Chapter 10 from BASICS OF NUCLEAR PHYSICS AND OF RADIATION DETECTION AND MEASUREMENT - An open-access textbook for nuclear and radiochemistry students by Jukka Lehto

Photons and particles emitted in radioactive decay ionize gas molecules which phenomenon is utilized in detection and measurement of radiation. In detectors based on the gas ionization, the ionizable gas is inside a metal chamber, which has typically a cylinder shape and is called tube. A voltage is applied to the tube so that the metal wall acts as cathode and a metal wire in the middle of the tube as anode (Figure X.1).

Figure X.1. Gas ionization detector (http://www.equipcoservices.com/support/tutorials/introduction-to-radiation-monitors/).

Gamma radiation penetrates the tube wall and ionizes the filling gas whereas beta and alpha radiations are not able to penetrate the wall. For the detection of alpha and beta active sources they either need to be placed inside the tube or the tube needs to have a penetrable window made of glass, mica or plastic. For the detection of external alpha radiation the window thickness should be very small. The filling gas is typically noble gas, such as argon, that the radiation ionizes to Ar⁺ ions. Due to electric field applied between the electrodes these argon cations transfer towards the cathode, the tube wall, while the electrons transfer towards the anode, the metal wire in the middle of the tube. From the anode wire the electrons are transported through an external circuit to the tube wall where they neutralize Ar⁺ ions back to Ar atoms. The electrons going through the external circuit are registered as an electric pulse representing an individual radiation absorption event. Thus the number of electric pulses corresponds to the number of radiation absorptions in the tube which in turn corresponds to the number particles or photons hitting the tube, i.e. the number of pulses corresponds to the activity of the source detected. As will be explained below the height of a pulse corresponds to the energy of a particle or a photon being absorbed in the tube in the case of two modes of gas ionization detectors (ionization chamber and proportional counter) but not in the third mode (Geiger-Műller counter). Depending on the voltage applied across the tube there are three types of gas ionization detectors (Figure X.2).

• Ionization chamber
• Proportional counter
• Geiger-Műller counter

Figure X.2. Operation ranges of three gas ionization detectors as a function of high voltage applied across the tube (http://www.canberra.com/literature/fundamental-principles/).

In the low voltage region, below about 50 V in Figure X.2, the velocities of the electrons and the Ar⁺ ion, induced by radiation absorption, towards the electrodes are so low that part of them are recombined back to Ar atoms before reaching the electrodes. This area (I in Figure X.2) is called recombination area. As the voltage is high enough to prevent recombination, all electrons and cations are collected to the electrodes. This area (II) is seen in Figure X.2 as about a 200 V wide area in the range of 130-330 V. In this range the number of ions (or electrons) collected on the electrodes is independent of the voltage applied. Gas ionization detectors operating at this voltage area are called ionization chambers. Since all ions and electrons are collected on the electrodes the height of the electric pulse recorded is proportional to the energy of the particle losing its kinetic energy in the filling gas. The higher is the initial energy the more there is ionization in the chamber and consequently the higher is the electric pulse recorded. Alpha particles have typically very high energies and they also cause very high specific ionization. Therefore, the pulses observed from alpha particles are much higher than those from beta particles. Ionization chambers are typically used for the detection of alpha radiation, for the measurement of absolute activities of radioactive sources and in radiation monitoring and dosimetry. In typical radionuclide laboratories, however, ionization chambers are very seldom used for activity measurements.

As the voltage is further increased from ionization chamber operation range the electrons have such a high energy that they cause additional, secondary ionization. In this range (III) the height of the electric pulse is dependent on the voltage applied. A gas ionization detector working in this range is called proportional counter since the height of the electric pulse, at constant voltage, is proportional to the energy of the photon or particle losing its energy in the filling gas by ionizations. This is because the amplification of the electrons due to secondary ionizations is constant providing that the voltage remains the same. Thus, as in case of ionization chamber the proportional counter can be used in nuclear spectrometry, i.e. in determination of alpha and beta particle energies. As seen in Figure X.2 the amplification factor of electrons in proportional counters is up to about 10⁵. Since the pulse height is highly dependent on the voltage proportional counters need very stable high voltage sources. The advantage of proportional counter compared to ionization chamber is that the observed pulse is much higher and thus easier to detect.

