Volume 92, Issue 4, 15 August 2002
Index of content:
- APPLIED BIOPHYSICS (PACS 87)
92(2002); http://dx.doi.org/10.1063/1.1494112View Description Hide Description
Compton scattering can be used to determine the electron densities of tissues for medical applications and those of materials for industrial applications. Much work has been devoted to solving the reconstruction problem. Hussein proposed a successive approximation process (SAP) for the reconstruction of the density image from the Compton scattered energy spectral data [IEEE Trans. Nucl. Sci., 42, 2155 (1995)]. This algorithm is adapted directly from the neutron scatteringtomographyreconstruction where a convergence condition for such an algorithm was presented [Hussein and Meneley, Int. J. Multiphase Flow,12, 1 (1986)]. However, we found that this condition is not correct in general cases and we cannot obtain such a simple form of convergence condition in terms of the maximum length(in unit of mean-free path) a neutron or a photon travels in any pixel in the test section. A detailed discussion of the convergence problem of SAP will be presented in this paper.
92(2002); http://dx.doi.org/10.1063/1.1495896View Description Hide Description
Laser ablation of dental enamel with subpicosecond laser pulses has been studied over the intensity range of using 95 and 150 fs pulses at a pulse repetition rate of 1 kHz. The experimentally determined ablation threshold of 2.2±0.1 J/cm2 was in good agreement with theoretical predictions based on an electrostaticablation model. The ablation rate increased linearly with the laser fluence for up to 15 times the ablation threshold. The absence of collateral damage was observed using optical and scanning electron microscopy. Pulpal temperature measurements showed an increase of about 10 °C during the 200 s course of ablation. However, air cooling at a rate of 5 l/min resulted in the intrapulpal temperature being maintained below the pulpal damage threshhold of 5.5 °C. The material removal rates for subpicosecond precision laser ablation of dental enamel are compared with other techniques.