US3326178A - Vapor deposition means to produce a radioactive source - Google Patents

Description

June 261% 1%? H. M. DE ANGELIS VAPOR DEPOSITION MEANS TO PRODUCE A RADIOACTIVE SOURCE 2 Sheets-Sheet 1 Filed Sept. 12, 1963 Jammie 2% WW? H. M. DE ANGELIS W VAPOR DEPOSITION MEANS TO PRODUCE A RADIOACTIVE SOURCE Filed Sept. 12, 1963 2 SheetSSheet IN VEN TOR.

United States Patent M 3,326,178 VAPOR DEPUSHTION MEANS T0 PRODUCE A RADIGACTIVE SQURQE Henry M. De Angelis, Westwood, Mam, assignor to the United States of America as represented by the Secretary of the Air Force Filed Sept. 12, 1163, Ser. No. 308,6ti7 1 Claim. (Cl. 11849.1)

The invention described herein may be manufactured and used by or for the United States Government for governmental purposes without payment to me of any royalty thereon.

The present invention relates to a device for the preparation of laminated radioactive sources.

These sources are prepared for four general classes of use:

(1) For reference sources;

(2) For test sources in radiation detection equipment; (3) For spark-gap, glow lamp, and cold cathode tubes; and

(4) For light sources, such as watch dials, etc.

It is to be understood however that the invention is not limited to these specific uses of the radioactive source specimens produced.

The object of the invention is the provision of Stratified or laminated radioactive sources and a device wherein, such radioactive sources can be prepared with ease, and wherein there is accurate control of intensity of radioactivity in the rsulting laminated radioactive source product, and wherein the pattern or geometry of radioactivity produced in the target or source can be easily controlled.

A further object of the invention is the production of radioactive sources of increased and controllable intensity and the means of producing these sources, wherein substrate material is subjected repetitively toalternate radioactive bombardment and deposition of substrate material, itself capable of receiving and absorbing positive ions when subjected to positive ion bombardment.

A still further object of the invention is the provision of a single unit of vacuum equipment wherein the alternating processes of radioactive bombardment and deposition of material over the bombarded substrate can both take place without transfer or removal of the substrate from one system to another.

In this process substrate materials are bombarded with positive ions of radio active species. During positive ion bombardment some of the ions penetrate the surface layers and are trapped. After a certain bombardment time, the surface layers are saturated with the trapped bombarding atoms. The saturation bombardment time depends upon the positive ion current density, while the saturation concentration of trapped atoms depends on the mass and energy of the bombarding ions and on the characteristics of the substrate material. Therefore, the concentration of trapped atoms and thereby the radioactive intensity is limited. The present invention has succeeded in producing a final product source of an intensity increased beyond these limits. The entrapped radioactive atoms are prevented from escaping by depositing a thin protective layer of the substrate material (or a different substrate material) over the bombarded layer.

The protective layer thus produced is then subjected to positive ion bombardment. The time of bombardment is controlled to produce radioactive intensity of each bombarded layer to saturation or less, as desired. The bombardment can be accomplished in an apparatus in which either gaseous or vaporized solid species may be ionized and accelerated to bombard the substrate material. Where electroplating is used, it is necessary to remove the target from the vacuum system for the electroplating process,

3,326,178 Patented June 20, 1967 and return it for further bombardment. For vacuum deposition, however, the invention provides a device where both bombardment and substrate deposit over the bombarded layer can be carried out in a single system without the physical removal of the target from one system to another.

The term substrate material, as used herein, includes any material capable of receiving and absorbing positive ions by positive ion bombardment. Examples of metals hucessfnlly used are, nickel, germanium, platinum, aluminum, tungsten, and molybdinum. Some of these can be deposited by vacuum deposit. Others may be deposited by electroplating.

The intensity of each layer is additive in the final product. It will thus be seen that sources of heightened intensities become possible. Target materials other than metals may be used such as Lucite, glass or other suitable insulating materials. The insulating material would be entirely coated with a conductor and then treated in the rnaner of the above described substrates. The unbombarded metallic coating may then be removed. The pattern, that is, the source geometry can be controlled by using suitable masks over the substrate material which define the area to be subjected to bombardment and protect the remaining area from exposure.

