Page 220 - Radiochemistry and nuclear chemistry
P. 220
204 Radiochemistry and Nuclear Chemistry
pair together and the sum of their individual rates, the resolving time is calculated using the
equations"
X = R a R b - Rab R 0
y = R a Rb (Rab + R 0) - Rab R0 (Ra + R b)
z = y (R a + R b - Rab -- Ro)/X 2
t r = x[1 - (1 - z) '/~ ]/y (8.9)
where R a, R b, and Rab, are the measured count rate of samples a and b separately, a and
b together, respectively. R 0 is the background count rate for the system. The correction for
the resolving time can then be made according to (8.8a).
A more accurate technique is based on the use of a short-lived radionuclide, e.g. 99roTe
or 116rain" The count rate is then measured a number of times during at least one half-life
with the source left untouched in position all the time. When the background count rate can
be neglected (which is usually the case) combining (8.8a) with the equation for radioactive
decay gives after some algebra
Rob se kt--R 0 -R 0t rRobs (8.10)
where t is the time of measurement, X the decay constant and R 0 the initial count rate. A
plot of Rob s e xt as function of Rob s should yield a straight line with slope - R 0 t r and
intercept R 0 on the vertical axis. The dead time is then obtained as the absolute value of
slope divided by intercept. If the plotted data deviates strongly from a straight line it
indicates that the system investigated is paralyzable.
8.3. Gas counters
All gas-filled counters are in principle ion chambers (with the exception of the less
common gas scintillation counters). The ionization produced in an ion chamber by a single
nuclear particle produces too low a charge pulse to be easily detectable except for c~-
particles. However, an ion chamber can be designed so that the number of ion pairs formed
in each event is multiplied greatly.
Consider an ion chamber with a hollow cylindrical cathode and a thin central wire as an
anode (Fig. 8.8(a) shows an old type GM-tube). The ion pairs formed in the gas by the
passage of the ionizing radiation are separated from each other by their attraction to the
electrodes. The small, very mobile electrons are rapidly collected on the anode, which is
maintained at a high positive potential above ground, > 1000 V. Most of the voltage pulse
which appears on the anode arises by induction by the positive ions as they move away
from the immediate region of the anode. This step is responsible for the rise time of the
pulse, curve I in Figure 8.3(b). The potential decrease is only momentary as the anode is
rapidly recharged by the power supply. The time necessary to restore the original potential