Basic Atomic Structure, Radioactive Decay

Please answer all questions True or False. There is
no negative marking.

1. Concerning the nucleus of atoms
a. It is made up of protons and electrons
b. It is made up of protons and neutrons
c. Nucleons are held together by the strong nuclear force
d. The mass number represents the number of protons
e. The number of protons and neutrons is always equal

The nucleus of an atom is composed of protons and neutrons. The charge
of protons is +1 and it is 0 for neutrons. The strong nuclear force is
responsible for holding the nucleus together. The mass number represents
the number of protons and neutrons, whereas it is the atomic number
that represents the number of protons only. The number of protons and
neutrons is not always equal, and for higher atomic number elements
neutrons tend to outnumber protons.

1a. False – it is made of protons and neutrons
1b. True – it is made of protons and neutrons
1c. True – the strong nuclear force holds nucleons (protons and neutrons) together
1d. False – the mass number is the number of protons and neutrons
1e. False – the number of protons and neutrons is not always equal

2. Concerning Electrons
a. In the Bohr model of atomic structure electrons orbit the
nucleus
b. The electron has +1 charge
c. The binding energy of the L-shell is higher than the
K-shell
d. The K-shell can hold 2 electrons
e. Electrons have a greater mass than protons

In the Bohr model of the nucleus electrons orbit the nucleus in discrete
energy shells. These energy shells start with the letter K and increase
alphabetically, e.g. K,L,M,N,O, etc. Each shell holds a certain number
of electrons. The energy required to remove an electron from its shell is
referred to as the Electron Binding Energy. This is greatest for the inner
shell electrons (K-shell) and decreases the further away the electron is from
the nucleus. Electrons have a smaller mass than protons, at approximately
9.1 ¥ 10–31 Kg. The mass of a proton is approximately 1836 times greater
than this
.
2a. True – in the Bohr model electrons orbit the nucleus
2b. False – the electron has a charge of -1
2c. False – the binding energy for the K-shell is higher than for the L-shell
2d. True – the K-shell can hold 2 electrons
2e. False – protons have much greater mass than electrons

3. Concerning the atomic of tungsten
a. It has an atomic number of 74
b. It has a physical density of approximately 19
c. The K-shell binding energy of tungsten is 20
d. The mass number of tungsten is 284
e. It is represented by the letter W

Tungsten is an important element in radiology as it is used to produce
X-rays. It is represented by the letter W. It has an atomic number of 74 and
mass number 184 (110 neutrons). The K-shell binding energy of tungsten
is 69.5 KeV. Molybdenum has a K-Shell binding energy of 20.0 KeV.

3a. True – atomic number is 74
3b. True – tungsten has a physical density of approximately 19
3c. False – the K-shell binding energy for Tungsten is 69.5 KeV
3d. False – the mass number for Tungsten is 184
3e. True – it is represented by the letter W on the periodic table

4. Concerning the isotopes of an element
a. They have the same number of neutrons
b. They have the same physical properties
c. They have the same chemical properties
d. All isotopes are stable
e. I123 decays by emitting gamma rays

Isotopes of an element have the same number of protons and different
numbers of neutrons. They have similar chemical properties and different
physical properties. Not all isotopes are stable and hence they can be used
in imaging using radio-nuclides. Iodine 123 decays by emitting 160 KeV
Gamma rays and it is used in the imaging of the thyroid gland.

4a. False – isotopes have different numbers of neutrons
4b. False – they have different physical properties
4c. True – they have the same chemical properties
4d. False – not all isotopes are stable
4e. True – I123 decays by gamma emission

5. Concerning nuclides
a. Unstable nuclides are called radionuclides
b. Nuclides with the same number of protons are called
Isotopes
c. Nuclides with the same number of neutrons are called
Isobars
d. Isobars have the same atomic mass numbers
e. An Isomer is a nucleus in an unexcited state
Isotopes have the same number of protons. Isotones have the same number
of neutrons. Isobars have the same atomic mass numbers. An Isomer is
the excited state of a nucleus.

5a. True – unstable nuclides are termed radionuclides
5b. True – isotopes have the same number of protons but a different number of neutrons
5c. False – nuclides with the same number of neutrons are called
isotones
5d. True – isobars have the same atomic mass number
5e. False – an Isomer is a nucleus in an excited state

6. Regarding radioactive half-life
a. It is constant for a particular radionuclide
b. Decay is a stochastic event
c. Half-life is defi ned as the time taken for half the material to
decay
d. Half-life is directly proportional to the Decay Constant
e. One Curie is one transformation per second

Radioactive half-life is the time taken for half the material to decay. The
decay constant is equal to 0.693/half-life. The Activity is the number of
transformations in unit time. The Becquerel is 1 transformation per second.
The Curie is 3.7 ¥ 1010 transformations per second. Radioactive decay is
an exponential process and as such the activity will NEVER become 0.

