watch the glowing orange ball dip through one of the blowing snow clouds. Some things were better than football. When the last edge of the sun dipped below the ridge line, he turned back, deciding to take another look at the box inside the truck. He would not make it.
35
THREE SHAKES
The timer just outside the bomb case reached 5:00:00, and things began to happen.
First, high-voltage capacitors began to charge and small pyrotechnics adjacent to the tritium reservoirs at both ends of the bomb fired. These drove pistons, forcing the tritium down narrow metal tubes. One tube led into the Primary, the other into the Secondary. There was no hurry here, and the objective was to mix the various collections of lithium-deuteride with the fusion-friendly tritium atoms. Elapsed time was ten seconds.
At 5:00:10, the timer sent out a second signal.
Time Zero.
The capacitors discharged, sending an impulse down a wire into a divider network. The length of the first wire was 50 centimeters. This took one and two-thirds nanoseconds. The impulse entered a dividing network using kryton switches — each of them a small and exceedingly fast device using self-ionized and radioactive krypton gas to time its discharges with remarkable precision. Using pulse-compression to build their amperage, the dividing network split the impulse into seventy different wires, each of which was exactly one meter in length. The relayed impulses required three-tenths of a shake (three nanoseconds) to transit this distance. The wires all had to be of the same length, of course, because all of the seventy explosive blocks were supposed to detonate at the same instant. With the krytons and the simple expedient of cutting each wire to the same length, this was easy to achieve.
The impulses reached the detonators simultaneously. Each explosive block had three separate detonators, and none of them failed to function. The detonators were small wire filaments, sufficiently thin that the arriving current exploded each. The impulse was transferred into the explosive blocks, and the physical detonation process began 4.4 nanoseconds after the signal was transmitted by the timer. The result was not an explosion, but an implosion, since the explosive force was mainly focused inward.
The high-explosives blocks were actually very sophisticated laminates of two materials, each laced with dust from light and heavy metals. The outer layer in each case was a relatively slow explosive with a detonation speed of just over seven thousand meters per second. The explosive wave in each expanded radially from the detonator, quickly reaching the edge of the block. Since the blocks were detonated from the outside-in, the blast front traveled inward through the blocks. The border between the slow and fast explosives contained bubbles — called voids — which began to change the shock-wave from spherical-shaped to a planar, or flat wave, which was refocused again to match exactly its metallic target, called “drivers.”
The “driver” in each case was a piece of carefully-shaped tungsten-rhenium. These were hit by a force wave traveling at more than nine thousand eight hundred meters (six miles) per second. Inside the tungsten-rhenium was a one-centimeter layer of beryllium. Beyond that was a one-millimeter thickness of uranium 235, which though thin weighed almost as much as the far thicker beryllium. The entire metallic mass was driving across a vacuum, and since the implosion was focused on a central point, the actual closing speed of opposite segments of the bomb was 18,600 meters (or 11.5 miles) per second.
The central aiming point of the explosives and the metallic projectiles was a ten kilogram (22 pound) mass of radioactive plutonium 239. It was shaped like a glass tumbler whose top had been bent outwards and down towards the bottom, creating two parallel walls of metal. Ordinarily denser than lead, the plutonium was compressed further by the million-atmospheres pressure of the implosion. This had to be done very quickly. The plutonium 239 mass also included a small but troublesome quantity of plutonium 240, which was even less stable and prone to pre-ignition. The outer and inner surfaces were slammed together and driven in turn towards the geometric center of the weapon.
The final external act came from a device called a “zipper.” Operating off the third signal from the still-intact electronic timer, the zipper was a miniature particle accelerator, a very compact mini-cyclotron that looked remarkably like a hand-held hair-dryer. This fired deuterium atoms at a beryllium target. Neutrons traveling ten percent of the speed of light were generated in vast numbers and traveled down a metal tube into the center of the Primary, called the Pit. The neutrons were timed to arrive just as the plutonium reached half of its peak density.
Ordinarily, a material weighing roughly twice an equivalent mass of lead, the plutonium was already ten times denser than that and still accelerating inward. The bombardment of neutrons entered a mass of still- compressing plutonium.
Fission.
The plutonium atom has an atomic weight of 239, that being the combined number of neutrons and protons in the atomic nucleus. What began happened at literally millions of places at once, but each event was precisely the same. An invading “slow” neutron passed close enough to a plutonium nucleus to fall under the Strong Nuclear Force that holds atomic nuclei together. The neutron was pulled into the atom's center, changing the energy state of the host nucleus and kicking it into an unstable state. The once symmetrical atomic nucleus began gyrating wildly and was torn apart by force fluctuations. In most cases a neutron or proton disappeared entirely, converted to energy in homage to Einstein's law E = MC2. The energy that resulted from the disappearance of the particles was released in the form of gamma and X-radiation, or any of thirty or so other but less important routes. Finally, the atomic nucleus released two or three additional neutrons. This was the important part. The process that had required only one neutron to start released two or three more, each traveling at over 10 percent of the speed of light—20,000 miles per second — through space occupied by a plutonium mass two hundred times the density of water. The majority of the newly-liberated atomic particles found targets to hit.
A chain-reaction merely means that the process builds on itself, that the energy released is sufficient to continue the process without outside assistance. The fission of the plutonium proceeded in steps called “doublings.” The energy liberated by each step was double that of the preceding one, and that of each subsequent step was doubled again. What began as a trivial amount of energy and just a handful of freed particles doubled and redoubled, and the interval between steps was measured in fractions of nanoseconds. The rate of increase — that is, the acceleration of the chain reaction — is called the “Alpha,” and is the most important variable in the fission process. An Alpha of 1,000 means that the number of doublings per microsecond is a vast number, 21000—the number 2 multiplied by itself one thousand times. At peak fission — between 250 and 253—the bomb would be generating 10 billion billion watts of power, one hundred thousand times the electrical-generating capacity of the entire world. Fromm had designed the bomb to do just that — and that was only ten percent of the weapon's total designed output. The Secondary had yet to be affected. No part of it had yet been touched by the forces only a few inches away.
But the fission process had scarcely begun.
Some of the gamma rays, traveling at the speed of light, were outside the bomb case while the plutonium was still being compressed by the explosives. Even nuclear reactions take time. Other gamma rays started to impact on the Secondary. The majority of the gammas streaked through a gas cloud that only a few microseconds earlier had been the chemical explosive blocks, heating it far beyond the temperatures chemicals alone could achieve. Made up of very light atoms like carbon and oxygen, this cloud emitted a vast quantity of low-frequency “soft” X-rays. To this point, the device was functioning exactly as Fromm and Ghosn had planned.
The fission process was seven nanoseconds—0.7 shakes — old when something went wrong.
Radiation from the fissioning plutonium blazed in on the tritium-impregnated lithium-deuteride that occupied the geometric center of the Pit. The reason Manfred Fromm had left the tritium extraction to last lay in his basic engineer's conservatism. Tritium is an unstable gas, with a half-life of 12.3 years, meaning that a quantity of pure tritium will, after that time, be composed half of tritium and half of 3He. Called “helium-three,” 3He is a form of that second-lightest of elements whose nucleus lacks an extra neutron, and craves another. By filtering the gas through a thin block of palladium, the 3He would have been easily separated out, but Ghosn hadn't known about that. As a result, more than a fifth of the tritium was the wrong material. It could hardly have been a worse material.
