29. Nuclear measurement technique.

Question 1

Describe the parts of a scintillation counter and their function

Answer 1

Consists of three parts:

1. Scintillator: Th activated NaI crystal converts E_kin of charged particles into E_excitation of the scintillator → fluorescence: photons → ground state
2. Photomulitplier tube (PMT): photoelectrode converts light (scintillations) via photoelectric effect to flow of e- multiplied further by secondary e- emission between the dynodes of the PMT → output voltage an be measured
3. Electronics: sort amplitudes and eliminate noise caused by electronics, or background interferences (i.e. radioactive stain, cosmic rays, walls, furniture, etc.)

Question 2

Describe the basic principle of gas-ionization detectors and explain the differences between measuring devices. Refer to the diagram below

Answer 2

Gas-ionization detectors:

1. U is connected to 2 electrodes
2. radiation entering the chamber ionizes the noble gas molecules
3. charges are attracted towards the oppositely charged electrode
4. movement of charges → current is measured → change of charge (exposure) is proportional to the dose (can be calculated)

Difference between measuring devices:

U applied to electrodes (none of devices work when too low U is applied → recombination (ion⁺ + e^- → atom)

Question 3

Explain the function of an ionization chamber

Answer 3

Ionization chamber:

1. U high enough → no recombination
2. measures amount of U pulses (change of charge) produced by radiation (→ exposure → dose)

Question 4

Explain the function of a Geiger-Müller (GM) tube

Answer 4

Geiger-Müller (GM) tube:

1. high U between the electrodes accelerates charges
2. gain E to further ionize → avalanche of charges (primary, secondary, tertiary ion pairs)
3. current can be measured → **no. of particles **

**BUT: **relatively insensitive to γ-radiation, but extremely sensitive to α-, β-radiation

Question 5

Explain the function of a thermoluminescent dosimeter (TLD). Refer to the diagram below

Answer 5

Thermoluminescent dosimeter (TLD):

contain one or more impurities (Mg or Mn) to produce trap states for energetic e^-

1. radiation → e^- in the crystal’s atoms jump to higher E states, where they stay trapped until heated
2. heating → e^- to drop back to their ground state, releasing a photon of E equal to ∆E between the trap state and the ground state (= luminescence)

Question 6

Explain the function of a semiconductor detector and the advantages compared to a gas detector

Answer 6

Semiconductor detectors:

1. Ionizing radiation → free e^- and holes (e^- from valence to conduction band, holes in valence band)
2. Electric field → e^- and holes travel to the electrodes
3. Pulse can be measured
4. no. of e^--hole pairs proportional to J of **incident radiation **

Advantages:

• E required to produce e^--hole-pairs very low compared to E required to produce paired ions in gas detectors → variation of the pulse height smaller and E resolution higher
• e^- travel fast → very good time resolution