However, like most polymers, PMMA is permeable to air, other gases, and water vapour. It can, for example reversibly absorb or de-sorb water from the atmosphere over the range of 0 to 2% by weight of incorporated water.
When PMMA dosimeter material is irradiated, chemical changes take place that produce the familiar radiation coloration effects used to measure dose. The chemical reactions are somewhat sensitive to incorporated water, so to ensure that the dosimeters are not affected by changes in atmospheric humidity, water concentration is optimised at the manufacturing stage (Red4034), and the dosimeters are sealed in water impermeable pouches. Therefore, providing the packing material meets specifications, the pouch seals remain intact, and the dosimeters are irradiated in the sealed pouches, atmospheric humidity is not a problem. The sealed pouches also offer some protection against other possible environmental influences, for example intense light, or unusual atmospheres.
Setting aside the effects of humidity, atmosphere and light, as environmental effects that can in principle be avoided completely, there are two other known potential problem areas, namely, possible effects of temperature during and after irradiation, and dose rate. At this point, it should be noted that there are in fact six different kinds of PMMA dosimeter material in current use. Of these, the most widely used and the best researched is Harwell Red 4034. It should be stressed here that the other kinds of PMMA dosimeter can and do have quite different dose rate and temperature characteristics, and cannot be expected to perform similarly.
In the early studies of Red4034 at Wantage Research Laboratory (UKAEA ,1960s), no significant dose rate effects were found in the medium to high dose rate range (0.5 to 100 Gy/s) with cobalt-60 gamma and 4 MeV electron irradiation (1). Rates lower than 0.5 Gy/s were not at that time studied, nor were irradiation temperature effects, because practically all irradiations were carried out at normal ambient temperatures, 15 to 25 C. Since the 1960s, these dosimeters have gained almost World-wide acceptance, but as a consequence, that also means that they are routinely used in a very wide range of facilities with widely different operating characteristics, in particular, irradiation temperatures vary, and in some cobalt facilities can be much higher than 25 C., probably up to 60 C. Lower temperatures, - 20 C. or below are also now of interest.
In 1975 Chu and Antoniades reported a low dose-rate effect in Red4034, in which the radiation response of the material increased by a few percent at low rates (2 ) . in the same year Miller and Mc. Laughlin reported a study of radiation response from - 80 to + 80 C. essentially finding little effect from - 30 to +40 C, but significant effects outside these limits (3). Following-up these papers, and other similar independent reports published over the period 1975-81, several detailed studies of rate and temperature effects were carried out at Harwell, and results were published first in 1985 (4), then most recently in 1993 (5 ). To date, at Harwell, five different batches of dosimeters have been studied, at cobalt-60 gamma dose rates ranging from 0.14 to 6.5 Gy/s, an EB dose rate of 100 Gy/s (mean rate), and temperatures from 20 to 50 C.
To summarise this Harwell work, low dose-rate effects were confirmed, at rates below 0.5 Gy/s; significant temperature effects were observed above 40 C., especially at low rates; some batches performed better than others, and rate and temperature effects appeared to be linked, i.e. temperature effects were most significant at low rates. The length of time that a dosimeter is held at a given temperature to absorb a given dose obviously must increase with decreasing dose rate, and this could be the reason for the apparent link between temperature and rate effects.
To conclude, in the Harwell studies Red4034 performed quite well in the environmental effects sense at rates above 0.5 Gy/s, and temperatures in the range of 20 to 40 C.
Clearly, when Red4034 dosimeters are used at lower rates or higher temperatures, or especially at combined low rates plus higher temperatures, they require calibration for actual conditions of use. This can best be accomplished in the irradiation facility, using reference dosimeters to measure the calibration doses given. As temperature and rate effects appear to be linked, other methods, such as the use of 'correction factors' may not be generally reliable. Conection factors, based on in-plant comparisons of Red4034 and reference dosimeters may of course be quite valid for a specific facility operating under specific temperature and rate conditions.
References
1. Whittaker, B., UKAEA Report AERE- R 3360, (1964 ).
2. Chu, R. D. and Antoniades, M. T., IAEA Report SM-192/ 14, (1975).
3. Miller, A., Bjergbakke, E. and Mc. Laughlin. W. L., Int. J. Appl. Radiat. Isot. 26, p 611, (1975 ).
4. Whittaker, B., Watts, M. F., Mellor, S. and Heneghan, M., IAEA Report SM- 272/5, (1985).
5. Glover, K. M. G., Plested . M. E., Watts, M. F. and Whittaker . B., Radiat. Phys. Chem. 42, p 739, (1993).
About the Author
Brian is a higher graduate of London University (a Ph.D.) and a graduate member of the British Royal Society of Chemistry. During 38 years of service in the U.K. Atomic Energy Authority (latterly AEA Technology), at Harwell, he published or co-authored many scientific papers on subjects as diverse as radiation dosimetry, photon-induced nuclear reactions, lasers, the preparation and chemistry of protactinium metal, solid-states spectra of actinide complexes, the neutron absorption cross-sections of Americium isotopes, and the formation and radiochemical purification of exotic cyclotron produced neutron-deficient actinide isotopes such as Neptunium-235.
He is named in U.K. patents related to low work-function electron-emitting filaments, high-dose dosimeters based on polymethylmethacrylate, and a unique method for manufacturing Neptunium-237 fission foils.
For several years he has been involved in the preparation of ASTM standards relating to high dose dosimetry, and continues to work in that field. In 1994 he decided to seek early retirement from AEA-Technology, and he now operates as a free-lance consultant.
He can be reached via e-mail at BW_Tech@Compuserve.com.
MCSaylor
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