Principle of operation

An ionization chamber is a type of radiation detector that measures ionizing radiation by detecting the charged particles (ions) produced when radiation interacts with the gas inside the chamber. The basic working principle relies on the ionization process. 

•  Radiation Interaction: When ionizing radiation (such as alpha, beta, or gamma radiation) enters the chamber, it interacts with the gas inside, typically air. This interaction causes the gas molecules to become ionized, producing free electrons and positive ions. 
•  Electric Field: The chamber contains two electrodes: a central anode and a surrounding cathode. A high voltage is applied between these electrodes, creating an electric field inside the chamber. 
•  Ion Collection: The free electrons, which are negatively charged, are attracted to the positive anode, while the positively charged ions are attracted to the cathode. As these ions move toward the electrodes, they create an electrical current. 
•  Current Measurement: The generated current is directly proportional to the number of ions produced, which in turn is related to the amount of radiation that has passed through the chamber. This current is then measured and used to calculate the intensity of the radiation.

Results

The device is not calibrated, as I do not have any calibrated samples. However, I was able to measure various radioactive samples; below is the collected data.

System architecture

For optimal sensitivity, the radioactive sample is placed directly on top of the ionization chamber. In the absence of the sample, the device will measure background radiation, which refers to the natural radiation present in the environment.

The ionization chamber requires a relatively high and stable polarization voltage, and the current drawn by the chamber is almost zero. Initially, I had planned to use a DC/DC converter (which is why it’s labeled 400V on the diagram), but later I found that four 12V batteries connected in series are sufficient to provide the necessary voltage.

The first stage of amplification consists of a transimpedance amplifier, which is used due to the very low signal from the photodiodes. The feedback resistor of this amplifier has a value of 2 * 50 GΩ. The second stage is a non-inverting amplifier, followed by a third stage, which is an inverting amplifier, and finally a buffer.

The first stage amplifier is crucial, both in terms of performance and the selection of the op-amp, PCB routing, and shielding. The op-amp must have a low input bias current and low input offset voltage. Ideally, it should come in a THT (Through-Hole Technology) package, and the input pin of the op-amp should be directly soldered to the electrode without touching the PCB to prevent leakage currents. The LMC6062 op-amp was selected because it meets all the required criteria and is cost-effective.

It is also important to clean the PCB after soldering, as the device will not function correctly without this step. 

The amplified signal is fed to the ADC and periodically checked by the microcontroller (note: it’s STM8, not STM32, as they are different families). The data is then sent via UART over USB to the Raspberry Pi for processing and storage.

Hardware

Hardware Revision History:

• 1.0 - Did not work
• 2.0 - Did not work
• 3.0 - Did not work
• 4.0 - Works; this is the version presented in this project
• 5.0 - Abandoned
• 6.0 - Work in progress

Hardware was designed using KiCad.

Software

Firmware was compiled using the SDCC compiler, as GCC does not support this family of microcontrollers.

A few scripts in Python and R were created to gather and analyze the results.

Mechanic

The device will not function properly without adequate shielding from Electromagnetic Interference (EMI). Due to the high amplification of the first-stage op-amp, the device will  oscillate or produce inaccurate data if exposed to noise. 

The ionization...