Nuclear Engineering

Gas Detectors (Contd.)

Let us now discuss design and operation of three major gas detectors namely, Parallel plate gas ionization chamber, Proportional counter and Geiger-Muller (GM) counter.

(A) Parallel plate gas ionization chamber

In the V-I characteristic (Fig. m2.11) we have seen at relatively lower voltage primary ionization is the dominant ionization mechanism, which eventually reaches to saturation after certain voltage. An ionization chamber in its simplest form [Fig. m2.13] consists of a chamber filled with gas (Ar, isobutene, P-10 etc.) as detection medium and two electrodes in parallel plate geometry.

FIGURE m2.13 Schematic representation of a parallel plate gas ionization chamber.

When energetic ions enters inside a parallel plate gas ionization chamber it ionizes gas atoms inside the chamber creating large number of electrons and positive ions. The positive ions move towards cathode (connected to the negative terminal of the voltage supply) and the electrons towards anode. The flow of charges in the outer circuit can be recorded as current signal or as voltage pulse by passing it across a resistance. Potential energy consideration² shows that the voltage induced at the collecting electrodes by the motions of an ion is directly proportional to the potential difference through which it has moved. Without considering recombination effect, which is basically, a recombination of negative and positive charge carriers the rate of change of voltage (rise time of signal) at the electrode is proportional to the velocity of the ions or electrons towards respective electrodes. In gas detectors, this result in a fast and slow component to the rise of the voltage pulse due to collection of the electrons and ions, respectively, since the electron velocity almost two orders of magnitude that of the ions. The magnitudes of the fast and slow component of the pulse depend on the relative distances of the two electrodes from the path of the ionizing particles.

In order to collect the fast pulse and get rid of the position dependence, a method is devised by Frisch⁴. A grid is introduced to separate the collector from the bulk of the chamber in which ionization occurs. Electrons are drawn through the grid, subsequently all moving through the same potential difference to the collector.

The maximum voltage in a gridded ionization chamber is¹

where, C is the capacitance associated with the chamber.

(B) Proportional counter

Let us now discuss the second type of gas detector, namely proportional counter. If the ionizing particle energy is relatively low, it cannot be detected by ionization chamber because of small amplitude of the output signal. If the field between electrodes is made higher enough so as to produce output pulses appreciably higher amplitude, low energy incident particles can be detected. We have seen in the previous lecture that generation of higher amplitude signal requires gas multiplication, as shown in the proportional region. Therefore, it is important to design another type of detector where a high voltage is applied between the electrodes. This detector having a cylindrical geometry, the outer shell of the cylinder (cathode) is operated at ground potential and an axial conducting wire (anode wire) the high voltage is applied as shown in Fig. m2.14.

FIGURE m2.14 Schematic representation of a cylindrical proportional counter.

In this case the electric field E is

where r is the radial distance from the axis; b: inner radius of the cylinder and a: radius of the central wire and V₀ is the positive voltage applied to central wire. Because of the 1/r dependence, the field is relatively weak at large r and is stronger near the surface of the anode wire. A high field causes multiplication resulting avalanche and the signal is generated. All multiplication takes place surrounding the anode and the position of ionizing event cannot influence the output signal. The choice of fill gas in proportional counter is very much important and depends on the following factors: low working voltage, high gain, good proportionality and high capability. Instead of filling with a single pure gas, a gas mixture is generally used to satisfy these conditions. Noble gases (e.g Ar) are usually chosen since they require the lowest electric field intensities for avalanche formation. However, due to higher excitation energy (^~11.6 eV), the discharge of the gas takes place and it cannot be operated for gains more than 10³ to 10⁴. This problem can be solved by adding some quenching gases like CH₄, CO₂, BF₃, which can absorb photons. It enhances the gain dramatically. In conventional proportional counters, the quencher gases generally used are P10 (90% Ar and 10% CH₄) and isobutene.

Gas detector operating even at higher voltage, i.e. in the Geiger Müller region, is known as Geiger Müller counter or G-M counter. In the proportional counter, each original electron leads to an avalanche, which is independent of all other avalanches formed from other electrons associated with initial ionization event. In G-M counter, substantially higher electric fields are created that creates multiple avalanches, where each avalanche create on the average, one or more avalanche, which eventually results in a chain of avalanches. A schematic of G-M counter is shown in Fig. m2.15.

FIGURE m2.15 Schematic representation in G-M counter.

At still greater electric field, the process becomes rapidly divergent and, in principle, an exponentially growing number of avalanches are created within a very short time. Consequently, collective effects of all the individual avalanches come into play and ultimately terminate this chain. All pulses from G-M tube are of same amplitude regardless of the number of original ion pairs that initiated the process. A Gieger tube can therefore function as a simple counter of radiation-induced events and cannot be applied in direct radiation spectroscopy because all the information on the amount of energy deposited by the incident radiation is lost. The output amplitude of a G-M tube is generally very high (typically of the order of volts) as it is generated due to collection of large amount of charges. Like proportional counter a quencher gas is used along with the fill gas so as to suppress excessive multiple pulsing effects. As G-M tube functions as a simple counter, its application requires that operating conditions, so as each pulse be registered by the counting system. In practice, this operating point is normally chosen by recording a plateau curve from the system under conditions in which a radiation source generates events at a constant rate within the tube. The minimum voltage at which the pulses are first registered by the counting system is often known as the starting voltage, whereas the transition between the rapid rise of the curve and the plateau is its knee (Fig. m2.16).

FIGURE m2.16 A typical count versus applied voltage in G-M counter showing different region.

In real cases, the counting plateau always shows some finite slope as shown in the figure. This may happen due to lower field strength near the end of the tube and the discharge originating in these regions may be smaller than the normal. This may also happen due to occasional failure of the quenching mechanism, which may lead to satellite or spurious pulse in addition to the primary Geiger discharge.

Nuclear Engineering

Nuclear Engineering

Curriculum

Gas Detectors (Contd.)

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