Fragment Kinetic Energy Spectra Research Articles

Small-angle fragmentation of carbon ions at 0.6 GeV/n: a comparison with models of ion-ion interactions

Momentum distributions of hydrogen and helium isotopes from 12C fragmentation at 3.5° were measured at 0.6 GeV/nucleon in the FRAGM experiment at ITEP TWA heavy ion accelerator. The fragments were selected by correlated time of flight and dE/dx measurements with a magnetic spectrometer with scintillation counters. The main attention was drawn to the high momentum region where the fragment velocity exceeds the velocity of the projectile nucleus. The momentum spectra of fragments span the region of the fragmentation peak as well as the cumulative region. The differential cross sections cover six orders of magnitude. The distributions measured are compared to the predictions of three ion-ion interaction models: BC, QMD and LAQGSM03.03. The kinetic energy spectra of fragments in the projectile rest frame have an exponential shape with two temperatures, being defined by their slope parameters.

CONTROLLING ABOVE-THRESHOLD DISSOCIATION BRANCHING RATIOS OF HD+ WITH FEMTOSECOND LASER PULSE TRAIN

Above-threshold dissociation (ATD) process of the molecular ions HD+ steered by a femtosecond laser pulse train (LPT) is investigated theoretically using the time-dependent quantum wave packet method. Energy-dependent distributions of ATD fragments are analyzed by using an asymptotic-flow expression in the momentum space. It is found that fragment kinetic energy spectra shift to low energy region with increasing pulse number of LPT. The photofragment branching ratio between the 1sσg and 2pσu dissociation channels is sensitive to the pulse number of LPT. The momentum distribution of the ATD fragments is discussed in detail.

Kinetic energy spectra for fragments and break-up density in multifragmentation

We investigate the possibility, in nuclear fragmentation, to extract information on nuclear density at break-up from fragment kinetic energy spectra using a simultaneous scenario for fragment emission. It is found that a decrease of peak centroids for kinetic energy spectra of fragments with increasing excitation energy can be observed at constant low density, which is different from recently published results of Viola et al. [V.E. Viola, K. Kwiatkowski, J.B. Natowitz, S.J. Yennello, Phys. Rev. Lett. 93 (2004) 132701].

Two characteristic volumes in thermal nuclear multifragmentation

The paper is devoted to the experimental determination of the space-time characteristics for the target multifragmentation in p(8.1GeV) + Au collisions. The experimental data on the fragment charge distribution and kinetic energy spectra are analyzed within the framework of the statistical multifragmentation model. It is found that the partition of hot nuclei is specified after expansion of the target spectator to a volume equal to Vt = (2.9 +- 0.2) Vo, with Vo as the volume at normal density. However, the freeze-out volume is found to be Vf = (11 +- 3) Vo. At freeze-out, all the fragments are well separated and only the Coulomb force should be taken into account. The results are in accordance with a scenario of spinodal disintegration of hot nuclei.

Nonadiabatic molecular response to short, intense laser pulses: a wave operatorgeneralized Floquet approach

A generalized Floquet formalism combined with Bloch’s wave operatormethodology is applied to the study of adiabatic versus sudden transportmechanisms in molecular responses to ultrashort and intense laser pulses. Themodel considers the laser pulse as a whole to be periodic, instead of theoptical cycle as in standard Floquet treatments. This leads to a betterunderstanding of the dynamics and a considerable simplification in termsof the number of generalized Floquet eigenstates needed to accuratelydescribe the dynamics. The method is illustrated for the phenomenon ofabove threshold dissociation and inelastic transitions in H2+ inthe presence of ultrashort (a few femtoseconds) and intense (of the order of TW cm−2)laser fields. It is found that, while the dissociation probabilities and fragmentkinetic energy spectra can be accurately described by a single generalized Floqueteigenstate within the adiabatic limit, an accurate description of inelastictransitions requires more than one generalized Floquet state, due to the role ofnon-adiabatic dynamics.

