Geiger - Muller Radiation detector

As part of my interest for studying ionizing radiation, I was attracted by the idea of designing and building my own radiation detector using a Geiger - Muller (GM) tube for quantizing radioactive events.

The main goal of this project is to design a device capable of counting the number of particles reaching the GM Tube, in order to calculate the radioactivity of certain objects containing radioactive isotopes. It is also of interest to calculate the biological damage of the emitted ionizing radiation, however, with this current device, it will only be possible to estimate it based on the calibration of the GM for a standard radioactive isotope, which in this case is Cobalt 60 (Co-60)

1. General description

This device consists of a circuit board with electronics components soldered to it, which will include the GM tube. The electronics will be protected by a 3D printed PLA casing, dotted with an LCD screen that will display the number of counts per minute. It is powered by a 6V battery pack. It does not possess a separated probe, but it has a extension of the casing containg the probe to improve the detection of radiation.

It is important to note that this detector, as it uses the GM tube J321/M4011 is only capable of detecting gamma and beta rays but not alpha particles. The detailed capabilities are explained in the sections below.

2. Geiger-Muller tube J321/M4011 Capabilities

2.1 GM J321/M4011 Introduction

The radiation detector element used in this device is the J321/M4011. This is a readily available and affordable GM tube, that may not provide the most accurate measurements for professional experimentation, but it is enough for the aims of this project, since it will meet the goals stated.

2.2 GM working principle

A GM tube is capable of detecting radiation when a particle collides with the atoms of the gas contained by the tube (typically Helium, Neon or Argon) the gas gets ionized, then the free electrons generated by ionisation are accelerated towards the positively charged anode (the wire in the center of the tube). This process happens multiple times, creating finally an electric pulse (containing enough electrons). This current triggers the piezoelectric speaker that produces the characteristic "click" sound. Following this, the signal will also be converted into a digital input to be interpreted by an Arduino controller (this is explained in detail in section X).

2.3 GM specifications

The Geiger-Muller tube J321/M4011 has the following detection capabilities:

Alpha particles: not capable of detecting them unless the glass enclosure of the tube is replaced by for instance a mica window. This is not a part of this experiment.
Beta particles: it is capable of detecting beta particles with an energy of at least 0.2 MeV or above.
Gamma rays: it is capable of detecting gamma radiation with an energy of at least 0.05 MeV or above.

Minimum working voltage is 350V (recommended 380V)

Minimum required current is 0.15mA

2.4 Preliminary calibration

For this preliminary calibration, the characteristics of the SBM-20 tube are considered. This tube outperforms the J321, but there is available data about it. The final conversion rate is therefore adjusted to compensate for this lower capacity of detecting radiation. The SBM-20 has been calibrated with Cobalt-60 (Co), which is a beta-emitter isotope with very well known radiation levels, which benefits the calibration process.

The value from the specifications of SBM-20 for Co60 is: 22cps = 1mR/hr, which is equivalent to: 1320 cpm = 1mR/hr

Converting to microSieverts (uSv), in which 1 mR=10 uSv: 1320cpm = 10 uSv

Then, the resulting conversion factor from cpm to uSv is: f=132

3. Electronics setup

3.1 Working principle

As mentioned earlier, the active component of the detector is the GM tube. In order to operate the tube a minimum voltage of 380V DC is required. However, it is the plan to power the detector with a conventional 9V battery, therefore, first of all, the voltage needs to be boosted from 9V to 380V DC.

First, in order to understand the working principles of increasing the voltage, a simple voltage booster has been designed, and it's presented in the schematic below:

Figure 3.1-1 Voltage booster schematic

The n-moset transistor M1 acts as a switch in this circuit, crating to different stages that make the boosting of the voltage possible. For doing this, the MOSFET opens and closes depending on the pulses generated by the 555 timer, which is connected to its trigger leg. Therefore, the stges are:

- MOSFET closed: the current flows through the inductor and then escapes through the MOSFET, directly into the battery negative again. This current flowing through the inductor allows the inductor to store energy in form of a magnetic field.

Figure 3.1-2 Voltage booster schematic

- MOSFET open: at this point, the current cannot escape anymore though the MOSFET (since it's open now). This makes the inductor magnetic field to collapse and to release the stored energy, which is now combined with the voltage from the battery, thus allowing to charge the capacitor at a higher voltage. This occurs many times per second.

Figure 3.1-3 Voltage booster schematic

In the following sections, the inductors is replaced by a 9 to 220V transformer.

3.2 General Component list

The main components that are used in the radiation detector are shown below:

Geiger Muller Tube J321/M4011
Timer 555
Transformer 9V - 220V
Diodes
LED Diodes
Resistors
n-MOSFET Transistor
Arduino UNO
Bipolar Capacitors
Piezoelectric speaker

3.3 Building the circuit - The oscillator side of the circuit

For this section, the oscillator part is built on a bread board. This part of the circuit is in charge of generating the pulses (oscillations) required to feed the transformer. For this articular transformer frequencies of 50Hz are desired for correct functioning. Therefore, using specific resistors and capacitors in line with the 555 Timer it is possible to generate pulses with a frequency of 50 Hz. In the image below this part of the circuit is already assembled.

Figure 3.3-1 Oscillator assembled

3.4 Building the circuit - The High Voltage section

Continuing the circuit, the following part to be design is the High Voltage section. For this part, the inductor showed in the. images above is replaced by a 9V 50Hz to 220 V 50Hz Transformer connected to a rectifier bridge circuit to convert the current to DC again. In the image below, the second section is assembled.

Figure 3.4-1 Oscillator and High Voltage sections

3.5 Building the circuit - Low voltage controller output

The current pulses created when the air is ionised in the geiger tube are passed through a relay that links the high voltage section to the second low voltage section. When current passes through the inductor in the relay ut creates a magnetic field that closes the switch and triggers an input for the microcontroller. This input is processed by the controller and then the output is displayed in a OLED display. Below the full circuit schematic including the 3 sections.

Figure 3.5-1 Full schematic (no voltage boost)

However, as it can be seen from the schematic above, the output voltage reaching the Geiger Tube is around 220 V, and a much higher voltage is required. For that, in the following schematic the voltage the the transformer outputs is boosted. The expected output voltage is estimated to be around 440V.

Figure 3.5-2 Full schematic (voltage boost)

< Work in progress1 Mar 2025 >

1. General description

2. Geiger-Muller tube J321/M4011 Capabilities

2.1 GM J321/M4011 Introduction​

2.2 GM working principle​

2.3 GM specifications​