6. Gas-Filled Detectors-Components
Variable voltage source
Gas-filled counting chamber
Two coaxial electrodes well insulated from
each other
Electron-pairs
produced by radiation in fill gas
move under influence of electric field
produce measurable current on electrodes
transformed into pulse
7. Gas-Filled Detectors
wall
fill gas
End Anode (+) Output
window
Or wall
Cathode (-) R or A
8. Gas-Filled Detector ที่มี
ความต่างศักย์ ต่าง ๆ กันไป
ก็จะเกิดเป็นเครื่องมือสารวจ
ต่างประเภทต่าง
วัตถุประสงค์การใช้งาน
I ; Ionization chamber region
P ; Proportional region
GM ; Geigur-Mueller region
13. Ionization Chamber
In the ionization chamber region
The number of ion pairs collected by the
electrodes is equal to the number of ion pair
produced by the radiation in the detector.
There is no change in the number of ion pairs
collected as the voltage increase.
จานวนคู่ของไอออน (Ion Pair) ที่ถูกเก็บสะสมโดย Electrode จะมีค่า
เท่ากับจานวน Ion pair ที่เกิดขึ้นจริงเมื่อรังสีตกกระทบหัววัด
ถึงแม้จะเพิ่มความต่างศักย์ไฟฟ้าให้มากขึ้น จะไม่ทาให้กระแสที่วัดได้เพิ่มขึ้น
14. Ionization Chamber
Main properties
High accuracy
Stable
Relatively low sensitivity
Slow response (used as integrator)
Wide range (uGy/hr – several thousand)
Portable
Used for
Monitoring instrument : Survey for radiation level >1 mR/hr
Main x-ray QC. Instrument
18. Proportional region
การเกิด gas multiplication สามารถทาให้เกิดคู่ไอออนประมาณล้านคู่ต่อ 1
ปฏิกิริยา ซึ่งต่างจาก ionization chamber ที่ 1 ปฏิกิริยาทาให้เกิดเพียง 1 คู่
ไอออน ถึงแม้จะเกิดการขยายตัวได้คู่ไอออนมากมายแต่อัตราการขยายยัง
เป็นสัดส่วนกับพลังงานของรังสีหรืออนุภาคที่เข้ามาทาปฏิกิริยา
Diagram of a proportional counter: (a) region of electron drift
and (b) region of gas amplification
20. Proportional counter
In the proportional region
The number of ion pairs collected is greater than the
number of ion pairs produced in the detector by the
radiation.
There is gas amplification.
ion pairs collected
The amplificat ion factor
ion pairs produced
The amplification factor at specific voltage is the same
for any type of radiation or energy of radiation.
The number of ion pairs collected is proportional to the
number of ion pairs originally produced.
21. Proportional counter
Operates in pulse mode
Most common general-purpose : 90% argon + 10%
methane
22. Proportional counter
Main properties
Laboratory instrument.
Accuracy.
A little higher sensitivity than the ion chamber.
Used for particles and low energy photon.
Used for
Monitoring instrument ; Assay of small quantities of
radionuclides
spectrometer
24. Geiger-Müller Counter
In the Geiger-Muller region
Any single ionizing event will produce so many secondary
ions that very large pulse is produce.
These ion pairs produce more ion pairs, until literally
millions of ion pairs are produced. This effect called
“Avalanching”
Because of the avalanche, It’s possible to tell that the
radiation is present, but it isn’t possible to determine the
type of radiation.
To stop the continual avalanche, another gas, called
“Quenching gas” is mixed with argon
34. Thermoluminescence (TL)
(TL) is the ability to convert energy from radiation
to a radiation of a different wavelength, normally in
the visible light range.
Two categories
Fluorescence - emission of light during or
immediately after irradiation
Not a particularly useful reaction for TLD use
Phosphorescence - emission of light after the
irradiation period. Delay can be seconds to
months.
TLDs use phosphorescence to detect radiation.
35.
36. Thermoluminescence
Radiation moves electrons into “traps”
Heating moves them out
Energy released is proportional to radiation
Response is ~ linear
High energy trap data is stored in TLD for a long
time
37. TL Process
Conduction Band (unfilled shell)
Electron trap
(metastable state)
-
Phosphor atom
Valence Band (outermost electron shell)
Incident
radiation
38. TL Process, continued
Conduction Band
Thermoluminescent
photon - Heat Applied
Phosphor atom
Valence Band (outermost electron shell)
39. TLD Reader Construction
To High DC Amp
Voltage To ground
PMT
Recorder or meter
Filter
TL material
Heated
Cup
Power Supply
•Output as “glow curve”
•Area under represents the radiation energy deposited in the TLD
40. Thermoluminescence
Main properties
High sensitivity.
Accuracy
Wild range (uGy-Gy)
Size-so small, can be taped to finger.
Versatility ; can use for long period.
Reusable - economical
Readout convenience - rapid (<30 sec)
No wet chemical ; data won’t be lost if it wet.
Used for
personnel monitoring
stationary area monitoring
44. Optically stimulated luminescence
(OSL)
Minimum detectable dose
1 mRem for gamma and x-ray radiation,
10 mRem for beta radiation.
Uses thin layer of aluminum oxide Al2O3 : C
Has a TL sensitivity 50 times greater than TLD-100
(LiF:Mg,Ti)
Almost tissue equivalent.
Strong sensitivity to light
Readout stimulated using laser
Intensity luminescence in proportion to radiation
dose.