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The statistical distribution of the number of emitted secondary electrons per primary incident on transmission alkali‐halide dynodes has been studied. For high primary energies (1 MeV), a model based on the theory of dielectric energy loss and the secondary electron‐phonon interaction is proposed to account for statistical distributions. Measurements of distributions from 500‐A‐thick films of CsI, KCl, and LiF have been carried out both at high energies and for primary energies near 9 keV corresponding to the maximum of secondary yield. It is shown that with some simplifying approximations, the model proposed can be used to calculate the distributions observed with high‐energy primaries. Qualitatively, it is also possible to use the ideas proposed to understand the experimental results at low energy. Although in the literature the statement is often made that secondary emission distributions can be expected to be Poissonian, it is shown in this paper that there is very little firm basis for such a stateme...

The general problem of determining photonuclear reaction energies in the region below 25 Mev is discussed, and the need for absolute calibration of betatron energy scales is demonstrated. A system for accomplishing such a calibration is briefly described and the results of some measurements made with it are given. The ${\mathrm{Cu}}^{63}(\ensuremath{\gamma}, n){\mathrm{Cu}}^{62}$ threshold was observed to be at 10.78\ifmmode\pm\else\textpm\fi{}0.03 Mev, and two well-defined "breaks" in the ${\mathrm{O}}^{16}(\ensuremath{\gamma}, n){\mathrm{O}}^{15}$ yield curve are at 16.19\ifmmode\pm\else\textpm\fi{}0.04 and 17.25\ifmmode\pm\else\textpm\fi{}0.04 Mev. These "breaks" lead to the assignment of energy levels in ${\mathrm{O}}^{16}$ which are in excellent agreement with those observed in the ${\mathrm{N}}^{15}(p, n){\mathrm{O}}^{15}$ reaction.

A NaI crystal was used to study the spectra of gamma rays associated with the giant resonance cross section of the nuclear photoeffect in carbon, oxygen, and copper. The x-ray source was a bremsstrahlung beam whose energy was varied from 19 to 61 Mev. Cross sections for elastic, and mixed elastic and inelastic scattering was measured it 135 deg to the x-ray beam. The copper cross section has a magnitude which is well predicted by particle photoproduction cross sections coupled with a dipole dispersion theory. The elastic scattering cross section for carbon can be predicted in a similar fashion, but the oxygen cross section cannot. For oxygen, the observed cross section is much larger than the predicted one and this result is explained if narrow, isolated resonances are an important part of the oxygen photonuclear cross section. The different behavior of carbon and oxygen which is found is consistent with other experiments. The angular dependence of the oxygen elastic scattering cross section near 21 Mev is predominantly dipole with a possible quadrupole admixture. For oxygen, an inelastic scattering cross section was observed which has a threshold at about 26 Mev, a peak at about 30 Me, and is due tomore » transitions to a state (or states) near 6 Mev. It is interpreted at the result of an overlap of two giant resonance cross sections--one for the ground state and one for the excited state. Thy consequences of this interpretation to the theory of the nuclear photoeffect is discussed. The yield of gamma rays from O/sup 15/ and N/sup 15/ which follow neutron or proton emission was also studied and several lines were observed, but none above 6.5 Mev. The photonuclear cross sections associated with these lines are estimated to be 45% of the whole photonuclear cross section at 23 Mev. (auth)« less

The interaction between conduction electrons with energies from 0.25 to 7.5 eV and longitudinal optical phonons in alkali halides is studied in detail by time‐dependent perturbation theory. Expressions for the rate and angular distribution of scattering are obtained. The electron‐transport problem is then solved with the exact quantum mechanical scattering results by a direct simulation Monte Carlo method. Probabilities of escape and average energy losses for electrons generated isotropically at a certain depth in the material, with a given initial energy, are computed for CsI, KCl, NaF, and LiF. A simple theory shows the effective mass and temperature dependence. The effect of including scattering to angles other than forward is quite apparent in the results.

