Index of content:
Volume 28, Issue 12, December 1957
28(1957); http://dx.doi.org/10.1063/1.1715823View Description Hide Description
A number of experiments are described which test the interactions of particles with rf accelerating voltages in an FFAG acclerator. The experiments confirm predictions of a theory of Symon and Sessler and demonstrate the feasibility of ``stacking'' successively accelerated groups of particles with a density close to that of a single group.
28(1957); http://dx.doi.org/10.1063/1.1715824View Description Hide Description
Monoenergetic neutron fluxes can be measured at neutron energies above 1 Mev by observing recoil protonsscattered at forward angles from a hydrogenous radiator. A counter telescope system for observing the recoil protons is described which has a relatively high efficiency for neutron detection. The major part of the paper is devoted to backgrounds and corrections, the calculation of telescope efficiency, and the accuracy of neutron flux measurements. Tables are given which make possible a simple determination of the efficiency of a counter system at neutron energies between 1 and 30 Mev. An appendix presents a semiempirical formulation of n‐pscattering which is used in obtaining counter efficiencies.
28(1957); http://dx.doi.org/10.1063/1.1715789View Description Hide Description
A method is described whereby the life of an oxide cathode can be substantially increased when subjected to repeated air exposures. This is accomplished by preventing the hydrate formation, and consequently reducing flaking of the cathode coating. Further limitations to cathode life caused by the effects of poisoning are also discussed.
28(1957); http://dx.doi.org/10.1063/1.1715790View Description Hide Description
A new electronic read‐out system has been devised which enables one to obtain a fast answer from a chronotron to be used with accelerator beams of particles. It allows for an analysis of time delay with variable and continuous time intervals. The timing is pulse‐height independent with an accuracy increasing for large pulses. The recovery time is in the range of one microsecond.
With delay‐line clipped coincidence pulses from two photomultipliers being fed into a 12‐channel chronotron with a 30‐mμsec range, a time resolution with half‐width at half maximum of 1.2 mμsec was readily obtained. Using only 4 channels, with a range of 10 mμsec, a half‐width at half maximum of 6×10−10 sec was observed. A simplified version with only three channels has made a useful fast coincidence circuit.
28(1957); http://dx.doi.org/10.1063/1.1715791View Description Hide Description
A fast scaler is described that employs a fast flip‐flop circuit of unique design. With techniques applied to fast trigger circuits, this flip‐flop circuit has been triggered at a rate higher than 50 megacycles per second and has a double‐pulse resolution of 20 millimicroseconds. A 40‐megacycle scaler, with a scale of eight and employing this fast flip‐flop as a basic element, has been built at this Laboratory.
28(1957); http://dx.doi.org/10.1063/1.1715792View Description Hide Description
An instrument to measure fluorescence lifetimes of the order of millimicroseconds is described. It utilizes a hydrogen flash lamp and high‐speed oscilloscope. The fluorescence decay is displayed on the oscilloscope, and using the method of moments the observed fluorescence is corrected for the response of the lamp and the detection equipment. The instrument was used to study the primary photochemical processes of photosynthesis. Measured lifetimes are given for the various photosynthetic pigments (in vitro), some organic dyes, and chlorophyll fluorescence in algae. With this instrument it has been possible to measure the delayed energy transfer in living systems.
28(1957); http://dx.doi.org/10.1063/1.1715794View Description Hide Description
A neutrondetector with a threshold at 20.6 Mev is described which is based on the C 12(n,2n)C 11 reaction in liquid organic scintillators.
The efficiency of the detector is almost independent of neutron energy from ∼40 Mev to ∼400 Mev. The detector can be used to measure with an error of 6% high‐energy neutron fluxes in the tolerance range (20–50 n/cm2 sec).
28(1957); http://dx.doi.org/10.1063/1.1715795View Description Hide Description
A theoretical analysis is made of the error in thermal conductivity, measured by the guarded hot‐plate apparatus, resulting from a temperature difference, or unbalance, between the test area and guard ring. The solution is obtained by the application of two successive Schwarz transformations, the assumptions having been verified by relaxation calculations. An expression is derived for the error heat flow in the test specimens. The agreement between calculated and measured values of the error heat flow for three different hot plates for which experimental data are available is 5% or better. It is therefore now possible to calculate the maximum tolerable unbalance to achieve any desired accuracy in thermal conductivity measured by the guarded hot plate.
28(1957); http://dx.doi.org/10.1063/1.1715796View Description Hide Description
A new experimental technique for measuring rotational hysteresis losses over the frequency range 3 to 30 cps is described. A sample disk of ferromagnetic material is mounted on a top. The top is caused to spin about a vertical axis normal to the plane of the disk and of the magnetic field provided by an electromagnet. After removal of the driving force, the top dissipates its stored energy in losses in the sample and in bearing and windage losses. The bearing and windage losses are obtained by also performing the experiment in a magnetic field‐free region. The rate of change of rotational energy of the top is determined by measuring the angular velocity of the top as a function of time.
