radiation detection and measurement

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

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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!!!

K.L.Ramakumar 3


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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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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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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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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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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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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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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1

Channel number

Intensity

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2

Channel number

In tensity

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3

Channel number

Intensity

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8

Channel number

Intensity

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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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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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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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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 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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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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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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+

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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Counting curve

αααα

αααα + ββββ

V

Count rate

GasGas

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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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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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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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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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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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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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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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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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Large size detector

E

dN/dE

Medium size detector

Detector

Double escape peak

E

dN/dE

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Gamma ray spectrum

Influence of surrounding material

1

2

3

4

1

23 4

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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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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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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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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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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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Thank you

radiation detection and measurement

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