2,217 research outputs found
Measurement of long-lived radionuclide activity induced in target components of a cyclotron used for [18F ]-[FDG] production
The evaluation of high activity induced in target components in using a medical cyclotron to produce positron-emitting radionuclides for PET (Positron Emission Tomography) diagnostic studies is one of important issue and involves radiation protection concepts when operators are engaged in maintenance and/or substitution of a target or its components. Most of replaced target components are generally classified and stored in a Pb-shielded container in order to wait for their radioactive decay. However, after some years, it can be necessary to start with the removal of the oldest parts, for a temporary storage of the fresh activated ones. The feasibility of these operations and the waste final disposal depends on the level of activity achieved and in particular on the residual concentration of radionuclides. In this work we perform a nuclide identification and activity evaluation of some activated target parts by high resolution gamma-ray spectrometry with various HPGe detectors. The measurements were performed over a decay period of more than 10 years from extraction, which allows to identify radionuclides with different half-lives. In particular, measurements on some Havar foils, stripper foils and titanium parts of a target used inside an IBA CYCLONE 18/9 cyclotron, allow to evaluate the largest activity values related to the most important radionuclides produced by activation of the materials (51Cr, 52Mn, 54Mn, 56Co, 57Co, 58Co, \u2026) with half-life of 70-80 days, while radionuclides with higher half-lives (22Na, 44Ti, 60Co, 207Bi, \u2026..) were detected in the same samples in measurements performed after a long time period. In this way it is possible to evaluate in advance the activity level at a time period after the end of maintenance and establish the correct procedures for storage or disposal of wast
Numerical study of metastable states in Ising spin glasses
We study numerically the structure of metastable states in the
Sherrington-Kirkpatrick spin glass. We find that all non-paramagnetic
stationary points of the free energy are organized into pairs, consisting in a
minimum and a saddle of order one, which coalesce in the thermodynamic limit.
Within the annealed approximation, the entropic contribution of these states,
that is the complexity, is compatible with the supersymmetry-breaking
calculation performed in [A.J. Bray and M.A. Moore, J. Phys. C, 13 L469
(1980)]. This result indicates that the supersymmetry is spontaneously broken
in the Sherrington-Kirkpatrick model
Generalized minority games with adaptive trend-followers and contrarians
We introduce a simple extension of the minority game in which the market
rewards contrarian (resp. trend-following) strategies when it is far from
(resp. close to) efficiency. The model displays a smooth crossover from a
regime where contrarians dominate to one where trend-followers dominate. In the
intermediate phase, the stationary state is characterized by non-Gaussian
features as well as by the formation of sustained trends and bubbles.Comment: 4 pages, 6 figure
Sampling rare fluctuations of height in the Oslo ricepile model
We have studied large deviations of the height of the pile from its mean
value in the Oslo ricepile model. We sampled these very rare events with
probabilities of order by Monte Carlo simulations using importance
sampling. These simulations check our qualitative arguement [Phys. Rev. E, {\bf
73}, 021303, 2006] that in steady state of the Oslo ricepile model, the
probability of large negative height fluctuations about
the mean varies as as with
held fixed, and .Comment: 7 pages, 8 figure
Multi-Temporal InSAR Structural Damage Assessment: The London Crossrail Case Study
Spaceborne multi-temporal interferometric synthetic aperture radar (MT-InSAR) is a monitoring technique capable of extracting line of sight (LOS) cumulative surface displacement measurements with millimeter accuracy. Several improvements in the techniques and datasets quality lead to more effective, near real time assessment and response, and a greater ability of constraining dynamically changing physical processes. Using examples of the COSMO-SkyMed (CSK) system, we present a methodology that bridges the gaps between MT-InSAR and the relative stiffness method for tunnel-induced subsidence damage assessment. The results allow quantification of the effect of the building on the settlement profile. As expected the greenfield deformation assessment tends to provide a conservative estimate in the majority of cases (~ 71% of the analyzed buildings), overestimating tensile strains up to 50%. With this work we show how these two techniques in the field of remote sensing and structural engineering can be synergistically used to complement and replace the traditional ground based analysis by providing an extended coverage and a temporally dense set of data
Leaf litter decomposition rates increase with rising mean annual temperature in Hawaiian tropical montane wet forests
Decomposing litter in forest ecosystems supplies nutrients to plants, carbon to heterotrophic soil microorganisms and is a large source of CO2 to the atmosphere.
