Spontaneous fission

Spontaneous fission (SF) is a form of radioactive decay characteristic of very heavy isotopes. It is theoretically possible for any atomic nucleus whose mass is greater than or equal to 100 atomic mass units (u), i.e. elements near ruthenium. In practice, however, spontaneous fission is only energetically feasible for atomic masses above 230 u (elements near thorium). The elements most susceptible to spontaneous fission are the high-atomic-number actinide elements, such as mendelevium and lawrencium, and the trans-actinide elements, such as rutherfordium.

For uranium and thorium, the spontaneous fission mode of decay does occur, but it is not seen for the majority of radioactive breakdowns, and it is usually neglected except for the exact considerations of branching ratios when determining the activity of a sample containing these elements. Mathematically, the criterion for whether spontaneous fission can occur is approximately:

${\hbox{Z}}^{2}/{\hbox{A}}\geq 45.$

Where Z is the atomic number and A is the mass number (e.g., 235 for U-235).

As the name suggests, spontaneous fission gives the same result as induced nuclear fission. However, like other forms of radioactive decay, it occurs due to quantum tunneling, without the atom having been struck by a neutron or other particle as in induced nuclear fission. Spontaneous fissions release neutrons as all fissions do, so if a critical mass is present, a spontaneous fission can initiate a chain reaction. Also, radioisotopes for which spontaneous fission is a nonnegligible decay mode may be used as neutron sources; californium-252 (half-life 2.645 years, SF branch ratio 3.09%) is often used for this purpose. The neutrons may then be used to inspect airline luggage for hidden explosives, to gauge the moisture content of soil in the road construction and building industries, to measure the moisture of materials stored in silos, and in other applications.

As long as the fissions give a negligible reduction of the amount of nuclei that can spontaneously fission, this is a Poisson process: for very short time intervals the probability of a spontaneous fission is proportional to the length of time.

The spontaneous fission of uranium-238 leaves trails of damage in uranium-bearing minerals as the fission fragments recoil through the crystal structure. These trails, or fission tracks, provide the basis for the radiometric dating technique known as fission track dating.

Spontaneous fission rates

Spontaneous fission rates:

Nuclide	Half-life	Fission prob. per decay	Neutrons per fission	Neutrons per (g.s)
	7.04x108 years	7.0x10-11	1.86	1.0x10-5
	4.47x109 years	5.4x10-7	2.07	0.0136
	2.41x104 years	4.4x10-12	2.16	2.2x10-2
	6569 years	5.0x10-8	2.21	920
	2.638 years	3.09x10-2	3.73	2.3x1012

In practice ²³⁹
Pu
will invariably contain a certain amount of ²⁴⁰
Pu
due to the tendency of ²³⁹
Pu
to absorb an additional neutron during production. ²⁴⁰
Pu
's high rate of spontaneous fission events makes it an undesirable contaminant. Weapons-grade plutonium contains no more than 7.0% ²⁴⁰
Pu
.

The rarely-used gun-type atomic bomb has a critical insertion time of about one millisecond, and the probability of a fission during this time interval should be small. Therefore only ²³⁵
U
is suitable. Almost all nuclear bombs use some kind of implosion method.

Spontaneous fission can occur much more rapidly when the nucleus of an atom undergoes superdeformation.

History

The first nuclear fission process discovered was the fission induced by neutrons. Because cosmic rays produce some neutrons, it was difficult to distinguish between induced and spontaneous fission events. Cosmic rays can be reliably shielded by a thick amount of rock or water. The spontaneous fission was identified in 1940 by Soviet physicists Georgy Flyorov and Konstantin Petrzhak by their observations of uranium in the Moscow Metro Dinamo station, 60 metres (200 ft) deep underground.

External links

• The LIVEChart of Nuclides - IAEA with filter on spontaneous fission decay, in Java or HTML