Gamma-Ray and Neutron Detection with Li Co-Doped Scintillation Detector

There is presently a great deal of interest in radiation detection materials which can simultaneously identify neutrons and gamma rays. In recent years, neutron sensitive scintillators have been studied heavily, particularly as a result of the increasing demand from security applications and the 3He crisis1.

Li-containing elpasolites such as Cs2LiLaBr6 (CLLB) and Cs2LiYcI6 (CLYC) have become especially interesting, courtesy of their ability to carry out the dual detection of gamma rays and neutrons, as well as their capacity for pulse shape discrimination (PDS)2-6.

Nevertheless, due to quaternary crystal growth’s complexity and cost, there is a limit of ⌀3x3”7 for elpasolite scintillators such as CLYC which are available commercially. Consequently, they are not available in large enough sizes for bulk cargo scanning, vehicle-mounted area monitors, or for portal monitors at border crossings.

However, Saint-Gobain regularly grows Nal(Tl) in ingots larger than 100 liters in volume. Via the incorporation of Li into the crystal matrix, Nal(Tl) can be engineered to be neutron-sensitive. In the past, crystal sizes of as much as ⌀2x2” (0.1 liter) with 6Li co-doping (commercially known as NaILTM) have been reported on8. Fantastic neutron-gamma PSD is exhibited by NaIL, which has a high figure-of-merit9.

Measurements indicate that only 1-3% of 6Li doping is needed by large NaIL detectors in order for them to outperform numerous current neutron detection solutions like CLYC scintillators or standard 3He tubes, in terms of cost and performance10.

Documented in this article are details of the advances made on large crystals, their absolute neutron detection efficiencies, and the first ever measurements of a large NaIL crystal (2000 centimetres cubed). The Nal-Lil phase diagram indicates that Nal can form a solid solution with as much as 100% Lil11. Consequently, a significant amount of Li can be introduced into the matrix of Nal without its crystalline structure being interrupted.

In the past, grown single crystals of Nal(Tl) which have been co-doped with Li at concentrations as much as 8 mol% (with respect to Na) have been reported8. However, if the crystal is large, concentrations of 6Li do not need to be this high.

6Li’s areal density determines the neutron detection efficiency. The solid angle presented by the detector multiplied by neutron detection efficiency determines the source detection capability. Consequently, extraordinarily capable dual gamma/neutron detectors can result from NaIL crystals with large cross-sectional areas and thicknesses. Using thermal neutron absorption cross-sections, a straightforward calculation demonstrates this notion.

Thermal neutron detection capability (efficiency * area) is plotted against NaIL crystal thickness and lithium concentration in Figure 1. A NaIL crystal with a cross-section measuring 10 by 40 centimeters squared is assumed, as this is the common industrial size. The thermal neutron detection increases as the crystal’s thickness increases. When the thickness is 5 cm and [6Li] = 1%, the NaIL crystal is about a third the size of a large 3He tube (⌀5x173 cm, 3 atm), a neutron detector which is typically used for vehicle scanning at border crossings in the USA.

Thermal neutron detection capability vs. NaIL crystal thickness and lithium concentration for a 400 cm2 crystal surface. The values are calculated from thermal neutron interaction cross-sections.

Figure 1. Thermal neutron detection capability vs. NaIL crystal thickness and lithium concentration for a 400 cm2 crystal surface. The values are calculated from thermal neutron interaction cross-sections.

Experimental Methods

Saint-Gobain Crystals was used to grow NaIL single crystals which were studied in this research. For all of the grown crystals, thallium concentration was kept at 0.1 at% in the melt. Li concentration was altered between 0-8 at. % as a means of assessing its effect on the neutron detection efficiency and scintillation performance.

With respect to Na, all of the Li concentrations in the crystal which are presented here are at. %. Inductively coupled plasma-optical emission spectrometry (ICP-OES) is used to verify the values. The production purification process of Saint-Gobin Crystals is used to originate Nal raw material. With a purity of at least 99.99%, Lil salts enriched to 95% 6Li are purchased from SAFC Hitech.

