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We have measured the temporal duration of 45 MeV picosecond electron beam bunches using a noninvasive electro-optical (EO) technique. The amplitude of the EO modulation was found to increase linearly with electron beam charge and decrease inversely with distance from the electron beam. The rise time of the temporal signal was limited by our detection system to ∼70 ps. The EO signal due to ionization caused by the electrons traversing the EO crystal was also observed. It has a distinctively long decay time constant and signal polarity opposite to that due to the field induced by the electron beam. The electro-optical technique may be ideal for the measurement of bunch length of femtosecond, relativistic, high energy, charged, particle beams.

For central collisions of 17-115 A MeV {sup 40}Ar+Cu , Ag, Au, an overall balance is determined for the average mass, energy, and longitudinal momentum. Light charged particles and fragments are separated into forward-focused and isotropic components in the frame of the heaviest fragment. Energy removal by the isotropic component reaches 1-2 GeV. For such high deposition energies, statistical multifragmentation models predict much more extensive nuclear disassembly than is observed. (c) 1999 The American Physical Society.

Experiments performed at Lab. Nat. SATURNE on neutron produced by spallation from proton beams in the range 0.8 - 1.6 GeV are presented. Experimental data compared with codes show a significant improvement of the recent intra-nuclear cascade (J. Cugnon). This is also true in the same way for the neutron production from thick targets. However the model underestimates the energetic neutrons produced in the backward direction and other quantities as residual nuclei cross sections are not accurately predicted.

Measurements are reported for fragment masses and velocities from the reactions $(17--115)A\mathrm{MeV}$ ${}^{40}\mathrm{A}\mathrm{r}+\mathrm{C}\mathrm{u},$ Ag, and Au. Charged particle multiplicities were used to select the most violent reactions, and systematics are reported for the momentum and energy deposition. These reactions are fusionlike for $\ensuremath{\lesssim}44A\mathrm{MeV}$ ${}^{40}\mathrm{Ar}$ with a large fraction of momentum and energy deposition in the heavy nuclear system. However, for $\ensuremath{\gtrsim}44A\mathrm{MeV}$ ${}^{40}\mathrm{Ar},$ a majority of the projectile energy and momentum is carried away by a multibody spray of light ejectiles and only a minority is deposited in the heavy nucleus. Nevertheless heavy composite systems are formed with up to 9--12 MeV per source nucleon for $115A\mathrm{MeV}$ ${}^{40}\mathrm{Ar}.$

We present the experimental method conceived to measure high energy neutrons in the range (200 ≤ E ≤ 1600 MeV). The neutrons produce recoil protons in a liquid hydrogen converter. Momentum evaluation and identification of these protons are made by using a magnetic spectrometer equipped with plastic scintillators and three double-plane ( X - Y ) wire chambers. The response functions of the apparatus are determined using quasi-monoenergetic neutron beams produced by the break-up of deuterons or 3 He on a Be target. The performance of the apparatus is illustrated in the form of a preliminary neutron spectrum.

We present an experimental method based on a time-of-flight technique between the tagged incident particles and a thick liquid NE213 scintillator to measure neutron energy spectra from 3 to 400 MeV at the Saturne synchrotron of Saclay. Efficiency measurements are made using a pulsed neutron source from the Tandem Van de Graaff of Bruyeres-le-Châtel at low energies and a quasi-monoenergetic neutron beam at Saturne up to 50 MeV.

This article describes a simple nonlinear mechanical oscillator suitable for an undergraduate physics laboratory. The oscillating system is a Plexiglas blade, with nonlinearity introduced by means of contact with stretched rubber bands. Using a variable driving frequency, students can make amplitude and phase lag measurements with sufficient accuracy for quantitative work. It is possible to study classical resonance with the linear system and then, with the modified system, produce nonlinear effects such as hysteresis.

