Three options for assembling a homemade dosimeter

Capabilities of a homemade device

Naturally, it will be quite difficult to assemble a professional or multifunctional dosimeter. Household portable or individual devices register beta or gamma radiation. The radiometer is designed to study specific objects and read the level of radionuclides. In fact, a dosimeter and a radiometer are two different devices, but household versions often combine both the first and the second. Subtle terminology plays a role only for specialists, which is why even combined models are called generically – dosimeter.

By choosing one of the proposed circuits for assembly, the user will receive a simple device with low sensitivity. There is still a benefit in such a device: it is capable of recording critical doses of radiation, this will indicate a real threat to human health. Despite the fact that a homemade device is several times inferior to any household dosimeter from a store, it can be used to protect your own life.

Useful tips

Before choosing one of the assembly schemes for yourself, read the general recommendations for manufacturing the device.

1. For a self-assembled device, choose 400 volt meters; if the converter is designed for 500 volts, then you need to adjust the feedback circuit setting. It is permissible to choose a different configuration of zener diodes and neon lamps, depending on what dosimeter circuit is used during manufacture.
2. The output voltage of the stabilizer is measured with a voltmeter with an input resistance of 10 MΩ. It is important to check that it is actually equal to 400 volts; charged capacitors are potentially dangerous to humans, despite their low power.
3. Near the counter, several small holes are made in the housing for the penetration of beta radiation. Access to high voltage circuits must be excluded; this must be taken into account when installing the device in the housing.
4. The circuit of the measuring unit is selected based on the input voltage of the converter. The connection of the unit is carried out strictly with the power turned off and the storage capacitor discharged.
5. With a natural radiation background, a homemade dosimeter will produce about 30 - 35 signals in 60 seconds. Exceeding the indicator indicates high ion radiation.

List of parts needed for the radio circuit

• 1 BPW34 photodiode
• 1 LM358 op amp
• 1 transistor 2N3904
• 1 transistor 2N7000
• 2 capacitors 100 NF
• 1 capacitor 100 µF
• 1 capacitor 10 nF
• 1 capacitor 20 nF
• 1 10 MΩ resistor
• 2 1.5 Mohm resistor
• 1 56 kohm resistor
• 1 150 kohm resistor
• 2 1 kohm resistor
• 1 250 kohm potentiometer
• 1 Piezo speaker
• 1 Power switch

As you can see from the diagram, it is so simple that it can be assembled in a couple of hours. After assembly, make sure the polarity of the speaker and LED are correct. Place copper tubes and electrical tape on the photodiode. It should fit snugly. Drill a hole in the side wall of the aluminum case for the toggle switch, and on top for the photosensor, LED and sensitivity control.

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There should be no more holes in the case, since the circuit is very sensitive to electromagnetic interference. Once all electrical components are connected, insert the batteries. We used three CR1620 batteries stacked together. Wrap electrical tape around the tubes to prevent them from moving.

This will also help block light from reaching the photodiode. Now everything is ready to begin detecting radioactive particles. You can check it in action on any test radiation source, which you can find in special laboratories or in school classrooms for practical work.

Scheme No. 1 - elementary

To design a detector for detecting beta and gamma radiation “quickly and easily”, this option is perfect. What you will need before construction:

• a plastic bottle, or rather a neck with a lid;
• tin can without a lid with processed edges;
• regular tester;
• a piece of steel and copper wire;
• transistor kp302a or any kp303.

To assemble, you need to cut off the neck of the bottle so that it fits tightly into the tin can. A narrow, tall can, like condensed milk, is best. Two holes are made in the plastic cover, where you need to insert a steel wire. One of its edges is bent into a loop in the shape of the letter “C” so that it holds securely on the lid; the second end of the steel rod should not touch the can. Afterwards the lid is screwed on.

The KP302a gate leg is screwed to a loop of steel wire, and the tester terminals are connected to the drain and source. You need to wrap a copper wire around the can and secure one end to the black terminal. The capricious and short-lived field-effect transistor can be replaced, for example, by connecting several others using a Darlington circuit, the main thing is that the total gain must be equal to 9000.

The homemade dosimeter is ready, but it needs to be calibrated. To do this, a laboratory radiation source is used; as a rule, the unit of its ion radiation is indicated on it.

Assembly

The first thing to do is to adjust the voltage on the high voltage DC-DC potentiometer. For STS-5 we need approximately 410V. Then simply solder all the modules according to the diagram, I used solid wires, this increases the stability of the structure and makes it possible to assemble the device on the table, and then simply place it in the case. The important point is that we need to connect the minus at the input and output of the high-voltage converter, I just soldered the plug.

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Since we can't just connect the Arduino to 400V, we will need a simple circuit with a transistor, I just soldered them using the hinged method and wrapped them in heat shrink tubing, a 10MΩ resistor from +400V was attached directly to the connector. It is better to make a copper bracket for the tube, but I just wound the wire in a circle, everything works fine, do not change the plus and minus of the Geiger counter. We connect the display with a removable cable, carefully insulating it, since it was located very close to the high-voltage module.

Scheme of a homemade dosimeter.