Each of the NaIL crystals presents as colorless and clear. No phase separation, precipitation, or cloudiness was seen, even for the highest doping level. A Photonis XP20Y0 photomultiplier tube was used to detect scintillation light.

A multichannel analyzer (Aptek model S5008, 1 µs shaping time, 11-bit digitization, bi-polar shaping) was used to collect and analyze scintillation pulses for energy resolution and light yield. A waveform digitizer (CAEN model DT5720, 12-bit digitization, 250 MS/s) was used to digitize scintillation pulses for PDS analysis.

Neutron Detection Capability

The length of the scintillation pulse can be increased by the addition of Li to the Nal(Tl) matrix. The pulse becomes longer as more is added. Additional electron traps which are caused by lithium atoms distorting the crystal lattice are believed to lead to the lengthening of the pulse. Electrons are eventually released as the traps are shallow enough, however, this delay leads to an increase in the length of the pulse.

It is interesting that pulses caused by interactions with neutrons do not cause as great an increase in pulse length as do those caused by interactions with neutrons. This difference can be used to distinguish between neutron and gamma ray detection events. Shown in Figure 2 are representative scintillation pulses for neutrons and gamma rays at three different Li concentrations.

Averaged gamma-ray and neutron scintillation pulses for NaIL crystals with (a) 0.7% Li, (b) 2.2% Li and (c) 7.7% Li.

Figure 2. Averaged gamma-ray and neutron scintillation pulses for NaIL crystals with (a) 0.7% Li, (b) 2.2% Li and (c) 7.7% Li.

It is worth noting that, in Figure 2, the difference in pulse shape between gamma rays and neutrons decreases with an increase in [Li]. Regardless of this, there is enough of a difference to discriminate gammas from neutrons, even as [Li]= 7.7%.

The separation is clearly shown in Figure 3. This figure illustrates data collected on a ⌀2.5x2.5 cm crystal with [Li]= 0.6%. 252Cf was the source of radiation. This emits spontaneous fission neutrons as well as gamma rays. 5 cm of polyethylene was used to moderate the neutrons.

The gamma equivalent energy of each individual pulse is represented by the x-axis. The x-axis is, specifically, Energy = , where S(t) is the photosensor signal at time t. It is worth noting that the signal from 6Li(n,t)α reaction appears at approximately 3.4 MeV gamma equivalent energy.

The PSD value of the scintillation pulses is represented by the y-axis. The ratio of the amount of light contained at the end of the pulse divided by the total quantity of light emitted gives the PSD value. This is called “tail-to-total", and is a traditionally used PSD method. Specifically, the PSD value in Figure 3 is calculated for each pulse using the following equation:

(1)

PSD-energy density contour plot for NaIL scintillation waveforms under irradiation from a moderated 252Cf source.  Note the good separation between neutrons and gammas.

Figure 3. PSD-energy density contour plot for NaIL scintillation waveforms under irradiation from a moderated 252Cf source.  Note the good separation between neutrons and gammas.

A figure-of-merit (FoM) is often used to describe the quality of PSD. The FoM quantifies the degree of signal separation9. There is an FoM of 4.3 exhibited by the data in Figure 3. As a general rule, when the PSD FoM exceeds 1.5, for the majority of practical purposes, the complete separation of neutrons and gammas is able to be obtained4.

Neutron Detection Efficiency

At the Ohio State University Nuclear Reactor Laboratory (OSU-NRL), two NaIL crystals of varying sizes and lithium contents were measured for neutron detection efficiency. Beams of thermal neutrons with known fluxes were used to measure the crystals.

The crystal sizes are shown in Table 1, as well as the measured thermal neutron detection efficiency, the predicted efficiency from MCNPX simulations, and the lithium concentration13. As a result of small [6Li] differences between the samples which underwent ICP-OES and the actual crystals, slight discrepancies between the calculated and measured efficiencies are presumed.

