The relationship between radiation-induced chemical processes and transverse relaxation times in polymer gel dosimeters

The effects of ionizing radiation in different compositions of polymer gel dosimeters are investigated using FT-Raman spectroscopy and NMR T-2 relaxation times. The dosimeters are manufactured from different concentrations of comonomers (acrylamide and N,N'-methylene-bis-acrylamide) dispersed in different concentrations of an aqueous gelatin matrix. Results are analysed using a model of fast exchange of magnetization between three proton pools. The fraction of protons in each pool is determined using the known chemical composition of the dosimeter and FT-Raman spectroscopy. Based on these results, the physical and chemical processes in interplay in the dosimeters are examined in view of their effect on the changes in T-2 The precipitation of growing macroradicals and the scavenging of free radicals by gelatin are used to explain the rate of polymerization. The model describes the changes in T-2 as a function of the absorbed dose up to 50 Gy for the different compositions. This is expected to aid the theoretical design of new, more efficient dosimeters, since it was demonstrated that the optimum dosimeter (i.e, with the lowest dose resolution) must have a range of relaxation times which match the range of T-2 values which can be determined with the lowest uncertainty using an MRI scanner.

Modelling of post-irradiation events in polymer gel dosimeters

The nuclear magnetic resonance (NMR) spin-spin relaxation time (T-2) is related to the radiation-dependent concentration of polymer formed in polymer gel dosimeters manufactured from monomers in an aqueous gelatin matrix. Changes in T-2 with time post-irradiation have been reported in the literature but their nature is not fully understood. We investigated those changes with time after irradiation using FT-Raman spectroscopy and the precise determination of T-2 at high magnetic field in a polymer gel dosimeter, A model of fast exchange of magnetization taking into account ongoing gelation and strengthening of the gelatin matrix as well as the polymerization of the monomers with time is presented. Published data on the changes of T-2 in gelatin gels as a function of post-manufacture time are used and fitted closely by the model presented. The same set of parameters characterizing the variations of T-2 in gelatin gels and the increasing concentration of polymer determined from Fr-Raman spectroscopy are used successfully in the modelling of irradiated polymer gel dosimeters. Minimal variations in T-2 in an irradiated PAG dosimeter are observed after 13 h.

Ultrasound evaluation of polymer gel dosimeters

A new method for the evaluation of radiotherapy 3D polymer gel dosimeters has been developed using ultrasound to assess the significant structural changes that occur following irradiation of the dosimeters. The ultrasonic parameters of acoustic speed of propagation, attenuation and transmitted signal intensity were measured as a function of absorbed radiation dose. The dose sensitivities for each parameter were determined as 1.8 x 10(-4) s m(-1) Gy(-1), 3.9 dB m(-1) Gy(-1) and 3.2 V-1 Gy(-1) respectively. All parameters displayed a strong variation with absorbed dose that continued beyond absorbed doses of 15 Gy. The ultrasonic measurements demonstrated a significantly larger dynamic range in dose response curves than that achieved with previously published magnetic resonance imaging (MRI) dose response data. It is concluded that ultrasound shows great potential as a technique for the evaluation of polymer gel dosimeters.

Investigation of ultrasonic properties of PAG and MAGIC polymer gel dosimeters

Ultrasonic speed of propagation and attenuation were investigated as a function of absorbed radiation dose in PAG and MAGIC polymer gel dosimeters. Both PAG and MAGIC gel dosimeters displayed a dependence of ultrasonic parameters on absorbed dose with attenuation displaying significant changes in the dose range investigated. The ultrasonic attenuation dose sensitivity at 4 MHz in MAGIC gels was determined to be 4.7 +/- 0.3 dB m(-1) Gy(-1) and for PAG 3.9 +/- 0.3 dB m(-1) Gy(-1). Ultrasonic speed dose sensitivities were 0.178 +/- 0.006 m s(-1) Gy(-1) for MAGIC gel and -0.44 +/- 0.02 m s(-1) Gy(-1) for PAG. Density and compressional elastic modulus were investigated to explain the different sensitivities of ultrasonic speed to radiation for PAG and MAGIC gels. The different sensitivities were found to be due to differences in the compressional elastic modulus as a function of dose for the two formulations. To understand the physical phenomena underlying the increase in ultrasonic attenuation with dose, the viscoelastic properties of the gels were studied. Results suggest that at ultrasonic frequencies, attenuation in polymer gel dosimeters is primarily due to volume viscosity. It is concluded that ultrasonic attenuation significantly increases with absorbed dose. Also, the ultrasonic speed in polymer gel dosimeters is affected by changes in dosimeter elastic modulus that are likely to be a result of polymerization. It is suggested that ultrasound is a sufficiently sensitive technique for polymer gel dosimetry.

