E.R. van der Graaf

The uptake of water by freeze-dried human dentine sections

Abstract Ten sections of dentine, cut perpendicular to the dentinal tubules, from human mature non-carious third molars were freeze-dried and then rehydrated by immersion in water. The uptake of water by the sections was determined as a function of time by weighing. Rehydration was complete. On the basis of 3 theoretical models the uptake could be described by a combination of the capillary suction of water into the dentinal tubules and the diffusion of water into the mineralized matrix parallel to the tubules. The calculated diffusion coefficient describing the diffusion of water into the intertubular dentine was 1.74 (±0.42) × 10 −10 m 2 /s . ( x ± SEM , n = 10) . A further calculation found that 75.2 (±1.5)% of the water is in the tubules and thus 24.8 (±1.5)% is in the mineralized matrix.

Methods for measuring diffusion coefficients of radon in building materials

Two methods for determining the Rn-222 diffusion coefficient in concrete are presented. Experimentally, the flush and adsorption technique to measure radon release rates underlines both methods. Theoretically, the first method was developed for samples of cubical geometry. The radon diffusion equation was solved for boundary conditions imposing zero flux conditions successively on each side. In practice, a 100% effective covering would ensure this condition to be satisfied. The diffusion coefficient is then determined by comparing the computed and respectively measured normalized ratios of the radon release rates with respect to the rate corresponding to the open-boundaries (uncovered) specimen. As in practice, none of the investigated coverings showed to be effective in reducing radon exhalation, indicating that radon-tight sealing of surfaces is far from trivial, no clear conclusions could be drawn with respect to the diffusion coefficient. The second method can be applied to specimens that are first reshaped into hollow cylinders. A one-dimensional situation can be inforced by requiring that the flux vanishes at the two ends of the hollow cylinder. The theoretical and experimental ratios are again compared. In practice, the radon flux originates from a radon source enclosed inside the hollow cylinder, the effectiveness of the sealing being previously tested on an aluminum dummy of similar dimensions. The radon bulk diffusion coefficient for the used concrete sample resulted in a value of D = (4.6 +/- 0.4) x 10(-6) cm2 s(-1).

Short-lived positron emitters in beam-on PET imaging during proton therapy

The only method for in vivo dose delivery verification in proton beam radiotherapy in clinical use today is positron emission tomography (PET) of the positron emitters produced in the patient during irradiation. PET imaging while the beam is on (so called beam-on PET) is an attractive option, providing the largest number of counts, the least biological washout and the fastest feedback. In this implementation, all nuclides, independent of their half-life, will contribute. As a first step towards assessing the relevance of short-lived nuclides (half-life shorter than that of (10)C, T1/2  =  19 s) for in vivo dose delivery verification using beam-on PET, we measured their production in the stopping of 55 MeV protons in water, carbon, phosphorus and calcium The most copiously produced short-lived nuclides and their production rates relative to the relevant long-lived nuclides are: (12)N (T1/2  =  11 ms) on carbon (9% of (11)C), (29)P (T1/2  =  4.1 s) on phosphorus (20% of (30)P) and (38m)K (T1/2  =  0.92 s) on calcium (113% of (38g)K). No short-lived nuclides are produced on oxygen. The number of decays integrated from the start of an irradiation as a function of time during the irradiation of PMMA and 4 tissue materials has been determined. For (carbon-rich) adipose tissue, (12)N dominates up to 70 s. On bone tissue, (12)N dominates over (15)O during the first 8-15 s (depending on carbon-to-oxygen ratio). The short-lived nuclides created on phosphorus and calcium provide 2.5 times more beam-on PET counts than the long-lived ones produced on these elements during a 70 s irradiation. From the estimated number of (12)N PET counts, we conclude that, for any tissue, (12)N PET imaging potentially provides equal to superior proton range information compared to prompt gamma imaging with an optimized knife-edge slit camera. The practical implementation of (12)N PET imaging is discussed.

Spectra of clinical CT scanners using a portable Compton spectrometer

PURPOSE Spectral information of the output of x-ray tubes in (dual source) computer tomography (CT) scanners can be used to improve the conversion of CT numbers to proton stopping power and can be used to advantage in CT scanner quality assurance. The purpose of this study is to design, validate, and apply a compact portable Compton spectrometer that was constructed to accurately measure x-ray spectra of CT scanners. METHODS In the design of the Compton spectrometer, the shielding materials were carefully chosen and positioned to reduce background by x-ray fluorescence from the materials used. The spectrum of Compton scattered x-rays alters from the original source spectrum due to various physical processes. Reconstruction of the original x-ray spectrum from the Compton scattered spectrum is based on Monte Carlo simulations of the processes involved. This reconstruction is validated by comparing directly and indirectly measured spectra of a mobile x-ray tube. The Compton spectrometer is assessed in a clinical setting by measuring x-ray spectra at various tube voltages of three different medical CT scanner x-ray tubes. RESULTS The directly and indirectly measured spectra are in good agreement (their ratio being 0.99) thereby validating the reconstruction method. The measured spectra of the medical CT scanners are consistent with theoretical spectra and spectra obtained from the x-ray tube manufacturer. CONCLUSIONS A Compton spectrometer has been successfully designed, constructed, validated, and applied in the measurement of x-ray spectra of CT scanners. These measurements show that our compact Compton spectrometer can be rapidly set-up using the alignment lasers of the CT scanner, thereby enabling its use in commissioning, troubleshooting, and, e.g., annual performance check-ups of CT scanners.

