Radiation Detection And Measurement

Radiation Detection and Measurement Radiation • Charged particles (α α, β, other ions) • Neutral particles (neutrons) • Electromagnetic radiation (γγ, x rays) Detection • Confirm the presence of radiation Measurement • Quantification of radiation — Nature — Energy — Intensity

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Radiation Detection and Measurement Detection systems : Different Why so ? α particles, β particles Heavy ions, fission fragments

Neutrons

γ Ray photons X-ray photons

Each interacts in a different way with matter

That is why!!! K.L.Ramakumar

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Radiation Detection and Measurement Typical Detector Configuration

Radiation

Detector

M S

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

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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 I

Detector

Time average of current signal I(t) I0

t Time dependent fluctuating component superimposed on steady state signal

Random nature of radiation events in the detector Radiation dosimetry instruments K.L.Ramakumar

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Radiation Detection and Measurement Modes of Detector Operation Mean Square Voltage Mode Ion Chamber

Squaring Circuit

Averaging

Steady state average current is blocked Fluctuating component is passed and squared (It – I0) is measured,squared and integrated α 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 C

Detector

Time constant RC <<< tc

R

V(t)

V(t) = R.I(t) V(t) tc

t

Current through R = Current flowing in the detector Mode useful for high event rates when timing information and not energy information is important K.L.Ramakumar

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Radiation Detection and Measurement Pulse mode of detector operation Time constant RC >>> tc tc Vmax = Q/C V(t) t

Each pulse is the result of interaction of a single radiation within the detector Pulse amplitude Q

α

α

Q

Energy of incident radiation

(Capacitance assumed constant) K.L.Ramakumar

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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 K.L.Ramakumar

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Intensity

Radiation Detection and Measurement

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Channel number

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Intensity

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Channel number

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Intensity

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Channel number

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Intensity

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Channel number

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Radiation Detection and Measurement

Intensity

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Channel number

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Intensity

Radiation Detection and Measurement

Channel number

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Intensity

Radiation Detection and Measurement

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 keV resolution (Bertolini,NIMM 223(1984)) K.L.Ramakumar

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Radiation Detection and Measurement Applications of ion chambers Gamma ray exposure measurements Exposure: Amount of ionisation charge 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) K.L.Ramakumar

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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 K.L.Ramakumar

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Radiation Detection and Measurement Cathode

Anode wire

RL V

- +

Charge collected : proportional to the number of ion-pairs created by the incident radiation Multiplication needs high electric field ε(r) V = voltage 2000 V

ε(r) =

V r ln(b/a)

a = anode radius 0.008 cm b = cathode radius 1 cm

ε(r) = 50000 V/cm Parallel plate geometry : 50000 V/cm K.L.Ramakumar

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Radiation Detection and Measurement

Count rate

Counting curve α + β α V

Gas

Gas

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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 G-M tube R

V(t)

Signal

C

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) K.L.Ramakumar

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Radiation Detection and Measurement Dead Time in G-M Counters +++++++++++++++++++ +++++++++++++++++++ +++++++++++++++++++ Anode wire

+ ve ions massive drift slowly towards cathode Electrons move fast towards anode wire Cathode

Decrease in electric field below critical point Subsequent discharges cannot occur Counter is “dead” K.L.Ramakumar

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Radiation Detection and Measurement Dead time of a GM-Counter

Dead time Recovery time

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) K.L.Ramakumar

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Radiation Detection and Measurement Scintillation Detectors One of the oldest radiation detection techniques (Rutherford’s α scattering experiment) Ideal scintillation material o High scintillation efficiency o Linear conversion : Light output α deposited 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 K.L.Ramakumar

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Radiation Detection and Measurement Scintillation detection systems Organic scintillation detectors Liquids Plastics Fast response time Fluorescence process independent of physical state Low light output, Low Z, poor efficiency for γ Inorganic scintillation detectors ZnS NaI(Tl) Best light output, best linearity, High z, Good efficiency for γ Long response time Regular crystalline lattice for fluorescence

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Radiation Detection and Measurement Gamma ray detection and measurement Three main types of interaction Photoelectric absorption Compton scattering Pair 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 dN/dE

Detector

E

Compton scattering dN/dE

Continuum Edge E dN/dE

Double escape peak

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Radiation Detection and Measurement Large size detector

dN/dE

E

Medium size detector Double escape peak

dN/dE

Detector

E

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Radiation Detection and Measurement

Gamma ray spectrum Influence of surrounding material

2 3 1 4

2

3

4

1

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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 K.L.Ramakumar

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Radiation Detection and Measurement

Usage of radiation detectors and understanding of signal measurement Detectors as simple counters Number of pulses (signals) per unit time No information about the radiation (neither type nor energy but only the intensity)

Detectors in pulse mode Pulse (signal) amplitude (height [volts]) distribution Useful to deduce information about the incident radiation (type, energy as well as intensity) K.L.Ramakumar

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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

Counting rate

Operating voltage is found out by establishing counting plateau

Plateau

V No energy information. All are counted and bunched together K.L.Ramakumar

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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 H4 dN dH H1

H2

H3 K.L.Ramakumar

H5 H

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Radiation Detection and Measurement H4 dN dH H1

H2

H3

H5 H

Typical pulse height spectrum dN/dH has no physical significance Number of pulses between H1 and H2 is given by

H2

∫

H1

dN dH dH

Integral gives number of pulses under the curve

For mono-energetic radiation, a line is expected. But broad peak is seen. Why? K.L.Ramakumar

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Radiation Detection and Measurement

Thank you

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