Lanthanum halides

The general chemistry of Ac3 in both solid compounds and solution, where known, is very similar to that of lanthanum, as would be expected from the similarity in position in the Periodic Table and in radii (Ac3, 1.10 La3, 1.06 A) together with the noble gas structure of the ion. Thus actinium is a true member of Group 3, the only difference from lanthanum being in the expected increased basicity. The increased basic character is shown by the stronger absorption of the hydrated ion on cation-exchange resins, the poorer extraction of the ion from concentrated nitric acid solutions by tributyl phosphate, and the hydrolysis of the trihalides with water vapor at 1000°C to the oxohalides AcOX the lanthanum halides are hydrolyzed to oxide by water vapor at 1000°C. 

The materials that have found particular application for gamma-ray measurements are all inorganic crystals sodium iodide (Nal), caesium iodide (Csl), calcium fluoride (CaF2), bismuth germanate (BGO) and, recently, lanthanum halides. Of these, the first is the most important and the last are materials rapidly gaining in importance. 

Reactions with Transition Metals Forming Carbon-Carbon Bonds. The combination of certain lanthanides and TMS-Br has been found to produce lanthanum halides (LaX n = 2 or 3) that are very active reducing reagents (eq 19). So far, the only metals to be used in these reactions are samarium (Sm) and ytterbium (Yb). In addition to TMS-Br, these reactions have been accomplished using Sm/TMS-Cl/Nal under similar conditions with corr5>arable yields. 

In the first section a brief overview of the properties of Lanthanum Halides crystals is presented. In section 2 the performances of LaBrsiCe and LaClsiCe gamma-rays deteetors are discussed in fom subsections on internal activity, radiation hardness, particles identifieation time and energy resolution. Section 3 presents some recent results on the use of such detector in a position sensitive mode. 

The LaCls Ce and LaBrs Ce are crystals with an hexagonal (UCL3 type) structure with a P63/m space group [8]. They are extremely hygroscopic (more than NaI Tl) and their crystal structure produces an anisotropic thermal expansion and make them birelxangent. In addition Lanthanum Halide crystals have a relatively weak (100) cleavage plane which makes the growth of crystals complex [4]. 

The Lanthanum Halide frequency spectra, as figure 1 shows, depend on the temperature and on the concentration of the Cerium dopant. Even though several studies have shown that the seintillation properties depend on the Ce eoncentration, there are still several open problems or contradictoiy data that need a more systematic study. In fact, as already stated, it is veiy difficult to produce crystals identical except for the concentration of the Ce. As an example in one of the first papers on LaBr3. Ce [10] it was measured that a low concentration of Ce (0.5%) provides the most efficient scintillation output and with the increase of Ce concentration the light output decreases. In reference [11], instead, no significant changes in the light output with Ce concentration was reported. 

The temperature dependence of the light yield is a second important factor which could affect in a significant way its performance as a scintillation detector, in particular it directly affects the detector energy resolution. The Lanthanum Halide materials have an uncommon temperature stability in a veiy large temperature range which make them suitable to use also in very harsh or hostile environment. 

A similar feature has been observed for Lanthanum Halide detectors [33-35]. The scintillation light in such crystals, as discussed in the first section, does not have two separate components with very different time constant. However, it has been observed a small difference in the line-shape of alpha and gamma induced pulses [33,35]. 

At the moment it is not yet available a PMT especially designed for LaBr iCe or LaCls- Ce. In fact, PMT has a number of dynodes between 8 to 12 which provide a too high gain in the case of Lanthanum Halide scintillation detectors which produce scintillation light with an extremely high yield and short time constant. The interplay between the PMT time properties and signal saturation is probably the key aspect for the optimal use of such scintillators. 

Okamoto Y, Madden PA (2005) Structural study of molten lanthanum halides by X- ray diffraction and computo simulation techniques. J Phys Chem Sol 66 448—451... 

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