Heavy residual nuclei are shown to result from the most violent (i.e., central) collisions for {sup 40}Ar + Ag reactions of up to 1.36 GeV; their average velocities are {approx_gt}80{percent} of the c.m. velocity. Angular and energy distributions for {sup 1,2,3}H, {sup 3,4}He, and Li are measured in coincidence with these heavy nuclei. The dominant light particle components are nearly isotropic in a frame of reference having the velocity of the heavy residues. In addition there are forward-peaked high-energy components of the H, He, and Li emission attributable to prethermalization emission. Fractional abundances of these prethermalization components increase markedly with increasing incident energy. Mass and momentum balance preclude the presence of a projectilelike fragment and thus indicate fusionlike reactions with large but incomplete linear momentum transfer. The remainder of the momentum is carried away by the spray of forward-peaked ejectiles. For 1.36 GeV {sup 40}Ar {approximately}1/2 of the 900 MeV available is completely thermalized, and {approximately}1/2 goes into prethermalization emission after strong collisional mixing. {copyright} {ital 1996 The American Physical Society.}

This article describes lecture demonstration apparatus for displaying free and forced oscillations of a mechanical system to a large class. The setup described won first prize at the AAPT apparatus competition in August 1994. A Plexiglas blade clamped at one end is magnetically driven by a low‐frequency generator–amplifier. The frequencies are so low (between 0.2 and 1.2 Hz) that visual observations of displacements are possible. The driving voltage, associated with the force applied, is visualized with a row of 20 light‐emitting diodes, a laser pointer projecting displacements, and a photosensor providing sound cues for phase references. Dependence of amplitude and phase on frequency can be seen, heard, and recorded by a large group of students. The apparatus is simple enough to be replicated and used at all levels of physics teaching. Technical details on the LED display unit, and some theoretical considerations, are described in two appendices.

Lifetime measurements are crucial for our understanding of the dynamics of reaction processes. Nuclear reactions often occur too rapidly for direct measurements of the times involved. Here we describe a newly developed and actively used method for studying decay processes for very hot nuclei (e.g., with temperatures of ≳1010 K or ≳2 MeV) and in particular the associated time scales of ≊10−20–10−22 s (or ≊3000–30 fm/c).

In-plane and out-of-plane angular correlations have been measured between fragments of {ital Z}{gt}3, Li fragments, {sup 3,4}He, and {sup 1,2,3}H. The changing patterns for {sup 40}Ar induced reactions of 7{ital A}, 17{ital A}, 27{ital A}, and 34{ital A} MeV give an overview of the decreasing importance of mass-symmetric fissionlike reactions at the expense of a broad range of more mass-asymmetric breakups. Evidence is given that these fragments come from a central collision group of reactions that have similar violence and from which many combinations of fragments and particles are ejected. Very similar azimuthal angular correlations are observed for particles with a Li fragment and for particles with a pair of heavier fragments ({ital Z}{gt}3). This similarity suggests comparable strengths of association with the reaction plane for single Li fragments and for fragment pairs of {ital Z}{gt}3. Azimuthal angular correlations for Li-Li pairs exhibit distinct asymmetries; their interpretation via trajectory-model calculations indicates mean delay times of {approx}5{times}10{sup {minus}22} s.

1) A year ago I would have replied that cold fusion is pseudoscience. But I am no longer comfortable with this kind of reply. Why am I not comfortable? My first opinion was based on Huizenga’s famous ERAB report. I knew the author personally and I respected him. His criticism of cold fusion was convincing because it was based on the idea that cold fusion is a thermonuclear reaction between two colliding hydrogen ions. Experimental data certainly do not support such an idea. But who made this claim? Only the popular press and those who criticized cold fusion. Fleischmann and Pons, who discovered excess heat, speculated that some kind of nuclear processes might be responsible for what they observed. The very name “cold fusion” was highly unfortunate because it misled many scientists, including myself. Who invented this term? Was it introduced deliberately (as a straw man) to discredit the nascent field of research? I have no answers to such questions. But I suspect that this misleading term will survive the controversy; that is why I decided to use it here.

We have made an observation of 45 MeV electron beam bunches using the nondestructive electro-optical (EO) technique. The amplitude of the EO modulation was found to increase linearly with electron beam charge and decrease inversely with the optical beam path distance from the electron beam. The risetime of the signal was bandwidth limited by our detection system to \~70ps. An EO signal due to ionization caused by the electrons traversing the EO crystal was also observed. The EO technique may be ideal for the measurement of bunch structure with femtosecond resolution of relativistic charged particle beam bunches., 3 Pages, 1 figure. Presented at DPF2000 conference at Ohio

We have made the first observation of a charged particle beam by means of its electro-optical effect on the polarization of laser light in a LiNbO3 crystal. The modulation of the laser light during the passage of a pulsed electron beam was observed using a fast photodiode and a digital oscilloscope. The fastest rise time measured, 120 ps, was obtained in the single shot mode and was limited by the bandwidth of the oscilloscope and the associated electronics. This technology holds promise for detectors of greatly improved spatial and temporal resolution for single relativistic charged particles as well as particle beams., 7 pages, 2 figures. Published in Nucl.Instrum.Meth.A452:396-400,2000