6a. True – for a particular radionuclide the half-life is constant
6b. True – radioactive decay is a stochastic event
6c. True – this is the defi nition
6d. False – it is inversely proportional to the half-life
6e. False – a Becquerel is one transformation per second

7. Concerning Alpha decay
a. It occurs only in light nuclei
b. It results in the atomic number decreasing by 4
c. The alpha particle is equal to the hydrogen nucleus
d. Alpha particles have an energy between 4 and 7 MeV
e. It does not occur in nature

The alpha particle consists of 2 neutrons and 2 protons. It is the equivalent
of a Helium nucleus. It generally occurs with heavy atoms with atomic
numbers greater than 82. Alpha particles have a high energy and as such
are very damaging to living tissues. They carry a charge of +2 and can
travel up to 10cm in air. In tissues they travel less than 0.1 mm. They
have an energy between 4-7MeV. They cause the atomic number to fall
by 2 and the atomic mass number to fall by 4.

7a. False – it occurs in heavy nuclei
7b. False – the atomic number decreases by 2
7c. False – the alpha particle is the equivalent to a helium nucleus
7d. True – they do have an energy between 4-7MeV
7e. False – alpha decay does occur in nature

8. Concerning beta plus decay
a. It is also called positron emission
b. It occurs in nuclei that are neutron rich
c. The atomic number decreases by 1
d. A neutron is emitted with the positron
e. A positron has a charge of – 1

Beta plus decay is also called positron emission decay. A positron is an
electron with a +1 charge and is a form of anti-matter. It occurs in nuclei
which are neutron poor. A proton is converted to a neutron and positron
which is ejected from the atom. In addition to the positron a neutrino
(not a neutron) is ejected.

8a. True – beta plus decay is also called positron emission
8b. False – it occurs in neutron poor nuclei
8c. True – the atomic number decreases by 1
8d. False – the neutron is not emitted
8e. False – a positron has a charge of +1

9 Regarding positrons
a. Positrons have a mass equal to electrons
b. They annihilate with electrons and release one 511 KeV
photon
c. Fluorine18 (F18) is a positron emitter
d. Elements that decay by positron emission have long halflives
e. Positrons only exist while they have kinetic energy

Positrons only exist while they have kinetic energy. When they come to rest
they spontaneously annihilate with an electron. The mass of the positron
and the electron is converted into two photons with energy of 511 KeV
that are emitted in exact opposite directions (180 degrees apart.)
Some important positron emitters are F18, O15 and C11. Positron emitters
generally have short T1/2.

9a. True – positrons and electrons have the same mass but opposite
charge
9b. False – two photons are released
9c. True – F18 is a positron emitter
9d. False – they tend to have short half-lives
9e. True – positrons only exist while they have kinetic energy

10. Concerning beta minus decay
a. Phosphorus32 (P32) is a pure beta minus emitter
b. Beta minus decay occurs in neutron-rich radionuclides
c. The electrons are emitted from the orbital shells
d. The atomic number increases by 1
e. An anti-neutrino is emitted alongside the electron

In beta minus decay, a neutron in the nucleus is converted to a proton.
An electron and an anti-neutrino are ejected in the process. This process
occurs in neutron-rich nuclei. The electrons released have a wide range of
energies up to a maximum dependent on the emmiting nuclide. The mass
number remains the same, but the atomic number will increase by 1.

10a. True – P32 is a pure beta minus emitter
10b. True – it occurs in neutron rich radionuclides
10c. False – the electrons are ejected from the nucleus
10.d True – the atomic number increases by 1
10.e True – an anti-neutrino is ejected alongside the electron
Basic Atomic Structure,

11. The following are part of the Electromagnetic (EM) spectrum
a. Sound waves
b. Microwaves
c. Ultraviolet light
d. Alpha particles
e. X-rays

The Electromagnetic spectrum is a continuum stretching from Radio waves
to Gamma and X-rays. Electromagnetic radiation travels in straight lines
at the speed of light – c (3 ¥ 108 m/sec in a vacuum). An electromagnetic
wave is made up of two waves, a electric wave and magnetic wave that
oscillate perpendicular to their direction of motion and at 90 degrees to
each other. These are examples of transverse waves.

