Nickel Target Research Articles

Intensities of Monochromatic Continuous X-Rays from Atomic Targets of Nickel

Intensities of Monochromatic Continuous X-Rays from Atomic Targets of Nickel

New Technique for Making Thin Targets

This paper describes a new technique for preparing thin nickel targets ranging upward in thickness from 10−6 cm. The targets are approximately 10 cm2 in area and are backed by films of cellulose acetate 5·10−6 cm thick. The periphery of the target is supported by a flexible array of fine wires.

The Relative Secondary Electron Emission Due to He, Ne, and A Ions Bombarding a Hot Nickel Target

The number of electrons emitted per positive ion from a degassed Ni target bombarded by He, Ne, and A ions was measured. For He, in an energy range from 450 to 1650 ev the emission increased from 49 to 107 percent; for Ne having energies from 900 to 1275 ev the increase was 43 to 57 percent; for A of 680 to 1480 ev from 11 to 18 percent. These results confirmed those found earlier for $\mathrm{H}_{2}^{+}$ and $\mathrm{D}_{2}^{+}$ ions in showing a smaller emission for the heavier ion at the same energy. Similarly, the percentage of positive ions reflected increased with the energy of the primary ions and was smaller for the heavier ions.

Proton Induced Radioactivities II. Nickel and Copper Targets

Nickel bombarded with 6.3-Mev protons shows activities of half-lives of 10.5\ifmmode\pm\else\textpm\fi{}0.6 min., 3.4\ifmmode\pm\else\textpm\fi{}0.3 hr., and 12.8\ifmmode\pm\else\textpm\fi{}0.8 hr., corresponding to known periods of ${\mathrm{Cu}}^{62}$, ${\mathrm{Cu}}^{61}$ and ${\mathrm{Cu}}^{64}$, respectively. The reactions are principally of the $p\ensuremath{-}n$ type but there is evidence that in the case of ${\mathrm{Cu}}^{62}$ proton capture occurs at energies below the $p\ensuremath{-}n$ threshold. The maximum $\ensuremath{\beta}$-ray energies obtained by absorption method are 2.8, 1.2 and 0.68 Mev for ${\mathrm{Cu}}^{62}$, ${\mathrm{Cu}}^{61}$ and ${\mathrm{Cu}}^{64}$, respectively. Thick target excitation curves are given. Copper bombarded with 6.3-Mev protons shows two activities of half-lives 38.3\ifmmode\pm\else\textpm\fi{}0.5 min. and 235\ifmmode\pm\else\textpm\fi{}20 days due to ${\mathrm{Zn}}^{63}$ and ${\mathrm{Zn}}^{65}$, respectively. Both must be formed by $p\ensuremath{-}n$ reactions. The ${\mathrm{Zn}}^{63}$ positrons have a maximum energy of 2.3\ifmmode\pm\else\textpm\fi{}0.15 Mev. Thick target excitation curve shows a threshold proton energy of 4.1\ifmmode\pm\else\textpm\fi{}0.1 Mev in good agreement with the energy relations. Average (thick target) cross section for the ${\mathrm{Cu}}^{63}(p\ensuremath{-}n){\mathrm{Zn}}^{63}$ reaction is 0.28\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}25}$ ${\mathrm{cm}}^{2}$. For protons of energy 6.1 Mev it is 0.95\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}25}$ cm.

A Determination ofhefrom the Short Wave-Length Limit of the Continuous X-Ray Spectrum

