The usual design is based on a combination of
beryllium-9 and
polonium-210, separated until activation, then placed in 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 cannot 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 design. The "urchin" is the polonium–beryllium sphere.
Urchin was the code name for the internal neutron initiator used by the
Los Alamos Laboratory as a
neutron generating device to trigger 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 initiator used in the early devices, located at the center of the bomb's
plutonium pit, consisted of a beryllium pellet and a beryllium shell with polonium-210 between the two. The pellet, in diameter, was coated with
nickel and then a layer of
gold. The beryllium shell was of outer diameter with wall thickness of . The inner surface of that shell had 15 concentric, wedge-shaped latitudinal grooves and was, like the inner sphere, coated with gold and nickel. 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 from
alpha particles emitted by the polonium. The whole urchin weighed about 7 grams ( ounce) and was attached to mounting brackets in a 2.5 cm (1-inch) diameter inner cavity in the pit. When the
shock wave from the implosion of the plutonium core arrives, it crushes the initiator. Hydrodynamic forces acting 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 emit
neutrons at a rate of about 1 neutron every 5–10 nanoseconds (See
Beryllium). These neutrons trigger the
chain reaction in the compressed supercritical plutonium. 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 of
decay heat, noticeably warming the small sphere. The grooves in the inner surface of the shell shaped the
shock wave into jets by the
Munroe effect, 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 omit the central sphere, being hollow instead. The advantage of a hollow design is possibly managing a smaller size while retaining reliability. The short
half-life of polonium-210 (138 days) necessitated frequent replacement of initiators and a 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. The US government used "Postum" as a code name for polonium. Use of polonium for the neutron initiator was proposed in 1944 by
Edward Condon, although polonium as an initiator was mentioned as a possibility in the "
Los Alamos Primer" lectures given in April 1943. The final "urchin" initiator was designed by
James L. Tuck and
Hans Bethe and its development and testing was carried out at
Los Alamos National Laboratory in "
Gadget" division's initiator group led by
Charles Critchfield. Other polonium-beryllium initiator designs were considered, but the choice of "urchin" as the production design was made in early May 1945, with input from
Enrico Fermi and
Niels Bohr. It has been estimated that the initiators used in the wartime weapons produced on the order of 100 neutrons during the critical ~1 microsecond of assembly time.
Abner 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.
TOM initiator 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. The first live fire test of a TOM initiator occurred on 28-Jan-1951 during the Baker-1 shot of
Operation Ranger. 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.
Flower 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 a uranium shell with embedded beryllium pellets. According to other sources, the design was 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".
Other designs Uranium deuteride (UD3) can be used for construction of a neutron multiplier.
Boosted fission weapons and weapons using external
neutron generators offer the possibility of
variable yield, allowing selection of the weapon's power depending on the tactical needs. ==Development==