MeV 12C Research Articles

The Non-statistical Population of the 11·10 MeV (4+) State in 160 by 12C(6Li,d)

When compound nucleus calculations are normalized to the 13�88 MeV (4+) 12C(6Li, d) 160 cross section it is found that the population of the 11�10 MeV (4 +) state is underpredicted by a factor of 4. This result means that the anomalous population of the 11�10 MeV (4+) state in 160 by the 12C(6Li, d) reaction is not compound nuclear in origin.

A study of three-nucleon transfer reactions on 17O

The reactions 17O( 11B, 8Li) 20Ne, 17O( 12C, 9Be) 20Ne and 17O( 13C, 10B) 20F have been studied using beams of 115 MeV 11B and 12C and 105 MeV 13C incident on a gas target. Shell-model calculations have been performed for 20Ne and 20F, for comparison with the experimental data. It is found that the data can be interpreted using the shell model spectroscopic factors and a semi-classical reaction theory. We justify the use of the latter by applying it to the cases of three-nucleon transfer on 16O. Spin assignments are suggested for previously unidentified states in 20F.

Few-nucleon transfer reactions on 15N and 16O

We have studied one-, two- and three-particle transfer reactions induced by 115 MeV 11B and 12C, 126 MeV 18O and 105 MeV 13C ions on gas targets of 15N and 16O. These reactions are found to be very selective; in some cases only one excited state in the final nucleus is populated. The reactions on 16O excite mainly high spin states with n particles in the sd shell in a relative s-state. Reactions on 15N excite positive parity states by transferring one particle into the p-shell and n − 1 into the sd shell and negative parity states which have the structure p −1(sd) n . These latter states correspond to weak coupling of a p 1 2 hole to the states excited in the corresponding transfer reaction bn 16O. Shell model wave functions have been computed for the two-particle one-hole states of mass 17 and for the three-particle one-hole states of mass 18. The overlaps of these with weak coupling wave functions were calculated in order to justify the application of the weak coupling model in this mass region. The qualitative difference in the spectra of the T = 0 and T = 1 negative parity states of mass 18 is reproduced and explained in a simple fashion.

Measurement of specific energy, ash and moisture in bulk coal samples by a combined neutron and gamma-ray method

The specific energy, ash and moisture contents of bulk coal samples can be determined simultaneously by a method based on the accurate measurement of carbon and hydrogen contents. The method for specific energy depends on a combination of a measurement of 4.43 MeV 12C γ-rays from neutron inelastic scattering and a separate 60Co γ-ray scattering measurement on essentially the same sample volume. The γ-ray scattering measurement corrects the neutron gauge for variables such as sample bulk density, bulk density gradients and sample thickness. By “matching” the response of the two gauges, an accuracy can be obtained that is higher than has been previously reported for bulk coal samples. Ash and moisture are determined by combining the above measurements with a simultaneous measurement of 2.22 MeV H neutron capture γ-rays. The method has been tested by many laboratory measurements on ∼50 kg coal samples from coalfields in southern N.S.W., Australia. Rms deviations between chemical laboratory and nuclear gauge assays were 0.92 wt.%, 0.44 MJ/kg and 1.4 wt.% for respectively, carbon, specific energy and ash. However, if the data was restricted to coal samples containing > 70 wt.% C, these errors reduced to 0.38 wt.%, 0.26 MJ/kg and 0.81 wt.% respectively. Accurate moisture values can be obtained provided that the C/H ratio in the coal matter remains constant. Measured rms deviations for moisture varied from 0.2 wt.% for a single sample with a number of different added water contents to 0.85 wt.% for a variety of samples from the different mines. The main potential applications of the method are for the on-line analysis of coal on conveyor belts and in chutes or hoppers; this analysis is required for the control of power stations and coal washeries.

Evidence for giant quadrupole excitation in 208Pb by 200 MeV 12C scattering

In inelastic 12C scattering from 208Pb a peak, corresponding in excitation energy, width and strength to the well-known GQR is observed. The underlying broad continuum is probably due to 13C breakup rather than to excitation of giant resonances with L > 2.

The prompt γ-ray de-excitation of evaporation residues of 39K and Se

Average multiplicities 〈 M γ 〉 and average energies 〈 E γ 〉 have been measured for evaporation residues of the compound nuclei 39K and Se produced in reactions of 107, 155 and 197 MeV 12C projectiles with 27Al and natNi target nuclei. Both quantities are found to be essentially independent of the projectile energy. For the reaction 197 MeV 12C + 27A1, 〈 M γ 〉 increases as a function of product atomic number. The results are interpreted in terms of the angular momentum dissipation which occurs during the particle evaporation cascade.

