Radiation Sensors Zachariadou K. | TEI of Piraeus.

Radiation SensorsRadiation Sensors

Zachariadou K. | TEI of Piraeus

Part-IIPart-IIGeneral AspectsGeneral Aspects


The course is largely based on :

G. F. Knoll, “Radiation detection and measurement” ; 3rd ed., New York, Wiley, 2000

Gordon Gilmore & John D. Hemingway, “ Practical Gamma-Ray Spectrometry”; Willey , 21008

Part-IIPart-IIRadiation SensorsRadiation SensorsGeneral AspectsGeneral Aspects


Modes of operationModes of operation General propertiesGeneral properties

Pulse Counting modePulse Counting mode

Current modeCurrent mode

Mean square voltage modeMean square voltage mode

Sensitivity

Efficiency

Energy resolution

Time resolution

Pulse-pair resolution

Position resolution

Modes of Detection operationModes of Detection operationThe net result of the radiation interaction in a wide category of detectors is the appearance of a given amount of electric charge within the active volume of the detector

The charge must be collected as an electric signal.

The collection is accomplished by applying electric field within the detector causing the positive and negative charges created by the radiation to flow in opposite directions

Collection time:

Ion chambers: few ms

Semiconductor detectors: few ns

ct Qdtti0

)(

Modes of Detection operation Modes of Detection operation -cont-cont

Response of typical detector: Current that flows for a time equal to the charge collection time (tc)

ct Qdtti0

)(i(t)

time -ttc

Qdttict

0 )(

Modes of Detection operationModes of Detection operation

Most commonly applied

The detector records each individual radiation that interacts

Pulse mode is impractical for high event rates

Pulse Counting modePulse Counting mode(the signal from each interaction is processed individually)

Current modeCurrent mode(the electrical signals from individual interactions are averaged together, forming a net current signal)

Used when event rates are high

The time integral of each burst of current is recorded

All pulses above a low-level threshold are registered

(pulse counting)

C=equivalent capacitance of the detector +measuring circuit

(eg cable +preamplifier)

The voltage V(t) across R is the fundamental signal voltage on which pulse mode operation is based

Two cases:

Small RC (τ<<RC)

Large RC (τ>>RC) (more common)

Modes of Detection operation-Modes of Detection operation-Pulse modePulse mode


Small RC (τ<<RC)

The time constant of the external circuit is kept small compared with the charge collection time

Used when high event rates or time information is more important than accurate energy information

Large RC (τ>>RC) (more common)

Little current flows in R during the charge collection time

The detector current is integrated on the capacitance

If time between pulses is large the capacitance will discharge through R

Modes of Detection operation-Modes of Detection operation-Pulse height spectraPulse height spectra

Radiation detector in pulse mode:

The pulse amplitude distribution is used to deduce information about the incident radiation

Differential pulse height distributionDisplaying modes:

Integral pulse height distribution


Differential pulse height distribution

Ordinate: The differential (dN) number of pulses observed having an amplitude within dH, divided by dH

Total number of pulses at [H1, H2]:

1

1

H

H

dHdH

dNN


Integral pulse height distribution

Ordinate: number of pulses whose amplitude exceeds that of a given values of the abscissa H

Modes of Detection operation-Modes of Detection operation-an examplean example

The shape of the depends strongly on the mechanism via which the incident photon primarily interacts with the detector:

If the primary photon interaction is a photoelectric effect, its energy is fully absorbed and it contributes to the full energy peak (photo-peak) of the energy spectrum. In contrast, a primary Compton interaction creates a scattered electron that carries only a fraction of the initial photon energy and a scattered photon that carries the remaining energy. If the latter is absorbed by a sensitive material of the detector, the event contributes to the photo-peak of the spectrum. Otherwise, the event contributes to the plateau at energies below the photo-peak (Compton plateau).

simulated energy spectrum of 200keV incident γ- rays

The spectrum is obtained by summing the deposited energies in the sensitive materials a radiation sensor

Τhe number of incompletely absorbed events (off-peak part of the energy spectrum) increases compared to the photo-peak events as the incident photon energy increases.

Modes of Detection operation-Modes of Detection operation-an examplean example

The rise time of the pulse is determined by the charge time collection

The dead time of the pulse is determined by the time constant of the load circuit

Vmax : the amplitude of the signal is proportional to the charge generated within the detector : C

QV max

Large RC (τ>>RC) General properties


The proportionality holds if C is constant

General properties-General properties-Energy ResolutionEnergy Resolution

100% peak ofcenter at height Pulse

FWHM resolutionEnergy

100% 2.35

100% kN

2.35

100% H

2.35 resolutionEnergy

0

N

Nk

N=charge carriers, (large number)

