Using a NaIL Scintillation Detector for Neutron and Gamma-Ray Detection

Dr. Robert Hofstadter invented thallium-activated sodium iodide in 1948 and Harshaw Chemical was the first company to begin producing it commercially in the 1950s. It is an industrial scintillator with mature system and detector designs, and remains the most frequently used scintillator in the world by volume.

Incorporating Li Into Nal:Tl

For the interdiction of illicit nuclear material, neutron detectors are essential components:

  • Neutrons are only emitted by fissile isotypes
  • Affordable and efficient neutron detectors are needed
  • The simultaneous detection of neutrons and gamma rays is longed for

In order to introduce thermal neutron detection, Li can be incorporated into Nal:Tl:

  • A lot of scintillation light is created by 4.8 MeV events
  • n + 6Li ---> t (2.75 MeV) + α (2.05 MeV)

At any ratio, a solid solution is formed by Nal and Lil:

  • The efficiency of neutron detection is tunable
  • High Li gradients can be tolerated by crystal growth

This appears simple, as long as you can distinguish between gamma-rays and neutrons (there are no false positives). Previously, work in this field had been unable to differentiate between the two, as pulse shape discrimination was not feasible.

Phase diagram of Nal - Lil

Phase diagram of Nal - Lil

Adding Lithium Affects the Scintillation Pulse Shape

  • Pulses get longer as the Li gets higher
  • However, a greater increase is observed after interactions with gammas than with neutrons

Pulse Shape Difference Gives Excellent Gamma/Neutron Discrimination

∅2.5x2.5 cm crystal, [6Li]= 0.6%

Applications for Nail

A Few % 6Li Doping is Sufficient for Most Security Applications

Na & I have small σn abs, ∴ nth detection efficiency grows strongly with [6Li]

& nth detection efficiency grows with thickness

The True Advantage of Nail Detectors

Two important facts to consider are that it is affordable and simple to grow sizeable Nal crystals, and that, for neutron attenuation (0.53 and 6.3=25 vs. 940 barn), 6Li significantly outperforms Na and I.

There are two key benefits of using this method. Firstly, identical neutron detection abilities to those of 3He or CLLB or CLYC detectors can be achieved using low concentrations of 6Li and large thicknesses, at a lower cost. Secondly, an efficient gamma ray detection capacity is also contributed by large volumes.

Neutron Efficiency is as Expected

OSU reactor and neutron beam facility

OSU reactor and neutron beam facility

Size [6Li] in crystal thermal neutron ε MCNPX ε prediction
∅2.5x2.5 cm 1.37% 34.5±0.2% 32.8%
∅5.1x5.1 cm 0.24% 10.6±0.3% 11.1%

Achieving Large Nail Ingots

A 5X10X40 cm Cubed was Chosen as a Large Crystal for Testing

This large format is popular.

Gamma Test Results: 5x10x40 cm Cubed Detector

Light Yield and Energy Resolution Depend on [LI]

With [Li], scintillation light yield decreases. Energy resolution also increases marginally.

  • At 1% Li, there is approximately 34,000 ph/MeV
  • At 2% Li and over, there is approximately 31,000 ph/MeV
  • The strongest dependence occurs at low [Li]

Non-uniformity occurs in light output, as [Li] is not uniform.

From one end to the other, there is a 4.5% difference in light yield. This constitutes the primary source of degradation in energy resolution. Higher lithium concentrations and gradient reduction techniques will be used in future growths. The target is less than 8% at 662 KeV.

Neutron Detection Capability is Completely Acceptable

moderated 252Cf source fission neutrons + gammas PSD FoM = 2.0

moderated 252Cf source fission neutrons + gammas PSD FoM = 2.0

3He tube in Radiation Portal Monitors
Φ5x173 cm, 3 atm = ~3 n/s/ng of 252Cf at 2 m

∴Three of these NaIL detectors with [6Li]=1-2% will have same n detection capability as one 3He tube at similar cost

Summary and Outlook

For dual mode detection, NaIL represents the future as it operates at large volumes cheaply, and is spectroscopic. It is the primary production scale Nal(TI, 6Li) which is ingot grown (120 liters).

It is also the first large fabricated detector (2 liter capacity). Its capacity for neutron detection is fantastic, but it has a below average gamma energy resolution. This can be fixed either with lower [Li], or overall higher [Li].

The work which needs to be completed in future is the finalization of the optimization process. There will be more than 70 differently sized crystals to package for sales and demos. The next goal would be to offer NaIL at all sizes.

This information has been sourced, reviewed and adapted from materials provided by Saint-Gobain Crystals.

For more information on this source, please visit Saint-Gobain Crystals.


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