Some of the earliest and most commonly used detectors were based on the effect of particles passing through the gas and ionizing it. In this case, the particle moves through the gas and ionizes gas molecules along its path. Examples of these detectors are ion chamber (IC) and Geiger-Muller (GM) detectors. However, detectors differ in how they produce an output signal after gas ionization.
An IC detector is the simplest gas detector. Its mode of operation is based on collecting the charge created by the direct ionization of the gas by applying an electric field. Similar to other detectors, an IC can operate with a current or pulses produced as the output. Working in the current mode is a more common operation mode for IC detectors, but there are applications where an IC is used in pulse mode at the output.
Figure 1: The Basic Components of an Ionization Chambers
The initial interaction of the ionizing radiation with the chamber involves the emission of high-energy electrons from the chamber wall. This emission occurs because of the photoelectric effect, the Compton effect, or pair production. In these processes, secondary electrons are emitted, penetrate the sensitive volume of the chamber, and ionize air molecules. This ionization creates positive ions and low-energy electrons within the sensitive volume, as shown in Figure 1.
These low-energy electrons are attracted to the electronegative oxygen molecules in air, giving rise to negative ions. Consequently, in air-based IC, the collected charged particles consist of positive and negative ions (ion pairs) rather than positive ions and electrons.
Detectors based on ionization chambers have immense significance in radiation therapy, especially in the context of precise dosimetry management. One of the key advantages of using an IC-based detector is its ability to accurately measure short pulses, which cannot be achieved with Geiger-Muller tube-based detectors. Moreover, IC-based detectors can provide real-time measurements, which is not possible with thermoluminescent dosimeters.
Therefore, real-time measurement of pulses is a distinct advantage of IC over other technologies.
RAM-Ion, a well-established radiation survey-meter from Rotem Industries, is based on IC technology and has been successfully tested in pulsed fields. Its capability to measure the absorbed dose originating from an ultrashort radiation pulse within a certain limit of accuracy and range has been previously demonstrated.
Geiger-Mueller Based Detector
The GM detector is one of the oldest radiation detectors used. This detector is still widely used because of its simplicity, low cost, and ease of operation.
These detectors multiply the charge created by the original particles. In these detectors, a much larger electric field is used than in an ICs, which causes a type of avalanche after the interaction of a single particle. This leads to a high output signal on the order of magnitude of a few volts. When the output signal is very high, this advantage allows the use of simple electronics, thereby eliminating the need for external amplification and making it a sensitive and inexpensive detector. A detector based on a GM tube is very common in situations where only the radiation intensity needs to be measured, particularly because of its low cost.
One of the disadvantages of GM is its high dead time compared to all other detectors. Therefore, these detectors must be corrected for dead time. In addition, GMs have a lifetime limit, and they stop working properly after a certain number of pulses.
One of the key advantages of using an IC-based detector is its ability to accurately measure short pulses, which cannot be achieved using GM-based detectors. In addition, it has no lifetime limits.
Scintillator Based Detector
Scintillation is one of the earliest and most effective methods for detecting ionizing radiation. Special materials that emit light upon radiation exposure are used in this process. An ideal scintillator should efficiently convert particle energy into light, exhibit a linear light yield, be transparent to its emission, have a short decay time, and offer good optical quality. It must also allow efficient light coupling to sensors such as photomultiplier tubes. However, no material meets all of these criteria perfectly, thus requiring trade-offs. Photomultiplier tubes and photodiodes are used to convert the light into electrical signals. Some disadvantages of scintillator detectors include their sensitivity to humidity, temperature dependence, and the decay time of the scintillating material.
Semiconductor Based Detectors
Semiconductor detectors offer several advantages, particularly for high-energy electrons and gamma rays. They produce much larger electron-hole pairs per radiation event, enabling superior energy resolution, compact size, fast timing, and customizable thickness.
