The Partial Test Ban Treaty (PTBT) is the abbreviated name of the 1963 Treaty Banning Nuclear Weapon Tests in the Atmosphere, in Outer Space and Under Water, which prohibited all test detonations of nuclear weapons except for those conducted underground. It is also abbreviated as the Limited Test Ban Treaty (LTBT) and Nuclear Test Ban Treaty (NTBT), though the latter may also refer to the Comprehensive Nuclear-Test-Ban Treaty (CTBT), which succeeded the PTBT for ratifying parties. Negotiations initially focused on a comprehensive ban, but that was abandoned because of technical questions surrounding the detection of underground tests and Soviet concerns over the intrusiveness of proposed verification methods. The impetus for the test ban was provided by rising public anxiety over the magnitude of nuclear tests, particularly tests of new thermonuclear weapons (hydrogen bombs), and the resulting nuclear fallout. A test ban was also seen as a means of slowing nuclear proliferation and the nuclear arms race. Though the PTBT did not halt proliferation or the arms race, its enactment did coincide with a substantial decline in the concentration of radioactive particles in the atmosphere. The PTBT was signed by the governments of the Soviet Union, the United Kingdom, and the United States in Moscow on 5 August 1963 before it was opened for signature by other countries. The treaty formally went into effect on 10 October 1963. Since then, 123 other states have become party to the treaty. Ten states have signed but not ratified the treaty. Background Much of the stimulus for the treaty was increasing public unease about radioactive fallout as a result of above-ground or underwater nuclear testing, particularly given the increasing power of nuclear devices, as well as concern about the general environmental damage caused by testing. In 1952–53, the US and Soviet Union detonated their first thermonuclear weapons (hydrogen bombs), far more powerful than the atomic bombs tested and deployed since 1945. In 1954, the US Castle Bravo test at Bikini Atoll (part of Operation Castle) had a yield of 15 megatons of TNT, more than doubling the expected yield. The Castle Bravo test resulted in the worst radiological event in US history as radioactive particles spread over more than , affected inhabited areas (including Rongelap Atoll and Utirik Atoll), and sickened Japanese fishermen aboard the Lucky Dragon upon whom "ashes of death" had rained. In the same year, a Soviet test sent radioactive particles