Electron Paramagnetic Resonance Dosimeter Research Articles

A new EPR dosimeter based on 2:1 mixture of Li 2C 2O 4:Na 2C 2O 4 for gamma and neutron dosimetry

Electron paramagnetic resonance (EPR) investigations of Li 2C 2O 4, Na 2C 2O 4 mixed in proportion 1:0, 0:1, 1:1, 2:1 and 3:1 were carried out to measure the absorbed dose from photons and thermal neutrons in a mixed radiation field. A single line spectrum of CO 2 − radical anion centered around g=2.0045±0.0005 was obtained in the respective cases on gamma and neutron irradiation. Except Na 2C 2O 4, other mixtures had shown increase in line width on neutron irradiation possibly due to relaxation effects. Of all combinations, the 2:1 mixture is the more sensitive material for gamma and thermal neutrons. Intensity of CO 2 − radical signal in 2:1 Li 2C 2O 4:Na 2C 2O 4 mixture was found to be linear from 0.006 to 11 kGy for gamma and 40–1530 kGy for thermal neutron doses. Radical signal was found to be stable over a period of 300 days with marginal fading of less than 1 percent. Experimental results thus obtained suggest 2:1 Li 2C 2O 4:Na 2C 2O 4 mixture as the potential neutron dosimeter for medium and high dose range.

Dosimetry of stereotactic radiosurgery using lithium formate EPR dosimeters

Small lithium formate EPR (electron paramagnetic resonance) dosimeters (diameter 3 mm, height 2 mm) were produced and employed for 2D dosimetry of stereotactic radiosurgery (SRS). An anthropomorphic head phantom with an in-house made insert holding 45 lithium formate dosimeters was used. A spherical target was outlined centrally in planning CT images of the head and an SRS dose plan with three arcs was made using the iPlan planning system. Beam collimation was achieved with the BrainLAB m3 micro-MLC. The minimum target dose was 15 Gy. The planned dose distribution was compared to measurements. For dosimetry, a dosimeter calibration series was generated with doses from 1 to 20 Gy. At the treatment unit, three replicate measurement series were performed. The measurements gave on average 2.2% lower dose at the plateau of the dose distribution compared to the dose plan. Larger differences were seen in the penumbra, where the dose plan underestimated the dose gradients. By repeated measurements, the systematic and random error in the SRS delivery was estimated to less than 1 mm. In conclusion, the planning system produced an intracranial dose distribution with tolerable accuracy. Furthermore, small lithium formate EPR dosimeters were useful for measuring SRS dose distributions.

EPR EMERGENCY DOSIMETRY WITH PLASTIC COMPONENTS OF PERSONAL GOODS

Radiation-induced electron paramagnetic resonance (EPR) signals were studied in samples of plastic materials of various origin: buttons, details of underwear, elements of mobile phones, etc. The following parameters were investigated: dose response curve in the range 0-25 Gy; stability of potential dosimetric signals at different temperatures of storage after exposure; and influence of solar radiation on the dosimetric properties of materials. Plastics from personal goods were found to be a potentially acceptable material for use as individual EPR dosimeters with sensitivity threshold below 5 Gy. Radiation-induced EPR signals in plastic demonstrated clear saturation for doses above 10 Gy. Fading of dosimetric signals was best described by the two-exponential decay function with fast and slow decay components. Values of slow decay constant were approximately 2 and 15 d, while the corresponding values for the fast decay component were approximately 2 and 15 h for temperatures of +25 degrees C and -18 degrees C, respectively. Strong influence of solar light radiation on EPR spectra was observed for unexposed and gamma-irradiated plastic samples, which may affect drastically the results of dose reconstruction.

Monte Carlo simulation of the response of ESR dosimeters added with gadolinium exposed to thermal, epithermal and fast neutrons

Monte Carlo numerical calculations of the response of alanine and ammonium tartrate ESR (electron spin resonance) dosimeters exposed to neutron fields with different energy spectra are reported. Results have been obtained for various gadolinium concentrations inside the dosimeters. Furthermore, in order to simulate the in-phantom response we have carried out calculations by varying the depth of the dosimeter. We have found that a large enhancement is obtained for thermal neutrons, because of the very high capture cross section of gadolinium to thermal neutrons. A good enhancement was obtained for epithermal neutrons, whereas the sensitivity improvement in the case of fast neutron irradiation is poor. Furthermore, the simulations carried out by varying the depth suggests that an appreciable sensitivity to thermal and epithermal neutrons could be observed for in-phantom measurements in the 2–3 cm depth range. These results can provide useful insight for future experiments with epithermal neutron beams (such as those used in neutron capture therapy) and for future applications in neutron capture therapy dosimetry.

