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PancakeGeiger

Purpose

For experimenting with weakly radioactive items, and for sensing of low-energy x-rays and α-radiation, a Geiger counter more sensitive than the old Soviet-era DRSB01 was desired.

A SBT10A Geiger tube (also described here) was obtained from eBay and the rest of electronics was built around it. Original approach with knocking off a third-party design wholesale did not work as the attempts either malfunctioned outright or consumed too much current; a bottom-up design was therefore attempted and proved to be successful, with the resulting operating current being kept below 1.5 milliamps.

The Mk1 Geiger counter was successfully used in the Geiger On A Plane experiment.

Construction

The entire design was subject to severe constraints required by the desire to maintain simplicity and as long as possible battery life.

Power supply

For powering the unit, a Li-poly battery was chosen for its relative robustness and long life. For in-circuit charging a MAX1555 chip was chosen due to simplicity and availability. As the chip allows two charging modes, one current-limited to 100mA for USB charging and one limited to 350mA for charging from a wall-wart, it was decided to use two microUSB connectors - one for slower and one for faster charging. This gives the flexibility for deciding between low and high charging rate, allowing use of virtually any USB-grade power supply. The charging is monitored by a red LED, active when the battery charging current is sufficiently high; when it goes dark, it signals the end of charging (or the loss of cable contact - this is a potential problem with this design approach, rectifiable by using a two-color LED).

High voltage supply

A Geiger tube requires fairly high voltage for operation, in the ballpark of 350-1000 volts. The halogen-quenched ones are usually at the lower end of the range. Most common operating voltage for them is at about 400V. A boost converter is therefore necessary.

Finding a schematics of a boost converter that would require minimal current and could run from a Li-ion battery turned out to be somewhat a challenge. It turned out to be easier to design one from scratch, using just fragments of third-party circuits and generic approaches.

Oscillator

The heart of a step-up converter is the oscillator. A simple circuit with a LM358 dual op-amp was designed. The waveform required was asymmetric, short narrow positive spikes. The diodes were therefore employed to maintain this asymmetry; the high period was set to be as short as achievable with a common 1N4148 diode, the low period (and therefore the frequency) can be adjusted with a trimpot for maximum efficiency and lowest power consumption.

The LM358 was chosen due to its cost, availability, low operating voltage, and decent overall parameters. Its current consumption is also low (though significant for this application, a lower-power alternative could be worth investigating).

Transformer

A custom transformer had to be built, as none of the available scrap ones provided any reasonable efficiency. A ferrite core was salvaged from a small transformer obtained from a dead ATX power supply; a tall transformer with a fairly large space on the bobbin was chosen due to the predicted amount of winding. The primary was wound with a roughly 0.3 mm copper magnet wire, until about 100 µH inductance was achieved - about 20-25 turns. The secondary was calculated to be about 1200 turns for over 200 volts of output; the thinnest magnet wire available (from an old relay) was used. The insulation between the primary and the secondary was made with kapton tape. A single layer of kapton tape was placed between each 300 turns of the secondary and then over the entire coil.

The polarity of the secondary winding in respect to the primary has to be maintained. One of the two possibilities results in higher secondary voltage - that's the one to choose.

The performance of the transformer was optimized by adjusting the air gap between the halves of the core. Shims from two layers of thin kapton tape were found to be the best.

Multiplier

The output of the transformer is doubled with a Cockcroft-Walton generator, composed of three 1N4007 diodes and three high-voltage capacitors. The output capacitor was chosen to be of higher value (10 nF), in order to provide smoother feed for the tube. The first stage of the multiplier is tapped for the feedback, a compromise between output voltage stability and power wasted on the feedback resistor divider (and therefore power drawn from the battery).

Discharge pins, colored yellow, are available in the circuit for the purpose of removing residual charge after powering the circuit off and preventing getting a mild but unpleasant shock.

Feedback

The second op-amp was used for feedback control of the generator. The Cockroft-Walton cascade was tapped at its first stage. A resistor divider was made from a 100 MΩ resistor (built from 10 pieces of 10 MΩ SMD resistors) and a 3.3 MΩ trimpot, used for setting the desired voltage. The trimpot output was connected to the inverting input of the op-amp.

The noninverting input is connected to a resistor divider, maintaining a half of the battery voltage. This is sufficient for short-term tests, but has to be replaced with a real voltage stabilizer; whether a low-drop stabilizer or even just a low-voltage Zener diode.

