arXiv subjects

S. J. Sanders

Publications and source records attributed to S. J. Sanders.

17 recordsLinked to original sources

arXiv2016

Rapidity and centrality dependence of particle production for identified hadrons in Cu+Cu collisions at $\sqrt{s_{NN}} = 200$ GeV

The BRAHMS collaboration has measured transverse momentum spectra of pions, kaons, protons and antiprotons at rapidities 0 and 3 for Cu+Cu collisions at $\sqrt{s_{NN}} = 200$ GeV. As the collisions become more central the collective radial flow increases while the temperature of kinetic freeze-out decreases. The temperature is lower and the radial flow weaker at forward rapidity. Pion and kaon yields with transverse momenta between 1.5 and 2.5 GeV/c are suppressed for central collisions relative to scaled $p+p$ collisions. This suppression, which increases as the collisions become more central is consistent with jet quenching models and is also present with comparable magnitude at forward rapidity. At such rapidities initial state effects may also be present and persistence of the meson suppression to high rapidity may reflect a combination of jet quenching and nuclear shadowing. The ratio of protons to mesons increases as the collisions become more central and is largest at forward rapidities.

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arXiv2011

Rapidity dependence of deuteron production in Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV

We have measured the distributions of protons and deuterons produced in high energy heavy ion Au+Au collisions at RHIC over a very wide range of transverse and longitudinal momentum. Near mid-rapidity we have also measured the distribution of anti-protons and anti-deuterons. We present our results in the context of coalescence models. In particular we extract the "volume of homogeneity" and the average phase-space density for protons and anti-protons. Near central rapidity the coalescence parameter $B_2(p_T)$ and the space averaged phase-space density $ (p_T)$ are very similar for both protons and anti-protons. For protons we see little variation of either $B_2(p_T)$ or the space averaged phase-space density as the rapidity increases from 0 to 3. However both these quantities depend strongly on $p_T$ at all rapidities. These results are in contrast to lower energy data where the proton and anti-proton phase-space densities are different at $y$=0 and both $B_2$ and $f$ depend strongly on rapidity.

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arXiv2009

Forward-rapidity azimuthal and radial flow of identified particles for $\sqrt{s_{NN}}$ = 200 GeV Au+Au collisions

A strong azimuthal flow signature at RHIC suggests rapid system equilibration leading to an almost perfect fluid state. The longitudinal extent of the flow behavior depends on how this state is formed and can be studied by measuring the pseudorapidity and transverse momentum dependence of the second Fourier component ($v_{2}(p_{T})$) of the azimuthal angular distribution. We report on a measurement of identified-particle $v_{2}$ as a function of $p_{T}$ (0.5-2.0 GeV/c), centrality (0-25%, 25-50%), and pseudorapidity ($0\leqη<3.2$) for $\sqrt{s_{NN}} = 200 \rm GeV$ Au+Au collisions. The BRAHMS spectrometers are used for particle identification ($π$, K, p) and momentum determination and the BRAHMS global detectors are used to determine the corresponding reaction-plane angles. The results are discussed in terms of the rapidity dependence of constituent quark scaling and in terms of models that develop the complete (azimuthal and radial) hydrodynamic aspects of the forward dynamics at RHIC.

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arXiv2007

Differential elliptic flow at forward rapidity in 200 GeV AuAu collisions

Identified particle elliptic flow results are presented for the Au+Au reaction at sqrt[s_{NN}] = 200$ GeV as a function of transverse momentum and pseudorapidity. Data at pseudorapidities eta approximately equal to 0, 1, and 3.4 were obtained using the two BRAHMS spectrometers. Differential v2(eta, pt) values for a given particle type are found to be essentially constant over the covered pseudorapidity range, in contrast to the integral v2 values which have previously been observed to decrease at forward rapidities. A softening of the particle spectra at forward angles is found to account for at least part of the integral v2 falloff. The data are found to be consistent with existing constituent quark scaling systematics.

