multi-channel
analyzer /
computer
system
Photomultiply tube
Photo-
cathode
Na(
Tl)I
crystal
Thin Al
window
X- or

rays


The output pulses from a scintillation counter are proportional to the energy of the radiation.
Electronic devices have been built not only to detect the pulses, but also to measure the pulse
heights. The measurements enable us to plot the i
ntensity (number of pulses) versus energy
(pulse height), yielding a spectrum of the source.

Scintillation counters and solid state detectors are used to determine the energy of the
incoming particles. The former uses a doped NaI crystal, which may be kept

at room
temperature, and needs no special care. Most solid
-
state detectors must be maintained at low
temperatures (cooled by liquid nitrogen or liquid helium) to achieve excellent resolution,
i.e.
, to
distinguish radiation particles of various energy. Pul
ses from solid
-
state and scintillation
detectors are counted by multi
-
channel analyzers or computers. Each measurement gives a
spectrum of the source of radiation.

Review Questions

1.

How does a scintillation counter work?

What does it measure and what type
of results is obtained.

2.

What are the advantages of solid
-
state detectors and scintillation counters for radiation measurement?

Fluorescence Screens

J.J. Thomson used fluorescence screens to see electron tracks in cathode ray tubes. In 1895,
Röntgen saw the

shadow of his skeleton on fluorescence screens. His screen was made of
barium
-
platinocyanide. Rutherford observed alpha particle on scintillation material zinc sulfide,



318


ZnS. Fluorescence screens are important detectors for ionizing radiation and high ener
gy
photons.

Fluorescence material absorbs invisible light and the energy excites the electron. De
-
exciting of
these electrons results in the emission of visible light. By mixing different materials together,
we have engineered many different fluorescence
materials to emit lights of any desirable
colors.

Fluorescence screens are convenient detectors of high
-
energy radiation. They are used in many
other applications such as fluorescence tubes, UV detectors, computer and TV screens and
movie screens. Even la
undry detergents contain fluorescence material to emit blue light to
make the cloth whiter than white after washing with them.

Review Questions

1.

What is a fluorescence material?

Give some applications of fluorescence material.

2.

How do TV screens work?

Cloud
and Bubble Chambers

Studies of particle tracks have led to the discovery a zoo of particles. Cloud and bubble
chambers have contributed greatly to these discoveries. These chambers show tracks of
ionizing particles.



Why do cloud and bubbles form along the
trail of these particles?

Where did the ideas of using cloud and bubble chambers to record the tracks of particles
come from?

Cloud and bubble chambers for the detection of radiation particles are based on the ionization
effect of energetic particles. How
ever, working functions for cloud and bubble chambers are
different from ionization chambers, proportional and Geiger counters.




319


The ion pairs on the tracks of ionizing radiation form
seeds of gas bubbles and droplets. Formations

of
droplets and bubbles provide visual appearance of
their tracks. Therefore, cloud and bubble chambers are
called
path
-
, 3
-
dimensional
-
, or
track
-
detectors
. In
the good old days, photographs of these tracks were
taken for detailed analyses. In modern sci
ence, high
-
energy particle track detectors are built using
sophisticated electronics and computers for the study
of particle behavior.

The story on the development of cloud chambers is
fascinating because the chambers were originally
studied for a complete
ly different reason than the
study of radiation. Furthermore, a young man at his
teens initiated the studies.

At age 15, the Scottish physicist C.T.R. Wilson (1869
-
1959) spent a few weeks in the
observatory on the summit of the highest Scottish hill Ben Ne
vis. He was intrigued by the
color of the cloud droplets. He also learned that droplets would form around dust particles.
He built apparatus according to an earlier study of Coulier and Aitken to expand moist air to
study the formation of cloud. Between 18
96 and 1912, he found dust
-
free moist air formed
droplets at some over
-
saturation points. He repeated the experiments with the same air and
found cloud drops always form at some saturation points. He concluded that although dust
particles were nucleation c
enters of cloud drops, there was
something else

that would also
nucleate cloud drops. His meticulous experiments showed that these centers were always
present in air, and he considered them
ions

rather than dust particles. He further suspected
that these i
ons were produced by energetic particles, and he was determined to confirm that.

