Large Number Of Neutrons Research Articles

Laser-driven vehicles – from inner-space to outer-space

Laser-supported propulsion of a micro-airplane with a water-covered ablator is demonstrated. The repetitive use of an overlay structure is experimentally demonstrated with a specially designed water supply. Various transparent overlays are investigated by the CIP-based hydrodynamic code and by experiments using a pendulum and using a semi-conductor load cell. A momentum-coupling efficiency of ∼104 N s/MJ is achieved by water–exotic-target experiments, in agreement with the simulation code, which predicts a maximum efficiency of ∼105 N s/MJ. The concept of laser-supported propulsion can also be used for driving a Mach 5 airplane in the stratosphere, a micro-ship inside the human body, and a robot in a nuclear power reactor accident, during which large numbers of neutrons make electronic devices useless.

Research into accelerator driven systems using particle track detectors

In the interaction of relativistic protons with heavy and extended targets such as lead, large number of neutrons is produced in the course of the so-called spallation process. These neutrons can be used to drive a sub-critical nuclear assembly for energy generation and/or for the transmutation of the long-lived nuclear waste isotopes to environmentally safer nuclear species. Such nuclear assemblies are referred to as accelerator driven systems (ADS). Knowledge of the neutron yield in the spallation process and an understanding of the behaviour of these neutrons in the desired sub-critical assembly are the most important and determining factors in the design and operation of these systems. Many parameters related to the neutronics of an ADS can be studied qualitatively as well as quantitatively using solid-state nuclear track detectors (SSNTD). In some circumstances SSNTDs provide the best and the most logical detector option for these investigations. In this paper applications of the SSNTDs into research related to ADS are discussed and some experimental and theoretical results presented.

Light and heavy transfer products in58Ni+208Pbat the Coulomb barrier

Multinucleon transfer reactions in Ni+Pb have been studied at ELAB=328.4 MeV, corresponding to ' 3% above the Coulomb barrier. Light reaction products have been detected with the time-of-flight spectrometer PISOLO [1], while the associated heavy partners produced in the binary reaction have been detected in kinematic coincidence using a transmission-type multiwire parallel-plate avalanche counter [2]. The main motivation of the experiment was to simultaneously detect both light and heavy nuclei produced in multinucleon transfer reactions, for a suitable projectile and target combination where large number of neutrons and protons could be transferred at energies close to the Coulomb barrier. The Ni+Pb system is in fact a good candidate, since its Q-value matching conditions, close to optimum up to 8 proton stripping, allows one to investigate in detail the mass and charge partitions of the light reaction products and, at the same time, to see if the same transfer mechanism is suited for producing a significant yield of the corresponding high Z heavy partners [3,4]. A first report on the experiment is given in Ref. [5] while the final work including the comparison between data and theory is reported in Ref. [6]. We refer to the cited references (and references therein) for the details and for the discussion of the physics involved.

