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Radiation Protection in Nuclear Medicine
PRACTICAL SESSION 1
PULSE HEIGHT ANALYSIS
BACKGROUND
The most commonly used detector in nuclear medicine applications is the scintillation
detector. It is used in sample counters, probes, scanners and gamma cameras. One
important feature of the scintillation detector is its ability to give a signal that is proportional to the energy absorbed in the detector. This can be used in detection of different radionuclides with different photon energies and to separate scattered photons
from the useful ones. The signals from a radioactive source e.g. a patient will show a
distribution of pulse heights with a full energy peak representing photons that have
been completely absorbed in the detector and a continuous distribution representing
scattered photons (from the source or only partially absorbed in the detector). The
width of the full energy peak is called the energy resolution of the detector.
Figure 1: Pulse height distribution from a patient with Tc99m registered with a gamma camera
The signal from the photomultiplier tube is led into a pulse height analyzer in order to
do this energy discrimination. Only signals of a certain size (height) will pass the analyzer and become registered. This pulse height is preset by defining a voltage window (ΔE) between a lower level (LL) and an upper level (UL). In a scintillation detector system the lower level and the window width can generally be adjusted. The settings will depend on the radionuclide to be measured.
AIM
The aim of this practice is to learn the function and properties of a pulse height analyzer and to be able to interpret a pulse height distribution.
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Radiation Protection in Nuclear Medicine
PRACTICAL SESSION 1
MATERIAL
Radionuclides
 Co57 (122 keV)
 Cs137 (662 keV)
Instrumentation
 Scintillation detector system with a manual pulse height analyzer
PROCEDURE
a) Pulse height distribution
Use Co57. Set a suitable window width ΔW on the pulse height analyzer. Start a
measurement with the low level discriminator at 0. Record the count rate. Increase
the low level setting with ΔW and record the count rate. Continue to increase the
lower level stepwise with ΔW until the full energy peak has been passed. Draw the
measurements on a graphic paper in linear scale and determine the pulse height value for the center of the full energy peak. Measure the energy resolution of the detector as the full width half maximum (FWHM) of the peak. Discuss a suitable pulse
height analyzer setting for the radionuclide.
b) Energy linearity
Use Cs137. Determine the pulse height for the center of the full energy peak. Calculate the relation between pulse height and photon energy. Use this value to determine the energy of the photons from Co57 using the data from the previous measurement.
CONCLUSIONS
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Radiation Protection in Nuclear Medicine
PRACTICAL SESSION 1
Pulse height analysis
Radionuclide:
Window width:
Lower level
Count rate
Lower level
Count rate
Co 57 (122 keV)
Full energy peak pulse height:
FWHM (%):
Suggested window setting:
Cs137 (662 keV)
Full energy peak pulse height:
Pulse height/keV:
Calculated energy for Co57 photons:
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