Index of content:
Volume 30, Issue 11, November 1959
30(1959); http://dx.doi.org/10.1063/1.1716441View Description Hide Description
A versatile neutron time‐of‐flight spectrometer suitable for use in the millimicrosecond region is described. Either the rf beam deflection method or the gamma coincidence method is used to provide the zero time indication. Details of these methods, along with spectra obtained for some (d,n) reactions, are given. The two systems are compatible and make use of the same simple time‐to‐pulse‐height converter. The dependence of the decay time of a liquid scintillator on the specific ionization of the detected particle has been used to make the neutrondetector insensitive to gamma‐rays.
30(1959); http://dx.doi.org/10.1063/1.1716442View Description Hide Description
An automatic recording filmbalance system is described. It consists of a film trough and two pendulum type balances with pressure ranges of 0–400 (800) millidynes/cm and 0–40 (80) dynes/cm, respectively. Each balance is connected to a float when in operation (Langmuir type balance) and the sensing element in both cases is a linear variable differential transformer, the iron core of which is connected to the pendulum. The transformer signal after amplification drives the pen motor of a strip chart recorder. Special features like a constant pressure device together with auxiliary instruments such as a second X‐Y recorder, an automatic dipping device for film transfer, a volta potential meter and a slit viscosimeter permit the automatic recording of the following functions:.F=film pressure, A=film area, T=temperature, t=time, Aw /As =deposition ratio, η=film viscosity, and V=volta potential. Test and performance data are presented.
30(1959); http://dx.doi.org/10.1063/1.1716444View Description Hide Description
Vacuum‐tight cryogenic containers can easily be made from Mylar plastic sheet and metal, or from Mylar alone, for use in vacuum‐insulated liquid hydrogen target assemblies. Mylar is bonded very firmly to metals or to itself by an Epon‐Versamid mixture or by Armstrong A‐4. These adhesives maintain strong vacuum‐tight joints throughout the temperature range from 300 to 4°K. Mylar sheet has also proved satisfactory as vacuum‐window material for beam ports. Fabrication of these structures is described in detail, and explosionsafety measures are mentioned. Some examples are given of structures which have been used or tested, with data on the ultimate strength of each expressed as the internal pressure required to rupture it. Helium leak testing must be carried out at 77°K or below, because Mylar is relatively impermeable to helium only at low temperatures. The primary electron beams commonly obtained from electron linear accelerators are sufficiently intense to cause Mylar beam windows to fail under vacuum loading, probably because of radiation damage. However, the authors know of no case of radiation damage to Mylar sustained in exposure to secondary beams from high‐energy accelerators.
30(1959); http://dx.doi.org/10.1063/1.1716445View Description Hide Description
An electron current monitor for linear accelerators has been developed. Its sensing element was a toroidal coil through which the beam passes undisturbed. With appropriate electronic circuity this permitted the true shape of the current pulse to be viewed and also provided an indication of the average beam current on a meter. The average beam current that could be measured ranged from 3×10−8 to 3×10−6 amp. Furthermore, the use of the current monitor greatly simplified the process of initially obtaining yield from the machine.
30(1959); http://dx.doi.org/10.1063/1.1716446View Description Hide Description
Two conically tapered single crystals of Al2O3, with small ends abutting, are used to simultaneously effect a high pressure seal and provide a matched transformation from a standard 1‐cm circular wave guide at atmospheric pressure to a circular wave guide terminated by a high‐Q cavity resonator with an internal hydrostaticpressure up to 104 bars.
30(1959); http://dx.doi.org/10.1063/1.1716447View Description Hide Description
A time analyzer has been designed which is capable of utilizing the memory of any multichannel analyzer which employs pulse height to time conversion or any similar instrument. The circuit described here has been specifically adapted for use with the Argonne type 256‐channel analyzer. Channel widths from 1 μsec to >38 μsec are possible with a minimum dead time of 38 μsec.
30(1959); http://dx.doi.org/10.1063/1.1716448View Description Hide Description
The extension of optical pumping techniques to solids has been hampered by the lack of suitable light sources. The utilization of photoluminescent effects is proposed as a method for obtaining light sources applicable to optical pumping experiments in some solids, and a ruby source which emits 0.1 w of light at 6934 A and 6920 A is described.
30(1959); http://dx.doi.org/10.1063/1.1716449View Description Hide Description
30(1959); http://dx.doi.org/10.1063/1.1716403View Description Hide Description
A discriminator circuit for use with millimicrosecond counting equipment is described here. The circuit utilizes a diode voltage comparator which drives a secondary‐emission tube univibrator after amplification. Two shaped outputs are provided: one, of short duration, for use with 10‐Mc scalers, and another longer pulse for microsecond scalers.
The main characteristics of this unit are its good response to pulses as short as 3 m μsec and the fast recovery time, which is less than 0.1 μsec at the fast output.
30(1959); http://dx.doi.org/10.1063/1.1716404View Description Hide Description
An absolute method of standardization and measurement of the magnetic susceptibility of small samples is presented which can be applied to most techniques based on the Faraday method. The fact that the susceptibility is a function of the area under the curve of sample displacement versus distance of the magnet from the sample, offers a simple method of measuring the susceptibility without recourse to a standard sample. Typical results on a few substances are compared with reported values, and an error of less than 2% can be achieved.
