The German cosmic ray mine unit (figure 257) was an experimental attempt to pro-duce a mine-firing unit which operates on changes in the cosmic ray background due to the transit for a ship over the mine. It was in the developmental stage, and was not de-veloped sufficiently for production.

The unit consists, essentially, of a group of Geiger-Muller counter tubes in a ground mine and a suitable amplifier to use the cosmic ray pulses. When such a system is sub-merged, the layer of water above it will cause a constant amount of aborption to cosmic rays. If a ship passes overhead, a certain mass of water is displaced by thin layers of iron; i.e., the ship's bottom and decks. The absorption is thereby decreased and the cosmic ray level increases. If the gradient is sufficiently large, a rate-of-change circuit operates, and the mine fires.

The Counter Tubes. The counter tubes are Geiger-Muller counters of standard manu-facture. The ones experimented with were of glass, with a silver lining as one electrode and an iron wire on the axis as the other. The tubes are air-filled (possibly Ne or A) at 0.1 mm pressure. These were found too fragile for dropping from aircraft, and an experi-mental type of steel tube was in the development. The tubes, 24 in number, were moun-ted in an LMB mine-case, and cast into the explosive. They were mounted in a circle, with their axis parallel to the axis of the mine.

The Circuit. If one counter tube were used in this type of mechanism, the effect on the rate-of-change circuit would be unsatisfactory, since the interval between cosmic ray pluses passing though one tube would be too long. As well, sharp peaks are produced in the output circuit of one counter tube. If several tubes are used, the resultant mixing of pulses produces a smoother background level. In the German experiments, 24 such tubes were used to make a fairly smooth background, and the effect of a ship on such a background is shown in figure 257. It was intended to use a circuit of the type shown, using a one-tube extinguisher for each counter tube, and feeding a common rate-of-change circuit. This type of circuit was used to discriminate against tidal changes, con-stant small changes in comsic ray background level, and changes caused by waves and swells.

Power Supply. The principal difficulty encountered was proper voltage regulation. A potential of 1,000 to 1,500 volts was used, and potential regulation of 0.1% was consi-dered necessary for proper extinguishing characteristics of the counter tubes. At one time the work on this unit was nearly abandoned as a result of the "nearly insuperable difficulties" with potential regulation. There is also, of course, some difficulty in handling such a high potential in a mine case. The main effort to produce a satisfactory high-po-tential supply was directed to the co-called "Volta-Pillar", consisting of a great number of very thin copper and zinc plates piled up with sheets of wet paper between.

Directional Effect. While this unit was under development, one of the proposed vari-ations was to make the unit directional. This was to be accomplished by making tubes operate in pairs, and requring that a cosmic ray pulse operate the two tubes in a pair before it registered in the circuit. This proposal was entirely in the idea stage, and the Germans considered that it would require too many counter tubes and would unnecessa-rily complicate the circuit.

Figure 257 – Cosmic Ray Mine Unit Circuit