Other advantages, features and objects of this invention will be developed relative to the following description taken in connection with the accompanying drawings wherein:

FIGURE 1 is a partially schematic elevational view of the device with portions broken away to show selected features of the device in cross section;

FIGURE 2 is a partially schematic elevational view of a modified form of the invention; and

FIGURE 3 is a highly enlarged view in cross section of a source specimen produced by the present invention.

Referring more in detail to the drawing, the numeral 10 represents a hot cathode discharge tube which in the construction model of the present invention is Pyrex, or quartz; but the invention need not be limited to these materials. Transparent material, of course, allows visual observation.

In the embodiment of the invention shown in FIGURE 1 both operations of bombardment and metal coating of the target take place within one piece of equipment Wherein means for both bombarding and metal coating are provided within one tube without the necessity of removal or disturbance of the target.

The tube 10 is of substantially Y-configuration. It is comprised of a main body 12, an inlet-outlet tube 14 for governing vacuum conditions, and for introduction of radioactive gaseous species, a section 16 wherein the radioactive ions are accelerated, a section 18 for providing metal vaporization for metal deposit, and a movable lower section 20 in which the target holder 22 is anchored.

The system is capable of sustaining a vacuum. A vacuum seal is provided by the rubber, or other gaskets 24 and 26 which join respectively the sections 16 and 18 to the main body 12, and by a vacuum seal effected in the area where section 23 joins the body 12. The sections 20 and 28 form a vacuum tight ball and socket joint which allows movement of the stem 30 from bombarding position to metal deposit position.

The target support 22 has a stem 30 mounted at the base of the section 20 by means of any vacuum tight medium, which provides also for the lead in 32.

For providing acceleration of radioactive species and bombardment of the target 34, a tungsten or other filament 36, acting as a cathode, is led from a power supply 43 (see FIGURE 2) into the section 16 through lead-in anchors 4t and 42. A helical anode 33 is led in at 44 and anchored at 46 and 47.

The target 34, shown with a high degree of magnification in FIGURE 3, is of suitable substrate target material such as nickel, germanium, silicon, platinum, or other metals having the necessary characteristics. It is introduced into the system by removing the lower element 20 and placing the substrate target 34 on the target holder 22. Electrical contact is made with the source substrate target 34 through lead-in 32 to the target support 22. The system is then evacuated. Vacuum conditions used are of the order of from less than 1 to 5 10" mm. Hg. The tube 12 is then filled with radioactive material, such as Krypton-85 to pressures of the order of 1 to 2 microns. A dis-charge voltage of the order of 40 v. and discharge currents up to 100 ma. provide substrate positive ion cur rent densities of the order of from 500 to 600 microamperes per square centimeter when a potential of 1000 volts is applied between the target substrate 34 and the anode 38. Saturation of the surface layers of the substrate with Krypton under these conditions of bombardment is reached in approximately -1S seconds. It is to be understood that the specific pressure, time, voltage, and current quantities given are by way of example only; the invention not being limited to these specific parameters.

The target support is then pivoted to the position shown in phantom lines in FIGURE 1 for metal deposition over the saturated substrate. vaporized aluminum, molybdenum, tungsten or other metal having the required characteristics are vaporized and deposited by vacuum on the target. For example, the metal rod 52 is heated and vaporized by heating coil 50.

The target is then moved back into bombardment position, and activation of a surface layer of the newly acquired substrate layer is obtained. In repeated bombardments and deposits of this nature, a radioactive source has been obtained equivalent to approximately two microcuries per cm.

FIGURE 3 shows the process in schematic form. A layer B of the original substrate A reaches saturation, or any lesser degree of radioactivity desired. A layer of substrate represented at C is then deposited over the radioactive layer B. The surface of the layer C is then bombarded, producing a radioactive surface layer D, which is in turn covered with a substrate deposit E, and so on, until the desired intensity is reached.

FIGURE 2 shows a form of the invention wherein only the bombardment step takes place in the discharge tube, the target being removed for the metal layer deposit.

The embodiment of the device shown in FIGURE 1 allows only metal vapor deposit in vacuum. In cases where other means of metal deposit are to be used, for example, electroplating, the target is removed from the discharge tube to another environment.

