On the basis of nonproportionality data obtained for several scintillators, we have developed a model to describe the carrier dynamics to fit the light yield versus electron energy. The theory of ...Onsager was adapted to explain how the carriers form excitons or sequentially arrive at the activators to promote the ion to an excited state, and the theory of Birks was employed to allow for exciton-exciton annihilation. We then developed a second model to deduce the degradation in resolution that results from nonproportionality by evoking Landau fluctuations, which are essentially variations in the deposited energy density that occur as the high energy electron travels along its trajectory. In general there is agreement with the data, in terms of fitting the nonproportionality curves and reproducing the literature values of nonproportionality's contribution to the scintillator resolution.
Single crystals of SrI 2 :Eu and SrI 2 :Ce/Na were grown from anhydrous iodides by the vertical Bridgman technique in evacuated silica ampoules. Growth rates were of the order of 5-30 mm/day. ...Radioluminescence spectra of SrI 2 :Eu and SrI 2 :Ce/Na exhibit a broad band due to Eu 2+ and Ce 3+ emission, respectively. The maximum in the luminescence spectrum of SrI 2 :Eu is found at 435 nm. The spectrum of SrI 2 :Ce/Na exhibits a doublet peaking at 404 and 435 nm attributed to Ce 3+ emission, while additional impurity-or defect-related emission is present at approximately 525 nm. The strontium iodide scintillators show very high light yields of up to 120 000 photons/MeV, have energy resolutions down to 3% at 662 keV (Full Width Half Maximum) and exhibit excellent light yield proportionality with a standard deviation of less than 5% between 6 and 460 keV.
Efficient, readily-available, low-cost, high-energy neutron detectors can play a central role in detecting illicit nuclear weapons since neutrons are a strong indication for the presence of fissile ...material such as Plutonium and Highly-Enriched Uranium. The main challenge in detecting fast neutrons consists in the discrimination of the signal from the gamma radiation background. At present, the only well-investigated organic crystal scintillator for fast neutron detection, in a n/gamma mixed field, is stilbene, which while offering good pulse shape discrimination, is not widely used because of its limited availability and high cost.
In this work we will present the characterization, in terms of gamma response and internal activity of newly developed crystals that contains Lanthanum in their chemical formula. In particular we ...tested two LaBr3:Ce,Sr, one CLLBC and two CLLB crystals with different volumes. These crystals just overcome the prototype stage and, even if the production in still not standardized at least for large optics, they have been very recently commercialized in sizes interesting for high-energy gamma-ray spectroscopy, as for application in nuclear physics experiments. In particular, we will report on the study of the decay time, light yield and energy resolution with gamma rays, on the response as a function of the gamma interaction point and on the internal activity due to the presence of Lanthanum.
In the effort of the AugerPrime scintillator surface detector R&D activity, we investigated the performances of different extruded and cast plastic scintillators that were read out with ...wavelength-shifting (WLS) optical fibers and then coupled to a PMT. In particular we compared the light yield of eighteen scintillator/fiber configurations, obtained combining eight different scintillator bars with six fiber types, in order to investigate which was satisfying the AugerPrime specifications in terms of light production (>12photoelectrons per minimum ionizing particle). In this paper, we present the results of the study on different scintillator bar geometries, scintillator production techniques, and wavelength-shifting optical fiber types. We also propose an effective way to optically couple the fibers to the PMT entrance window.
New scintillators for high-resolution gamma ray spectroscopy have been identified, grown and characterized. Our development efforts have focused on two classes of high-light-yield materials: ...europium-doped alkaline earth halides and cerium-doped garnets. Of the halide single crystals we have grown by the Bridgman method-SrI 2 , CaI 2 , SrBr 2 , BaI 2 and BaBr 2 -SrI 2 is the most promising. SrI 2 (Eu) emits into the Eu 2+ band, centered at 435 nm, with a decay time of 1.2 mus and a light yield of up to 115,000 photons/MeV. It offers energy resolution better than 3% FWHM at 662 keV, and exhibits excellent light yield proportionality. Transparent ceramic fabrication allows the production of gadolinium- and terbium-based garnets which are not growable by melt techniques due to phase instabilities. The scintillation light yields of cerium-doped ceramic garnets are high, 20,000-100,000 photons/MeV. We are developing an understanding of the mechanisms underlying energy dependent scintillation light yield non-proportionality and how it affects energy resolution. We have also identified aspects of optical design that can be optimized to enhance the energy resolution.
In this work, we describe the optical properties of the single photoelectron (SPE) calibration system designed for NectarCAM, a camera proposed for the Medium Sized Telescopes (MST) of the Cherenkov ...Telescope Array (CTA). One of the goals of the SPE system, as integral part of the NectarCAM camera, consists in measuring with high accuracy the gain of its photo-detection chain. The SPE system is based on a white painted screen where light pulses are injected through a fishtail light guide from a dedicated flasher. The screen – placed 15 mm away from the focal plane – is mounted on an XY motorization that allows movements over all the camera plane. This allows in-situ measurements of the SPE spectra via a complete scan of the 1855 photo-multiplier tubes (PMTs) of NectarCAM. This calibration process will enable the reduction of the systematic uncertainties on the energy reconstruction of γ-rays coming from distant astronomical sources and detected by CTA.
We discuss the design of the screen used in the calibration system and we present its optical performances in terms of light homogeneity and timing of the signal.
We have constructed a second-generation Compton coincidence instrument, known as the Scintillator Light Yield Non-proportionality Characterization Instrument (SLYNCI), to characterize the electron ...response of scintillating materials. While the SLYNCI design includes more and higher efficiency HPGe detectors than the original apparatus (five 25%-30% detectors versus one 10% detector), the most novel feature is that no collimator is placed in front of the HPGe detectors. Because of these improvements, the SLYNCI data collection rate is over 30 times higher than the original instrument. In this paper, we present a validation study of this instrument, reporting on the hardware implementation, calibration, and performance. We discuss the analysis method and present measurements of the electron response of two different NaI:Tl samples. We also discuss the systematic errors of the measurement, especially those that are unique to SLYNCI. We find that the apparatus is very stable, but that careful attention must be paid to the energy calibration of the HPGe detectors.
Optical quality lithium salicylate (LiSal) single crystals have been grown from water and methanol solutions for thermal neutron detection studies. The results of structural studies showed that LiSal ...crystallizes from water as a stable modification different from the monohydrate LiSal·H2O reported previously. Growth from methanol results in the formation of unstable crystals of LiSal·MeOH that quickly degrade in air due to the loss of methanol. Initial radiation detection tests showed a good neutron/gamma separation in water-grown modification making LiSal in its single crystal form a potential material for thermal neutron detection.
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