US2407381A - Means and method for transmitting secret intelligence - Google Patents

Description

Patentecl Sept. 1, 1946 SITES ATENT OFFICE MEANS AND METHOD FOR TRANSMITTING SECRET INTELLIGENCE tion of New York No Drawing. Application June 2, 1941, Serial No. 396,363

3 Claims.

My invention relates to a means and method for transmitting secret intelligence, and is particularly applicable for the transmission of secret military intelligence, including writing, maps, photographs, and the like.

Broadly, my invention comprises the use of a radioactive material which has a predetermined and preferably short half-life period, this material being used in dispersed form in the general manner of a secret and invisible ink for the reproduction of writing, drawings or photographs.

Artificial radioactive materials suitable for my purpose are capable of being produced in relatively large quantities by bombardment of inert materials with high velocity deuterons, for example, in devices such as the sixty inch cyclotron at present being operated at the University of California at Berkeley, California. Such materials have half-life periods of different lengths. Furthermore the type of radiation of the mateterials may differ, and materials can be selected for alpha, beta or gamma-radiation as desired, all of which radiation will aiiect and radiographicaly expose a radiation sensitive material such as the emulsions on plates or films or films customarily used in taking X-ray radiographs. Furthermore, such artificial radioactive products are relatively safe for use, for example, in the transmission of military intelligence, because after a certain length of time, which can be predetermined by the selection of the material, the material will lose its radioactivity and the intelligence, if captured, becomes wholly innocuous and incapable of being detected or reproduced. Because of the fact that they can be used in such extreme dispersal, they are practically incapable of being detected by chemical analysis. Even in extreme dilution they are capable of producing a radiographic print or reproduction of satisfactory detail, during their active life.

Furthermore, materials having a relatively soft beta-radiation only may be selected, and radiation from such materials is readily absorbed by containers so that while being transmitted the danger of detection by an electroscope is small.

It is the main object of my invention to provide a means and method of utilizing radioactive materials in extreme dispersal to transmit writing, printing, drawings, or photographs, which will normally be completely invisible, but which can readily be made visible by the application thereto of a surface sensitive to alpha, beta or gamma-radiation.

In practicing my invention, radioactive material such as, for example, radioactive disodium phosphate produced by bombardment in a cyclotron may be used. This material has a half life of about seven days with a relatively soft beta-radiation only. This material is dissolved in an aqueous or equivalent solution and in extreme dilution. The dilution can be varied, of course, in accordance with the necessity of protecting the material from discovery by chemical analysis, and also by the exposure requirements in the eventual visible reproduction. The solution is, of course, absolutely colorless. The material in watery solution may be applied to ordinary paper, for example, or, in fact, to any other substance or carrier, even on the skin, as an ink, by using a pen or equivalent instrument. If extraordinary precautions are to be taken to avoid pen scratches on the carrier, it may be applied with a camels-hair brush. If extreme precautions are necessary to avoid even solution stain, the Writing, printing, drawing or other representation may be made directly on an auxiliary member. A piece of paper or similar carrier may then be pressed against the auxiliary memher and by rubbin or pressure a part of the radioactive material that has been previously deposited on the auxiliary member will be transferred to the paper or carrier. A sufiicient amount will adhere to the latter so that thereafter, Within its active life, the radioactive material can be detected by making a radiograph thereof,

The paper or other carrier which has had the radioactive material applied thereto, either directly or indirectly, may then be written on with ordinary ink, have other drawings placed thereon, be typewritten on or, in fact, be treated in any manner that may be desired to convey other information which is not secret. Such extra writing, not being radioactive, will in no way affect or obscure the secret intelligence being transmitted.

After the paper, or other carrier, has been received at its proper destination, the carrier is then placed in contact with, or closely adjacent, a sensitive emulsion such as that used on X-ray film, for example, and left in such position until the radioactive material has had time to expose the plate or film. The proper exposure time will, of course, be correlated with the known half-life period of the radioactive material being used, and the known date of production.

It can be readily seen that inasmuch as the radioactive material deposited on the carrier paper will deteriorate in radioactive power at a known rate, the writing or drawing secretly placed upon the carrier will, in a predetermined length of time, become impossible to decipher radiographically. Thus, if the carrier goes astray and is not tested radiographically within a predetermined length of time, the secret intelligence can never thereafter be detected.

