Search Results (36 total)

We have performed a multifractal (G-moment) analysis of 14.6-200 GeV/nucleon nucleus-nucleus and 200-800 GeV proton-nucleus interactions from KLM and Fermilab E-90 and E-508 emulsion data, including explicit corrections for the finite statistical sample. The corrected slopes of the G-moments for protons, ¹⁶O, ²⁸Si, and ³²S nuclei show only slight evidence for departures from random behavior, while the normalized entropies appear to show a more consistent departure from randomness, particularly for protons. Given the size of the uncertainties, the results of the fractal analysis are not inconsistent either with results of intermittency analyses for nucleus-nucleus collisions or with the nonrandom behavior previously reported for leptonic and hadronic collisions. However, because of the effects of statistical noise, the fractal analysis is not as sensitive as the intermittency analysis for detecting nonrandom fluctuations.

Oxygen and sulfur nuclei with energies of 200 GeV/nucleon have been allowed to interact in nuclear emulsions exposed at CERN. These emulsions have been scanned with a minimum bias so that essentially all the interactions occurring were detected. Nearly 1000 interactions of each projectile have been analyzed. We present results on the multiplicity distributions, the pseudorapidity distributions, and the fragmentation of the projectile and target nuclei. It is shown that the mean number of intranuclear collisions in each interaction, calculated from a superposition model, provides a useful parameter for organizing the data. We conclude that there are no significant deviations even at these energies from models, such as the venus model, describing the interactions as being the superposition of individual nucleon-nucleon collisions.

Scaled factorial moments, corrected for the shape of the single-particle pseudorapidity distribution, are analyzed in pseudorapidity and in two-dimensional (pseudorapidity and azimuth angle) space. An intermittent, power-law growth of the moments with decreasing bin size is found, with two-dimensional analysis revealing a much stronger effect than for one-dimensional for nucleus-nucleus data. The intermittent patterns are more evident for proton-nucleus than for nucleus-nucleus collisions, with the heaviest nucleus, ³²S, showing the weakest effect.

The angles of all charged secondaries have been measured in central collisions of ¹⁶O with AgBr at 14.6, 60, and 200 GeV per nucleon. The pseudorapidity distributions are approximately Gaussian in shape. The widths increase monotonically with beam energy, but are essentially independent of multiplicity at a fixed beam energy. Quantitatively, the widths are consistent with Landau’s hydrodynamical model and inconsistent with a spherical fireball model and with Bjorken’s hydrodynamical scaling model. There are no statistically significant correlations in the azimuthal angle of the observed charged secondaries. Analysis of the two-particle pseudorapidity correlation functions and of the pseudorapidity gap distributions provide no evidence of significant cluster production. The data are consistent with a Monte Carlo simulation based on independent emission of the secondary particles, although intrinsic few-particle correlations could be hidden by the high multiplicities in these events.

The method of scaled factorial moments is used to study short-range fluctuations in the pseudorapidity distributions of particles produced in high-energy interactions in nuclear emulsion. An intermittent behavior of the fluctuations is clearly observed in both proton (200 and 800 GeV) and oxygen (60 and 200 GeV/nucleon) beam interactions in emulsion.

Central collisions of ¹⁶O nuclei with the ¹⁰⁷Ag and ⁸⁰Br nuclei in nuclear emulsion at 14.6, 60, and 200 GeV/nucleon are compared with proton-emulsion data at equivalent energies. The multiplicities of produced charged secondaries are consistent with the predictions of superposition models. At 200 GeV/nucleon the central particle pseudorapidity density is 58±2 for those events with multiplicities exceeding 200 particles.

Transverse-momentum data are analyzed for the presence of collective flow of nuclear matter in interactions of Au and Xe in nuclear emulsion at energies from 0.5 to 1.2 GeV/nucleon. Evidence of such flow is obtained from 122 interactions involving AgBr in nuclear emulsion by adapting a recently proposed method of transverse-momentum analysis.

A systematic study of meson multiplicity as a function of energy at energies up to 100 GeV/u in nucleus-nucleus collisions has been made, using cosmic-ray data in nuclear emulsion. The data are consistent with simple nucleon-nucleon superposition models. Multiplicity per interacting nucleon in AA collisions does not appear to differ significantly from pp collisions.

