Peter R. Menge

Improvement of γ-ray energy resolution of LaBr3:Ce3+ scintillation detectors by Sr2+ and Ca2+ co-doping

Commercially available LaBr3:5% Ce3+ scintillators show with photomultiplier tube readout about 2.7% energy resolution for the detection of 662 keV γ-rays. Here we will show that by co-doping LaBr3:Ce3+ with Sr2+ or Ca2+ the resolution is improved to 2.0%. Such an improvement is attributed to a strong reduction of the scintillation light losses that are due to radiationless recombination of free electrons and holes during the earliest stages (1–10 ps) inside the high free charge carrier density parts of the ionization track.

Behavior of Cs 2 LiYC l6 :Ce scintillator up to 175°C

Cs2LiYCl6:Ce (CLYC) is a promising crystal scintillator for gamma ray and neutron detection. Like many elpasolite crystal scintillators, it exhibits outstanding energy linearity. The crystal contains lithium and is capable of excellent gamma ray/neutron discrimination using pulse shape differences. The largest contributor to pulse shape differences is gamma interactions which produce very fast core-valence luminescence (CVL). Current interest exists in examining the potential for CLYC in neutron well logging and other applications which require operating at extreme temperatures. Experiments have been performed which investigate the scintillation properties of CLYC from −40° C to 175° C. Light output, pulse shape discrimination (PSD) ability, and the gamma ray equivalent energy (GEE) of the neutron reactions are among the properties examined. At higher temperatures, the light output from gamma rays remains strong, but the GEE decreases. The PSD also degrades due to reduction of CVL.

Use of LaBr 3 for downhole spectroscopic applications

The use of lanthanum bromide (LaBr3) in commercial applications has increased rapidly over the last few years owing to its very favorable properties, e.g., excellent spectral resolution, high speed, high light output, and efficiency.

Enhanced Discrimination in Co-doped LaBr3Ce

LaBr3:Ce crystal scintillator can be co-doped with various alkaline earth metals to improve light output and energy resolution of the basic scintillator. Another benefit is improvement of alpha/gamma discrimination via pulse shape analysis. LaBr3:Ce contains a low level of actinium contamination, which produces an alpha particle background. This background is difficult to discriminate from gamma rays. Conversely, the addition of co-dopant into the crystal makes the alpha response much easier to distinguish. LaBr3:Ce,Sr, for example, produces a second, longer decay component in the scintillation pulse when excited by radiation. The amplitude of this second decay component changes in response to a gamma ray versus a heavy charged particle. The change in pulse shape is used to eliminate the alpha background and enable detection of neutron reaction products.

Enhanced α-γ discrimination in co-doped LaBr3:Ce

LaBr3:Ce crystal scintillator can be co-doped with various alkaline earth metals to improve light output and energy resolution of the basic scintillator. Another benefit is improvement of alpha/gamma discrimination via pulse shape analysis. LaBr3:Ce contains a low level of actinium contamination, which produces an alpha particle background. This background is difficult to discriminate from gamma rays. Conversely, the addition of co-dopant into the crystal makes the alpha response much easier to distinguish. LaBr3:Ce,Sr, for example, produces a second, longer decay component in the scintillation pulse when excited by radiation. The amplitude of this second decay component changes in response to a gamma ray versus a heavy charged particle. The change in pulse shape is used to eliminate the alpha background and enable detection of neutron reaction products.

Design and performance of a compact Cs 2 LiLaBr 6 (Ce) neutron/gamma detector using silicon photomultipliers

Cs2LiLaBr6(Ce) (CLLB) crystal scintillator shows great potential as a radiation detection material with excellent energy resolution for gammas, sensitivity to neutrons, and the ability to separate the two using pulse shape discrimination (PSD). Experiments have been performed testing this material using silicon photomultipliers for creation of compact, easily-portable detectors for dual gamma ray spectroscopy and neutron detection. Pulse shape discrimination in CLLB is achieved by analyzing the scintillation pulse decay on the time scales of >1 μs. This feature enables the silicon photomultipliers with low afterpulsing to achieve suitable discrimination. Experiments have been conducted attempting to find a suitable cost/efficiency compromise that maximizes performance by varying crystal cerium concentration along with silicon photomultiplier type and placement position. A disk of CLLB (diameter = 52 mm, thickness = 6 mm, 6Li enriched) coupled to a 6×6 mm2 silicon photomultiplier can achieve 4.4% gamma ray energy resolution at 1275 keV, and 74% thermal neutron detection efficiency with a high pulse shape discrimination figure-of-merit of 1.9.