ABS>The importance of narrow, isolated, resonances in the photonuclear absorption cross sections of C and O was investigated by means of an absorption experiment performed with 30.7-Mev bremsstrahlang. The experiment yielded values for the cross sections averaged over the ( gamma ,n) cross sections for C/sup 12/ and O/sup 16/. The results for C were the same as those expected for a cross section without narrow resonances, while for O a definite resonance effect was demonstrated. The O photonuclear absorption cross section, averaged over the O/ sup 16/ ( gamma ,n)O/sup 15/ cross section, was found to be 26.7 plus or minus 3.5 millibarns, l6 plus or minus 4 millibarns higher than the value expected if narrow resonances contribute insignificantly. Estimated values for the parameters of the O resonances near 22 Mev arei average peak height, 106 plus or minus 14 mb; ratio of the radiative width (to ground state) to total width, (6.6 approximately 0.9) x 10/sup -3/; total width about 30 kev. The experimental results are insensitive to uncertainties in the non-nuclear x-ray attenuation cross sections for C and O. (auth)

The charge-exchange scattering of negative pions by liquid hydrogen was measured at 61 plus or minus 1 and 95 plus or minus 2 Mev bombarding energy. The measurements were made with a gamma spectrometer which employs a lead glass Cherenkov counter. If the charge exchange scattering cross section is expanded as a series of Legendre polynomials in the center-of-mass system of the collision, it is found that at 61 Mev, d sigma /d OMEGA = (1.00 plus or minus 0.05)STA0.613 plus or minus and at 95 Mev, d sigma /d OMEGA = (1.00 plus or minus 0.03)STA1.05 plus or minus 0.05 -- (1.15 plus or minus 0.12)P/sub 1/ (cos theta ') + (0.33 plus or minus 0.25)P/sub 2/(cos theta ')!. The total cross section for charge exchange, obtained by intteration, (is sigma /sub tot/( pi /sup -/ yields pi /sup 0/)= 7.7 plus or minus 0.6 mb at 6l Mev and sigma / sub tot / ( pi /sup -/ yields pi /sup 0/) == 13.2 plus or minus 0.8 mb at 95 Mev. A table summarizing the measurements performed at 61, 95, 128, and 150 Mev is given. (auth)

A materials research program is described. The program objectives are to evaluate materials and develop processing and fabrication techniques which it will be economically feasible to use in the construction of a high-gradient (10 MeV/ft) two-mile linear accelerator. RF properties are evaluated by cavity Q measurements at selected frequencies in the range 3 to 11 GHz. Techniques for reducing field emission are being sought. At present, this phenomenon is being studied by making dc measurements at room temperature. RF superconducting properties are known to be very sensitive functions of material purity and crystal structure. For this reason, various methods of deposition, surface purification and heat treatment are being tried. Most work is being done with lead and niobium, but technetium plating is being actively investigated.

An electronic gate circuit is described which has better than 1% linearity for pulses in the range 1 to 70 volts. The circuit gives an inverted output with a gain very close to unity and accepts positive inputs. The leakage of a 70‐volt pulse through the closed gate is 0.6 volt and the largest negative going portion of the gating transient is 0.2 volt. The gate can be reliably opened with positive trigger pulses in the range 4 to 9 volts and the time jitter over this range is the same as the rise time of the trigger pulses. The inherent rise time of the gate output is about 50 millimicroseconds.

The charge exchange scattering of negative pions by liquid hydrogen has been measured at 128\ifmmode\pm\else\textpm\fi{}2 Mev bombarding energy. A lead-glass \ifmmode \check{C}\else \v{C}\fi{}erenkov counter was used to measure the energy spectrum of the gamma rays emitted in the decay of the neutral pions. The gamma rays were detected at four angles relative to the incident beam: 45\ifmmode^\circ\else\textdegree\fi{}, 80\ifmmode^\circ\else\textdegree\fi{}, 116\ifmmode^\circ\else\textdegree\fi{}, and 135\ifmmode^\circ\else\textdegree\fi{}. If the charge exchange scattering cross section is expanded as a sum of Legendre polynomials which are functions of the ${\ensuremath{\pi}}^{0}$ scattering angle in the center-of-mass system, we find that $\frac{d\ensuremath{\sigma}}{d\ensuremath{\Omega}}({\ensuremath{\pi}}^{\ensuremath{-}}, {\ensuremath{\pi}}^{0})=(1.00\ifmmode\pm\else\textpm\fi{}0.04)[(2.04\ifmmode\pm\else\textpm\fi{}0.06)+(\ensuremath{-}1.61\ifmmode\pm\else\textpm\fi{}0.13){P}_{1}+(1.43\ifmmode\pm\else\textpm\fi{}0.24){P}_{2}],$ when only $s$ and $p$ waves are considered. The confidence level for the least-squares fit used to determine the coefficients inside the square brackets is 65%. The integrated cross section is ${\ensuremath{\sigma}}_{\mathrm{tot}}({\ensuremath{\pi}}^{\ensuremath{-}}, {\ensuremath{\pi}}^{0})=25.6\ifmmode\pm\else\textpm\fi{}1.3$ mb, which is in good agreement with other work.

