Trap levels in Y-aluminum garnet scintillating crystals

Point defects acting as trap levels were investigated on undoped, Ce- and (Ce, Si)-doped Y₃Al₅O₁₂ (YAG) crystals by TSL measurements performed over a wide temperature range (10–800 K). Below room temperature, a composite glow curve was observed, whose intensity strongly increased after Ce doping. Moreover, Ce doping introduced new trap levels giving rise to glow peaks in the 100–200 K range. On the other hand, Si co-doping did not influence the low T glow curve in a significant way. The spectral emission of the TSL was found to be governed by the Ce³⁺ 5d–4f radiative transition, while defect related higher energy emission bands were detected only in the undoped crystal. Above RT, the glow curve was found to be much more influenced by Si co-doping since a strong increase of a glow peak at about 250°C was noticed. Scintillation time decays of Ce- and Ce,Si-doped samples are also reported and compared with TSL data. The significance of the results and the potential impact of defect states on the scintillation properties are discussed.     

Novel applications of the modified polonovski reaction - VII preparation of isoquinuclidines

The first successful preparations of the biochemically important isoquinuclidine ring system by the modified Polonovski reaction are described.     

Finite groups and subgroup-permutability

Subgrouppermutability in a finite group is studied, in order to distinguish between irreducible and reducible groups and to give a bound for the diameter. A simple nonabelian finite group is irreducible and its diameter is less or equal16 while a reducible group is forced to be soluble and the bound of the diameter of such a group is 4.Research partially supported by G.N.S.A.G.A. of C.N.R. and M.U.R.S.T. of Italy.     

Time-resolved EPR study of 3C60 in solid matrices

Using time-resolved cw EPR and pulsed EPR techniques, a study was made of the characteristics of the photoexcited triplet state of C₆₀ adsorbed in silica gel pores and embedded in a polymethylmethacrylate matrix. It was found that the time-resolved spectra from ³C₆₀ in these matrices at room temperature retain the absorption/emission features of spectra from ³C₆₀ in frozen solution (≤120 K). Apparently, the combination of molecular rotation and pseudo rotation (resulting from interconversion between Jahn-Teller states) is not fast enough, to lead to complete averaging of the dipole-dipole interaction between the unpaired electrons. The study explored the effect of introduction of solvent molecules in the silica gel pores on the ³C₆₀ spectrum and quenching of the triplet by electron donors.     

Approximation and regular perturbation of optimal control problems via Hamilton-Jacobi theory

We present two convergence theorems for Hamilton-Jacobi equations and we apply them to the convergence of approximations and perturbations of optimal control problems and of two-players zero-sum differential games. One of our results is, for instance, the following. LetT andT _h be the minimal time functions to reach the origin of two control systemsy = f(y, a) andy = f _h (y, a), both locally controllable in the origin, and letK be any compact set of points controllable to the origin. If f _h –f Ch, then |T(x) – T _h (x)| C _K h , for all x K, where is the exponent of Hölder continuity ofT(x).     

Some remarks on the degrees of faithful irreducible representations

We study a class of finite p-groups admitting faithful irreducible complex representations of distinct degrees. In particular, for every prime p, we produce an example of such a p-group having minimal order p ⁸.Dedicated to A. Wagner on the occasion of his 60th birthdayLavoro eseguito nell'ambito dei finanziamenti M.U.R.S.T.     

Applications of a graph related to conjugacy classes in finite groups

This article was written by the authors with a during the forth author's visit to Milano and, independently.