Radiation Detection  Measurement II

Radiation Detection Measurement II PowerPoint PPT Presentation


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Pulse height analyzers. Many radiation detectors produce electrical pulses whose amplitudes are proportional to the energies deposited in the detector by individual interactionsPHAs are electronic systems that may be used with these detectors to perform pulse height spectroscopy and energy-selective countingIn energy-selective counting, only interactions that deposit energies within a certain energy range are counted.

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Radiation Detection Measurement II

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1. Radiation Detection & Measurement II Pulse height spectroscopy

2. Pulse height analyzers Many radiation detectors produce electrical pulses whose amplitudes are proportional to the energies deposited in the detector by individual interactions PHAs are electronic systems that may be used with these detectors to perform pulse height spectroscopy and energy-selective counting In energy-selective counting, only interactions that deposit energies within a certain energy range are counted

3. PHAs (cont.) Energy-selective counting can be used to: Reduce the effects of background radiation Reduce the effects of scatter Separate events caused by different radionuclides in a mixed radionuclide sample Two types of PHAs – single-channel analyzers (SCAs) and multichannel analyzers (MCAs) Pulse height discrimination circuits incorporated in scintillation cameras and other nuclear medicine imaging devices to reduce effects of scatter

4. Single-channel analyzer systems High-voltage power supply typically provides 800 to 1,200 volts to the PMT Raising voltage increases magnitude of voltage pulses from PMT Preamp connected to PMT using very short cable Amplifies voltage pulses to minimize distortion and attenuation of signal during transmission to remainder of system

6. SCA systems (cont.) Amplifier further amplifies the pulses and modifies their shapes – gain typically adjustable SCA allows user to set two voltage levels, a lower level and an upper level If input pulse has voltage within this range, output from SCA is a single logic pulse (fixed amplitude and duration) Counter counts the logic pulses from the SCA for a time interval set by the timer

8. SCA energy modes LL/UL mode – one knob directly sets the lower level and the other sets the upper level Window mode – one knob (often labeled E) sets the midpoint of the range of acceptable pulse heights and the other knob (often labeled ?E or window) sets a range of voltages around this value. Lower-level voltage is E - ?E/2 and upper-level voltage is E + ?E/2

10. Plotting a spectrum using a SCA The SCA is placed in window mode, the E setting is set to zero, and a small window (?E) is selected A series of counts is taken for a fixed length of time per count, with the E setting increased before each count but without changing the window setting Each count is plotted on graph paper as a function of baseline (E) setting

11. Energy calibration of SCA On most SCAs, each of the two knobs permits values from 0 to 1,000 to be selected By adjusting the amplification of the pulses reaching the SCA – either by changing the voltage applied to the PMT or by changing the amplifier gain – the system can be calibrated so that these knob settings directly indicate keV A Cs-137 source, which emits 662-keV gamma rays, is often used for calibration

12. Multichannel analyzer systems An MCA system permits an energy spectrum to be automatically acquired much more quickly and easily than does a SCA system The detector, HV power supply, preamp, and amplifier are the same as for SCA systems The MCA consists of an analog-to-digital converter, a memory containing many storage locations called channels, control circuitry, a timer, and a display

17. Interactions of photons with a spectrometer An incident photon can deposit its full energy by: A photoelectric interaction (A) One or more Compton scatters followed by a photoelectric interaction (B) A photon will deposit only a fraction of its energy if it interacts by Compton scattering and the scattered photon escapes the detector (C) Energy deposited depends on scattering angle, with larger angle scatters depositing larger energies

19. Interactions (cont.) Even if the incident photon interacts by the photoelectric effect, less than its total energy will be deposited if the inner-shell electron vacancy created by the interaction results in emission of a characteristic x-ray that escapes the detector (D)

21. Interactions (cont.) Detectors normally shielded to reduce effects of natural background radiation and nearby radiation sources An x-ray or gamma-ray may interact in the shield of the detector and deposit energy in the detector: Compton scatter in the shield, with the scattered photon striking the detector (E) A characteristic x-ray from the shield may interact with the detector (F)

22. Spectrum of Cesium-137 Cs-137 decays by beta particle emission to Ba-137m, leaving the Ba-137m nucleus in an excited state The Ba-137m nucleus attains its ground state by the emission of a 662-keV gamma ray 90% of the time In 10% of decays, a conversion electron is emitted instead, followed by a ~32-keV K-shell characteristic x-ray

24. Reasons for differences in spectra First, there are a number of mechanisms by which an x-ray or gamma-ray can deposit energy in the detector, several of which deposit only a fraction of the incident photon energy Second, there are random variations in the processes by which the energy deposited in the detector is converted into an electrical signal

25. NaI(Tl) crystal/PMT Random variations in: The fraction of deposited energy converted into light The fraction of the light that reaches the photocathode of the PMT The number of electrons ejected from the back of the photocathode per unit energy deposited by the light Cause random variations in the size of the voltage pulses produced by the detector, even when the incident x-rays or gamma rays deposit exactly the same energy

27. Pulse height spectrum of Cs-137 Photopeak corresponding to interactions in which the energy of an incident 662-keV photon is entirely absorbed in the crystal Compton continuum caused by 662-keV photons that scatter in the crystal, with the scattered photon escaping the crystal The Compton edge is the upper limit of the Compton continuum

29. Pulse height spectrum (cont.) Backscatter peak caused by 662-keV photons that scatter from the shielding around the detector into the detector Barium x-ray photopeak caused by absorption of barium K-shell x-rays (31 to 37 keV) Photopeak caused by lead K-shell x-rays (72 to 88 keV) from the shield

30. Spectrum of Technetium-99m Tc-99m is an isomer of Tc-99 that decays by isomeric transition to its ground state, with the emission of a 140.5-keV gamma ray In 11% of the transitions, a conversion electron is emitted instead of a gamma ray

32. Tc-99m (cont.) Photopeak caused by total absorption of the 140-keV gamma rays Escape peak caused by 140-keV gamma rays that interact with the crystal by photoelectric effect but with resultant iodine K-shell x-rays (28 to 33 keV) escaping the crystal Photopeak caused by absorption of lead K-shell x-rays from the shield Compton continuum is quite small because the photoelectric effect predominates in iodine at 140 keV

34. Energy resolution Energy resolution of a spectrometer is a measure of its ability to differentiate between particles or photons of different energies Determined by irradiating detector with monoenergetic particles or photons and measuring width of resulting peak in the pulse height spectrum Statistical effects in the detection process cause the amplitudes of pulses from detector to randomly vary about the mean pulse height, giving the peak a Gaussian shape

35. Energy resolution (cont.) Width is usually measured at half the maximal height of the peak – called the full width at half-maximum (FWHM)

37. Count-rate effects If two interactions occur in a detector, separated by a very short time interval, the detector produces a single pulse Sum of the individual signals from the two interactions Higher amplitude than the signal from either individual interaction Operating a pulse height spectrometer at a high count rate causes loss of counts and misplacement of counts in the spectrum

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