Two precision determinations of the short wave-length limits of the continuous x-ray spectrum have been made, one in the region of 10,000 volts, the other in the region of 20,000 volts by the of isochromats under very steady applied voltage. The two crystal spectrometer was used as a monochromator. The $K{\ensuremath{\beta}}_{2}$ line of molybdenum and the $L{\ensuremath{\beta}}_{1}$ line of tungsten were used for setting the monochromator. The experimental technique is fully described. A thin window tube with a nickel target coated with tungsten by evaporation was used as the x-ray source. The effect of the finite resolution of the two crystal spectrometer, its behavior as a function of wave-length, and the pitfalls resulting from the tails of the spectral window as defined by the selective bicrystalline reflection are carefully considered. The advantage of using a thin layer target of high atomic number on a back support of lower atomic number in order to minimize the danger from these pitfalls is explained. Arguments for the applicability of a correction to the measured voltage for the work function of the cathode are presented. All such as the finite resolution of the spectrometer tending to render the voltage threshold for a given wave-length setting less well defined are here called for brevity smearing effects and five different new methods of desmearing the isochromats or of locating the thresholds are considered. The presence of a hitherto unsuspected knee in the isochromat within a few volts of the quantum limit is shown to render inapplicable the only so far in use for locating the threshold voltage, the method of the projected tangent, which is also shown to contain other objectionable pitfalls. The experiment permits of determining either $\frac{h}{e}$ or $\frac{e}{{h}^{\frac{3}{4}}}$ according to whether we assume the ruled grating wave-lengths of x-rays to be correct or make no such assumption but include the data from selective x-ray crystal reflection, crystal density, molecular weight and the Faraday constant as part of the computation. By these respective methods we obtain $\frac{h}{e}=1.3762\ifmmode\pm\else\textpm\fi{}0.0003\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}17}$; $\frac{e}{{h}^{\frac{3}{4}}}=2.0720\ifmmode\pm\else\textpm\fi{}0.0004\ifmmode\times\else\texttimes\fi{}{10}^{10}$. These results agree with those of Kirkpatrick and Ross and those of Schaitberger on the same experiment with deviations which are small compared to the disagreement with two other entirely different experiments for determining functions of $e$ and $h$. This discrepancy on the Birge-Bond diagram already emphasized by Birge is carefully discussed and possible causes for it are considered. Two possible objections to the present experiment are raised and we believe successfully disposed of. The practical impossibility of blaming this discrepancy on the short wave-length limit experiment is, we believe, convincingly proven by our results.

The depth at which secondary electrons are liberated

A smooth and a rough carbon target are bombarded with electrons. If the angle of incidence is increased, the number of secondary electrons liberated increases for the smooth target; it changes however very little for the rough one, because the angle of incidence in this case is not well-defined. The cause for the increased secondary emission when using a smooth target is supposed to be the liberation of secondary electrons at a smaller depth. The experiments show that in this case it is justified to accept the ordinary absorption law for the secondary electrons. The absorption of these electrons increases with increasing energy of the primaries. This explains the maximum appearing in the curve, giving the relation between secondary emission and velocity of the primary electrons. The mean depth of origine for the secondary electrons emerging from a nickel target is estimated to be 30Å.U. under the surface for a primary energy of 500 Volt.

Secondary Electron Emission from a Hot Nickel Target Due to Bombardment by Hydrogen Ions

Secondary Electron Emission from a Hot Nickel Target Due to Bombardment by Hydrogen Ions

The Scattering of Lithium Ions by a Polycrystalline Nickel Surface

Positive ions scattered by a nickel target were measured both as to energy and quantity in various directions. The ions are scattered in a forwardly directed beam whose width at one-half the maximum intensity is about 45\ifmmode^\circ\else\textdegree\fi{}. The direction of the maximum is independent of the incident energy and nearly so of the angle of incidence. The ratio of the most probable energy of the scattered ions to that of the incident ions is found to decrease as the total angle of scattering increases and to be independent of the energy or the angle of incidence. To a first approximation the energy distribution is found to be the same as that for projectiles of macroscopic size undergoing elastic impacts at a rough surface with a small coefficient of restitution.

Secondary electron emission from a nickel surface produced by positive ions of mercury

(1)A beam of positive ions of mercury produced from an arc in mercury vapour and fired upon a surface of nickel produces an emission of electrons.(2)For a fresh untreated nickel target the electron emission is of the order of 1·5 per cent. for ion energies less than about 600 electron-volts, and rises to about 15–20 per cent. at 2000 electron-volts.(3)After degassing thoroughly at a red heat the secondary electron emission is found to fall to about half the value observed with a dirty target. Continued bombardment with mercury ions leads to a progressive decrease in the emission, which approaches a final steady state after some hours. In this steady state the ratio is about 2·3 per cent. at 2000 electron-volts. After degassing afresh, the same process is repeated.(4)Tests were made to assure that the emission is in reality one of electrons from the struck target, and that it is due to the impact of the positive ions.

Emission of Negative Electricity from Nickel when Bombarded by Positive Lithium Ions

A nickel target was bombarded with positive lithium ions and observations were made of the number of negative charges emitted per positive ion striking the target. The energies of the bombarding ions were within the ranges 1000 to 2000 electron-volts and 5000 to 20,000 electron-volts, approximately. Observations were made with the target at room temperature and at a yellowish-red heat. There is a marked difference between the curves found for the two cases. For the cold target the curve has a maximum between 10,000 and 11,000 volts, while the number of negative charges emitted per positive ion from the hot target increases from 0.13 at 1000 volts to 2.35 at 20,000 volts, no maximum having been found.