The news of Röntgen's discovery reached Wilson, who also learned of J.J. Thomson's
investigation of air conductivity due to X
-
rays. He set up his cloud chamber apparatus in f
ront
of an X
-
ray tube, and after the X
-
rays were turned on, he expanded the air in the chamber. To
his astonishment, he found many cloud
-
like small drops, not the rain
-
like large drops as he
usually saw. The X
-
rays have created a large number of cloud nucl
eation centers. This marked
the beginning of his research in trying to perfect an apparatus for the detection of ionizing
particles. He carefully designed the apparatus so that the expansion will not disturb the air,
leaving the tracks of ionizing radiatio
n undistorted. This apparatus enabled scientists to study
tracks of radiation. Moreover, the tracks marked by cloud
-
like droplets can be seen,
photographed, studied, reported, and published.

The perfection of the cloud
-
chamber techniques had a much farther

impact in the
development of nuclear science and particle physics. Later using oil vapors, Milikan studied the
force exerted on a drop by an electric field, and determined the amount of a fundamental
charge (of an electron). Cloud chambers showed

-
particle tracks being fatter and shorter than
those of


particle, and they enabled scientists to study behavior of particles under the
influence of electric and magnetic fields. Tracks in cloud chambers revealed the


rays
Photographing the Particle Track
s
Cloud or bubble chamber
radia
tion




320


(electrons ejected by


partic
les), origins of secondary electrons, ranges of


and


particles,
variations of ionization along the tracks, charge densities of ionization, and absorption of X
-
rays by atoms.

Using the cloud
-
chamber technique, Compton discovered that high energy photons
gave
portions of their energies to electrons, and they became less energetic with longer wavelength.
This is now known as Compton scattering. Compton and Wilson shared the 1927 Nobel prize
for physics. Furthermore, using the cloud chamber while working as

Rutherford's student,
P.M.S. Blackett (1897
-

1974) studied elastic collisions of


particles with atoms, and
transmutation of nitrogen when bombarded by


particles,
14
N (

, p)
17
O. The


tracks were
fatter than the proton tracks, and the angles of deflection agreed with his calculated results.
Blackett attached Geiger counters on both

sides of large cloud chambers to catch the tracks of
elusive cosmic rays. This further development led to other achievements including Anderson's
discovery of positrons, and the visual demonstration of the processes of pair production and
annihilation of
electrons and positrons. Blackett (1948) received the 1948 Nobel prize in
physics "for his development of the Wilson cloud chamber method, and his discoveries
therewith in the fields of nuclear physics and cosmic radiations". The cloud chamber
contribute
d to the discovery of the transmutation of atomic nuclei carried out by Cockroft
(1951) and Walton (1951). Rutherford once remarked that “
the cloud chamber was the most original
and wonderful instrument in scientific history
.” The cloud chamber had been
evolved into a
continuously sensitive detector by diffusing warm vapor into a cool chamber.

Like the formation of droplets from saturated vapor, the formation of bubbles in a liquid also
requires nucleation, without which overheating results. Ion pairs due

to radiation serve as
nucleation centers, and the tiny bubbles mark their tracks in
bubble chambers
, which were
developed by the U.S. physicist Donald A. Glaser (1926
-
). He began his research in elementary
particles, some of which had energy in the order
of GeV (10
9

eV), and the diffusion cloud
chambers he constructed could not covered the entire tracks. In order to keep the chambers to
a reasonable size and yet covered the entire track, Glaser (1960) thought of using a
superheated liquid to observe the en
tire track of his particles. He contributed to both the
theories of bubble nucleation and engineering of instruments. Among the liquid he had used
were diethyl ether, propane, xenon, and hydrogen. The idea of using bubble chamber was
quickly adopted by oth
ers. The success of the bubble chambers is marked by the discovery of
a large number of new particles and phenomena. Precise information on masses, spins,
lifetimes, parity, and decay had been determined. The importance of these developments was
highlighte
d by the Nobel Prize for physics awarded to him at age 34 (in 1960). The citing of
the prize was
for his invention of the bubble chamber
.




