LET spectra measurements of charged particles in the P0006 experiment on LDEF

Measurements are under way of the charged particle radiation environment of the Long Duration Exposure Facility (LDEF) satellite using stacks of plastic nuclear track detectors (PNTD's) placed in different locations of the satellite. In the initial work the charge, energy, and linear energy transfer (LET) spectra of charged particles were measured with CR-39 double layer PNTD's located on the west side of the satellite (Experiment P0006). Primary and secondary stopping heavy ions were measured separately from the more energetic particles. Both trapped and galactic cosmic ray (GCR) particles are included, with the latter component being dominated by relativistic iron particles. The results from the P0006 experiment will be compared with similar measurements in other locations on LDEF with different orientation and shielding conditions. The remarkably detailed investigation of the charged particle radiation environment of the LDEF satellite will lead to a better understanding of the radiation environment of the Space Station Freedom. It will enable more accurate prediction of single event upsets (SEU's) in microelectronics and, especially, more accurate assessment of the risk - contributed by different components of the radiation field (GCR's, trapped protons, secondaries and heavy recoils, etc.) - to the health and safety of crew members

Measurement of trapped proton fluences in main stack of P0006 experiment

We have measured directional distribution and Eastward directed mission fluence of trapped protons at two different energies with plastic nuclear track detectors (CR-39 with DOP) in the main stack of the P0006 experiment on LDEF. Results show arriving directions of trapped protons have very high anisotropy with most protons arriving from the West direction. Selecting these particles we have determined the mission fluence of Eastward directed trapped protons. We found experimental fluences are slightly higher than results of the model calculations of Armstrong and Colborn

Linear-Time Algorithms for Computing Maximum-Density Sequence Segments with Bioinformatics Applications

We study an abstract optimization problem arising from biomolecular sequence analysis. For a sequence A of pairs (a_i,w_i) for i = 1,..,n and w_i>0, a segment A(i,j) is a consecutive subsequence of A starting with index i and ending with index j. The width of A(i,j) is w(i,j) = sum_{i <= k <= j} w_k, and the density is (sum_{i<= k <= j} a_k)/ w(i,j). The maximum-density segment problem takes A and two values L and U as input and asks for a segment of A with the largest possible density among those of width at least L and at most U. When U is unbounded, we provide a relatively simple, O(n)-time algorithm, improving upon the O(n \log L)-time algorithm by Lin, Jiang and Chao. When both L and U are specified, there are no previous nontrivial results. We solve the problem in O(n) time if w_i=1 for all i, and more generally in O(n+n\log(U-L+1)) time when w_i>=1 for all i.Comment: 23 pages, 13 figures. A significant portion of these results appeared under the title, "Fast Algorithms for Finding Maximum-Density Segments of a Sequence with Applications to Bioinformatics," in Proceedings of the Second Workshop on Algorithms in Bioinformatics (WABI), volume 2452 of Lecture Notes in Computer Science (Springer-Verlag, Berlin), R. Guigo and D. Gusfield editors, 2002, pp. 157--17

Hot dense capsule implosion cores produced by z-pinch dynamic hohlraum radiation

Hot dense capsule implosions driven by z-pinch x-rays have been measured for the first time. A ~220 eV dynamic hohlraum imploded 1.7-2.1 mm diameter gas-filled CH capsules which absorbed up to ~20 kJ of x-rays. Argon tracer atom spectra were used to measure the Te~ 1keV electron temperature and the ne ~ 1-4 x10^23 cm-3 electron density. Spectra from multiple directions provide core symmetry estimates. Computer simulations agree well with the peak compression values of Te, ne, and symmetry, indicating reasonable understanding of the hohlraum and implosion physics.Comment: submitted to Phys. Rev. Let

Techno-Economic Feasibility Analysis of a Fully Mobile Radiation Oncology System using Monte Carlo Simulation

