Thermal neutron scintillation detectors

As lithium-containing gas is not available, scintillation detectors are common as lithium-based neutron detectors. LiI is chemically similar to NaI, and therefore its scintillation performance is reasonably good. As an activator for LiI, Eu is doped.

Neutrons do not produce ionization directly in scintillation crystals but can be detected through their interaction with the nuclei of a suitable element. In a 6 LiI(Eu) scintillation crystal, for example, neutrons interact with 6 Li nuclei to produce an alpha particle and a triton (tritium nucleus), which both produce scintillation light that can be detected. For this purpose, also enriched 6 Li-containing glasses can be used, doped with an activator.

The thickness of a LiI(Eu) crystal is good enough to fully stop the neutron interaction products (alpha particle and triton). Thus, each neutron interaction event can make a signal pulse height equivalent to 4.78 MeV. When a gamma-ray with similar energy makes an interaction of full-energy deposition, the resultant pulse height is the same as the pulse height of the neutron event.

  • 3 mm Lil(Eu) stops 90% of thermal neutrons
  • Rugged crystal (> 20 degrees C temperature change per hour allowed)
  • Excellent neutron / gamma discrimination (neutron peak > 3.6 MeV)
  • Neutron peak resolution < 7% FWHM (PMT readout)

Crystal in daylight and under UV light:

High neutron cross-section, high light output, rugged
Thermal neutron detection and spectroscopy, Oil & Gas, neutron/gamma discrimination

Thermal neutron detection via nuclear reaction:

63Li+10n→42He+31H+Q (Q = 4.78 MeV)

Since the Helium atom and the triton are both particles with a limited range in the
scintillator, a thermal neutron interaction results in a peak in the pulse height spectrum.

Property Data
Density 4,1 g/cc
Emission wavelength 440 nm
Effective decay time 1.4 μs (neutrons and gammas)
High neutron peak position > 3.6 MeV
Photoelectron yield 30-35 % of NaI(Tl)
6-Li enrichment >96%

Pulse height spectrum showing the presence of the thermal neutron peak relative to the Cs-137 and Co-60 interactions. The thermal neutron peak is located at 4 MeV. Here the spectrum shows an excellent neutron/gamma separation. LiI(Eu) detectors can be manufactured in many different geometries. Since the decay time for neutrons is essentially different from other scintillation crystals, it is possible to combine LiI(Eu) crystals to other scintillators in phoswhich geometries. By means of digital pulse shape discrimination techniques, neutron and gamma interaction can be separated.

It is possible to read out LiI(Eu) crystals using PIN photodiodes. Such a detector is rugged and is operated at low voltage. The same properties for PMT readout apply.

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