Geiger–Müller meter circuit project

If electrons or ions are introduced into the cylinder, then the ion is ionised and therefore becomes conductive, so there is a current between anode and cathode (discharge). The gas consists of a mixture of noble gases and a gas that exhausts the discharge, the latter has the purpose of ending the ionization once the cause of it has ceased.

The voltage drop on the resistor connected in the series is enough to stop the discharge. Each evacuation is the proof that a charged particle has entered the cylinder and results in the generation of a current pulse that is seen in the ammeter or transformed into a voltage by passing it through a resistor.

The measure of the intensity of a radiation is the number of pulses in the unit of time e.g. In seconds or minutes. Some cylinders (tubes) have a glass casing, some have a metal casing (these are called casing counters) and some have a mica membrane in the opening of the cylinder and are called window meters.

Similar to the housing is the sensitivity of the meter, the thinner the casing (cathode), the greater the sensitivity and the more attention it needs to use. In the window pipes, the mica material lets it pass through almost no resistance to all the particles in the tube and so we have great sensitivity. However, since the membrane is very thin it is not allowed to touch it either. Metallic meter counters are for Gamma and Beta radiation measurements and window counters are for measurement of Alpha, Beta and Gamma radiation.

The circuit

The meter consists of three parts, the high-voltage generator, the tube and the device for measuring the pulses. A large part of the circuit is used to generate high voltage. The input of the regulating circuit is in the branch of a voltage divider consisting of the resistors A1, R2a to R2f and the P1 with which the high voltage is set between 300 and 1000V. When the voltage regulated by P1 is less than the desired Then T1 is not conducting because the voltage at its base is less than 0.7V. As a result of this we have a voltage in the collector of T1 (logic 1), so the oscillation of the circuit constructed with the NAN Schmitt-trigger of N1 begins. The oscillator signal enters the N2 used as an amplifier and then into T2 and the T3 supplying the transformer. Because of the oscillator's low frequency, a simple transformer for 50Hz 220V can be used.

Its secondary transformer will display a square pulse of about 250V, which with the voltage amplifier consisting of the diodes D4 to D11 and the C4 to C11 capacitors will be converted to a high continuous voltage. As the oscillator operates, the voltage rises. When the high voltage reaches the limit that we have set with P1, then at the base of T1 we have 0.7V and then it is conducting. The voltage on its collector is about 0V, causing the oscillations to stop and the high voltage increase.

If it is now reduced due to the discharger of the capacitor of the multiplier, then T1 stops flowing so the oscillations begin again. This way we have a high voltage setting with an accuracy of + 2%. This accuracy is sufficient for our purpose. The diode D1 connected between IC1 and Earth raises the voltage to 0.7V and so we always have a certainty that the oscillator will work in any case.

The power supply consists of T8, R4 and D2 having a voltage of 5.6V. So if a charged particle enters the tube, then due to ionization, the gas will briefly flow. This has the effect of generating a pulse that passes through T4 and T5 and then from the loudspeaker, and a characteristic noise will be heard. To have a visual indication we connect the T5 collector to the T6 base. T6 acts as a driving step. From the collector voltage divider we can get the pulses for a digital rpm. The direct current through R15 and D13 generates a 0.7 V voltage at the base of T7 which determines the operating point of the transistor.

Capacitor C13 completes (smooths) pulses originating from T6. The D12 diode is used to avoid C13 evacuation via R18, R13 and R14, so that the time constant depends only on the capacitor and P2. P2 is for adjusting the sensitivity according to the instrument. The T7 transmitter follower is used to drive the micro-ammeter after the signal has been filtered by the R13, C14 element.

The indication of the instrument is proportional to the frequency of the pulses and hence the intensity of the radiation and is regulated by the P2. The current consumption of the circuit is too little about 10mA. A small 9V battery is enough for ten hours of operation. For more time we can use two 9V in parallel.

