Experiments with a Slow Neutron Velocity Spectrometer

Neutron bursts were produced by modulation of the arc source of a small cyclotron. Neutrons subsequently slowed down in paraffin were detected by a BF_3 ionization chamber and linear amplifier. Modulation of an additional amplifier allowed the counting of all detected neutrons as well as those arriving in a specified time interval. Time of flight experiments were carried out at 3.0 meters with "on" times of 50 or 100 μsec. (microseconds) out of a period of 2500 μsec. The modulation of the source and detector was accurate to about 5 μsec. The absorption of thin Cd (0.052 g/cm^2) shows a resolved resonance absorption with maximum at 0.14 ev. The thick Cd (0.45 g/cm^2) absorption was also examined and it was found that the absorption edge was not sharp, the transmission being 0.5 at 0.47 ev. Experiments with thick absorbers of Rh and In showed definite evidence of resonance absorption at 1.0 ev for both elements. Experiments were also carried out to study the delayed emission of thermal neutrons from the paraffin surrounding the source. An effective mean life of 170 μsec. was found. The effect of the resolution of the apparatus was examined and found to be small except for the resonances at 1.0 ev. The effect of the delayed emission of thermal neutrons on the time of flight measurements was also examined. From the thin Cd absorption curve T, the total width of the resonance level, was found to be 0.12 ev and from the cross section at resonance [1±(1/2i+1)]Γn, where Γn is the neutron width and i is the angular momentum of the absorbing nucleus, it was found to be 5×10^(−4) ev. In this calculation 13 percent abundance of the effective isotope was assumed

Nuclear Isomerism in Rhodium

Pontecorvo has recently pointed out that Rh^(104) emits numerous electrons of energy 35—60 kev. He ascribes these electrons to the internal conversion of the gamma-radiation emitted when the Rh^(104) nucleus goes from an excited state (4.2-minute period) to the ground state. From the ground state there is beta-emission which gives Pd^(104) and is ascribed to the 44-second period. The emission of the gamma-ray from the excited state is an alternative process to the direct emission of a beta-ray from the excited state. In this latter connection the beta-ray spectra associated with the two periods are of interest

γ-H2AX foci as in vivo effect biomarker in children emphasize the importance to minimize x-ray doses in paediatric CT imaging

Objectives: Investigation of DNA damage induced by CT x-rays in paediatric patients versus patient dose in a multicentre setting. Methods: From 51 paediatric patients (median age, 3.8 years) who underwent an abdomen or chest CT examination in one of the five participating radiology departments, blood samples were taken before and shortly after the examination. DNA damage was estimated by scoring gamma-H2AX foci in peripheral blood T lymphocytes. Patient-specific organ and tissue doses were calculated with a validated Monte Carlo program. Individual lifetime attributable risks (LAR) for cancer incidence and mortality were estimated according to the BEIR VII risk models. Results: Despite the low CT doses, a median increase of 0.13 gamma-H2AX foci/cell was observed. Plotting the induced gamma-H2AX foci versus blood dose indicated a low-dose hypersensitivity, supported also by an in vitro dose-response study. Differences in dose levels between radiology centres were reflected in differences in DNA damage. LAR of cancer mortality for the paediatric chest CT and abdomen CT cohort was 0.08 and 0.13% respectively. Conclusion: CT x-rays induce DNA damage in paediatric patients even at low doses and the level of DNA damage is reduced by application of more effective CT dose reduction techniques and paediatric protocols

Exciton lifetime in InAs/GaAs quantum dot molecules

The exciton lifetimes $T_1$ in arrays of InAs/GaAs vertically coupled quantum dot pairs have been measured by time-resolved photoluminescence. A considerable reduction of $T_1$ by up to a factor of $\sim$ 2 has been observed as compared to a quantum dots reference, reflecting the inter-dot coherence. Increase of the molecular coupling strength leads to a systematic decrease of $T_1$ with decreasing barrier width, as for wide barriers a fraction of structures shows reduced coupling while for narrow barriers all molecules appear to be well coupled. The coherent excitons in the molecules gain the oscillator strength of the excitons in the two separate quantum dots halving the exciton lifetime. This superradiance effect contributes to the previously observed increase of the homogeneous exciton linewidth, but is weaker than the reduction of $T_2$. This shows that as compared to the quantum dots reference pure dephasing becomes increasingly important for the molecules

Experiments with a Slow Neutron Velocity Spectrometer II

The apparatus previously used for the determination of neutron energy by time of flight method has been completely rebuilt with a view to increasing the accuracy of measurement as well as the high energy limit. The repetition frequency is obtained from a series of relaxation oscillators and a 50-kc oscillator and frequencies of 100, 200, 500, 1000, and 2500 c.p.s. are available. Four detector channels have been built so that neutrons in four different time of flight groups can be counted simultaneously. When used with the highest repetition frequency, this decreases the time to obtain data by a factor of 24 and has thus made the present experiments feasible with the small Cornell cyclotron. The study of the transmission of Ag with a 1.35-g/cm^2 absorber shows a single strong resonance at 5.8 ev. A re-examination of In with considerably higher resolution than was previously used shows a single resonance at about 1.35 ev. The effective mean life of neutrons in several different sources was examined and a thin paraffin source with Cd backing devised for measurements in the thermal region. The absorption of B has been examined and found to be proportional to 1/v within the limits of error of the experiment, from 0.028 to 50 ev. An experiment to determine the B cross section of the Cd stopped neutrons gave 540×10^(−24) cm^2. The B absorption curve shows that the cross section of thermal neutrons (0.025 ev) is 708×10^(−24) cm^2. It is concluded that the effective energy of the Cd stopped neutrons is not that of kT at thermal energy, for the geometry used, but is 0.041 ev. This conclusion is confirmed by the measured resonances in Ag and In which are higher than the values obtained by the boron absorption method. Correction of these values, as measured by Horvath and Salant (reference 4) for the effective energy of the Cd stopped neutrons, leads to 1.32 ev for In and 5.2 ev for Ag, in agreement with the present results. It is concluded that resonances measured by the boron absorption method are in error by an amount which depends upon the geometry of the experiment, and are probably too low by a factor of 0.041/0.025 = 1.64

