Monte Carlo calculation and experimental verification of the photon energy response of tooth enamel in a head-sized plexiglas phantom

The use of electron paramagnetic resonance (EPR) tooth dosimetry for calculation of organ doses requires conversion of the measured absorbed dose in enamel. Before deriving conversion factors from simulation calculations with a realistic anthropomorphic human phantom, in the current study a simplified phantom was chosen to compare EPR measurement and Monte Carlo calculation. The dose response of tooth enamel of molars at various positions inside a cylindrical Plexiglas phantom of head-size was calculated hy Monte Carlo modelling in parallel photon beams of X rays of 63 keV equivalent energy and 60Co gamma rays (1.25 Mev). For X ray exposure, preliminary results of EPR dosimetry with tooth enamel samples prepared from molars irradiated in the phantom were in agreement with calculation. The mean value of the ratio of the measured to the calculated dose was 0.93 +/- 0.08.     

Aluminablasting of human tooth enamel

As part of a programme aimed at enhancing the degree of bonding that can be developed between dental polymer and human tooth enamel, the enamel has been subjected to two types of pretreatment, namely, aluminablasting (a mechanical etching) and aluminablasting followed by surface etching with orthophosphoric acid. Analysis of the surfaces so prepared by scanning electron microscopy has shown that enamel patterns with different degrees of retention ability have been obtained, and that the surfaces should be capable of bonding with dental polymer.     

Spherical indentation of tooth enamel

Spherical indentation measurements were made on enamel surfaces of intact human teeth. From the load versus depth of indentation curve Young's modulus was found to be typically 8.3×10¹⁰ Pa and the yield stress to be 3.3×10⁸ Pa. Young's modulus was observed to vary with moisture content and with the orientation of the enamel surface. Viscoelastic behaviour was evident, also depending on the moisture content of the enamel.     

Conductivity in human tooth enamel

When human tooth enamel is heated either in vacuum or air it presents drastic changes in electrical susceptibility, conductivity and structural properties. In this paper we report an insulator-conductive transition which is observed in air around 350°C where enamel conductivity changes drastically and its electrical resistance decreases from 10¹⁵ to 10⁵ that is, it goes from an insulator to a super-ionic ceramic behavior. This transition, first evidenced in vacuum by electron microscopy observations, is now completely determined by a.c. impedance spectroscopy technique and its characterization was carried out as a function of the frequency and temperature. X-ray in situ heating diffraction experiments show that there is no structural phase transition during a wide range of temperature including the one where the conductivity transition occurs.     

Calibration of EPR signal dose response of tooth enamel to photons: experiment and Monte Carlo simulation

The experimental energy dependence of the electron paramagnetic resonance (EPR) radiation-induced signal at irradiation by photons in the energy range of 13 keV-1.25 MeV was analysed in terms of the absorbed dose in human tooth enamel. The latter was calculated using a Monte Carlo simulation of the photon and electron transport. The dependence of the calculated absorbed dose on the sample thickness was analysed. No energy dependence of the EPR signal on the absorbed dose in enamel was verified in the range of 37 keV-1.25 MeV. At 13 and 20 keV the EPR signal dose response was reduced by 8% probably due to sample powdering. Dose-depth profiles in enamel samples irradiated by 1.25 MeV photons in polymethylmethacrylate and aluminium build-up materials were calculated. It was concluded that secondary electron equilibrium conditions are better fulfilled for irradiation in aluminium, which makes this material preferable for calibration.     

Analyses of absorbed dose to tooth enamel against external photon exposure

Absorbed dose to tooth enamel was examined against external photon exposure by measurements with thermoluminescence dosemeters (TLDs) and Monte Carlo calculations. TLDs were placed in a realistic physical phantom to measure dose to the teeth region in a head. A voxel-type phantom was constructed from computed tomography (CT) images of the physical phantom. Monte Carlo calculations with this voxel-type phantom were performed to analyse the results of the experiments. The data obtained were compared to the enamel doses, which were calculated with a modified MIRD-type phantom and already given in a previous paper. It was confirmed that the data derived with the MIRD-type phantom are applicable for retrospective individual dose assessments by electron spin resonance (ESR) dosimetry using teeth for the photon energy region above 300 keV. The analysis, however, indicated that the configuration of the head can affect the enamel dose relative to external exposure to photons with energy below 100 keV.     

In-situ monitoring of dental erosion in tooth enamel when exposed to soft drinks

The prevalence of dental erosion and decay in human teeth has increased dramatically in the few decades as a result of an increasing consumption of soft drinks and commercial fruit juices. A simple weighting method is described for the in-situ monitoring of dental erosion in tooth enamel when immersed in a soft drink. When compared to the conventional method, this new method is far superior in terms of accuracy of data collected and the ease of data collection in real-time. Atomic absorption results suggest that the increasing weight loss in tooth enamel during dental erosion in soft drinks can be attributed to the continuous leaching of Ca²⁺ ions, in addition to phosphorus, oxygen, and hydrogen. The potential application of this new weighing technique in other research areas is discussed.     

