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Modulated neutron initiator

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Amodulated neutron initiatoris aneutron sourcecapable of producing a burst ofneutronson activation. It is a crucial part of somenuclear weapons,as its role is to "kick-start" the chain reaction at the optimal moment when the configuration isprompt critical.It is also known as aninternal neutron initiator.The initiator is typically placed in the center of theplutonium pit,and is activated by impact of the convergingshock wave.

One of the key elements in the proper operation of a nuclear weapon is initiation of the fission chain reaction at the proper time. To obtain a significant nuclear yield, sufficient neutrons must be present within the supercritical core at the right time. If the chain reaction starts too soon ( "predetonation"), the result will be only a 'fizzleyield', well below the design specification. If it occurs too late, the core will have begun to expand and disassemble into a less-dense state, leading to a lowered yield (less of the core material undergoes fission) or no yield at all (the core is no longer a critical mass). Therefore, low spontaneous neutron emission of the pit material is crucial.

Forboosted fission weapons,the size of the centrally placed initiator is critical and has to be as small as possible. The use of an external neutron source allows more flexibility, such as variable yields.

Design[edit]

The usual design is based on a combination ofberyllium-9 andpolonium-210,separated until activation, then placed in intimate contact by the shock wave.Polonium-208andactinium-227were also considered as Alpha sources. The isotope used must have strong Alpha emissions and weak gamma emissions, as gamma photons can also knock neutrons loose and cannot be so efficiently shielded as Alpha particles.[1]Several variants were developed, differing by the dimensions and mechanical configuration of the system ensuring proper mi xing of the metals.

Urchin[edit]

Urchinwas the code name for the internal neutron initiator used by theLos Alamos Laboratoryas aneutrongenerating device to trigger thenuclear detonationof the earliestplutoniumatomic bombssuch asThe GadgetandFat Man,once thecritical masshad been 'assembled' by the force of conventional explosives.

The initiator used in the early devices, located at the center of the bomb'splutonium pit,consisted of aberylliumpellet and a beryllium shell withpoloniumbetween the two. The pellet, 0.8 cm in diameter, was coated withnickeland then a layer ofgold.The beryllium shell was of 2 cm outer diameter with wall thickness of 0.6 cm. The inner surface of that shell had 15 concentric, wedge-shaped latitudinal grooves and was, like the inner sphere, coated with gold and nickel.[2][3]A small amount of polonium-210 (50 curies, 11 mg) was deposited in the grooves of the shell and on the central sphere: the layers of gold and nickel served to shield the beryllium fromAlpha particlesemitted by the polonium. The whole urchin weighed about 7 grams and was attached to mounting brackets in a 2.5 cm diameter inner cavity in the pit.[4]

When theshock wavefrom the implosion of the plutonium core arrives, it crushes the initiator.Hydrodynamic forcesacting on the grooved shell thoroughly and virtually instantly mix the beryllium and polonium, allowing the Alpha particles from the polonium to impinge on the beryllium atoms. Reacting to Alpha particle bombardment, the beryllium atoms emitneutronsat a rate of about 1 neutron every 5–10 nanoseconds (SeeBeryllium). These neutrons trigger thechain reactionin the compressedsupercriticalplutonium. Placing the polonium layer between two large masses of beryllium ensures contact of the metals even if the shock wave turbulence performs poorly.

The 50 curies of polonium generated about 0.1 watts ofdecay heat,noticeably warming the small sphere.[5]

The grooves in the inner surface of the shell shaped theshock waveinto jets by theMunroe effect,similar to ashaped charge,for fast and thorough mi xing of the beryllium and polonium. As the Munroe effect is less reliable in linear geometry, later designs used a sphere with conical or pyramidal inner indentations instead of linear grooves. Some initiator designs omit the central sphere, being hollow instead. The advantage of a hollow design is possibly managing a smaller size while retaining reliability.

The shorthalf-lifeof polonium (138.376 days) required frequent replacement of initiators and a continued supply of polonium for their manufacture, as their shelf life was only about 4 months.[6]Later designs had shelf life as long as 1 year.

The US government used Postum as a code name for polonium.[7]

Use of polonium for the neutron initiator was proposed in 1944 byEdward Condon,although polonium as an initiator was mentioned as a possibility in the "Los Alamos Primer"lectures given in April 1943. The initiator itself was designed byJames L. Tuck,[8]and its development and testing was carried out atLos Alamos National Laboratoryin "Gadget"division's initiator group led byCharles Critchfield.[9]

Abner[edit]

A different initiator (code namedABNER) was used for theLittle Boyuranium bomb. Its design was simpler and it contained less polonium. It was activated by the impact of the uranium projectile to the target. It was added to the design as an afterthought and was not essential for the weapon's function.[10]

TOM initiator[edit]

An improved construction of the initiator, probably based on conical or pyramidal indentations, was proposed in 1948, put into production byLos Alamosin January 1950, and tested in May 1951. The TOM design used less polonium, as the number of neutrons per milligram of polonium was higher than of the Urchin. Its outer diameter was only 1 cm. The first live fire test of a TOM initiator occurred on 28-Jan-1951 during the Baker-1 shot ofOperation Ranger.[11]A series of calibration experiments for initiation time vs yield data of the TOM initiators was done during theOperation Snapper,during the Fox test on 25 May 1952.