Perpendicular dissociation ofD2+in intense Ti:sapphire laser pulses

Detailed results of numerical one-dimensional wave-packet simulations are reported for the dynamics of a ${\mathrm{D}}_{2}^{+}$ molecule subjected to separated and combined 400-nm and 800-nm intense laser pulses with their polarization vectors lying either parallel or perpendicular to the internuclear axis of the molecule. Depending on the laser excitation conditions, the model employed involves from four to a maximum of eight electronic channels, comprising molecular states correlating, in the dissociative limit, with the $n=1$ shell of atomic hydrogen and those correlating with the $n=2$ shell. Preliminary results of calculations including molecular rotations within a two-dimensional model are also reported. Overall, calculated fragment kinetic-energy spectra, the main results of the present work, are in accord with the findings of recent experimental works [Talebpour et al., Phys. Rev. A 62, 042708 (2000); Vijayalakshmi et al., ibid. 053408 (2000)], which reported evidences of perpendicular dissociation induced by intense 400-nm and 800-nm laser pulses. These results are thus supportive of the hypothesis made therein of perpendicular transitions to upper molecular excited states, through a ${\ensuremath{\pi}}_{u}$ doorway. Deviations from the predictions of the Floquet schemes proposed in these works are discussed in terms of the laser-driven rotational dynamics of the molecular ion and in terms of bond-softening versus bond-hardening effects.

Thermal expansion effects in the 8 GeV/cπ−+197Aureaction

Fragment kinetic energy spectra for reactions induced by 8.0 GeV/c $\rm{\pi^-}$ beams incident on a $\rm{^{197}}$Au target have been analyzed in order to deduce the possible existence and influence of thermal expansion. The average fragment kinetic energies are observed to increase systematically with fragment charge but are nearly independent of excitation energy. Comparison of the data with statistical multifragmentation models indicates the onset of extra collective thermal expansion near an excitation energy of E*/A $\rm{\approx}$ 5 MeV. However, this effect is weak relative to the radial expansion observed in heavy-ion-induced reactions, consistent with the interpretation that the latter expansion may be driven primarily by dynamical effects such as compression/decompression.

Above-threshold dissociation in the long-wavelength limit

Detailed measurements of fragment kinetic energy spectra coming from the interaction between intense laser pulses and hydrogen and deuterium molecules have been made. Both the proton and the deuteron spectra exhibit a broad continuum distribution with a finer peak structure superimposed. These fragments are associated with the dissociation stage of the interaction. The broad continuum is understood using a simple two-step model in which the first stage involves the barrier-suppression dissociation of the molecular ion followed by a second step involving the interaction between the dissociated, charged fragment and the electric field. This model is analogous to the `Simpleman theory' (van Linden van den Heuvell and Muller 1988 Multiphoton Processes, Cambridge Studies in Modern Optics vol 8 ed S J Smith and P L Knight (New York: Camridge University Press)) which has been used in the interpretation of long-wavelength above-threshold ionization results (Gallagher 1988 61 2304, Corkum et al 1989 62 1259). The fine peak structure is related to the above-threshold dissociation picture. The deuteron spectra also contain a set of higher-energy fragments which are associated with a secondary, enhanced ionization process. The absence of corresponding higher-energy protons can be understood in the context of the proposed two-step dissociation model. The experimental spectra are also compared with the results of both quantum and semiclassical calculations; these agree qualitatively well.

Nonadiabatic response to short intense laser pulses in dissociation dynamics

Molecular nonadiabatic response to the sudden switching on (rise times of few femtoseconds) of intense (tens of TW/cm${}^{2})$ laser pulses may lead to some unexpected enhancements of dissociation rates. These effects are accounted for by the spreading of the molecule-plus-field wave packet over several resonances (instead of a single one predicted when the laser is adiabatically switched on) interfering during the excitation process. Typical time-resolved signatures of such highly nonlinear responses are thoroughly investigated in terms of the evolution of vibrational survival probabilities and fragment kinetic energy spectra of ${\mathrm{H}}_{2}^{+}$ molecular ion taken as an illustrative example. A plausible 532-nm wavelength single-photon dissociation scenario bringing into the system less energy than the minimum required for the fragmentation to occur is examined. The suggested mechanism, which may be termed below-threshold dissociation, as opposed to above-threshold dissociation, refers to a very sharp rise of the laser pulse resulting into temporal excitation of some resonances lying above the single-photon dissociation energy, with efficient decay rates.

