Current trends in scintillator detectors and materials,

Abstract The last decade has seen a renaissance in inorganic scintillator development for gamma ray detection. Lead tungstate (PbWO 4 ) has been developed for high energy physics experiments, and possesses exceptionally high density and radiation hardness, albeit with low luminous efficiency. Lutetium orthosilicate or LSO (Lu 2 SiO 5 :Ce) possesses a unique combination of high luminous efficiency, high density, and reasonably short decay time, and is now incorporated in commercial positron emission tomography (PET) cameras. There have been advances in understanding the fundamental mechanisms that limit energy resolution, and several recently discovered materials (such as LaBr 3 :Ce) possess energy resolution that approaches that of direct solid state detectors. Finally, there are indications that a neglected class of scintillator materials that exhibit near bandedge fluorescence could provide scintillators with sub-nanosecond decay times and high luminescent efficiency

Large scale motions of Neptune's bow shock: Evidence for control of the shock position by the rotation phase of Neptune's magnetic field

The Voyager 2 spacecraft observed high levels of Langmuir waves before the inbound crossing of Neptune's bow shock, thereby signifying magnetic connection of the bow shock. The Langmuir waves occurred in multiple bursts throughout two distinct periods separated by an 85 minute absence of wave activity. The times of onsets, peaks, and disappearances of the waves were used together with the magnetic field directions and spacecraft position, to perform a 'remote-sensing' analysis of the shape and location of Neptune's bow shock prior to the inbound bow shock crossing. The bow shock is assumed to have a parabolidal shape with a nose location and flaring parameter determined independently for each wave event. The remote-sensing analysis give a shock position consistent with the time of the inbound shock crossing. The flaring parameter of the shock remains approximately constant throughout each period of wave activity but differs by a factor of 10 between the two periods. The absence of waves between two periods of wave activity coincides with a large rotation of the magnetic field and a large increase in the solar wind ram pressure' both these effects lead to magnetic disconnection of the spacecraft from shock. The planetwards motion of the shock's nose from 38.5 R(sub N) to 34.5 R(sub N) during the second time period occurred while the solar wind ram pressure remained constant to within 15 percent. This second period of planetwards motion of the shock is therefore strong evidence for Neptune's bow shock moving in response to the rotation of Neptune's oblique, tilted magnetic dipole. Normalizing the ram pressure, the remotely-sensed shock moves sunwards during the first wave period and planetwards in the second wave period. The maximum standoff distance occurs while the dipole axis is close to being perpendicular to the Sun-Neptune direction. The remote-sensing analysis provides strong evidence that the location of Neptune's bow shock is controlled by Neptune's rotation phase

Evaluation of production samples of the scintillators LaBr3:Ce and LaCl3:Ce

We report on the evaluation of the performance of two recently developed scintillator materials, LaCl{sub 3}:Ce and LaBr{sub 3}:Ce, at the task of gamma ray spectroscopy. Their performance is compared to a standard scintillator used for gamma ray spectroscopy--a 25 mm diameter 25 mm tall cylinder of NaI:Tl. We measure the pulse height, energy resolution, and full-energy efficiency of production LaBr{sub 3}:Ce and LaCl{sub 3}:Ce scintillation crystals of different sizes and geometries for a variety of gamma-ray energies. Using production rather than specially selected crystals will establish whether immediate large-scale use is feasible. The crystal is excited by gamma rays from one of six isotopic sources ({sup 125}I, {sup 241}Am, {sup 57}Co, {sup 22}Na, {sup 137}Cs, and {sup 60}Co) placed 15 cm away from the scintillator. Our measurements show that both LaCl{sub 3} and LaBr{sub 3} outperform NaI:Tl in almost all cases. They outperform NaI:Tl at all energies for the photopeak fraction and counting rate measurements, and for energy resolution at higher energies (above 200 keV for LaCl{sub 3} and 75 keV for LaBr{sub 3}). The performance of production crystals is excellent and these scintillators should be considered for immediate use in systems where stopping power and energy resolution are crucial

Temperature dependence of CsI(Tl) gamma-ray excited scintillation characteristics

