Lyoluminescent dosimetric materials and their response to ionizing radiation

Nuclear Instruments and Methods 175 (1980) 119-121 © North-Holland Publishing Company

Part VIII. L yoluminescence - Other techniques

LYOLUMINESCENT DOSIMETRIC MATERIALS AND THEIR RESPONSE TO IONIZING RADIATION Kamil V. ETTINGER *, Ralph G. FAIRCHILD Medical Department, Brookhaven National Laboratory, Upton, NY 119 73, USA

and Clifford I. ANUNUSO, Somsri SRIRATH, Eugenio Del VIGNA FILHO Department of Biomedical Physics and BioEngineering, University of Aberdeen, Foresterhill, Aberdeen AB9 2ZD, Scotland, UK

A survey of known lyoluminescent phosphors, suitable for use in radiation dosimetry is provided, giving information on the sensitivity, range of doses which can be measured and on the factors affecting reproducibility. A brief description of the application of sensitizers is also given.

A very large number of organic compounds exhibit lyoluminescence when dissolved after exposure to ionizing radiation. Saccharides, amino acids, amino sugars, nucleic acids, acrylic and vinyl polymers have been investigated in the Aberdeen laboratory as potential dosimetric materials. Recently it was found, that not only saccharides, but carbohydrates in general, are strongly lyoluminescent, as long as a suitable solvent can be found in which dissolution takes place quickly enough to be of practical value. In particular, starch, cellulose and their derivatives are strongly lyoluminescent. Lyoluminescence has also been found in many proteins particularly in globulins. It has been observed not only in purified, well defined chemical compounds, but also in dried biological materials, including dried foods, desiccated animal tissues, etc. It is likely that there exists a common mechanism which operates in all cases, even if the actual structure o f interacting molecules and perhaps details of chemical reactions may differ. It is certain that a radical mechanism underlies the phenomenon of lyoluminescence, free trapped radicals being formed as a result of exposure to ionizing radiation. The proposed mechanism, described in more detail elsewhere by Ettinger and Buchan [1], assumes that originally formed radicals are oxidized to peroxy radicals and then undergo disproportionation on dissolution. This process, known as Vasil'evRussel scheme yields singlet oxygen and a carbonyl

compound, which can be formed in an excited state. The light emitting step is thus: 2 ( R I R z C H O O ' ) ~ RIRzCHOH + 02 + R1R2C=:O where R1, R2 are radical groups specific for the given m~iterial. De-excitation o f the carbonyl compound gives rise to emission in the blue part of the visible spectrum, the exact position of the emission band depending upon the particular compound. This scheme appears to explain the experimental facts and is strongly supported by research on oxidation processes in hydrocarbons and is also implicated in bioluminescence [2]. Obviously, in order to be useful for the purpose of radiation dosimetry the lyoluminescent material must show such favourable characteristics as longevity of induced radicals, must be easily soluble in a suitable solvent and its energy response must be adequate to the task. Until recently, most applications of lyoluminescence involved the use of water or an aqueous solution as a solvent, but the effect has been observed in other solvents, namely organic polar and non-polar liquids. In fact, the use of methyl or ethyl alcohols may have distinct advantages over water, particularly in work with sensitizers. On various occasions tables and graphs have been produced given relative sensitivities of various LL phosphors [ 3 - 5 ] . These comparisons must be taken as a very rough measure of the ability of a compound to exhibit lyoluminescence, because there is a large and unexplained variation in the sensitivity between materials supplied by different producers and even

* On leave from Dept. of Biomedical Physics and BioEngineering, University of Aberdeen. 119

VIII. LYOLUMINESCENCE

K. V Ettinger et al. / L yoluminescent dosimetric materials

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between different batches from the same source. LL phosphors are not available commercially as such, but are purchased as laboratory reagents in various degrees of purity, without any regard to their LL properties. In consequence the sensitivity of the same material (expressed as light yield at a given dose, e.g. 100 Gy) varies, often by a factor of 2 - 5 . It is uncertain what physical or chemical agent is behind this variability, but is has been established that the process of crystallization plays a crucial role in determining the sensitivity of a lyoluminescent compound. One of the best investigated and most reproducible phosphors is mannose. Fig. 1 shows dose response curves for most sensitive saccharides using 6°Co as a source of radiation. Due to a very good rate of dissolution, saccharides are preferred lyoluminescent phosphors. The energy response of LL phosptrors can be calculated on the basis of their composition. Another group of LL compounds of practical importance are amino acids. Of these glutamine is particularly useful as a dosimetric material for a range up to about 8 X 104 Gy. Fig. 2 shows the response of man-