PURPOSEDisparities in radiation oncology (RO) can be attributed to geographic location, socioeconomic status, race, sex, and other societal factors. One potential solution is to implement a fully mobile (FM) RO system to bring radiotherapy to rural areas and reduce barriers to access. We use Monte Carlo simulation to quantify techno-economic feasibility with uncertainty, using two rural Missouri scenarios.METHODSRecently, a semimobile RO system has been developed by building an o-ring linear accelerator (linac) into a mobile coach that is used for temporary care, months at a time. Transitioning to a more FM-RO system, which changes location within a given day, presents technical challenges including logistics and quality assurance. This simulation includes cancer census in both northern and southeastern Missouri, multiple treatment locations within a given day, and associated expenditures and revenues. A subset of patients with lung, breast, and rectal diseases, treated with five fractions, was simulated in the FM-RO system.RESULTSThe FM-RO can perform all necessary quality assurance tests as suggested in national medical physics guidelines within 1.5 hours, thus demonstrating technological feasibility. In northern and southeastern Missouri, five-fraction simulations\u27 net incomes were, in US dollars (USD), $1.55 ± 0.17 million (approximately 74 patients/year) and$3.65 USD ± 0.25 million (approximately 98 patients/year), respectively. The number of patients seen had the highest correlation with net income as well as the ability to break-even within the simulation. The model does not account for disruptions in care or other commonly used treatment paradigms, which may lead to differences in estimated economic return. Overall, the mobile system achieved a net benefit, even for the most negative simulation scenarios.CONCLUSIONOur simulations suggest technologic success and economic viability for a FM-RO system within rural Missouri and present an interesting solution to address other geographic disparities in access to radiotherapy

A pilot study of same-day MRI-only simulation and treatment with MR-guided adaptive palliative radiotherapy (MAP-RT)

We conducted a prospective pilot study evaluating the feasibility of same day MRI-only simulation and treatment with MRI-guided adaptive palliative radiotherapy (MAP-RT) for urgent palliative indications (NCT#03824366). All (16/16) patients were able to complete 99% of their first on-table attempted fractions, and no grades 3-5 toxicities occurred

Comment on "X-ray resonant scattering studies of orbital and charge ordering in Pr1-xCaxMnO3"

In a recent published paper [Phys. Rev. B 64, 195133 (2001)], Zimmermann et al. present a systematic x-ray scattering study of charge and orbital ordering phenomena in the Pr1-xCaxMnO3 series with x= 0.25, 0.4 and 0.5. They propose that for Ca concentrations x=0.4 and 0.5, the appearance of (0, k+1/2, 0) reflections are originated by the orbital ordering of the eg electrons in the a-b plane while the (0, 2k+1, 0) reflections are due to the charge ordering among the Mn3+ and Mn4+ ions. Moreover, for small Ca concentrations (x<0.3), the orbital ordering is only considered and it occurs at (0, k, 0) reflections. A rigorous analysis of all these resonance reflections will show the inadequacy of the charge-orbital model proposed to explain the experimental results. In addition, this charge-orbital model is highly inconsistent with the electronic balance. On the contrary, these reflections can be easily understood as arising from the anisotropy of charge distribution induced by the presence of local distortions, i.e. due to a structural phase transition.Comment: 10 pages, 2 figures.To be published Phys. Rev.

Nanosized superparamagnetic precipitates in cobalt-doped ZnO

The existence of semiconductors exhibiting long-range ferromagnetic ordering at room temperature still is controversial. One particularly important issue is the presence of secondary magnetic phases such as clusters, segregations, etc... These are often tedious to detect, leading to contradictory interpretations. We show that in our cobalt doped ZnO films grown homoepitaxially on single crystalline ZnO substrates the magnetism unambiguously stems from metallic cobalt nano-inclusions. The magnetic behavior was investigated by SQUID magnetometry, x-ray magnetic circular dichroism, and AC susceptibility measurements. The results were correlated to a detailed microstructural analysis based on high resolution x-ray diffraction, transmission electron microscopy, and electron-spectroscopic imaging. No evidence for carrier mediated ferromagnetic exchange between diluted cobalt moments was found. In contrast, the combined data provide clear evidence that the observed room temperature ferromagnetic-like behavior originates from nanometer sized superparamagnetic metallic cobalt precipitates.Comment: 20 pages, 6 figures; details about background subtraction added to section III. (XMCD