Mean Emission Time Research Articles

Fusion and decay in 24Mg+ 12C at [formula omitted] MeV

Complete events with at least 83% of the total charge of the 24Mg+ 12C system has been investigated at 45 A MeV with a large multidetector array. Central single-source events are selected by use of the Statistical Discriminant Analysis Method. Two-fragment reduced-velocity correlation functions are measured for the emission of intermediate-mass fragments from single-source events. Many-body trajectory calculations with a surface sequential model indicate an emission time of ≈400 fm/ c suggesting that a large part of the single-source data can be described in a sequential decay picture. At the highest reconstructed excitation energy limit (≈8–10 A MeV), a mean emission time of ≈200 fm/ c is deduced showing that part of the data may be explained by an instantaneous multifragmentation scenario.

Emission time scales for light charged particles from symmetric and asymmetric fission processes for40Ar+197Aureactions at 25 MeV/nucleon

Two-particle correlation functions at small relative momenta were measured in the binary fission processes for ${}^{40}\mathrm{Ar}{+}^{197}\mathrm{Au}$ reactions at 25 MeV/nucleon. Compared to trajectory calculations the mean emission times of light charged particles were determined from two-particle correlation functions. The emission times varied weakly with the mass of the particles, but strongly with the kinetic energy of the particles. A slightly shorter emission time was determined for the particles emitted in the fission plane compared to out of the fission plane due to preferential, in-plane preequilibrium emission with shorter emission time. The emission time of light particles was nearly independent of the mass asymmetry of the fission fragments, depending mainly on the degree of equilibration in the emitting nuclei. A very short emission time of less than 100 fm/$c$ was deduced for the preequilibrium emission of high-energy particle pairs with kinetic energy $E>~$20 MeV/nucleon while a long emission time of about 600--1000 fm/$c$ was deduced for emission of low-energy particles with $E<~$9 MeV/nucleon. Such a long time suggests evaporation for these low-energy particles from thermalized compound nuclei or fission fragments.

The correlation functions and emission time scales for light particles and light fragments in 40Ar+ 197Au reactions at 25 MeV/nucleon

Two-particle correlation functions were measured for the emission of protons, deuterons, tritons, α, and lithium fragment at small relative momenta in 40Ar-induced reactions on 197Au at 25 MeV/nucleon. Based on three-body trajectory calculations the emission time scales for the particles were extracted from p-d, d-d, t-t, and lithium-lithium correlation functions. The mean emission time was found to decrease with increasing sum of the kinetic energies of the particle in a correlated pairs, indicating the emission of more energetic particles at earlier stages of the reaction than at later stages. The time scale for the emission of lithium fragments emission suggests that sequential binary disassembly occurs in 40Ar+ 197Au reaction at MeV/nucleon. Mean emission temperature of 3.5 −0.8 +1.3 and 3.6±0.4 MeV were also extracted from the relative populations of the excited and ground states of the respective emitted nuclides 8Be and 4He.

Evolution from Sequential Decay to Simultaneous Multifragmentation with Increasing Beam Energy

Two-fragment correlation functions have been measured for 40Ar+197Au collisions at E/A = 25 MeV. Based on the three-body trajectory calculation, a mean emission time of τ similar 450 fm/c for light fragments was extracted from the measured correlation functions. This result suggests that the sequential decay occurs at this energy.

Emission times for energy selected 1,2,3H ejectiles from central collisions: 1360 MeV 40Ar+Ag.

For central collisions of 1360 MeV $^{40}\mathrm{Ar}$ + Ag we report correlations in relative momentum and in velocity difference for $^{1,2,3}\mathrm{H}$ ejectiles at 32\ifmmode^\circ\else\textdegree\fi{} and 68\ifmmode^\circ\else\textdegree\fi{}. Comparison to trajectory calculations gives a measure of the mean emission times, and, for unlike pairs, the average emission orders. There is a strong variation of the average emission times as a function of ejectile energy that is very similar for each H isotope; they change from \ensuremath{\gtrsim}1000 fm/c for \ensuremath{\sim}10 MeV in the c.m. to \ensuremath{\lesssim}50 fm/c for \ensuremath{\gtrsim}30 MeV. This indicates a broad spectrum of emission sources and associated characteristics. The longer times suggest evaporative emission from thermalized systems for $^{1,2,3}\mathrm{H}$ of energies \ensuremath{\sim}10 MeV (i.e., those near the emission barrier). The shorter times, along with the observed energy spectra, suggest extensive prethermalization or direct emission from the central collision zone for $^{1,2,3}\mathrm{H}$ ejectiles of much higher energy.

Space-time characteristics of fragment emission in the E/A=30 MeV 129Xe+natCu reaction.

