neutron emitting sources

Isotopic neutron sources

The Sources of neutrons with suitable examples and sketches are discuss here advanced. Sources of neutrons is needed to read as engineer student.

Types of neutron sources:

  1. Isotopic (ampoule) sources.
  2. The accelerator as a source of neutrons.
  3. Neutron generators.
  4. Reactors:
  5. industrial research: – continuous; – impulse.
  6. Plasma traps.

(α, n) sources

In laboratory practice (α, n) sources have found widespread use. It was the bombardment of Be with alpha particles that led to the discovery of the neutron. There are two branches of the reaction on Be: 9Be + α = 12C + n + 5.708 MeV; 9Be + α = 12C * + n + 1.29 MeV. In the second branch, the carbon core is formed in an excited state, which is removed by emission of a photon with an energy of 4.42 MeV. The reaction is exoenergetic.

A high reaction energy allows one to obtain a neutron with a maximum energy of 7.7 to 10.6 MeV when using α particles with an energy of 2 to 5 MeV. At lower energy, the yield decreases by two orders of magnitude. Alpha-active isotopes 210Po, 226Ra, 239Pu, 238Pu, 241Am are used as sources of alpha particles. As a rule, the target has a considerable thickness in comparison with the range of α particles. The neutron spectrum (α, n) of the source is continuous and has a complex form (Fig. 2.1). The neutron energy varies from 0.1 to 12 MeV. The source may be a mixture of radium bromide (or another alpha-active nuclide) and metallic beryllium powder. The disadvantages of such sources are: 1) a continuous complex spectrum; 2) the presence of concomitant gamma radiation; 3) for 210Po – a short half-life.

The neutron energy spectra of Po-α-Be and Pu-α-Be sources almost coincide. This is because the energies of the alpha particles Po and Pu coincide with an error of about 0.15 MeV. In a Po-α-Be source, an excited 12C nucleus emits about 1 photon / neutron. with an energy of 4.42 MeV, as well as photons with an energy of 2.9 and 7.3 MeV of low intensity. 210Ро itself emits photons with an energy of 0.803 MeV with an output of 1.2 • 10-5 photons / dec.

measured energy spectra
measured energy spectra

The measured energy spectra of (α, n) sources: a) Po-α-Be; Pu-α-Be; b) Ra-α-Be; c) the calculated spectrum for the Ra-α-Be source

Photoneutron sources – Sources of neutrons

Irradiation of targets with photons with an energy exceeding the binding energy of a nucleon in the nucleus leads to the emission of a neutron. Two stable nuclides have the lowest binding energy: deuterium (2.226 MeV) and beryllium (1.666 MeV). Radionuclides, as a rule, do not emit photons with an energy greater than about 3 MeV; therefore, targets in nuclide photoneutron sources are made only from Be and D. Beryllium is usually used in elementary form, deuterium in the form of deuterium oxide D2O. Photoneutron sources are prepared by surrounding a radionuclide with a layer of beryllium or deuterium. One neutron in such sources accounts for 103–104 photons. As radionuclides use 24Na, 88Y, 124Sb, Ra (in equilibrium), 140La. The half-life is from several hours to several thousand years. The neutron energy is several tens of kiloelectron-volts.

Fission neutrons – Sources of neutrons

Fission neutrons are formed either as a result of nuclear reactions of neutrons or photons with nuclei of heavy nuclides, or in acts of spontaneous fission. The yield of spontaneous fission neutrons U or Pu is too small to be used as a source. At present, neutron sources generated in acts of spontaneous fission of 252Cf are widely used. The neutron spectrum differs little from the neutron spectrum U, but a detailed examination shows the enrichment of high-energy neutrons. In fig. 2.2 shows the neutron spectrum of fission 252Сf. The maximum of the spectrum in the range of 0.6 – 0.8 MeV.

Sources of neutrons Isotope Fission Neutron Spectrum
Isotope Fission Neutron Spectrum

Isotope Fission Neutron Spectrum

The average energy value (taken in the usual way – by integration over the spectrum) 2.26 ± 0.04 MeV for Cf. At energies above 2–3 MeV, the spectrum has the form of an exponential, and the number of neutrons decreases by a factor of 10 with an increase in energy by 4 MeV. The neutron energy spectrum of the 252Cf source is close to the fission neutron spectrum and is well described by the dependence

Sources of neutrons
Sources of neutrons

where T is the temperature of the spectrum, T = (1.40 ± 0.02) MeV. In a rough approximation, for E> 3 MeV, the spectrum can be approximated by N (E0) = const exp (-k E0), where k = 0.67 MeV-1 (Cf) and k = 0.76 MeV-1 (U) . The number of neutrons per decay reaches 3.8, and the number of photons is about 3. The neutron yield is 2.7 · 1012 s-1 per 1 g of 252Cf. The time dependence of the yield is determined by α-decay, the half-life of 252Cf relative to α-decay is 2.64 g, and relative to spontaneous fission – 82 g. Therefore, the decrease in the neutron yield of the source occurs with a half-life of 2.56 years. The characteristics of the most common ampoule neutron sources are given in table.

Characteristics of the most common ampoule neutron sources

Nuclear reactionHalf lifeThe number of neutrons in 1 s per 1 curieNeutron Energy, MeVThe number of gamma rays per neutron
Reaction (α, n)
226Ra + Be 1620 years107Continuous spectrum from 0.1 to 12 with a maximum in the range 3-5~ 104
222Rn + Be3.8 days.10^7~ 1
210Po + Be 139 days.10^6~ 1-40
239Pu + Be 24 thousand years10^6~ 10^2
241Am + Be 470 years10^6
Reaction (γ, n)
226Ra + D2O 1620 years10^4-10^50.12~ 10^4
MsTh + Be6.7 years10^4-10^50, 83~ 10^4
MsTh + D2O 6.7 years10^4-10^50.2~ 10^4
140La + Be 40 hr10^4-10^50.62~ 10^4
140La + D2O 40 hr10^4-10^50.15~ 10^4
124Sb + Be 61 days10^4-10^524 keV~ 10^4
72Ca + D2O 14.1 h10^4-10^50.13~ 10^4
24Na + Be 14.8 h10^4-10^50.83~ 10^4
24Na + D2O 14.8 h10^4-10^50.22~ 10^4
38Y + Be 104 days10^4-10^50.972~ 10^4
38Y + D2O 104 days10^4-10^50.265~ 10^4
144Ce + Be 285 days10^4-10^50.400~ 10^4
228Ra + D2O 6.7 years10^4-10^50.200~ 10^4
Spontaneous fissionNumber of neutrons per 1 mg
236Pu 2.9 years26Continuous spectrum 0.1-12 with a maximum in the region of 1, 53 – 10
240Pu 6.6 × 10^3 years1.13 – 10
244Cm 18.4 years9 × 10^33 – 10
252Cf 2.6 years2.7 × 10^93 – 10
Characteristics of the most common ampoule neutron sources

The Sources of neutrons with suitable examples and sketches are discuss here advanced. Sources of neutrons is needed to read as engineer student.

Reference Sources of neutrons

  1. Page Sources of neutrons
  2. Page Sources of neutrons

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