Total secondary electron emission from a single crystal face of nickel

In an accompanying paper, secondary electron experiments on ordinary nickel are described. These were conducted mainly to study the conditions of secondary electron emission and to find how far the experimental results of Retry on a polycrystalline nickel target could be reproduced. It was found that a large number of inflections were obtained some of which coincided with Petry’s values. Most of these inflections had corresponding values in Thomas’s results for soft X-rays from nickel. In this paper, the results of experiments on total secondary electron emission from the 100 face of a nickel crystal are given.

Absorption in the Region of Soft X-Rays

Energy of soft x-rays from a tungsten-coated nickel target as a function of potential from 40 to 610 v---Soft x-rays from the target were allowed to fall on a solid solution consisting of CaS${\mathrm{O}}_{4}$+2 percent MnS${\mathrm{O}}_{4}$. The square of the time of duration of the thermoluminescence produced was assumed to be proportional to the total energy of the x-rays. The results indicate that in the range 40-610 v., the total energy varies approximately as the square of the potential applied to the tube.Absorption of soft x-rays (40-610 v.) by thin celluloid films.---By interposing a thin celluloid film, about 25 m\ensuremath{\mu} thick, between the target and the thermoluminescent substance, the transmission of the film was measured for various values of the applied potential from 40 to 610 volts. The transmission varied from 0.0 percent at 40 volts (minimum wave-length 310A) to 57 percent at 610 volts (minimum wave-length 20A). The results are in general agreement with the hypothesis that the absorption varies as the cube of the wave-length, that K absorption discontinuities in celluloid occur between 300 and 600 volts and that the x-rays cover a wide spectral region. A photographic method of investigating absorption is also described. The results are fairly consistent with those obtained otherwise but the method suffers from the fact that the photographic plate is not equally sensitive to energy of different wave-lengths.Absorption of soft x-rays (300-600 v.) in air and ${\mathrm{H}}_{2}$.---Computation based on previous measurements give for air values of $\frac{\ensuremath{\mu}}{\ensuremath{\rho}}=6.0, 6.25, 7.0, 7.6\ifmmode\times\else\texttimes\fi{}{10}^{3}$ at 600, 500, 400, and 300 volts respectively. These figures agree fairly well with those given by Holweck in ${\mathrm{O}}_{2}$ and ${\mathrm{N}}_{2}$. For ${\mathrm{H}}_{2}$ the value of $\frac{\ensuremath{\mu}}{\ensuremath{\rho}}$ is calculated to be about 1.8\ifmmode\times\else\texttimes\fi{}${10}^{4}$ which is larger than the value given by Holweck and larger than is to be expected from the results for air.

On the Influence of Temperature upon the Photo-electric Effect

Photo-electric threshold frequency for metals.---Free electron theory. A discussion is given of the suggestion by Millikan that in the photo-electric effect the energy of the light is transferred to the free electrons of the metals as well as to the bound electrons, and the threshold frequency ${\ensuremath{\nu}}_{0}$ is interpreted by the equation $h{\ensuremath{\nu}}_{0}={\ensuremath{\phi}}_{p}\ensuremath{-}{E}_{k}$ where ${\ensuremath{\phi}}_{p}$ is the work necessary to remove a free electron from the metal. In cases where ${E}_{k}$ is not small compared with ${\ensuremath{\phi}}_{p}$ the theory leads to a lack of sharpness in the definition of ${\ensuremath{\nu}}_{0}$. Neglecting variations of kinetic energy, the difference of stopping potentials comes out equal to the Peltier coefficient; hence the uniformity of stopping potentials for different metals observed by Millikan is due, according to the theory, to the smallness of the Peltier effect. The variation of the long wave-length limit with temperature comes out $\frac{\ensuremath{\partial}{\ensuremath{\lambda}}_{0}}{\ensuremath{\partial}T}=(\frac{\ensuremath{\lambda}_{0}^{}{}_{}{}^{2}}{\mathrm{hc}})e\ensuremath{\sigma}$, $\ensuremath{\sigma}$ being the Thomson coefficient. In most cases ${\ensuremath{\lambda}}_{0}$ should be practically independent of temperature. In the case of aluminum, after prolonged heating, the photo-electric current due to $\ensuremath{\lambda}2537$ was found to remain constant within 1/2 per cent as the target was cooled from 400\ifmmode^\circ\else\textdegree\fi{} to 100\ifmmode^\circ\else\textdegree\fi{}C and this constancy is interpreted as evidence that the shift of the long wavelength limit with change of temperature is less than 1A. The limit was found to be at about 2700A. Similar observations with a nickel target and 2412A gave inconclusive results, as in spite of heating to 1300\ifmmode^\circ\else\textdegree\fi{}C and reduction of the oxide on the surface by heating in hydrogen, reproducible results were not obtained.Photo-electric current from aluminum for $\ensuremath{\lambda}2537$.---(1) Effect of prolonged heating in vacuum by high frequency induction for many hours to near the melting point. The current remained strong. (2) Effect of cooling from 400\ifmmode^\circ\else\textdegree\fi{} to 100\ifmmode^\circ\else\textdegree\fi{}C. The current remained constant within 1/2 per cent.