The comparator op-amp output is used to pull down the noninverting input of the oscillator op-amp; this increases the interval between the pulses and lowers the amount of power fed into the transformer. To avoid pushing it up (which turned out to be shortening the periods too much and widening the positive pulse, which impaired the circuit stability and power consumption), a Schottky diode is used.

Sensor

The SBT-10A is a very sensitive tube suitable for detection of α, β and γ radiation, down to fairly low energies. It is composed of ten individual chambers, with separate cathodes and common anode, and clad with a mica window. Its recommended operating voltage is 390 volts.

Due to lack of a suitable socket the tube's pins were directly soldered to wires and secured with heat-shrink tubes.

Each cathode is fed through a dedicated 10 MΩ resistor to facilitate separate operations and limit the effect of the tube dead time to the individual chambers.

The tube was bought as new-old-stock from Ukraina. The seller was reasonable and declared it on the paperwork as an old vacuum tube with low price, so it sailed through the customs without delay and extortion. Shopping with East and Far East is a pleasure; Americans in turn tend to insist on doing things "correctly", which only brings annoyance to the recipients.

Detection circuit

Due to the desired simplicity, a simple approach was used. The anode of the tube was connected to a small resistor, the current pulses manifested as voltage spikes, and these were used to open a transistor.

The transistor operates a blue LED (which indicates the ionization events with a distinct flash), and a small 16-ohm speaker that produces audible clicks. No pulse widening was found to be necessary for satisfying indication. Due to possible annoyance factor the speaker has a switch allowing disabling it. Due to the desire to operate the detector semiclandestinely, a 3.5 mm $ was added to allow connecting headphones (which also automatically disconnects the internal speaker).

The detector circuit turned out to be sensitive to external noise; holding the case in hand with fingers underneath the transistor produced a distinct mains hum in the headphones. A layer of aluminium adhesive tape, placed under the circuitboard and connected to the circuit ground, solved this issue.

Mechanical assembly

A plastic box was chosen for the enclosure. Hole was cut with a hot knife for the geiger tube, the tube was set in the case with a pair of aluminium strips bent around it and secured with double-sided foam adhesive around its window.

Due to the very high mechanical sensitivity of the tube's mica window an aluminium flip-off shield was attached to the box as a protection for routine operations when the low-energy and alpha sensitivity is not needed. The tube was expensive and it is not so easy to get.

Tube protection

As the mica window of the pancake tube is rather annoyingly sensitive to mechanical damage, it was decided to protect it with a grille. A combination of a mechanically strong grille from thick steel wires with a denser thin-wire mesh was chosen to provide a compromise between mechanical strength and area consumption (and therefore alpha- and beta- shielding).

An off-the-shelf grille for a 92x92 mm cooling fan was chosen for the main structure. As the probe window is rectangular and the fan was square, the sides of the outermost ring were cut off. The plastic box was slightly filed off at the middles of the longer sides of the window, to accommodate the sides of the second outermost ring. (This was done in an excessive way, resulting in somewhat ugly edges.) The mounting brackets of the grille were bent in to avoid interference with the screws holding in the tube-mount strips but to rest against the plastic edges of the tube. (One mountpoint was partially broken off by an accident; the material is pretty intolerant to bending.) The brackets were secured with hot glue.

A rectangle was cut from an aluminium mesh and placed over the grille, overhanging to the sides. A hot knife on the soldering iron was used to locally heat the mesh and push it into the plastic, then to smoothen the plastic over the mesh, resulting in firm embedding of the wires in the plastic.

To prevent inward bending of the mesh, thin wires were used to tie it to the main grille at several points and were secured with drops of hot glue to prevent them from getting their ends loose and poking through the mica window.

Endurance

At normal level of background radiation (few pulses per second) the device can run for over 4 weeks on a single charge of its 850mAh battery.

TODO

Images


SBT10A pancake tube

SBT10A pancake tube

Inside view

Detail of the electronics

Detail of the electronics

Detail of the battery

Detail of the charger

Detail of the battery and charger

Detail of the indicators

Detail of the electronics

Detail of the electronics

Detail of the electronics

Detail of the electronics

Bottom side, cover closed

Bottom side, cover opened

Top side with indicator

Control panel

Main grille

Main grille

Main grille

Fine mesh over grille

Fine mesh over grille, after melting-in

Fine mesh over grille, from outside

Edge cushioning

Edge cushioning

Finished tube protection

Finished tube protection

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