Potential use of wallboard (drywall) for EPR retrospective dosimetry

Concern regarding the possibility of criminal or terrorist use of nuclear materials has led to an interest in developing the capability to measure radiation dose in a variety of natural and manufactured materials. Electron paramagnetic resonance (EPR) measurements of radiation dose following a radiological incident may aid in screening affected populations (triage) and in reconstruction of doses following accidents. One such EPR dosimeter is wallboard (drywall), a common construction material composed largely of gypsum (calcium sulphate dihydrate). We have identified the CO 3 − and SO 3 − dose-sensitive lines in drywall and developed a measurement protocol using the intensity of CO 3 − line. Proper background subtraction is a major difficulty, and we demonstrate a procedure based on alignment of a contaminant Mn 2+ line. As a proof-of-concept, a wallboard panel was irradiated with a 60Co source, and a two-dimensional map of the absorbed dose was measured. While most aliquots yielded reasonably accurate doses, a spatially contiguous region of apparent dose-insensitivity in one panel was identified.

An international intercomparison on “self-calibrated” alanine EPR dosimeters

The results, obtained by six independent electron paramagnetic resonance (EPR) laboratories, of the dose response coefficients ( K dr) of “self-calibrated” solid-state EPR dosimeters containing alanine as a radiation-sensitive material and Mn 2+/MgO as an internal reference material, are reported. The intercomparison trial was divided into three steps. It started with the distribution of dosimeters among the participating EPR laboratories with the purpose of irradiating them with known doses of γ-rays and to estimate the K dr. The percentage standard deviation (PSD) of the K dr obtained at individual labs was in the range of 1.4–4.6%. The interlaboratory PSD of the K dr was 8%, primarily pointing to variations in irradiation procedures and EPR spectrometer settings. Further investigations showed that the main source of the interlaboratory PSD is differences in the calibrations of irradiators and settings of EPR acquisition parameters. In order to provide reproducible estimates of the K dr, low microwave power and modulation amplitude using a combination of sweep time and time constant that gives a distortion-free EPR spectrum should be utilized. In the third step following such a procedure, measuring the same irradiated alanine dosimeter at the respective laboratories, spectrometers (12 instruments of 6 different models and 3 producers) and 10 operators gave an interlaboratory PSD of 3.1%. In conclusion, EPR dosimetry using “self-calibrated” alanine dosimeters may be used as a secondary standard, although a careful calibration of the EPR spectrometer must be performed in order to further reduce the uncertainty.

LET effects following neutron irradiation of lithium formate EPR dosimeters

Lithium formate electron paramagnetic resonance (EPR) dosimeters were irradiated using 60Co γ-rays or fast neutrons to doses ranging from 5 to 20 Gy and investigated by EPR spectroscopy. Using a polynomial fitting procedure in order to accurately analyze peak-to-peak line widths of first derivative EPR spectra, dosimeters irradiated with neutrons had on average 4.4 ± 0.9% broader EPR resonance lines than γ-irradiated dosimeters. The increase in line width was slightly asymmetrical. Computer simulated first derivative polycrystalline EPR spectra of a CO 2 − radical gave very good reconstructions of experimental spectra of irradiated dosimeters. The spectrum simulations could then be used as a tool to investigate the line broadening observed following neutron irradiation. It was shown that an increase in the simulated Lorentzian line width could explain both the observed line broadening and the asymmetrical effect. The ratio of the peak-to-peak amplitude of first derivative EPR spectra obtained at two different microwave powers (20 and 0.5 mW) was 7.8 ± 1.2% higher for dosimeters irradiated with neutrons. The dependence of the spectrum amplitude on the microwave power was extensively investigated by fitting observations to an analytical non-linear model incorporating, among others, the spin–lattice ( T 1) and spin–spin ( T 2) relaxation times as fitting parameters. Neutron irradiation resulted in a reduction in T 2 in comparison with γ-irradiation, while a smaller difference in T 1 was found. The effects observed indicate increased local radical density following irradiation using high linear energy transfer (LET) neutrons as compared to low LET γ-irradiation. A fingerprint of the LET may thus be found either by an analysis of the line width or of the dependence of the spectrum amplitude on the microwave power. Lithium formate is therefore a promising material for EPR dosimetry of high LET radiation.

