16O Projectiles Research Articles

Studies of observed trends in values of Bethe–Bloch parameters extracted from stopping power measurements

AbstractIn the course of previous investigations, trends in values of the Bethe–Bloch parameters, mean excitation energy (I) and Barkas‐effect parameter (b), have been observed. Generally I values decrease with increasing projectile atomic number (z) whereas b values increase. However, the opposite trends have been previously observed for ZnSe and GaAs targets traversed by 1H and 4He projectiles. In the present study, that opposite trend was also followed when 7Li projectiles were included. When selected measurements for 4He, 12C, 14N, and 16O projectiles impinging on Si targets were analyzed, extracted I and b values followed essentially the original trends in z dependence. Stopping power measurements for 12 elemental targets with atomic numbers (Z) lying between 12 and 48 traversed by 29Si projectiles have been analyzed for determination of values of a single effective charge parameter, as part of a continuing study of systematics in such values in terms of both z and Z. Two other sets of measurements featuring 7Li projectiles and eight elemental targets with Z values between 5 and 48 have been similarly analyzed. In some of the effective charge studies the data proved amenable to extraction of three parameters, so that results could be included in the ongoing study of z dependences of extracted values of I and b. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004

Excitation functions of heavy ion induced nuclear reactions between 16O ion beam and natural copper: Measurements, analysis and its applicability in TLA study

Experimental cross sections of a number of reaction channels of 16O ion induced reactions on natural copper target have been determined at different energies in the range of 50–110 MeV of 16O projectile by stacked foil activation technique. The cross sections have been compared with theoretical calculations using the computer code ALICE-91. The experimental values compared reasonably well with the corresponding theoretical estimates. The results indicate no significant role of incomplete fusion process in the 16O induced reactions on natural copper in the energy range of ⩽7 MeV/nucleon. As heavy ion beam produces an extremely narrow layer of activities in the surface of a material, these reactions could be useful for thin layer activation (TLA) study. The purpose of this work is to apply heavy ion activation in TLA technique for the study of surface wear with increased sensitivity.

Fragment angular correlation in the breakup of 16O ions at 95 MeV/ A

Fragment angular correlations in projectile breakup reactions are very sensitive to interference between different multipolarities entering in the excitation-dissociation process of the projectile. In particular, it was proposed to disentangle L = 1 and L = 2 contributions in direct breakup reactions of 16O with low relative energy between the α and 12C fragment, which are of astrophysical interest. We studied the experimental aspects of extracting those angular correlations in extreme kinematical conditions usually encountered in breakup experiments of astrophysical interest. The breakup of 95 MeV/ A 16O projectiles induced by a 208Pb target was measured using the high-resolution spectrometer SPEG at Ganil for the coincident detection of the fragments. Sequential breakup via the 12.53 MeV level of 16O is analyzed in this framework and it favors an one-step M2 excitation of this level.

The 12C + α reaction rate from the elastic 16O breakup

Evidence for direct elastic breakup of 16O into the α- 12C continuum with relative energies ranging from 900 to 1800 keV has been obtained in the scattering of 1527 MeV 16O projectiles off 208Pb. An interpretation of E2 breakup including nuclear and Coulomb contributions leads to reduced electromagnetic transitions probabilities and astrophysical S e2 factors in reasonable agreement with direct measurements, showing that the method can be applied to extract the E2 part of the 12C(α, γ) reaction rate.

Coulomb dissociation of incident 16O at 4.5 GeV/c

Projectile multifragment breakup of 16O, 2C and 7Li at energies 3.0–4.5 A-GeV is studied by means of the Weizsacker-Williams method. The fragmentation channels of the 16O projectile at 4.5 A-GeV are investigated and compared with that of 16O at 200 A-GeV. The events characterized by Nh=0 and the events due to both Coulomb and diffraction dissociation have been selected and analyzed as a function of impact parameter. Also, the dependence of the electromagnetic dissociation cross-section on incident energy and the charge of projectile and target is found.