321


Tracks of single particles and their decay
products have been recorded in
bubble
chambers
.

For example, when an
antiproton,
p
, entered a propane bubble
chamber, it underwent a charge exchange
reaction with a proton, p, to give a neutron,
n, and an antineutron
n
,


p

+ p


n

+ n(charge exchange)

A moment later, the antineutron
annihilated with another neutron giving a
star of tracks due to the many particles in
the reaction. Agnew
et al.
, (1958) suggested
the five tracks to be due to

+

and

-

pions
i
n the reaction,



n

+ n


3

+

+ 2


.

A sketch of the tracks as seen in a propane bubble chamber is shown here. The pions have a
mass of about 140 MeV, and a life time of 2.6 x 10
-
8

s. They were discovered by Cecil F.
Powell and his c
o
-
workers in 1947. The discovery of pions in the cosmic radiation used yet
another track detectors using photographic emulsions, which will be discussed in the next
section.

Millions of particle
-
track photographs have been taken using the bubble chambers

for the
study Each photograph contains many tracks, and each has to be analyzed. Almost all tracks
are left by particles already well known and understood. The few tracks of new particles are
buried in trillions of tracks. Their discoveries are getting ha
rder as more and more have been
discovered.

Review Questions

1.

What cause the formation of droplets in clouds?

And what causes the formation of bubbles in overheated fluids?

2.

Why bubble chambers can cover the entire track whereas cloud chambers can not?

Photo
graphic Emulsions and Films

Everyone knows that when photographic films are exposed to light, the silver bromide grains
of the emulsion are sensitized and they developed into blackened grains. From the stories of
discoveries of radioactivity and X
-
rays, yo
u have also learned that photographic emulsions
played important in nuclear technology. Yet, we often forget to count photographic films and
emulsions are detectors of radiation. In fact, they are two
-
dimensional detectors. When several
A Sketch of the Tracks of Charge Exchange
and Antineutron-Proton Annihilation.
antiproton
Charge
exchange
Antineutron-
neutron
annihilation



+




322


films are stacked t
ogether to record particle tracks directly, these are three
-
dimensional
detectors. On the other hand, dentists still use films to record X
-
ray images of teeth.

Photographic films and emulsions of various speed and sensitivity towards light. Special films
have been made for X
-

and

-
rays. Often auxiliary devices such as fluorescence screens are
used in medical applications. When photons of high energy strike the fluorescence screens,
visible lights are emitted that give images on the films. Images on film a
re permanent, and they
may be reinvestigated.

An emulsion sensitive to fast moving protons was independently developed by Zhdanov in
Leningrad and by Ilford Laboratories in 1935. The method has some success, but it is not
widely used because it did not gi
ve constant ranges for energy calculation. This is probably due
to the consistency of particle size in the emulsion.

Review Question

1.

Give some examples where photographic films are used for the detection of ionizing radiation.

(Discoveries of X
-
rays, radio
activity, and many high
-
energy particles are made via using
photographic plates)




323


Exercises

1.

Most significant scientific discoveries require instruments for their dectection. However,
instruments alone were insufficient. What are other ingredients for sc
ientific discoveries?
Describe some examples of discoveries, including instruments used experiment
performed.

2.

At 273 K and 1 atm pressure, 1.0 mol of N
2

occupies 22.4 L. Calculate the number of N
2

molecules in 1.0

L. At 277 K, 1.0 L of water weighs 1.0
kg. Calculate the number of
moles of water in 1.0 L. Calculate the number of H
2
O molecules in 1.0 L at 277 K. The
density of lead (
Z
, 82; at. wt. 207.2) is 11.29

g/mL, calculate the number of atoms in 1.0 L.
(1.0 mole of N
2

an avogadro's number, 6.02 x
10
23
, of N
2

molecules).

