Electron Paramagnetic Resonance Cavity Research Articles

Design Considerations of a Dual Mode X-Band EPR Resonator for Rapid In-Situ Microwave Heating

This paper describes the design considerations for a dual mode X-band continuous wave (CW) Electron Paramagnetic Resonance (EPR) cavity, for simultaneous EPR measurement and microwave heating of the same sample. An elliptical cavity geometry is chosen to split the degeneracy of the TM110 mode, allowing for a well resolved EPR signal with the TM110,a and TM110,b modes resonating at around 10 GHz and 9.5 GHz, respectively, the latter of which is used for EPR measurements. This geometry has the benefit that the TM010 mode used for microwave heating resonates at 6.1 GHz, below the cut off frequency of the X-band waveguide used for the EPR channel, providing effective isolation between the heating and EPR channels. The use of a pair of 9 µm thick copper clad laminates as the flat cavity walls allows for sufficient penetration of the modulation field (Bmod) into the cavity, as well as maintaining a high cavity Q factor (> 5700) for sensitive EPR measurements. Locating the heating port at an angle of 135° to the EPR port provides additional space for easier coupling adjustment and for larger sample access to be accommodated. The associated decrease of EPR signal strength is fully compensated for by using a 7.2 GHz low pass filter on the heating port. EPR spectra using 1.6 mm and 4.0 mm sample tubes are shown at room temperature (298 K) and 318 K for a standard Cu(acac)2 solution, demonstrating the effectiveness of this dual-mode EPR cavity for microwave heating during EPR detection.

Hydride Conformers of the Nitrogenase FeMo-cofactor Two-Electron Reduced State E2(2H), Assigned Using Cryogenic Intra Electron Paramagnetic Resonance Cavity Photolysis.

Early studies in which nitrogenase was freeze-trapped during enzymatic turnover revealed the presence of high-spin (S = 3/2) electron paramagnetic resonance (EPR) signals from the active-site FeMo-cofactor (FeMo-co) in electron-reduced intermediates of the MoFe protein. Historically denoted as 1b and 1c, each of the signals is describable as a fictitious spin system, S′ = 1/2, with anisotropic g′ tensor, 1b with g′ = [4.21, 3.76, ?] and 1c with g′ = [4.69, ∼3.20, ?]. A clear discrepancy between the magnetic properties of 1b and 1c and the kinetic analysis of their appearance during pre-steady-state turnover left their identities in doubt, however. We subsequently associated 1b with the state having accumulated 2[e–/H+], denoted as E2(2H), and suggested that the reducing equivalents are stored on the catalytic FeMo-co cluster as an iron hydride, likely an [Fe–H–Fe] hydride bridge. Intra-EPR cavity photolysis (450 nm; temperature-independent from 4 to 12 K) of the E2(2H)/1b state now corroborates the identification of this state as storing two reducing equivalents as a hydride. Photolysis converts E2(2H)/1b to a state with the same EPR spectrum, and thus the same cofactor structure as pre-steady-state turnover 1c, but with a different active-site environment. Upon annealing of the photogenerated state at temperature T = 145 K, it relaxes back to E2(2H)/1b. This implies that the 1c signal comes from an E2(2H) hydride isomer of E2(2H)/1b that stores its two reducing equivalents either as a hydride bridge between a different pair of iron atoms or an Fe–H terminal hydride.

Pseudo flow-injection ESR technique combining spin-trapping and application to the evaluation of superoxide scavenging capacity of phenolic compound