Nuclei of ₅₄Xe, ₆₇Ho, ₇₉Au, and ₉₂U accelerated at the Lawrence Berkeley Laboratory Bevalac to energies between 1200 and 900 MeV/n have been stopped in nuclear emulsions. The observed residual ranges have been compared with those calculated from various models of energy loss and shown to be most consistent with a calculation that includes those higher-order correction terms proposed previously to describe the energy loss of highly charged particles, for which the first Born approximation is not valid.

The nuclear interactions of _{¹ 79⁹⁷}Au nuclei of initial energy 990 MeV/nucleon have been studied in nuclear emulsions as they slow down and stop. Typically, these interactions are fragmentations of the incident nucleus into an average of 2.0±0.3 fragments with Z≥3 and 4.2±0.2 helium nuclei. As many as eight Z≥3 fragments or 22 helium nuclei have been observed to be produced in a single interaction. The dependence on incident energy and target nucleus is discussed. Some 7% of the interactions appear to be fission in flight of the excited projectile nucleus emerging from the interaction, with the release of two highly charged fragments that contain most, if not all, of the incident charge. The charge distribution of these fragments ranges from symmetric to highly asymmetric.

The residual ranges of ₇₉¹⁹⁷Au nuclei stopping in nuclear emulsions has been measured for nuclei with an incident energy of 991 MeV/amu. The mean ranges observed are appreciably less than those predicted from measurements made on energetic particles of lower charge. However, by the consideration of higher-order correction terms to the rate of energy loss, good agreement can be obtained between the predicted and observed ranges.

We use low-energy data on pion production in collisions between nuclei to study some characteristics of high-energy cosmic-ray showers. The fact that pion production is suppressed in collisions initiated by heavy nuclei has the result that fluctuations in showers generated by such nuclei are larger than would be expected in some previously used models, though not so large as to impair use of air showers for studies of composition and cross section. We emphasize here implications for new cosmic-ray experiments around 10⁹ GeV.

Cosmic-ray nuclei have been observed to produce 2072 secondary fragments in nuclear interactions detected in nuclear emulsions. These fragments produce 557 further interactions. Previous reports that these secondary nuclei show anomalously short mean free paths near their point of origin are confirmed. This effect can be interpreted as being due to all fragments having cross sections about twice normal for some 10^-10 sec after creation, or to a small fraction having cross sections an order of magnitude greater than normal.

An attempt has been made to understand the form of the observed energy spectrum of the highly charged (20≤Z≤30) nuclei in the primary cosmic radiation. It is shown that the behavior of this spectrum relative to that of the helium nuclei also present in the cosmic radiation can be explained by assuming that the highly charged nuclei have two components. The first of these components is dominant at high energies and has a spectrum at the source that can be represented as a simple power law with a slope somewhat flatter than that of the source spectrum of the helium nuclei. The nuclei in this component may be regarded as having traversed several g/cm² of matter between the times of acceleration and of detection. The second component is of importance only at relatively low energies (a few hundred MeV/nucleon), has a spectrum that falls steeply with increasing energy, and cannot simply be represented as a power law. The possible consequences of such a model are briefly considered.

The intensities and energy spectrum of the medium- and very heavy, MH and VH, nuclei, 16≤Z≤30, present in the primary cosmic radiation have been studied using three stacks of nuclear emulsions exposed on high-altitude balloons flown over Fort Churchill (Canada), Texas, and India. Integral intensities of Z≥20 nuclei above energies of 0.225, 1.58, and 7.1 BeV per nucleon were, respectively, 1.50±0.06, 0.403±0.023, and 0.090±0.006 (VH nuclei)/m²sr sec. The energy spectrum of these nuclei was measured in detail between 225 MeV per nucleon and 1 BeV per nucleon, and was found to exhibit a maximum in the 300- to 400-MeV per nucleon range, where the differential intensity was of the order of 1.7×10^-3 nuclei/m² sr sec (MeV per nucleon). Integral intensities of 16≤Z≤19 nuclei above energies of 1.58 and 7.1 BeV per nucleon were, respectively, 0.086±0.010 and 0.020±0.003 nuclei/m² sr sec; differential intensities were also measured at lower energies.

The results are presented of thirteen separate observations made between June 1954 and August 1960 of the characteristics of the helium nuclei in the primary cosmic radiation. These observations were made using photographic nuclear emulsions flown on high-altitude balloons to detect the particles during times ranging from the minimum of solar activity to the maximum. Attention is confined to the presentation of the experimental data, with a detailed discussion of the physical significance being deferred to a later paper. The results are compared with those published previously from this laboratory on the higher charged nuclei in the cosmic radiation.