Heavy-ion excitation of 3 − states in deformed nuclei

Previously measured data for 70.4 MeV 12C excitation of 3 − states in 148, 150Nd are reanalyzed using a third-order rotational-vibrational model. Calculations using 3 − quadrupole moments which are expected if they are K = 0 states are in reasonable agreement with the magnitude of the large-angle data, but the quality of the fit in the Coulomb-nuclear interference region is only fair. It is found that the matrix element 〈2 +∥ M(E3)∥3 −〉 plays an important role in the calculations making the “measurement” of the 3 − quadrupole moments very difficult, if not impossible.

Fusion and direct reactions between 92 MeV 19F and 12C

The gross features of the reaction mechanism in the sytem 19F + 12C are studied. At 92 MeV 19F energy ( E = 35.6 MeV c.m.) cross sections for evaporation residues and products from direct reactions were measured. The results are discussed together with similar data obtained previously at lower energies. From the energy dependence of the fusion cross section the radius and height of the interaction barrier and the corresponding values for the “critical” fusion distance are deduced. The analysis of the evaporation residue mass distribution at 92 MeV indicates a considerable (30%) contribution of fusion after excitation of the projectile 19F and break-up into 15N+α. This break-up process is also observed to be the strongest direct channel.

Nuclear reorientation effects for the 26Mg, [formula omitted] reactions at E = 41 MeV

Angular distributions for elastic and inelastic scattering of 41 MeV 12C from 26Mg and 28Si have been measured. Corresponding angular distributions from coupled-channels calculations show significant differences depending on whether a prolate or an oblate intrinsic shape is assumed. The 26Mg and 28Si data are best described by calculations with prolate and oblate shapes, respectively, in agreement with previous Coulomb excitation rorientation measurements. The Hendrie scaling procedure fails to accurately predict the measured nuclear β 2 deformation for 26Mg.

Can projectiles traversing solids bind electrons at all velocities?

PAC measurements on the 4.43 MeV 12C(2 +) state on recoil in magnetized iron at velocities υ ion = 0.058 c, 0.071 c yield integral nuclear precession angles of Φ = −0.18 (30), +0.17 (39) mrad respectively. Evidently, the high electron-spin-polarization transfer to the innermost carbon orbits implied by a previous measurement (υ ion = 0.03 c, Φ = +0.85 (14) mrad) is suppressed at higher ion velocities.

Void formation in solution-treated aisi 316 and 321 stainless steels under 46.5 mev ni6+ irradiation