Evaluation methods of superoxide scavenging capacity have been studied by many researchers. One of the methods is a spintrapping electron spin resonance (ESR) technique, in which 5,5-diemthyl-1-pyrroline 1-oxide (DMPO) was usually used as a spintrapping reagent. In previous reports, the evaluation method based on a competition reaction and spin-trapping ESR technique were proposed. However, a big key issue and unavoidable problem remained in this evaluation. This is the rapid decrease of DMPO-O2 adduct by the following chemical reactions. Therefore, the initial signal intensity of DMPO-O2 has to be estimated with an approximation from a decreasing curve. This approximation, however, includes at least a thirty-second time lag before starting an ESR measurement. So, the measurement of a shorter time lag is required to obtain a more accurate initial intensity of DMPO-O2 - adduct. In this study, the pseudo flow-injection ESR method using a peristaltic pump was adopted to inject the test solution directly into a quartz cell in ESR cavity. This method achieved a 10 times shorter time lag (ca. 3 seconds) compared with the previous method (ca. 30 seconds) using a hematocrit capillary. As a consequence, the initial signal intensity within 10 % error was obtained without any estimation. The second-order rate constants of the reaction between various phenolic compounds and superoxide were obtained as one of the applications of this method.

Characterization of the TiO2/dye/electrolyte interfaces in dye-sensitized solar cells by means of a titania-binding nitroxide.

Dye-sensitized solar cells (DSSCs) have been characterized in several literature examples by using relatively complex methods and/or modified DSSC conditions with respect to the usual working ones. In this study, we propose a method for the investigation of the interfaces TiO2/dye/electrolyte in a DSSC at its usual working conditions. This method implies the use of a computer-aided analysis of the electron paramagnetic resonance (EPR) spectra of the spin probe 4-carboxy-2,2,6,6-tetramethylpiperidine 1-oxyl (4-carboxy-TEMPO, indicated as 4-cT). This probe well-mimics the dyes in their interactions with TiO2 surface, but does not perturb dye adsorption onto TiO2 surface, as verified by UV-vis measurements. First, we investigated the interacting ability toward 4-cT of commercially available TiO2 used for assembling the DSSC. It was found that interactions are modulated by the different distribution of interacting sites at the solid surface and powder aggregation. Further, experiments on 4-cT were carried out in the presence of a series of other molecules coded as N3, N719, and D149, which are commonly used as dyes in DSSCs. Then, the effect of solutions added to the electrodes was investigated. On the basis of the interactions occurring at the TiO2/dye/electrolyte interfaces, we selected the ingredients of the DSSCs. Electrical and EPR characterizations of these DSSCs miniaturized to enter the EPR cavity, together with time-dependent laser-light on-off experiments, were carried out, which demonstrated the ability of the EPR analysis to monitor the types and strengths of the interactions occurring at the cell's different interfaces. This method using the standard continuous wave EPR technique at room temperature may be profitably used to characterize the quality and performances of a DSSC.

Experimental setup for investigation of nanoclusters at cryogenic temperatures by electron spin resonance and optical spectroscopies.

We present the design and performance of an experimental setup for simultaneous electron spin resonance (ESR) and optical studies of nanoclusters with stabilized free radicals at cryogenic temperatures. A gas mixture of impurities and helium after passing through a RF discharge for dissociation of molecules is directed onto the surface of superfluid helium to form the nanoclusters of impurities. A specially designed ESR cavity operated in the TE011 mode allows optical access to the sample. The cavity is incorporated into a homemade insert which is placed inside a variable temperature insert of a Janis (4)He cryostat. The temperature range for sample investigation is 1.25-300 K. A Bruker EPR 300E and Andor 500i optical spectrograph incorporated with a Newton EMCCD camera are used for ESR and optical registration, respectively. The current experimental system makes it possible to study the ESR and optical spectra of impurity-helium condensates simultaneously. The setup allows a broad range of research at low temperatures including optically detected magnetic resonance, studies of chemical processes of the active species produced by photolysis in solid matrices, and investigations of nanoclusters produced by laser ablation in superfluid helium.

