CN0536

Geiger-Mueller Tube Principles of Operation

The Geiger-Muller tube is an ionizing radiation detector that is sensitive to alpha, beta, gamma, and x-ray radiation. While it cannot discriminate between radiation types, its sensitivity, simplicity, and reliability make it a practical choice in a wide range of radiation monitoring applications, either alone or in conjunction with other sensors.

Geiger-Muller tubes are commonly biased to a voltage ranging from 250 V to 500 V with a near zero current drawn when no radiation events are taking place. Several typical Geiger-Muller tubes are shown in Figure 2.

 

When an ionized particle enters the tube, ion pairs consisting of a free electron and a positively charged gas molecule are created. Electrons accelerate toward the anode, creating additional ion pairs, causing an avalanche of additional electrons. Electrically, this effect produces a momentary low impedance path between the anode and the cathode that can be detected as a voltage pulse across an external resistance. The external resistance is also necessary to allow the anode to the cathode voltage to drop to a point where the avalanche stops (known as quenching).

The output signal can be measured at either the cathode (see Figure 3) or anode (see Figure 4).

While taking the signal from the cathode avoids the need for a high voltage, level shifting capacitor, it is more sensitive to electrical noise because the cathode impedance to ground is relatively high. The CN-0536 takes the signal from the anode (see Figure 4) with the cathode grounded to minimize susceptibility to noise.

Geiger Pulse Detection

Figure 5 shows the LTspice^® schematic of the Geiger-Mueller pulse level shifting and detection circuitry.

When no radiation events are occurring, the anode is biased at 400 V. C28 and C26 form a capacitive voltage divider, whose output is biased at IO_VREF through R21. When an event occurs, the anode is momentarily pulled toward ground potential, producing a 400 V negative going step. When the output of the attenuator drops to less than the 1.98 V threshold set by R27 and R28, the output of the LTC1441 comparator drives high. The comparator output is routed to an interrupt pin on the platform board, such that software can respond to the event. Figure 6 shows the attenuator and comparator outputs for a typical radiation event.

High Voltage Supply

The Geiger-Mueller bias is provided by the micropower, high voltage power supply shown in Figure 7. The circuit is a fixed frequency, fixed 50% duty cycle, nonsynchronous boost converter followed by a Cockroft-Walton multiplier, with hysteretic voltage mode control.

An LTC6906 resistor-programmable oscillator is configured to produce a 250 kHz, 50% duty cycle square wave. An AND gate buffers this clock signal and functions as a gate driver for the boost power metal-oxide semiconductor field effect transistor (MOSFET). Because the LTC6906 draws its full 12.5 μA supply current whenever power is applied, the converter is enabled and disabled by simply switching the V+ supply of the LTC6906 on and off. The AND gate ensures that the switching FET is either off or fully enhanced, even as the supply voltage of the LTC6906 is ramping up or down.

Voltage feedback is taken from the output of the first multiplier stage, which greatly reduces power dissipation in the feedback divider at the expense of a slight reduction in load regulation. Figure 7 is the LTspice schematic simulation of the high voltage power supply that is included in the design support package.

High Voltage Generation (280 V to 500 V)

When power is first applied, the feedback voltage is at ground potential, driving the output of U1 high, which in turn powers up the LTC6906, and the boost stage begins switching at 350 kHz, 50% duty cycle. L1, Q2, D1, and C5 form a conventional nonsynchronous boost converter, and once the voltage at C5 rises to 64 V, U1 trips off. The output voltage slowly decays to 55 V, determined by the hysteresis of U2, and the cycle begins again. The initial turn-on transients are shown in Figure 8 and Figure 9.

The output voltage is set to 400 V nominal and maintained between 390 V and 410 V. The duty cycle is low, approximately 0.1%, with a 200 µs burst every 600 ms. This results in an average current consumption of 33 µA. Probing the output of the multiplier is challenging because even the 10 MΩ impedance of a typical scope probe can affect the operation of the circuit. However, probing earlier stages illustrates the principle. Figure 10 and Figure 11 show the voltage at C11 at the output of the second multiplier stage, while Figure 12 and Figure 13 show the pulse switching of the 100 V, N-channel, enhancement mode MOSFET Q2 driven to its gate terminal by the 350 kHz switching frequency using a combination of an AND gate and the LTC6906 frequency clock generator.

Geiger Pulse Visual/Audible Indicators

The 18 µs output pulse from the Geiger-Mueller event detector circuit is too short to reliably generate a visual indication (LED) or audible click from a buzzer. An LTC6994 programmable delay block/debouncer is configured with a master timebase (t_MASTER) of 2.5 µs, N divider value of 512, and a delay falling edge (POL) = 1. This results in a 1.28 ms pulse to an LED indicator and buzzer, producing an easily visible flash and audible click. Figure 14 shows the detailed schematic of the timer block delay.

where:

High Voltage Power Good

The second half of the LTC1441 comparator functions as a high voltage power-good indicator (see Figure 15).

The circuit uses the REF output (V_REF) of the LTC1540 as a reference, which is the same reference that is used to set the regulation voltage. If the potentiometer wiper falls to less than the reference voltage, the comparator output asserts, indicating a loss of regulation.