Crystallography of human tooth enamel: Initial structure refinement

A first crystal structure refinement of human tooth enamel, per se, has been achieved. For this, both x-ray and neutron powder diffraction data were analyzed with the Rietveld method of whole-pattern fitting. The dense fraction (sp.g. >2.95) of pooled human tooth enamel was used. Refinement in space group P6₃/m led to $\text{a}\text{= 9.441(2)}\text{A}\text{?}$, $\text{c}\text{= 6.878(1)}\text{A}\text{?}$ and locations of the principal atoms similar to those in hydroxyapatite. The observed scattering density distributions along the hexad axis, including the differences between x-ray and neutron measures, would accommodate H₂O in some orientational disorder centered at z= 0.11 to 0.14 and the known small quantity of Cl^? ions partially substituting for OH^? (e.g. 1 in 20). This structure refinement constitutes the first direct evidence of both similarity and differences in the atomic-scale structural details of hydroxyapatite and human tooth enamel.     

Determination of neutron energy spectra inside a water phantom irradiated by 64 MeV neutrons

A NE230 deuterated liquid scintillator detector (25 mm diameter x 25 mm) has been used to investigate neutron energy spectra as a function of position in a water phantom under irradiation by a quasi-monoenergetic 64 MeV neutron beam. Neutron energy spectra are obtained from measurements of pulse height spectra by the NE230 detector using the Bayesian unfolding code MAXED. The experimentally measured energy spectra are compared with spectra calculated by Monte Carlo simulation using the code MCNPX.     

The toughness of tooth enamel,a natural fibrous composite

It is suggested on the basis of experiments carried out in vitro on whole caries-free human premolars that the energy absorbing capacity (mechanical toughness) of the enamel depends upon the explusion of liquid from between the mineral fibre of this natural composite material. Classical theory of flow through narrow channels with recent modifications to allow for the influence of the electrical double layers at the surface of the fibres has been applied to show how toughness can be influenced and controlled by wholly chemical methods. In particular the effect of replacing the hydroxyl groups with flouride ions is shown to result in an increase in toughness. Finally, it is suggested that the flow of liquids through fine capillaries may be responsible for toughness in other natural fibrous composites such as wood.Dr. Peter Fox died shortly after the acceptance of this paper for publication. It has been my pleasure to publish several of Dr Fox's distinctive papers over the years and his thoughtful contributions to the subject of Materials Science will be missed.     

The response function of organic scintillator NE213 to 59.5 MeV neutrons

The response function of a 12.7 cm diameter × 12.7 cm cell of an organic liquid scintillator NE213 has been studied at E_n = 59.5 MeV. In the pulse shape spectra bands due to deuterons, besides protons and alpha particles produced by neutron interactions are identified. In order to discriminate the contribution from the ¹²C (n, np) reaction, a coincidence experiment between the protons in the scintillator and the escaping neutrons from the scintillator was made. To confirm the deuteron band further, a liquid scintillator cell was directly bombarded by a faint deuteron beam with an energy of 56 MeV. The response functions for individual bands are very different from the calculated response using the Monte Carlo method.     

Characteristic response of plastic track detectors to 40-80 MeV neutrons

This paper investigates the characteristic response of plastic track detectors to high-energy neutrons. Three types of plastic nuclear track detector (PNTD), Baryotrak made of pure CR-39, TD-1 made of CR-39 containing an antioxidant and TNF-1 made of a copolymer of CR-39/N-isopropylacrylamide, were exposed in quasi-monoenergetic neutron fields generated by p-Li reactions. The total efficiencies for TD-1 and TNF-1 were more than double and triple that of Baryotrak respectively. In addition, the species of particles were classitied into three groups, i.e. proton relatives, alpha particles and heavy ions, by analysing the etch-pit growth curve obtained by step-by-step etching. In a 65 MeV neutron field about half of the tracks recorded in pure CR-39 were due to heavy ions, whereas the TNF-1 detector could effectively register the protons, accounting for 70% of the tracks. The results could be explained by the difference in the sensitivity to high-energy protons.     

Effects of different preparation procedures during tooth whitening on enamel bonding

The objective of this study was to assess effects of some clinically related preparation procedures during tooth whitening on enamel bonding properties. Sixty-two extracted human teeth were cleaned and divided into four groups. Forty-two of the teeth were left with their natural surface intact while 20 teeth were polished to form a flat surface. Half of the tooth served as the experimental side and received one of the two whitening products: Opalescence (10% carbamide peroxide) and Crest Whitestrips (6.5% hydrogen peroxide), for 2 weeks. Post-bleaching intervals included: 1 day, 1 week, and 2 weeks. On these days, tooth (10 mm × 1.5 mm × 1.5 mm) sections were evaluated using Raman spectroscopy, scanning electron microscopy and tensile bond strength tests. T-test, ANOVA test, and mixed model regression analysis were used to assess the differences. No significant difference existed between natural surface and polished surface teeth for all groups at both Day One and Week Two (P > 0.05). On Day One, both treated groups had significant lower bond strength than the control group (P = 0.002). After 2 weeks, no significant difference existed between any group (P = 0.381). SEM indicated that resin–enamel interfaces in bleached enamel exhibited more defects in granular formations when compared to the control. Raman results indicated a lower degree of polymerization (DP) of adhesive at the interface for treated teeth surfaces. In summary, pre-bleaching surface treatments such as polish or non-polish, had no effect on bond strength. Bleaching significantly decreased bond strength initially, but after 2 weeks, bleaching had no significant effect on bond strength. Storage time had significant effect on Opalescence treated enamel, but not on control and Whitestrip treated enamel. The decrease of bond strength may be related to interfacial defects and low DP due to oxygen release after bleaching.     