Electron Spin Resonance Imaging Probes and a Sample Holder for Thin and Long One-Dimensional Samples

Most electron spin resonance (ESR) experiments involve the use of samples that can be easily placed in millimeter-size tubes and measured efficiently in conventional resonators. However, in some cases, the samples must remain intact, due to which conventional commercial resonators may not be suitable to measure them. Here, we describe a set of three resonators, which can be combined and incorporated as part of a 1-D continuous wave ESR imaging probe to measure and image very thin (~50–500 μm) and very long (~10–30 mm) objects. The dielectric resonators we employ make it possible to greatly enhance spin sensitivity per unit of length––compared to the use of a rectangular ESR cavity, at ~9.3 GHz. In addition, a special sample holder was developed to facilitate the handling and measurement of such thin and long delicate objects, which in our case are the Arabidopsis roots. A detailed design of the resonators, imaging probe, and the sample holder is provided, along with experimental results for the resonator properties, its spin sensitivity, and imaging capability.

Development of X-Band TE111 Mode ESR Cavity for In Vivo Tooth Dosimetry

Electron spin resonance (ESR) tooth spectroscopy offers the possibility of in vivo dosimetry for radiation accident emergency. We developed an X-band ESR cavity for in vivo measurement of tooth enamel, which is a cylindrical cavity working at TE111 mode. A narrow rectangular aperture to fix incisors was opened on the cavity wall, where there were strong microwave magnetic field component H1 and weak microwave electronic field component E1 inside the cavity. The external modulated magnetic field was applied to the incisors from both sides of the aperture. Theoretical calculations and simulations were discussed to optimize the characteristics and predict the cavity working conditions and performances. The characteristics were evaluated by measuring DPPH and whole incisor samples. Remarkable radiation induced signal of 2Gy irradiated tooth could be obtained by one 5-min ESR spectroscopy. This result indicated that the cavity was sensitive enough to detect the radiation-induced signal in human teeth. This paper describes the operation principle, configuration, and characteristics, and reports the DPPH and tooth experiments to verify the cavity.

Temporal shift from magnetoplasma resonance to cyclotron resonance of photo-carriers generated from 1s-exciton in cuprous oxide crystal

We have established a method for time-resolved cyclotron resonance measurements using an electron spin resonance cavity with a temporal resolution 15 ns under photo-excitation in an undoped semiconductor, cuprous oxide. The observed photo-carriers are generated via long-lived 1s-excitons excited at the phonon assisted absorption edge below the yellow gap. This work focuses on the fast response that is within 80 ns after photo-excitation of high-density excitons. A shift of the photo-carriers from a magnetoplasma resonance to a cyclotron resonance, previously known as a steady resonance in highly doped semiconductors, is observed for the first time in the temporal domain. We successfully explain the real and imaginary parts of the transient microwave absorption using spectral simulations, taking account of the depolarization factors in two different sample shapes and configurations in the cavity.

EPR Method for ex vivo Detection of Reactive Oxygen Species in Tissue

Measurement of free radicals, ex vivo, in biological tissue with continuous wave electron paramagnetic resonance (EPR) has been problematic. Here, a novel EPR spin trapping technique has been developed that places one or two large segments of porcine arterial tissue along with supernatant directly into a standard EPR spectrometer cavity at liquid nitrogen temperature. This approach is validated in porcine arterial tissues treated as controls in vivo. To investigate that this method has lower extraneous ROS generation than methods which slice tissue into thin slivers before assaying for reactive oxygen species (ROS), we take two sets of coronary arterial tissue from the same animal. In the first set, we excise one or two large segments of tissue, and in the second set we section the tissue into ten pieces. In both cases, tissue was harvested, placed in physiological saline solution with 190-mM PBN (N-tert-butyl-α-phenylnitrone) and incubated in the dark for 2 hours at physiological temperature while gently being stirred. Tissue and supernatant were then loaded into a syringe and frozen at minus eighty degrees Celsius until EPR analysis. Although absorption of microwaves by an aqueous sample typically at room temperature limits the size of the sample; this method can accommodate as large as 20-mm3 tissue volume into the EPR cavity at liquid nitrogen temperature. In our experiments the tissue volumes were in a range of values between 2 mm3 and 5 mm3,and the EPR spectra were normalized with respect to tissue volume.