Despite its essential role in carbon and nutrient cycling, the temperature sensitivity
of leaf litter decay in tropical forest ecosystems remains poorly resolved, especially in
tropical montane wet forests where the warming trend may be amplified compared
to tropical wet forests at lower elevations. We quantified leaf litter decomposition
rates along a highly constrained 5.2 ◦C mean annual temperature (MAT) gradient in
tropical montane wet forests on the Island ofHawaii. Dominant vegetation, substrate
type and age, soil moisture, and disturbance history are all nearly constant across this
gradient, allowing us to isolate the effect of rising MAT on leaf litter decomposition
and nutrient release. Leaf litter decomposition rates were a positive linear function
of MAT, causing the residence time of leaf litter on the forest floor to decline by ∼31
days for each 1 ◦C increase in MAT. Our estimate of the Q10 temperature coefficient
for leaf litter decomposition was 2.17, within the commonly reported range
for heterotrophic organic matter decomposition (1.5–2.5) across a broad range of
ecosystems. The percentage of leaf litter nitrogen (N) remaining after six months
declined linearly with increasing MAT from ∼88% of initial N at the coolest site to
∼74% at the warmest site. The lack of net N immobilization during all three litter
collection periods at all MAT plots indicates that N was not limiting to leaf litter
decomposition, regardless of temperature. These results suggest that leaf litter decay
in tropical montane wet forests may be more sensitive to rising MAT than in tropical
lowland wet forests, and that increased rates of N release from decomposing litter
could delay or prevent progressive N limitation to net primary productivity with
climate warming
Statistical mechanics of the mixed majority-minority game with random external information
We study the asymptotic macroscopic properties of the mixed majority-minority
game, modeling a population in which two types of heterogeneous adaptive
agents, namely ``fundamentalists'' driven by differentiation and
``trend-followers'' driven by imitation, interact. The presence of a fraction f
of trend-followers is shown to induce (a) a significant loss of informational
efficiency with respect to a pure minority game (in particular, an efficient,
unpredictable phase exists only for f<1/2), and (b) a catastrophic increase of
global fluctuations for f>1/2. We solve the model by means of an approximate
static (replica) theory and by a direct dynamical (generating functional)
technique. The two approaches coincide and match numerical results
convincingly.Comment: 19 pages, 3 figure
Predictability of large future changes in a competitive evolving population
The dynamical evolution of many economic, sociological, biological and
physical systems tends to be dominated by a relatively small number of
unexpected, large changes (`extreme events'). We study the large, internal
changes produced in a generic multi-agent population competing for a limited
resource, and find that the level of predictability actually increases prior to
a large change. These large changes hence arise as a predictable consequence of
information encoded in the system's global state.Comment: 10 pages, 3 figure
Chemotactic smoothing of collective migration
Collective migration-the directed, coordinated motion of many self-propelled agents-is a fascinating emergent behavior exhibited by active matter with functional implications for biological systems. However, how migration can persist when a population is confronted with perturbations is poorly understood. Here, we address this gap in knowledge through studies of bacteria that migrate via directed motion, or chemotaxis, in response to a self-generated nutrient gradient. We find that bacterial populations autonomously smooth out large-scale perturbations in their overall morphology, enabling the cells to continue to migrate together. This smoothing process arises from spatial variations in the ability of cells to sense and respond to the local nutrient gradient-revealing a population-scale consequence of the manner in which individual cells transduce external signals. Altogether, our work provides insights to predict, and potentially control, the collective migration and morphology of cellular populations and diverse other forms of active matter. eLife digest Flocks of birds, schools of fish and herds of animals are all good examples of collective migration, where individuals co-ordinate their behavior to improve survival. This process also happens on a cellular level; for example, when bacteria consume a nutrient in their surroundings, they will collectively move to an area with a higher concentration of food via a process known as chemotaxis. Several studies have examined how disturbing collective migration can cause populations to fall apart. However, little is known about how groups withstand these interferences. To investigate, Bhattacharjee, Amchin, Alert et al. studied bacteria called Escherichia coli as they moved through a gel towards nutrients. The E. coli were injected into the gel using a three-dimensional printer, which deposited the bacteria into a wiggly shape that forces the cells apart, making it harder for them to move as a collective group. However, as the bacteria migrated through the gel, they smoothed out the line and gradually made it straighter so they could continue to travel together over longer distances. Computer simulations revealed that this smoothing process is achieved by differences in how the cells respond to local nutrient levels based on their position. Bacteria towards the front of the group are exposed to more nutrients, causing them to become oversaturated and respond less effectively to the nutrient gradient. As a result, they move more slowly, allowing the cells behind them to eventually catch-up. These findings reveal a general mechanism in which limitations in how individuals sense and respond to an external signal (in this case local nutrient concentrations) allows them to continue migrating together. This mechanism may apply to other systems that migrate via chemotaxis, as well as groups whose movement is directed by different external factors, such as temperature and light intensity
Intraoperative electrocochleographic characteristics of auditory neuropathy spectrum disorder in cochlear implant subjects
Auditory neuropathy spectrum disorder (ANSD) is characterized by an apparent discrepancy between measures of cochlear and neural function based on auditory brainstem response (ABR) testing. Clinical indicators of ANSD are a present cochlear microphonic (CM) with small or absent wave V. Many identified ANSD patients have speech impairment severe enough that cochlear implantation (CI) is indicated. To better understand the cochleae identified with ANSD that lead to a CI, we performed intraoperative round window electrocochleography (ECochG) to tone bursts in children (n = 167) and adults (n = 163). Magnitudes of the responses to tones of different frequencies were summed to measure the “total response” (ECochG-TR), a metric often dominated by hair cell activity, and auditory nerve activity was estimated visually from the compound action potential (CAP) and auditory nerve neurophonic (ANN) as a ranked “Nerve Score”. Subjects identified as ANSD (45 ears in children, 3 in adults) had higher values of ECochG-TR than adult and pediatric subjects also receiving CIs not identified as ANSD. However, nerve scores of the ANSD group were similar to the other cohorts, although dominated by the ANN to low frequencies more than in the non-ANSD groups. To high frequencies, the common morphology of ANSD cases was a large CM and summating potential, and small or absent CAP. Common morphologies in other groups were either only a CM, or a combination of CM and CAP. These results indicate that responses to high frequencies, derived primarily from hair cells, are the main source of the CM used to evaluate ANSD in the clinical setting. However, the clinical tests do not capture the wide range of neural activity seen to low frequency sounds
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