A system for the energy analysis of scattered gamma rays of up to 70‐Mev energy is described. A 5‐in. diam, 4‐in. long NaI(Tl) crystal was the energy sensitive element. The net amplification between the ten‐stage photomultiplier output and the pulse‐height analyzer input was 1.8. The pulse‐height‐to‐channel transfer characteristic of the system departs from linearity by less than 0.05 channel between the 50th and 16th channel and by one channel between the 16th and sixth. It is demonstrated that the light‐to‐voltage transfer characteristic of the photomultiplier is linear up to at least 30‐v output levels. Provision for direct determination of the effect of superposed pulses (pileup) is incorporated into the system. Careful shaping of the pulses reduced pileup to a minimum, commensurate with obtaining good resolution from the NaI(Tl) crystal. Some typical pulse‐height spectra are given, and the novel features of the circuits are described.

The mechanism of field‐enhanced secondary emission has been studied experimentally in detail for KCl low‐density transmission dynodes transferred in a dry nitrogen atmosphere. By means of a Kelvin probe located at the center of a collector, dynode surface potentials and their relationship with collector potential, secondary yield, primary energy, and current density have been studied. A unique relationship between yield and surface potential has been found, independent of all other parameters for current densities between 1 and 10 nA/cm2. This study clearly shows that an avalanche process, together with an increase in escape length, is responsible for the high yields obtainable. A study of transient effects reveals that the high surface potential needed for high yield decays rapidly after the primary beam is turned off. A smaller surface potential, with a long decay time constant, can be expected to provide slight field enhancement for single particle bombardment at low rates. The statistical distributions of the number of emitted secondary electrons per primary incident at high (1‐MeV) and low (9.5‐keV) energies have been measured for low‐density films of CsI and KCl at low count rates. The results do not differ very much from those obtained previously from normal density films except that the yields are somewhat higher, particularly after the film has been charged with a dense primary beam. The distributions obtained are non‐Poissonian in nature (more nearly exponential), and this is explained by their similarity to normal density films.

This paper deals with recent developments of power absorbing devices such as beam dumps, slits, and collimators. A brief discussion is given of the heat transfer experiments that have been conducted at the Stanford Linear Accelerator Center to establish design heat flux conditions for the various materials to be used for the power absorbing devices. Design concepts of a 2.2 MW, 11 to 25 GeV beam dump, as presently being built at SLAC, are also discussed. Finally, there is a typical analysis which leads to the design of a collimator, with emphasis on beam definition, heat transfer, thermal stress development and material selection.

The absorbed electron current for a clean Fe(100) surface as a function of energy rises steplike at the vacuum‐energy cutoff with an absorption close to 1. The smooth decrease of absorbed current at higher electron energy due to secondary electron emission is superimposed by a considerable amount of fine structure, the amplitude of which decreases with increasing energy. These features are found in good agreement with the results of a calculation of the elastic part of the electron reflection coefficient. Further, they are compared with the ferromagnetic electronic bulk band structure calculated above the vacuum energy. From the comparison with the experimental data, the energy dependence of the real and imaginary parts of the inner potential is determined.

We are investigating the RF properties of thin films of materials which are known to have low secondary emission coefficients, such as NbC, NbN and TiN. Preliminary measurements on the latter material have been performed by depositing a 15 nm film on parts of a doubly re-entrant Nb cavity designed to favor electron multipacting which, in the uncoated cavity, occurs copiously between the posts'tips. The measurements performed with TiN films sputtered onto Nb indicate that the RF current losses are increased by the presence of the films while the dielectric losses are negligible, within the measurement sensitivity of the system. The electron multipacting cannot be excited between the posts coated with the material tested.