Semiconductor diodes exhibit high sensitivity; however, this is influenced by temperature, as even a slight increase can cause some electrons to transition into the conduction band. Because the diode sensitivity depends on the photon energy, filters are often placed in front of them to reduce the over-response of lower-energy photons.
Another disadvantage is the performance degradation caused by radiation-induced damage.
In this chapter, we present several practical problems associated with the use of radiation monitoring devices and the considerations involved in designing the IC3 device to overcome these problems.
Ionization chambers are often considered the gold standard for radiation monitoring because of their ability to measure short pulses at high frequency, flat energy response, ability to measure background or leakage radiation, and wide dynamic range.
Ionization chambers come in different configurations: some are ventilated with standard air pressure, and some are sealed and even maintain high pressure. Each configuration has advantages and disadvantages.
The advantage of sealed and pressurized chambers is the higher sensitivity owing to the higher number of molecules that can be ionized, thus increasing the signal amplitude. In addition, sealed ionization chambers are less affected by temperature, humidity, and pressure from the external environment.
The disadvantage is the energy response, especially at energies below 100KeV, due to the need for thick walls. A radiation-measuring device without a flat energy response requires correction factors, making it challenging to accurately measure radiation. In practice, when working with ionizing radiation from X-ray machines, photons will be created over a broad energy spectrum owing to interaction with items in the room; therefore, it is of great importance for the device to have a flat response to obtain an accurate measurement. Standards define permissible deviation over a range of energies for a particular energy.
Another notable disadvantage is the risk of classifying the device as a hazardous material because of the pressure in the chamber. This restricts air shipping, and considering that the device has to be shipped from place to place in order to be calibrated periodically, this is a major disadvantage. The last disadvantage of a sealed chamber is its inability to measure the beta radiation.
By contrast, ventilated ionization chambers require thin walls that provide a flat energy response over a wide range of energies. These chambers are defined as unhazardous materials and measure beta radiation. Ventilated ionization chambers are affected more by temperature, humidity, and pressure from the external environment and can require the use and maintenance of moisture- absorbing materials such as silica gel.
This section introduces how IC3 addresses the challenges identified in section 3.
Humidity
A new electrometer in the IC3 device was developed based on reliable devices that have been on the market for years. The disadvantage of the old electrometers is the need to use silica gel because of the sensitivity of the electrometer to humidity. Dependence on silica gel would have been complicated for the user because of the need to occasionally maintain and replace the material with a new desiccant. The new IC3 electrometer has been designed to not be affected by humidity like the electrometers of the past, and there is no need to use silica gel.
Temperature
The change in temperature affects not only the electronic components, but also the medium in the chamber, causing a deviation in the readings. To overcome these changes in readings, a circuit was designed that incorporates a Negative Temperature Coefficient resistor (thermistor) that changes the resistance according to the temperature. Resistance change depending on the temperature leads to amplification modification of the electrometer and provides inherent compensation without the need for software compensation by factors or tables.
Ambient pressure
A change in pressure also causes a deviation in the readings, owing to a change in the density of the medium in the chamber. A pressure sensor was used to overcome this phenomenon. It has a compensation function for measuring the pressure, and compensation is implemented on the readings.
Photon Dependence
IC3 has a flat energy response and does not require correction factors owing to the wall thickness. The wall thickness of the chamber, which is the active area of the sensor, is tissue equivalent 1000𝑚𝑔/𝑐𝑚2. In addition, there is a Mylar beta window for radiation measurements.
Validation tests were performed using IC3 to validate new modifications. IC3 was connected to a laptop. The instrument was monitored online during the test procedure using Rotem Meter View Calibration - RMVC software.
Humidity
According to the standard, the mean instrument response should be within 15% of the mean instrument response determined at 40% relative humidity at 22°C during and after exposure to a non-condensing relative humidity range of 40% to 93% at 30°C.
The unit-under-test (UUT) was located in the environmental chamber (as shown in figure 2) exposed to the Th-232 source; during the test, the temperature and humidity were regulated by the environmental chamber.