Ultrasonic absorption in polymer gel dosimeters

Ultrasonic absorption in polymer gel dosimeters was investigated. An ultrasonic interferometer was used to study the frequency (f) dependence of the absorption coefficient (alpha) in a polyacrylamide gel dosimeter (PAG) in the frequency range 5-20 MHz. The frequency dependence of ultrasonic absorption deviated from that of an ideal viscous fluid. The presence of relaxation mechanisms was evidenced by the frequency dependence of alpha/f(2) and the dispersion in ultrasonic velocity. It was concluded that absorption in polymer gel dosimeters is due to a number of relaxation processes which may include polymer-solvent interactions as well as relaxation due to motion of polymer side groups. The dependence of ultrasonic absorption on absorbed dose and formulation was also investigated in polymer gel dosimeters as a function of pH and chemical composition. Changes in dosimeter pH and chemical composition resulted in a variation in ultrasonic dose response curves. The observed dependence on pH was considered to be due to pH induced modifications in the radiation yield while changes in chemical composition resulted in differences in polymerisation kinetics. (C) 2003 Elsevier B.V. All rights reserved.

NMR study of the radiation-induced cross-linking of poly(tetrafluoroethylene-co-perfluoromethyl vinyl ether)

The gamma-radiolysis of poly(tetrafluoroethylene-co-perfuoromethyl vinyl ether) (TFE/PMVE) was investigated using solid state F-19 and C-13 NMR spectroscopy. Chain scission products identified in the polymer were saturated chain ends -CF2CF3 (G = 1.0), methyl ether end groups -CF2OCF3 (G = 0.9), acid end groups -CF2COOH (G = 0.5), and a small amount of terminal unsaturation -CF=CF2 (G = 0.2). A mechanism for the formation of these scission products was proposed and the G value for main chain scission, G(S), was determined to be 1.4. Cross-linking of TFE/PMVE was found to proceed via a Y-linking mechanism. The G value for cross-linking, G(X), was determined to be 0.9. A maximum of 0.2 mol % cross-links were formed under the experimental conditions.

Radiation use efficiency increases when the diffuse component of incident radiation is enhanced under shade

Theoretical analyses have shown the radiation use efficiency of maize, soybean, and peanut to increase with a decrease in the level of incident radiation and an increase in the proportion of diffuse radiation. This study compared the growth and radiation use efficiency of Panicum maximum cv. Petrie (green panic) and Bothriochloa insculpta cv. Bisset (creeping bluegrass) beneath shading treatments (birdguard and solarweave shadecloths) with that in full sunlight. A level of incident radiation reduced by 25% under birdguard shadecloth decreased final yield and final leaf area index, but increased canopy leaf nitrogen concentration and radiation use efficiency (19-14%) (compared with the full sun treatment). A similar level of reduced incident radiation under solarweave shadecloth (which provided an increased proportion of diffuse radiation), increased final yield and radiation use efficiency (46-50%). An understanding of the effects of composition of incident radiation on radiation use efficiency of tropical grasses enables more accurate estimation of potential pasture growth in shaded environments. It also has impact upon crop production in glasshouses and greenhouses.

Thermal and mechanical properties of radiation crosslinked poly(tetrafluoroethylene-co-perfluoromethyl vinyl ether)

The gamma-radiolysis of poly(tetrafluoroethylene-co-perfluoromethyl vinyl ether) (TFE/PMVE) was investigated using chemical and mechanical analyses. The polymer was found to form an insoluble network with a dose of gelation of 15.8 kGy. Tensile and glass transition temperature measurements indicated the predominance of crosslinking, with optimal elastomeric properties reached in the dose range of 120 to 200 kGy. Photoacoustic FTIR spectroscopy CPAS) showed the formation of new carboxylic acid end groups on irradiation. These new end groups were shown to decrease the thermal oxidative stability of the crosslinked network as determined by thermal gravimetric analysis. Electron spin resonance (ESR) studies of the polymer at 77 K indicated the presence of radical precursors. A G-value of 1.1 was determined for radical production at 77 K. Comparison of radical concentrations for a copolymer with a different mole ratio of PMVE, indicated that the PMVE units contribute to scission reactions. (C) 1998 Elsevier Science Ltd. All rights reserved.

A comparative study of the effects of UV- and gamma-radiation on copolymers of acrylonitrile/butadiene

The radiolysis of nitrile rubbers with different acrylonitrile/butadiene composition and the homopolymers, poly(butadiene) (PBD) and poly(acrylonitrile) (PAN) has been investigated and compared with the photolysis of the same polymers. A significantly different mechanism of degradation was found for the two types of radiation. The results obtained by ESR, FTIR and measurements of soluble fractions of irradiated samples, indicated that the acrylonitrile units of the nitrile rubbers are more sensitive units to gamma-radiation, with the effects of irradiation increasing with the acrylonitrile content. The reactions observed were consumption of double bonds, crosslinking, and cyclization with the formation of conjugated double bonds. No chain-scission reactions were detected. In contrast to gamma-irradiation, the effects of photolysis were centred at the butadiene units, and increases in the acrylonitrile content resulted in a proportional decrease in the sensitivity of the copolymers. Crosslinking and chain scission were identified as the main effects of photolysis of NBR rubbers. (C) 1999 Society of Chemical Industry.

