Dr. Samuel Goudsmit was the head of the US intelligence mission to Europe codenamed ALSOS, whos objective was to discover to what extent the Nazis had been working on an atomic weapon. In his book "ALSOS - The Failure in German Science" (New York, 1947), there appears a sketch of the zenith of German scientists' achievement in the field. The same diagram appears in the book authoured by Lt. Leslie Groves, military chief of the Manhattan Project. Both Goudsmit and Groves stated that the diagram and photos represent "the German atom bomb".

The bomb was an aluminium sphere, about the size of a medicine ball, and had a tall chimney. The latter enabled the radium-beryllium radio-active source to be introduced into the core of the reaction. Within the sphere was layered alternately natural uranium powder (551 kilos) and paraffin wax.

The Nobel Prize winner Professor Heisenberg was looked to as the pioneering genius of Germany's atomic project. This was outwardly aimed at building a working atomic pile, a target which had not been reached by the end of hostilities five years later. The excuse offered was that there was not enough heavy water available for the final successful experiment. Since Heisenberg's assistant Dr Karl Wirtz stated in his 1987 book "Im Umkreis der Physik" that there was easily enough heavy water in aggregate to moderate a nuclear pile in 1944, and he could not understand the reluctance to go ahead and do so, our attentions are drawn to the possibility that the heavy water was needed in another area.

As he admitted, Heisenberg's experiments B-III and L-IV at Leipzig made calculations regarding the effectiveness of paraffin wax as a barrier and measured the capture of neutrons by U-238 uranium material after they had been emitted by the radioactive source and been slowed by passage through heavy water. Dr. Flannen, a US physicist, explained in an internet article that these two experiments could only be explained if the aim was to design not a reactor, but a bomb.

By 1941 the Germans knew that isotopes of U-238 in capturing neutrons became transformed into isotopes of plutonium, and Heisenberg was measuring where most such transformations took place. This would not be of much interest for reactor technology, but would be vital if building a bomb. The paraffin wax would have a function as a bomb part in connection with a technical problem associated with plutonium isotopes.

In June 1942 at Leipzig, Heisenberg placed within an aluminium sphere about 750 kilos of natural uranium, placed a concentric sphere of heavy water at its centre, dropped the radioactive source down the chimney and sat back. Five weeks later there was a disastrous fire and the experiment was terminated. But - what was this experiment intended to prove?

The United States invested hundreds of millions of dollars into uranium enrichment plants and plutonium breeder reactors. Germany, under heavy aerial bombardment and on a tight budget, could never have competed. What was needed was a nuclear device of small magnitude which could be mass-produced at small cost.

When an aluminium sphere of natural uranium powder is left to breed in the manner of Heisenberg's device, within about two years the plutonium bred by U-238 capturing neutrons exceeds the figure of 7%. This is the magic figure for a nuclear explosion of some sort.

If several hundred such spheres were left to breed for two years in mid-1942, by late 1944 Germany would have had a small arsenal of little nuclear devices. All that was needed would be some means of setting them off.

The target was London. If Britain could be forced out of the war, even in late 1944 there was still a slim chance of success for Germany. The obvious means of delivering the weapons on London was the V-2 rocket. The little bombs weighed less than a ton, and could fit easily into the space for the V-2 warhead. There was no need for tonnes of conventional explosive to explode the device - the rocket hit the ground at 3,500 per second. This speed was fast enough to assemble the plutonium-enriched uranium material into a critical mass. In the split-second before the reaction collapsed, the resulting blast would be in the region of 20 tonnes TNT with nuclear fallout. The paraffin wax prevented the unstable plutonium isotope Pu 240 from reacting too smartly and so ruin the nuclear reaction.

How long could London have withstood two or three such rockets fired on London every day? Each crater region would be unapproachable for years, maybe decades. The effect of the fallout need not be mentioned. No surprise then, that Lt. Gen Putt, Deputy Head of United States Air Force Intelligence, should state shortly after the war that if the invasion of Europe had been delayed by six months, the course of the war would have been changed, for Germany had "rocket surprises in store for the whole world in general and England in particular".

The range of a V-2 was 200 miles. In June 1944, London was in range from anywhere along the French and Belgian coasts. Six months after the invasion - December 1944 - the German front line was far back from this 200 mile point. The Germans had no intermediate rocket to hit London from Germany - the critical failure of German science. Hence the need for the Ardennes Campaign to recapture Antwerp which is 200 miles from London.

Note that although this page is titled atom bomb, the weapon developped by the Germans could not be called a nuclear device in the sense of it being an atomic explosive. The Americans decided in 1944 that the term "nuclear device" or "atom bomb" should not be applied to any nuclear explosive with an equivalent yield less than 500 tons TNT. The yield of the V-2 warhead would not have exceeded 30 tons TNT or so. If you have a conventional explosive to scatter radioactive dust, that weapon is a radiological device. Similarly the weapon described would have used the effects of meltdown as a localized radiological weapon.

Originally published under "The German Atomic Device - The Evidence" by Geoffrey Brooks at the Axis History forums.