Page 226 - Radiochemistry and nuclear chemistry

210 Radiochemistry and Nuclear Chemistry

Table 8.2. Properties of some representative counter tubes
Geiger-Miiller counters Proportional counters

Purpose B,~ a,a n a,t~
Wall thickness 0.1ram glass 1.5 mg/cm 2 mica 1 mm steel 0.3 mg/cm 2 foil
Filling gas Ne + Ar + halogen Ar + organic BF 3 enriched in l~ Pure CH 4
Operating voltage 700 V 1250 V 2200 V c~ 1900, ~ 2600 V
Plateau length > 250 v -300 v > 300 v 0t>800 v, ~-4(Dv
Plateau slope 8%/100 v <4%/100 v <2%/100 v <2%/100 v
Lifetime > 3 x 109counts 3 x 10Scoums Infinite
Dead time 140 p.s 300 ~ 3 #s
Background 20- 30 cpm 80 cpm 1 - 2 cpm ot 0.1 cpm
count rate B 20- 25 cpm
Background 50 cm Pb 10 cm Pb 50 cm Pb
shielding
Special Insensitive Insensitive to 7
features to overvoltage

Another technique for neutron detection uses a fission chamber. One design contains a
stack of alternate anodes and cathodes, one of the electrodes being covered by a thin layer
of uranium enriched in 235U. The fission fragments produce large ionization even though
the gas multiplication is quite low. This detector is more sensitive to fast neutrons than the
BF 3 counter, and can be used for fast neutron fluxes up to ~ 10 l~ n m -2 s -1 with a
background of a few cps.

8.3.3. Geiger-Maller counters

In region IV (Fig. 8.7), the proportionality between the primary ionization and the output
pulse disappears and the latter becomes the same size for all initial ionization whether it be
a 6 MeV a-particle or a 50 keV X-ray. Geiger-Mfiller (GM) counters which operate in this
region have high sensitivity to all different kinds of radiation and the large size of the
output pulse (from a tenth of a volt to one volt, compared to the several tenths of a millivolt
output of ionization chambers) requires much less external amplification. This considerably
reduces the complexity of the auxiliary electronic equipment. The detector tubes for GM
counters are quite simple and allow a great deal of flexibility in design. Figure 8.8(a) shows
a GM tube with a jacket for flowing liquids; Table 8.2 gives the properties of some other
typical GM tubes. In general GM counters are limited to handling lower count rates than
proportional counters.
As in the case of proportional counters, the primary electrons from the ionizing radiation
cause secondary ionization near the center anode wire in GM detectors. This initial
avalanche ends when the very mobile electrons are all collected by the anode. However,
the neutralization of the electrons at the wire produces photons, which react with the gas
leading to the emission of photoelectrons. These trigger further avalanches and an overall
avalanche spreads along the complete length of the center wire and continues until the
build-up of the positive ion sheath progresses to a point sufficient to reduce the field
strength sufficiently to prevent further ionization. This build-up takes place because the