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arXiv2007

Rapidity and pt dependence of identified-particle elliptic flow at RHIC

Elliptic flow has been measured by the BRAHMS experiment as a function of transverse momentum and pseudorapidity for the Au+Au reaction at sqrt[s_{NN}] = 200 GeV. Identified-particle v2 (eta, pt) values were obtained with the two BRAHMS spectrometers at pseudorapidities eta approximately equal to 0, 1, and 3.4. The results show that the differential v2(eta, pt) values for a given particle type are essentially constant over the covered pseudorapidity range. It is suggested that the dominant cause of the observed fall-off of the integral v2 values going away from mid-rapidity is a corresponding softening of the particle spectra .

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arXiv2006

Rapidity dependence of high p_T suppression at sqrt{s_{NN}}=62.4{GeV}

We present measurements of charged hadron p_T spectra from Au+Au collisions at sqrt(snn)=62.4 GeV for pseudorapidities eta=0, eta=1 and eta=3.2. Around midrapidity (eta=0, eta=1) we find nuclear modification factors at levels suggesting a smaller degree of high p_T suppression than in the same reaction at higher energy sqrt(snn)=200 GeV. At the high pseudorapidity, eta=3.2, where nuclear modification factors cannot be constructed due to the lack of p+p reference data, we find a significant reduction of R_{CP} (central to peripheral ratio) as compared to midrapidity.

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arXiv2002

Highly deformed $^{40}$Ca configurations in $^{28}$Si + $^{12}$C

The possible occurrence of highly deformed configurations in the $^{40}$Ca di-nuclear system formed in the $^{28}$Si + $^{12}$C reaction is investigated by analyzing the spectra of emitted light charged particles. Both inclusive and exclusive measurements of the heavy fragments (A $\geq$ 10) and their associated light charged particles (protons and $α$ particles) have been made at the IReS Strasbourg {\sc VIVITRON} Tandem facility at bombarding energies of $E_{lab} (^{28}$Si) = 112 MeV and 180 MeV by using the {\sc ICARE} charged particle multidetector array. The energy spectra, velocity distributions, and both in-plane and out-of-plane angular correlations of light charged particles are compared to statistical-model calculations using a consistent set of parameters with spin-dependent level densities. The analysis suggests the onset of large nuclear deformation in $^{40}$Ca at high spin.

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arXiv2001

Deformation effects in $^{56}$Ni nuclei produced in $^{28}$Si+$^{28}$Si at 112 MeV

Velocity and energy spectra of the light charged particles (protons and $α$-particles) emitted in the $^{28}$Si(E$_{lab}$ = 112 MeV) + $^{28}$Si reaction have been measured at the Strasbourg VIVITRON Tandem facility. The ICARE charged particle multidetector array was used to obtain exclusive spectra of the light particles in the angular range 15 - 150 degree and to determine the angular correlations of these particles with respect to the emission angles of the evaporation residues. The experimental data are analysed in the framework of the statistical model. The exclusive energy spectra of $α$-particles emitted from the $^{28}$Si + $^{28}$Si compound system are generally well reproduced by Monte Carlo calculations using spin-dependent level densities. This spin dependence approach suggests the onset of large deformations at high spin. A re-analysis of previous $α$-particle data from the $^{30}$Si + $^{30}$Si compound system, using the same spin-dependent parametrization, is also presented in the framework of a general discussion of the occurrence of large deformation effects in the A$_{CN}$ ~ 60 mass region.

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arXiv2001

Deformation effects in the $^{28}$Si+$^{12}$C and $^{28}$Si+$^{28}$Si reaction Search

The possible occurence of highly deformed configurations is investigated in the $^{40}$Ca and $^{56}$Ni di-nuclear systems as formed in the $^{28}$Si+$^{12}$C,$^{28}$Si reactions by using the properties of emitted light charged particles. Inclusive as well as exclusive data of the heavy fragments and their associated light charged particles have been collected by using the {\sc ICARE} charged particle multidetector array. The data are analysed by Monte Carlo CASCADE statistical-model calculations using a consistent set of parameters with spin-dependent level densities. Significant deformation effects at high spin are observed as well as an unexpected large $^{8}$Be cluster emission of a binary nature.

