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It is important that accurate estimates of crew exposure to radiation are obtained for future long-term space missions. Presently, several space radiation transport codes, all of which take as input particle interaction cross sections that describe the nuclear interactions between the particles and the shielding material, exist to predict the radiation environment. The space radiation transport code HZETRN uses the nuclear fragmentation model NUC-FRG2 to calculate electromagnetic dissociation (EMD) cross sections. Currently, NUCFRG2 employs energy-independent branching ratios to calculate these cross sections. Using Weisskopf-Ewing (WE) theory to calculate branching ratios for compound nucleus reactions, however, is more advantageous than the method currently employed in NUCFRG2. The WE theory can calculate not only neutron and proton emission, as in the energy-independent branching ratio formalism used in NUC-FRG2, but also deuteron, triton, helion, and alpha-particle emission. These particles can contribute significantly to total exposure estimates. In this work, photonuclear cross sections are calculated using WE theory and the energy-independent branching ratios used in NUCFRG2 and then compared to experimental data. It is found that the WE theory gives comparable but mainly better agreement with data than the energy-independent branching ratio. Furthermore, EMD cross sections for single neutron removal are calculated using WE theory and an energy-independent branching ratio used in NUCFRG2 and compared to experimental data. [ABSTRACT FROM AUTHOR]

The radiative characteristics and the decay properties of a Λ-type excited atom embedded in anisotropic photonic crystals are studied. It is found that the dynamic properties of the atom and the propagating characteristics of the emitted field are apparently influenced by the relative position of the upper level from the forbidden gap and the two lower levels. By Von Neumann measurement, the effects of measurements on the decaying atom are investigated. The frequent projection on the excited state can lead to decay suppression or an acceleration effect at rather low repetition rates, depending on the frequency of measurements and the relative position of the upper level from the photonic crystals band edge. [ABSTRACT FROM AUTHOR]

We create mechanically stable (MS) packings of bidisperse disks using an algorithm in which we successively grow or shrink soft repulsive disks followed by energy minimization until the overlaps are vanishingly small. We focus on small systems because this enables us to enumerate nearly all distinct MS packings. We measure the probability of obtaining an MS packing at packing fraction φ and find several notable results. Firstly, the probability is highly non-uniform. When averaged over narrow packing fraction intervals, the most probable MS packing occurs at the highest φ and the probability decays exponentially with decreasing φ. Even more striking, within each packing-fraction interval, the probability can vary by many orders of magnitude. By using two different packing-generation protocols, we show that these results are robust and the packing frequencies do not change qualitatively with different protocols. [ABSTRACT FROM AUTHOR]

We derive an energy landscape interpretation of dielectric relaxation times in undercooled liquids, comparing it to the traditional Debye and Gemant-DiMarzio-Bishop pictures. The interaction between different local structural rearrangements in the energy landscape explains qualitatively the recently observed splitting of the flow process into an initial and a final stage. The initial mechanical relaxation stage is attributed to hopping processes, the final thermal or structural relaxation stage to the decay of the local double-well potentials. The energy landscape concept provides an explanation for the equality of thermal and dielectric relaxation times. The equality itself is once more demonstrated on the basis of literature data for salol. [ABSTRACT FROM AUTHOR]

The paper reports an investigation of the dissociative recombination of PD2+ at the heavy-ion storage ring CRYRING. The absolute cross-section has been measured as a function of centre-of-mass energy ranging from 1meV to 0.1 eV. The experiment performed has shown the dissociative recombination of PD2+ to be dominated by three-body break-up, with a branching ratio of about 78%. Competition between the available three-body channels producing the ground state, P(4 S ), and the first two excited states, P(2 D ) and P(2 P ), is observed. The formation of the first excited state dominates over the other two almost equally probable channels with about 75% of all three-body events. The results indicate that the kinetic energy released in the three-body break-up of PD2+ is randomly shared between the deuterium atoms. The intra-molecular angle on dissociation has also been investigated. A comparative analysis of the dissociative recombination dynamics for the two isovalent systems, PD2+ and NH2+ , is undertaken. [ABSTRACT FROM AUTHOR]

The mesotrons, or mesons, were the first elementary particles observed to be inherently unstable. This essay offers a reconstruction of the stream of researches related to mesotron decay, and examines how these researches shaped some of the basic concepts and practices of the emerging field of particle physics. Mass measurements could not settle the question of whether the mesons were a homogeneous kind of particles or an assortment of particles with different masses. The assumption of a single mass prevailed not on experimental grounds but because the mesons were identified tentatively with the carriers of the nuclear force according to a theory formulated by Hideki Yukawa. The identification gained currency because it entailed the prediction of meson decay, and thereby upheld the promise of a unified explanation of nuclear and cosmic-ray phenomena. In turn, the observation of decay and the measurement of the mean lifetime created the conditions for investigating the nuclear interactions of mesons at rest. Interest in these interactions was heightened, immediately after WWII, by the prospect of building and using accelerators to acquire knowledge about fundamental nuclear processes. Using decay to study nuclear capture, however, led to the realization that there exist not only different kinds of mesons but also two nuclear forces. [ABSTRACT FROM AUTHOR]

