Detector For Gaseous Ionization

Ionizing radiation is measured with gaseous ionization detectors, which are also employed in particle physics to detect the presence of ionizing particles. These detectors are based on the ionizing action of radiation and are meant to quantify the ionization created when an incident particle crosses some medium. The kinetic energy of ionizing radiation particles (photons, electrons, etc.) must be adequate for ionizing radiation to occur, and the particle must be able to ionize (lose electrons from target atoms to create ions). Ionizing radiation can just knock electrons out of an atom.

The most basic gaseous ionization detector consists of a chamber filled with an easily ionizing medium (air or a specific fill gas). The effects created when a charged particle passes through a gas are the most extensively employed types of these detectors.

The most common gases used in detectors are argon and helium, however boron-triflouride (BF3) is employed when measuring neutrons. For the most part, gaseous ionization detectors are used in nuclear power plants to measure alpha and beta particles, neutrons, and gamma rays. With a configuration most sensitive to the type of radiation being measured, the nuclear radiation detectors function in the ionization, proportional, and Geiger-Mueller zones. Ionization chambers or proportional counters of appropriate design are used in neutron detectors. Neutron detectors include compensated ion chambers, BF3 counters, fission counters, and proton recoil counters.

The Fundamentals of Gaseous Ionization Detectors

The device is characterized by a capacitance that is defined by the geometry of the electrodes, and it has a cathode and anode that are both kept at a high relative voltage. A finite number of ion-pairs are produced as ionizing radiation penetrates the gas between the electrodes. The potential gradient of the electric field within the gas, as well as the type and pressure of the fill gas, influence the behavior of the resulting ion-pairs. Positive ions will travel toward the negatively charged electrode (outer cylinder) under the influence of the electric field, while negative ions (electrons) will migrate toward the positively charged electrode (central wire). This region's electric field prevents the ions from recombining with the electrons. The ions will collect on the electrodes, causing a charge on them and an electrical pulse across the detection circuit. Because the average energy required to generate an ion in air is around 34 eV, a 1 MeV radiation absorbed completely in the detector produces roughly 3 x 104 pairs of ions. Despite the fact that it is a little signal, it may be amplified significantly with basic electronics.

Ionizing Detector Operating Regions – Detector Voltage

In a detector, the connection between applied voltage and pulse height is quite complicated. The number of ion pairs collected is proportional to the pulse height. The voltages might vary greatly based on the detector shape as well as the gas type and pressure, as previously stated. The voltage areas for alpha, beta, and gamma radiation are shown schematically in the figure. There are six primary practical working areas, three of which are helpful for detecting ionizing radiation (ionization, proportional, and Geiger-Mueller region). Due to the larger number of ion pairs formed by the incident radiation's first reaction, the alpha curve is higher than the beta and gamma curves from the recombination region to part of the limited proportionality region.

Region of Recombination. The electric field isn't strong enough to accelerate electrons and ions at low voltage. Only a small fraction of the created electrons and ions reach their respective electrodes because they can recombine quickly after they are formed. However, as the detector voltage is raised, a larger proportion of the ions created will reach the electrodes. This rise continues until the voltage reaches "saturation." The recombination region refers to the operational voltage range where this occurs. Because neither the number of recombinations nor the number of ion-pairs initially formed can be precisely calculated, detectors are not used in this region.

Region of Ionization. An increase in voltage does not result in a significant rise in the number of ion-pairs gathered in the ionization area. The number of ion-pairs gathered by the electrodes is the same as the number of ion-pairs produced by the incident radiation, and is determined by the type and energy of the particles or rays in the incident radiation. As a result, the curve is flat in this area. The voltage must be higher than the recombination point of dissociated ion pairs. The voltage, on the other hand, is insufficient to induce gas amplification (secondary ionization). Detectors in the ionization area have a low electric field intensity, which is chosen to prevent gas multiplication. They are chosen for high radiation dose rates because they have no "dead time," a phenomena that affects the accuracy of the Geiger-Mueller tube at high dose rates, and they have no "dead time."

Region with a proportional population. In the proportional region, as the detector voltage is increased, the charge collected grows while the number of primary ion-pairs remains constant. Increasing the voltage allows the initial electrons to accelerate and gain enough energy to ionize additional atoms in the medium. These secondary ions are also accelerated, resulting in a Townsend avalanche effect, which produces a single enormous electrical pulse. Despite the fact that each primary event produces a huge number of secondary ions (about 103–105), the chamber is always set up so that the number of secondary ions is proportional to the number of primary events. It's crucial because the kind and energy of the particles or rays in the intercepted radiation field determine primary ionization. The gas amplification factor is calculated by dividing the number of ion pairs collected by the number of ion pairs produced by primary ionization. The gas amplification that takes place in this location has the potential to raise the total amount of ionization to a noticeable level. Charge amplification improves the detector's signal-to-noise ratio while also reducing the amount of electronic amplification required. The voltage must be kept constant when instruments are operated in the proportional range. The gas amplification factor does not change if the voltage remains constant. Low quantities of radiation are highly sensitive to proportional counter detection equipment. Furthermore, proportional counters may identify particles as well as measure energy (spectroscopy). Because primary ionization differs dramatically between different energy and types of radiation, the pulse height can be used to distinguish them.

Proportional Region with a Limited Size. The gas amplification factor does not continue to increase proportionally to the voltage in the limited proportional area. Because of additional ionizations and nonlinear effects, the output signal is not proportional to the deposited energy at a given applied voltage. Due to the high positive ion concentration in the chamber, the electric field is distorted. Because free electrons are lighter than positive ions, they are attracted to the positive central electrode more faster than positive ions are attracted to the chamber wall. The resultant cloud of positive ions around the electrode causes gas multiplication distortions. As a detection region, this area is normally avoided.

Region of Geiger-Mueller. The voltage and hence the electric field in the Geiger-Mueller area is so high that secondary avalanches can develop. Photons emitted by atoms excited in the original avalanche can cause and propagate these avalanches. Because these photons are unaffected by the electric field, they can interact far from the initial avalanche, involving the entire Geiger tube. These avalanches with form and height independent of the initial ionization and the energy of the detected photon provide a significant signal (the amplification factor can reach about 1010). Detectors in the Geiger-Mueller zone are capable of detecting gamma rays as well as any other form of charged particle that may enter the detector. Geiger counters are the name for these detectors. The main benefit of these instruments is that they do not usually require signal amplifiers. A positively charged ion cloud disrupts the electric field and ends the avalanche process because positive ions do not migrate far from the avalanche location. The adoption of "quenching" tactics improves the avalanche's termination in practice. Geiger counters, unlike proportional counters, cannot distinguish between energy or even incident radiation particles since the output signal is independent of the amount and kind of original ionization.

Region of Discharge. Finally, at even higher voltages, the electric field causes a continual discharge of the medium, rendering the chamber impervious to incident ionization. This region is not used for ionizing radiation detection or measurement. If the Geiger tube voltage is raised above the plateau's end, the count rate begins to rise rapidly again, until continuous discharge occurs, at which point the tube is unable to detect radiation and may be damaged.