- Modulated neutron initiator
A modulated neutron initiator is a neutron source capable of producing a burst of neutrons on activation. It is a crucial part of some nuclear weapons, as its role is to "kick-start" the chain reaction at the optimal moment when the configuration is prompt critical. It is also known as an internal neutron initiator. The initiator is typically placed in the center of the plutonium pit, and is activated by impact of the converging shock 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 just the right time. If the chain reaction starts too soon, the result will be only a 'fizzle yield', well below the design specification; if it occurs too late, there may be no yield whatsoever. Low spontaneous neutron emission of the pit material is crucial to prevent predetonation.
For boosted fission weapons, the size of the centrally placed initiator is critical and has to be as small as possible; replacement with an external neutron source allows for more flexibility, such as variable yields.
The usual design is based on a combination of beryllium-9 and polonium-210, separated until activation, then put into intimate contact by the shock wave. Polonium-208 and actinium-227 were 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 can not be so efficiently shielded as alpha particles. Several variants were developed, differing by the dimensions and mechanical configuration of the system ensuring proper mixing of the metals.
Urchin was the code name for the internal neutron initiator, a neutron generating device that triggered the nuclear detonation of the earliest plutonium atomic bombs such as The gadget and Fat Man, once the critical mass had been "assembled" by the force of conventional explosives.
The detonator used in the first devices, located at the center of the bomb's plutonium pit, consisted of a 0.8 cm beryllium pellet coated with nickel and then a 0.1 mm layer of gold. The coated pellet was surrounded by a beryllium shell with 2 cm outer diameter and 0.6 cm thick walls, with 15 concentric, wedge-shaped, 2.09 mm deep latitudal grooves in its inner surface that was also coated with gold and nickel. The grooves were filled with 30 curies (1.1 TBq) of polonium-210, which emits alpha particle radiation; the inner sphere was covered with another 20 curie of polonium; its total amount was about 11 milligrams. The hollow sphere is formed from two halves made by hot pressing. The nickel layer is deposited by exposition of the beryllium metal to nickel tetracarbonyl atmosphere. The thin layers of gold and nickel maintained the slight separation between the beryllium and polonium necessary to shield the beryllium from alpha particles emitted from 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.
When the shock wave from the implosion of the plutonium core arrives, it crushes the detonator. Hydrodynamic forces acting on the grooved shell thoroughly 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 emit neutrons in a rate of about 1 neutron each 5–10 nanoseconds; these neutrons then trigger the chain reaction in the compressed supercritical plutonium. Placing the polonium layer between two large masses of beryllium ensured contact of the metals even if the shock wave turbulence performed less than expected.
The grooves in the inner surface of the shell play the role of shaping the shock wave into jets by the Munroe effect, in a way similar to a shaped charge, for fast and thorough mixing 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 omitted the central sphere, being hollow instead; the advantage of a hollow design is its possible smaller size while retaining reliability.
The short half-life of polonium (138.376 days) requires frequent replacement of initiators and continued supply of polonium for their manufacture, as their shelf life was only about 4 months. Later designs had shelf life as long as 1 year.
A different initiator (code named ABNER) was used for the Little Boy uranium 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.
An improved construction of the initiator, probably based on conical or pyramidal indentations, was proposed in 1948, put into production by Los Alamos in 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. A series of calibration experiments for initiation time vs yield data of the TOM initiators was done during the Operation Snapper, during the Fox test on 25 May 1952.
In 1974, India performed the Smiling Buddha nuclear test. The initiator, codenamed "Flower", was based on the same principle as the Urchin. It is believed the polonium was deposited on lotus-shaped platinum gauze to maximize its surface and enclosed in a tantalum sphere 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".
The Polonium used in the urchin initiator was created at Oak Ridge and then extracted and purified as part of the Dayton Project under the leadership of Charles Allen Thomas. The Dayton Project was one of the various sites comprising the Manhattan Project
In 1949 Mound Laboratories in nearby Miamisburg, Ohio opened as a replacement for the Dayton Project and the new home of nuclear initiator research & development. Polonium-210 was produced by neutron irradiation of bismuth. Production and research of polonium at Mound was phased out in 1971.
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 as screwballs. The test assemblies were imploded and their remains studied to examine how well the polonium and beryllium mixed.
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. The sealed 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.
Urchin style initiators were later superseded by other means of generating neutrons such as pulsed neutron emitters that do not use polonium. Another possibility is a boosted fission weapon, using a hollow pit, injected with gaseous deuterium and tritium mixture; tritium, while also radioactive, has much longer half-life (13.2 years) than polonium-210, allowing longer shelf life of the weapons.
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