Study on Influence of X-ray Baggage Scan on ESR Dosimetry for SNTS using Human Tooth Enamel

The influence of X-ray baggage scanning on electron spin resonance (ESR) dosimetry studies around the Semipalatinsk Nuclear Test Site (SNTS) has been examined at Incheon Airport in Korea, which is a transfer point of the routes from Kazakhstan to Japan. Utilized dosimeters are Japanese human tooth enamel for ESR and glass dosimeters. The difference between the estimated doses with the X-ray scan and those without it is below the evaluation errors for both ESR and the glass dosimeters. For glass dosimeters, the dose from the X-ray scan is estimated to be lower than the detection limit for the utilized glass dosimeters of ten microGy. This supports the absence of significant difference for the ESR results, which have an error in the order of ten mGy. Since ESR dosimetry for SNTS usually has similar errors, the dose by the X-ray scan in this study is concluded to be negligible in ESR dosimetry using tooth enamel from residents near SNTS.

ESR dosimetric properties of window glass

The general dosimetric characteristics of commercially window glass were investigated with electron spin resonance (ESR) technique. Irradiated window glasses exhibit a paramagnetic oxygen hole center with an ESR signal at g=2.0128. The study was carried out on g=2.0128 signal because of the accuracy of measurements and the possibility of using it as ESR dosimeter. The thermal stability and kinetic features of the ESR signal were studied by annealing the samples at predetermined temperature for predetermined time over the temperature range of 60–150°C. The response of the signal at g=2.0128 to 60Co γ-ray doses ranging from 5Gy to 20kGy has been studied. Dose–response was found to be appropriate for dosimetry in the range 5Gy–20kGy, taking into account its thermal fading. The other ESR dosimetric parameters of window glass samples, like light effect, dose-rate dependence, reproducibility of measurement, batch uniformity and photon energy dependence, have also been studied in detail. Apart from its non-tissue equivalence character, it showed very good dosimetric properties with visible light independency, good reproducibility, batch uniformity, wide dose–response range (5Gy–20kGy), dose-rate independency and independency from photon energy above 100keV. These features of window glass can open application fields for high-dose measurements and in accident dosimetry in near future.

Enhanced sensitivity of lithium dithionates doped with rhodium and nickel for EPR dosimetry

Electron paramagnetic resonance (EPR) studies of X-irradiated lithium dithionate, Li 2S 2O 6·2H 2O, doped with Ni and Rh have shown that these impurities enhance the yield of radicals formed by X-irradiation at room temperature. The signal in the doped samples, measured peak-to-peak of the single EPR derivative line attributed to the SO 3 − anion was about 3–4 times that of the pure lithium dithionate and more than 10 times stronger than the alanine signal. These impurities also shortened the spin-lattice relaxation time, T 1, which gives the possibility to measure the doped samples at a higher microwave power. This implies that sensitivity could be further enhanced in the already sensitive EPR dosimeter material lithium dithionate.

Electron paramagnetic resonance (EPR) dosimetry using lithium formate in radiotherapy: comparison with thermoluminescence (TL) dosimetry using lithium fluoride rods

Solid-state radiation dosimetry by electron paramagnetic resonance (EPR) spectroscopy and thermoluminescence (TL) was utilized for the determination of absorbed doses in the range of 0.5–2.5 Gy. The dosimeter materials used were lithium formate and lithium fluoride (TLD-100 rods) for EPR dosimetry and TL dosimetry, respectively. 60Co γ-rays and 4, 6, 10 and 15 MV x-rays were employed. The main objectives were to compare the variation in dosimeter reading of the respective dosimetry systems and to determine the photon energy dependence of the two dosimeter materials. The EPR dosimeter sensitivity was constant over the dose range in question, while the TL sensitivity increased by more than 5% from 0.5 to 2.5 Gy, thus displaying a supralinear dose response. The average relative standard deviation in the dosimeter reading per dose was 3.0% and 1.2% for the EPR and TL procedures, respectively. For EPR dosimeters, the relative standard deviation declined significantly from 4.3% to 1.1% over the dose range in question. The dose-to-water energy response for the megavoltage x-ray beams relative to 60Co γ-rays was in the range of 0.990–0.979 and 0.984–0.962 for lithium formate and lithium fluoride, respectively. The results show that EPR dosimetry with lithium formate provides dose estimates with a precision comparable to that of TL dosimetry (using lithium fluoride) for doses above 2 Gy, and that lithium formate is slightly less dependent on megavoltage photon beam energy than lithium fluoride.

Possibility of precipitated CaCO 3 with vitamin C as a new dosimetric material

Precipitated CaCO 3 in the presence of vitamin C has been investigated as an ESR dosimeter. γ-ray irradiation produced a sharp electron spin resonance (ESR) signal with a linewidth (0.015 mT) with high thermal stability. Required microwave power (0.02 mW) and magnetic field modulation (0.02 mT) for ESR measurement were about 250 and 30 times lower than those of DL- α-alanine, respectively. The lower detection limit was reduced from ∼3 Gy for a commercial alanine dosimeter to ∼20 mGy for γ-rays for this material.