3.

Calculate the velocities of the alpha
-
particle, the proton, and the electron if they all have
the following kinetic energies:10.0 MeV, 1.0 MeV, 1.0 keV, 100 eV, and 1 eV.

4.

Calculate the ratios of the rates of energy loss (stopping po
wer) for the alpha
-
particle, the
proton, and the electron by using the simple equation:





-

(
dE
/
dx
) = k (
M z
2

/
E
).


Assume these particles have the same kinetic energy.

5.

Explain the three processes by which beta particles lose energy when they pass by a

nucleus.

6.

Gamma
-
rays lose their intensity exponentially,




I

=
I
0

e
-
a x
.

where
Io

is the initial incident intensity,
x

is the distance in the medium,
a

is the
absorption coefficient, and
I

is the intensity of gamma
-
rays after passing the medium by a
distance of
x
. Find the distance required to reduce
I

to 0.25
I
0
., express this result in terms
of
a
.

7.

For Al (aluminum) and Pb (lead),
a

= 0.1 and 10 respectively for certain gamma rays.
Calculate the ratio of half thickness (
i.e.
,
I

= 0.5
Io
) for these
two substances.

8.

Describe the mechanisms by which the gamma rays interact with matter. Explain each
mechanism separately.

9.

Explain how a Geiger
-
Muller counter works.

10.

How do bubble chambers and cloud chambers work? What are the similarities and
differences b
etween bubble chambers and cloud chambers?

11.

What roles do photographic films and emulsions play for the detection of ionizing
radiation?




324



Reference and Further Reading


Agnew, L.E., Eliot, T., Fowler, W.B., Gilly, L., Lander, R., Oswald, L., Powell, W., Seg
rè,
Steiner, H., White, H., W. Wiegand, C., and Ypsilantis, T. (1958),
Phys. Rev.

110
, 994.

Brackett, P.M.S. (1948)
Cloud chamber researches in nuclear phyusics and cosmic radiation
, in
Nobel
lectures, physics
.
3
. Published for the Nobel Foundation in 1965

by Elsvier Publishing Company.

Chadwick, J. (1935),
Neutron and its properties
, in
Nobel lectures, physics
.
3
. Published for the Nobel
Foundation in 1965 by Elsvier Publishing Company.

Cockcroft, J.D. (1951)
Experiments on the interaction of high
-
speed nu
cleons with atomic nuclei
, in
Nobel
lectures, physics
.
2
. Published for the Nobel Foundation in 1965 by Elsvier Publishing Company.

Walton, E.T.S. (1951)
Artificial production of fast particles
, in
Nobel lectures, physics
.
3
. Published for
the Nobel Founda
tion in 1965 by Elsvier Publishing Company.

Compton, A.H. (1926),
X
-
rays and electrons
, Chap. 9, Nostrand

Compton, A.H. (1927),
X
-
ray as a branch of optics
, in
Nobel lectures, physics
.
2
. Published for the
Nobel Foundation in 1965 by Elsvier Publishing Com
pany.

Glaser, D.A. (1960),
Elementary particles and bubble chambers
, in
Nobel lectures, physics
.
3
. Published
for the Nobel Foundation in 1965 by Elsvier Publishing Company.

B.G. Harvey,
Introduction to nuclear physics and chemistry
, 2nd Ed., Prentice Hall
, 1969.

Z. M. Haissinsky,
Nuclear chemistry and its applications
, Addison
-
Welsley, 1964

Wilson, C.T.R. (1927),
On the cloud method of making visible ions and the tracks of ionizing particles
, in
Nobel lectures, physics
.
2
. Published for the Nobel Foundatio
n in 1965 by Elsvier Publishing
Company.