The relative abundances of protons, deuterons, and tritons have been studied in photographic nuclear emulsions flown during the 18 July 1961 solar-flare event. These emulsions were exposed at a time and locality where there was a geomagnetic threshold. The effects of this threshold on the different particles have been calculated on the basis of theoretical assumptions, and the resulting enrichment of the abundance ratios has been used to assist in reducing the abundance limits to lower values than those previously quoted. By including the results of previous α-particle measurements in these emulsions, it has been found that the relative abundances of solar-produced H¹, H², H³, and He above a given rigidity at this time were 1: ≤1.4×10^-2: ≤2.8×10^-2: 0.17, and above an energy of 50 MeV per nucleon, i.e., above a given velocity, were 1: ≤2×10^-3: ≤6×10^-4: 2.4×10^-2. The consequences of these observations are briefly discussed.

The intensities of the primary cosmic-ray heavy nuclei, Z>~3, have been studied during several Forbush decreases. Fifteen values of the intensity were measured using nuclear emulsion detectors flown during a balloon cosmic-ray monitoring program. These values are those observed before, during, or after four of the largest Forbush decreases that occurred in the last solar cycle. Examination of these data, together with those previously available in the literature, suggests that the heavy nuclei are modulated in a similar manner to the α particles and protons of the primary radiation, showing that the modulation process is not a strongly charge-dependent one.

The fluxes of primary cosmic-ray α particles over Minnesota and Texas have been measured during the present period of maximum solar activity. A value of 136±9 α particles/m² sec sterad was measured over Minnesota and of 68±4 α particles/m² sec sterad over Texas. In both cases these values are significantly lower than those observed at solar minimum. The energy spectrum of these particles has been determined between 200 Mev/nucleon and 3.0 Bev/nucleon. It is shown that the slope of the integral spectrum is less than that observed at solar minimum and that a significant number of low-energy particles is still present. A possible mechanism for these changes is discussed briefly. The determination of energies of particles from a measurement of their ionization is discussed in detail in an appendix.

In order to resolve the controversy concerning the abundance of the light elements (3<~Z<~5) in the primary cosmic radiation, a further experiment has been performed. The charge spectrum of the cosmic radiation has been determined on October 19, 1957 in a stack of nuclear emulsions exposed nearer the top of the atmosphere than before. The results of the experiment prove that these light elements make up an appreciable fraction of all those nuclei with Z>~3 present in primary cosmic radiation. The flux values found after correction to the top of the atmosphere are, in particles/m² sec sterad: Li, Be, B (L nuclei) 1.9±0.3 CNOF (M nuclei) 5.1±0.5 Z>~10 (H nuclei) 1.7±0.3 These values appear to be lower than observed previously in agreement with the decrease in the α-nparticle flux associated with the sun's increased activity.

The zenith and azimuthal angular distribution of primary cosmic rays with charges Z≥5 has been measured at Texas using horizontal emulsions with known orientation relative to the earth. The magnitude of the azimuthal asymmetry is consistent with Störmer theory down to a zenith angle of about 65° while for larger zenith angles the effect of the solid earth (shadow cone) is observed. The direction of the asymmetry seems to be consistent with the centered dipole approximation of the earth's magnetic field (north pole at 79°N and 70°W). The flux at the top of the atmosphere is 1.6±0.3 particles/m² sec sterad for nuclei with Z≥10 and 4.8±1.0 particles/m² sec sterad for CNOF nuclei.

Three interactions occurring in nuclear emulsions and caused by primaries of charge ≥2 and energy >10⁴ Bev/nucleon are described. The electromagnetic cascade from one of these showers has been studied in detail. The mean free path for direct pair production (trident) has been measured in various intervals. The mean free path at energies <10 Bev is shorter than the theoretical one calculated by Racah, but the small number of cases (7) at these energies allows large fluctuations. For the mean free path of the high-energy electrons whose energies measured by scattering have a lower limit of 6 Bev placed on them, we find a singificant disagreement with theory only if the average electron energy is as low as 10 Bev. We have shown that the average electron energy is more like 50-100 Bev, thus giving no disagreement with the theoretical cross section for direct pair production.