EPR characterization of graphitized and activated micro- and meso-porous carbons

Porous carbons are widely used for adsorbing pollutants from gas or liquid phases, for chromatographic separation, and for catalysis. A computer aided electron paramagnetic resonance (EPR) analysis was performed on 11 commercially available carbons, at different activation and graphitization levels. A heating treatment of H 3PO 4-preactivated carbons dispersed into an inert glass fibres matrix was performed using a steam-N 2 flow directly in the EPR cavity, following the line-shape/line-width variations over temperature up to 673 K, at different heating speeds. The surface area and the pore size distribution (PSD) of the differently treated carbons were evaluated by nitrogen adsorption isotherms. Both PSD and transmission electron micrographs (TEM) indicated an heterogeneous porosities of some carbons. The variation of the EPR line shape as a function of the surface area allowed us to identify three main carbon structures at different porosities, classified as graphitized, activated and highly polar. In several cases, two or three structures were contemporaneously present and corresponded to different zones of heterogeneous carbons and to superimposing contributions in the EPR spectra. In these cases, the EPR analysis of the spectra allowed us to relatively quantify these three zones and to extrapolate the true area of each carbon region at different porosity.

Novel multisample dielectric resonators for electron paramagnetic resonance spectroscopy

We have developed and tested two types of novel dielectric resonators for simultaneous recording of electron paramagnetic resonance (EPR) spectra from two to four samples. The resonator of the first type contains two holes, and the other resonator contains four holes for introduction of the samples. Also, the resonator structure includes a pair of gradient coils. Dielectric resonators made of materials with high dielectric constant with low losses can be inserted into the standard EPR cavity or waveguide in the maximum microwave magnetic field. Gradient coils are located outside the cavity (or waveguide) so that their axes are parallel to the static magnetic field. Computer simulations were made to obtain microwave characteristics of the resonators such as resonant frequency, sizes, and distribution of the fields. Spacing of the point samples and optimum value of the magnetic-field gradient have been chosen correctly. The designed resonators can be applied in express analysis using EPR technique, for instance.

Electron Spin Resonance of Charge Carriers in Organic Field-Effect Devices

We have developed a field-induced electron spin resonance (ESR) technique to detect charge carriers with spins that are generated at the interface of organic field-effect devices. ESR signals have been successfully observed in the devices of high-mobility organic materials, such as regioregular poly(3-alkylthiophene) (RR-P3AT), in the case of polymer, and pentacene, in the case of small organic molecules. ESR signals of these devices provide valuable information concerning the spin–charge relationship, the spatial extent of charge carriers and molecular orientations at the device interfaces. Instead of Al2O3 gate insulators initially used in devices for ESR, more conventional SiO2 gate insulators on the relatively low-doped Si substrates have been introduced in obtaining ESR signals without lowering the Q-value of the ESR cavity. We present gate-bias dependence of ESR signals of RR-P3AT devices with different gate insulators, where the spins tend to saturate above the charge concentration of about 0.2%, regardless of gate insulating materials. This fact indicates that the charge carriers of the polymers change from polarons with spins to spinless bipolarons or polaron pairs at higher doping levels. Anisotropic ESR spectra of unpaired π-electrons in the devices with SiO2 gate insulators have revealed preferential orientation of the molecular plane, consistent with the formation of self-organized lamellar structures of RR-P3AT as in the case of Al2O3 gate insulators. Motional narrowing of ESR spectra has been observed, depending on the alkyl-chain lengths of RR-P3AT, which could be related to the reported alkyl-chain dependence of carrier mobility.