11a. False – not part of EM spectrum
11b. True – part of the EM spectrum
11c. True – part of the EM spectrum
11d. False – represents 2 neutrons and 2 protons
11e. True – part of the EM spectrum

12. Concerning electromagnetic radiation
a. It travels at a constant speed independent of the matter
through which it travels
b. Wavelength and frequency are directly related
c. The product of wavelength and frequency is constant
d. Electromagnetic radiation exists as photons
e. X-rays and gamma rays always have different frequency and
wavelength

Electromagnetic radiation exists in discrete packages of energy called
Photons. These photons can behave as waves and particles, so-called ‘Wave
Particle Duality.’ The energy of a photon is related to the wavelength and
frequency.
Photon energy E = h ¥ f where f = frequency and h is Planck’s
Constant.
Since f = 1/wavelength, the energy is directly related to the frequency
and inversely related to wavelength.
Electromagnetic radiation does travel at a constant speed, but this speed is
related to the medium through which it travels. The product of wavelength
and frequency is equal to the speed of light, c.
X-rays and gamma rays cannot be distinguished in terms of frequency
and wavelength. They are called X-rays if they are produced by electron
interactions and gamma rays if they are produced by nuclear reactions.

12a. False – the speed is dependant on the matter
12b. False – wavelength and frequency are indirectly related
12c. True – constant is 1
12d. True – electromagnetic radiation exists as photons
12e. False – they cannot be distinguished by frequency and wavelength
Basic Atomic Structure,

13. Regarding electron capture
a. It occurs in radionuclides with a neutron defi cit
b. It is most likely with valence electrons
c. A neutron combines with the electron to produce a proton
d. The atomic number decreases by 1
e. I123 decays wholly by electron capture

In this form of decay a proton captures an electron from one of the shells
and is converted into a neutron. It occurs in nuclei that are neutron poor.
The electron is most likely to be absorbed from the K-shell then from the
L, M, N, etc. The loss of the electron from the shell leaves a hole which
needs to be fi lled. This hole can be fi lled by an electron from another
shell. This process can cause the emission of K characteristic X-rays.
I123 decays by electron capture and produces 160 KeV gamma rays and
28 KeV X rays.

13a. True – it occurs in neutron poor radionuclides
13b. False – it is most likely to occur with K-shell electrons
13c. False – an electron combines with a proton to form a neutron
13d. True – since a proton is converted into a neutron
13e. True – after electron capture gamma and X-rays are produced

14. The following are correctly paired
a. Alpha decay decreases the mass number by 4
b. Beta minus decay increases the atomic number by 1
c. Electron capture increases the atomic number by 1
d. Beta plus decay increases the number of neutrons by 1
e. Alpha decay decreases the atomic number by 2

There are several decay modes. In alpha decay the radionuclide emits an
alpha particle that consists of two neutrons and two protons (a helium
nucleus). This is most common in atoms with atomic numbers above 82.
The loss of 2 neutrons and 2 protons causes the mass number to decrease
by 4 and the atomic number to decrease by 2.
In beta minus decay, a neutron inside the nucleus is converted into a
proton, and the excess energy is released in the form of a beta particle
and an antineutrino. Beta minus decay occurs in nuclei that are neutron
rich. Hence, in beta minus decay the atomic number increases by 1 but
the mass number remains the same.
In beta plus decay (also known as positron emission) a proton inside the
nucleus is converted into a neutron and the excess energy is emitted as a
positron (a positively charged electron) and a neutrino. Beta plus decay
tends to occur in nuclei with too few neutrons. In beta plus decay, the
atomic number decreases by 1 and the mass number remains the same.
Electron capture occurs when a proton inside the nucleus is converted
into a neutron by capturing an electron from one of its’ atomic shells. A
neutrino is emitted in the process. It occurs in nuclei with too few neutrons
and may compete with beta plus decay. If the electron is captured from
the K-shell, an outer shell electron fi lls the vacancy left in the K-shell.
The excess energy is emitted as either an Auger electron or characteristic
X-rays. Electron capture results in a decrease in the atomic number by 1
and an increase in the mass number by 1.