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arXiv2001

Search for emission of unstable $^8$Be clusters from hot $^40$Ca and $^56$Ni nuclei

The possible occurence of highly deformed configurations is investigated in the $^{40}$Ca and $^{56}$Ni di-nuclear systems as formed in the $^{28}$Si + $^{12}$C and $^{28}$Si + $^{28}$Si reactions, respectively, by using the properties of emitted light charged particles. Inclusive as well as exclusive data of the heavy fragments (A $\geq$ 6) and their associated light charged particles (p, d, t, and $α$-particles) have been collected at the IReS Strasbourg VIVITRON Tandem facility with two bombarding energies $E_{lab}(^{28}$Si) = 112 and 180 MeV by using the ICARE charged particle multidetector array, which consists of nearly 40 telescopes. The measured energy spectra, velocity distributions, in-plane and out-of-plane angular correlations are analysed by Monte Carlo CASCADE statistical-model calculations using a consistent set of parameters with spin-dependent level densities. Although significant deformation effects at high spin are needed, the remaining disagreement observed in the $^{28}$Si + $^{12}$C reaction for the S evaporation residue suggests an unexpected large unstable $^{8}$Be cluster emission of a binary nature.

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arXiv1999

Cluster Emission of $^8$Be in the $^{28}$Si+$^{12}$C Fusion Reaction at Low Temperature

Inclusive as well as exclusive energy spectra of the light charged particles emitted in the $^{28}$Si ($E_{lab}$=112.6 MeV) + $^{12}$C reaction has been measured using the {\bf ICARE} multidetector array. The data have been analysed by statistical-model calculations using a spin-dependent level density parametrization. The results suggest significant deformation effects at high spin and cluster emission of $^8$Be.

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arXiv1999

Vanishing spin alignment : experimental indication of triaxial $\bf ^{28}Si + {^{28}Si}$ nuclear molecule

Fragment-fragment-$γ$ coincidences have been measured for $\rm ^{28}Si + {^{28}Si}$ at an energy corresponding to the population of a conjectured resonance in $^{56}$Ni. Fragment angular distributions as well as $γ$-ray angular correlations indicate that the spin orientations of the outgoing fragments are perpendicular to the orbital angular momentum. This differs from the $\rm ^{24}Mg+{^{24}Mg}$ and the $\rm ^{12}C+{^{12}C}$ resonances, and suggests two oblate $\rm ^{28}Si$ nuclei interacting in an equator-to-equator molecular configuration.

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arXiv1999

Binary Decay of Light Nuclear Systems

A review of the characteristic features found in fully energy-damped, binarydecay yields from light heavy-ion reactions with $20\leq A_{target} + A_{projectile}\leq 80$ is presented. The different aspects of these yields that have been used to support models of compound-nucleus (CN) fission and deep-inelastic dinucleus orbiting are highlighted. Cross section calculations based on the statistical phase space at different stages of the reaction are presented and compared to the experimental results. Although the statistical models are found to reproduce most of the observed experimental behaviors, an additional reaction component corresponding to a heavy-ion resonance or orbiting mechanism is also evident in certain systems. The system dependence of this second component is discussed. The extent to which the binary yields in very light systems $(A_{CN} \leq 32)$ can be viewed as resulting from a fusion-fission mechanism is explored. A number of unresolved questions, such as whether the different observed behaviors reflect characteristically different reaction times, are discussed.

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arXiv1999

Role of deformation in the decay of $^{56}$Ni and $^{40}$Ca di-nuclei

Inclusive as well as exclusive energy spectra of the light charged particles emitted in the $^{28}Si(E_{lab}=112.6 MeV) + ^{28}Si,^{12}C$ reactions have been measured at the Strasbourg VIVITRON facility in the angular range 15^0 - 150^0, using the ICARE multidetector array. The experimental energy spectra of $α$-particles are generally well reproduced by the statistical model with a spin-dependent level density indicating the onset of defomations at high spin.