In the embodiment of the invention shown in FIG- URE 2, the target 34, supported on an immovable target holder 22, is placed within the discharge tube 10' in bombardment position. The anchor member which secures the stem 30 into the lower bowl section 20 and provides for the lead-in 32' is stationary. The section of the tube providing metal deposit is absent. In all other respects the devices of FIGURES 1 and 2 are identical. An inlet-outlet tube 14' allows pressure control, while vacuum seal measures are provided at 26 and 28'.

The cathode 36' and anode coil 38' are anchored in the bell element 16', in the same manner as before.

When the first period of bombardment of the target has been completed, the vacuum is relieved, the section 20 removed, and a metal layer deposited on the saturated substrate by electroplating or in any other manner.

The radioactive source specimens formed by the method of the invention have been found to have a high degree of thermal stability. Aluminum bombarded with Krypton at 1000 v. was heated in vacuum to 600 C. After ten minutes at 600 C. the aluminum still retained approximately 50% of the originally trapped Krypton. With ion bombardment on aluminum, Krypton sources of an intensity of 20-30 microcuries per sq. cm. can be prepared with five bombardments and five aluminum platings.

The intensity of the sources obtained can be controlled in three ways: (1) the conditions of bombardment may be controlled; i.e., discharge voltage and currert, bombardment voltage and current, pressure of radioactive species and bombardment time, (2) the specific activity of the bombarding species, and (3) the number of layers of bombardment and deposit.

Further, the geometry or configuration of the sources obtained can be controlled by positioning appropriate masks over the bombarded material, both the first substrate layer and the subsequent stratified or laminated layers to define the area to be bombarded. Electrostatic and/or magnetic control of the positive ions can also be used to define the area to be activated.

Thus, there has been presented an apparatus for producing a radioactive source wherein each successive layer is additive with respect to the total amount of radioactivity to be produced by the source and reduced by the shielding effect of the substrates. When gases are utilized, the successive layers provide for maintenance of the entrapment, and with certain leaky materials such as nickel, an unbom barded layer may be provided as a cap.

Although the invention has been described with refer ence to a particular embodiment, it will be understood to those skilled in the art that the invention is capable of a variety of alternative embodiments within the spirit and scope of the appended claim.

What I claim:

A device for the preparation of a laminated radioactive source; said device comprising a tube-like body capable of sustaining a vacuum, said body comprising a vertically arranged prirnarychamber including a ball, and socket type bottom section and a secondary tubular chamber open to the primary chamber at an intermediate side wall section thereof; an electric heating coil associated with a metal to be evaporated by said coil and the combination located in said secondary chamber; electrode means to establish an ionic stream disposed in said primary chamber; means to introduce a radioactive, gaseous material into the ionic stream whereby to establish a positive ionic stream of the radioactive material; and a substrate support fixed to the ball portion of said bottom section and oscillatory therewith for alternate alignment of the substrate with respect to the metal evaporant and the radioactive ionic stream.

References Cited UNITED STATES PATENTS 1,530,555 3/1925 Greenslade 117-220 X 2,031,102 2/1936 Gettcken et a1. 117-220 X 2,239,642 4/ 1941 Burkhardt et al.

2,391,595 12/1945 Richards et a1. 11849 X 2,440,999 5/1948 Anderson 252301.1 2,463,180 3/1949 Johnson 11849 X 2,553,289 5/1951 Alexander et a]. 11849 X 2,685,535 8/1954 Nack 11849.5 X 2,724,663 11/1955 Bond 117-71 2,728,321 12/1955 Davis 11849 2,799,600 7/1957 Scott 11849 X 2,868,990 1/1959 Reardon et a1. 250-84.5 2,910,039 10/1959 Patton et al. 11849 3,087,838 4/1963 Lubin 11849.1 X 3,089,785 5/1963 Lewis et a1 11771 3,099,579 7/1963 Spitzer et a1. 11848 X 3,192,892 7/1965 Hanson et al 11849.1

MORRIS KAPLAN, Primary Examiner.

L. D. RUTLEDGE, Assistant Examiner.

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