Intermediate-mass-fragment emission has been studied in central {ital E}/{ital A}=30 MeV {sup 129}Xe+{sup nat}Cu reactions. The measured fragment multiplicities, reduced-velocity correlation functions, and emission velocities have been compared with schematic three-body trajectory calculations and with three statistical models with input based upon a dynamical BNV code. The statistical models which include expansion either explicitly or implicitly are able to generate a sufficient number of fragments. The three-body trajectory calculations indicate a mean emission time of {approx}200 fm/{ital c}, consistent with sequential decay. Dynamical expanding-emitting source calculations predict a similar time scale for fragment emission and give satisfactory agreement with the experimental correlation functions if the experimental angular distributions are incorporated into the model. The Berlin multifragmentation model gives good agreement with the experimental charge distributions, and, depending upon the choice of radius parameter, can provide agreement with either the correlation functions or the fragment emission velocities, but not with both simultaneously. Although an overall good agreement is obtained in the statistical model comparisons, even in the most violent collisions the angular distributions and fragment emission velocities are incompatible with completely equilibrated decay from a single source.

Time scale for multifragmentation in intermediate energy heavy-ion reactions.

Fragment-fragment correlations are used to probe the spatial-temporal extent of the emitting source in central $^{36}\mathrm{Ar}$${+}^{197}$Au reactions at E/A=35, 50, 80, and 110 MeV. The experimental two particle correlations are compared both with the Koonin-Pratt two-body formalism as well as a three-body Coulomb trajectory calculation. The spatial-temporal extent of the emitting system decreases with increasing incident energy. Within the context of a three-body Coulomb trajectory model the mean fragment emission time rises sharply as a function of the assumed density of the system until \ensuremath{\rho}/${\mathrm{\ensuremath{\rho}}}_{0}$\ensuremath{\approxeq}0.3. If one assumes a fixed density, the extracted mean emission time decreases with increasing assumed charge of the emitting system. Assuming \ensuremath{\rho}/${\mathrm{\ensuremath{\rho}}}_{0}$\ensuremath{\approxeq}0.3 the mean emission time \ensuremath{\tau} according to calculations using a three-body Coulomb trajectory model, is \ensuremath{\approxeq}115--135 fm/c at E/A=50 MeV and \ensuremath{\approxeq}75--100 fm/c at E/A=110 MeV. Comparisons with a generalized N-body Coulomb trajectory model demonstrate that the effect of interactions with other emitted particles is negligible. The prediction of a microcanonical model which includes pre-emission correlations between the fragments is compared to the measured correlation function at E/A=110 MeV.

Dynamics of high index step equilibrium fluctuations as observed by scanning tunneling microscopy

We present experimental evidence for a dramatic influence on the aspect of tunneling images of the orientation relative to a [ 1 110] direction of the steps on vicinal copper surfaces. A model is proposed which relates these observations to the mobility of geometrical kinks of the step edges. This model allows one to deduce from the experimental data the mean emission time of an adatom from a kink on a step oriented near the [ 1 110] direction on Cu(100).

Pattern of Cercarial Emergence of Schistosoma curassoni from Niger and Comparison with Three Sympatric Species of Schistosomes

The emergence pattern of Schistosoma curassoni cercariae from Bulinus umbilicatus, whose adult worms parasitize bovine, caprine, and ovine ungulates in Niger, is of a circadian type with a mean emission time at 0855 hr +/- 1 hr 6 min, characteristic of the schistosome species parasitizing domestic or wild cattle. The comparison of this cercarial emergence pattern with those of the other 3 sympatric species of schistosomes (Schistosoma haematobium, Schistosoma bovis, and Schistosoma mansoni) shows a significant difference between the chronobiology of the cercariae infective for human and those infective for bovine hosts. This difference may improve epidemiological surveys based on snail prevalences by allowing the distinction between bulinids infected with human and bovine parasites.

A Monte Carlo reaction simulation for small-angle correlations between light charged particles

Nuclear reactions often proceed via fusion of projectile and target into a thermalized compound nucleus. For hot compound nuclei of temperature ≈1–5 MeV one expects that a chain of particles (neutrons, protons, alphas…) will be evaporated with a variety of time intervals of ≈10 -22-10 -20 s. The determination ofthese emission times is a major challenge for nuclear physicists. Such time intervals are too short to measure directly, but they do lead to final-state interactions between charged-particle pairs due to their mutual Coulomb repulsions. Therefore, one expects correlations between the charged-particle pairs that depend on their velocities and separation distances at birth, i.e. the flight distance of the first particle before birth of the second. The KALLIOPI program is a reaction simulation designed to predict the particle-particle correlations as a function of predicted or assumed values of the mean emission times. By comparison of the simulated results to experimental data one can test the basic predictions of decay rates for particle ejection from hot nuclear emitter.

Structural studies of tetrachloride liquids

The differential scattering cross-sections for neutrons scattering by liquid CCl4, SiCl4, TiCl4, GeCl4 and SnCl4 at 20°C have been obtained using the pulsed neutron beam from the Harwell Linac. The measured data at two scattering angles (150° and 90°) have been analysed using a molecular form factor with wavelength-dependent Debye-Waller factors. Molecular bond length parameters extracted from a x 2-fit to the observations are compared with data obtained by other techniques. Special consideration is given to the calibration of the momentum-transfer scale and the data are found to be sensitive to the variation of mean emission times for different wavelength neutrons leaving the moderator material. Correction for this effect gives an improved fit to the form factor but does not alter the fitted bond length values.