The samples were extracted from the ingots in positions which were adjacent to those from which the detector crystals below were cut. As this is a destructive test, the detector crystals have not themselves undergone ICP-OES.

Shown in Figure 4 is a plot of detected neutron count rate against the neutron particle flux intercepted by the crystals. The detection efficiency is indicated by the fitted lines’ slopes.

Table I. NaIL calibrated thermal neutron measurements Crystal name Size 6Li concentration in crystal

Crystal name Size 6Li concentration in crystal Thermal neutron detection efficiency MCNPX efficiency prediction
F24-35A ⌀2.5x2.5 cm 1.37% 34.5±0.2% 32.8%
F12-01C ⌀5.1x5.1 cm 0.23% 10.6±0.3% 11.1%

Plot of the detected neutron count rate vs the thermal neutron flux from the OSU-NRL reactor beam. The NaIL crystal, F23-35A, has dimensions ⌀2.5x2.5 cm and [6Li] = 1.37% (filled circles). F12-01C has dimensions ⌀5.1x5.1 cm and [6Li] = 0.23% (open circles).

Figure 4. Plot of the detected neutron count rate vs the thermal neutron flux from the OSU-NRL reactor beam. The NaIL crystal, F23-35A, has dimensions 2.5x2.5 cm and [6Li] = 1.37% (filled circles). F12-01C has dimensions 5.1x5.1 cm and [6Li] = 0.23% (open circles).

In terms of Figure 4 and Table 1, the important things to take into account are that even at a low concentration such as [6Li] = 1.37%, there is a significant neutron detection efficiency of 35.5%, which accords well with what was predicted. Due to its superior thickness, the efficiency of F12-01C is only one third that of F2335A, despite the fact that the former’s 6Li concentration is only about one sixth of the latter.

An advantage unique to NaIL is illustrated by these points. The other two central constituent elements, I (6.15 barn) and Na (0.54 barn), have significantly smaller neutron absorption cross sections than that of 6Li (940 barn)14. This enables the use of large thicknesses and low Li concentrations in order to achieve the same neutron detection capabilities as CLLB, CLYC, or 3He detectors.

Scintillation Light Yield and Energy Resolution

Figure 5 shows the energy resolution of NaIL and scintillation light yield with different Li concentrations. When compared to standard Nal:Tl, both energy resolution and light yield of NaIL degrade gradually as the Li concentration increases.

A light output of approximately 34,000 photons/MeV is shown by NaIL crystals with 1% Li doping and approximately 31,000 photons/MeV with 2% Li doping. For the NaIL crystal containing 7.7% Li, the scintillation light yield is still above 30,000 photons/MeV.

Nevertheless, the Li doping concentration does not seem to strongly influence the energy resolution of NaIL, at least for crystals whose Li concentration is less than 8%. For a broad variety of Li concentrations, the averaged energy resolution of NaIL is stabilized at approximately 7%.

(a) Scintillation light yield and (b) energy resolution (at 662 keV) of NaIL with different Li concentrations in crystal

Figure 5. (a) Scintillation light yield and (b) energy resolution (at 662 keV) of NaIL with different Li concentrations in crystal.

The degradation in NaIL’s scintillation performance can be partly put down to decreased intrinsic scintillation efficiency, and partly attributed to the crystal synthesis process’ variability. Figure 5 presents results which are averages of 20 crystals. The best results achieved were an energy resolution of 6.3% with 1% Li doping and, with 2% Li doping, an energy resolution of 6.6%. As more crystal growth process refinements take place, it is anticipated that energy resolution and light yield of NaIL will be improved.

A possible problem of growing crystals with low concentrations of lithium is indicated by Figure 5. At these low values, there is the greatest dependence on energy resolution and light yield. Consequently, significant nonuniformities within a large crystal or ingot can be caused by even small lithium concentration gradients. Energy resolution and, potentially, the quality of PSD can be degraded by these non-uniformities, as it also changes with [Li] (as shown in Figure 2).

