Page 224 - Radiochemistry and nuclear chemistry

208 Radiochemistry and Nuclear Chemistry

sensitive instruments, with very high resistors (~ 1015 9) or special circuitry as in the
vibrating reed electrometer. The chamber must always be designed with extreme care to
avoid leaking currents from the anode over the chamber casing to the cathode. One way to
minimize this is to ground the casing, as shown in Figure 8.90a). In the best of these
instruments, currents as low as 10-18 A can be measured, corresponding to less than 1 et
h-1, 10 B rain-1, or 10 "y s-1. These instruments are best suited for measurement of
radioactive gases, like tritium or radon in nature.
The beta-current neutron detector is a solid state ion chamber which is used in nuclear
reactor technology. It consists of an emitter in which a nuclear reaction occurs, leading to
the emission of primary B- particles (e.g. through the reaction ( 103Rh(n,-y)l~ 4.2
s) l~ or secondary electrons (e.g. through absorption of the prompt "g's emitted in the
neutron capture). These electrons represent a current and are collected by a collector. The
radioactive decay type detectors have a response time depending on the product half-life,
which the capture-'y detectors lack. These detectors have a limited lifetime; for the
59Co(n,-y)6~ it amounts to 0.1% per month at 1013 n cm-2s - 1. The lifetime depends on
the On3 , value (37 b for 59Co, 146 b for l~

8.3.2. Proportional counters

Values of a of 103 - 105 are commonly achieved in proportional counter operation. If a
= 103, essentially all the gas multiplication occurs within 10 mean free path lengths from
the wire for the electron in the gas (2 l~ = 1024). At 1 atm the mean free path length is
approximately 10 -6 m, which means that the gas multiplication occurs within 0.01 mm of
the wire.
The gas multiplication factor varies with the applied voltage but for a given voltage a is
constant so the detector pulse output is directly proportional to the primary ionization. As
a result it is possible to use a proportional counter to distinguish between a- and B-particles
and between identical particles of different energies inasmuch as different amounts of
primary ionization are produced in these cases.
The output pulse in proportional counter operation is not dependent on the collection of
the positive ions by the cathode. Consequently the rate of detection depends on the time
necessary for the primary electrons to drift into the region of high field strength near the
anode wire. As a result, proportional counters have a much shorter resolving time than ion
chambers which depend on the slow-moving positive ions. In fact the detector tube in a
proportional counter can amplify a new pulse before the positive ion cloud of the previous
pulse has moved very far if the new ionization occurs at a different location on the center
wire. Time intervals necessary to enable the counter to measure two distinct pulses can be
as low as 0.2- 0.5 #s. Frequently the associated measuring equipment is a greater
determinant of the resolving time than the detector itself. If, however, a proportional
counter is being used for the measurement of particle energies, any residual positive ion
cloud must have time to drift an appreciable distance before a new pulse is generated. In
this case the resolving time is closer to 100/xs.
Counting gases consists usually of one of the noble gases mixed with a small amount of
polyatomic gas. The latter makes the gas multiplication factor less dependent on applied
voltage, and increases the speed of electron collection. Typical counting gas mixtures are