Dosimeter circuit on a microcontroller

The device is designed to measure ionizing radiation caused by beta and gamma rays and has the following parameters:

• Measured dose range: 0 - 250 milliroentgen/hour
• Supply voltage: 2 – 3.3 V two AA batteries
• Average current consumption: 0.5 mA with sound indication disabled
• Time to reach operating mode: 30 seconds
• Reading update period: 1 second

The device consists of the following functional blocks: a high-voltage generator for powering the gas-discharge counter, a counter pulse shaper, a liquid crystal display control unit, an audio indication unit, and voltage stabilizers for powering various circuits of the device.

Synchronous control of all blocks is provided by the DD2 microcontroller. The high voltage is generated by the converter on transistor VT2 and transformer T1. The VT2 gate receives pulses with a frequency of 244 Hz and a duty cycle of approximately 4-15% from the DD2 microcontroller. At the moment of the pulse, the transistor is open and magnetic energy accumulates in the magnetic circuit T1.

Diagram of a homemade radiometer.

When the transistor is closed, a self-induction EMF is formed in winding I of the transformer, leading to a short pulse of positive polarity with an amplitude of about 60 V at the drain of VT2. This voltage is increased by winding II and supplied to the voltage tripler on diodes VD3-VD5 and capacitors C12-C14. The use of a voltage tripler reduces the requirements on the transformer and simplifies its design. A high voltage of about 400 V is supplied to the Geiger counter BD1 through load resistor R10.

Expert commentary

Lagutin Vitaly Sergeevich

Engineer with a degree in Computer Software and Automated Systems, MEPhI, 2005–2010.

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At this voltage, the meter operates in the middle of the plateau of its operating range. Zener diodes VD6-VD8 limit the voltage at the tripler output to a level of 430 V and protect capacitors C11-C13 with a rated operating voltage of 630 V from breakdown. Such protection is necessary during the setup process or when the dosimeter is suddenly removed from the radiation source.

Without zener diodes, the voltage on the capacitors can exceed 800-900 V and lead to their breakdown. The average current consumption in the T1-VT2 circuit does not exceed 0.3 mA with a load resistance of 40 MOhm and higher.

Scheme No. 2 - installation of the meter

In order to assemble a dosimeter with your own hands, a regular SBM-20 meter will do - you will have to buy it at a specialized radio parts store. An anode, a thin wire, passes along the axis through the sealed cathode tube. The internal space is filled with gas at low pressure, which creates an optimal environment for electrical breakdown.

The voltage of SBM-20 is about 300 - 500 V, it must be adjusted so as to prevent arbitrary breakdown. When a radioactive particle hits, it ionizes the gas in the tube, creating a large number of ions and electrons between the cathode and anode. Similarly, the counter is triggered for each particle.

It is important to know! For a homemade device, any meter designed for 400 volts is suitable, but the SBM-20 is the most suitable; you can purchase the popular STS-5, but it is less durable.

The dosimeter circuit consists of two blocks: an indicator and a network rectifier, which are assembled in plastic boxes and connected with a connector. The power supply is connected to the network for a short period of time. The capacitor is charged to a voltage of 600 W and is the power source for the device.

The unit is disconnected from the network and from the indicator, and high-impedance telephones are connected to the connector contacts. A capacitor should be of good quality; this will extend the operating time of the dosimeter. A homemade device can function for 20 minutes or more.

Technical features:

• the rectifier resistor should be optimally selected with a power dissipation of up to 2 W;
• capacitors can be ceramic or paper, with the appropriate voltage;
• You can choose any counter;
• eliminate the possibility of touching the resistor contacts with your hands

Natural background radiation will be registered as rare signals in phones; the absence of sounds means that there is no power.

What is a dosimeter

A dosimeter is actually a very simple device, we need a sensitive element, in our case a Geiger tube, power for it, usually about 400V DC and an indicator, in the simplest case it can be a regular speaker. When ionizing radiation hits the wall of a Geiger counter and knocks electrons out of it, it causes the gas in the tube to become a conductor, so a current goes straight to the speaker and causes it to click, if you're interested there is a much better explanation online.

I think everyone will agree that clicks are not the most informative indicator, however, it has the ability to notify about an increase in background radiation, but counting radiation using a stopwatch for more accurate results is a rather strange thing, so I decided to add some brains to the device . A dosimeter is actually a very simple device, we need a sensitive element, in our case a Geiger tube, power for it, usually about 400V DC and an indicator, in the simplest case it can be a regular speaker.

How to make a geiger counter with your own hands.

When ionizing radiation hits the wall of a Geiger counter and knocks electrons out of it, it causes the gas in the tube to become a conductor, so a current goes straight to the speaker and causes it to click, if you're interested there is a much better explanation online. Clicks are not the most informative indicator, however, it has the ability to notify about an increase in background radiation, but counting radiation using a stopwatch for more accurate results is a rather strange thing, so I decided to add some brains to the device.

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Scheme No. 3 with a two-wire detector

You can construct a homemade dosimeter with a two-wire detector; for this you need a plastic capacitor, a pass capacitor, three resistors and a single-channel damper.

The damper itself reduces the amplitude of oscillations and is installed behind the detector, directly next to the feed-through capacitor, which measures the dose. For this design, only resonant rectifiers are suitable, but expanders are practically not used. The device will be more sensitive to radiation, but will require more time to assemble.

There are other schemes on how to make a dosimeter yourself. Radio amateurs have developed and tested many variations, but most are based on the circuits described above.