Nuclear fragmentation in nucleus–nucleus reaction within a two-lattices percolation picture

It is shown that elemental multiplicity distributions, relative abundances of intermediate mass fragments as well as fragment kinetic energy spectra and charged-particle correlations in nucleus-nucleus induced multi-fragmentation reactions can be satisfactorily reproduced by site percolation procedure in the case that the occupation probability is taken explicitly as a function of the geometry of the collision.

A hot expanding source in 50 A MeV Xe + Sn central reactions

The INDRA multidetector has been used to study multifragmentation processes in central collisions for the Xe + Sn reaction at 50 A MeV. A single isotropic source formed at an excitation energy of 12 A MeV exhausting most of the emitted charged products has been isolated in such collisions. The fragment kinetic energy spectra indicate a fast disintegration of the system with a radial collective motion of about 2 A MeV. The light charged particle characteristics within this scenario are also discussed.

Temperatures of fragment kinetic energy spectra.

Multifragmentation reactions without large compression in the initial state (proton-induced reactions, reverse-kinematics, projectile fragmentation) are examined, and it is verified quantitatively that the high temperatures obtained from fragment kinetic energy spectra and lower temperatures obtained from observables such as level population or isotope ratios can be understood in a common framework.

Collective motion in selected central collisions of Au on Au at 150A MeV.

Using the FOPI facility at GSI Darmstadt complete data of Au on Au collisions at 150[ital A] MeV were collected for charged products ([ital Z]=1--15) at laboratory angles 1[degree][le][Theta][sub lab][le]30[degree]. Central collisions were selected by applying various criteria. The kinetic energy spectra of fragments from an isolated midrapidity source are investigated in detail for center-of-mass angles 25[degree][le][Theta][sub c.m.][le]45[degree]. The heavy products ([ital Z][ge]3) are used to determine the collective energy which is found to be at least 10[ital A] MeV.

The effects of the interfragment nuclear interaction on the kinetic energy spectra in the multifragmentation process

The importance of the interfragment nuclear interaction for the mass and isotopic yields and the kinetic energy spectra is shown. Ad hoc reduction in the Coulomb energies of the emitted fragments, usually resorted to in the description of fragment kinetic energy spectra, is accounted for by the introduction of the interfragment nuclear interaction in a statistical model. It is shown that the mass and isotopic yields and the kinetic energy spectra can be described in a consistent manner, using a single model with one temperature.

Selective detection of O(1S) following electron impact dissociation of O2 and N2O using a XeO* conversion technique

We have observed the metastable species O(1S0) following the dissociation of molecules by pulsed electron impact. A crossed beam apparatus was used to obtain time-of-flight spectra of the fragments. We have employed a novel detector which consists of a layer of freshly deposited Xe. O(1S0) atoms impinging on the surface quickly form XeO* excimers which rapidly decay (<1 μs) producing easily detected photons. Our present results indicate that this method is sensitive (with high quantum efficiency) solely to the 1S state of oxygen. Other metastables (such as 5S and 1D) and ground state oxygen atoms have not been detected. Low resolution optical spectra of the XeO emission reveal bands at 375, 550, and 725 nm. Time-of-flight and fragment kinetic energy spectra for both target gases at various electron impact energies are presented together with excitation functions from threshold to 1000 eV. These have been made absolute using a Bethe–Born calibration technique for N2O. O2 data are calibrated relative to N2O. Maximum cross sections for O(1S) production are 2.25×10−17 cm2 at 45 eV and 2.1×10−18 cm2 at 80 eV for N2O and O2 targets, respectively.