Effective charge parametrization for 7Li, 11B, 12C, 14N, 16O, 27Al, 28Si, 31P, 32S, and 35Cl projectiles traversing Mylar targets

Some recent measurements of the stopping powers of Mylar foils for 0.2–2.1 MeV/u 7Li, 11B, 12C, 14N, 16O, 27Al, 28Si, 31P, 32S, and 35Cl projectiles have been subjected to analysis in terms of the modified Bethe–Bloch theory in order to obtain values of projectile effective charge parameters. Although both one- and two-parameter fits were attempted, only the former yielded meaningful results. The one-parameter fits manifested considerable internal consistency, and reasonable consistency with results obtained in recent similar analyses of Kapton, Havar, and nickel stopping powers for 7Li, 11B, 12C, 14N, and 16O projectiles.

Effective charge parameters for 7Li, 11B, 12C, 14N, and 16O projectiles traversing Kapton targets

Some recent measurements of the stopping powers of Kapton foils for 0.2–2.1 MeV/u 7Li, 11B, 12C, 14N, and 16O projectiles have been subjected to analysis in terms of modified Bethe-Bloch theory so as to extract values of projectile effective charge parameters. Although one-parameter and two-parameter fits were attempted, only the former provided meaningful results. The one-parameter fits showed excellent internal consistency, and reasonable consistency with results obtained in a recent similar analysis of nickel and Havar stopping powers for the same projectiles.

Effective charge parameters for 7Li, 11B, 12C, 14N, and 16O projectiles traversing nickel and Havar targets

Recent measurements of the stopping powers of nickel and Havar foils for 0.2–2.1 MeV/u 7Li, 11B, 12C, 14N, and 16O projectiles have been analyzed in terms of modified Bethe-Bloch theory in order to extract values of projectile effective charge parameters. A single-parameter form of effective charge proved generally adequate, although a slightly improved fit was attained with a two-parameter form.

Population effects in the highly-deformed bands of 131Ce and 135Nd from 18,16O-induced reactions

The highly-deformed bands in 131Ce and 135Nd have been studied with 18O and 16O projectiles, respectively. Compared to heavy-ion reactions, the amount of sidefeeding increases in 131Ce but not in 135Nd This suggests 135Nd is insensitive to the maximum angular momentum brought in. The Doppler Shift Attenuation Method was used to extract a value for the highly-deformed quadrupole moment in 131Ce of Q 0 = 5.5 ± 0.5 eb. It is concluded that the increased sidefeeding has little effect on the measured Q 0.

Coulomb excitation of 226Ra

Radioactive targets of 226Ra were Coulomb excited by 4He projectiles (particle spectroscopy), and 16O, 32S and 208Pb projectiles (particle-γ coincidences). The K = 0± rotational bands could be followed up to spin 18 + and 17 −, respectively. All experiments were simultaneously analyzed in terms of E1, E2, E3 and E4 matrix elements coupling the positive- and negative-parity rotational states. The systematics of level energies, as well as dipole, quadrupole and octupole matrix elements are discussed in the framework of models assuming stable octupole deformation.

On the breakup of excited 16O projectiles into four alphas at intermediate energies

Mechanisms of the breakup into four alpha particles of 16O projectiles excited from peripheral collisions with 197Au target nuclei have been investigated at 50 and 70 MeV/N. The experimental distributions of relative angles between alpha particles in the rest frame of the primary oxygen nucleus are compared to theoretical predictions based on prompt fragmentation and binary sequential decay. The results are compared to similar studies for that channel at loweer energy, 32.5 MeV/N, where the breakup of 16O was shown to be consistent with sequential emission of alpha particles.

Excitation energy evolution and multi-particle correlations in heavy ion peripheral collisions at intermediate energies

Quasi-elastic and transfer reaction cross section systematics, excitation energy partition and multi-particle correlations are presented for projectile fragmentation following the reaction of 16O projectiles at 50–70 A MeV on 197Au target. Results are compared to breakup mechanism predictions from sequential and multifragmentation models.

A compact velocity filter for evaporation residue measurements

A velocity filter with crossed electric and magnetic fields has been built and used to measure evaporation residue angular distributions for a series of reactions using 16O, 19F and 28Si projectiles. The filter is compact, ∼ 20 cm long, and uses permanent magnets. The spatial separation between the residues and the intense elastic scattering is such that the latter can be physically blocked without affecting the observation of residues; this allows measurements to be made at angles as close as 0.5° to the beam direction. The measured efficiency of the filter is very close to 100%.

