GeigerWatch, a Sensitiv Radiation Detector Watch

Description

The GeigerWatch is a digital Watch with the additional function of a radiation detector. It is not a real Geiger counter, as instead of a counting tube, three modified PIN photo diodes serve as sensor. The aim is to achieve high sensitivity to alpha, beta, and gamma radiation with a small size, low power consumption, and power supply from a single Li-Po cell. This was achieved by applying a negative bias (-4.5 V) to the PIN photo diodes and using a complicated laser ablation, in which the plastic package is removed almost down to the silicon chip.

Details

A CAD program was designed for laser ablation of the BPW34 in order to exclude the bonding wire and leave a frame that ensures mechanical stability. The laser parameters were selected so that the silicon chip would not be damaged. In the experiment, the count rate for radium (light points on an old compass) increased by 70% and for potassium tablets by 50% compared to unmodified BPW34 (experiment in a cookie jar).

Laser can be seen here: https://www.dropletwatch.com/laser-tools

Later, the photo diodes must be sealed light-tight. Either 10 µm aluminum foil can be used for this purpose, although this would absorb some of the alpha particles, or < 1 µm aluminum leaf, which is of course very sensitive and difficult to work with.

To increase sensitivity, the BPW34 are operated with a negative bias voltage of 4.5 V. For this purpose, a constant voltage of 5.2 V, independent of the battery's state of charge, is generated via U2(LTC3459), which feeds U3(LTC1261L), which in turn generates a negative voltage of -5 V, which charges C1 to 4.5 V via D1. D1 (BAS45) is characterized by very low leakage current (< 1nA), which, together with the reverse dark current of the three BPW34 (~ 3nA), prevents C1 (insulation resistance 10 GΩ) from discharging too quickly. The charging cycle takes place no later than one minute (whenever the ePaper display is updated) after the Attiny3226 is woken up by the DS3231 RTC. During one minute, the voltage drops by < 0.2 V (measured with HP 3478A, Ri = 10¹⁰Ω).

U1A (LTC6241) operates as a low-noise transimpedance amplifier to amplify the current pulses of a few nA per decay and generate a voltage that is further amplified via U1B and U4A. U4B is connected as a comparator. The voltage drop across D2 (~170 mV) serves as the reference voltage. U6 (PCF8593 low power clock and calendar) is operated as an event counter and transmits the count value to the Attiny3226 via the I²C bus. The counting process only takes place in the Attiny's deep sleep mode, which is periodically awakened by the DS3231 RTC to update the display. During this time (~0.5 s), VEE is switched off from U4 to avoid interference (crackling noises and incorrect counting pulses).

As in previous projects, the circuit board was manufactured using a combination of LDI and etching. (https://www.dropletwatch.com)

Next step: Mechanical processing of the Watchy CNC Anodized Aluminum Case from SQFMI to accommodate the BPW34 (in progress)

The aluminum foil proved to be unsuitable: it is translucent!! and has a thickness of approx. 200 nm!

I lightly anodized the 10 um aluminum foil and colored it brown. I cut out the aperture with my laser and also anodized it brown.