The gamma-ray excited, temperature dependent scintillation characteristics of CsI(Tl) are reported over the temperature range of -100 to + 50[deg]C. The modified Bollinger-Thomas and shaped square wave methods were used to measure the rise and decay times. Emission spectra were measured using a monochromator and corrected for monochromator and photocathode spectral efficiencies. The shaped square wave method was also used to determine the scintillation yield as was a current mode method. The thermoluminescence emissions of CsI(Tl) were measured using the same current mode method. At room temperature, CsI(Tl) was found to have two primary decay components with decay time constants of [tau]1 = 679+/-10 ns (63.7%) and [tau]2 = 3.34+/-0.14 [mu]s (36.1%), and to have emission bands at about 400 and 560 nm. The [tau]1 luminescent state was observed to be populated by an exponential process with a resulting rise time constant of 19.6+/-1.9 ns at room temperature. An ultra-fast decay component with a 1 and [tau]2 were determined to be 2.22+/-0.33 [mu]s and 18.0+/-2.59 [mu]s, respectively, while the 400 nm emission band was not observed below -50[deg]C. At +50[deg]C the decay constants were found to be 628 ns (70.5%) and 2.63 [mu]s (29.3%) and both emission bands were present. The scintillation yield of CsI(Tl) was observed to be only slightly temperature dependent between -30 and +50[deg]C, peaking at about -30[deg]C (about 6% above the room temperature yield) and monotonically decreasing above and below this temperature. Four different commercially available CsI(Tl) crystals were used. Minimal variations in the measured scintillation characteristics were observed among these four crystals. Thermoluminescence emissions were observed to have peak yields at -90, -65, -40, +20, and possibly -55[deg]C. The relative magnitudes and number of thermoluminescence peaks were found to vary from crystal to crystal.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/30984/1/0000659.pd

Strontium and barium iodide high light yield scintillators

Europium-doped strontium and barium iodide are found to be readily growable by the Bridgman method and to produce high scintillation light yields

Large scale numerical investigation of excited states in poly(phenylene)

A density matrix renormalisation group scheme is developed, allowing for the first time essentially exact numerical solutions for the important excited states of a realistic semi-empirical model for oligo-phenylenes. By monitoring the evolution of the energies with chain length and comparing them to the experimental absorption peaks of oligomers and thin films, we assign the four characteristic absorption peaks of phenyl-based polymers. We also determine the position and nature of the nonlinear optical states in this model.Comment: RevTeX, 10 pages, 4 eps figures included using eps

Dry matter yields and hydrological properties of three perennial grasses of a semi-arid environment in east Africa

Enteropogon macrostachyus (Bush rye), Cenchrus ciliaris L. (African foxtail grass) and Eragrostis superba Peyr (Maasai love grass) are important perennial rangeland grasses in Kenya. They provide an important source of forage for domestic livestock and wild ungulates. These grasses have been used extensively to rehabilitate denuded patches in semi-arid environment of Kenya. This study investigated the dry matter yields and hydrological properties of the three grasses under simulated rainfall at three phenological stages; early growth, elongation and reproduction. Laboratory seed viability tests were also done. Hydrological properties of the three grasses were estimated using a Kamphorst rainfall simulator. Results showed that there was a significant difference (p > 0.05) in dry matter yields and soil hydrological properties at the different grass phenological stages. Generally, all the three grasses improved the soil hydrological properties with an increase in grass stubble height. C. ciliaris gave the best soil hydrological properties followed by E. macrostachyus and E. superba, respectively. E. macrostachyus recorded the highest seed viability percentage. C. ciliaris and E. superba were ranked second and third, respectively. C. ciliaris yielded the highest biomass production at the reproductive stage followed by E. superba and E. macrostachyus, respectively. (Résumé d'auteur

Outcomes of bailout percutaneous ventricular assist device versus prophylactic strategy in patients undergoing nonemergent percutaneous coronary intervention

OBJECTIVES: To compare in-hospital outcomes of bailout support to prophylactic support with percutaneous ventricular assist devices (pVAD) for high-risk nonemergent percutaneous coronary intervention (HRPCI). BACKGROUND: Prophylactic support with pVAD for a HRPCI is used in patients felt to be at risk for hemodynamic collapse during PCI. An alternative strategy of bailout pVAD support in the event of hemodynamic collapse is also entertained. METHODS: We compared the outcomes of patients entered in the cVAD database who underwent Impella Protected PCI (ProPCI group) with patients from the cVAD and USpella databases receiving bailout Impella support for hemodynamic collapse during HRPCI (Bailout group). RESULTS: A total of 1,028 patients supported with Impella pVAD were entered into the cVAD database as of July 2019 and were included in this analysis. Of those 971 were in the ProPCI group and 57 in the Bailout group. Patients in the Bailout group were more often female (50.9%vs. 27.2%, p = .0002) with higher median baseline left ventricular ejection fraction (LVEF) (40%vs. 30%, p \u3c .0001) and with lower prevalence of both heart failure (42.1%vs. 56.9%, p = .0385) and left main disease (40.0%vs. 56.1%, p = .0250) compared to the ProPCI group. Unadjusted and adjusted in-hospital mortality was significantly higher in the Bailout group (49.1%vs. 4.3%, and 57.8%vs. 4.4%, p \u3c .0001 for both). CONCLUSIONS: In our study population, the bailout group was associated with significant increased mortality compared to ProPCI group. Female gender was more frequently observed in patients requiring bailout pVAD. Further investigation is warranted in order to generalize the findings of our study