Impact and Application: Quest for a Nuclear Georeactor

In a time when astronauts orbit the Earth and visit the Moon, and mankind has brought vehicles to Mars and telescopes into orbit, we seldom realize that we have penetrated the Earth by only 12 km, a distance smaller than we commute daily to our work or equivalent to the cruising altitude of airplanes. Consequently, we know little about the interior of our planet, except from seismic information and study of the composition of meteorites. Recently our knowledge about antineutrinos has reached the stage that they can be used as a tool, allowing us to look at radiogenic heat sources in the interior part of the planet and associated processes.

Radiation performance index for Dutch dwellings : Consequences for some typical situations

This paper describes the new approach to control radiation exposure from natural sources to inhabitants of dwellings that is presently being considered in the Netherlands. The goal of this approach is to uphold the current rather favorable situation (average annual effective dose due to indoor radon and external radiation in dwellings is approx. 1 mSv). To achieve this goal a model is foreseen to predict the potential effective dose an inhabitant may receive from a dwelling on basis of its building plan. A scheme to calculate this dose is proposed in this paper. In future, such a scheme will be included in the Dutch Building Codes and houses to be built will be evaluated by using this scheme and comparing the results with, yet to be posed, limits to the potential effective dose.

Temperature dependence of water transport into the mineralized matrix of freeze-dried human dentine

Abstract Ten dentine sections cut perpendicular to the dentinal tubules from human mature non-carious third molars, were freeze-dried and then rehydrated by immersion in water at four temperatures, 10, 25, 40 and 70 °C. The uptake of water by the sections was assessed as a function of rehydration time. The data were analysed using a theoretical model in which the uptake is ascribed to two processes. The first is capillary suction of water into the dentinal tubules, the second is the diffusion of water from the two faces of the section into the mineralized matrix. By fitting theory to experiment the diffusion coefficient of water in intertubular dentine at the different temperatures was found. Using the Arrhenius relation the activation energy of diffusion of water in intertubular dentine was calculated as E a = 29.5 ± 2.2 kJ/mol, which is of the order of the strength of a hydrogen bond. Transport of water in the mineralized matrix during rehydration is thus most likely a hopping of water molecules along the surfaces of the collagen and/or mineral, each jump involving the breaking of one hydrogen bond.

A theoretical model for the hydraulic conductance of human dentine

A theoretical model was devised that describes the dependence of the hydraulic conductance of dentine disks on the decrease in dentine tubule radius and dentine tubule density (number per area of cross-section) in the pulp-enamel direction. A parameter that describes the decrease in tubule radius was calculated from experimental results from the literature.

Proton radiography to improve proton therapy treatment

The quality of cancer treatment with protons critically depends on an accurate prediction of the proton stopping powers for the tissues traversed by the protons. Today, treatment planning in proton radiotherapy is based on stopping power calculations from densities of X-ray Computed Tomography (CT) images. This causes systematic uncertainties in the calculated proton range in a patient of typically 3-4%, but can become even 10% in bone regions [1-8]. This may lead to no dose in parts of the tumor and too high dose in healthy tissues [9]. A direct measurement of proton stopping powers with high-energy protons will allow reducing these uncertainties and will improve the quality of the treatment. Several studies have shown that a sufficiently accurate radiograph can be obtained by tracking individual protons traversing a phantom (patient) [4, 6, 10]. Our studies benefit from the gas-filled time projection chambers based on GridPix technology [11], developed at Nikhef, capable of tracking a single proton. A BaF2 crystal measuring the residual energy of protons was used. Proton radiographs of phantom consisting of different tissue-like materials were measured with a 30 x 30 mm(2) 150 MeV proton beam. Measurements were simulated with the Geant4 toolkit. First experimental and simulated energy radiographs are in very good agreement [12]. In this paper we focus on simulation studies of the proton scattering angle as it affects the position resolution of the proton energy loss radiograph. By selecting protons with a small scattering angle, the image quality can be improved significantly.

An assessment of the radiological consequences of using phosphorus slag in concrete foundation poles

Publisher Summary This chapter presents an assessment of the extra effective dose that can be associated with the use of foundation poles made of concrete with phosphorus slag addition (PP-concrete). The assessment is based on measurements of the radon release rate and activity concentrations of this PP-concrete. The study showed that for a typical Dutch crawl space dwelling with a concrete floor and with 30 foundation poles, the use of PP-concrete in the poles might lead to an extra effective dose between approximately 3 to 40 μSv a −1 . This is about 4% of the total effective dose that inhabitants receive from their dwelling in the Netherlands. The maximum extra effective dose (due to both radon and external radiation) is approximately 4% of the total effective dose connected to living in Dutch dwellings. Before application of PP-concrete foundation poles in construction of new houses can become an option in the Dutch building practice these radiological consequences have to be judged by the Dutch government in a larger framework in that also, for example, the environmental impacts of storage and disposal and the economical benefits of re-use of the phosphorus slag are considered.