Bombardment of Metal Surfaces by Slow-Moving Electrons

Secondary Electrons Produced by Electronic Bombardment of Nickel.---The main features of the apparatus used are: an equi-potential nitrate-coated Pt cathode heated by radiation from a tungsten spiral filament; a series of insulated diaphragms to limit the beam of primary rays and for use in determining their velocity; and a long Faraday cylinder in front of which the nickel target could be alternately interposed and withdrawn. By baking out the tube and using liquid air traps an extremely low vacuum corresponding to less than ${10}^{\ensuremath{-}7}$ mm. was attained. The secondary current was determined from the difference between the current to the Faraday cylinder and that to the interposed target. (I) Ratio of the secondary (emergent) to the primary (incident) electron current was found to be independent of the roughness of the surface, but to vary with the treatment. After heating the target red-hot by high frequency induction for some minutes, a limiting curve was reached which probably represents the characteristics of nickel itself, free from surface contamination. In this case the secondary electrons begin when the primary velocity is as low as 0.2 volt; the ratio to primary current then increases rapidly with primary velocity to about 4 volts, remains constant to about 9 volts, then again increases reaching a value of unity for a primary velocity corresponding to about 260 volts. The effect of exposure to air or hydrogen is to increase very considerably the secondary current and to round out the flat part of the curve between 4 and 9 volts. (2) Velocity distribution curves of the secondary electrons indicate that for primary electrons of less than 9 volts velocity, most of the secondary electrons have velocities nearly equal to the primary velocity, while for primary velocities above 9 volts, the percentage of secondary electrons having small velocities increases with the primary velocity, although a small proportion have velocities nearly equal to the primary velocity up to at least 110 volts.Reflection and Emission of Electrons from a Nickel Surface Bombarded with Electrons of Velocity 0 to 260 Volts.---It is suggested by the above results, that reflection occurs for all the primary velocities investigated, and that emission or ionization begins at about 9 volts and increases with the primary velocity.Contact difference of potential between a gas-free nickel surface and an ordinary baked nickel surface was found to be 0.8 volt.Method of making a gas-tight glass joint for high vacuum work by soldering with Wood's metal, the platinized and copper-coated surfaces of a ground joint, is described.

A Tungsten Target for X-Ray Tubes

GREAT advances have recently been made in the production of X-rays, chiefly by the employment of very heavy currents. The exposures necessary for producing radiographs of the thorax have been reduced from minutes to fractions of a second. To make this possible, much attention has been devoted to the target or anti-kathode, which is the critical part of the tube, for here it is that the focus of the kathode stream strikes, and the energy of the bombarding electrons is transformed into X-radiation. The early English tubes were furnished with substantial targets of platinum, but in the later foreign tubes with which the market was flooded the platinum was often reduced to a sheet of very thin foil laid upon a plate of nickel. For weak currents, and with an imperfectly focused kathode stream, this plan answered moderately well, but if heavy currents were used the heat generated at the focus was often so great that the platinum skin alloyed with the nickel backing, when fusion and destruction of the whole apparatus followed immediately. This is well illustrated by the ac co nip a nying photograph of such a fused target which appeared some time ago in the Journal of the Rontgen Society. Platinised nickel target damaged by the kathode focus. Recently attention has been directed to the exceptional properties of pure metallic tungsten, now produced in quantity for the manufacture of metal filament lamps, and its suitability for the purpose was at once recognised, the metal having a fusing point of about 3000° C., as against 1750° C. for platinum. Tungsten is also very tough, and does not readily disintegrate by the kathodic discharge (kathode sputtering); its atomic weight, 180, is not much below that of platinum. The British Thomson-Houston Company, Ltd., has introduced a special target of this metal that is being largely used by manufacturers of X-ray tubes. The tungsten is in the form of a thick button brazed into a solid block of copper, in some cases weighing as much as half a pound; this forms a lasting and efficient target, even when heavy currents are used for considerable periods of time, as is often necessary when using X-rays for therapeutic purposes. The adaptation of tungsten for this purpose is an example of the great value that lies hidden in the rare and little-known elements, and doubtless other instances of a similar nature will develop as the metals become available.