Measurement of Radioactivity
9
light. The molecules of the secondary fluor, POPOP, absorb this light and
reemit it at a longer wavelength. The purpose in introducing this secondary
fluor is to achieve a better match between the emission spectrum of the ~particles and the spectral response of the photocathode of the photomultiplier,
which detects the light quanta thus formed.
The liquid scintillation counter consists of two photomultiplier tubes
placed in dark cells on both sides of a chamber with the counting sample
placed in a glass or plastic (polyethylene) vial (Fig. 1.4). The cell with the
sample and the phototubes of the multiplier are covered a ca. 3-5-cm layer of
lead, protecting them from external high-energy rays, such as cosmic rays or
the radioactive background from natural or contamination sources. This
decreases the level of background count in the instrument. The output pulses
of light produced by the quanta emitted from the counting sample reach
the photocathode, the photosensitive layer on the inner side of the tube of the
multiplier, and induce the electrons' emission. The electrons produced are
accelerated by a positive potential of about lOOV and, interacting with the
dynodes, produce each time three to five secondary electrons, thus generating
the current pulse. This pulse is collected at the anode and is converted to a
voltage pulse, which can be counted by the scaler. The number of counts is
proportional to the amount of primary radioactive particles produced in the
counting sample, just as the magnitude of the output signal from the detector
is proportional to the energy corresponding to the detector by the primary
emitted ~-particles, e.g., to the energy of ~-radiation of a given radioisotope
expressed as MEV (see Table 1.1). This signal is then amplified and goes into
a pulse height analysis of the discriminator, which compares it with the reference voltages. The signal is passed through the discriminator circuit only if it
falls within the limits (channel) between the preselected voltage levels, the
window. The window can be preselected by setting it on the scaler. The upper
level may be set at infinity in order to count all pulses above the lower level,
c
DODO
E @@
Fig. 1.4. Scheme of liquid scintillation counter. A Scintillation vial with the emitting
sample; B photomultiplier tubes; C amplifer; D discriminator; E scaler
9
light. The molecules of the secondary fluor, POPOP, absorb this light and
reemit it at a longer wavelength. The purpose in introducing this secondary
fluor is to achieve a better match between the emission spectrum of the ~particles and the spectral response of the photocathode of the photomultiplier,
which detects the light quanta thus formed.
The liquid scintillation counter consists of two photomultiplier tubes
placed in dark cells on both sides of a chamber with the counting sample
placed in a glass or plastic (polyethylene) vial (Fig. 1.4). The cell with the
sample and the phototubes of the multiplier are covered a ca. 3-5-cm layer of
lead, protecting them from external high-energy rays, such as cosmic rays or
the radioactive background from natural or contamination sources. This
decreases the level of background count in the instrument. The output pulses
of light produced by the quanta emitted from the counting sample reach
the photocathode, the photosensitive layer on the inner side of the tube of the
multiplier, and induce the electrons' emission. The electrons produced are
accelerated by a positive potential of about lOOV and, interacting with the
dynodes, produce each time three to five secondary electrons, thus generating
the current pulse. This pulse is collected at the anode and is converted to a
voltage pulse, which can be counted by the scaler. The number of counts is
proportional to the amount of primary radioactive particles produced in the
counting sample, just as the magnitude of the output signal from the detector
is proportional to the energy corresponding to the detector by the primary
emitted ~-particles, e.g., to the energy of ~-radiation of a given radioisotope
expressed as MEV (see Table 1.1). This signal is then amplified and goes into
a pulse height analysis of the discriminator, which compares it with the reference voltages. The signal is passed through the discriminator circuit only if it
falls within the limits (channel) between the preselected voltage levels, the
window. The window can be preselected by setting it on the scaler. The upper
level may be set at infinity in order to count all pulses above the lower level,
c
DODO
E @@
Fig. 1.4. Scheme of liquid scintillation counter. A Scintillation vial with the emitting
sample; B photomultiplier tubes; C amplifer; D discriminator; E scaler