Irradiation of solid polycrystalline alanine creates a free radical, whose concentration can be measured from its ESR signal. The radical CH3
HCOO– of practically unlimited stability at room temperature shows an electronic absorption spectrum in the UV. Modern methods of diffuse reflectance spectrophotometry allow to measure the radical concentration which is proportional to the absorbed dose of radiation. The alanine dosimeter is prepared in a thin layer, adequate both for the congested isodose curves in the case of accelerated electrons irradiation and the method of measurement. Thus the proposed dosimeter is applicable not only for gamma, but also for EB radiation processing in the range of 0.1 to 50 kGy. The application of the dosimeter does not demand to use the ESR spectrometer, even of the type dedicated to alanine dosimetry only, but may be performed by a UV-VIS spectrophotometer equipped with an integrating sphere, permitting measurements of the Kubelka-Munk function.
A preliminary pulse radiolytic study is reported on transient optical absorption spectra in solid amino acids. Spectra have been obtained by improved Cherenkov Light Self-absorption Method applied during 5.5 s pulses of 10 or 13 MeV linac electrons. Amino acids exhibit a very different type of behaviour: from lack of absorption signal (at the present sensitivity of the system) to strong absorptions in the visible part of the spectrum and the near UV. The absorption may be due to the electron trapped in lattice imperfections or to the formed free radical. Comparison of spectra in a variety of preparations (hydrochlorides, salts of amino acids and sets of related compounds) suggests a free radical origin of spectra. All transient absorption disappear within a fraction of a second. Free radicals turn by deamination and/or decarboxylation into more stable species detected by ESR.
Characteristics of a new alanine dosimeter in the shape of a thin film, with the measurement of optical absorption of the CH3CHCOO– radical is described. That type of dosimeter, ALA/DRS (for diffuse reflection spectrophotometry) is compared, to an alanine dosimeter with EPR evaluation (ALA/EPR for short). In many respects the simple ALA/DRS version, as the alanine-polyethylene composite is superior. The paper shows the importance of the new experimental approach to free radical research in solid state radiation chemistry.
Optical absorption by electrons trapped in natural anionic vacancies in NaCl has been used for the construction of a dosimeter for radiation processing. To meet the demands of electron beam processing, characterised by congestion of isodoses, the active part of the dosimeter, i.e., the microcrystals of NaCl are embedded in a 0.3 mm thick polyethylene film, in which doses from 10 MeV electrons do not exceed ±2% difference in extreme parts of the dosimeter body. The dosimetric film is opaque and the optical density at the wavelength
max = 465 nm, i.e., the maximum absorption of the F band, is measured by diffuse reflected light spectrophotometry (DRS). The measurement is performed against the unirradiated film as reference, thus increasing the accuracy, by self-compensation of signals not belonging to the absorption of F-centres. The spectrum obtained in such a way is identical with that of F-centres in irradiated single NaCl crystals. The calibration curve of the height of the band is almost linear versus the dose in the range of several tens of kilograys. As ordinary grades of sodium chloride may be used, the dosimeter developed is cheap and enables to map the irradiation field in objects of complicated geometry.
There is no statistically significant difference in the response of the enantiomers and the racemate of -alanine to -radiation (60Co) and accelerated electrons (10 MeV). The response was measured by the yield of ammonia, one of the separated final products of solid alanine radiolysis and the concentration of free radical CH3·CH·COO–, the intermediate product of radiolysis. Some of the properties of the racemate may affect the precision of measurements. TheDL-alanine shows different crystal structure, morphology of crystals, specific density and bulk density than the enantiomers. The response ofL andDL alanines built into composites may be different because of different reactivity of the enantiomer and the racemate with components during the processing and irradiation.
The objects of investigation were single crystals of L-α-alanine, in which radical anion CH3 C˙H CO
has been formed by radiation induced deamination of alanine. Previously, this stable radical has been spectrally identified
(λmax, ε=1100 M−1·cm−1), and its characteristics have found to be identical with characteristics of the same radical obtained by pulse radiolysis
in aqueous solution. The mechanism of radical formation in the solid state is not known. Time resolved pulse radiolysis of
single crystal alanine has shown more complicated way of the formation of the same radical in solid state than in aqueous
solution. The electrons abstracted from the solid alanine molecule neutralise positive of zwitter-ion alanine. Ammonia is
leaving the reaction-complex in time of milliseconds, leaving the stable radical anion.
The alanine dosimeter made for evaluation by diffuse light reflection spectrophotometry (ALA/DRS) does not show the effect of orientation of crystals. Supposed deviation from random orientation has been investigated by EPR spectroscopy. EPR investigation shows that in spite of the very fine size of L-alanine crystals, they are oriented in thin layers of the polyethylene matrix. Specially prepared films with deliberately well oriented crystals have confirmed this observation. Our ALA/DRS dosimeter can be evaluated by the EPR method for the concentration of free radicals, providing that the dominating crystal orientation in the dosimetric film is indicated on it as an arrow, and the sample is inserted into the magnetic cavity always in the same orientation as has been done during the calibration operation.