No. 5 Vol. 42, No. 3, pp. 231‐236, 2008 Establishment of ex vivo stratum corneum lipid ordering analysis by electron spin resonance

Electron spin resonance (ESR) spectra have been used for studying stratum corneum (SC) lipid ordering. Order parameter S obtained from ESR spectra is a good index for evaluating the fluidity of SC lipids in vitro. However, the method for evaluation of ex vivo SC lipids has not been established. A glass plate using a cyanoacrylate resin was used to strip the SC lipids. A single‐chain aliphatic spin probe, 5‐Doxylstearic acid (5‐DSA), was used. The SC samples were incubated in the 0.001% 5‐DSA in a 0.1% ethanolic aqueous solution for 60 min at 37°C. After the incubation, the excess spin probe was removed with distilled water, and then an ESR glass holder with the incubated SC sample was mounted in the ESR cavity. The newly designed ESR glass holder improved accuracy and reproducibility of sample preparations as well as ESR measurements. In conclusion, we have optimized the preparation method of ex vivo SC specimen and established accurate ex vivo ESR analyses of SC lipid ordering.Keywords: stratum corneum, lipid ordering, electron spin resonance, ESR, 5‐Doxylstearic acid, 5‐DSA, order parameter, SC, spin probe

Dietary flavonoids with a catechol structure increase α-tocopherol in rats and protect the vitamin from oxidation in vitro

To identify dietary phenolic compounds capable of improving vitamin E status, male Sprague-Dawley rats were fed for 4 weeks either a basal diet (control) with 2 g/kg cholesterol and an adequate content of vitamin E or the basal diet fortified with quercetin (Q), (-)-epicatechin (EC), or (+)-catechin (C) at concentrations of 2 g/kg. All three catechol derivatives substantially increased concentrations of alpha-tocopherol (alpha-T) in blood plasma and liver. To study potential mechanisms underlying the observed increase of alpha-T, the capacities of the flavonoids to i) protect alpha-T from oxidation in LDL exposed to peroxyl radicals, ii) reduce alpha-tocopheroxyl radicals (alpha-T (.) ) in SDS micelles, and iii) inhibit the metabolism of tocopherols in HepG2 cells were determined. All flavonoids protected alpha-T from oxidation in human LDL ex vivo and dose-dependently reduced the concentrations of alpha-T (.) . None of the test compounds affected vitamin E metabolism in the hepatocyte cultures. In conclusion, fortification of the diet of Sprague-Dawley rats with Q, EC, or C considerably improved their vitamin E status. The underlying mechanism does not appear to involve vitamin E metabolism but may involve direct quenching of free radicals or reduction of the alpha-T (.) by the flavonoids.

Analysis of the longitudinal, “sloping plateau” effect of a planar sample in a single TE 102 rectangular electron paramagnetic resonance cavity

The response of the cavity to the movement of a concentrically positioned “over full-length cavity” planar sample of variable length, L = 30, 40, and 50 mm, and width, w = 4 , 6, and 8 mm, along the vertical, common sample–cavity x-axis in a Bruker single TE 102 rectangular cavity (longitudinal effect) has been analysed. The observed dependence of the EPR signal intensity, Ipp, on the sample position, x, showed the following: (i) A plateau, over which the Ipp value remained constant within experimental error of 0.62–2.48%, for sample length of 30 and 40 mm. (ii) An “sloping plateau” region, which could be approximated by a linear function (correlation, R = 0.83–0.93), for sample length of 50 mm. (iii) An additional oscillating component superimposed on the “sloping plateau” region. The trends of these dependencies with the plateau and “sloping plateau” region were independent of the planar sample width, w , and its angular position, ϕ, in the rectangular cavity. But, the EPR signal amplitude on the plateau and “sloping plateau” region obtained for the sample positioned at angles, ϕ = 0° and ϕ = 180° (which corresponded to the experimental configuration, in which the planar sample area is parallel to the waveguide in the microwave cavity) was ca 1.8 times higher compared to those obtained for the sample positioned at angles ϕ = 90° and ϕ = 270° (which corresponded to the experimental configuration, in which the planar sample area is perpendicular to the waveguide). Theoretical predictions of the experimental dependencies of the EPR signal intensity versus sample position along the common sample–cavity x-axis can be calculated using the “sine-squared” function (given by longitudinal integral) modified for an “over full-length cavity” planar sample, and the correlation between observed and theoretically computed dependencies is very good. Each of the above-mentioned phenomenon could be a serious source of significant errors in quantitative EPR spectroscopy. For maximum accuracy in quantitative EPR an “over full-length cavity” planar sample should be compared to a reference sample of identical length which must be identically positioned in the microwave cavity.