As the voltage is further increased from the proportional counter area all individual particles or photons cause complete ionization of the filling gas (area IV). This means that the observed electric pulses have the same heigth and are thus independent of the energy of the particle or photon losing its energy in the tube. Thus Geiger-Műller counter cannot be used in nuclear spectrometry but only in pulse counting, i.e. determination of activities or radiation intensities. The amplification of electrons in a Geiger-Műller tube is in the range 10⁶-10⁷. Thus the pulses are in the volt range and no amplifiers are needed unlike in ionization chambers and proportional counters. As seen in Figure XI.2 the number of electrons (pulse height) is more or less constant in about 200 V wide voltage range. Since the plateau has not exactly a constant value the high voltage source needs to be stable. In good Geiger-Műller tubes the slope of the plateau is below 1%. As the voltage is still increased from the Geiger-Műller voltage range there will be a continuous electric discharge (area V) which can destroy the tube rather quickly.

In addition to argon (or neon) the filling gas in GM tubes contains about 10% of halogen or organic gas, such as ethyl alcohol, which act as quenching gases. As the argon ions approach the cathode or when they hit it they may cause additional ionization which in turn causes additional erroneous pulses. As the ionization potentials of halogens and ethyl alcohol are lower than that of argon, Ar⁺ ions transfer their positive charges to them when hitting them. These in turn do not cause additional ionization and their positive charge is neutralized on the surface of the cathode.

When recording high pulse rates in GM tubes and in proportional counter (as also in most other radiation detectors) one needs to take into account the dead-time. As the argon gas ionizes, the induced electrons travel very fast to the anode while the positive argon ions travel much slower which causes a very low electric field near the anode (Figure X.3). The detector is then unable to record pulses that are caused from new radiation absorption events due to the travel of argon ions towards the cathode and recovery of the filling gas back to argon atoms. The time when the detector cannot record new pulses is called dead-time and it is marked with $\tau$.

Figure X.3. Pulse shape in proportional and Geiger-Műller counters.

When so high count rates are measured that the dead-time becomes important the observed count rates need to be corrected for dead-time $\tau$ (unit s). For that we mark observed count rate by $R$ (imp/s) and the true count rate by $R_0$ (imp/s) that would be observed if there was no dead-time. Because of the dead-time, $R_0 - R$ impulses in each second remain unrecorded. On the other in each second the tube is unable to record impulses a time $R \times \tau$ during which $R_0 \times R \times \tau$ photons or particles hit the detector. Thus

$$R_0 - R = R_0 \times R \times \tau$$

[X.I]

from which we solve $R_0$

$$R_0 = \frac{R}{1 - R \times \tau}$$

[X.II]

This equation can be used to correct the observed count rate to true count rate as far as the dead-time of the tube is known. For example, if the observed count rate is 1000 imp/s and the dead-time is 0.2 ms the true count rate is $\frac{1000}{1 - 1000 \times 0.0002} = 1250 \, \text{imp/s}$ or 25% higher than the observed one. At ten times lower count rate 100 imp/s the true count rate is only 2% higher.

In GM tubes the dead-time is 0.1-0.4 ms while in proportional counters it is much shorter, only a few microseconds. Therefore, a proportional counter can be used to measure a hundred times higher count rates without the essential effect of dead-time. If proportional counter is used not only for pulse counting but also for nuclear spectrometry the highest count rates should, however, be avoided since the tube has, in addition to dead-time, also a recovery time (Figure X.3). If a new particle is recorded during the recovery time the pulse height response of the tube is higher than when each particle is recorded completely individually without overlap. The total recovery time in proportional counters is much higher than the dead-time, around 0.1 ms.