27_-_Radiation_Detection_II

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:
Energy discrimination occurs by rejection of pulses above or below the energy window set by the operator
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.
Spectrum of Cesium-137
• Cs-137 decays by beta particle emission to Ba137m, 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
– Lower-level voltage is E - E/2 and upper-level voltage is E + E/2
Example of a single-channel analyzer
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
• 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
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
• 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
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)
Radiation Detection & Measurement II
Pulse height spectroscopy Nonimaging detector applications Counting statistics
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
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
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)

Modern, computer-based multichannel analyzer
After the analog pulses are digitized by the ADC, they are sorted into bins (channels) by height, forming an energy spectrum.
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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
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