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202 Radiochemistry and Nuclear Chemistry
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I-- / DISCRIMINATOR LEVEL
"1- / a:i i.,. I l
m
6i ta. # t
RI JI ,= [ I
ffl
..1
NOISE
I I -"
I i
,, ~
TIME ENERGY (PULSE HEIGHT)
& b
FIG. 8.5. Idealized pulse spectrum obtained (a) from a shaping amplifier, and Co) from a
multichannel analyzer using a solid state detector. The figure shows the effect of different
discriminator settings.
In modern counters the number of pulses counted are often shown on light-emitting diode
(LED), gas-plasma, or liquid crystal displays.
8.2.3. Pulse shape and dead time
From Figure 8.3(b) it is obvious that it takes some time to restore proper measuring
conditions. Such conditions may be assumed to have occurred when the potential Vp has
returned above a certain level. During the first part of this time the detector is unable to
collect a new charge because working conditions have not been sufficiently restored. This
RECOVERY TIME --~
]
~ RESOLVING TIME----~, i 1
DEAD TIME~ I ',
t
...................................
-Vt (a)
V
t t
(b) (c)
FIG. 8.6. Effects of short interval between events registered in detectors. (a) G.M. tube, (b)
peak pile-up and (c) tail pile-up for fast detectors.