Performance of a Very Large NaIL Crystal

Recently, an extremely large NaIL ingot was grown - ⌀80x23 cm, with [6Li]=1% in the melt (added as 6LiI). In order to make sure that general Nal(Tl) production does not get contaminated with Li, a standard production furnace has been specifically dedicated to NaIl growth.

In order to suppress Lil evaporation, the crucible is covered with a lid, and the grown ingot was cut into numerous large crystals. A photo of a 5.1x10.2x40.6 cm cubed crystal which was cut from an ingot before being sealed in a housing is shown in Figure 6a. The housing is shown in Figure 6.b.

a) photo of a large (~2100 cm3) crystal cut from a 120,000 cm3 NaIL ingot. b) the same crystal packaged in a hermetic housing for testing.

Figure 6. a) photo of a large (~2100 cm3) crystal cut from a 120,000 cm3 NaIL ingot. b) the same crystal packaged in a hermetic housing for testing.

A side-on 662 keV source measured a gamma ray energy resolution of 9.8%. Generally, a standard Nal(Tl) crystal of this manufacture and size produces an energy resolution of between 7.0-8.0 %. Non-uniformity in light yield led to the worsening of the result. This was largely a result of a non-uniformity in [Li] between the two ends of the crystal.

ICP-OES tests carried out on samples adjacent to each end of the crystal measured [6Li]=0.25% and [6Li]=0.47%. Based on Figure 5, therefore, a non-uniformity of light yield of approximately 5% is to be expected. The majority of the worsening is believed to have been caused by this additional broadening.

A 252Cf source which was moderated by 5 cm of polyethylene before being placed 2 m from the detector was used to collect neutron data. Figure 7 shows the PSD-energy plot.

PSD and energy density contour plot for a large NaIL crystal (~5x10x40 cm3) irradiated with a moderated 252Cf source.

Figure 7. PSD and energy density contour plot for a large NaIL crystal (~5x10x40 cm3) irradiated with a moderated 252Cf source.

Even though the PSD quality is worse than the small crystal illustrated in Figure 3, the FoM of 2.0 is still impressive. This is good enough to make sure that, at a rate lower than 10-7 gamma detection, a gamma ray can be mistaken for a neutron. The detection rate of neutrons was 0.40 count/s/ng of 252Cf.  When considering data obtained from simulations, this count rate necessitates an average [6Li]= 0.37%, which is probably reasonably close to the actual crystal value, as the crystal ends are 0.25 and 0.47%.

Discussion and Conclusions

Large dual neutron/gamma scintillation detectors are being industrialized by Saint-Gobain Crystals. A proof-of-concept, 120 liter, large NAl(Tl) ingot co-doped with 6Li has been grown. Fantastic neutron/gamma discrimination and a decent gamma detection performance are exhibited by a proof-of-concept, large (2000 cm cubed) crystal detector cut from this ingot.

With an overall increase in [Li] or with improved uniformity of [Li], the gamma detection performance can be increased. In future growths, both of these methods will be tried. The efficiency of neutron detection scales with [6Li] as the simulations anticipated.

For a wide range of security applications, the prospect of detectors available in large volumes makes NaIL a useful and unique fit. As an example, for vehicle scanning at the borders of the USA, a typical 3He neutron detector (a “portal” monitor) is generally ⌀5x173 cm and pressurized to 3 atm. Typically, these detectors’ neutron detection efficiency is approximately 3.0 count/s/ng 252Cf at 2 m15.

As a result of the above, there will be approximately identical efficiency from three large NaIL detectors such as are shown in Figure 6 – with [6Li] increased to 1% - as there is from a 3He tube (i.e. 0.40 cps/ns * 1%/0.37% * 3 = 3.2 cps/ng). In terms of cost, it will be similar to that of one 3He tube. However, NaIL detectors bring the added bonus of isotope identification and gamma ray spectroscopy.

References

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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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