This work is devoted to an analysis of thermal decay processes of the stable colour centres (CC) created in gadolinium gallium garnet (GGG) Gd 3 Ga 5 O 12 , crystals under irradiation by gamma-quanta ( E =1.25 MeV, D =10 6 Gy). An analysis of the model of CC accumulation under irradiation as well as an isochronous heating model is presented. The decay activation energy (Δ E ) for O - and F centres in GGG crystals is calculated in accordance with this model. [ABSTRACT FROM AUTHOR]

Ultrafast pump-probe measurement was applied to investigate the excited state deactivation dynamics of bisimidazolyl radical, 1,4-bis-(4,5-diphenylimidazol-2-ylidene)cyclohexa-2,5-diene (BDPI-2Y). BDPI-2Y is non-fluorescent and the recovery of the ground state took place in the time scale of femtoseconds to picoseconds, indicating a direct ultrafast nonradiative decay from the excited state. Coherent intermolecular vibrations were also observed and the low-frequency mode at 150 cm -1 and 128 cm -1 had a phase shift of ~ p compared to the mode at 290 cm -1 . [ABSTRACT FROM AUTHOR]

Purpose: γH2AX foci formation was investigated after γ irradiation and after accumulating 125IdU decays to study the DNA double strand break (dsb) damage repair response in human breast cancer cells, MCF-7. Materials and methods: Confocal laser scanning microscopy (CLSM) was used to detect γH2AX foci formed in response to DNA dsbs induced by 0, 0.5, 1, 2 and 5 Gy γ irradiation and 125IdU decays accumulated at -90°C in human breast cancer cells, MCF-7, 125IdU treated cells were labeled with 4 different concentrations of 125IdU and then accumulated decays for 6, 19 or 35 days. γH2AX foci formation time for all experiments was 1 hour at 37°C. Visual confirmation of γH2AX foci was achieved by digital imaging (histogram analysis or profile analysis) and by standardizing the scored number of foci. The average numbers of γH2AX foci formed per cell after γ irradiation or accumulated 125IdU decays were determined by counting red voxels or counting γH2AX foci in propidium iodide (PI) counterstained nuclei by eye in optically sectioned cells. Results: Control, unirradiated MCF-7 cells had an average of 1.7 γH2AX foci per cell and an average of 32 γH2AX foci were scored for cells irradiated with 1 Gy γ rays. The data for doses up to approximately 1 Gy was a good linear fit (r² = 0.97} indicating that the assay is sensitive to low doses of γ rays. The average number of γH2AX foci scored in control cells that were frozen and thawed but not irradiated (= 2.3) was not statistically significantly different from controls that were not frozen and thawed. The average number of γH2AX foci was linearly related (r² = 0.98) to low numbers (< 200 decays/cell) of 125IdU decays indicating that the assay is also sensitive to low numbers of accumulated 125IdU decays. At 125I decays greater than 200 decays/cell, the average number of γH2AX foci plateaued. Regression analysis of the data for 0-140 125IdU decays per cell was used to calculate the rate of γH2AX foci formation (= 0.26 foci per 125I decay). Conclusions: The γH2AX foci formation assay is sensitive to low doses of γ rays and accumulated 125I decays. When 125IdU decays were accumulated at -90°C (to overcome confounding DNA damage repair processes that occur during simultaneous 125IdU incorporation and decay accumulation at 37°C), 0.26 γH2AX foci were formed per 125IdU decay. Methods used to incorporate 125 decay may modulate the number of γH2AX foci scored in cells. [ABSTRACT FROM AUTHOR]

Purpose: To assess DNA breakage by DNA-associated decay of the Auger electron emitter 125I, using an improved analytical approach. Material and Methods: Breakage of pBR322 plasmid DNA following incubation with DNA-binding ligand Hoechst 33258 labelled with 125I was studied by measuring the conversion of native supercoiled to relaxed and linear forms. The analytical approach that was developed considers two distinct sources of breakage: DNA damage at the site of DNA-associated Auger decay (local effect), and DNA damage from the radiation field created by all decay events in the incubation volume (external breakage). In addition, to enable application of Poisson statistics to local breakage events, relaxation events and linearisation events are considered rather than single-stranded and double-stranded breaks. Results: The results indicated that a decay event of DNA-associated I induced relaxation and linearisation of the plasmid molecule with probabilities of 0.090 ± 0.0035 and 0.82 ± 0.04, respectively. Addition of a radical scavenger DMSO increases the probability of relaxation to 0.15 ± 0.02 and reduces the probability of linearisation to 0.65 ± 0.05. Conclusions: Accurate evaluation of the local effect in plasmid breakage requires consideration of external breakage, the contribution of which depends on experimental conditions. The highly efficient production of linearisation events by 125I-decay restricts the detection of relaxation events by the plasmid assay. [ABSTRACT FROM AUTHOR]