The ensemble of charged isotropically emitted ejectiles is studied for central collisions of $(17\ensuremath{-}115)A$ MeV ${}^{40}\mathrm{Ar}\mathrm{}+\mathrm{}\mathrm{Cu},$ Ag, Au. Measurements of average multiplicities, spectral slopes, and masses of the heaviest fragments are compared to statistical models for multifragmentation or sequential evaporation. The multifragmentation models predict much more complete nuclear disassembly than is observed. The evaporation model reproduces the data much more closely except for the spectra of $Z=1$ ejectiles. The kinetic energies of $Z=1$ and 2 ejectiles are much less than found for $1A$ GeV ${}^{197}\mathrm{Au}{+}^{12}\mathrm{C}$ for similar energy depositions. Entrance channel dynamics seem to affect the isotropic emission ensembles, often taken to define an equilibrated emission source.

We have made the first observation of a charged particle beam by means of its electro-optical effect on the propagation of laser light in a birefringent crystal at the Brookhaven National Laboratory Accelerator Test Facility. Polarized infrared light was coupled to a LiNbO/sub 3/ crystal through a polarization maintaining fiber of 4 micron diameter. An electron beam in 10 ps bunches of 1 mm diameter was scanned across the crystal. The modulation of the laser light during passage of the electron beam was observed using a photodiode with 45 GHz bandwidth. The fastest rise time measured, 120 ps, was made in the single shot mode and was limited by the bandwidth of the oscilloscope and the associated electronics. Both polarization dependent and polarization independent effects were observed. This technology holds promise of greatly improved spatial and temporal resolution of charged particle beams.

Spallation neutron production in proton induced reactions on Pb targets at 0.8, 1.2, and 1.6 GeV has been measured at the SATURNE accelerator. Double-differential cross sections were obtained over a broad angular range from which averaged neutron multiplicities per reaction were inferred for energies above 2 MeV. The results are compared with calculations performed with a high energy transport code including two different intranuclear cascade (INC) models: it is shown that the Cugnon INC model gives a better agreement with the data than the Bertini one, mainly because of improved nucleon-nucleon cross sections and Pauli blocking treatment. © 1999 The American Physical Society.

Light charged particles from the reactions 247 and 337 MeV $^{40}\mathrm{Ar}$${+}^{\mathrm{nat}}$Ag have been measured in singles and in coincidence with fission-like products. Most of the $^{1}\mathrm{H}$ and $^{4}\mathrm{He}$ in coincidence with fusion-fission and quasifission can be accounted for by evaporation from the composite nucleus or by the fragments. Multiplicities are determined for $^{1}\mathrm{H}$ and $^{4}\mathrm{He}$ in association with evaporation residues, fusion-fission, and quasifission. The probability for composite nucleus emission decreases with increasing spin zone of the emitter and gives a view of the role of emitter lifetime. The ratio of composite-nucleus multiplicities for quasifission/fusion-fission is about 1/2 ((1/4) for 337 MeV $^{40}\mathrm{Ar}$ (247 MeV $^{40}\mathrm{Ar}$) which implies a similar ratio for the lifetimes of the respective composite nuclei between thermalization and scission. Energy spectra and angular distributions of particles (in coincidence with fusion-fission and quasifission) are consistent with emission from a strongly distorted nuclear system en route toward scission. Prethermalization emission of $^{4}\mathrm{He}$ is observed at 10\ifmmode^\circ\else\textdegree\fi{} in comparable amounts for peripheral collisions (quasifission and fusion-fission) and for central collisions.

The binary fission process in Au and U induced by 12 C at 30, 60 and 84 Me u has been investigated. Excitation energies of the fused nuclei are inferred from the mass measurements as a function of the transferred linear momentum. Incomplete-fusion models appear to account for the bulk of the data. The variance of the mass distribution of the fragments increases roughly linearly with the temperature derived at saddle point as expected in a thermal process.

Cross sections for evaporation residues following complete fusion have been measured for the bombardment of $^{27}\mathrm{Al}$ with 81-, 105-, 126-, and 168-MeV $^{16}\mathrm{O}$, 138- and 210-MeV $^{20}\mathrm{Ne}$, and 336-MeV $^{32}\mathrm{S}$. The results were obtained using a counter telescope and, in some cases, differ considerably from previous work with mica track detectors. The experimental critical angular momenta for fusion are in good agreement with recent theoretical calculations for the O and Ne bombardments. Our data suggest parameters for the critical distance of approach which are significantly smaller than previously reported values.