Selection of the Sensor Tube

The selection of the pipe depends on its use and its cost. A simple mantle tube such as The ZP1310 is for Gamma and strong Beta radiation.

The ZP1430 is a window type and is for measurement of weak Alpha, Beta and Gamma radiation, but its cost is 3 to 4 times higher than the previous one.

ZP1400 is an intermediate type of the two previous ones. In the next table you will see the main technical data of those three types.

Operating Voltage V_B: The voltage at which the pipe must be fed.

Plateau Potential: This is the area of ​​the supply voltage in which the pulse rate is almost independent of the supply voltage.

Background: This is the indication of the instrument when there is no radiation. This indication is usually derived from cosmic radiation or from the radiation of the tube material.

Dead Time: It is the time it takes for the tube to return to its normal state after an ionization (the ionisation time is extinguished). During this time, the tube does not react to any other radiation entering it.

Value Limits: These values should not be overcome in any case.

Construction

The construction of the circuit is easy. The printed circuit PCB for the construction is shown. Because of the high voltage, only attention is needed at the point of welding. The gluing should be a little and no spikes should be created because there is a risk of leakage.

The only difficulty in finding, is the capacitor C15 (1p / 1000V) which is not very common, however, you may not place it.

The transformer is 1.6VA with 220V / 9V and 33mmX 27mmX37mm. Higher power transformers should not be used due to the collector current of T3.

Instead of 74LS13 the 7413 can be used, but then the current instead of 10mA will be about 20mA.

Resistance R19 and capacitor C15 (if present) are not placed on the printed but as close as possible to the tube's rise to prevent large capacity between anode and resistor.

The connection to the anode is done with the special connector that each tube has. The connection must be made after the R19 is glued onto the connector. The rise of the pipe must not be heated or mechanically stressed.

Function and setting

Before the appliance is switched on for the first time, the pipe must not be connected. The cursor of P1 must be in the minimum voltage state towards the R1 side.

After the connection, the capacitors begin to charge and after a few seconds the charging current, which was initially about 120mA, dropped to 10mA. With a multimeter (1 KV region) at the output of the multiplier and with the help of P1 we raise the voltage to V_B.

Because high voltage is dangerous, a lot of attention is needed during handling. The measuring terminals must be well insulated and must never touch high voltage points even when disconnected (at least for a few seconds until the capacitors are discharged). For a somewhat faster discharge of the capacitors we can use a resistance of several thousand Amps, but we can not short-circuit them.

For setting, we need a 5V pulse generator with a frequency of 5Hz to 20Hz (eg an unstable multiplier made with IC 555 or the like).

The meter's instrument is so set that the highest sensitivity (P2 cursor at the (+) of C13) gives us a full reading (50μA) at 7 pulses per second. An instrument of 100μA gives full indication at 15 beats per second. When we use the ZP1310 tube then we have a sensitivity of about 4mR per hour (instrument 50μA), for ZP1400 we only have 0.4mR per hour.

The device is set up as follows:

1) From the characteristic tube we find the dosing rate (eg for ZP1400 and for 20 pulses per second (20imp/s) we find a dose rate of 1mR per hour (or 10^-3R / h).

2) The pulse frequency we need (for our example) from the generator is 20Hz to be connected parallel to the R8.

3) With P2 set the rotating coil M1 to show full indication.

To control the pulse frequency, we can connect a frequency meter to the D point. After that, the system is fully ready for operation after the tube has been connected to its position before closing it.

The geiger meter - Muller when it comes into operation will start to sound inside our room. If we want more clues, we can only get close to asbestos, the reading will be about 50 beats per second because asbestos contains thorium oxide.

We also have a strong enough indication when we approach the meter in potassium salts that we can buy from pharmacies (potassium chlorate, potassium carbonate, etc.).

List of materials

Update: If the power consumption is greater than normal, then raise the value of R6 to 3KΩ and remove D3.