28(1957); http://dx.doi.org/10.1063/1.1715797View Description Hide Description
The two point boundary value problem of ordinary differential equations is reduced to two initial value problems, each of which can be easily solved by analog equipment. The computer can judiciously combine these results to yield a solution of the boundary value problem. The methods involved are described analytically and an example solved on the REAC is provided. A complete diagram of the electronic mechanization is provided.
28(1957); http://dx.doi.org/10.1063/1.1715798View Description Hide Description
Crystal monochromators can provide the exceptionally fine collimation requisite for the study of small‐angle diffraction phenomena. To obtain sufficient intensity and contrast it is necessary not only to bend the crystal but also to use two crystals in tandem with their focal circles mutually orthogonal.
The apparatus described uses the x‐ray ``weak window'' line source to illuminate the (200) planes of mica in transmission and the (101̄1) planes of a cylindrically ground, polished, and etchedquartz crystal in reflection. Symmetrical couple mounts were used to minimize aberrations. The camera essentially isolates the Kα1 component and consequently reduces diffuse background and general radiation artifacts as shown by pertinent applications to collagen and feather keratin. Although operating in air and using exposure times comparable (50–250 hr) to those encountered with aperture‐collimated systems, the camera has resolved the central first order of the 640‐A macroperiod of collagen fibers and is capable of resolving consecutive orders of spacings as high as ca 3800 A.
The Johansson, Johann, and Cauchois geometries are illustrated in a new manner to show the importance of weak and large sources of x‐rays, to demonstrate the focal pattern characteristic of two‐crystal point‐focusing arrangements, and to show why mosaic crystals are incapable of the fine focus required.
28(1957); http://dx.doi.org/10.1063/1.1715799View Description Hide Description
A rapid, convenient, and reproducible method has been developed for the radioassay of volatile materials labeled with beta emitters of more than 0.15‐Mev energy. The sensitive detection element is a thin‐walled aluminum G. M. tube sealed into an annular jacket and attached directly to a high‐vacuum manifold.Analysis data for SiCl4 36 is presented, and application of the method to several tracer problems is suggested.
28(1957); http://dx.doi.org/10.1063/1.1715800View Description Hide Description
By designing an ionization gauge such that the electron paths are short and well‐defined and that the collection efficiency of the ion collector is near unity, one can extend the range of ionization gauges to pressures of the order of a millimeter of mercury. Several specific tube constructions are proposed and the characteristics of the gauges are discussed.
28(1957); http://dx.doi.org/10.1063/1.1715801View Description Hide Description
A laboratory technique for the quantitative environmental testing and comparison of various types of subsurface transducers is described. Such transducers are currently in use to determine the transient surface temperature response to intense surface heating of a metal slab that is a good heat conductor. The simulator provides surface heating whose uniformity is estimated to be within about 3% of the heating rate over an area of 1 in.2 with flux capability exceeding 1500 w/cm2. (Surface melting of a semi‐infinite slab of pure copper occurs in less than 10 sec at this rate.) Because the method of heating is electric, a shaped surface heat pulse may be produced by programing an input Variac. An important feature of the device is its symmetry which permits identical heating of two opposing surfaces simultaneously.
28(1957); http://dx.doi.org/10.1063/1.1715803View Description Hide Description
A cryostat is described for temperature control from 300° to − 150°C. Novel features of this apparatus include a combined heating‐cooling thermal exchange coil, good temperature homogeneity over the experimental volume combined with simplicity in design and ease of demountability and maintenance. By using other bath liquids the range of the cryostat can be extended to higher and lower temperatures.
28(1957); http://dx.doi.org/10.1063/1.1715804View Description Hide Description
A system for the generation, amplification, and detection of extremely short, monochromatic pulses in the 5–6 mm region is briefly described. The pulses are 3 mμsec wide at the base, with a peak power of a few milliwatts at 55 kMc/sec; they may be viewed on a commercially available oscilloscope.
28(1957); http://dx.doi.org/10.1063/1.1715805View Description Hide Description
A controlled pressure mass spectrometer for the study of ion‐molecule collision processes is described. A pressure ratio of 200 to 1 can be maintained between the ion chamber and the analyzer tube by means of differential pumping. Ion beams of the sample gas undergo collisionreactions with molecules of the same or other gases in the analyzer tube.
28(1957); http://dx.doi.org/10.1063/1.1715806View Description Hide Description
In connection with a precision adiabatic calorimeter used for heat‐capacity measurements up to 500°C, there is described apparatus for automatically controlling and recording the temperature difference between the calorimeter and its surroundings and for evaluation of the heat leak by electronic integration of the time‐temperature difference function.