A new ESR dosimeter based on bioglass material

Bioglass (Bio-G) samples were irradiated with 60 Co γ -rays to study radicals for dosimetric materials with electron spin resonance (ESR). The ESR spectrum of Bio-G is characterized by two main signals. The first signal at g≅4.3 corresponds to Fe 3+ impurities and the second signal at g≅2.0130 with line-width 10.85 G is ascribed as a hole center. The γ-ray dose response and thermal stability were studied to establish the suitability of Bio-G as an ESR dosimeter. A radical formation efficiency, G-value, of 0.53±0.11 was obtained. The lifetime of radicals and the activation energy were estimated from Arrhenius plots to be approximately 255±46 days and 0.71 eV , respectively.

The use of ESR spectroscopy for the investigation of dosimetric properties of egg shells

Electron spin resonance (ESR) spectroscopy was used to investigate the dosimetric properties of chicken egg shells. The ESR spectra of the irradiated egg shell were found to have an asymmetric absorption characterized by a major resonance at g ⊥=2.0019 and a minor resonance at g ||=1.9980. The study was carried out on g ⊥=2.0019 signal because of the accuracy of measurements and the possibility of using it as ESR dosimeter. The activation energy ( E), frequency factor ( k 0) and mean-life ( τ) were calculated to be 1.50±0.10 eV, 2×10 13 s −1 and (4.4±0.4)×10 4 year respectively. Dose–response was investigated between dose ranges of 1 Gy and 10 kGy for 60Co γ-rays. Dose–response was found to be appropriate for dosimetry in the range 3 Gy to 10 kGy. The lower limit of observable doses for egg shell sample was about 3 Gy. The other ESR dosimetric parameters of egg shell samples, fading characteristic, light effect, dose-rate dependence and energy dependence, have also been studied in detail. Apart from its non-tissue equivalence, egg shell has very good dosimetric properties with insignificant fading, light independence, linearity in dose–response (3 Gy–10 kGy), dose-rate independence and independence from energy above 500 keV. It suggests that egg shell may be used as a retrospective γ radiation dosimetry after nuclear accidents or other short accidental radiation events.

Response characterization of ammonium tartrate solid state pellets forESR dosimetry with radiotherapeutic photon and electron beams

Solid state pellets (1 mm thick) for electron spin resonance (ESR) dosimetrywere made using ammonium tartrate as the radiation-sensitive substance. Theirbehaviour was experimentally investigated as a function of dose with 60Cogamma rays. The calibration function obtained permits measurements of absorbeddose in the 2-50 Gy range, with a combined uncertainty of ±4%. Thelowest detectable dose was about 0.5 Gy. These properties are comparable withor even better than those of ESR dosimeters made from other materials. Thetime stability of the ESR signal of ammonium tartrate dosimeters at differentstorage conditions after irradiation was studied. A rather complex behaviourwas observed, which suggests that more species of free radicals are producedby radiation and that migration processes may be effective. No dependence ofthe response on beam quality was found for high-energy photon and electronbeams produced by a linear accelerator used in radiotherapy, whereas dosewas underestimated with low-energy x-rays.

Radical Formation in Lithium and Magnesium Oxalate

Lithium oxalate (Li-oxalate: Li2C2O4) and magnesium oxalate (Mg-oxalate: MgC2O4·2H2O) were investigated by electron spin resonance (ESR) spectroscopy as new ESR dosimeter materials. The ESR spectra of Li- and Mg-oxalates irradiated by γ-rays have a singlet with a spectroscopic splitting factor (g-factor) of g=2.0043±0.0004 and are ascribed to a self-trapped hole, the oxalate radical C2O4-. A broad signal formed by high dose irradiation is considered to be due to the zero field fine structure splitting, DS2 (D/gβ\\cong0.65 mT) for the triplet state (S=1) of a dimer of C2O4- or a pair of electron and hole centers. The response to the γ-ray dose and thermal stability as well as the effect of illumination have been studied with respect to using these materials as ESR dosimeter elements. The radical formation efficiencies (G-value) for Li- and Mg-oxalates were 0.4±0.1 and 0.21±0.06 and the activation energies (E) from the Arrhenius plot were 1.16±0.24 eV and 1.28±0.26 eV, respectively. These lead to the respective lifetimes of 2.6±0.9 and 3.2±1.1 years at 25°C, which are sufficient for practical dosimetry.

