Topic 3 Pulsehight Spectrometry

1
Topic 3 Pulse-hight Spectrometry

• Basic Principle
• Spectrometry with NaI(Tl)
• Spectrometry with Other Detectors

2
Basic Principles

• The amplitude of a pulse is proportional to the
  radiation energy therefore pulse-hight spectrum
  is energy spectrum
• Pulse-hight spectrum is a display showing number
  of events detected (counts) versus the amplitude
  of those events.
• The spectrum recorded from a radiation source
  depends not only on the energy of the emissions
  but also on the type of detector used, as well as
  the mechanisms of radiation energy deposition.
• The amplitude of the signal from a detector
  reflects only the amount of energy deposited in
  it by the radiation event.

3
(No Transcript)
4
Spectrometry With NaI(Tl)

• The Ideal Pulse-Hight Spectrum
• The Actual Spectrum
• General Effects of ?-Ray Energy
• Effects of Detector Size
• Effect of Counting Rate
• Energy Resolution
• Energy Linearity

5
The Ideal Pulse-Hight Spectrum

• Foe simplicity, we assume mono-energy ? rays and
  Elt1.02MeV (no paired production).
• Pulse hight is proportional to the photon energy
  DEPOSITED into the detector.
• Compton Scattering electron has a maximum energy
  (at 180o)

6
(No Transcript)
7
The Actual Spectrum for Cs

• 30 KeV Barium x-ray peak (radioactive daughter
  of Cs)
• Spectrum smeared out (imperfect energy
  resolution)
• Backscattering Peak (180o scattered photons
  re-enter the detector and be detected)

8
(No Transcript)
9
Iodine Escape Peak

• Absorption by Iodine K-shell (30 KeV), emitted as
  x rays which are escaped from the detector.
• Escape peak at E?-30 keV shown in the spectrum.
• Often occurs for low energy photons and at the
  detector entrance (easy for x-rays to escape).
• Lead x-ray peaks occur at systems with lead
  shielding and collimation (80-90 keV x-rays enter
  the detector - different mechanism from Iodine
  absorption).

10
(No Transcript)
11
Pair Production Interaction

• Pair Production occurs when the input ?-ray
  energy exceeds 1.02 MeV.
• If both of the pair electrons are absorbed by the
  detector, it shows at the photopeak.
• If one of the pair electrons escaped from the
  detector, it shows at the Single Escape Peak.
• If both of the pair electrons escaped from the
  detector, it shows at the Double Escape Peak.

12
(No Transcript)
13
Effect of Object Scattering

• Photon scattering within or around radiation
  source (such as radiation within patients) is
  called Object Scattering.
• The effect of object scattering is to add events
  in the lower energy region of the spectrum (lost
  some energy before reaching the detector)

14
(No Transcript)
15
Coincident Summing

• Occurs when a radionuclide emits two or more ?
  rays from single disintegration.
• Prominent in detector system with high geometric
  efficiency, such as well counter.
• Summing also occurs between x and ? rays as well
  as two 511 KeV annihilation photons

16
(No Transcript)
17
General Effects of ? Ray Energy

• With the increase of photon energy, Compton
  scattering increases.
• With the increase of ? ray energy, it becomes
  easier to separate object scatter from the
  photo-peak (change of Compton scatted ? ray
  energy increases)

18
(No Transcript)
19
Effects of Detector Size

• Larger detector size helps the absorption of
  scattered ? rays and annihilation photons. This
  leads to more counts at photo-peak and less
  counts in Compton scattering region, and less
  annihilation escape if the ? ray energy is
  greater than 1.02MeV.

20
(No Transcript)
21
Effects of Counting Rate

• Hight counting rates cause pulse pile-up and
  baseline shift.
• Pulse pile-up between photo-peak and lower energy
  events cause a general broadening of the
  photo-peak (on the right hand side).
• Baseline shift causes the shift of the photo-peak
  towards the lower energy region.

22
(No Transcript)
23
Energy Resolution (1)

• Reasons of energy broadening from a sharp line
  (1) statistical variation in (a) numbers of
  photons produced per KeV, (b) numbers of the
  photons detected at the cathode of the PM tube,
  (c) numbers of photoelectrons released from the
  photocathode, (d) electron multiplication factors
  of dynodes. (2) Non-uniformity over the area of
  the PM tube (3) fluctuation of high voltage
  supply (4) electrical noise in PM tube.

24
Energy Resolution (2)

• The principle source of variation for NaI(Tl) is
  the number of electrons released from the
  photocathode.
• The average number is about three per KeV for
  NaI(Tl).
• Variation follows Poisson (or Gaussian)
  statistics.

25
Energy Resolution (3)

• Energy resolution is measured as full width at
  half maximum (FWHM) and is often expressed as
  percentage of the photo-peak energy

26
(No Transcript)
27
Energy Resolution (4)

• The higher the ? energy, the better energy
  resolution (more photoelectrons therefore better
  statistics)
• The resolution of NaI(Tl) detector is usually
  specified for 662 KeV ? ray from Cs (6.5-7 for
  Cs and 10-13 99mTc)
• Degradation of energy resolution is often caused
  by the poor light coupling between the NaI(Tl)
  crystal and PM tubes.

28
(No Transcript)
29
Energy Linearity

• Energy linearity refers to the proportionality of
  the pulse amplitude and the absorbed energy in
  the detector.
• In general, it is quite linear for NaI(Tl)
  detector from 0.1 to 2 MeV photons but there
  could be trouble if the system is calibrated by
  using single low energy and attempt to use for
  high energy acquisition, and vice versa.

30
Energy Linearity
31
Spectrometry With Other Detectors

• Liquid Scintillation Spectrometry
• Proportional Spectrometers
• Semiconductor Detector Spectrometers

32
Liquid Scintillation Spectrometry

• Liquid scintillation detectors are used primarily
  for counting low energy ß emissions from 3H,
  14C,35S, 45Ca and 32P
• Poor energy resolution for ? rays (fewer
  scintillation photons produced and poor optical
  coupling)
• The energy of ß emission from zero to a maximum
  (shared with neutrino).

33
(No Transcript)
34
Proportional Counter Spectrometers

• Limited application in Nuclear Medicine
• Energy resolution is better than NaI(Tl) but
  poorer than semiconductor detectors.
• Poor detection efficiency of ? rays is the major
  problem in nuclear medicine application.

35
Semiconductor Detector Spectrometers

• Super energy resolution (20-80 times better than
  NaI(Tl) and 6-9 time better than proportional
  counter).
• Large number of electrons produced by the ? rays
  (one per 3 eV in comparison with 3 electrons per
  KeV in NaI(Tl)).
• High energy resolution comes from the large
  electron numbers (good statistics, small
  variation).

36
(No Transcript)
37
Cadmium Zinc Telluride (CZT)
38
Monte Carlo Simulation
39
Experimental Acquisition