Volume 27, Issue 7, July 1956
Index of content:
27(1956); http://dx.doi.org/10.1063/1.1715606View Description Hide Description
Multichannel systems for pulse‐height and time‐of‐flight analysis based on the original development of Hutchinson and Scarrott are described. The equipment can be divided into three basic categories:
(a) A time channel analyzer using serial storage and arithmetic operations. This unit provides approximately 500 2‐μsec time channels each having a capacity of 10 binary digits. It is now being used for slow neutron time‐of‐flight spectroscopy.
(b) A pulse‐height‐to‐time converter which when used in conjunction with the above or similar unit forms a multichannel pulse‐height analyzer with 200 useful channels.
(c) Data display systems of two types, one for continuous visual monitoring and one for permanent recording in chart form.
27(1956); http://dx.doi.org/10.1063/1.1715607View Description Hide Description
The design of an apparatus for applying uniaxial compression to specimens with cross sections up to ¼ inch square is described. One feature of the apparatus is the construction of the force transmitting linkage. This linkage was designed to minimize the transmission of shear forces, and thus to avoid bending moments on the samples. The apparatus enables the compressive forces acting on the samples to be determined to within 0.1%.
27(1956); http://dx.doi.org/10.1063/1.1715608View Description Hide Description
The control circuit described utilizes a logarithmic difference amplifier to measure the ratio of the positive ion current to the electron current drawn by the gauge. This eliminates the necessity of stabilizing the electron current within close limits and permits direct readings of pressures ranging from 10−3 to 10−7 mm of mercury on a single logarithmic scale.
27(1956); http://dx.doi.org/10.1063/1.1715609View Description Hide Description
A microwaveinterferometer has been developed for measuring the phase retardation of a wave passing through a radome wall. The instrument is used to analyze radomes which are subsequently corrected so that the phase retardations are of the desired values everywhere. High accuracy is required in the measurements. Automatic balancing of the interferometer is employed so that continuous measurements can be made.
27(1956); http://dx.doi.org/10.1063/1.1715610View Description Hide Description
The problem of the atmospheric infrared heat balance has been solved graphically and analytically, but almost no experimental evidence exists to verify the calculations. The ``Black Ball'' is a device which can be flown on an ordinary radiosonde and indicates a temperature which is related to the total incident infrared flux at any point. The actually observed temperatures must be corrected for convection effects and thermal time constants. Construction and correction data are presented in this paper.
27(1956); http://dx.doi.org/10.1063/1.1715611View Description Hide Description
A neutronenergy spectrometer is described, suitable for the region 50 kev to 1 Mev. Hydrogen recoils from CH4 in a proportional counter are collimated and enter a second counter. The sum of the pulse heights is analyzed whenever a coincidence occurs. A third counter in anticoincidence discriminates against cases in which all the energy is not lost in the first two. With the collimation used the energy spread is and the efficiency an approximately linear function of energy, being 10−5 at 180 kev.
27(1956); http://dx.doi.org/10.1063/1.1715612View Description Hide Description
A proton‐recoil telescope with thin polyethylene radiator has been designed for neutron spectroscopy and for neutron detection with known efficiency in the energy region from about 2 to 20 Mev. Five radiator thicknesses may be chosen over this energy region. Protons recoiling near zero degrees from the radiator pass through two proportional counters and terminate in a NaI(Tl) crystal. Triple coincidences gate an analyzer for observing the spectrum from the crystal. A spectrum of 13.7‐Mev neutrons was observed with a width at half‐maximum of 5.3% and 6% background. The efficiency with this resolution at 13.7 Mev is 3.6×10−6.
27(1956); http://dx.doi.org/10.1063/1.1715613View Description Hide Description
A linear amplifier with a 1.2‐μsec clipping time and a gain of 50 000 suitable for scintillation spectrometry is described. Low‐energy photoelectric peaks can be resolved under conditions in which the unwanted background has a rate in excess of 100 000 cps and has components 40 times more energetic than the desired spectrum. No shift in the energy axis is observed under these conditions.
The amplifier consists of a White cathode follower preamplifier, three feedback groups, and a White cathode follower output stage. Each of the three feedback groups contains four stages, three of which are amplifiers. Blocking is eliminated by the use of grid resistors low in value compared to the preceding stage plate load resistors. This feedback group configuration is also more linear by an order of magnitude than the conventional configuration. It is shown how RLC coupling between White cathode follower halves can conserve power supply voltage.
It is shown how a cable correctly terminated at the transmitting end rather than at the receiving end can eliminate the difficulties usually associated with the use of a long cable between preamplifier and main amplifier.
27(1956); http://dx.doi.org/10.1063/1.1715614View Description Hide Description
It is shown that the optimum pulse shape for driving a Schmitt trigger circuit is one in which the peak of the pulse occurs near the leading edge, followed by a linear decay to the trailing edge. The magnitude of the decay should be less than the trigger hysteresis. The circuit for obtaining such a pulse shape is shown.
27(1956); http://dx.doi.org/10.1063/1.1715615View Description Hide Description
As part of the program to develop high current accelerators several model Thomas cyclotrons employing three‐phase rf were built. The three phase signal was produced by a phase generator which was excited by a crystal oscillator. Each phase was amplified and applied to the corresponding dee, which was kept in tune by means of servomechanisms. Balanced dee voltages require a symmetrical resonator with equal inter‐dee capacitances; stability of the servomechanism requires that the effective inter‐dee capacitances be zero. This last condition was fulfilled by means of neutralizing transmission lines connected from dee stem to dee stem. Thomas cyclotrons appear to be capable of producing megawatts of beam power with an rf efficiency in excess of 70%.