Assessment and comparison of the Marennes-Oléron Bay (France) and Carlingford Lough (Ireland) carrying capacity with ecosystem models

Based on the individual growth, food limitation, population renewal through seeding, and individual marketable size, a theoretical model of the cultured species population dynamics was used to assess the carrying capacity of an ecosystem. It gave a domeshape curve relating the annual production and the standing stock under the assumption of individual growth limited by the available food in an ecosystem. It also showed the influence of mortality rate and marketable size on this curve and was introduced as a means to explore the global properties resulting from the interactions between the ecophysiology of the reared species and the environment at the ecosystem level. In a second step, an ecosystem model was built to assess the carrying capacity of MarennesOl ´eron bay, the most important shellfish culture site in France, with a standing stock of Crassostrea gigas around 100 000 tonnes fresh weight (FW) and an annual production of 30 000 tonnes FW. The ecosystem model focused on the oyster growth rate and considered the interaction between food availability, residence time of the water, oyster ecophysiology and number of individuals. It included a spatial discretization of the bay (box design) based on a hydrodynamicmodel, and the nitrogen or carbon cycling between phytoplankton, cultured oysters, and detritus. From simulations of the oyster growth with different seeding values, a curve relating the total annual production and the standing stock was obtained. This curve exhibited a dome shape with a maximum production corresponding to an optimum standing stock. The model predicted amaximum annual production of 45 000 tonnes FWfor a standing stock around 115 000 tonnes FW. The prediction confirmed some results obtained empirically in the case of MarennesOl ´eron bay and the results of the theoretical model. Results were compared with those obtained in Carlingford Lough (Ireland) using a similar ecosystem model. Carlingford Lough is a small intertidal bay where the same species is cultured at a reduced scale, with current biomass less than 500 tonnes FW. The model showed that the standing stock can be increased from 200 tonnes FW to approximately 1500 tonnes FW before any decrease of the production

Atomic Alchemy

We consider the transitions between electromagnetic bound states, such as the exclusive weak decay of a muonic atom into an electronic atom: $(\mu^- Z) \to (e^- Z) {\bar \nu_e }\nu_\mu .$ We show that relativistic effects in the atomic wavefunctions are crucial for determining the rate. In the case of heavy atoms, the exclusive channel branching ratios exceed $10^{-6},$ possibly bringing the study of these rare decays within experimental reach. Such processes thus provide a detailed laboratory for studying the high momentum tail of wavefunctions in atomic physics; in addition, they provide a simple toy model for investigating analogous exclusive heavy hadronic decays in quantum chromodynamics such as $B \to \pi e \nu.$Comment: 16 pages and 5 Figures, SLAC-PUB-648

Assessment and comparison of the Marennes-Oléron Bay (France) and Carlingford Lough (Ireland) carrying capacity with ecosystem models

Based on the individual growth, food limitation, population renewal through seeding, and individual marketable size, a theoretical model of the cultured species population dynamics was used to assess the carrying capacity of an ecosystem. It gave a domeshape curve relating the annual production and the standing stock under the assumption of individual growth limited by the available food in an ecosystem. It also showed the influence of mortality rate and marketable size on this curve and was introduced as a means to explore the global properties resulting from the interactions between the ecophysiology of the reared species and the environment at the ecosystem level. In a second step, an ecosystem model was built to assess the carrying capacity of MarennesOl ´eron bay, the most important shellfish culture site in France, with a standing stock of Crassostrea gigas around 100 000 tonnes fresh weight (FW) and an annual production of 30 000 tonnes FW. The ecosystem model focused on the oyster growth rate and considered the interaction between food availability, residence time of the water, oyster ecophysiology and number of individuals. It included a spatial discretization of the bay (box design) based on a hydrodynamicmodel, and the nitrogen or carbon cycling between phytoplankton, cultured oysters, and detritus. From simulations of the oyster growth with different seeding values, a curve relating the total annual production and the standing stock was obtained. This curve exhibited a dome shape with a maximum production corresponding to an optimum standing stock. The model predicted amaximum annual production of 45 000 tonnes FWfor a standing stock around 115 000 tonnes FW. The prediction confirmed some results obtained empirically in the case of MarennesOl ´eron bay and the results of the theoretical model. Results were compared with those obtained in Carlingford Lough (Ireland) using a similar ecosystem model. Carlingford Lough is a small intertidal bay where the same species is cultured at a reduced scale, with current biomass less than 500 tonnes FW. The model showed that the standing stock can be increased from 200 tonnes FW to approximately 1500 tonnes FW before any decrease of the production

Modeling active electrolocation in weakly electric fish

In this paper, we provide a mathematical model for the electrolocation in weakly electric fishes. We first investigate the forward complex conductivity problem and derive the approximate boundary conditions on the skin of the fish. Then we provide a dipole approximation for small targets away from the fish. Based on this approximation, we obtain a non-iterative location search algorithm using multi-frequency measurements. We present numerical experiments to illustrate the performance and the stability of the proposed multi-frequency location search algorithm. Finally, in the case of disk- and ellipse-shaped targets, we provide a method to reconstruct separately the conductivity, the permittivity, and the size of the targets from multi-frequency measurements.Comment: 37 pages, 11 figure