the relatively low-density fusion fuel—leading to unacceptably large radiation losses—the stand-off gap also acted as a buffer to mitigate the unavoidable and undesirable Taylor mixing. Use of boron Boron was used at many locations in this dry system; it has a high cross-section for the absorption of slow neutrons, which fission U and Pu, but a low cross-section for the absorption of fast neutrons, which fission U. Because of this characteristic, B deposited onto the surface of the secondary stage would prevent pre-detonation of the spark plug by stray neutrons from the primary without interfering with the subsequent fissioning of the U of the fusion tamper wrapping the secondary. Boron also played a role in increasing the compressive plasma pressure around the secondary by blocking the sputtering effect, leading to higher thermonuclear efficiency. Because the structural foam holding the secondary in place within the casing was doped with B, the secondary was compressed more highly, at a cost of some radiated neutrons. (The Castle Koon MORGENSTERN device did not use B in its design; as a result, the intense neutron flux from its RACER IV primary predetonated the spherical fission spark plug, which in turn "cooked" the fusion fuel, leading to an overall poor compression.) The plastic's low molecular weight is unable to implode the secondary's mass. Its plasma-pressure is confined in the boiled-off sections of the tamper and the radiation case so that material from neither of these two walls can enter the radiation channel that has to be open for the radiation transit. Detonation The device was mounted in a "shot cab" on an artificial island built on a reef off Namu Island, in Bikini Atoll. A sizable array of diagnostic instruments were trained on it, including high-speed cameras trained through an arc of mirror towers around the shot cab. The detonation took place at 06:45 on March 1, 1954, local time (18:45 on February 28 GMT). When Bravo was detonated, within one second it formed a fireball almost across. This fireball was visible on Kwajalein Atoll over away. The explosion left a crater in diameter and in depth. The mushroom cloud reached a height of and a diameter of in about a minute, a height of and in diameter in less than 10 minutes and was expanding at more than . As a result of the blast, the cloud contaminated more than of the surrounding Pacific Ocean,