NEUTRON FLUCTUATIONS IN TRADITIONAL AND ACCELERATOR DRIVEN REACTORS

Classical particle transport, such as the diffusion and slowing down of neutrons in a multiplying system, is a random process. As a result, the number of neutrons in the system, or the number of detector counts, is a random number. Because of the branching property of the cascade, i.e. producing two or more neutrons in the fission process, correlations will exist between the neutrons in the system. This will lead to non-trivial statistics in the cascade, which, among others, carries information on the dynamical properties of the system. Neutron fluctuations have thus been used for a long time to measure either nuclear physics parameters such as the effective delayed neutron fraction, or the so-called multiplication constant, which describes the criticality properties of the system. In addition to the above described inherent random property of the particle transport in steady-state systems, another possibility of neutron fluctuations arises in systems whose properties fluctuate in time. This type of noise is observed in systems with high power. The reason for neutron fluctuations, that is neutron noise, is the fluctuations of the reactor material. There is boiling of the coolant, vibration of various components such as control rods, the core barrel etc., which all will lead to parametric excitation of the neutron noise. In this paper we shall however mostly deal with the neutron noise in low power systems, i.e. with inherent fluctuations of the neutron transport process, and the case of power reactor noise will only be touched upon. The neutron noise in low power systems has received a renewed strong interest recently, due to the appearance of a new concept, the so-called accelerator driven systems (ADS). An ADS is a subcritical reactor, driven by a strong external source, which utilizes spallation. The advantages of such a system is that it has excellent operational safety properties (a criticality accident is practically impossible), can utilize fertile nuclides as fuel such as thorium or U-238 easier than traditional reactors (hereby having access to more abundant fuel reserves as the current reactors), and finally, on the long run and with proper design, it would produce less high-level nuclear waste. As a matter of fact, an ADS can incinerate more waste than it produces, so it can also be used primarily for transmutation of nuclear waste with energy production as a by-product. In an ADS, however, the statistical properties of the neutron source are different from those of the classical radioactive sources that were used in the development of the theory in the past. A radioactive source emits one neutron at a time, and thus all source neutrons are independent (they obey Poisson statistics). In a spallation source, on the other hand, a large number of neutrons are produced in each spallation event, and the number of neutrons generated is a random number. Thus the source neutrons are correlated. In addition to time correlations, energy correlations may exist between spallation neutrons even if the neutron energies in a single spallation even are independent. This fact will be illustrated i the paper. All of the above mentioned (i.e. time and energy) correlations will influence the statistics of the neutrons in an ADS and thus they need to be accounted for.

Multi-nucleon transfer reactions and fusion with unstable nuclei

Abstract We show that a large number of neutrons are expected to be transferred from the projectile to the target if a neutron rich unstable nucleus with a neutron skin is used as the projectile in heavy-ion collisions at energies about twice the Coulomb barrier. We then show that though the neutron halo enhances the fusion cross section through the size effect, the additional effect due to the molecular bond formation is not significant in the fusion between 11Li and 9Li at energies near and below the Coulomb barrier.

Diagnostic System for the Deuterium-Tritium Phase of the IGNITEX Experiment

A fusion ignition experiment will produce large numbers of neutrons and alpha particles. The detection and characterization of these particles will be important in understanding the physics of igni...

Quantum coherence among phonons generated by fast processesin elastic targets. II

The excited state of the quantum phonon field, generated by very fast processes is deduced. γ-ray emission, orabsorption, by unstable nuclei (occurring in Mössbauer experiments) and neutron scattering by a crystal are in particular considered. We show that coherent states of the phonon field are generated. The appropriate response functions are written in terms of these states. The phonon field generated by the scattering of a large number of neutrons is discussed, since phonons are found to be distributed according to an exponential distribution, heat is generated.

Suggested relaxation of plasma focus discharges to helical force-free configurations

It appears difficult to account for the large number of neutrons produced per shot in plasma foci, as well as for the long duration of the neutron emission phase, unless one assumes that the fast ions, which are presumably produced in the collapsing phase, are actually confined for a substantial length of time within the focus region, before being lost to the electrodes. The authors suggest that such a confinement may be provided by the mirror properties of the quasi force-free, helical configurations, which were predicted by Taylor (1976) to be the likely end point for highly-sheared, dissipative pinch discharges. In fact, in plasma foci, particularly of the Filippov type, evidence has accumulated in favor of the existence of long lasting ( approximately 100 nsec) helical plasma-magnetic structures, which seem to occur simultaneously with the second neutron outburst (Rager, 1976).

Pulsed spallation neutron sources

The discovery of the neutron in 1932, which indirectly led to the potentially catastrophic development of nuclear weapons on the one hand and to the beneficial development of nuclear‐generated electricity on the other, has had another, less well‐known, but scientifically important consequence: the provision of a uniquely sensitive and pervasive tool for probing condensed matter. Because they are neutral and because they interact mainly with nuclei, neutrons can penetrate into bulk material and provide information that is difficult to obtain with x rays or charged particles. Until recently, nuclear reactors provided the highest‐intensity neutron sources. But lately another kind of source is promising to overcome the limitations of reactors. In these sources high‐energy protons are made to collide with heavy nuclei, splitting off a large number of neutrons. The process is rather like making chips fly by hitting a rock with a hammer—what the geologists call “spallation.” These neutron spallation sources together with equipment such as that shown in figure 1 have already begun to provide us with useful information accessible in no other way.