Electron paramagnetic resonance of human tooth enamel at high gamma ray doses

Powdered human tooth enamel was exposed to 60Co gamma rays up to a dose of 100 kGy. The electron paramagnetic resonance (EPR) signal intensity (1) of the radiation-generated carbon dioxide radicals was measured for dependence on absorbed dose (D). The EPR dose response can be fitted with an exponential saturation function I = I(M)[1 - exp(-D/D37)] with the saturated signal intensity (I(M)) and the dose saturation value (D37). The obtained value D37 = 9.64 (+/- 0.96) kGy (measured at least one month after irradiation) exceeds those given in the literature. The saturated concentration of orthorhombic CO2- radicals was estimated at 6.5 x 10(17) per gram of enamel by comparing the integrated EPR spectra of enamel and a standard MgO:Cr probe. For enamel samples, which were heated before irradiation for one hour at +405 degrees C, the value of D37T = 3.89 (+/- 0.44) kGy and the saturated value of CO2- radicals 3.4 x 10(17) per gram of enamel were lower than for unheated samples. The initial rise of the signal with the dose was slightly higher (8.8 x 10(13) radicals/g x Gy) for heated compared with unheated samples (6.8 x 10(13) radicals/g x Gy).     

Organ dose conversions from ESR measurements using tooth enamel of atomic bomb survivors

Dose conversions were studied for dosimetry of atomic bomb survivors based upon electron spin resonance (ESR) measurements of tooth enamel. Previously analysed data had clarified that the tooth enamel dose could be much larger than other organ doses from a low-energy photon exposure. The radiation doses to other organs or whole-body doses, however, are assumed to be near the tooth enamel dose for photon energies which are dominant in the leakage spectrum of the Hiroshima atomic bomb assumed in DS02. In addition, the thyroid can be a candidate for a surrogate organ in cases where the tooth enamel dose is not available in organ dosimetry. This paper also suggests the application of new Japanese voxel phantoms to derive tooth enamel doses by numerical analyses.     

Mechanisms and causes of wear in tooth enamel: implications for hominin diets

The wear of teeth is a major factor limiting mammalian lifespans in the wild. One method of describing worn surfaces, dental microwear texture analysis, has proved powerful for reconstructing the diets of extinct vertebrates, but has yielded unexpected results in early hominins. In particular, although australopiths exhibit derived craniodental features interpreted as adaptations for eating hard foods, most do not exhibit microwear signals indicative of this diet. However, no experiments have yet demonstrated the fundamental mechanisms and causes of this wear. Here, we report nanowear experiments where individual dust particles, phytoliths and enamel chips were slid across a flat enamel surface. Microwear features produced were influenced strongly by interacting mechanical properties and particle geometry. Quartz dust was a rigid abrasive, capable of fracturing and removing enamel pieces. By contrast, phytoliths and enamel chips deformed during sliding, forming U-shaped grooves or flat troughs in enamel, without tissue loss. Other plant tissues seem too soft to mark enamel, acting as particle transporters. We conclude that dust has overwhelming importance as a wear agent and that dietary signals preserved in dental microwear are indirect. Nanowear studies should resolve controversies over adaptive trends in mammals like enamel thickening or hypsodonty that delay functional dental loss.     

The response of a large Ge(Li)/NaI anti-compton system to evaporation neutrons

The spectrum due to scattering of evaporated neutrons in a large Ge(Li) detector used with a sodium-iodide anti-Compton shield is presented in a typical situation.     

Comparative studies on the CO2- signal in tooth enamel and carbonates

When a swift ion is slowed down through a plastic detector it creates a latent track. In nuclear track detectors, this latent track can be specifically etched by an appropriate chemical solution. This enlargement process is due to a higher etch velocity (VT) along the ion's path than in the non-damaged part of the detector. The etched track velocity is definitely linked to the damage created by the incoming ion in the detector material. A relationship between the physical parameters of the energy deposition and the variation in this etched track velocity with the ion energy cannot easily be explained. We present here our study on the chemical damage created by several ions in a cellulose nitrate type detector and our first attempt to simulate them by the use of the hit theory.     

Dose estimation by EPR spectroscopy of tooth enamel in Chinese medical diagnostic X-ray workers

Individual accumulated doses were determined by electron paramagnetic resonance spectroscopy of tooth enamel in Chinese medical diagnostic X-ray workers. Dose determination was performed using a specially developed automatic spectrum processing procedure. The determined dose values owing to X rays for the three workers with entry year (the year they began their career as medical diagnostic X-ray workers) in the 1950s ranged from 435 to 903 mGy, the converted effective doses ranged from 91 to 190 mSv.