27(1956); http://dx.doi.org/10.1063/1.1715616View Description Hide Description
As part of the program to investigate the properties of the Thomas cyclotron, a 20‐in. diam proton cyclotron was constructed. In such a three‐dee three‐phase system it is possible to accelerate protons, deuterons, and tritons at the same setting of frequency and magnetic field but on different modes of the rf. For stable operation in the proper mode and with balanced voltages, it has been found necessary to provide both phase servos and amplifier efficiency servos. The dees could not be servoed individually until the inter‐dee capacity was neutralized. Under such conditions it was possible to attain steadily 6.0 ma of protons at 1.0 Mev in the forward mode and 6.5 ma of deuterons at 0.5 Mev in the reverse mode.
27(1956); http://dx.doi.org/10.1063/1.1715617View Description Hide Description
Two model constant‐frequency cyclotrons based on the principle of L. H. Thomas, as extended by David L. Judd, are described. Both accelerated electrons to speeds of half that of light in magnetic fields of three‐fold azimuthal periodicity. Three 60°‐wide wedge‐shaped electrodes, driven 120° out of phase, provided an energy gain per revolution of 3 eV 0, where V 0 is the peak electrode‐to‐ground voltage. Electrons were accelerated to 75 kv with V 0=23 v, implying a minimum of one thousand revolutions in the cyclotron. The beam reached full energy without axial loss and it was demonstrated that essentially all of the circulating current will emerge from this type of accelerator without the use of additional deflecting systems. The success of this development program has shown the feasibility of a high‐current, high‐energy cyclotron based on the Thomas principle.
27(1956); http://dx.doi.org/10.1063/1.1715618View Description Hide Description
A mass spectrometer employing a cycloidal‐focusing mass resolving system is described. The mass analyzer proper, including the magnet and a high‐speed continuous‐flow inlet system, weighs approximately seventy pounds. The resolving system shows good mass resolving power, sensitivity, and analytical accuracy up to about mass 150 amu, and adequate service life. The mass resolving power can be extended substantially in cases where some loss of sensitivity and analytical accuracy is tolerable.
27(1956); http://dx.doi.org/10.1063/1.1715619View Description Hide Description
In a cycloidal‐focusing mass spectrometer, nonuniformities in the magnetic and electric fields may cause either an actual deterioration of the image quality or merely a shift in the focal point. The latter effect is the more usual one and it can be compensated easily.
27(1956); http://dx.doi.org/10.1063/1.1715620View Description Hide Description
In a curtate‐cycloidal‐focusing mass spectrometer, the first‐order effect of space charge is not to impair the quality of the focus but to shift the focal point out of the plane of the resolving slit. For a given ion mass and ion current, the magnitude of the effect is about four times as large in a cycloidal‐focusing instrument as in a 180° magnetic sector mass spectrometer operating in the same magnet.
27(1956); http://dx.doi.org/10.1063/1.1715621View Description Hide Description
A description is given of a machine designed to produce microelectrodes from a standard 3‐mm Pyrex glass tube. The glass tube is heated by a platinum coil and drawn into microelectrodes by a vertical two‐stage pull. The first pull is by gravity and the second by an electromagnet. The operation is entirely automatic. After the machine is switched on adjustable microswitches control the sequence of operation and finally switch the machine off when the microelectrodes are drawn. Two identical microelectrodes are produced per minute. The microelectrode tip diameters may be varied from 0.5 to 10 μ. The taper towards the tip may also be varied within wide limits to suit different experiments.
Advantages of the machine over earlier microelectrode pullers are discussed together with a description of a rapid method of filling microelectrodes with conducting solutions.
27(1956); http://dx.doi.org/10.1063/1.1715622View Description Hide Description
The instrumentation for the magnetic focusing and analysis of the external beam of the Michigan cyclotron is described. It provides to a scattering chamber in an adjacent room a beam of deuterons of a few tenths microampere monoenergetic to within 15 kev at a mean energy of 7.8 Mev. A double‐focusing analyzer magnet of mean radius one meter, focuses reaction products of magnetic rigidity less than or equal to that of a 20‐Mev proton. At the optimum energy resolution of the system, proton groups from (d,p) reactions differing in Q by about 20 kev can be resolved. Particles can be observed over a continuum of angles from −7° to +112° relative to the incident beam. The determination of reaction energies and the measurement of relative differential cross sections are discussed.
27(1956); http://dx.doi.org/10.1063/1.1715623View Description Hide Description
A thin‐walled liquid hydrogen‐deuterium target has been developed which makes use of the boil‐off vapors from liquid helium as a coolant. Approximately three liters of liquid helium are required to condense the hydrogen and 0.13 liter per hour thereafter.
27(1956); http://dx.doi.org/10.1063/1.1715624View Description Hide Description
One of the major problems of analysis in the soft x‐ray region is the limitation due to low transmission of the window materials of sealed‐off x‐ray detectors. This paper describes the properties and use of a gas‐flow proportional counter system for the x‐ray analysis of elements in the third period of the periodic table, with special reference to aluminum and silicon. The use of gases flowing continuously through the counter at atmospheric pressure in conjunction with helium also at atmospheric pressure in the optical path permits the use of ultra‐thin nonvacuum‐tight windows with high transmission in this region. Comparative data, obtained on a Norelco fluorescence spectrometer, show a tenfold increase in counting efficiency for AlKα over detectors previously available for this application.
27(1956); http://dx.doi.org/10.1063/1.1715625View Description Hide Description
An all‐electronic generator is described which provides a voltage which varies linearly with time between adjustable positive and negative limits. Such a generator lends itself well to the automatic recording of current‐dependent phenomena. Linear variation is obtained on both the rising and falling half‐cycles, so that complete reversal of slow processes may be assured. The maximum current provided in either direction is 3 amps. The limit is determined by the number of output tubes connected in parallel. Cycling periods up to 1 min in length are provided.