Multi-detector arrangements for on-line measurements of γ-ray and neutron multiplicities

This paper describes multi-detector arrangements for measuring the number of quanta emitted in a cascade. Two set-ups are described, one with NaI(Tl) detectors and another with liquid scintillators. The latter system has the advantage of being able to separate between neutron and γ-ray events, which is of particular importance for reactions with a large number of neutrons emitted. The set-ups are used on-line the Stockholm cyclotron. Examples of experiments with 51 MeV α-particles and 118 MeV 12C-ions are presented.

Search for Superheavy Elements in Nature

The results of a search for elements in the region of atomic number 114 are reported. More than 40 large samples of ores, naturals minerals, and other possible sources have been investigated. The method of detection made use of a large liquid scintillator to detect spontaneous fission events in which unusually large numbers of neutrons are emitted. The apparatus was located in a tunnel under about 250 m of earth shielding in order to reduce the interfering effects of cosmic rays. The limit set for the ratio of half-life/concentration for all the materials tested was greater than ${10}^{23}$ yr/concentration. If the half-life were ${10}^{9}$ yr this limit would correspond to a concentration of less than ${10}^{\ensuremath{-}14}$ moles of superheavy elements per mole of sample.

Réactions de transfert d'un grand nombre de nucléons induites par 84Kr sur 232Th: étude des seuils de réaction

Abstract Excitation functions have been measured for the formation of 225 Th, 226 Th, 223 Ac and 224 Ac, when 232 Th was bombarded by 84 Kr ions at energies ranging between 450 and 494 MeV. The thresholds of all the reactions appear at 450 MeV (330 MeV c.m.), a higher energy than expected. Cross sections of the order of 1 mb are reached for these transfer reactions corresponding to a large number of neutrons. The results are compared with the same reactions induced with lighter projectiles.

New Keys to Life Processes

NEW KEYS TO LIFE PROCESSES MELVIN CALVIN* It seems entirely appropriate as part of this occasion to have a tribute from biology to physics in general and to the reactor in particular and to Fermi as its creator. Physics has frequently and repeatedly provided biochemistry and biology with new tools for the advancement ofthe sciences. However, in the provision ofan almost unlimited supply ofradioisotopes ofvery nearly all of the elements in the periodic table, physics has provided a tool of immense power and diverse application in biology that has rarely been equaled in the history ofthe two sciences. It is my hope that I will be able to review for you some ofthe highlights ofthe contributions which the availability of these new keys has made possible in the evolution of our knowledge of the construction and behavior of living organisms, from the molecular level to that of man. This is a task which would have been done much better by George Von Hevesy, the man who first conceived of the utility of radioisotopes in learning how living organisms are made and work. And I regret very much that he is not here to do it for you; however, time moves on, and I will do my best. Shortly after the discovery by the physicists of their ability to induce radioactivity in elements which normally were not so, it was clearly evident that thisability, ifitcould be brought to bear on theprimary elements of living organisms, such as hydrogen, carbon, oxygen, nitrogen, phosphorus , and some of the others, would have an enormous value to the investigation ofthe structure, construction, and behavior ofliving organisms . And the search was on. Even with the development ofthe accelera- * Professor ofchemistry and molecular biology, University of California, Berkeley. This paper was presented at the Twenty-fifth Anniversary Observance ofthe First Nuclear Chain Reaction, at the University ofChicago, December I, 1967. Publication in Perspectives is supported by the Dow Chemical Company. 555 tors, particularly the early cyclotrons ofErnest Lawrence in Berkeley, the idea that they might be used for such a purpose grew concomitantly. As soon as cyclotron time became available, it began to be used under Lawrence's prodding for the development of biological problems, both directly as an irradiation instrument and indirectly as a device for producing possibly useful radioisotopes. Since the principal material ofwhich living organisms are constructed is carbon, it was quite obvious that a radioisotope ofcarbon would have immense value in disentangling the current activities ofa living organism from the structure of the machinery which was performing the work. For this reason and other such reasons, the search for radiocarbon isotopes was under way almost as soon as the accelerator was functioning. And it didn't take long for Drs. Ruben and Kamen to produce the first ofthese, namely, the carbon of mass 11 which was radioactive with a half-life of about 20.5 minutes. As you are undoubtedly aware, it is produced by a deuteron bombardment of boron-?? with the emission of a neutron to produce Cu. Most ofthe early work using radiocarbon was with this isotope . However, its short life limited its usefulness, and Ruben had clear reason to search on the other side ofthe stable range for another radioisotope having mass greater than 13. He and Kamen first produced such an isotope by bombarding C13 with deuterons from the 60-inch cyclotron— by a (d,p) reaction giving rise to C14. But they knew that a large number ofneutrons were escaping the cyclotron whenever a deuteron bombardment was underway, and they recognized thepossibility ofcreating C14 by an (n,p) reaction on N14. They therefore surrounded the cyclotron with cans ofammonium nitrate solutions which after a period ofmonths were worked up for C14 and its production confirmed. But since its lifetime was so long (half-life about 5,000 years) and the number ofneutrons available by this method so small, only minuteamounts ofC14were available by this route. About this time, the work toward this end had to stop because ofthe beginning ofthe war, which in addition to everything else took the life of Sam Ruben. Some three years later, the event we were celebrating here today took place, and then...