Molecular dynamics (MD) method is used to investigate time features of the transition of a superheated ideal crystal into liquid. Different levels of statistical description of the process are used. An approach to the calculation of the lifetime of a metastable crystal is developed. The homogeneous nucleation rate dependence on temperature is calculated. The decay under conditions of the constant rate heating is simulated as well. The limits of predictability of the MD method arising from chaotic properties of many-particle systems are shown for the problem concerned. [ABSTRACT FROM AUTHOR]

In the context of studies on long-time storage of irradiated spent fuel, molecular dynamics simulations have been carried out in order to understand the physical phenomena, on the atomic scale, linked to modifications and damage that the uranium dioxide structure undergoes during α-decay irradiation in repository conditions. Simulations of atomic displacement cascades over an energy range from 1 to 20 keV for the initial primary knock-on atom (PKA) do not show any amorphization of the structure in agreement with what has been found experimentally, and there is very little correlation between the initial orientation of the PKA and the cascade morphology. The number of Frenkel pairs, as a function of the initial energy of the PKA, exhibits a power-law behaviour with an exponent of 0.9 which is contrary to the theoretical linear Norgett-Robinson-Torrens law. Finally, for both species the vacancies have a tendency to aggregate and cluster near the core of the cascade while interstitial atoms are preferentially located at the periphery of the branches corresponding to subcascades. [ABSTRACT FROM AUTHOR]

Ludwig distributions are a generalization of circular Taylor distributions that generate radiation patterns with wide-angle side lobe levels decaying faster than for Taylor patterns. In this paper we show that by means of an appropriate optimization technique the zeros of Ludwig patterns can be perturbed so as to improve or modify pattern and/or aperture distribution characteristics without altering the wide-angle decay behavior. The examples presented include footprint patterns, pencil beam patterns with individually controlled side lobe heights, footprint beams generated by real excitations, and pencil beam patterns with aperture distributions that are smoother than the original Ludwig distribution. [ABSTRACT FROM AUTHOR]

The response observed after application of a large-area pulse to an inhomogeneously broadened spin system is analysed in this paper. Particular attention has been devoted to the elucidation of the complex physical mechanisms underlying the single-pulse experiment. The theoretical analysis has been split into three steps. First, the evolution of a generic isochromatic subset of the spin system is calculated by the density-operator formalism. Then, general properties of the macroscopic response are derived from the structure of the expressions describing the coherent signal emitted by each isochromatic subsystem. Finally, numerical simulations of the response are carried out for a large variety of experimental conditions and are explained by reference to the developed theory. It is found that the response comprises three partial signals, namely two free induction decays (FIDs) and one true single-pulse echo, resulting from three different coherence transfer pathways occurring during the experiment. The very first part of the response is the FID arising from the spins which have been locked by the pulse. The other FID is usually negligible. The variety of patterns observed in the response is due to the widely different shapes taken on by the single-pulse echo when the experimental conditions are varied. For off-resonance excitation, the echo is an oscillating signal with a smooth bell-shaped envelope, well separated in time from the locked FID. In the case of on-resonance excitation, the echo is a long oscillating signal, which is already known as oscillatory FID. [ABSTRACT FROM AUTHOR]

The photo induced three body decay : ABC hnu A B C, where a molecule ABC decays into three fragments A, B and C upon irradiation, is reviewed. Various experimental and theoretical techniques for the investigation of this reaction and their application to a wide range of molecular species are discussed. Emphasis is laid on the distinction between concerted and stepwise processes, consisting of one single or two consecutive kinetic events, respectively. The concerted fragmentation scheme is further classified as being of either synchronous or asynchronous character, depending on whether or not the bond breaking processes take place in unison. The three body decays of acetone, azomethane and s-tetrazine are discussed in detail as prototypes for these mechanisms. A novel kinematic analysis approach, based on the evaluation of fragment kinetic energy distributions, is presented and applied to the ultraviolet photodissociation of phosgene. Competing pathways are found to be operative, dominated by the asynchronous concerted mechanism with preferential forward scattering of the carbon monoxide fragment. The synchronous concerted decay plays a minor role under significant excitation of the in-plane and out-of-plane bending modes of the parent molecule. Finally the power of the newly developed method for the analysis of the three body decay of a small polyatomic molecule is highlighted. [ABSTRACT FROM AUTHOR]