Thermoluminescence of ultra-high dilutions of lithium chloride and sodium chloride

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Abstract

Ultra-high dilutions of lithium chloride and sodium chloride (10−30gcm−3) have been irradiated by X- and γ-rays at 77K, then progressively rewarmed to room temperature. During that phase, their thermoluminescence has been studied and it was found that, despite their dilution beyond the Avogadro number, the emitted light was specific of the original salts dissolved initially.

Introduction

Thermally stimulated luminescence—often called thermoluminescence—is a well-known phenomenon amongst the thermally stimulated processes (thermally stimulated conductivity—thermally stimulated electron emission—thermogravimetry—differential thermal analysis and differential scanning calorimetry, etc.). Its theory and applications have been fully developed inter alia by McKeever [1], Chen [2] and Visocekas [3] and it proved to be a most interesting tool to study the structure of solids, mainly ordered crystals. To that end, the studied material is “activated” at low-temperature, usually by radiant energy (UV, X-rays, gamma rays, electron beams, α-particles or neutrons) which most generally creates electrons–holes pairs which become separately “trapped” at different energy levels. Then, when the irradiated material is warmed up, the heating serves as a trigger to release the initially accumulated energy and the trapped electrons and holes move and recombine. A characteristic glow is emitted most often under the shape of different successive peaks according to the depths of the initial traps. As a general rule this phenomenon is observed in ordered crystals though it can be equally seen in disordered materials such as glasses [2]. In that mechanism, imperfections in the lattice play a major role and are considered to be the place where luminescent centres appear. Thus, thermoluminescence is a good tool to study these imperfections and understand how they appear in the crystal.

This is exactly along those lines that we have carried our first investigations, starting, this time, from liquids which were turned into stable solids by low-temperature cooling. Working essentially with water—mainly deuterium oxide—we have shown [4], [5] that the thermoluminescent glow of irradiated hexagonal ice consisted in two major peak areas—Peak 1 near 120K and Peak 2 near 166K—(Fig. 1) having well-defined emission spectra (Fig. 2), the D2O samples giving a much higher signal than the H2O ones. In both cases, unirradiated samples gave no signals whatsoever. For both D2O and H2O we equally showed that the relative intensity of the thermoluminescence glow was a function of the irradiation dose and, that at least for Peak 2, it did show a maximum between 1 and 10kGy (Fig. 3).

As a first hypothesis on the nature of the emission itself it has been suggested by Teixeira [6] that Peak 2 could be connected to the hydrogen-bond network within the ice which, in turn, could result from the structure of the original liquid sample, whilst Peak 1 looked to be closely related to the molecule. Actually, for a totally different substance, such as formamide which is known to present strong hydrogen bonds, our experiments show a similar glow in the Peak 2 region (Fig. 4). This strengthens our views on the involvement of hydrogen bonds in this mechanism.

To develop this concept further we did select to study the effect of lithium chloride on the thermoluminescence of irradiated D2O ice since this particular substance is known to suppress hydrogen bonds. The result, indeed, is spectacular and, at the relatively low concentration of 0.1M Peak 2 is totally erased (Fig. 5) whereas the basic emission of Peak 1 remains almost unchanged.

At that point we thought that it would be of interest to challenge the theory according which pre-existent “structures” in the original fluid, developed around some added chemicals, could survive a great number of successive dilutions when done under vigorous mechanical stirring.

To that end we did prepare ourselves, courtesy of the BOIRON LABORATORIES, ultra-high dilutions of lithium chloride and sodium chloride by successive dilutions to the hundredths, all done under vigorous mechanical stirring (initially 1g in 100cm3, then 1cm3 of this solution in 99cm3 of pure D2O ... and so on) until we reached—theoretically—at the 15th dilution, a “concentration” of 10−30gcm−3. A reference sample of D2O alone was also prepared according to this technique, still keeping vigorous agitation (150strokes/7.5s at each successive “dilution” step).

We did proceed, then, to the “activation” of these materials by irradiation according the following experimental protocol.

Section snippets

Experimental

One cubic centimeter of each solution is placed in aluminium test cavities of 20mm diameter and 2mm depth and frozen to −20°C on a cold metallic block. The frozen systems are kept 24h at −20°C to achieve stability into their crystallization pattern and they are finally immersed into liquid nitrogen and kept at −196°C for 24h.

In a first set of experiments the frozen ice disks are irradiated at 77K with 100kV X-rays to achieve a dose of 0.4kGy(30min). Previous determinations were done to check

Results

Much to our surprise, the experimental results do show—without any ambiguity—that for an X-ray dose of 0.4kGy the thermoluminescence glows of the three systems were substantially different (Fig. 6). These findings did prove to be reproducible in the course of many different identical experiments.

To compare the curves between them we normalised the emitted light readings taking Peak 1 as the reference. In doing so, we obtain for Peak 2 the different curves presented in Fig. 7 which show quite

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