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arXiv1998

Spin-Alignment and Quasi-Molecular Resonances in Heavy-Ion Collision

Fragment-fragment-$γ$ triple coincidence measurements of the $^{28}$Si~$+$~$^{28}$Si reaction at E$_{\rm c.m.}$~=~55.8 MeV, carefully chosen to populate a well known quasi-molecular resonance in $^{56}$Ni, have been performed at the VIVITRON tandem facility by using the Eurogam Phase~II $γ$-ray spectrometer. In the $^{28}$Si~$+$~$^{28}$Si reaction, the resonant behavior of the $^{28}$Si $+$ $^{28}$Si exit-channel is clearly observed by the present fragment-fragment coincidence data. The more unexpected result is the spin disalignment of the $^{28}$Si $+$ $^{28}$Si resonance. This has been demonstrated first by the measured angular distributions of the elastic 0$^{+}$, inelastic 2$^{+}$ and mutual excitation channels 2$^{+}-2^{+}$, which are dominated by a unique and pure partial wave with L = 38 ${\rm \hbar}$, and has been confirmed by measuring their particle-$γ$ angular correlations with Eurogam Phase II. The spin disalignment supports new molecular model predictions, in which the observed resonance would correspond to the ''Butterfly mode''. A discussion concerning the {\it spin alignment and spin disalignment} for different systems : $^{12}$C~$+$~$^{12}$C, $^{24}$Mg~$+$~$^{24}$Mg and $^{28}$Si~$+$~$^{28}$Si will be given.}

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arXiv1997

Fission Fragment Spectroscopy on a $^{28}$Si+$^{28}$Si Quasimolecular Resonance

Fragment-fragment-$γ$ triple coincident measurements of the $^{28}$Si + $^{28}$Si reaction at E$_{lab.}$ = 111.6 MeV carefully chosen to populate J = 38$^{+}$ resonance have been performed at the VIVITRON tandem facility by using Eurogam Phase II $γ$-ray spectrometer. In the $^{28}$Si + $^{28}$Si exit-channel, the resonance behavior of the $^{28}$Si + $^{28}$Si reaction at the beam energy is clearly confirmed. An unexpected spin disalignment has been observed in the measured angular distributions in the elastic, inelastic, and mutual excitation channels. This disalignment is found to be consistent with particle-$γ$ angular correlations and supported by the molecular model prediction of a "butterfly motion". The K$^π$ = 0$^{+}_{3}$ band corresponding to the large prolate deformation of the $^{28}$Si is more intensely fed in the resonance region. The selective population of high-excited states are discussed within a statistical fusion-fission model. In the $^{32}$S + $^{24}$Mg exit-channel, the spectroscopic study of the $^{32}$S, has revealed the contribution of a new $γ$-ray transition $0^{+} (8507.8 keV) \to 2^{+}_{1} (2230.2 keV) $.

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arXiv1996

$^{35}$Cl+$^{12}$C Asymmetrical fission excitation functions

The fully energy-damped yields from the $^{35}$Cl+$^{12}$C reaction have been systematically investigated using particle-particle coincidence techniques at a $^{35}$Cl bombarding energy of $\sim$ 8 MeV/nucleon. The fragment-fragment correlation data show that the majority of events arises from a binary-decay process with rather large numbers of secondary light-charged particles emitted from the two excited exit fragments. No evidence is observed for ternary break-up events. The binary-process results of the present measurement, along with those of earlier, inclusive experimental data obtained at several lower bombarding energies are compared with predictions of two different kinds of statistical model calculations. These calculations are performed using the transition-state formalism and the Extended Hauser-Feshbach method and are based on the available phase space at the saddle point and scission point of the compound nucleus, respectively. The methods give comparable predictions and are both in good agreement with the experimental results thus confirming the fusion-fission origin of the fully-damped yields. The similarity of the predictions for the two models supports the claim that the scission point configuration is very close to that of the saddle point for the light $^{47}$V mass-asymmetry-dependent fission barriers needed in the transition-state calculation.

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