14a. True – the mass number decreases by 4 in alpha decay
14b. True – beta minus decay increases the atomic number by 1
14c. False – electron capture decreases the atomic number by 1
14d. True – beta plus decay increases the number of neutrons by 1
14e. True – alpha decay decreases the atomic number by 2
Basic Atomic Structure,

15. The Linear Energy Transfer (LET)
a. LET represents the energy deposited in tissue per unit
length
b. High LET radiations are easily stopped
c. Gamma rays have lower LET than neutrons
d. Alpha particles have the highest LET
e. High LET radiation damage is more likely to be nonrepairable

The Linear Energy Transfer is a way of expressing the energy deposited in
a medium per unit path length. Larger particles and more heavily charged
particles are more easily stopped. These large charged particles have a
higher LET since they deposit their energy in a shorter length. Alpha
particles have the highest LET followed by neutrons, beta particles and
X-Rays, and gamma rays have the lowest LET.
15a. True – this is a basic defi nition for the linear energy transfer
15b. True – larger, more highly charged particles are more easily
stopped
15c. True – gamma rays have a lower LET than neutrons
15d. True – alpha particles are the heaviest and highest charged therefore
have the highest LET
15.e True – the damage is more severe because of the higher energy
deposited

16. Radioactive material
a. Does not occur in nature
b. Is important in radiology
c. Decays to produce sound waves
d. Decays to a level of 0
e. Decays by a factor of approx. 250 times after 8 half lives

Radioactive material does occur in nature from many sources. Sound waves
are not produced in the decay process, but electromagnetic radiation may
be produced. Radioactive material will never reach a level of 0 because
the decay is exponential. After 8 half-lives the level of radioactivity will
be 256 times lower than the initial level.

16a. False – radioactive material is found in nature
16b. True – it forms the basis of nuclear imaging
16c. False – it does not decay to produce sound waves
16d. False – it never reaches 0 but becomes asymptotic with 0
16e. True – 28 is 256

17. The following are positron emitters
a. Carbon 11
b. Indium 111
c. Nitrogen 13
d. Oxygen 15
e. Fluorine 18

Positron emitters are not widely used in medical radiology. They are
cyclotron-produced. A cyclotron bombards nuclei with high energy protons
to create unstable nuclei that are proton rich. These will then tend to decay
by Positron emission (beta plus emission).

17a. True – emits positrons
17b. False – emits gamma rays
17c. True – emits positrons
17d. True – emits positrons
17e. True – emits positrons

18. Metastable states
a. Are stable
b. Must have a T1/2 longer than 10–12 seconds
c. Can be generator-produced
d. Can decay by isomeric transition
e. Both the parent and daughter nuclides may have the same
atomic number and mass number after decay

Metastable states are unstable. They are represented by a lower case ‘m’
after the mass number. By defi nition a metastable state must have a halflife
over longer than 10–12 seconds. They can be generator-produced, and
Technetium 99 m is an important generator-produced radionuclide used in
medical radiology. After decay by isomeric transition the mass number,
atomic number and neutron number all remain unchanged.

18a. False – they are unstable
18b. True – this is part of the defi nition
18c. True – they can be produced in a generator
18d. True – they can decay by isomeric transition
18e. True – after isomeric decay atomic number, mass number and neutron
number are unchanged

19. I123 and I125 have different
a. Chemical properties
b. Physical properties
c. Numbers of neutrons
d. Numbers of protons
e. Numbers of valence electrons if uncharged

Isomers have different physical properties and the same chemical properties.
They have different numbers of neutrons and the same number of protons.
Change in the number of valence electrons will cause the atoms to become
ions not isomers.

19a. False – isotopes have the same chemical properties
19b. True – isotopes have different physical properties
19c. True – isotopes have different number of neutrons
19d. False – isotopes have the same number of protons
19e. False – isotopes will have the same number of valence electrons if
uncharged

20. Regarding the Inverse Square Law
a. The intensity will be 1/8 if the distance doubles
b. It applies to all electromagnetic radiation
c. It represents energy conservation principles
d. Applies to all sources of radiation
e. Can be used in protection radiation

The Inverse Square Law is an important principle in physics. For a Point
source it states that the intensity will be proportional to the square of the
distance from the point source. So if the distance doubles the intensity
will decrease by 4 times, etc. The law applies to all radiation from the
Electromagnetic spectrum as long as it is a point source. It does represent
a conservation of energy principle. The inverse square law is useful in
radiation protection, as the radiologist can relatively easily increase the
distance between themselves and the radiation source and hence cut the
intensity they are exposed to.

20a. False – the intensity will 1/4
20b. True – the inverse Square law applies to all electromagnetic
radiation
20c. True – it represents energy conservation
20d. False – it apples to all radiation from a point source
20e. True – it is very important in radiation protection