644
23 Electricity-to-Light Conversion
23.2.2 Display Applications
The once ubiquitous amber-colored monochrome displays are mostly fabricated using ZnS:Mn [1929],
having broad emission (540–680 nm) with its spectral peak at 585 nm (x = 0.50, y = 0.50) with an
efficiency of 2–4 lm/W. In color television (and similar applications such as color computer monitors,
tubes for aviation use, projection television) the image is reproduced by selective and time-multiplexed
cathode excitation of three phosphors (blue, green and red) deposited on the internal face of the screen.
The chromaticity coordinates of the standard CRT phosphors P-22B, P-22G and P-22R are given
in Table 23.3. They cover about the color range labeled ‘sRGB’ in Fig. 23.3c. For blue and green
ZnS:Ag (x = 0.157, y = 0.069), ZnS:Ag,Cl, ZnS:Ag,Al and ZnS:Cu,Al (x = 0.312, y = 0.597),
ZnS:Cu,Au,Al are used as phosphors, respectively. Y 2 O 2 S:Eu (x = 0.624, y = 0.337) activated
with trivalent europium (Eu
3+ ) facilitated such a gain in the brilliance of red over ZnS:Ag (more
than doubled it) that it has totally replaced it at about one fifth of the cost. For reproducible image
quality, precise grain-size control (median size for CRT phosphors is about 8 µm), dispersion control
and surface treatment are necessary. Flat-panel displays with their lower excitation voltage require
different phosphors for optimal efficiency.
23.2.3 Radiation Detection
The most commonly used scintillation detector for alpha measurements is ZnS activated with silver,
ZnS:Ag. This material is not very transparent to light and is usually prepared as a large number of
crystals with sub-mm size attached with an adhesive to a flat piece of plastic or other material. The
flat screen is optically coupled to a photomultiplier tube that is attached to associated electronics. The
voltage and discriminator levels are selected so that the detector is sensitive to the rather large pulses
from alpha interactions but insensitive to beta- or gamma-induced pulses. The alpha particles deposit
all of their energies in a small thickness of material compared to beta and gamma radiations.
Scintillation detectors for beta radiation are often made from organic materials. In an organic
scintillator, the light emission occurs as a result of fluorescence when a molecule relaxes from an excited
level following excitation by energy absorption from ionizing radiation. Molecules such as anthracene,
trans-stilbene, para-terphenyl, and phenyl oxazole derivatives are among the many organic species
that have useful scintillation properties. The organic molecules are dissolved in organic solvents and
used as liquid scintillators. A classic application is in the measurement of low-energy beta radiation
from, e.g. tritium,
14 C, or
35 S. In such cases, the sample containing the radioactive beta emitter is
dissolved in, or in some cases suspended in, the liquid scintillation solution. The emitted beta radiation
transfers energy through the solvent to the scintillator molecule that emits light, subsequently detected
by photomultiplier tubes. Organic scintillator molecules can also be dissolved in an organic monomer
that can then be polymerized to produce a plastic scintillator in a wide variety of shapes and sizes. Very
thin scintillators have been used for alpha detection, somewhat thicker scintillators for beta detection.
Table 23.3 CIE color coordinates, peak emission wavelength and decay time (10%) of standard CRT phosphors
Phosphor
x
y
λ p (nm)
Decay time
P-22B
0.148
0.062
440
∼20 µs
P-22G
0.310
0.594
540
∼60 µs
P-22R
0.661
0.332
625
1 ms
23 Electricity-to-Light Conversion
23.2.2 Display Applications
The once ubiquitous amber-colored monochrome displays are mostly fabricated using ZnS:Mn [1929],
having broad emission (540–680 nm) with its spectral peak at 585 nm (x = 0.50, y = 0.50) with an
efficiency of 2–4 lm/W. In color television (and similar applications such as color computer monitors,
tubes for aviation use, projection television) the image is reproduced by selective and time-multiplexed
cathode excitation of three phosphors (blue, green and red) deposited on the internal face of the screen.
The chromaticity coordinates of the standard CRT phosphors P-22B, P-22G and P-22R are given
in Table 23.3. They cover about the color range labeled ‘sRGB’ in Fig. 23.3c. For blue and green
ZnS:Ag (x = 0.157, y = 0.069), ZnS:Ag,Cl, ZnS:Ag,Al and ZnS:Cu,Al (x = 0.312, y = 0.597),
ZnS:Cu,Au,Al are used as phosphors, respectively. Y 2 O 2 S:Eu (x = 0.624, y = 0.337) activated
with trivalent europium (Eu
3+ ) facilitated such a gain in the brilliance of red over ZnS:Ag (more
than doubled it) that it has totally replaced it at about one fifth of the cost. For reproducible image
quality, precise grain-size control (median size for CRT phosphors is about 8 µm), dispersion control
and surface treatment are necessary. Flat-panel displays with their lower excitation voltage require
different phosphors for optimal efficiency.
23.2.3 Radiation Detection
The most commonly used scintillation detector for alpha measurements is ZnS activated with silver,
ZnS:Ag. This material is not very transparent to light and is usually prepared as a large number of
crystals with sub-mm size attached with an adhesive to a flat piece of plastic or other material. The
flat screen is optically coupled to a photomultiplier tube that is attached to associated electronics. The
voltage and discriminator levels are selected so that the detector is sensitive to the rather large pulses
from alpha interactions but insensitive to beta- or gamma-induced pulses. The alpha particles deposit
all of their energies in a small thickness of material compared to beta and gamma radiations.
Scintillation detectors for beta radiation are often made from organic materials. In an organic
scintillator, the light emission occurs as a result of fluorescence when a molecule relaxes from an excited
level following excitation by energy absorption from ionizing radiation. Molecules such as anthracene,
trans-stilbene, para-terphenyl, and phenyl oxazole derivatives are among the many organic species
that have useful scintillation properties. The organic molecules are dissolved in organic solvents and
used as liquid scintillators. A classic application is in the measurement of low-energy beta radiation
from, e.g. tritium,
14 C, or
35 S. In such cases, the sample containing the radioactive beta emitter is
dissolved in, or in some cases suspended in, the liquid scintillation solution. The emitted beta radiation
transfers energy through the solvent to the scintillator molecule that emits light, subsequently detected
by photomultiplier tubes. Organic scintillator molecules can also be dissolved in an organic monomer
that can then be polymerized to produce a plastic scintillator in a wide variety of shapes and sizes. Very
thin scintillators have been used for alpha detection, somewhat thicker scintillators for beta detection.
Table 23.3 CIE color coordinates, peak emission wavelength and decay time (10%) of standard CRT phosphors
Phosphor
x
y
λ p (nm)
Decay time
P-22B
0.148
0.062
440
∼20 µs
P-22G
0.310
0.594
540
∼60 µs
P-22R
0.661
0.332
625
1 ms