RADIATION DETECTION AND MEASUREMENT Lecture 03 Shahid Younas
Transcript
1. RADIATION DETECTION AND MEASUREMENT Lecture 03 Shahid
Younas
2. Once upon a time.. Lecture 03 Gas Filled Detectors Detection
Efficiency
3. Gas Filled Detector- GM Counters Lecture 03 1. Radium Hen
Detectors 2. Based on gas-amplification as that proportional
counters. 3. Billions of ion-pairs; additional electronic
amplification (increased voltage) is required. 4. Used as
inexpensive survey meters.
5. GM Detectors- Characteristic & Usage Lecture 03 1.
Inefficient detectors of x-rays and gamma rays. 2. Size of voltage
pulse is independent of the incident energy. 3. Cant be used as
spectrometers and dose-rate meter.
6. Gas Filled Detector- GM Counters Lecture 03 Do you know how
the Proportional Counter differ from GM Counter? Chain of
Avalanches
7. Gas Filled Detector- GM Counters Lecture 03 In the
proportional counter, each original electron leads to an avalanche
which is basically independent of all other avalanches formed from
other electrons associated with original ionizing event.
8. Gas Filled Detector- GM Counters Lecture 03 In the GM
counter, substantially higher electric fields are created that
enhance the intensity of each avalanche.
9. Gas Filled Detector- GM Counters Lecture 03 Under proper
conditions, a situation is created in which one avalanche can
itself trigger a second avalanche at a different position within
the tube.
10. Gas Filled Detectors Lecture 03 Do you know how many ion
pair are produced once a radioactive tracer hits the GM tube. 109
to 1010
11. Gas Filled Detector- GM Counters Lecture 03 1. GM survey
meters can not truly measure exposure rates. 2. Their reading must
be considered as crude approximation. 3. Calibration source of 662
keV. 4. Compensated detectors.
12. Gas Filled Detector- GM Counters Lecture 03 GM Counters
have the highest dead time amongst the detectors. Please someone
guess the reason.
13. Scintillation Detectors Lecture 03
14. Scintillation Detectors Lecture 03 Scintillators are
materials that emit visible or ultraviolet light after the
interaction of ionizing radiation with the material.
15. Scintillation Detectors Lecture 03 Roentgen discovered
x-radiation. Do you know the material in which x-radiation were
induced by Roentgen? Scintillator Detector Barium
Plantino-cyandide
16. Scintillation Detectors Lecture 03 Scintillators are used
in conventional film-screen radiography, many digital radiographic
receptors, fluoroscopy, scintillation cameras, most CT Scanners and
PET Scanner. Scintillation detectors consist of a scintillator and
a device, such as a PMT, that converts the light into an electrical
signal.
17. Scintillator Detectors Lecture 03 1. Amount of light
emitted after an interaction increases with energy deposited by the
interaction. 2. May be operated in pulse mode as spectrometers. 3.
High conversion efficiency produces superior energy
resolution.
18. Development- Anger Gamma Camera Lecture 9: Nuclear
Imaging-I What is the suitable photon energy range of usage of
gamma camera? 100 to 200 keV
19. Scintillator Detectors Lecture 03 Luminescence,
Fluorescence and phosphorescence are synonyms of word scintillator.
Do you know the difference between these terms? Luminescence is the
emission of light after excitation Fluorescence is the prompt and
phosphorescence is the delayed emission of light respectively.
20. Scintillator Detectors- Ideal Properties Lecture 03 High
conversion efficiency Decay times of excited states should be short
Material transparent to its own emissions Color of emitted light
should match spectral sensitivity of the light receptor. For x-ray
and gamma ray detectors, should be large. Rugged , unaffected by
moisture, and inexpensive to manufacture.
21. Scintillation Detectors Lecture 03 For x-ray and gamma ray
detectors, linear attenuation coefficient should be large. Do you
know the advantage of large ?
22. Scintillator Detectors- Materials Lecture 03 1. Sodium
iodide activated with thallium [NaI(Tl)], coupled to PMTs and
operated in pulse mode, is used for most nuclear medicine
applications. 2. Fragile and hygroscopic. 3. Bismuth germinate
(BGO) is coupled to PMTs and used in pulse mode as detectors in
most PET scanners.
23. Scintillator Detectors- Photomultiplier Tubes Lecture 03
PMTs perform two functions: Conversion of ultraviolet and visible
light photons into an electrical signal. Signal amplification , on
the order of millions to billions. Consists of an evacuated glass
tube containing a photocathode, typically 10 to 12 electrodes
called dynodes, and an anode.
24. Scintillator Detectors- PM Tube Lecture 03
25. Scintillator Detectors- Process in PM Tube Lecture 03
26. Scintillation Detectors Lecture 03 Do you know how many
PMTs and their diameter in modern Gamma Cameras? 60-120 PMTs with
diameter ~ 2 inches
27. Scintillator Detectors - Dynodes Lecture 03 Electrons
emitted by the photo-cathode are attracted to the first dynode and
are accelerated to K.E. equal to the potential difference between
the photocathode and the first dynode.
28. Scintillator Detectors - Dynodes Lecture 03 When these
electrons strike the first dynode, about 5 electrons are ejected
from the dynode for each electron hitting it. These electrons are
attracted to the second dynode, and so on, finally reaching the
anode.
29. Scintillator Detectors PMT Amplification Lecture 03 Total
amplification of the PMT is the product of the individual
amplifications at each dynode. If a PMT has ten dynodes and the
amplification at each stage is 5, the total amplification will be
approximately, 5x5x5x5x5x5x5x5x5x5= 510= 10,000,000
30. Scintillator Detectors PMT Amplification Lecture 03 Applied
Voltage on PMT has significant role. Do you know its effect on
amplification? Amplification can be adjusted by changing the
voltage applied to the PMT.
31. Scintillator Detectors with Trapping of Excited Electrons
Lecture 03 Thermo luminescent Dosimeters Electrons become trapped
in excited states after interaction with ionizing radiation.
Heating & Reading Lithium fluoride (LiF) most useful for
personal and area radiation monitoring.
32. Scintillator Detectors PMT Amplification Lecture 02 LiF is
most useful because its effective atomic number is close to tissue.
Do you know the minimum threshold of exposure where it starts
counting? 100 uSv/ hr
33. Scintillator Detectors with Trapping of Excited Electrons
Lecture 03 Photo-stimulable Phosphors (PSP) Alternative to TLDs.
Reading is done by Laser Light Use in Radiology as image receptors
instead of film-screen cassettes. Barium fluorohalide activated
with europium.
34. Scintillator Detectors PMT Amplification Lecture 03 Today
we shall perform Detection Efficiency of Dose Calibrators in
Injection Room at 4 pm.