Figure 2: UUT was located in the environmental chamber
A summary of these results is presented in Table 1.
Table 1 Environmental Parameters and Obtained Readings
| No. | Temperature [C°] | Humidity [%] | Obtained Reading [mR/h] | Period [hours] | Deviation from 40% relative humidity at 22 °C [%] |
|---|---|---|---|---|---|
| 1 | 22 | 40% | 2.06 | 4 | 0 |
| 2 | 30 | 93% | 2.12 | 24 | 3 |
| 3 | 22 | 40% | 2.05 | 4 | -0.29 |
In conclusion, IC3 withstands the humidity requirements.
Temperature
According to the standard, the mean instrument response over the temperature range from 0°C to 40°C should be within 15% of the mean response determined at 22°C. Additionally, the mean instrument response over the temperature range from − 10°C to 0°C and 40°C to 50°C should be within 20% of the mean response determined at 22°C.
The UUT was located in an environmental chamber (as shown in figure 2) exposed to the Th-232 source, and the temperature and humidity were regulated by the environmental chamber.
Figure 3 shows the IC3 response to the Th-232 radiation source over a temperature range of -10ᵒC to 50ᵒC.
Figure 3: IC3 response to the Th-232 radiation source over the temperature range of -10ᵒC to 50ᵒC.
In conclusion, IC3 withstands the temperature requirement, according to the standard.
Ambient pressure
According to the standard, the mean instrument response should be within 15% of the mean instrument response at an ambient pressure of 101 kPa over the ambient pressure range of 70 kPa to 106 kPa.
The UUT was located in the pressure chamber ( Figure 4) exposed to the Th-232 source. During the test, pressure was regulated using a pressure chamber. Tests were performed using open and closed beta windows.
Figure 4: UUT was located in the pressure chamber
A summary of the results is presented in Table 2.
Table 2 Pressure Parameters and Obtained Readings
| Pressure [kPa] | ALTITUD E [feet] | Deviation From Nominal (Open Beta Window) | Deviation from Nominal (Closed Beta Window) |
|---|---|---|---|
| 101 | 0 | Nominal | Nominal |
| 92 | 2650 | -2.3% | 4.2% |
| 84 | 5100 | -2.8% | 2.2% |
| 76 | 7740 | -0.4% | 4.6% |
| 70 | 10000 | 1.8% | 8.4% |
| 106 | -1250 | -0.7% | 1.7% |
In conclusion, IC3 withstands the ambient pressure requirement, according to the standard.
Photon Dependence
The useful energy range for the photon dose rate measuring instruments should be at least 80 keV to 1.25 MeV. It is based on the range of energies where the following conditions are met: The photon energy dependence was measured in the 300-500 mR/h range, and the radiation energy of 100 keV – N120 was used as the reference energy. A sufficient number of instrument readings were obtained in accordance with the standard.
A summary of the results is presented in Table 3.
Table 3 Energies and Response
| Nominal effective energy of Radiations used (keV) | Normalised Response |
|---|---|
| 33 | 0.99 |
| 48 | 0.98 |
| 65 | 1.05 |
| 83 | 1.04 |
| 100 | 1.00 |
| 118 | 0.93 |
| 164 | 0.83 |
| 661 | 0.84 |
Figure 5: IC3 Energy Response
In conclusion, the requirement according to ANSI N42.17A was met.
The tests demonstrated that the IC3 design preserved the key advantages of the ionization chambers as the gold standard for radiation monitoring. These include a flat energy response, capability to measure high-frequency short pulses, sensitivity to background and leakage radiation, and a wide dynamic range of up to 100Rem/h. Additionally, the IC3 addresses common drawbacks of ionization chambers: it is neither sealed nor classified as hazardous due to pressure, and it is resistant to environmental factors such as humidity, temperature, and pressure variations.