Projectile excitation energy evolution in peripheral collisions for 16O + 197Au at 32.5, 50 and 70 MeV/N

A comparison of the multiple breakup of 16O projectiles scattered by a Au target at three different energies (32.5, 50 and 70 MeV/N) is presented. The excitation energy spectra of the primary projectile-like nuclei decaying into specific output channels were reconstructed. The excitation energy of the target is found to increase faster with beam energy than the one for the quasi-projectile.

Search for direct photons in heavy-ion collisions

A search for direct photon production has been performed using p and 16O projectiles at 60 and 200 A-GeV interacting with a C and a Au target. Photon and π 0 spectra have been measured in the pseudorapidity range 1.5 ⩽ η ⩽ 2.1 for the transverse momentum region 0.4 GeV/c ⩽ PT ⩽ 3 GeV/c with the lead-glass spectrometer SAPHIR. An upper limit of 15 % at the 90 % CL for the direct photon signal relative to the neutral pion production is obtained from the measured photon spectra and the Monte Carlo simulations of the hadronic background based on reconstructed π 0 and η spectra. Consequences for a possible phase transition to a quark-gluon plasma (QGP) are discussed.

Measurement of the transverse energy flow in nucleus-nucleus collisions at 200 GeV per nucleon

The transverse energy distributions have been measured for interactions of 32S nuclei with Al, Ag, W, Pt, Pb, and U targets, at an incident energy of 200 GeV per nucleon in the pseudorapidity region −0.1 < ν lab < 5.5. These distributions are compared with those for 16OW interactions in the same pseudorapidity region and with earlier measurements performed with 16O and 32S projectiles in the region −0.1 < ν lab < 2.9. These comparisons provide both a better understanding of the dynamics involved and improved estimates of stopping power and energy density.

Coulomb and photo cross sections for nucleon emission from relativistic 16O projectiles

Coulomb cross sections for nucleon emission out of 16O projectiles scattered with laboratory energies of 2.1, 60 and 200 GeV/nucleon on various target nuclei are calculated in first-order perturbation theory by using the corresponding photo cross sections. The photo cross sections are obtained with an 1-particle-1-hole basis in the giant resonance region and with the quasi-deuteron model up to photon energies of 140 MeV. Whereas the 2.1 GeV/nucleon results agree well with the experimental data points, the 60 and 200 GeV/nucleon cross sections are roughly one quarter lower than the measured ones, because also photons with energies higher than 140 MeV contribute at these incident energies.

Observation of scaling in nucleus-nucleus interactions at ultra-relativistic energies

The minimum bias events produced by interactions of 32S ions at 200 GeV/n and 16O ions at 200 and 60 GeV/n with emulsion target exhibit scaling not only in multiplicity distributions of the produced particles but also in their pseudorapidity distributions in the target (for all projectiles) as well as in the projectile rest frames (for 16O projectiles). Similar scalings are also seen in central events of 16O beams in their corresponding pseudorapidity distributions. The data agree well with the predictions of the Lund model FRITIOF.

Nuclear fusion between heavy ions by the gamma-ray spectroscopy method

The in-beam and off-beam gamma-ray spectroscopy method, as a tool for the study of nuclear fusion between heavy ions, is described. These methods allow the study of the production and decay of the compound nucleus, inelastic scattering and transfer reactions. In this work we report the determination of fusion cross sections at energies near and below the Coulomb barrier for a series of systems: targets of 46,50Ti, 59Co and 64,66Zn bombarded by 14N and 16O projectiles. Cross sections from 0.4 to 1000 mb were measured.

π0 and photon spectra from central and peripheral 16O + Au collisions at 200 A GeV

The production of neutral pions by the interaction of 200 A GeV proton and 16O projectiles with an Au target has been studied for 1.5 ≤ η ≤ 2.1. Transverse momentum spectra have been measured between 0.4 GeV/c and 2.8 GeV/c and their dependence on the centrality of the reaction has been investigated. The peripheral spectrum shows a marked change of slope with a hard component starting at about 1.8 GeV/c in close similarity to p + p data. Preliminary analyses of photon data yield an excess over known photon sources for central data which is absent for peripheral reactions.