Multiparticle Detector Research Articles

Nuclear neck-density determination at Fermi energy with CHIMERA detector

Binary collisions of either stable or radioactive heavy ions at Fermi energies allow to study the nuclear reaction mechanisms under dynamical conditions of non-equilibrium and the formation of transient pieces of nuclear matter of very short mean life times at sub-normal density. An important role in the evolutionary phase of the collision is played by the gradient of the nuclear density affecting the isospin asymmetry of the reaction products by typical transport phenomena such as the isospin diffusion and drift. Experimental determination of the value of the nuclear matter density in the early phase of the collision is a crucial step towards understanding the underlying mechanism responsible for the production of clusters. In this paper a method for evaluating the nuclear density in semi-peripheral collisions in the reaction $$^{124}Sn + ^{64}Ni$$ at 35 MeV/nucleon as studied with the CHIMERA multi-particle detector is described.

Моделирование и анализ основных характеристик внутренней трековой системы многофункционального детектора частиц MPD методом Монте-Карло

Моделирование и анализ основных характеристик внутренней трековой системы многофункционального детектора частиц MPD методом Монте-Карло

Fragmentation patterns of multiply ionized hydrocarbons in collisions with swift ions

The multiple ionization and fragmentation of methane and chloromethane was studied in collisions with swift projectile ions utilizing a position- and time-sensitive multi-particle detector. The applied coincidence mapping technique allows a detailed study of the characteristic fragmentation patterns. Even more information about the fragmentation dynamics can be derived from the measured momenta of coincident fragment ions. The measured kinetic energy release spectra of fragmenting CH 4 and CH 3Cl dications significantly depend on the projectile ion and the collision velocity.

Ion-impact induced excitation and fragmentation of C 60

We studied the multiple ionization and fragmentation of C 60 in collisions with 50– 300 keV H + , He +, Ne +, and 6.5– 300 keV·z Ar z+ ( z=1–3) ions using a time- and position-sensitive multi-particle detector. Several aspects of C 2 evaporations, fullerene fission and multi-fragmentation processes are discussed. For Ar ++C 60 the multi-fragmentation patterns depend on the projectile velocity and especially for slow ions strong variations occur. This behaviour is in accordance with recent non-adiabatic quantum molecular dynamic (NA-QMD) studies (Phys. Rev. Lett. 86 (2001) 5258).

Multiple ionization and fragmentation of CH4 in collisions with slow highly charged ions

The multiple ionization and fragmentation of methane was studied in collisions with slow highly charged Xe-ions utilizing a position- and time-sensitive multi-particle detector. The coincident measurement of the momenta of correlated fragment ions allows a detailed analysis of the fragmentation kinematics and the kinetic energy release can be derived for each break-up into charged fragments. Among the observed processes the spectra for H++CH3+ and H2++CH2+ showed the highest sensitivity to projectile characteristics.

Orientation dependence of N2 and O2 multiple ionization in slow and fast collisions with highly charged Xe-ions

The influence of the molecular orientation on the multiple ionization of N2 and O2 was studied in collisions with highly charged Xe-ions at velocities of 0.2–0.3 and ≈15 a.u. The applied technique is based on a position- and time-sensitive multi-particle detector which allows to determine the ionization cross-section as a function of the orientation of the molecular axis. At lower collision energies, e.g. in collisions with 360 keV Xe18+, no anisotropy was found, whereas in collisions with 770 MeV Xe18+ significant orientation effects appear at higher degrees of ionization.

A Charged-Particle Detector Based on Proportional Tubes with a Segmented Cathode and Cathode Readout

A charged-particle detector based on proportional tubes with cathode readout is described. The cathode is sectionalized into 2.2 × 2.2-cm pads. The detector was tested on high-energy particle beams. At an anode-to-cathode distance of 1.1 cm, the spatial resolution was 1.25 mm along the anode wire. A simulation model of the detector on the basis of the measured pad-response function was proposed, compared to the experimental data, and used to calculate the two-track resolution. The detector can be used in various experiments as a large-area multiparticle two-coordinate detector.

