Response and noise characteristics of small-sized inorganic and organic scintillation detectors measured with vacuum and solid-state photodetectors

Response and noise properties of scintillation detectors were investigated using a ¹³⁷Cs radioactive source. The experimental setup consisted of a scintillator, photodetector, amplifier, and spectrometric analog-to-digital converter. Small-sized, cubic crystals were manufactured of stilbene, p-terphenyl and thallium-doped cesium iodide, as well as of a plastic scintillator material based on polystyrene. Vacuum and solid-state photomultipliers and a silicon PIN photodiode were used as photodetectors. The energy equivalent noise was determined for all used photodetectors. The response of various scintillators when using vacuum and semiconductor photomultipliers was found to be closely similar.     

Crosslinking of mixtures of diglycidylether of bisphenol‐A with 1,6‐dioxaspiro[4.4] nonan‐2,7‐dione initiated by tertiary amines: III. Effect of hydroxyl groups on network formation

BACKGROUND: Blends of epoxy resin oligomers, diglycidylether of bisphenol‐A (DGEBA), and a bislactone, 1,6‐dioxaspiro[4.4]nonan‐2,7‐dione (s(γBL)), were anionically copolymerized using two tertiary amines as anionic initiators. Their curing rheology and gelation behaviour were studied to provide a more comprehensive knowledge of the curing of these previously studied systems. RESULTS: The activation energy for gelation was found to be similar to that previously measured using differential scanning calorimetry and appeared to increase in the presence of the bislactone. The reaction rate during copolymerization of DGEBA with s(γBL) was slower than DGEBA homopolymerization alone because the alkoxide attack on the epoxide is faster than the reaction of the carboxylate ion and the epoxy group. The effect of the initiator type on the gel conversion was small and was presumably due to differences in the kinetic chain length caused by amine regeneration from the quaternary amine. For the same initiator and at a constant ratio of DGEBA/s(γBL), an increase in the hydroxyl concentration of the DGEBA oligomer raised the gel conversion. For a DGEBA oligomer with low hydroxyl levels, an increase in the concentration of s(γBL) increased the gel conversion; however, for a DGEBA oligomer with high hydroxyl levels, increasing s(γBL) concentration decreased the gel conversion. CONCLUSION: These results are interpreted in terms of the effect of initiation rate and chain transfer rate on the kinetic chain length. The glass transition temperature of the gel was found to be controlled by the fraction of the aliphatic s(γBL) and the amount of plasticizing sol in the matrix. Copyright © 2009 Society of Chemical Industry     

Synthesis of a useful class of transmission line networks

A general class of networks consisting of cascaded commensurate transmission lines and shunt imaginary conductances is considered. This kind of network performs an excellent approximation to transmission line networks with shunt stubs or lumped elements. The realizability conditions for the driving-point admittance and transfer function have been given in a previous paper. The imaginary conductances were generated by introducing an extra phase angle into the scattering function. In the present paper two alternative methods are given

• 1 by identifying in different ways the roots of the numerator polynomial in the input scattering function.
• 2 by using non-symmetric gain functions.

The influence of different factorizations and non-symmetric gain functions upon the element values variation is examined.     

Curing of mixtures of epoxy resins and 4‐methyl‐1,3‐dioxolan‐2‐one with several initiators

Diglycidyl ether of bisphenol A or 3,4‐epoxycyclohexylmethyl 3,4‐epoxycyclohexane carboxylate were mixed with different proportions of 4‐methyl‐1,3‐dioxolan‐2‐one and cured using lanthanide triflates as initiators. In order to compare the materials obtained, conventional initiators such as boron trifluoride complexes and N,N‐dimethylaminopyridine were also tested. The curing process was followed by differential scanning calorimetry (DSC) and Fourier transform IR in attenuated total reflectance mode. This technique proved that the carbonate accelerates the curing process because it helps to form the active initiating species, although it was not chemically incorporated into the network and remained entrapped in the material. The DSC kinetic study was also reported. © 2006 Wiley Periodicals Inc. J Appl Polym Sci 102: 2086–2093, 2006     

New thermosets obtained by cationic copolymerization of DGEBA with γ‐caprolactone with improvement in the shrinkage. II. Time–temperature–transformation (TTT) cure diagram

