Atomic structure and production of radiotherapy

Question 1

<p>What does Z in the atomic nomenclature indicate?</p>

What about N?

Answer 1

<p>Total protons</p>
Total neutrons (N = A-Z)

Question 2

<p>What does A in the atomic nomenclature indicate?</p>

Answer 2

<p>Total nucleons (protons + neutrons)</p>

Question 3

Define isotope

Answer 3

Some number of Protons, different nucleons.

Ex: I-125 and I-131 are isotopes

Question 4

Define isotone

Answer 4

Same number of Neutrons, different protons

Ex: not really used

Question 5

Define isobar

Answer 5

Same number of nucleons, different proportion
Bar = barbell = mass
Ex: I-131 and Xe-131

Question 6

What are the four fundamental forces, in declining strength?

Answer 6

Strong nuclear > Electromagnetic/Coulombic > Weak nuclear > gravity

Question 7

What equation converts energy to mass and vice versa?

Answer 7

E=mc2

Question 8

Which number is larger, the atomic mass or the sum of the particles, why?

Answer 8

Atomic mass is slightly smaller as some of that mass is converted to the binding energy of the nucleus

Question 9

What is the mass (AMU) of an electron?

Answer 9

0.005 AMUs (1/2000th of a proton)

Question 10

What is the energy equivalent (MeV) of 1 AMU? 1 electron?

Answer 10

931.5 MeV, 0.511 MeV

Question 11

What is the expected relationship of the neutron:proton ratio?

Answer 11

The ratio is approx 1:1 as the nucleus becomes larger (Z=20, Calcium). For elements heavier, n:p >1

Question 12

What is the significance of the n:p ratio above Bismuth (Z=83)

Answer 12

N:P ratio >1, up to a point where every increase yields a decreasing binding energy per nucleon. Above 83, the nuclei become unstable and decay

Question 13

What impacts the electron binding energy of an atom?

Answer 13

The closer to the nucleus, the higher the BE. The more charge in the nucleus (Z), the higher the BE.

Question 14

How is an Auger electron formed? What is its energy?

Answer 14

When an inner orbital electron is lost, an outer orbital electron drops into that shell. Characteristic energy is lost in the outer-inner transition. If this energy leaves the atom, it is a photon ‘characteristic x-ray’. In some cases, this excess energy is transferred to another orbital electron, giving it enough energy to eject. The ejected electron is an ‘Auger electron’.

The kinetic energy of an Auger electron is the difference in binding energy between the outer-inner shells (initial energy released) minus the binding energy of the auger electron.

Question 15

Describe alpha decay

Answer 15

Since particles are stable in pairs, a large heavy atom emits a pair of neutrons and protons (2n, 2p, He). The decay energy is split between the daughter nucleus and alpha particle (typical energy is 2-8 MeV)

Question 16

Describe beta particle decay

Answer 16

A nucleus is unstable and wants to convert a neutron to a proton or vice versa. In either decay, mass is not lost (isobaric). A positron/electron is produced, and an anti-matter particle.

Question 17

What is the beta-minus decay?

Answer 17

A neutron turns into a proton. To preserve charge, an electron is released (hence the -minus decay) as well as an antineutrino (antimatter)

Question 18

What is beta-positive decay?

Answer 18

A proton turns into a neutron. An anti-electron (positron) and neutrino are formed.

Question 19

What happens with the particle produced in beta-positive decay?

Answer 19

A positron is released, which annihilates with a nearby electron almost immediately. With annihilation, all mass is converted to energy. Since the rest mass energy of each particle is 0.511 MeV, two 0.511 MeV photons are produced

Question 20

What is electron capture?

Answer 20

Similar to beta-positive decay, but with lower available energy. An orbital electron is consumed by the nucleus, joining a proton to become a neutron. This releases a neutrino (just like beta-positive decay) and a photon. That photon may be emitted (gamma ray) or transferred to an orbital electron ejecting it (internal conversion electron). With the empty inner shell, a second characteristic x-ray (or auger electron) can be produced as well. If an IC electron is ejected, this can produce another characteristic x-ray.

Question 21

Explain internal conversion

Answer 21

The nucleus has extra energy, and transfers this to an inner shell electron. The inner shell electron is ejected, leaving an unfilled shell. The outer shell electron fills this, producing characteristic x-rays (or Auger electrons)
Ex; I-125 decays by absorbing an inner shell electron –> Te-125 + 35.5 keV. That 35.5 keV is transferred to another orbital electron, lets say with a BE of 8.5 keV –> empty shell + 27 keV electron.

Question 22

What is the value of 1 Ci is dps? In Bq?

Answer 22

1 Ci = 3.7 e10 disintegrations per second
1 Bq = 1 dps
Thus, 1 Ci = 3.7 e10 Bq
1 mCi = 37 MBq

Question 23

What is the equation for exponential decay?

Answer 23

A(t) = Ao e^-(lambda t)

Question 24

What is the equation for half life?

Answer 24

t1/2 = 0.693/t

Question 25

What is the equation for mean half life?

Answer 25

tavg = 1.44(t1/2)

Question 26

What is secular equilibrium?

Answer 26

The daughter half-life is much shorter than the parent, producing a bottleneck. After buildup, the daughter activity and half-life are the same as the parent

Question 27

What is transient equilibrium?

Answer 27

The daughter half-life is similar to the parent, thus the daughter activity builds up as the the parent decays.

Question 28

How are frequency, wavelength, and light related?

Answer 28

c = v lambda

Question 29

What is the equation for energy of a photon?

Answer 29

E = hv
E(keV) = 1.24/lambda

Question 30

What approximate energy is needed to ionize?

Answer 30

10-30 keV

Question 31

How do electrons produce x-rays?

Answer 31

As electrons bombard a target, their close approximation with matter (high Z material) causes deceleration and loss of photons (‘braking radiation’, brehmsstrahlung)

Question 32

What is the significance of the angle in an x-ray tube target?

Answer 32

X-rays preferentially come out at a right angle; thus a slight angle will allow a small ‘focal spot’.

Question 33

How are electrons produced/modulated in an electron gun?

Answer 33

A tungsten filament is heated, giving off electrons by thermionic emission. The speed of these electrons can be increased by increasing the tube voltage. The frequency of emission can be increased by increasing the current through the filament.

Question 34

What is the function of the klystron?

Answer 34

Low power microwave are used to ‘bunch’ electrons in a drift tube. As they exit the tube they emit high powered microwaves. Thus, a klystron amplifies microwaves.

Question 35

What is the role of a waveguide?

Answer 35

The waveguide carries high powered amplified microwaves to the accelerator tube, where they then accelerate electrons toward the speed of light

Question 36

What is the sequence of applicances in a treatment head

Answer 36

Target > flattening filter > primary collimator > ion chamber > secondary collimator > MLCs

For electrons, the target/flattening filter are replaced by a scattering foil

Question 37

What is a microtron?

Answer 37

This is a specialized linear accelerator where electrons travel in a circle. Each pass around the circle re-enters them into the linear accelerator to pick up more energy. Electrons of a desired energy (up to 1500 MeV!) can be collected by moving the collection duct to various loop diameters

Question 38

What is a cyclotron?

Answer 38

Alternating D-shaped electrodes accelerate a particle in the setting of a static perpendicular magnetic field. Primarily used to accelerate protons

Question 39

What is a synchotron?

Answer 39

Essentially the LHC; a long loop with magnetic segments where the segments can be alternated to accelerate a particle to GeV energies.