A lasertron (a microwave "triode" with an RF output cavity and an RF modulated laser to illuminate a photocathode) is a possible high power RF amplifier for TeV linear colliders. As the first step toward building a 35 MW, S-band lasertron for a proof of principle demonstration, a 400 kV dc diode is being designed with a GaAs photocathode, a drift-tube and a collector. After some cathode life tests are made in the diode, an RF output cavity will replace the drift tube and a modelocked, frequency-doubled, Nd:YAG laser, modulated to produce a 1 us-long comb of 60 ps pulses at a 2856 MHz rate, will be used to illuminate the photocathode to make an RF power source out of the device. This paper discusses the plans for the project and includes some results of numerical simulation studies of the lasertron as well as some of the ultra-high vacuum and mechanical design requirements for incorporating a photocathode.

An in vacuo translational‐rotational motion based on a ball bearing package is described. Lubrication is accomplished by making the ball retainer from a solid dry lubricant, boron nitride (BN). This eliminates the need to separately coat the balls with lubricant prior to assembly; however, the balls are continuously coated while the bearing is in use.

Dynamical calculations are performed to determine the differential cross section of low energy electrons scattered from antiferromagnetic NiO (111) surfaces. We find that the spin-dependence of this quantity with respect to the incident electron polarization depends strongly on: (1) the magnetization of the topmost layer, (2) the exchange potential model used, and (3) the incident beam angle.

Resistivity measurements have been made on Nb cavities, as well as on Pb and Cu, at 296, 77, and 4.2 K by means of a contactless induced-current method. For superconductors, a constant magnetic field drives the material normal below the transition temperature. These measurements provide a simple means for initial material evaluation as well as a direct means of monitoring the effects of material parameters (purity, heat treatment, gas incorporation, etc.) on the electron mean free path. Approximate determinations of Hc, Hc1, and Hc2 can also be derived from these measurements. Normal-state thermal conductivity and the Ginzburg-Landau parameter kappa are calculated from the resistivity measurements., 8 pages. 1 table

The width of the central resonance absorption line in the Mossbauer effect in Fe/sup 57/ being wider than the natural linewidth suggests that an external perturbation exists which affects the nuclear levels. Accordingly, experiments were carried out to detect whether anisotropy in inertia (due to nonuniform distribution of matter in the galaxy) may be responsible for the line broadening. The linewidth was measured for a Fe/sup 57/ source and a Fe absorber with (1) no magnetic field present and (2) both source and absorber parallel to magnetic fields of about 10/sup 3/ gauss; no difference was found. Next, the amplitudes of the center of the line and of four Doppler-shifted points on the side of the line were measured with 600-gauss magnetic fields oriented at +45 deg and --45 deg in the vertical (north-south) plane at different times so that the galactic center exerts its maximum and minimum effects on the level shift, The amplitudes were found to be constant within the statistical limits, which give a result of DELTA M/M (fraction of inertia change) of (5 x 10/sup -16/ (D.L.C.)

The thirty accelerator sectors (each 330 feet long) of the two-mile machine will be evacuated with sputter-ion pumps to pressures low enough to limit rf breakdown and multipactor problems. After the beam leaves the accelerator it will be "switched" by pulsed magnets to one or more of the experimental areas through the 1000-foot beam switchyard. Due to the nature of the equipment in the beam switchyard and the requirements for inexpensive, non-elastomeric, remotely operable couplings, there will be a high probability for virtual and real leaks and a consequent need for continuous high throughput pumping. Oil diffusion pumps are therefore used to evacuate the four branching beam tubes of the beam switchyard. Some kind of in-line oil trap is needed to separate the clean, low pressure (10-7 torr), ion-pumped accelerator from the oil-pumped (10-4 torr) beam switchyard end of the machine. This trap should limit the amount of oil and organic fractions passing back through to the accelerator and should provide some of the impedance necessary to obtain the three-order-of-magnitude pressure difference between accelerator and switchyard. The pressure difference will be maintained by differentially pumping on each of two in-line vacuum impedances. One of these impedances will be a standard 10-foot section of disk-loaded waveguide and will act as an oil trap. When refrigerated, such a trap had measured transmission probabilities of 10-9 to 10-6 for heavy hydrocarbons.

Development of a high intensity, rapidly reversible, polarized electron source to serve as an injector for the Stanford Linear Accelerator is underway. Polarized electrons are obtained by illuminating negative electron affinity GaAs with circularly polarized light of wavelength correponding to the band gap. The present status of the project is described. It is anticipated that this source will provide sufficient intensity of ?50−5 to be measured in reasonable periods of time.