30(1959); http://dx.doi.org/10.1063/1.1716405View Description Hide Description
At low‐voltage levels a diode ring forms an extremely simple four‐quadrant passive analog multiplier characterized by reliability, stability, and wide band width. Operation is analyzed theoretically, and experimental tests are described. These show that with care in selecting diodes and adjusting circuit values, 1% accuracy is obtainable at input levels up to 150 mv.
30(1959); http://dx.doi.org/10.1063/1.1716406View Description Hide Description
An electromechanical analog integrator for obtaining a continuous time integral of varying voltage is described. It is suited for applications where integrating times of greater than one minute are encountered. Particular consideration is given to its applicability to magnetic resonancedata reduction.
30(1959); http://dx.doi.org/10.1063/1.1716407View Description Hide Description
A dosimeter for electron beams has been constructed which utilizes the luminescence of plastic phosphors under electron bombardment. A phosphor, in sheet form, was placed in a Lucite medium and the luminescence of a small volume of the phosphor was focused on a photomultiplier. By scanning the phosphor, the three‐dimensional dose distribution within the medium was determined. Special attention was paid to obtaining the dose close to the surface of the medium by which the electron beam entered.
30(1959); http://dx.doi.org/10.1063/1.1716408View Description Hide Description
A device is described for obtaining powder diffraction patterns of thin sheets pressed between opposing diamond pistons. The pressure, initially applied by a small auxiliary laboratory press, is clamped onto the sample by a lock‐nut. The whole unit is designed to replace the sample mount of a standard x‐ray diffraction unit. Sample patterns taken on two of the high pressure modifications of bismuth are exhibited.
30(1959); http://dx.doi.org/10.1063/1.1716409View Description Hide Description
An electronic coulometer is described which has 15‐ohms input impedance, precision of 0.003 coulomb per minute, and long time stability of 0.1% of full scale value. Its calibration and testing are described, and means of adapting it to other operating conditions are given.
30(1959); http://dx.doi.org/10.1063/1.1716410View Description Hide Description
A high‐precision capacitor divider method for measuring the voltage on the terminal of a 2‐Mv Van de Graaff accelerator is described. Calibration of the system was done using the Be(γ,n) threshold reaction. Linearity was tested using a total beam calorimeter and a beam current integrator.
30(1959); http://dx.doi.org/10.1063/1.1716411View Description Hide Description
The effect of temperature on the stability of the Varian V‐4300–2 NMR spectrometer has been investigated. The spectrometer was most sensitive to changes in room‐air temperature. A change of 0.3°C per minute caused a spectrometer drift of about 20 cps per minute. A change of 0.1°C per minute in temperature of magnet cooling water caused a spectrometer drift of 1 cps per minute. Enclosing the magnet served to minimize effects of air temperature changes but amplified the effects of water temperature changes. Satisfactory control was attained by running the refrigeration units continuously and regulating the temperatures by controlling the suction pressure on the evaporators. Under these conditions spectra were reproducible to within ½ cps.
30(1959); http://dx.doi.org/10.1063/1.1716412View Description Hide Description
A grinder which can be used for sectioning diffusion specimens in which the average diffusion distance is a very few microns is described. Three fixed steel balls are used to determine the grinding plane of the instrument, and the specimen is held with constant orientation at the center of the triangle formed by the balls. The orientation of the grinding plane with respect to the specimen surface can be changed to make the two parallel by adjusting the lengths of the legs to which the steel balls are attached. These lengths can be adjusted over several centimeters, permitting a large variation in specimen thickness and orientation. A differential screw arrangement on each leg makes it possible to orient the grinding plane and crystal surface to within 5×10−6 radian with respect to one another. The shaft on which the specimen remains mounted during the entire grinding procedure can be moved in a line perpendicular to the grinding plane so that the specimen surface protrudes beyond the grinding plane. A solid grinding plate is then used to grind the specimen away until the surface lies in the plane. Since no loose grinding compound is used, the material removed from the specimen can easily be collected for preparing a radioactive slide. It is also possible to assay the radioactivity of the specimen itself. The motion of the shaft, and, consequently, the thickness ground away, is measured directly using an electronic indicator and is checked every 10 μ against gauge blocks. It is estimated that sections parallel to within 10−5 radian and with the thickness known to the order of a few hundredths of a micron can be removed with the device, the lower limit on the thickness of the slice being about 1 μ.
30(1959); http://dx.doi.org/10.1063/1.1716413View Description Hide Description
A circuit technique is described which reduces the rise times of high‐power pulses by means of exploding wires. This circuit is a nonlinear lumped parameter transmission line. The magnetic energy is stored in the interstage lead inductances and rapidly transferred into (or more correctly, shared with) succeeding stages by the vaporization of exploding wire resistive fuse elements connected in shunt between the leads. In our case, each of three resistive fuse elements consisted of 20 to 50 parallel 0.001‐in. diam copper wires held in place across a 2‐in. gap with pressure sensitive tape. An empirically determined arrangement is described wherein we have increased the maximum rate of current rise from 300 000 amp/μsec to 800 000 amp/μsec. Using this technique, a magnetic field of 10 000 gauss is built up in 0.15 μsec throughout a volume 4 cm in length and 10 cm in diameter.
It will be shown that the best results are obtained with high conductivity fuse materials such as copper or aluminum. Similarity theorems are presented for the design of pulse steepening elements for use with circuits having similar pulse shapes but different energies and characteristic impedances.