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Fig. 3. Response of mannose and glutamine to 7.6 MeV neutron beams in comparison with response to 6°Co gamma

rays. nose and glutamine to monoenergetic electron beams in comparison with the response to 1.25 MeV gamma rays. The response to fast neutron beams of 7.6 MeV average energy produced by deuteron bombardment of thick beryllium targets is shown for mannose and glutamine in fig. 3. The response to heavy charged particles, shown in fig. 4, is in agreement with reduction of yield with increasing LET generally observed in single-hit detectors. In general, the LET response can be expressed as 1/(1 + kL), where L is LET in keV/micron and k is a coefficient within limits 0 . 0 2 0.03 micron/keV. The energy response to fast neutrons, reported previously [6] has now been remeasured in Brookhaven and is shown, together with earlier data, in fig. 5. Samples of mannose and organic lithium salts: lithium oxalate, lithium pyruvate and 6Li pyruvate were-exposed to thermal and epithermal beams from

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by Laflin and Baugh [8] a colloidal suspension of sodium lauryl sulphate with diphenylisobenzofuran (DPBF) in water was prepared as a solvent for mannose. Enhancement by a factor of 10 was gained in this way. It is also worthwhile to mention that incorporation in the LL phosphors resulted in materials with light yields increased by more than an order of magnitude [9].

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10 15 ENERGY [ MeV] Fig. 5. Fast neutron response of mannose in comparison with response to 6°Co gamma rays. Horizontal bars indicate the energy spread of neutron sources measured at half maximum. For fission spectrum and for neutrons produced by bombardment of thick Be target with 30 MeV 3He ions only mean energy is indicated.

the Brookhaven Medical Reactor. Numerical values for the radiation field, which include fast and slow nuetron fluxes as well as gamma contamination, are given in table 1, together with the response of irradiated materials. Organic salts of lithium may find application as neutron flux monitors and in the mapping of therapeutic slow neutron distributions such as is needed for boron neutron capture therapy

[71. It is possible to enhance the amount of emitted light by using sensitizers, some of which are known to work as oxidation indicators (luminol, lucigenin) and some as energy transfer agents with high quantum yields (fluorescent dyes). Following a communication

References [1] K.V. Ettinger and G. Buchan, in: Radiation Biology and Chemistry, Vol. 6 in the series: Studies in physical and theoretical chemistry, ed. H.E. Edwards et al. (Elsevier, Amsterdam, 1979). [21 Yu.A. Vladimirov, Ultraweak luminescence during biochemical reactions (in Russian) (Nauka, Moscow, 1966) Ch. 1 p. 26. [3] D.I. Thwaites, G. Buchan, K.V. Ettinger, J.R. Mallard and A. Takavar, Int. J. Appl. Radial, Isotopes 27 (1976) 663. [4] K.V. Ettinger, J.R. Mallard, S. Srirath and A. Takavar, Phys. Med. Biol. 22 (1977) 481. [5] A. Takavar, PhD Thesis, Dept. of Medical Physics, University of Aberdeen, 1979. [6] K.V. Ettinger, A. Takavar and J.R. Mallard, in: Proc. Illrd Symp. Neutron Dosimetry in Biology and Medicine eds. G. Burger and H.G. Ebert (Neuherberg 1977) EUR 5848. [71 R.G. Fairchild, Phys. Med. Biol. 10 (1965) 491. [81 P. Laflin and B.J. Baugh, J. Chem. Soc. Chem. Commun. (1979) 239. [91 J.A. BurkiU, MSc Thesis. Dept. of Medical Physics, University of Aberdeen, 1977.

Table 1 Thermal and epithermal neutron response of lithium pyruvate (natural and enriched to 97% in 6Li)

Thermal flux : ~th = 3 X 101 o n/cm 2 s (1 MW power of Brookhaven Medical Research Reactor) Dose rate from 6Li(n,a)aT reaction: 5.69 kGy/min (enriched sample), 0.435 kGy/min (natural sample) Fast flux dose rate: 0.18 Gy/min Gamma ray dose rate: 0.12 Gy/min Ratio of 6 Li concentration in enriched and natural samples of lithium pyruvate: Observed ratio of lyoluminescent responses of enriched and natural samples:

13.2 10.5 -+ 0.13

Epithermal flux (cadmium filtered) Ratio of epithermal flux per energy decade to thermal flux ~0.02 Expected ratio of lyoluminescent responses calculated from cross section for 6Li(n,c03T reaction: epithermal beam response thermal beam response - 0.048 Observed lyoluminescent response ratio for enriched sample: Observed lyoluminescent response ratio for natural sample:

0.052 _+0.005 0.055 _+0.005

VIII. LYOLUMINESCENCE