Kinetic energy release distributions in the Coulomb explosion ofN2molecules induced by fast, highly-charged-ion impact

The dissociation of molecular ions $({\mathrm{N}}_{2}{)}^{q+}$ produced by 4.7 MeV/amu ${\mathrm{Bi}}^{57+,25+}$ and 5.9 MeV/amu ${\mathrm{Xe}}^{43+,18+}$ ion impact on ${\mathrm{N}}_{2}$ has been studied using the coincidence technique with a position- and time-sensitive multiparticle detector. The kinetic energy release distributions for all observed ion pairs ${\mathrm{N}}^{{q}_{1}+}+{\mathrm{N}}^{{q}_{2}+}$ are measured. Analysis of the distributions is made with emphasis on the highly charged-molecular ions, $q>3.$ In these cases the most probable total kinetic energy of the two fragments is quite well described by the Coulomb explosion model for point charges, while the measured width of the distributions is much larger than that predicted by this simplified model. We therefore suggest a more elaborate model of Coulomb fragmentation based on a statistical description of the individual potential energy curves of the molecular ion. This enables us to describe both the position and the width of the kinetic energy release distributions. We suggest that the main cause of the large width of the distributions for the high q values is the spread of the potential energy curves at the equilibrium internuclear distance; this is mainly due to the dispersion interaction arising from the mutual polarization of the atomic ions.

Photoelectron–multiple-photofragment coincidence spectrometer

A new photoelectron–photofragment-coincidence spectrometer is described. Using a multiparticle time- and position-sensitive detector, this apparatus allows the study of dissociation processes of negative ions yielding three photofragments in coincidence with a photoelectron. The photoelectron spectrometer uses two detectors and works in time of flight mode, detecting 10% of the photoelectrons with an energy resolution of 5% at 1.3 eV as shown in studies of the photodetachment of O2−. A third detector is used for collection of multiple photofragments (up to 8) in coincidence. This multiparticle detector uses a crossed-delay-line anode and fast timing signals to encode the time- and position-of-arrival of multiple photofragments. The detector was demonstrated to record all three particles produced in a single three-body dissociation event, yielding an energy resolution of ≈15% ΔE/E at 0.7 eV in experiments on the three-body dissociative photodetachment of O6−.

Multiple Ionisation and Fragmentation of Molecules

The multiple ionisation and dissociation of molecules, e.g. H2, N2, H2O, CH4 and C60, by fast ions was studied using a position- and time-sensitive multi-particle detector. The data obtained allow a clear separation of various reaction channels. Of special interest are the ‘Coulomb explosion’ processes like N2 → Nq+ +Nn+ or H2O → H+ + H+ +On+. For these reactions the coincident measurement of the momenta of correlated fragment ion yields a kinematically complete image of the molecular break-up process, and the fragmentation energy as well as angular correlations can be derived for each individual event. In the case of H2 as target molecule the CE model describes the fragmentation energy rather well, whereas in the case of more complex molecules, e.g. N2, CO and CH4, the simple CE model is insufficient to explain the measured energy and angular spectra. Better agreement was achieved with ab initio MCSCF calculations which take into account several molecular states of the fragmenting highly charged molecular ion.

Ion-impact Induced Multiple Ionization and Fragmentation of N2

The multiple ionization and fragmentation of N2 in collisions with fast H+, He+, Bi25+, and Bi57+ ions was studied using a position- and time-sensitive multi-particle detector. By the coincident measurement of correlated fragment momenta a kinematically complete image of the fragmentation process can be obtained for each individual event. Of special interest are the “Coulomb explosion” processes N2 → Nq+ + Np+ where the dissociation energy and the ionization cross section as a function of the orientation of the molecular axis can be derived.

Multifragmentation of molecules and clusters

The multiple ionization and fragmentation of small polyatomic molecules and clusters,e.g. H2O, CH4, and C60 by fast H+, He+, and Oq+ ions were studied utilizing a position- and time-sensitive multiparticle detector which allows the coincident measurement of the momenta of correlated fragments. Thereby, akinematically complete image of the molecular break-up can be derived for each individual event. Of special interest are the “Coulomb explosion” processes, like H2 O→ H++ H++Oq+, where the measured dissociation energies and angular correlations show significant deviations from the pure Coulomb explosion model. For larger molecules and clusters, new phenomena may be expected. For example, using a classical molecular dynamics simulation, we have studied the break-up of hot Arn (n ≥ 100) clusters. For initial temperatures around 500 K various analytical tools consistently indicate critical behaviour in the expansion phase.

Orientation Effects in Multiple Ionization of Molecules by Fast Ions

The multiple ionization and dissociation of ${\mathrm{N}}_{2}$ molecules by fast ${\mathrm{He}}^{+}$ ions was studied by the coincident measurement of correlated fragment-ion momenta using a position- and time-sensitive multiparticle detector. The dependence of the cross section on the alignment of the molecular axis with respect to the beam axis was determined. The results show that for higher degrees of ionization the cross section has a pronounced minimum for molecules oriented perpendicular to the beam. The quantitative interpretation of the observed results is based on an extended version of the statistical energy deposition model.

