The first atom bomb was known as an “implosion device.” It was big, unwieldy looking thing called “the Gadget.” Its fissionable core was plutonium, a recently discovered element at that time.
The Gadget. The cables are hooked to HE (high explosive) charges. The plutonium core is a spherical mass. The charges—very carefully timed—go off at once compressing the plutonium sphere at every point along its surface at the same instant, causing the core to go supercritical and explode. To ensure the charges go off at exactly the same instant, the cables of each charge had to be same exact length.
To work, the Gadget was designed to keep two subcritical masses of fissile material apart and then allowing them to combine into a supercritical mass. For example, a polonium pellet wrapped in foil inside the fissile core will prevent supercriticality until the radioactivity of the polonium eats through the foil causing the generation of fast neutrons for a chain reaction. Then instead of slowing down the fast neutrons as with ordinary nuclear power, they instead use a reflector of beryllium, for example, to reflect fast neutrons back into the fissile material which would enable the critical mass to be much smaller and therefore lighter and yet much more powerful. Or they might use a tamper. A tamper is a dense shell of material, usually depleted uranium, that spherically surrounds the fissile core which then gets heated and expands when the fission process starts. This expansion of the tamper puts pressure back on the core slowing its expansion enabling more of the core material to undergo fission. In addition, the tamper is also a neutron reflector which keeps fast neutrons from escaping from the core. In other words, the majority of the fissionable material gets used before detonation is allowed to occur thus increasing the yield. If the tamper is made of depleted uranium, this too will undergo fission contributing yet more power to the yield.
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As the neutrons are ejected from an unstable atomic nucleus, they release energy in the form of a gamma ray—a high-energy electromagnetic wave/photons. Because the neutrons are not slowed down, the amount of neutrons liberated from nuclei increases exponentially in the chain reaction, i.e. the mass goes supercritical.
The site where the Gadget was to be detonated was part of an Air Force base at Alamogordo, New Mexico. Oppenheimer dubbed it “Trinity” after 14th Holy Sonnet of John Donne:
Batter my heart, three-person'd God, for you
As yet but knock, breathe, shine, and seek to mend;
That I may rise and stand, o'erthrow me, and bend
Your force to break, blow, burn, and make me new.
I, like an usurp'd town to another due,
Labor to admit you, but oh, to no end;
Reason, your viceroy in me, me should defend,
But is captiv'd, and proves weak or untrue.
Yet dearly I love you, and would be lov'd fain,
But am betroth'd unto your enemy;
Divorce me, untie or break that knot again,
Take me to you, imprison me, for I,
Except you enthrall me, never shall be free,
Nor ever chaste, except you ravish me.
We’ll examine Oppenheimer’s strange choice of designations for this project in a mind-bending, pysch-warping tour of the atom bombs and those who built them. But, for now, let us continue on with our little history.
Detonation of the Gadget was scheduled for 0400 hours on 16JULY1945. The day before, however, thunder was heard rolling by the afternoon. Some 10 miles from Ground Zero, the wind grew so wild that it collapsed tents while dumping sheets of rain. By the early morning hours, the storm crossed Ground Zero knocking out the power including the main searchlight throwing the camp into darkness. The detonation event was postponed by one-and-a-half hours.
As the Gadget was detonated at 0529 hours and 45 seconds, the nearby mountains were illuminated “brighter than daytime” for a couple of seconds with the color going from purple to green to intensely white. The base camp reported the temperature as “being as hot as an oven.” The shock wave reached the closest observers in 40 seconds and was felt over 100 miles (160 km) away.
The Gadget tower at the Trinity site.
The Gadget being hoisted into place on the tower.
As the Gadget detonates. cameras capture the event in fractions of a second. This a rapidly expanding ball of plasma released from the bomb.
The plasma ball at 0.025 seconds.
The fireball at a tenth of a second.
Trinity cloud at 12 seconds.
Military observers of the Trinity explosion.
Sixty seconds into the blast. The mushroom cloud rose to 7.5 miles (12.1 km).
The Gadget explosion fused the desert sand into radioactive glass forming a crater about 10 feet (3.0 m) deep and 1,100 feet (340 m) wide.
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