Thermal and radiation curing of phenylethynyl terminated macromers

The thermal and gamma-irradiation induced curing of two phenylethynyl terminated composite resin systems, DFB/BPF and PETI5A, was investigated. Thermal curing of these matrix resin samples was performed at a temperature of 360 degrees C, gamma irradiation of the samples was conducted at 300 degrees C at a dose rate of 2.2 kGy h(-1). The reaction and subsequent loss of ethynyl groups in the resins for both cure methods was demonstrated by observing the decrease of the 2215 cm(-1) peak in the Raman spectra of the resins. Fully cured resin samples were found to have glass transition temperatures of 244-246 degrees C and 278-280 degrees C for DFB/BPF and PETI5A respectively. Similar relationships between T-g and fractional conversion were observed in both resins. The apparent polymerization rate, R-p, for thermal cure at 360 degrees C, was found to be 4.79 x 10(-2)% s(-1) in PETI5A and 3.22 x 10(-2)% s(-1) in DFB/BPF. Catastrophic degradation under nitrogen was observed to commence near 450 degrees C and 530 degrees C, with 5% weight losses occurring at 455 degrees C and 540 degrees C for DFB/BPF and PETI5A respectively. Gamma radiation induced cure at 300 degrees C was shown to be feasible, with full cure being reached with doses of 40 kGy for DFB/BPF and 100 kGy for PETI5A.

The radiation chemistry of poly(dimethylsiloxane)

The radiation chemistry of poly(dimethyl siloxane) has been investigated with respect to identification of the nature of the small molecule chain scission products. Low molecular weight linear and cyclic products have been identified through the use of Si-29 solution NMR, GPC and MALDI-TOF mass spectrometry. It has been suggested that the low molecular weight cyclic products are formed by back-biting depolymerization reactions.

C-13-NMR, H-1-NMR, and FT-Raman study of radiation-induced modifications in radiation dosimetry polymer gels

H-1- and C-13-NMR spectroscopy and FT-Raman spectroscopy are used to investigate the properties of a polymer gel dosimeter post-irradiation. The polymer gel (PACT) is composed of acrylamide, N,N'-methylene-bisacrylamide, gelatin, and water. The formation of a polyacrylamide network within the gelatin matrix follows a dose dependence nonlinearly correlated to the disappearance of the double bonds from the dissolved monomers within the absorbed dose range of 0-50 Gy. The signal from the gelatin remains constant with irradiation. We show that the NMR spin-spin relaxation times (T-2) of PAGs irradiated to up to 50 Gy measured in a NMR spectrometer and a clinical magnetic resonance imaging scanner can be modeled using the spectroscopic intensity of the growing polymer network. More specifically, we show that the nonlinear T-2 dependence against dose can be understood in terms of the fraction of protons in three different proton pools. (C) 2000 John Wiley & Sons, Inc.

Magnetization transfer imaging for polymer gel dosimetry

Off-resonance RF pre-saturation was used to obtain contrast in MRI images of polymer gel dosimeters irradiated to doses up to 50 Gy. Two different polymer gel dosimeters composed of 2-hydroxyethyl-acryl ate or methacrylic acid monomers mixed with N, N'-methylene-bisacrylamide (BIS), dispersed in an aqueous gelatin matrix were evaluated. Radiation-induced polymerization of the co-monomers generates a fast-relaxing insoluble polymer. Saturation of the polymer using off-resonance Gaussian RF pulses prior to a spin-echo read-out with a short echo time leads to contrast that is dependent on the absorbed dose. This contrast is attributed to magnetization transfer (MT) between free water and the polymer, and direct saturation of water was found to be negligible under the prevailing experimental conditions. The usefulness of MT imaging was assessed by computing the dose resolution obtained with this technique. We found a low value of dose resolution over a wide range of doses could be obtained with a single experiment. This is an advantage over multiple spin echo (MSE) experiments using a single echo spacing where an optimal dose resolution is achieved over only very limited ranges of doses. The results suggest MT imaging protocols may be developed into a useful tool for polymer gel dosimetry.

Resistance to UVB radiation in Antarctic yeasts

Yeast cells were used as a model system to study the inter-relationship among free radicals, antioxidants and UV-induced cell damage. In particular, the effects of UV-radiation in newly isolated yeasts from the Antarctic have been studied.