A new ESR dosimeter based on polytetrafluoroethylene

A new electron spin resonance (ESR) dosimetry system based on the ESR response of polytetrafluoroethylene is presented. The dose response of these dosimeters irradiated with 60Co gamma radiation was linear in the range from 6×10 2 up to 1.5×10 5 Gy, having a repeatability of ±1% and a high degree of reliability ( C f =1.054 ). The persistence of the signal was monitored over a period of one month. ESR signal decay was 5–7% in the temperature range from room temperature up to 40°C.

Electron paramagnetic resonance dosimetry: Methodology and material characterization

Electron paramagnetic resonance(EPR) methodologies for radiationdosimetry are investigated using various dosimeter materials. Specifically, methodologies were developed and used which were intended to improve the accuracy and precision of EPR dosimetric techniques, including combining specimen rotation during measurement, use of an internal manganese standard, instrument stabilization techniques, and strict measurement protocols. Characterization and quantification of these improvements were performed on three specific EPR dosimeter materials. The dosimeter materials investigated were walrus teeth, human tooth enamel, and alanine dosimeters. Walrus teeth showed the least desirable properties for EPRdosimetry yielding large native signals and low radiation sensitivity. The methods for tooth enamel and alanine resulted in large improvements in precision and accuracy. The minimum detectable dose (MDD) found for alanine was approximately 30 mGy (three standard deviations from the measured zero dose value). This is a sensitivity improvement of 5 to 10 over other specialized techniques published in the literature that offer MDDs in the range of 150–300 mGy. The accuracy of the method on tooth-enamel was comparable to that typically reported in the literature although the measurement precision was increased by about 7. This improvement in measurement precision enabled a more nondestructive testing evaluation procedure to be developed (where the whole sample need not be additively irradiated in order to calibrate its radiation sensitivity). Using this virtually nondestructive evaluation procedure on numerous samples showed that the method could reconstruct the same doses to within 10 mGy of those evaluated destructively. Doses used for this assessment were in the range of 100–250 mGy.

Radiation-induced defects in magnesium lactate as ESR dosimeter

Magnesium lactate (Mg–lactate: (CH 3CH(OH)COO) 2Mg), magnesium lactate doped with lithium lactate (Mg(Li)–lactate) and nominal pure lithium lactate (CH 3CH(OH)COOLi) doped with Mg–lactate (Li(Mg)–lactate) were irradiated by γ-rays to study radicals for materials of radiation dosimeter with electron spin resonance (ESR). Quartet spectra were ascribed to lactate radicals in Mg–lactate and Li(Mg)–lactate with the spectroscopic splitting factors ( g-factor) of 2.0032±0.004 and 2.0029±0.004 and the intensity ratio of 1:3:3:1 due to the hyperfine coupling constants of ( A/ gβ) of 1.92±0.06 and 1.82±0.06 mT, respectively. The response to γ-ray dose and the thermal stability as well as the effect of UV-illumination have been studied to establish this material as an ESR dosimeter. The number of free radicals per 100 eV (G-value) was obtained to be 1.15±0.32, 1.35±0.35, 0.46±0.14 and 0.78±0.24 for Mg–lactate, Mg(Li)–lactate, Li–lactate and Lie(Mg)–lactate, respectively. Thermoluminescence (TL) curves gave activation energies ( E) of 0.75, 0.79 and 0.77 eV and frequency factors s (1/ ν 0) of 5×10 7, 4.5×10 7 and 8×10 7 for Mg–lactate, Mg(Li)–lactate and Li(Mg)–lactate, respectively. The lifetimes of radicals in both Mg–lactate and Mg(Li)–lactate were estimated from Arrhenius plots to be approximately 50.7±20 years, while they were 4.9±2.5 and 10±3.5 years for those of Li–lactate and Li(Mg)–lactate, respectively.

Lithium lactate as an ESR dosimeter

The feasibility of lithium lactate (Li-lactate: CH 3CH(OH)COOLi) as an electron spin resonance (ESR) dosimeter material has been studied and compared with lactose and a conventional alanine dosimeter. The ESR spectra of Li-lactate and lactose with the spectroscopic splitting g-factors of 2.0029 ± 0.0003 and 2.0043 ± 0.0003 respectively show hyperfine splitting with an intensity ratio of 1:3:3:1 and hyperfine coupling constants ( A/ gβ) of 1.82 ± 0.06 and 1.88 ± 0.06 mT, respectively. The radical formation efficiency ( G-value), i.e. the number of free radicals per 100 eV for alanine, Li-lactate and lactose were 0.95, 1.0 and 3.0, respectively. The sensitivity of Li-lactate to γ-rays is 10 times higher than that of alanine and 0.3 of lactose. The signal stability of Li-lactate was greater than that of lactose.