Nuclear Engineering

Gas Detectors

From the previous two lectures we got the idea about energy loss of various energetic particles, which includes charged and photons in the material medium. The challenge in nuclear detector design is how to obtain information about the incident particles from this energy loss and make it useful to understand various properties of the incident particle. In the simplest manner we can think that the energy given by the incident particles inside the medium produces an electron-positive ion pairs and those can be collected by electrodes eventually allowed to flow it through a circuit generating a current/voltage signal, one can obtain information about the energy of the incident particle. Let us first discuss the response of a gaseous medium under the application of an electric field. The response is basically the number of ion pairs collected in the electrodes placed in the gaseous medium. In Fig. m2.11, the number of ion pairs generated in a gas medium versus applied voltage for alpha and beta particles interacting with the medium are depicted.

FIGURE m2.11 V-I characteristics of a gas medium in a single wire gas chamber geometry.

Initially at zero applied voltage, a very low current in the outer circuit is observed. This is because of the ion pairs generated due to the energy deposited by charged particles and collected by the electrodes. Therefore, to generate a proper signal it is important to apply voltage. As shown in the above figure there are six regions in this characteristics curve with increasing voltage. In the first region (R1) where applied voltage is very low the ion pairs get recombined before collected in the respective electrodes. With increase in applied voltage, the recombination force is overcome and the ion pairs are collected, thereby increasing current. After a certain voltage, all ion pairs are collected in the electrodes and saturation in current is observed. This region, designated as R2 is known as ionization region, where the ion pairs generated by primary ionization process are completely collected. Application of further voltage will result in an electric field capable enough to accelerate the freed electrons which will ionize further gas molecules in the chamber. This is known as secondary ionization, which eventually turns to ionization avalanche or cascade if the secondary electrons cause further ionization of gases. The number of electron ion pair in the avalanche is proportional to the number of primary ionization and generate a proportional amplification of the current, with a multiplication factor depending on the working voltage, V. This factor can be as high as 10⁶ so that the output signal is much larger than that from an ionization chamber. A detector working under this operating condition is known as proportional chamber and the region of operation is known as proportional region (R3) as shown in the above figure. In proportional chamber basically the positive electrode or anode is a wire inside a chamber and the entire chamber body acts as cathode. This is a different design as compared to parallel plate ionization chamber and I am going to discuss about these as the lecture progresses. Now, the avalanche process occurs within a few radii of the anode wire, as the electric field is strongest near it. Further increase in voltage will create space charge, distorting the electric field near this region and hence the proportionality is no more strictly valid. This is known as region of limited proportionality (R4). Further increase in voltage creates very strong electric field resulting in discharge in the gas medium. Multiple avalanches spread throughout the anode length in which photons also contribute to ionization of gases. The output current thus gets completely saturated, always giving the same amplitude regardless of the energy of the initial event. Detectors working in this voltage region are known as Geiger-Müller counters (R5), which is characterized by a plateau over which the count rate varies little. A quenching gas is introduced in the medium in order to minimize the discharge by absorbing the photons. Finally, if the voltage is increased further a continuous breakdown occurs (R6) with or without radiation, which is of course avoided to prevent the damage of the detector.

Now before going to discuss on detectors, let us first know the general properties of radiation detectors.²

(A) Detector Sensitivity

The main purpose of the nuclear detector is to produce a usable signal corresponding to certain radiation and energy. All detectors are not useful to all types of radiations and energy ranges. Sensitivity of the detector depends on the cross section of the ionizing reaction, detector mass, inherent noise etc., which basically defines the capability of the detectors to generate signals corresponding to certain radiation and energy. A certain minimum amount of ionization is required to generate a usable signal. This lower limit is determined by the noise from the detector and the electronics associated with it.

(B) Detector Response

In a detector, the amount of ionization is proportional to the energy it loses in the sensitive volume. If the radiation is completely absorbed, ionization gives a measure of energy of the radiation. Output signal of the detector is generally in the form of a current pulse. The charge contained in this pulse basically measures the amount of ionization and hence the radiation energy. The relation between the radiation energy and the total charge or pulse height of the output signal is known as the response of the detector.

If a mono energetic radiation is sent to a detector, one would expect a delta function kind of spectrum as output signal. However, in reality a distribution (Gaussian like) is seen with some finite width. This width occurs mainly due to statistical fluctuations in the number of ionization and excitation produced in the detection. Apart from this, intrinsic noise of the system and the errors of the instruments associated with the detector also cause the broadening. All these factors overall decide the resolution of the detection system i.e. the extent to which detectors can distinguish two close lying energies. For monoenergetic radiation, resolution is usually given in terms of the full width at half maximum (FWHM) of the peak. The FWHM is illustrated in the Fig. m2.12 and is defined as the width of the distribution at a level that is just half the maximum ordinate of the peak intensity . Energies, which are closer than this interval, are usually considered un-resolvable. Resolution is defined as the ratio of FWHM and the centroid of the peak height H₀.

Therefore, Resolution,

Equation m2.23 is usually expressed in percentage. A NaI detector has about 8% or 9% resolution for 1 MeV incident energy, for example, while Ge detectors have resolution of about 0.1%.

FIGURE m2.12 Resolution of a detector is defined in terms of FWHM and H₀.

(D) Detector Efficiency

A detection system has certain capacity to generate signal corresponding to the events emitted by the source. The absolute or total efficiency of a detector is defined as that fraction of events emitted by the source that is actually registered by the detector, i.e.

ξ_tot = events registered/events emitted by the source ——————- (m2.24)

It depends upon the detector geometry and the probability of an interaction in the detector. There is another efficiency of the detector known as intrinsic efficiency, which is defined as the ratio of events registered to the events impinging on the detector. Therefore,

ξ_int = events registered/events impinging on the detector ——————- (m2.25)

When this quantity is plotted against mass number, the plot obtained is shown in Fig. m1.9.

This probability depends on the interaction cross section of the incident radiation on the detector medium. Therefore, it is a function of the type of radiation, its energy and the detector material. Considering a cylindrical symmetry of the detector it can be shown that the geometrical efficiency depends on the geometrical configuration of the detector and source and also on the angular distribution of the incident radiation¹.

(E) Detector Dead time

For all detection system a minimum amount of time is required to generate an electrical signa corresponding to a radiation event and to separate signals corresponding to two different events. This time limit is decided either by the inner mechanism of the detector or by the associated electronics. This minimum time separation is usually called the dead time of the counting system. During this time, the detection system is insensitive and any event arriving during this period is lost. In case of high counting rates the dead time losses can become rather severe. For measurement of dead time in a typical detection system go through the references.^1,2

Nuclear Engineering

Nuclear Engineering

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Gas Detectors

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