1. Radiation Sensors Zachariadou K. | TEI of Piraeus

2. Part-II General Aspects Radiation Sensors

3. 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-II Radiation Sensors General Aspects

4. Radiation Sensors Modes of operation General properties General properties  Pulse Counting mode  Current mode  Mean square voltage mode  Sensitivity  Efficiency  Energy resolution  Time resolution  Pulse-pair resolution  Position resolution

5. Modes of Detection operation The 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

6. Modes of Detection operation -cont Response of typical detector: Current that flows for a time equal to the charge collection time (t c ) i(t) time -t tctc

7. Modes of Detection operation Most commonly applied The detector records each individual radiation that interacts Pulse mode is impractical for high event rates  Pulse Counting mode ( the signal from each interaction is processed individually)  Current 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)

8. 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- Pulse mode

9.  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

10. Modes of Detection operation- Pulse height spectra Radiation detector in pulse mode: The pulse amplitude distribution is used to deduce information about the incident radiation  Differential pulse height distribution Displaying modes:  Integral pulse height distribution

11. Modes of Detection operation- Pulse height spectra  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 [H 1, H 2 ]:

12. Modes of Detection operation- Pulse height spectra  Integral pulse height distribution Ordinate: number of pulses whose amplitude exceeds that of a given values of the abscissa H

13. Modes of Detection operation- an 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

14. 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- an example

15.  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  V max : the amplitude of the signal is proportional to the charge generated within the detector : Large RC (τ>>RC) General properties Modes of Detection operation- Pulse mode The proportionality holds if C is constant

16. General properties- Energy Resolution N=charge carriers, (large number) Statistical fluctuations:

17. General properties- Energy Resolution  Scintillators for gamma spectroscopy: ~5-10%  Semiconductors : ~1% Larger number of carriers (Semiconductors )  better resolution Any other fluctuations will combine with the statistical fluctuations

18. General properties- Detection Efficiency For isotropic sources: Absolute Efficiency Intrinsic Efficiency Solid Angle of the detector The efficiency (sensitivity) of a radiation sensor is a measure of its ability to detect radiation

19. General properties- Detection Efficiency Ω=Solid Angle of the sensor 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

20. General properties- Detection Efficiency Ω=Solid Angle of the sensor For the case of point-source located along the axis of a cylindrical radiation sensor (of radius a),close to the source: a r d In the far field (d>>a)

21. Use the detection efficiency to measure the absolute activity of a radiation source Assume isotropic emission Given: N recorded events Detector intrinsic peak efficiency E ins The number of events (I o ) emitted by the source over the measurement period: General properties- Detection Efficiency Ω: solid angle (in steradians) subtented by the detector in a given source position

22. For the case of a parallel beam of mono-energetic gamma-rays incident on a detector of uniform thickness: General properties- Detection Efficiency Absorption law

23. General properties- Detection Efficiency 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

24. Peak efficiency Peak efficiency Only full energy deposition interactions are counted  Photopeak area Most common for Gamma ray detectors : Intrinsic peak efficiency General properties- Detection Efficiency total efficiency total efficiency All interactions are counted  Entire area under the spectrum  Entire area under the spectrum

25. General properties- Dead 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

26. General properties- Dead Time 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 time At 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

27. General properties- Dead Time Recorded count rate vs true interaction rate for an ideal (no dead time) paralyzable and non- paralyzable sensor

28. Gas detectors Gas detectors Gas-filled detectors consist of a volume of gas between two electrodesScintillators the interaction of ionizing radiation produces UV and/or visible light Solid state detectors crystals of silicon, germanium, or other materials to which trace amounts of impurity atoms have been added so that they act as diodes Other, Cerenkov etc… Types of detectors

29. Detectors may also be classified by the type of information produced: Counters: Counters: Detectors, such as Geiger-Mueller (GM), that indicate the number of interactions occurring in the detector spectrometers spectrometers Detectors that yield information about the energy distribution of the incident radiation, such as NaI scintillation detectors dosimeters dosimeters Detectors that indicate the net amount of energy deposited in the detector by multiple interactions Types of detectors (cont.)