Angular correlations have been measured between heavy fragments in $^{86}\mathrm{K}$R + $^{197}\mathrm{Au}$ reactions using two gas telescopes. The data have been analyzed to yield a separation of two-body from multibody events. The assumption that sequential fission accounts for all multibody breakup allows the determination of sequential fission probability as a function of inelastic energy transfer. Comparison to fission excitation functions for similar heavy compound nuclei provides insight into the deposition of energy and angular momentum. For $\ensuremath{-}Q$ of \ensuremath{\approx} 200 MeV or E* \ensuremath{\approx} 140 MeV typical spin values of $30\ensuremath{-}40\ensuremath{\hbar}$ can be estimated for the heavy (Aulike) fragments.

Reactions of /sup 1/2C and /sup 1/9F have been used to produce /sup 1/94Hg compound nuclei at excitation energies of 98 and 142 MeV. This combination of entrance channels permits the separation of energy and spin effects on the exit channels. Fission probability clearly increases with angular momentum at fixed energy. However, there is no increase in the cumulative fission fraction from 98 to 142 MeV for essentially constant spin domain. In contrast the probabilty of /sup 1/H and /sup 4/He evaporation increases rapidly with energy and seems to shield the system from fission.

Measurements of longitudinal momenta transferred to mass-identified products of the fragmentation of Cu by $^{12}\mathrm{C}$ ions give clear evidence for a change in reaction mechanism between 22 and 84 MeV/u. Results at 84 MeV/u are generally consistent with peripheral interactions. However, at 22 MeV/u large momentum transfers observed for near-target products suggest that strongly damped proceses have become important. Limits to momentum transfer of the type reported by Galin et al. are shown to arise in a natural way from this transition.

Energy and angular distributions have been measured for H and He both in singles and in coincidence with a heavy fragment. Large cross sections have been observed for both a low-temperature component at backward angles and a high-temperature component at forward angles. Both the preequilibrium and evaporationlike components are apparently emitted prior to scission, predominantly in coincidence with fragments of Z>27. The angular anisotropy and out-of-plane correlation for /sup 4/He are not consistent with emission from the composite systems of highest spin ( approx. = 190 h) but are consistent with emission from the /sup 237/Bk composite with average spin approx. = 70 h. Extensive energy sharing and H/He emission occur in many reactions so rapidly as to precede fission which is heavily favored by energetic and phase space considerations.

Measurements are reported for correlated energies and angles of heavy fragments from reactions of 343 MeV40Ar with197Au. The results show a continuous evolution of the mass distributions from fission-like products at angle pairs of ≈60 and 300 degrees to deeply inelastic projectile-like products at ≈45° and 300°. Certain difficulties are emphasized for the use of the “symmetric fragmentation” cross sections in the context of current theoretical models (e.g. the “extra push” theory). The fission-like reactions have been studied in some detail and the results have been compared to a Monte Carlo kinematic simulation. To account for the experimental results forward-peaked light particle emission (n, H, He) must occur in a large fraction of the fission-like reactions.

We have measured coincidences between $^{4}\mathrm{He}$ and $^{1}$,2,3H or a second $^{4}\mathrm{He}$ (large-angle correlations) in the reactions 247 and 337 MeV $^{40}$Ar${+}^{\mathrm{nat}}$Ag. These coincidences are dominated by reactions leading to evaporation residues with spins of about 0--70\ensuremath{\Elzxh}. Large values of the angle-integrated double and triple coincidence cross sections indicate that many particle evaporation chains involve 2 or 3 protons and/or 2 or 3 alphas. The angular and energy distributions are well described by an evaporation model with Hauser-Feshbach coupling. However, one must employ effective evaporation barriers for $^{1}\mathrm{H}$ and $^{4}\mathrm{He}$ that are strongly reduced compared to empirical fusion barriers. The barrier reduction required for $^{2}\mathrm{H}$ is rather small, and for $^{3}\mathrm{H}$ no barrier reduction is needed. Evaporation calculations that include emitter deformation do not give a satisfactory explanation for these results. We suggest a new aspect of composite nucleus excitation and evaporation-like decay. A hot, very diffuse nuclear surface may be formed that promotes evaporative decay prior to its relaxation. The low barriers for $^{1}\mathrm{H}$ and $^{4}\mathrm{He}$ reflect the radial extent of this transitory, diffuse surface.

