radiation detection and measurement
DESCRIPTION
This lecture on radiation detection and measurement has been given to university college studentsTRANSCRIPT
K.L.Ramakumar 1
Radiation Detection and Measurement
Radiation
• Charged particles (αααα, ββββ, other ions)• Neutral particles (neutrons)• Electromagnetic radiation (γγγγ, x rays)
Detection
• Confirm the presence of radiation
Measurement
• Quantification of radiation— Nature— Energy— Intensity
K.L.Ramakumar 2
Radiation Detection and MeasurementDetection systems : Different
Why so ?
αααα particles, ββββ particles
Heavy ions, fission fragments
Neutrons
γγγγ Ray photons
X-ray photons
Each interacts in a different way
with matter
That is why!!!
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Radiation Detection and Measurement
DetectorRadiation MS
Grossly simplified steps
— Radiation falls, enters detector
— Radiation interacts with the detector material (interaction of radiation with matter)
— Charge carriers (signatures of radiation) are produced
—The intensity is then measured
Typical Detector Configuration
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Radiation Detection and Measurement
Types of radiation detectors
Depends on detector material
— Gas
— Liquid
— Solid
Depends on the radiation
— Heavy charged particles
— Light charged particles
— Neutral particles
— Electromagnetic radiation
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Radiation Detection and Measurement
Modes of detector Operation
Current mode
Detector I
Time average of current signal
t
I(t)
I0
Time dependent fluctuating component superimposed on steady state signal
Random nature of radiation events in the detector
Radiation dosimetry instruments
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Radiation Detection and Measurement
Modes of Detector Operation
Mean Square Voltage Mode
Ion ChamberSquaring
CircuitAveraging
Steady state average current is blocked
Fluctuating component is passed and squared
(It – I0) is measured,squared and
integrated αααα rQ2/T
Used in mixed radiation environments (neutrons and gamma radiation)
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Radiation Detection and Measurement
Modes of Detector Operation
Pulse Mode
Detector C R V(t)
Time constant
RC <<< tcV(t)
t
V(t) = R.I(t)
tc
Current through R = Current flowing in the detector
Mode useful for high event rates when timing information and not energy information is important
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Radiation Detection and Measurement
Pulse mode of detector operation
Time constantRC >>> tc
V(t)t
tcVmax = Q/C
Each pulse is the result of interaction of a single radiation within the detector
Pulse amplitude α Q
Q α Energy of incident radiation
(Capacitance assumed constant)
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Radiation Detection and Measurement
Advantages of Pulse Mode of Operation
Each quantum of radiation is detected as individual pulse signal
(Lower LOD set by background radiation level)
Sensitivity far greater than that in current mode
Each individual pulse amplitude carries useful information (energy of radiation)
Pulse mode is widely employed
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Radiation Detection and Measurement
1
Channel number
Intensity
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Radiation Detection and Measurement
2
Channel number
In tensity
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Radiation Detection and Measurement
3
Channel number
Intensity
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Radiation Detection and Measurement
8
Channel number
Intensity
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Radiation Detection and Measurement
12
Channel number
Intensity
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Radiation Detection and MeasurementIntensity
Channel number
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Radiation Detection and MeasurementIn tensity
Channel number
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Radiation Detection and Measurement
Applications of ion chambers
• Gamma ray exposure measurement
• Absorbed dose measurement
• Radiation survey instruments
• Radiation source calibrators
• Measurement of radioactive gases
• Smoke detectors
All are used in current mode operation
Charged particle spectroscopy measurements require pulse mode
Advantages over semiconductor detectors
Alpha spectroscopy 11.5 keVresolution (Bertolini,NIMM223(1984))
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Radiation Detection and Measurement
Applications of ion chambers
Gamma ray exposure measurements
Exposure: Amount of ionisationcharge created in air.
Air-filled ionisation chamber is suited for this purpose.
Ionisation charge gives the measure of exposure
Ionisation current indicates exposure rate.
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Radiation Detection and Measurement
Applications of ion chambers
Absorbed dose measurement
Measurement of absorbed dose
Energy absorbed per unit mass of material
Bragg-Gray Principle
Dm = WSmP
Dose measurements in biological tissues:
Tissue equivalent ion chambers with walls made from material with similar composition as tissue
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Radiation Detection and Measurement
Radiation survey meter
Closed air volume
Saturation current is measured using a battery powered electrometer
Walls are air-equivalent (Al or plastic)
Measurement of radioactive gases
Radioactive gas (e.g. tritium) can itself be filled gas
(Tritium ionisation chambers)
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Radiation Detection and Measurement
Proportional Counters
Gas-filled radiation detectors
Almost always operated in pulse mode
Gas multiplication to amplify the signal due to original ion pairs
Hence small signals can also be measured
Used in low-energy x-ray spectroscopy
Alpha, beta, neutron detection
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Radiation Detection and Measurement
Fill gases
Gas multiplication is dependent on the migration of electrons rather than negative ions
(Negative ion formation should be negligible)
Air is not suitable (Oxygen !!!)
