Nuclear Medicine

Question 1

Structure of the atomic nucleus

Answer 1

• Protons + Neutrons
• Similar mass
• Mass number is P + N
• Atomic number is only P

Question 2

Alpha decay

Answer 2

• Atomic nucleus emits an alpha particle (Helium)
• Decays into different atomic nucleus
• Mass number reduced by 4
• Atomic number reduced by 2

Question 3

Energy spectra of alpha, beta and gamma radiations

Answer 3

• Alpha spectrum: Line, all excess energy goes to alpha particle that releases from unstable nucleus.
• Beta spectrum: Continuous, because emitted particles are electron/positron and neutrino/antineutrino.
• Gamma spectrum: Line, because gamma radiation is packed in quanta

Question 4

Stability of the atomic nucleus

Answer 4

• Depends on Proton/Neutron ratio
• Isotopes can be stable or unstable
• Unstable atoms will decay until they are stable

Question 5

Beta negative decay

Answer 5

Unstable nuclei convert a neutron into a proton, electron and antineutrino

Question 6

Production of isotopes

Answer 6

• Some isotopes are unstable, so emit radiation
• Radiation can be harvested for medical use
• Isotopes produced in nuclear reactors by bombardment of stable nuclei with high energy particles.
• Tc generator can be used specifically when gamma radiation isotope of short half life is required.

Question 7

Definition and types of isotopes

Answer 7

Two or more atoms that have the same atomic number but different atomic mass (dif. Number of neutrons)

Question 8

Beta positive decay

Answer 8

• Positron and antineutrino are emitted.
• Proton in nucleus becomes a neutron
• Daughter nucleus will have a smaller atomic number but same mass number.

Question 9

Types of radioactive decay

Answer 9

• Alpha
• Beta +/-
• Gamma

Question 10

Gamma decay

Answer 10

• Product of excited nucleus that tried to achieve stable state.
• Products of either alpha/beta decay or of some other nuclear process.

Question 11

Activity

Answer 11

Rate of decaying atoms per unit time. (Becquerel)
1Bq = 1 decay/second

Question 12

Interaction of alpha radiation with matter

Answer 12

• Linear Ion density: Amount of ions produced over a certain length decreases after the particle loses its energy.
• Effective range: Distance covered by particle until energy is lost. (Mass and charge influence this)
• Beta particle is much smaller so it has a higher effective range, but much lower linear ion density.

Question 13

Interaction of gamma radiation with matter I: photoeffect

Answer 13

• Gamma-photon removes an electron from the inner shell of an atom while being absorbed.
• K.E of electron = incident photon energy

Question 14

Interaction of gamma radiation with matter II: Compton-scatter

Answer 14

Gamma-Photon removes electron from outer shell “Compton electron” and a photon is emitted.

Question 15

Interaction of gamma radiation with matter III: pair production

Answer 15

High energy gamma-photon is absorbed near the nucleus and electron and positron are created.
Process called annihilation where 2 gamma photons will release.

Question 16

Differential and integral forms of the decay law

Answer 16

Integral: N/T (1/s) (Bq)
Differential: N(t) = N^-(1/s)*t

Question 17

Interaction of beta negative radiation with matter

Answer 17

• Directly ionizing atoms by coulomb’s force
• Scatter on electrons resulting in zigzag path
• Braking radiation (X-ray)
• More penetration compared to alpha (Lower mass and charge)

Question 18

Half-life and average lifetime of an isotope

Answer 18

Average life: time required for number of undeclared nuclei to decrease to 1/e of initial amount.
Half life: Amount of time taken for undecayed nuclei to decrease to 1/2 of initial amount.

Question 19

Interaction of beta positive radiation with matter

Answer 19

• Radiative particle is positron
• Collides with electron in annihilation, emitting 2 gamma rays

Question 20

Neutron radiation, proton radiation, the Bragg-peak

Answer 20

• Neutron radiation: Excited nucleus expels a neutron. Neutron does not ionize directly so it collides and energy exchanges to atom. (Elastic scatter, inelastic collision, neutron capture)
• Proton radiation: Acts similar to alpha particle. (Large mass, shorter effective range)
• Bragg peak: Relationship between penetration depth and amount of radiation deposited.

Question 21

Scintillation counter I.: the scintillation crystal

Answer 21

• NaI crystal (thallium activated by dropping)
• Has to be transparent to emission wavelength
• Absorbs energy of radiation

Question 22

Scintillation counter II.: the photomultiplier tube

Answer 22

A tube with a photocathode, converting scintillations from crystal to electrons (photoeffect).
- Electrons multiplied by dynodes before reaching anode.
- Electrons accelerated by a voltage through the tube
-Every collision with dynode produces secondary electrons
- Multiplication factor corresponds to number of secondary electrons.