Mixtures of diglycidylether of bisphenol A (DGEBA) with different proportions of γ‐caprolactone (γ‐CL) were cured with ytterbium triflate as initiator. The curing was studied with differential scanning calorimetry (DSC) and thermo mechanical analysis (TMA). The results are presented in the form of a time–temperature–transformation diagram. The kinetic analysis was performed by means of the isoconversional integral procedure and the kinetic model was also determined using the Coats–Redfern method. Gelation was determined by means of combined experiences of DSC and TMA. The relationship between the glass transition temperature (T_g) and the degree of conversion α was determined by DSC. Using the isoconversional lines and the T_g‐α relationship, the vitrificacion curve was obtained. The methodology developed makes it possible to obtain the TTT diagram using only no‐isothermal experiments with equivalent results to those using classical isothermal procedures. The addition of γ‐CL accelerates the curing and reduces the shrinkage after gelation and consequently the internal stresses in the material. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

New thermosets obtained by copolymerization of DGEBA with 1,5,7,11-tetraoxaspiro[5,5]undecane catalyzed by lanthanide triflates

DGEBA epoxy resin was cured with different proportions of 1,5,7,11-tetraoxaspiro[5,5]undecane (TOSU) with ytterbium and lanthanum triflates as catalysts. The curing was studied by differential scanning calorimetry (DSC) and Fourier transform infrared in the attenuated-total-reflexion mode (FTIR/ATR).FITR/ATR was used to monitor the competitive reactive processes and to quantify the evolution of the epoxide, orthocarbonate, linear carbonate and other groups formed in the curing process. The T_g of the cured materials decreased when the proportion of TOSU increased. The kinetics were studied by DSC experiments and analyzed with iso-conversional procedures. The systems catalyzed by ytterbium triflate had a higher curing rate and reached a higher degree of cross-linking. The addition of TOSU reduced the degree of overall shrinkage and even led to one material expanding. The flexibility and the degradability of these materials were improved by introducing higher proportions of TOSU.     

Role of infection as a risk factor for atherosclerosis, myocardial infarction, and stroke

An increasing body of evidence has linked infections to atherosclerosis and thrombosis. Herpesviruses cause atherosclerosis in experimental animals. Herpesviruses can also be detected in atherosclerotic lesions in humans. Cytomegalovirus may play a role in arteriosclerosis in transplanted hearts, and this virus, together with tumor suppressor protein p53, can be found in restenosis lesions following angioplasty. Chlamydia pneumoniae and dental infections are associated with coronary heart disease in cross-sectional and longitudinal studies, and preceding respiratory infections are associated with ischemic stroke. Infections may favor formation of atherosclerosis and thrombosis by elevation of blood levels of fibrinogen, leukocytes, clotting factor, and cytokines and by alteration of the metabolism and functions of endothelial cells and monocyte macrophages. Low-grade infections may also be one of the causes of the inflammatory reaction observed in atherosclerotic lesions and acute ischemic symptoms, reflected in elevated levels of C-reactive protein. These observations warrant further studies in this field.     

Effect of alpha 2-adrenergic drugs dexmedetomidine and atipamezole on extracellular amino acid levels in vivo

alpha 2-Adrenoceptors are known to be involved in a variety of physiological functions and pathological conditions, including epilepsy and the extent of excitotoxin-induced cell death. In this study we evaluated whether selective alpha 2-adrenergic drugs can modulate the release of neurotransmitter amino acids. The effect of the alpha 2-adrenoceptor agonist dexmedetomidine (5 micrograms/kg, s.c.) and the alpha 2-adrenoceptor antagonist atipamezole (0.1 mg/kg and 1 mg/kg, s.c.) on the release of extracellular glutamate, aspartate and gamma-aminobutyric acid (GABA) was studied with microdialysis in the hippocampus of freely moving rats under basal and K(+)-evoked conditions. Atipamezole (1 mg/kg) decreased K(+)-evoked glutamate efflux by 30% compared to the control group (P < 0.05) but did not affect significantly the effluxes of aspartate and GABA. Dexmedetomidine and the lower dose of atipamezole (0.1 mg/kg) did not significantly alter the evoked overflow of amino acids. The results suggest that alpha 2-adrenergic drugs have only modest effects on the K(+)-stimulated overflow of extracellular neurotransmitter amino acids in rat hippocampus.