The swelling and radiation damage structure developed in solution-treated 316 and 321 stainless steels bombarded by 46.5 MeV Ni6+ ions in the Variable Energy Cyclotron (VEC) have been determined. Foils were pre-injected with 10−5 a/a He at room temperature and subsequently bombarded by Ni6+ ions in the temperature range 450–750°C at a damage rate of 1–3 × 10−3 dpa per second to doses up to 300 dpa and specimens from the foils were examined by transmission electron microscopy. The data obtained were compared with data from other experiments aimed at simulating the fast-neutron irradiation of 316 and 321 steels, in particular previous work with 20 MeV C2+ ions and with data on fast-reactor bombarded material. The swelling rates in Ni-ion bombarded specimens were about a factor two less than those in C-ion bombarded specimens and in good agreement with swelling rates in 5 MeV Ni+- and neutron-bombarded material. The peak swelling temperature after a dose of 40 dpa was 650°C in 316 steel and 625°C in 321 steel where the swelling was about 5.8% and 4.6% respectively. Le gonflement et la structure des dommages par irradiation développés dans des aciers inoxydables 316 et 321 traités à haute température pour mise en solution solide et bombardés par des ions Ni6+ de 46,5 MeV dans un cyclotron à énergie variable (VEC) ont été déterminés. Des feuilles furent au préalable soumises à l'implantation de 10−5 a/a He à la temperature ambiante puis bombardées par des ions Ni6+ dans l'intervalle de températures de 450 à 750°C à un taux d'endommagement de 1à 3 × 10−3 dpa par seconde jusqu'à atteindre des doses de 300 dpa. Les échantillons furent ensuite examinés par microscopie électronique par transmission. Les données obtenues furent comparées à celles d'autres expériences réalisées en vue de simuler l'irradiation par des neutrons rapides d'aciers inoxydables 316 et 321. En particulier ces observations furent comparées à celles d'un travail antérieur sur des aciers inoxydables bombardés par des ions C2+ de 20 MeV et à des données obtenues sur des matériaux bombardés dans un réacteur rapide. Les vitesses de gonflement des échantillons bombard 'es par des ions Ni étaient inférieures d'un facteur 2 environ à celles d'échantillons bombardés par des ions C et en bon accord avec les vitesses de gonflement observées dans ces matériaux bombardés par des ions Ni+ de 5 MeV et par des neutrons. a température de gonflement maximum après une dose de 40 dpa était de 650°C dans l'acier 316 et de 625°C dans l'acier 321, aciers dont le gonflement était respectivement égal à environ 5,8% et 4,6% respectivement. Das Schwellen und die durch Strahlenschädigung entstandene Struktur wurden in den lösungsgeglühten rostfreien Stählen 316 und 321 untersucht, die im Zyklotron variabler Energie mit 46,5 MeV Ni6+-Ionen beschossen worden waren. Die Folien waren zuvor mit 10−5 a/a He bei Raumtemperatur und darauf mit Ni6+-Ionen zwischen 450 und 750°C bei einer Schädigungsrate zwischen 1 und 3 × 10−3 Verlagerungen pro Atom und Sekunde und einer Dosis von 300 Verlagerungen pro Atom beschossen worden. Die Folienabschnitte wurden transmissibnselektronenmikroskopisch untersucht. Die Ergebnisse werden mit denen anderer Experimente verglichen, die auf die Simulation einer Bestrahlung der Stähle 316 und 321 mit schnellen Neutronen abgezielt haben; insbesondere mit einer früheren Arbeit mit 20 MeV C2+-Ionen und mit Ergebnissen an Material, das im schnellen Reaktor bestrahlt worden war. Die Schwellrate der mit Ni-Ionen beschossenen Proben liegt um den Faktor zwei niedriger als die der mit C-Ionen beschossenen Proben. Die Übereinstimmung mit der Schwellrate des mit 5 MeV Ni+-Ionen und mit Neutronen bestrahlten Materials ist gut. Die Temperatur des maximalen Schwellens bei einer Dosis von 40 Verlagerungen pro Atom liegt für Stahl 316 bei 650°C und für Stahl 321 bei 625°C; hier beträgt das Schwellen etwa 5,8% bzw. 4,6%.

A microscopic description of inelastic 12C scattering from 208Pb

Heavy-ion inelastic scattering is described microscopically by using an effective nucleon-nucleon interaction and RPA hole-particle wave functions. The relative cross sections for different multipoles can be sensitive to the range of the interaction. The strength is determined by fitting elastic scattering. The model is used to analyze 98 MeV 12C ions exciting 208Pb. The range required is shorter than that for the bare interaction between nucleons. Collective model fits are also presented for comparison.

Multiple scattering of heavy ions through ΔE Si surface barrier detectors

The multiple scattering of 30–60 MeV 12C ions and 40–70 MeV 16O ions by ΔE Si surface barrier detectors of thicknesses 10.3 μm and 14 μm was measured with a position sensitive detector. The horizontal spatial distributions of the heavy ions were extracted from position measurements made both with and without the ΔE counter in place. Calculations based on the theories of Molière and Nigam, Sundaresan, and Wu (NSW) were made and compared to the data. The much simpler form of the NSW theory predicted fwhm which were much too large when compared to the experimental data while the Molière theory gave a reasonable description of the experimental distributions at the higher incident particle energies. However, the Molière calculations overestimated the fwhm by 28% in the case of 30 MeV 12C ions incident upon the 10.3 μm detector. Particle detection efficiencies for a typical time-of-flight system are presented.

Total kinetic energies in the fission of 101Rh, 110Cd, 119I and 138Ba nuclei

Singles and coincidence experiments with solid state detector telescopes have been used to determine the total fragment kinetic energies in the symmetric fission of 101Rh, 110Cd, 119I and 128Ba nuclei produced in reactions induced by 197 MeV 12C projectiles. These kinetic energies are found to be in excellent agreement with the predictions of Davies et al. based upon a dynamic liquid drop model calculation. For the reactions of 12C with Ag, the dependence of the kinetic energy release on angular momentum has been explored. The increase in kinetic energy with increasing angular momentum is found to be ≦ 0.7 keV h ̵ 2 .