Nucleosynthesis in the early history of the solar system

This paper revises the model of Fowler, Greenstein and Hoyle (FGH) for the nucleosynthesis of D, Li, Be and B by high energy particles from the Sun during the early history of the solar system. In this model these nuclei are produced by spallation reactions, mainly on O 16, in metric-sized planetesimals. Large numbers of neutrons are also produced. A fraction of these are thermalized and react by B 10( n, α) Li 7, Li( n, α) H 3, to produce the terrestrial Li and B isotopic ratios. Additional D is produced by H 1( n, γ) on hydrogen retained in the planetesimals as H 2O. The total energy required in high energy particles is about 2 × 10 44 ergs. The nuclear calculations have been generalized in an approximate manner to include a dependence on the duration of the irradiation caused by the long lifetime of Be 10. The first stage of the calculation yields the required spallation yields and time-integrated neutron flux to produce the terrestrial Li, Be and B abundances and isotopic ratios. The required flux is 4 × 10 21 n/cm 2, identical with that obtained by FGH, and does not depend significantly on the choice of irradiation time. The predicted spallation yields depend more strongly on the irradiation time. Those are compared with cloud chamber data for O 16 + 300 MeV neutrons. The predicted low spallation yield for Be 9, which merely reflects its low relative abundance, is consistent with the cloud chamber data. Good agreement is not obtained for the B Li spallation ratio; however, for large values of the irradiation time, this could be due to uncertainties in the parameters. This does not seem possible for short irradiation times, however. Tho nuclear processes are less reasonable if C is present in the planetesimals or if higher energy particles are assumed in order to get appreciable amounts of LiBeB from spallation of SiFe as well as from O 16. The second stage of the calculation yields the required hydrogen concentration, H Si ≈ 1 , and “dilution factor,” F d ≈ 20 (approximately the ratio of unirradiated to irradiated material) to give the terrestrial D H ratio. FGH calculated F d = 10 and H Si = 8 . The amount of water is considerably reduced in the present calculations. Although a loss of O during the formation of the Earth still must be postulated, the amount to be lost is much less than in tho FGH case. We point out that the FGH model is compatible with suggestions that the Moon has a high water content and that biotic material has formed in the carbonaceous chondrites and on the lunar surface. The nucleosynthesis of C 13 in its present terrestrial abundance appears quite feasible; however, in this case the solar C 13 C 12 ratio will be much less than that observed terrestrially. Conflicting experimental results exist on this point at the present time. Observed C 13 C 12 variations in meteorites appear to be due to chemical fractionation. The fact that the isotopic composition of Li, Gd, and K in stone meteorites is identical with that found terrestrially requires that both terrestrial and meteoritic material were subjected to the same particle flux and had the same fraction of material irradiated. This implies that the Earth and the meteorites had a common initial history if the basic features of this model are to be retained. A lunar origin for stone meteorites could very well provide the required astrophysical situation to meet this requirement, whereas an asteroidal origin presents many more difficulties.