Microwave leakage from field modulation slots in TE011 electron paramagnetic resonance cavities

This article considers a Q-band cylindrical TE011 electron paramagnetic resonance cavity with slots cut parallel to wall currents to allow penetration of magnetic field modulation and with mode suppression gaps in the end regions of zero wall currents to reduce interference from nearby modes. Origins of the small amount of rf leakage generally observed from modulation slots have been studied using a finite element computer program and ways to eliminate this leakage have been found. The leakage is caused by field distortions from the waveguide coupling iris, which is centered on the cylindrical sidewall. The distortion is of two distinct types, which result in comparable leakage levels. One is from iris fields that have components perpendicular to the fields of the mode of interest. The iris near fields couple to a nonresonant circumferential radial mode in the modulation slots, like that of a sectorial waveguide horn. The other type of rf leakage is caused by coupling of the aperture to nearby cavity modes that leak from the modulation slots if they have currents across the slots. End gaps have been used to “suppress” the TM modes that, without the gaps, are degenerate with the TE011. We find that the TM modes are not suppressed, but rather altered, as are most modes. The modes are combinations of coaxial modes that are excited in the end-gap regions coupled to normal cavity modes. The most desirable way to reduce leakage is the combination of a thin slotted iris, thin modulation slots and sufficiently thick cavity walls. Electric discharge machining techniques can be used to produce slots of the required dimensions. A less desirable way to reduce the rf leakage is to place a floating conducting shield just outside the modulation slots. If resistive material is placed in the end gaps, rf leakage from the modulation slots decreases because the amount of energy coupled to an adjacent cavity mode is reduced, which increases the Q of the TE011 mode.

Low-field microwave response and electron paramagnetic resonance identification of valence states of manganese including octahedral Mn5+ in YAlO3 and CaYAlO4

Microwave response near zero magnetic field has been observed in YAlO3 and CaYAlO4 crystals dilutely doped with Mn in concentration ranging from 0.005 to 0.2 at. %. The response is due to magnetic-field-dependent nonresonant microwave losses, which co-exist with normal electron paramagnetic resonance (EPR) absorption due to Mn2+, Mn4+, and Mn5+ ions in YAlO3 and CaYAlO4. The valence states of manganese have been identified using a dual mode EPR cavity that allowed spectrum recording with polarization of the magnetic vector B1 of the microwave perpendicular or parallel to the static magnetic field B0. The Mn2+ ion, electron spin S=52, and Mn4+, S=32 have been identified using normal B1⊥B0 polarization, while Mn5+, S=1, in unusual octahedral position, has been identified using B1∥B0 polarization. The low-field response did not depend on mutual orientation of the vectors B1 and B0 and had the opposite phase with respect to the paramagnetic absorption. This shows that Mn-doped YAlO3 and CaYAlO4 exhibit magnetically induced microwave absorption, which has a minimum at zero magnetic field and increases with the applied magnetic field. The response in Mn-doped YAlO3 increases with the introduction of Ca2+ ions. The existence of manganese in different valence states, particularly metastable Mn5+ in octahedral position, and the effect of calcium co-doping suggest the formation of relatively free charge carriers that can migrate in the vicinity of paramagnetic manganese ions. The spin-dependent mechanism of the charge migration is discussed to explain magnetoinduced microwave losses.