When the carrier is enclosed in an envelope, for example, and is in a mail sack or covered by other letters, the carrier is incapable of being detected by the electroscope when materials are used having a soft beta-radiation.

Invisible images that can be reproduced in the form of photographs can also be transmitted, utilizing a modification of my method, by making use of gelatin imbibition of the radioactive material. For example, if a photograph is available, paper may be impregnated with five per cent gelatin in water, and allowed to dry to form a gelatin-coated paper. Fifteen per cent potassium bichromate dissolved in water, or water and acetone for quick drying, may be then applied to the gelatinized paper in the dark, and dried in the dark. A contact print may then be made from negative of the photograph on the bichro-- mated paper. This exposed paper may then be developed by washing it in running water for an hour in light in order to prepare the under exposed portions of the gelatin for imbibition. The paper is then placed with the gelatin relief image face down in an aqueous solution of radioactive di-sodium phosphate, for example, and left there until suflicient of the radioactive material is selectively imbibed by the gelatin image. The excess liquid on the matrix may then be blotted off, and the matrix thus produced pressed carefully against another piece of paper or the carrier material, whereupon the radioactive disodium phosphate is transferred to the paper in accordance with its concentration on the gelatin matrix. This paper will then carry the radioactive image exactly as writing is carried, being completely invisible and under normal conditions being incapable of detection chemically or electrically. The carrier, when received, can be placed in contact with an X-ray emulsion and left in contacttherewith for a suflicient time for the photograph to be reproduced in visible form and in detail.

The gelatin relief method outlined above is, of course, only one manner in which an invisible image of radioactive material can be printed on a carrier, as any method capable of transferring the radioactive material in image form to a carrier may be used.

While I have described my invention as preferably including the direct application of the radioactive material to the carrier in intelligible form, I also deem my invention to include the spreading of the radioactive material over the carrier, with removal of the radioactive material in an intelligible manner. The resultant radiographs will merely be in reverse.

It will also be obvious that the various transfers may be made in proper order to give a true negative on the final film radiograph so that positive photographic prints may be made directly therefrom.

It should be pointed out that the word contact, as used herein to designate the relationship of the radioactive image to the radiation sensitive surface necessary to produce a radiograph, is deemed to include any adjacency close enough to maintain intelligibility of the radiograph.

Thus I have provided a means and method of forming invisible images on any carrier desired, of writing, printing, drawings or photographs, these images being capable of being covered or otherwise overlaid with any desired visible writing, printing, drawing or photograph, and which can be reproduced by proper exposure to a radiation sensitive emulsion, or other surface that is sensitive to alpha, beta or gamma-radiation of the radioactive material. The materials can be used in extremely high dilution, with consequent difficulty of chemical detection. Lastly the use of artificially produced radioactive materials having relatively short half-periods renders the intelligence completely undetectable after predetermined lengths of time. These times will be known and can be selected to protect the sender in case the material falls into unfavorable hands.

While I have described my preferred material as one having a relatively soft beta-radiation only, it is of course possible to utilize for specific purposes, radioactive materials having a different type of radiation. For example, radioactive yttrium has negligible beta-radiation and hard gamma-radiation, While element 85 is an alpha ray emitter, the former having an approximately one hundred day half life, and the latter a half life of seven and one-half hours. Many other materials are available. I am able therefore to select radio active materials for both the type of radiation and for life, as may suit, my purpose.

I claim:

l. The method of recording normally invisible intelligence capable of interpretation by making a contact radiograph thereof which comprises dissolving a colorless soluble radioactive material in a colorless liquid medium, applying the solution to a first carrier, applying said first carrier to a second carrier, and thereafter exposing said second carrier to a radiation sensitive surface to produce a visible image thereon.

2. The method of transmitting secret photographs which comprises forming a printing matrix from a photograph, charging said matrix with a colorless radioactive material, pressing said matrix against a carrier, and exposing said carrier to a radiation sensitive surface to form a visible image of said photograph thereon.

3. The method of transmitting secret photographs which comprises forming a gelatin matrix from a photograph, charging said matrix with a colorless aqueous solution of radioactive material, pressing said matrix on a carrier, and exposing said carrier to a radiation sensitive surface to form a visible image of said photograph