Noble gase (Ar, Kr, Xe)
90%Ar + 10% CH4 (P-10 gas)
Low energy x-rays: Kr, Xe
Thermal neutrons: BF3, 3He
Fast neutrons: H2, CH2, He
Dosimetry (biological tissues): 64.4% CH4 + 32.4% CO2 + 3.2% N2
Ethylene to enhance Penning effect
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Radiation Detection and Measurement
+
RL
V-
Anode wire
Cathode
Charge collected : proportional to the number of ion-pairs created by the incident radiation
Multiplication needs high electric
field εεεε(r)
εεεε(r) = Vr ln(b/a)
V = voltage 2000 V
a = anode radius 0.008 cm
b = cathode radius 1 cm
Parallel plate geometry : 50000 V/cm
εεεε(r) = 50000 V/cm
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Radiation Detection and Measurement
Counting curve
αααα
αααα + ββββ
V
Count rate
GasGas
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Radiation Detection and Measurement
Geiger-Muller Counters
One of the oldest and third general category of gas-filled radiation detectors
Gas multiplication employed to enhance the charge.
All pulses from a G-M counter have same amplitude (history of the radiation is lost).
G-M tube functions simply as a counter of radiation induced events and is not suitable for radiation spectroscopy
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Radiation Detection and Measurement
Signal
G-M tubeR
C
V(t)
Fill Gases : Same as in the case of proportional counters
Possibility of emission of electron from cathode surface when positive ions get neutralised
This electron triggers avalanche
Process repeats resulting continuous pulses
Quenchers added to prevent this
Ethyl alcohol/formate or Cl/Br)
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Radiation Detection and Measurement
Dead Time in G-M Counters
+++++++++++++++++++
+++++++++++++++++++
+++++++++++++++++++
Anode wireCathode
+ ve ions massive drift slowly towards cathode
Electrons move fast towards anode wire
Decrease in electric field below critical point
Subsequent discharges cannot occur
Counter is “dead”
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Radiation Detection and Measurement
Dead time
Recovery time
Dead time of a GM-Counter
Dead time : Period between the initial pulse and the time at which a second pulse can be detected.
(a few hundreds of microseconds)
During dead time, any radiation interactions within the detector are lost (not detected)
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Radiation Detection and Measurement
Scintillation Detectors
One of the oldest radiation detection techniques
(Rutherford’s αααα scattering experiment)
Ideal scintillation material
o High scintillation efficiency
o Linear conversion : Light output αdeposited energy
o Transparent medium to the wavelength
o Short decay time for fast signal generation
o Refractive index similar to glass
o Good physical properties
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Radiation Detection and Measurement
Scintillation detection systems
Organic scintillation detectors
Liquids Plastics
Fast response timeFluorescence process independent of physical state
Low light output, Low Z, poor efficiency for γγγγ
Inorganic scintillation detectors
ZnS NaI(Tl)
Best light output, best linearity,High z, Good efficiency for γγγγ
Long response timeRegular crystalline lattice for fluorescence
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Radiation Detection and Measurement
Gamma ray detection and measurement
Three main types of interaction
Photoelectric absorptionCompton scatteringPair production
All the three interactions lead to different peaks in a gamma spectrum
Three types of hypothetical gamma ray detectors
Small size (< 2 cm)Large size ( > 10 cm) Medium size ( >2 cm < 10 cm)
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Radiation Detection and Measurement
Small size detector
Photoelectric absorption
Detector
E
dN/dE
Compton scattering
Continuum
Edge
E
dN/dE
Double escape peak
E
dN/dE
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Radiation Detection and Measurement
Large size detector
E
dN/dE
Medium size detector
Detector
Double escape peak
E
dN/dE
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Radiation Detection and Measurement
Gamma ray spectrum
Influence of surrounding material
1
2
3
4
1
23 4
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Radiation Detection and Measurement
Suggested Reading
1. Glenn F. Knoll, Radiation Detection and Measurement, John Wiley & Sons, New York
2. G.Friedlander, J.W.Kennedy, E.S.Macias, and J.M.Miller, Nuclear and Radiochemistry, John Wiley & Sons, New York
3. R.D.Evans, The Atomic Nucleus, Mc Graw Hill Inc., New York
4. S.S.Kapoor and V.S. Ramamurthy, Radiation Detection and Measurement, Wiley (Eastern), New Delhi
5. H.J.Arnikar, Essentials of Nuclear Chemistry, Wiley (Eastern), New Delhi
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Radiation Detection and Measurement
Usage of radiation detectors and understanding of signal measurement
Detectors as simple countersNumber of pulses (signals) per unit time
No information about the radiation (neither type nor energy but only the intensity)
Detectors in pulse modePulse (signal) amplitude (height [volts]) distribution
Useful to deduce information about the incident radiation (type, energy as well as intensity)
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Radiation Detection and Measurement
Detectors as simple counters
Example: G-M counter
Each pulse is registered as a signal output (counts)
Count rate is measured
Operating voltage is found out by establishing counting plateau
Counti ng rate
V
Plateau
No energy information. All are counted and bunched together
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Radiation Detection and Measurement
Detectors in pulse mode
Each pulse amplitude (height) carried important information about the incident radiation
(Type, energy and strength)
Pulse amplitude information is obtained by differential pulse height distribution
Detector has a facility to accept only pulses of certain amplitude (height)
(Pulse height discriminator)
This is a variable discriminator
H
H4
H1 H2
H3H5
dN
dH
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Radiation Detection and Measurement
H
H4
H1 H2
H3H5
dN
dH
Typical pulse height spectrum
dN/dH has no physical significance
Number of pulses between H1 and H2 is given by
Integral givesnumber of pulsesunder the curve
2
1
H
H
dNdHdH∫
For mono-energetic radiation, a line is expected. But broad peak is seen.
Why?
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Radiation Detection and Measurement
Thank you