UV-Induced Degradation Rates of 1,3,5-Triamino-2,4,6-Trinitrobenzene (TATB)

We report a 2.5-year study of the photolytic degradation of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) with variations in temperature, humidity, and illumination by fluorescent and UV light (254, 312, and 365 nm). The free-radical decomposition product was monitored with electron paramagnetic resonance (EPR). The EPR spectrum of the green powder allowed reliable quantitation with a single peak (fwhm = 29.1 G). The variations in humidity showed little effect in accelerating the degradation of TATB. The only significant temperature effect was noticed at −10 °C, where fewer radicals formed. The radical production rates at −10 °C were some of the highest measured, however, suggesting that the rates under other temperature conditions had slowed, perhaps as a result of extensive conversion of surface molecules to radical species. We show that a substantial amount of radicals can be generated with UV light, and work is ongoing to modify our EPR spectrometer so that TATB can be irradiated in the EPR cavity to measure the initial rates of radical formation.

Electron paramagnetic resonance biophysical radiation dosimetry with tooth enamel

This thesis deals with the advancements made in the field of Electron Paramagnetic Resonance (EPR) for biophysical dosimetry with tooth enamel for accident, emergency, and retrospective radiation dose reconstruction. A methodology has been developed to measure retrospective radiation exposures in human tooth enamel. This entails novel sample preparation procedures with minimum mechanical treatment to reduce the preparation induced uncertainties, establish optimum measurement conditions inside the EPR cavity, post‐process the measured spectrum with functional simulation of dosimetric and other interfering signals, and reconstruct dose. By using this technique, retrospective gamma exposures as low as have been successfully deciphered. The notion of dose modifier was introduced in EPR biodosimetry for low dose measurements. It has been demonstrated that by using the modified zero added dose (MZAD) technique for low radiation exposures, doses in 100 mGy ranges can be easily reconstructed in teeth that were previously thought useless for EPR dosimetry. Also, the use of a dose modifier makes robust dose reconstruction possible for higher radiation exposures. The EPR dosimetry technique was also developed for tooth samples extracted from rodents, which represent small tooth sizing. EPR doses in the molars, extracted from the mice irradiated with whole body exposures, were reassessed and shown to be correct within the experimental uncertainty. The sensitivity of human tooth enamel for neutron irradiation, obtained from the 3 MV McMaster K.N. Van de Graaff accelerator, was also studied. For the first time this work has shown that the neutron sensitivity of the tooth enamel is approximately 1/10th of the equivalent gamma sensitivity. Parametric studies for neutron dose rate and neutron energy within the available range of the accelerator, showed no impact on the sensitivity of the tooth enamel. Therefore, tooth enamel can be used as a dosimeter for both neutrons as well as gamma radiation. We will continue experiments to develop this end point as a sensitive accident or emergency tool for our response capabilities.

Quantitative Electron Paramagnetic Resonance: The Importance of Matching the Q-Factor of Standards and Samples

Electron paramagnetic resonance (EPR) quantification of free radicals from different samples facilitates comparison of free radical concentrations. Stable free radicals, such as 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), in a suitable solvent (e.g., benzene) can be used as a quantification standard. Free radicals found in samples can be shorter lived than radicals in prepared standards and require stabilizing spin-trapping agents such as N-tert-butyl-α-phenylnitrone (PBN) in an appropriate solvent (e.g., benzene). Analysis in our laboratory showed that free radicals from spin-trapped samples quantified against a standard of TEMPO in benzene displayed large differences among identical samples measured on either a Micro-Now 8300, Micro-Now 8400, or Bruker EMX EPR instrument. The Bruker instrument reported that the typical TEMPO in benzene standard had a Q-factor of ∼4400 while the Q-factor of our PBN-containing samples was ∼2500. (The Q-factor is inversely proportional to the amount of dissipated microwave energy in an EPR cavity.) By placing the TEMPO standard in a PBN/benzene solvent matrix we were able to match the Q-factor of our standards and samples, resulting in each of the three EPR instruments giving the same quantified free radical yields for the samples. This result points out the importance of matching the Q-factor between samples and standards for any quantitative EPR measurement.