Neutron Source

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Neutron Source

 

a source of neutron beams. Neutron sources are used in nuclear physics research and in practical applications, such as neutron logging and neutron diffraction analysis. A neutron source is characterized by a number of factors—intensity (the number of neutrons emitted per second), energy distribution, angular distribution, neutron polarization, and mode of emission (continuous or pulsed). In early neutron sources, the neutrons were produced by using the nuclear reactions (α,n) in 9Be or 10B nuclei and the photodisintegration of a deuteron or a Be nucleus, that is, the reaction (γ,n). In the first case, the neutron source is a homogeneous mechanical mixture of powders of 9Be and a radioactive isotope that emits alpha particles sealed in an ampul; the radioactive isotopes used include radium, polonium, and plutonium. The ratio of Be to, for example, Ra is 1:5 (by weight). The intensity of the source is determined by the permissible amount of alpha-active preparation. The activity is usually 10 curies or less, corresponding to the emission of approximately 107–108 neutrons per second (see Table 1). Neutron sources with a Ra + Be or Am + Be mixture are also sources of intense gamma radiation (104–105 gamma quanta per neutron). Neutron sources using Po + Be or Pu + Be mixtures emit just one gamma quantum per neutron.

Table 1. Characteristics of most widely used ampul neutron sources
Nuclear reactionHalf-lifeNumber of neutrons*Neutron energy (MeV)
•The number of neutrons is taken per second per curie for the (α,n) and (y,n) reactions and per milligram for spontaneous fission fMesothorium
(α,n)
Ra + Be1,620 years107Continuous
Rn + Be3.8 days107spectrum from
Po + Be139 days1060.1 to 12,
Pu + Be24,000 years106with a maximum
Am + Be470 years106around 3–5
(γ,n)
Ra + D2O1,620 years 0.12
MsTh† + Be6.7 years 0.83
MsTht + D2O6.7 years 0.20
140La + Be40 hours 0.62
140La + D2O40 hours104-1050.15
124Sb + Be60 days 0.024
72Ca + D2014.1 hours 0.13
24Na + Be14.8 hours 0.83
24Na + D2O14.8 hours 0.22
Spontaneous fission
236Pu2.9 years26Continuous
240Pu6.6 × spectrum from
 103 years1.10.1 to 12, with a
244Cm18.4 years9 × 103maximum around
252Cf2.6 years2.7 × 1091.5

In the case of an ampul photoneutron source, the ampul contains a hollow cylinder or sphere made of Be or containing heavy water (D2O) and enclosing a gamma-radiation source. The energy of the gamma quanta must be greater than the threshold energy for photodisintegration of D or Be nuclei. A shortcoming of this source is the presence of intense gamma radiation; such a source is used when monoenergetic neutrons must be produced by simple means. The spontaneous fission of heavy nuclei is also used in ampul neutron sources.

Since the appearance of charged-particle accelerators, the (p,n) and (d,n) reactions in light nuclei and (d,pn) reactions have been used to produce neutrons. In special accelerator tubes, protons and deuterons are accelerated in an electric field generated by a voltage of approximately 105—107 V. Such neutron generators differ in size and other features. Some occupy an area of 50–100 m2 and have an intensity of approximately 1012-1013 neutrons/sec (the energy can be varied from 105 to 107 electron volts [eV]). For neutron logging there are also miniature accelerator tubes with diameters of approximately 25–30 mm that emit 107–108 neutrons per second.

To produce neutrons with energies of from 2 to 15 MeV, the reactions D (d,n)3He and T (d,n)4He are used most widely. The target is a metal hydride (usually the metal is Zr or Ti) containing deuterium or tritium instead of ordinary hydrogen. A significant neutron yield is observed in the D + d reaction when the deuteron energy reaches approximately 50 keV. At this value, the neutron energy is approximately 2 MeV and increases with increasing proton energy. For neutrons with an energy of from 13 to 20 MeV, the T + d reaction, which results in a larger neutron yield, is preferable. For example, when the deuteron energy is 200 keV, neutrons having an energy of approximately 14 MeV emerge from a thick tritium-zirconium target at a rate of 108 per second per microcoulomb of deuterons.

The (p,n) reaction in 7Li and other nuclei is convenient for producing monoenergetic neutrons over a broad range of energies. It usually is used in electrostatic generators. The (p,n) and (d,pn) reactions in beams of high-energy protons and deuterons are used to produce neutrons of higher energies (approximately 108 eV). The (p,n) reaction is brought about by the direct expulsion of a neutron from the nucleus (without any intermediate stage of excitation of the nucleus) and also by charge transfer in an emerging nucleon in the field of the nucleus. In this case, the neutrons emerge principally in the direction of the proton beam and are monochromatic for a fixed exit angle. The (d,pn) reaction (the disintegration of a deuteron in the field of a nucleus) leads to the production of a neutron with an energy equal to one half of the energy of the deuteron.

Electron accelerators are also used as neutron sources. Intense beams of fast electrons are directed onto thick targets made of heavy elements, such as lead or uranium. The resulting bremsstrahlung gamma quanta induce either the (γ,n) reaction or nuclear fission accompanied by the emission of neutrons. All neutron generators can operate in both continuous and pulse modes.

The most powerful neutron sources are nuclear reactors. A neutron beam extracted from a reactor contains neutrons with energies ranging from a fraction of an electron volt to 10–12 MeV. In powerful reactors, the neutron flux density at the center of the reactor core reaches 1015 neutrons per second per square centimeter (in the continuous mode of operation). Pulse reactors operating in the short-pulse mode produce a higher neutron flux density; for example, the fast-neutron pulse reactor at the Joint Institute for Nuclear Research has a flux density of 1020 neutrons per second per square centimeter at the center of the core at the time of a pulse.

REFERENCES

Vlasov, N. A. Neitrony, 2nd ed. Moscow, 1971.
Figure 1. Dependence of total effective neutron absorption and scattering cross section σ on neutron energy ε

Portativnye generatory neitronov ν iadernoi geofizike. Edited by S. I. Savosin. Moscow, 1962.

B. G. EROZOLIMSKII

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