Small Devices
- Radioisotopes which undergo spontaneous fission
- Certain isotopes undergo spontaneous fission with emission of neutrons. The most commonly used spontaneous fission source is the radioactive isotope californium-252. Cf-252 and all other spontaneous fission neutron sources are produced by irradiating uranium or another transuranic element in a nuclear reactor, where neutrons are absorbed in the starting material and its subsequent reaction products, transmuting the starting material into the SF isotope. Cf-252 neutron sources are typically 1/4" to 1/2" in diameter and 1" to 2" in length. When purchased new a typical Cf-252 neutron sources emit between 1×107 to 1×109 neutrons per second but, with a half life of 2.6 years, this neutron output rate drops to half of this original value in 2.6 years. The price of a typical Cf-252 neutron source is from $15,000 to $20,000.
- Radioisotopes which decay with alpha particles packed in a low-Z elemental matrix
- Neutrons are produced when alpha particles impinge upon any of several low atomic weight isotopes including isotopes of lithium, beryllium, carbon and oxygen. This nuclear reaction can be used to construct a neutron source by intermixing a radioisotope that emits alpha particles such as radium or polonium with a low atomic weight isotope, usually in the form of a mixture of powders of the two materials. Typical emission rates for alpha reaction neutron sources range from 1×106 to 1×108 neutrons per second. As an example, a representative alpha-beryllium neutron source can be expected to produce approximately 30 neutrons for every one million alpha particles. The useful lifetime for these types of sources is highly variable, depending upon the half-life of the radioisotope that emits the alpha particles. The size and cost of these neutron sources are also comparable to spontaneous fission sources. Usual combinations of materials are plutonium-beryllium (PuBe), americium-beryllium (AmBe), or americium-lithium (AmLi). The neutron initiators of early nuclear weapons used a device constructed of polonium-beryllium layers separated by nickel and gold, until a neutron pulse was desired. The implosion of the bomb then caused the mechanical mixing of the layers, by swirling them together and breaking the nickel and gold barriers between polonium and beryllium.
- Photoneutron sources, or radioisotopes which decay with high energy photons (gamma decay) co-located with beryllium or deuterium
- Gamma radiation with an energy exceeding the neutron binding energy of a nucleus can eject a neutron, called a photoneutron. Two examples and their decay products are:
- 9Be + >ṃ photon → 1 neutron + 2 4He
- 2H (deuterium) + >ṃ photon → 1 neutron + 1H
- Sealed tube neutron generators
- Some particle accelerator-based neutron generators exist that work by inducing nuclear fusion between beams of deuterium and/or tritium ions and metal hydride targets which also contain these isotopes.
Read more about this topic: Neutron Source
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