Yrast cascade in 60Ni

Yrast levels of 60Ni were investigated via the study of in-beam γ-rays induced by 15–25 MeV α-partictes on 58Ni, 26–48 MeV 12C ions on 50Cr and 30–60 MeV 16O ions on 46Ti. The compound nucleus formed by these target-projectile combinations is 62Zn, and its decay by two-proton emission populates yrast levels of 60Ni. The ordering and decay modes of the yrast levels were determined from the analyses of in-beam γ-ray angular distribution and γ-γ coincidence measurements. The new levels established are at 4.262, 5.345 and 6.807 MeV. These together with the known 21+ (1.332 MeV) and 41+(2.505 MeV) levels constitute the yrast cascade. The spin assignments based on the present study are 6, 7, 9 for the 4.262, 5.345 and 6.807 MeV levels, respectively. The excitation functions for the yrast γ-rays from the 50Cr(12C, 2p)60Ni reaction show peaks near 33 MeV incident energy.

Background-resonance interference effects in knockout reactions

Deep-hole excitations formed in knockout reactions decay rapidly by the ejection of additional nucleons. Coherent background processes that lead to the same final states are discussed. Distorted wave calculations are carried out for the 460 MeV 12C(p, 2p) reaction in the energy region of the s 1 2 knockout peak. Interesting threshold effects occur as a result of the proximity of the s 1 2 peak to the threshold for ( p 3 2 ) 2 ejection from 12C. The background processes provide small modifications to the resonance peak; this suggests that high-energy knockout reactions can serve as a useful probe of inner shells in nuclei. Complications associated with the exclusion principle in intermediate states are responsible for the principal uncertainties of the analysis.

Ground-state α-decay of N = 128 isotones 216Ra, 217Ac and 218Th

The α-decay properties of very short-lived N = 128 isotones, 216Ra, 217Ac and 218Th, were investigated by the pulsed-beam method. Alpha emitters of interest were produced in the bombardment of 208Pb or 209Bi with 65–96 MeV 12C or 14N ions and α-decays were measured between natural beam bursts of the cyclotron. The results obtained are Eα = 9.349±0.008 MeVand t 1 2 = 182±10 ns for 216 Ra, 9.650±0.010 MeV and 111±7 ns for 217 Ac, 9.665±0.010 MeV and 96±7 ns for 218 Th . The experimental reduced α-widths of N = 128 isotones from 212Po to 218Th are shown to agree well with the simple shell model calculation.

Investigation of the 208Pb( 12C, 11B) 209Bi and 208Pb( 12C, 13C) 207Pb reactions at high bombarding energies

States in 207Pb and 209Bi were populated in interactions of 208Pb with 77, 98 and 116 MeV 12C ions. Angular distributions indicated that the neutron-pickup and proton-stripping reactions were compatible, respectively, with mechanisms dependent on final and initial systems alone.

The cross sections for different channels in heavy ion nuclear reactions

Estimates have been made for the critical value lc of the orbital angular momentum above which a complete fusion nucleus cannot be formed. The results have been obtained by measurements of cross sections for noncompound inelastic processes (inelastic scattering, transfer. reactions, α particle emission) when Ag targets were bombarded by 86 MeV 12C ions and by 78 and 113 MeV 14N ions. The average value of lc was found between 38 and 45 ħ. A short discussion is given on the range of angular momenta at which transfer reactions occur. It is shown that quasi elastic single and multi-nucleon transfer reactions are possible only for large values of l called lT. When the bombarding energy is high enough, theses values lT are larger than lc and there is a region of l between l c and lT where a very inelastic mechanism takes place. A model called « fusion prompt scission » process is proposed. It is suggested that a deformed shape for the two fissioning nuclei is temporarly formed and breaks off very shortly after.

Mechanism of single-nucleon and multi-nucleon transfer reactions in grazing collisions of heavy ions on silver

A large number of products of transfer reactions has been identified when bombarding natural Ag and 109Ag by 86 MeV 12C and by 78 and 113 MeV 14N ions. From the analysis of angular and energy distributions, it has been shown that both multi-nucleon and single-nucleon transfer processes are ruled by two different mechanisms. (i) When the products are formed at the ground state or at relatively low excited states, the angular distribution can be interpreted in terms of a DWBA analysis of a quasi-elastic scattering. This first mechanism is responsible for the preferential emission at a particular angle which depends on the bombarding energy. (ii) When the products are formed with a very large defect of kinetic energy (more than 10 MeV), they are observed at very small angles. A qualitative explanation of this “grazing” process is tentatively given. The relative importance of the second mechanism is higher for a large number of transferred nucleons than for single-nucleon transfers and increases with the bombarding energy.