We have extended our earlier measurements of parity violating asymmetries in the inelastic scattering of longitudinally polarized electrons from deuterium to cover the range 0.15≤ y ≤0.36. The observed asymmetry shows only slight y dependence over this range. Our results are consistent with the expectations of the Weinberg-Salam model for a value of sin 2 θ w = 0.224±0.020.

When the monetary and political consequences of abortive or marginally successful space flights are considered, it becomes evident that preliminary research and proof testing under conditions encountered in the space environment are highly desirable. The recognized phenomena of material composition changes, optical property modification, surface friction enhancemen, cold welding, weight loss, sputtering, and many others may be studied under controlled conditions over extended periods of time by direct measurement in proper space environment simulators. By appropriate variation of the individual conditions, the effects on a particular phenomenon may be determined, with the concomitant possibility of accelerating effects experienced during space flights. New or unexpected phenomena may be discovered and investigated without risking a space mission to do so.

Klystrons manufactured at the Stanford Linear Accelerator Center are typically baked at temperatures of 550 °C for times as long as 200 h. During these long bakeouts the copper‐gasketed type 304 stainless steel flange joints (seven in all) diffusion bond so intimately that the flanges can be separated only with a jacking fixture, and copper is left on the sealing surfaces. Removal of this copper necessitates the use of abrasive materials, which can result in contamination of the klystron body and compromise its reuse. We report on the use of 50 and 150 A TiN thin films as a diffusion barrier between the Cu gasket and the stainless‐steel flange.

The linear gate circuit described utilizes a pair of diodes as the gating elements. Similar circuits have previously been used for gates, but stored charge effects in the diodes have limited them to gate durations of about 500-mμsec or more. Recently diodes have become available which have 6-mμsec recovery time and a back-voltage rating of 15 v. These allow the construction of linear gates with durations as short as 20 mμsec. (A.C.)

Diamondlike carbon films of 5−15 nm were deposited on copper gaskets which were sealed in 304−stainless steel flanges. The carbon films acted as a diffusion barrier during high temperature bakeout (550 °C at 1 x 10−5 Torr for up to 200 hours) which prevented carbon diffusion into the copper. (AIP)

Measuring the valence band density of states (DOS) of Fe provides a stringent test of models of metallic ferromagnetism at finite temperatures. The density of states of Fe above the Curie temperature TC, as obtained by self‐consistent calculations in the disordered local moment (DLM) picture by Oguchi et al. and Pindor et al. differs strongly from the T=0 DOS calculated by Moruzzi et al. between the Fermi energy and 2‐eV binding energy, i.e., the relatively broad peak in the T=0 DOS around 1‐eV binding energy is of much smaller amplitude in the T≳TC DOS. This effect should be observable by valence band XPS on Fe as a 30% decrease of intensity at about 1‐eV binding energy with a typical energy resolution of about 0.8 eV. We have measured the valence band XPS of Fe(100) at room temperature and temperatures above TC. The predicted intensity decrease has not been observed.

The higher energies planned for the next generation of particle accelerators and storage rings makes the use of superconducting hi-Q RF cavities highly desirable. Past efforts to produce reliable cavities for such projects have met with limited success. Among the barriers to achieving the maximum electric field gradient are oxide layer charging, single surface multipactoring and field emission. These are suface effects. At SLAC, a multi-technique surface analysis system has been constructed to examine possible sources of these problems and to suggest processes or surface coatings which will reduce or eliminate them. As one component of this analysis, we have investigated the time evolution of species on anodized Nb/sub 2/O/sub 5/-on-Nb surfaces as a function of electron bombardment. The surface concentration of C increases at an anomalously high rate under the exciting electron beam. Examination of the surface and gas phase indicate that the C source is in the underlying material. Estimates of the penetration depth of the beam are in agreement with the fact that there is a significant rate increase in the surface C buildup when the beam penetrates the anodized layer into the Nb bulk. Bulk analytical methods indicate, however, that the C concentration in the Nbmore » is very low. Grain boundary diffusion of C to the surface and/or an enhancement of the C diffusion coefficient due to localized beam pipe heating are examined as possible explanations of this effect.« less

A "peaking strip" is described which, when placed across the poles of a betatron, produces a sharp pulse whenever the magnetic field passes through zero. (A.C.)