Scintillation properties and temperature responses of Cs 2 LiLaBr 6 :Ce 3+

This report presents the scintillation properties and temperature dependent neutron and gamma responses of Cs2LiLaBr6 elpasolite crystals with 0.5, 2, and 3.5% Ce doping. Cs2LiLaBr6 has excellent scintillation light output proportionality, high light output, and good energy resolution. It also shows good light output temperature stability. Pulse shape differences between neutron and gamma excited pulses are analyzed as a function of temperature. Neutron-gamma pulse shape discrimination is possible in a wide temperature range from -10°C up to at least 140 °C.

Performance improvement of large Sr 2+ and Ba 2+ co-doped LaBr 3 :Ce 3+ scintillation crystals

The performance improvement of large size (φ = 60 mm, ℓ = 80 mm) Sr²⁺ and Br²⁺ co-doped LaBr₃:Ce³⁺ scintillation crystals is reported. The scintillation light output of both Sr²⁺ and Br²⁺ co-doped crystals are significantly improved. Compared to 70,000 ph/MeV (at 662 keV) for Ce³⁺ only LaBr₃, Sr²⁺ and Ba²⁺ co-doping increase the light output by ~25% to 88,000 ph/MeV and 89,000 ph/MeV, respectively. The energy resolutions of both Sr²⁺ and Ba²⁺ co-doped crystals are improved over a wide energy range as well. The scintillation decay time is slightly lengthened with co-doping. No secondary slow component is observed. Co-doped crystals exhibit very similar emission and excitation characteristics to the Ce³⁺ only crystal. Both Sr²⁺ and Ba²⁺ show improved light output proportionality in the low energy range.

Efficient positioning of silicon photomultipliers on large scintillation crystals

Silicon photomultipliers (SiPMs) are attractive replacements for photomultiplier tubes (PMTs). However, many radiation detector applications require large volumes of monolithic scintillator and correspondingly large numbers of SiPMs. When multiples of SiPMs are used, the dark count noise and cost increase proportionally with the surface area covered. When too few SiPMs are used, non-uniformity of light collection degrades the energy resolution of the detector. Strategic placement of a limited number of SiPMs on a large scintillator can reduce the number of SiPMs necessary and decrease the cost-to-performance ratio. Simulations and experiments have been performed to find general guidelines regarding optimal positioning of SiPMs on large NaI(Tl) crystal scintillators. For example, if the SiPMs are placed near edges and vertices on one cuboid face, the number of SiPMs necessary to achieve adequate energy resolution need only cover 40% or less of the light output face.

A cost effective means of extending the lifetime of plastic scintillators in portal monitors

Large plastic scintillators (5000 to >35000 cc) deployed as portal monitors and subjected to environmental extremes have been shown to suffer performance degradation due to water vapor penetration of the base material. Low temperatures can cause the absorbed vapor to condense and manifest itself as a fog within the scintillator that can dissipate as the material warms. Repeated cold cycles may lead to a permanent residual haze. Both polyvinyltoluene and polystyrene based scintillators are susceptible to this effect. The presence of internal haze causes scattering of scintillation light and results in reduced detector sensitivity. An obvious, but expensive solution would be to hermetically seal the plastic as one would a hygroscopic scintillating crystal. In this study we examine the performance of 12 plastic scintillation detectors packaged in various configurations and exposed to >60 days storage at 55C and 85% relative humidity. The detectors were cycled to 25C, 0C and −30C. Pulse height spectra were acquired continuously during the experiment. Count rate information along with the pulse height spectra are used to gauge the effectiveness of the various packaging configurations in preventing water vapor penetration and subsequent fogging at low temperatures.