Flower[edit]

In 1974, India performed theSmiling Buddhanuclear test.The initiator, codenamed "Flower", was based on the same principle as the Urchin. It is believed the polonium was deposited onlotus-shapedplatinumgauzeto maximize its surface and enclosed in atantalumsphere surrounded by uranium shell with embedded beryllium pellets. According to other sources, the design was yet more similar to the Urchin, with a beryllium shell shaped to create beryllium jets upon implosion. The initiator outer diameter is reported as 1.5 cm, or "about 2 cm".[12]

Other designs[edit]

Uranium deuteride(UD3) can be used for construction of a neutron multiplier.[13][14]

Boosted fission weaponsand weapons using externalneutron generatorsoffer the possibility ofvariable yield,allowing selection of the weapon's power depending on the tactical needs.

Development[edit]

The polonium used in the urchin initiator was created atOak Ridge National Laboratoryand then extracted and purified as part of theDayton Projectunder the leadership ofCharles Allen Thomas.TheDayton Projectwas one of the various sites comprising theManhattan Project.

In 1949,Mound Laboratoriesin nearbyMiamisburg, Ohioopened as a replacement for theDayton Projectand the new home of nuclear initiator research & development. Polonium-210 was produced by neutron irradiation ofbismuth.Production and research of polonium at Mound was phased out in 1971.[15]

Polonium from Dayton was used by the G Division of Los Alamos in initiator design studies at a test site in Sandia Canyon. The initiator group built test assemblies by drilling holes in large turbine ball bearings, inserting the active material, and plugging the holes with bolts. These test assemblies were known asscrewballs.The test assemblies were imploded and their remains studied to examine how well the polonium and beryllium mixed.[16]

The production of the beryllium-polonium TOM initiators ended in 1953. The initiators were replaced with a different design, which slightly reduced the weapon yield but its longer shelf life reduced the complexity of the logistics.[17]Thesealed neutron initiator,brought into inventory in late 1954, still required a periodic disassembly to access its capsule for maintenance checks. The capsules were phased out completely in 1962.[18]

Urchin style initiators were later superseded by other means of generating neutrons such aspulsed neutron emittersthat do not use polonium. Using tritium with a half-life of 12.3 years instead of polonium they have a much longer replacement interval. These are mounted outside the pit and electrically controlled, since neutrons easily pass through considerable mass without interactions. These initiators were more controllable and enable much improved weapon reliability.

See also[edit]

References[edit]

  1. ^Nuclear Weapons FAQ, Section 4.1, Version 2.04: 20 February 1999
  2. ^The Design of Gadget, Fat Man, and "Joe 1" (RDS-1)Archived2010-02-10 at theWayback Machine.Cartage.org.lb. Retrieved on 2010-02-08.
  3. ^On the Origins of the Soviet Atomic Project.Nuclearweaponarchive.org (1998-04-15). Retrieved on 2010-02-08.
  4. ^Nuclear Weapons FAQ, Section 8.0, Version 2.18: 3 July 2007
  5. ^4.1 Elements of Fission Weapon Design.Nuclearweaponarchive.org (1953-05-19). Retrieved on 2010-02-08.
  6. ^Abrahamson|The Sandia Pioneers.Unc.edu. Retrieved on 2010-02-08.
  7. ^Injecting Polonium into Humans,Federation of American Scientists, December 12th, 2006 by Steven Aftergood
  8. ^Ferenc Morton Szasz (1992).British scientists and the Manhattan Project: the Los Alamos years.Palgrave Macmillan. pp. 24–.ISBN978-0-312-06167-8.Retrieved22 April2011.
  9. ^"The Manhattan Project and predecessor organizations".Array of Contemporary American Physicists.American Institute of Physics. Archived fromthe originalon 2012-10-17.Retrieved2013-03-11.
  10. ^Carey Sublette,Section 8.0 The First Nuclear Weapons,The Nuclear Weapon Archive: A Guide to Nuclear Weapons(July 3, 2007).
  11. ^Carey Sublette. (6 August 2001). Gallery of U.S. Nuclear Tests
  12. ^India's Nuclear Weapons Program – Smiling Buddha: 1974.Nuclearweaponarchive.org. Retrieved 2010-02-08.
  13. ^[1]ArchivedJune 4, 2011, at theWayback Machine
  14. ^Uranium Deuteride Initiators.ArmsControlWonk (2009-12-14). Retrieved on 2010-02-08.
  15. ^Polonium.Globalsecurity.org (2005-04-27). Retrieved on 2010-02-08.
  16. ^The Making of the Atomic Bomb, Richard Rhodes, 1986, Simon & Schuster,ISBN0-684-81378-5p. 580
  17. ^Note by the secretary, Subject: part III – Weapons Progress Report to the Joint Committee, June – November 1953.Retrieved on 2010-02-08.
  18. ^United States Nuclear Weapons.Globalsecurity.org. Retrieved on 2010-02-08.
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