8. Nuclear medicine Flashcards
(42 cards)
What is scintillation counter?
a device that detects, counts and determines the energy distribution of particles or photons of the incident radiation.
3 parts of scintillation counter
(1) Scintillator
(2) Photomultiplier
(3) Electronics
What is SCINTILLATION?
an ionizing particle (e.g., - particle) or a high-energy photon (e.g., X-ray or -photon) causes a light flash (scintillation) in the material of the scintillator that can be used for detecting that particle or photon.
The role of Scintillator
In the scintillator material the energy of the particle (or photon) is converted totally or partially into the energy of scintillation light.
In the scintillator material the energy of the particle (or photon) is converted totally or partially into the energy of scintillation light.
-> the conversion is ___ (direct/indirect) (ionization) in the case of charged particles (e.g., )
direct
The role of PHOTOMULTIPLIER TUBE (PMT)
an amplifier device that consists of a photocathode, a dynode array and an anode. It converts the weak incident light into a measurable electric current.
The role of Electronics
amplifies the electric pulses, makes amplitude analysis and counts the selected pulses.
In the scintillator material the energy of the particle (or photon) is converted totally or partially into the energy of scintillation light.
-> the conversion is ___ (direct/indirect) (ionization) in the case of charged particles (e.g., )
direct
SCINTILLATOR
In the scintillator material the energy of the particle (or photon) is converted totally or partially into the energy of scintillation light.
-> the energy of X-ray- and gamma-photons is converted in a ___ process.
two-step
SCINTILLATOR
In the scintillator material the energy of the particle (or photon) is converted totally or partially into the energy of scintillation light.
- > the energy of X-ray- and gamma-photons is converted in a two-step process.
- > Describe this process
First, photoeffect, Compton scattering, or pair production occurs (Fig. 2), which generate charged particles (electrons).
Second, the generated charged particles evoke the scintillations.
SCINTILLATOR
which type of scintillation crystal is widely used for the detection of X-rays and gamma-rays.
thallium-activated (“doped”, 0.1-0.5 % Tl) NaI crystal
What happen in scintillator?
The kinetic energy of primary electron produced in secondary processes (photoeffect, Compton scattering, pair production) is dissipated during its zigzag pathway via ionizations, and it is converted into the excitation energy of the scintillator.
→ The scintillator returns from the excited state to the ground (stable) state via the emission of fluorescence photons (scintillation).
→ Energy of the emitted fluorescence photons is determined by the excitation energy levels of the activation material (Tl) (Fig. 3). (blue light photons)
The scintillator returns from the excited state to the ground (stable) state via the emission of fluorescence photons (scintillation).
→ Energy (“color”) of the emitted fluorescence photons is determined by the excitation energy levels of the activation material (Tl) (Fig. 3).
→ to be able to detect the scintillation light, what is the characteristics of photocathode?
the scintillation crystal has to be transparent at the emission wavelength
→ In the case of NaI(Tl) crystal, it is transparent to the blue light (3 eV).
Why is the time resolution of the scintillation (by NaI(Tl)) very good?
Flashes of the NaI(Tl) scintillator are very fast, shorter than 1 s.
PHOTOMULTIPLIER TUBE (PMT)
What does PMT consist of?
- A photocathode
- 8 to 14 dynodes
- An anode in a vacuum tube,
PHOTOMULTIPLIER TUBE (PMT)
A PMT consists of a photo- emissive cathode (photocathode), 8 to 14 dynodes and an anode in a vacuum tube, as shown in Fig. 4.
→ Describe structure of photocathode
a semitransparent photosensitive layer evaporated on the inner surface of the glass window of the PMT.
PHOTOMULTIPLIER TUBE (PMT)
A PMT consists of a photo- emissive cathode (photocathode), 8 to 14 dynodes and an anode in a vacuum tube, as shown in Fig. 4.
→ Describe Operation of photocathode
It converts light coming from the scintillator to the flow of electrons via photoelectric effect.
→ Photoelectrons emitted into the vacuum are directed towards a metal plate (dynode) which is at positive voltage compared to the photocathode.
PHOTOMULTIPLIER TUBE (PMT)
A PMT consists of a photo- emissive cathode (photocathode), 8 to 14 dynodes and an anode in a vacuum tube, as shown in Fig. 4.
→ Describe structure of dynodes
metal plates of the dynodes are connected to positive voltage which progressively increases towards successive dynode stages away from the photocathode.
PHOTOMULTIPLIER TUBE (PMT)
A PMT consists of a photo- emissive cathode (photocathode), 8 to 14 dynodes and an anode in a vacuum tube, as shown in Fig. 4.
→ Describe function of dynodes
Electrons, accelerated between the photocathode and the first dynode, or between the dynodes, hit the dynode and are multiplied by secondary electron emission.
→ Electric field between the dynodes drives the electron avalanche from one dynode to the other.
PHOTOMULTIPLIER TUBE (PMT)
A PMT consists of a photo- emissive cathode (photocathode), 8 to 14 dynodes and an anode in a vacuum tube, as shown in Fig. 4.
→ Describe operation of anode
collects the multiplied electrons
PHOTOMULTIPLIER TUBE (PMT)
A PMT consists of a photo- emissive cathode (photocathode), 8 to 14 dynodes and an anode in a vacuum tube, as shown in Fig. 4.
→ What is the relationship between Anode current and incident light intensity?
They are proportional to each other
PHOTOMULTIPLIER TUBE (PMT)
Anode: collects the multiplied electrons. Anode current is proportional to the incident light intensity (number of incident photons).
→ What is the relationship between Amplitude of this pulse and the energy of the incident gamma-photon?
Amplitude of this pulse is proportional to the energy of the incident -photon (in case, gamma-photon was absorbed by photoeffect).
PHOTOMULTIPLIER TUBE (PMT)
Inter-stage voltages for the dynodes are supplied by (1)___
The photocathode consisting of (2)___
- a voltage-divider circuit between the anode (1-2 kV
- series-connected resistors.
PHOTOMULTIPLIER TUBE (PMT)
As secondary electron emission depends on the (1)____
gain of the PMT is set by the (2)___
- voltage between the dynodes
- anode voltage.