Statistical fluctuations: N

General properties-General properties-Energy ResolutionEnergy Resolution

Scintillators for gamma spectroscopy: ~5-10%

Semiconductors: ~1%

Larger number of carriers (Semiconductors ) better resolution

...(FWHM)(FWHM)(FWHM)(FWHM) 2drift

2noise

2lstatistica

2all

Any other fluctuations will combine with the statistical fluctuations

General properties-General properties-Detection EfficiencyDetection Efficiency

emittedNumber

detectedNumber Eabs

detector on theincident Number

detectedNumber Eintr

For isotropic sources:

Absolute EfficiencyAbsolute Efficiency

Intrinsic EfficiencyIntrinsic Efficiency

4

E

E

intr

abs Solid Angle of the

detector

The efficiency (sensitivity) of a radiation sensor is a measure of its ability to detect radiation

emittedNumber


E

E

intr

abs


4

E

E

intr

abs

Ω=Solid Angle of the sensor

emittedNumber


E

E

intr

abs

dAA

2r

cosα

As the distance from a radiation source increases the absolute efficiency of a radiation sensor decreases

r= distance of the sensor’s surface element dA from a radiation source

a= angle between the normal to the sensor’s surface and the direction of the source


4

E

E

intr

abs

Ω=Solid Angle of the sensor

emittedNumber


E

E

intr

abs

dAA

2r

cosα For the case of point-source located along the axis of a cylindrical radiation sensor (of radius a) ,close to the source:

ar

d

2212

ad

d

In the far field (d>>a) 2

2

2 dd

A

Use the detection efficiency to measure the absolute activity of a radiation source

Assume isotropic emission

Given: N recorded events

Detector intrinsic peak efficiency Eins

4

int0

rabs E

I

E

II

The number of events (Io) emitted by the source over the measurement period:

0abs emittedNumber

detectedNumber E

I

I

4 Eabs


Ω: solid angle (in steradians) subtented by the detector in a given source position

For the case of a parallel beam of mono-energetic gamma-rays incident on a detector of uniform thickness:

x0

eII



detectedNumber Eintr

xe - 1 Eintr

Absorption law


the intrinsic efficiency increases with the increase of thickness x decreases with the increase of the photon energy

the intrinsic efficiency depends also on the energy of the incident gamma

For NaI(Tl) sensors:

For semiconductor detectors:

Intrinsic efficiency of a CdTe semiconductor gamma radiation sensor

PeakPeak efficiencyefficiencyOnly full energy deposition

interactions are counted Photopeak areaPhotopeak area

Most common for Gamma ray detectors : Intrinsic peak efficiency


total efficiencytotal efficiency All interactions are counted

Entire area under the Entire area under the spectrumspectrum

General properties-General properties-Dead TimeDead Time

Dead time:

Minimum amount of time between two events in order that they be recorded as two separate pulses

Severe for high counting rates

Main problem for detectors in pulse mode

time for a detector to recover before being sensitive to another radiation interaction (e.g. Geiger counter) pile-up: some detectors are forming an electrical pulse with a long tail when a new radiation interaction takes place distorts the pulse shape and possibly the energy measurement (based upon pulse amplitude) dead time of the ADC used for data acquisition


Paralyzable system, an interaction that occurs during the dead time after a previous interaction extends the dead time

Non-paralyzable system, does not extend the dead timeAt very high interaction rates, a paralyzable system will be unable to detect any interactions after the first, causing the detector to indicate a count rate of zero


Recorded count rate vs true interaction rate for an ideal (no dead time) paralyzable and non-paralyzable sensor

Gas detectorsGas detectorsGas-filled detectors consist of a volume of gas between two electrodes

ScintillatorsScintillatorsthe interaction of ionizing radiation produces UV and/or visible lightthe interaction of ionizing radiation produces UV and/or visible light

Solid state detectorsSolid state detectorscrystals of silicon, germanium, or other materials to which trace amounts crystals of silicon, germanium, or other materials to which trace amounts

of impurity atoms have been added so that they act as diodesof impurity atoms have been added so that they act as diodes

Other , Cerenkov etc…Other , Cerenkov etc…

Types of detectorsTypes of detectors

Detectors may also be classified by the type of information produced:Detectors may also be classified by the type of information produced: Counters:Counters: Detectors, such as Geiger-Mueller (GM), that indicate the number Detectors, such as Geiger-Mueller (GM), that indicate the number

of interactions occurring in the detectorof interactions occurring in the detector

spectrometersspectrometers Detectors that yield information about the energy distribution of Detectors that yield information about the energy distribution of

the incident radiation, such as NaI scintillation detectorsthe incident radiation, such as NaI scintillation detectors

dosimetersdosimetersDetectors that indicate the net amount of energy deposited in the Detectors that indicate the net amount of energy deposited in the

detector by multiple interactionsdetector by multiple interactions

Types of detectors (cont.)Types of detectors (cont.)

Radiation Sensors Zachariadou K. | TEI of Piraeus.

Documents

Transcript of Radiation Sensors Zachariadou K. | TEI of Piraeus.