Summary of Radioactivity detection using very simple ionization chamber
This article describes a simple, passive ionization chamber capable of detecting radioactivity from sources like smoke detectors without an external power supply. The device utilizes a multimeter's resistance mode to polarize electrodes and features robust shielding against electromagnetic interference (EMI) using a copper case and tin foil. It employs a BF256B J-FET transistor as the core sensing component, with readings stabilized via histogram mode on a high-precision multimeter.
Parts used in the Simple Ionization Chamber:
- Copper plate
- Bf256b n-channel j-fet transistor
- Tht resistor leg
- Multimeter (keysight 34460a)
- Tin foil
- Insulation tape
Today I will show a very simple ionization chamber that can detect radioactivity. I was able to detect with it ionizing radiation from a smoke detector (Am241 isotope). It’s also immune to electromagnetic interference (EMI) due to a good shielding.
This device doesn’t explicitly use any power supply. It’s connected to a multimeter set to measure resistance, in this mode, the multimeter provides a small voltage to its probes (R=I/U, so to measure resistance, it has to put voltage across measured element). This is sufficient here, because basically we just need to polarise electrodes of the ionization chamber and nothing more. My multimeter provides 5.6V in this mode.
My setup is presented below, note that the sensor is this metal box, not the PCB visible on the image.
The chamber is made from copper plate soldered carefully to prevent any holes where electromagnetic fields could flow and disturb readings. Inside is one BF256B (n-channel J-FET) transistor, its gate is connected to one of ionization chamber plate made from a leg of a THT resistor, source pin is available externally, this is where “plus” probe of the DMM is connected, drain is connected to the metal case (that is both and electrode and is shielded against EMI).
I’ve used Keysight 34460A as a multimeter here, because it has histogram mode, that will be useful to look if the measured value is stable over time. PLC was set to 0.2, it will reduce accuracy, but will give much more samples.
Below two images present what is inside of the sensor (only JFET, as mentioned above) and the sensor mounted. The front was shielded using tin foil, that was secured tightly by insulation tape.
There are many movies on YouTube with people constructing ionization chambers, however, those aren’t shielded completely and due to high wideband gain in those circuits, they will pick-up any electromagnetic radiation, so the results aren’t very useful.
Read more: Radioactivity detection using very simple ionization chamber
- How does the device receive power?
The device uses no external power supply; it relies on the small voltage provided by the multimeter when set to measure resistance. - What voltage is used to polarize the electrodes?
The multimeter provides 5.6V in resistance measurement mode to polarize the electrodes. - Which transistor is used inside the sensor?
A bf256b n-channel j-fet transistor is used inside the metal box. - How is the gate of the transistor connected?
The gate is connected to one of the ionization chamber plates made from a leg of a tht resistor. - Why was the keysight 34460a multimeter chosen?
It was chosen because it has a histogram mode to check if the measured value is stable over time. - What setting was used on the multimeter to reduce accuracy for more samples?
The plc was set to 0.2 to provide much more samples despite reduced accuracy. - How is the front of the sensor shielded?
The front was shielded using tin foil secured tightly by insulation tape. - Why do many online ionization chamber projects produce poor results?
Those projects often lack complete shielding and have high wideband gain, causing them to pick up electromagnetic radiation.