Radioactive Fallout

THE DETONATION of nuclear devices produces fission products in greater or lesser amounts, depending on the characteristics of the particular device employed. Fission devices which are in the low-yield range derive their explosive force from nuclear fission which gives rise to amounts of radioactive fission products roughly proportional to explosive yield. Thermonuclear devices may have a large fraction of their explosive force produced by a thermonuclear or fusion reaction. They derive only a part of their explosive force from the fission reaction. Of the fission products produced in the fission reaction, those of potential health significance are Sr 90 , Cs 137 ,I 131 , Sr 89 , Ba-La 140 , and Zr-Nb 98 (having half-lives of 28 yr, 30 yr, 8 days, 54 days, 13 days, and 65 days, respectively). Thermonuclear reactions may give rise to tritium (radioactive hydrogen, half-life, 12.8 yr). The very large number of neutrons released in the reactions

TIME-OF-FLIGHT FISSION STUDIES ON U233, U235 AND Pu239

The prompt mass and kinetic energy distributions resulting from the thermal neutron fission of U233, U235, and Pu239 have been reinvestigated using time-of-flight methods to measure simultaneously the velocities of the fragment pairs. A new feature shown by the present work is the existence of fine structure in the prompt mass yields. This fine structure is most pronounced at high total kinetic energies where the fragments have little excitation energy and may be associated with irregularities in the energy release as a function of mass. The fine structure is most noticeable in U235 and least in Pu239; the fragments of U235 have the lowest average excitation and those of Pu239, the highest. Another feature, which is confirmed by this work, is the large drop in total kinetic energy when the fragments are near symmetry. This decrease is about 35 Mev and is consistent either with a picture in which the nucleus with 50 protons is especially preferred or with one in which fragments at symmetric fission have an abnormally high excitation energy and a consequent large number of neutrons. The mean kinetic energies for thermal neutron fission of U233, U235, and Pu239 were found to be 167.6, 168.3, and 175.0 Mev with an error of ± 1.7 Mev.

Iron-55 from Nuclear Detonations

IT is well known that the large numbers of neutrons released during nuclear bomb detonations (∼0.5 kgm. neutrons per megaton explosion)1 can induce high levels of radioactivities in the hardware and associated structures of the apparatus. Of particular interest is iron-55, an X-ray emitter with a half-life of 2.6 years, formed by (n, 2n) and (n, γ) reactions on iron-56 and iron-54, respectively.

Intermediate Energy Neutron Leakage Through Iron

Neutron leakage through a reactor shield composed primarily of iron is discussed. This is of interest whenever the hydrogen content of a shield is reduced either by design requirements or thermal deterioration. Work done at several sites on individual aspects of the problem is combined to present an over-all description of the neutron streaming. In general there are two different phenomena involved, each determined by the geometry. In the case of a long thin streaming path, such as a structural member penetrating the shield, the leakage consists of neutrons which have suffered no collisions. These neutrons will have energies corresponding to energies at which the iron total cross section is small. Iron has several antiresonances in the interval 25 to 100 kev, with the largest dip apparently at 25 kev, so most of the neutron leakage will be at these energies. The other case involves the attenuation of neutrons by large slabs of iron with little or no hydrogen (or other good moderator) present. The 25 kev neutrons are still present, but they are augmented by a large number of neutrons of energy between thermal and 1 Mev. These neutrons may have collided elastically many times but with only a small energy loss each time. Above 1 Mev, inelastic scattering suppresses the leakage, and below a few volts, absorption removes the neutrons.

A Cloud-Chamber Analysis of Cosmic Rays at 14,120 Ft.

Protons and mesotrons near the end of their ranges are distinguished from each other by a method involving the degree of ionization in the gas of the cloud chamber and the scattering in the lead plates in the chamber. Photographs are shown of the production of protons and mesotrons in the lead by non-ionizing and ionizing radiation. Evidence is presented for the presence of an extremely large number of neutrons. Both neutrons and protons are shown to be secondary particles with a maximum energy near 200 Mev. The conclusion is reached that this maximum energy arises from the large cross section of the atmosphere for the production of pairs of mesotrons. The value of this maximum energy can be used as a measure of the mass of the mesotron. Photographs are shown of cascade showers produced by knock-on electrons, stars produced in the lead and the gas of the chamber, and Auger showers filling the whole chamber.