How molecules and clusters explode

The multiple ionization and fragmentation of small molecules, e.g. N 2 and H 2O, by fast H +, He +, Ar q+ and O q+ ions was studied utilizing a position- and time-sensitive multi-particle detector which allows the coincident measurement of the momenta of correlated fragments. Thereby, a kinematically complete image of the molecular break-up can be derived for each individual event. Of special interest are the “Coulomb explosion” processes like N 2 → N p+ + N q+ or H 2O → H + + H + + O q+. In these cases the pure Coulomb explosion model — though successful in case of H 2 — turns out to insufficient to explain the measured energy and angular spectra. In case of H 2O better agreement is achieved with ab initio MCSCF calculations of the intermediate H 2O ( q+2)+ parent ion.

Ion-impact-induced fragmentation of water molecules

The multiple ionization and fragmentation of H 2O by fast H +, He +, O 6+ and O 7+ ions was studied utilizing a position-and time-sensitive multi-particle detector which allows the coincident measurement of the momenta of correlated fragments. Thereby, the fragmentation energy and angular correlations for each individual event can be derived. Of special interest are the processes H 2O → H + + H + + O q+ , where a simple Coulomb-explosion model is insufficient to explain the measured energy and angular spectra. Better agreement was achieved with ab initio MCSCF calculations which take into account several molecular states of the fragmenting highly charged H 2O ( q+2)+ ion.

3D imaging of the collision-induced Coulomb fragmentation of water molecules.

The multiple ionization and fragmentation of ${\mathrm{H}}_{2}\mathrm{O}$ by fast ${\mathrm{H}}^{+}$ and ${\mathrm{He}}^{+}$ ions was studied using a position- and time-sensitive multiparticle detector. By the coincident measurement of correlated fragment momenta a complete three-dimensional image of the breakup process can be obtained for each individual event. Of special interest is the process ${\mathrm{H}}_{2}\mathrm{O}\ensuremath{\rightarrow}{\mathrm{H}}^{+}+{\mathrm{H}}^{+}+{\mathrm{O}}^{+}$ where a simple Coulomb explosion model is insufficient to explain the measured fragmentation energies and angular correlations. Better agreement is achieved with ab initio multiconfiguration self-consistent field calculations of the intermediate ${\mathrm{H}}_{2}{\mathrm{O}}^{3+}$ parent ion.

Measurement of γp→K + Λ and γp→K + Σ 0 at photon energies up to 1.47 GeV

The reactions γp→K + Λ and γp→K + Σ0 have been measured with the multiparticle detector system SAPHIR at ELSA in Bonn. Besides the differential cross sections the Λ polarization and, for the first time, the Σ0 polarization have been determined in a photon induced reaction. All data are presented as functions of the photon energy (from threshold up to 1.47 GeV) and of the kaon production angle (0°–180°). The polarization of both Λ and Σ0 is substantial at all energies and varies strongly with the production angle.

A system for correlated fragment detection in dissociation experiments

We describe a detector system for the study of the dissociation dynamics of small molecules, clusters or molecular ions. The system is based on a two-dimensional multiparticle detector which allows the position-sensitive and time-correlated detection of all fragments produced in one particular dissociation process. As a test example we discuss the double ionization and ensueing fragmentation of H 2 by fast H + and He + ions, in which case our detector allows an absolute measurement of the dissociation energy of the two correlated H +-fragments. The results are in excellent agreement with the prediction of a Franck-Condon calculation.

A system for Coulomb explosion imaging of small molecules at the Weizmann Institute

A description of a system for molecular structure imaging at the Weizmann Institute is presented. A novel method of controlled laser beam photo detachment inside the high voltage terminal of a tandem accelerator, enabling the study of neutral fast molecules by the Coulomb Explosion Imaging technique, is described. Also, a new type of three dimension multiparticle detector is presented.

A new type of multiparticle imaging detector

The foil-induced dissociation of molecular ions can provide a direct image of the stereochemical structures of small molecules. Such experiments require high-resolution measurements of the relative momenta of all dissociation fragments. In order to improve and extend previous studies of this type to a larger variety of molecules and clusters, we have developed a new type of low-pressure multiwire proportional counter. The novel feature of this large-area (25×50 cm2 ) multiparticle detector is that all the signals are recorded from a single two-level printed circuit board which serves as the anode. This segmented anode consists of three families of nonintersecting ‘‘wires’’ which are interwoven on the rear surface of the board. By redundantly determining positions and arrival times, the new segmented anode multiparticle detector provides good time resolution (100–200 ps) and high spatial resolution (0.2 mm) for detecting as many as six simultaneous particles.