Expansion effects in intermediate energy heavy-ion reactions

Transverse kinetic energy spectra of fragments in the reaction 36Ar + 197Au are used to probe the multifragmentation mechanism at intermediate energies. The mass dependence of the average transverse kinetic energy is examined in the context of the statistical decay of an expanding, emitting source. The role of source expansion during emission is explored. Results suggest the onset of volume emission at the highest incident energy.

Nonperturbative wave packet dynamics of the photodissociation of H2+ in ultrashort laser pulses

The wave packet dynamics of the photodissociation of H2+ under excitation by laser pulses of short durations at 329.7 nm are studied. The photodissociation process involves essentially two coupled channels, and the detailed mechanism for the formation of fragment kinetic energy spectra is examined by following the evolution of structures in the coupled-channel wave functions in momentum space. These structures appear in the channels’ momentum wave functions at P≠0, as the v=0 ground vibrational state is promoted to the dissociative channel then accelerated. The variations of these structures reflect the interplay between local laser-induced transitions and the accelerating–decelerating action of intrinsic molecular forces. The wave packet dynamics are studied for rectangular and Gaussian pulses of varying durations and peak intensities. In addition, two forms of channel couplings were considered corresponding to two different choices of the gauge: the electric-field (EF) gauge, in which the matter–field interaction is of the length form and the radiation-field (RF) gauge, in which it is of the velocity form.

Analyzing powers and isotope ratios for the natAg(p

Analyzing powers and isotope ratios have been measured for ejectiles with Z\ensuremath{\le}7 emitted at forward angles in the 200-MeV p\ensuremath{\rightarrow}${+}^{\mathrm{nat}}$Ag reaction. The observed analyzing powers are consistent with zero, and thus do not provide evidence for a significant contribution from cluster knockout, or similar direct formation mechanisms. Fragment kinetic-energy spectra above the Coulomb peak are compared with a coalescence calculation. The isotopic composition of the elemental kinetic-energy spectra is found to favor N/Z\ensuremath{\ge}1 nuclei for fragment energies near the exit-channel Coulomb energy, whereas species with N/Z\ensuremath{\le}1 are more abundant in the high-energy spectral tails. This behavior is consistent with the predictions of an accreting source calculation.

Complex fragment emission from the 3He+ natAg system between 0.48 and 3.6 GeV

The emission of complex fragments (3⩽ Z≲12) from reactions of 3He with natAg has been studied at bombarding energies of E = 480, 900, 1800, 2700 and 3600 MeV. Between 900 and 1800 MeV slopes of the fragment kinetic energy spectra at backward angles undergo a change in character, becoming much flatter; in addition, the Coulomb peak is found to broaden significantly and shift to lower energies. Power-law fits to the fragment charge distributions result in decreasing values of the exponent τ up to a bombarding energy of 1800 MeV; at this value and beyond, a constant value of τ=2.1±0.1 is observed. Elemental cross sections at the highest energy are found to increase by approximately two orders of magnitude relative to data near 100 MeV. The data suggest a change in reaction mechanism between incident energies of 900 and 1800 MeV.

Complex fragments emitted in particle-stable states for the 32S+natAg reaction at E/A=22.3 MeV.

The emission of complex fragments was studied for $^{32}\mathrm{induced}$ reactions on $^{\mathrm{nat}}\mathrm{Ag}$ at E/A=22.3 MeV for angles back of the grazing angle, ${\ensuremath{\theta}}_{\mathrm{IMF}=20\mathrm{\ifmmode^\circ\else\textdegree\fi{}}--50\mathrm{\ifmmode^\circ\else\textdegree\fi{}}}$. The measured fragment kinetic energy spectra peak at energies near the exit channel Coulomb barriers and decrease exponentially at higher energies. The angular distributions are forward peaked, indicating emission prior to the attainment of full statistical equilibrium of the composite system. The relative populations of a large number of particle stable states of these fragments were measured with a spin spectrometer. Comparisons with statistical calculations which include feeding from known discrete particle unbound states and high-lying continuum states indicate emission temperatures of about 3--4 MeV.