Emission of4He in the reaction 334 MeV40Ar+238U has been studied by triple coincidence measurements that allow the separate identification of fusion fission and sequential fission. For the4He evaporative spectra from fusion fission the composite system is shown to be the predominant contributor; whereas, for sequential fission the dominant emission is from the fragments. This result demonstrates a correlation between evaporative emission probability and lifetime expectancy of the composite system. To account for the observed4He spectra two other mechanisms are necessary in addition to nuclear evaporation. At forward angles, the4He spectra from both fusion fission and sequential fission exhibit higher intensities and larger energies than those expected from purely evaporative processes. This forward-peaked component must be related to a very rapid or pre-thermalization stage of the reaction. At backward angles yet another component is observed for fusion fission. As it is sensitive to the fragment masses but does not carry the kinematic shift characteristic of their full acceleration, this component must originate near to the time of scission. The average4He energy for this component is approximately 17 MeV (c.m.), and its intensity is correlated with a plane perpendicular to the fission fragment separation axis. These signatures are similar to those for long range alpha particle emission in low energy fission. Alpha particles evaporated from the composite nuclei in fusion-fission reactions are shown to be preferentially associated with fission events which result in the more symmetric masses. This result is consistent with the notion that mass asymmetric fission is a faster process than symmetric fission. Such a correlation between mass asymmetry and lifetime is an essential part of the “fast fission” or “quasifission” idea, which has attracted much current attention.

Projectile-like fragment production at the grazing angle has been studied in 27 Al + 63 Cu collisions at 13.4 MeV/nucleon. Isotopes are emitted with a roughly constant velocity which is significantly lower than the beam one. A specific dissociation mechanism of partially damped projectile-like nucleus has been observed in fragment-fragment coincidence measurements.

Multiplicities are compared for4He evaporation in reactions of40Ar and56Fe projectiles (8.5 MeV/u) with withnatAg and238U targets. Coincidence requirements involving fusion-fission and projectile-like fragments have been used to select separate entrance-channel spin zones. Evaporation-like4He emission from the composite nuclei is shown to be large for mean spins ≦100ℏ, and to decrease dramatically with increasing spin. Coincidence measurements between two4He particles have also been studied for40Ar+natAg. This requirement is shown to select reactions that lead to evaporation residues and therefore to a zone of relatively low spin. The shapes of the4He spectra provide an interesting probe of the mechanism and also of the thermodynamic properties of hot nuclei.

The angular distribution of the fission products formed in the reaction of 126-MeV ${\mathrm{C}}^{12}$ ions with ${\mathrm{Gd}}^{158}$ and 137-MeV ${\mathrm{O}}^{16}$ ions with ${\mathrm{Sm}}^{154}$, both of which give ${\mathrm{Yb}}^{170}$ compound nuclei at an excitation of 107 MeV, have been measured. It is found that the angular distribution in the ${\mathrm{C}}^{12}$-ion entrance channel is proportional to $\frac{1}{sin\ensuremath{\theta}}$ from about 25 to 155\ifmmode^\circ\else\textdegree\fi{} in the center-of-mass system, while the corresponding range is about 15 to 165\ifmmode^\circ\else\textdegree\fi{} for the ${\mathrm{O}}^{16}$-ion entrance channel. The total fission cross sections are 20 \ifmmode\pm\else\textpm\fi{} 2 and 39 \ifmmode\pm\else\textpm\fi{} 4 mb for the two entrance channels, respectively.

Angular and energy distributions have been measured for products of 3≦Z≦9 from reactions of 336MeV40Ar+107,9Ag/154Sm/197Au. The c.m. differential cross sections and mean energies increase as one moves from ≈100deg. to ≈170deg. similar to those for fission or evaporation-like emission. Most characteristics mimic those observed for4He, but the cross sections are smaller by a factor of as ≈1/300 for each reaction studied. A semiclassical analysis has been used to estimate the effective barriers to emission and root-mean-square spins of the emitters.

We have investigated the emission of 4 He in coincidence with mass-selected fragments from fission for 334 MeV 40 Ar + 238 U. Upper-limit multiplicities are assigned for fragment evaporation and composite-nucleus evaporation, but additional mechanisms are also required. A new low-energy component is found at backward angles; it is sensitive to the fragment masses but does not reflect their full kinematic shifts. Thus it must be due to near-scission emission (prior to full fragment acceleration).