Patentecl Sept. 1, 1946 25,47,381 SITES ATENT OFFICE 2,407,381 ' MEANS AND METHOD FOR TRANSMITTING SECRET INTELLIGENCE Charles Pecher, Berkeley, Calif., assignor to Re search Corporation, New York, N. Y., a corpora tion of New York No Drawing. Application June 2, 1941, Serial No. 396,363 3 Claims. I My invention relates to a means and method for transmitting secret intelligence, and is par ticularly applicable for the transmission of secret military intelligence, including writing, maps, photographs, and the like. Broadly, my invention comprises the use of ' (Cl. 250-106) 2 In practicing my invention, radioactive’ ma terial 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 a radioactive material which has a predeter in an aqueous or equivalent solution and in ex mined and preferably short half-life period, this material being used in dispersed form in the treme dilution. The dilution can be varied, of course, in accordance with the necessity of pro general manner of a secret and invisible ink for 10 tecting the material from discovery by chemical the reproduction of writing, drawings or photo analysis, and also by the exposure requirements in the eventual visible reproduction. The solu Arti?cial radioactive materials suitable for my tion is, of course, absolutely colorless. The ma purpose are capable of being produced in rela terial in watery solution may be applied to ordi tively large quantities by bombardment of inert 15 nary paper, for example, or, in fact, to any other materials with high velocity deuterons, for ex substance or carrier, even on the skin, as an ink, by using a pen or equivalent instrument. If ex ample, in devices such as the sixty inch cyclotron at present being operated at the University of traordinary precautions are to be taken to avoid California at Berkeley, California. Such mate pen scratches on the carrier, it may be applied graphs. rials have half-life periods of different lengths. 20 with a camel’s-hair brush. If extreme precau Furthermore the type of radiation of the mate tions are necessary to avoid even solution stain, the Writing, printing, drawing or other repre terials may differ, and materials can be selected for alpha, beta or gamma-radiation as desired, sentation may be made directly on an auxiliary all of which radiation will aiiect and radio member. A piece of paper or similar carrier graphicaly expose a radiation sensitive material 25 may then be pressed against the auxiliary mem her and by rubbing or pressure a part of the such as the emulsions on plates or ?lms or ?lms customarily used in taking X-ray radiographs. radioactive material that has been previously deposited on the auxiliary member will be trans Furthermore, such arti?cial 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 pre determined by the selection of the material, the material will lose its radioactivity and the intel ferred to the paper or carrier. A su?icient amount will adhere to the latter so that there after, Within its active life, the radioactive ma terial can be detected by making a radiograph thereof, ligence, if captured, becomes wholly innocuous The paper or other carrier ‘which has had the and incapable of being detected or reproduced. 35 radioactive material applied thereto, either di Because of the fact that they can be used in such rectly or indirectly, may then be written on with extreme dispersal, they are practically incapable of being detected by chemical analysis. Even in ordinary ink, have other drawings placed there on, be typewritten on or, in fact, be treated in extreme dilution they are capable of producing a radiographic print or reproduction of satis information which is not secret. factory detail, during their active life. any manner that may be desired to convey other Such extra writing, not being radioactive, will in no way affect or obscure the secret intelligence being transmitted. Furthermore, materials having a relatively soft beta-radiation only may be selected, and After the paper, or other carrier, has been re radiation from such materials is readily absorbed by containers so that while being transmitted 45 ceived at its proper destination, the carrier is the danger of detection by an electroscope is then placed in contact with, or closely adjacent, small. a sensitive emulsion such as that used on X-ray ?lm, for example, and left in such position until It is the main object of my invention to pro vide a means and method of utilizing radioactive the radioactive material has had time to expose materials in extreme dispersal to transmit writ 50 the plate or ?lm. The proper exposure time will, ing, printing, drawings, or photographs, which of course, be correlated with the known half-life will normally be completely invisible, but which period of the radioactive material being used, and can readily be made visible by the application the known date of production. . It can be readily seen that inasmuch as the thereto of a surface sensitive to alpha, beta or gamma-radiation. . 55 radioactive material deposited on the carrier pa 2,407,381 3 positive photographic prints may be made di rectly therefrom. per 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 prede termined length of time, the secret intelligence It should be pointed out that the word “con tact,” as used herein to designate the relation ship of the radioactive image to the radiation sensitive surface necessary to ‘produce a radio graph, is deemed to include any adjacency close enough to maintain intelligibility of the radio 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 de tected 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 modi?cation of my method, by making use of gelatin imbibition of the radioactive ma terial. For example, if a photograph is available, paper may be impregnated with ?ve 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 graph. 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 writ ’ ing, printing, drawing or photograph, and which can be reproduced by proper exposure to a radia tion sensitive emulsion, or other surface that is sensitive to alpha, beta or gamma-radiation of 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 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 hav ing relatively short half-periods renders the in telligence completely undetectable after prede termined 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 mate rial as one having a relatively soft beta-radiation an hour in light in order to prepare the under exposed portions of the gelatin for imbibition. 30 only, it is of course possible to utilize for speci?c purposes, radioactive materials having a differ The paper is then placed with the gelatin relief ent type of radiation. For example, radioactive image face down in an aqueous solution of radio yttrium has negligible beta-radiation and hard active di-sodium phosphate, for example, and gamma-radiation, While element 85 is an alpha left there until su?icient of the radioactive ma ray emitter, the former having an approximately terial is selectively imbibed by the gelatin image. one hundred day half life, and the latter a half The excess liquid on the matrix may then be life of seven and one-half hours. Many other blotted off, and the matrix thus produced pressed materials are available. I am able therefore to carefully against another piece of paper or the select radio active materials for both the type of carrier material, whereupon the radioactive di sodium phosphate is transferred to the paper in 40 radiation and for life, as may suit, my purpose. I claim: ' accordance with its concentration on the gela l. The method of recording normally invisible tin matrix. This paper will then carry the radio active image exactly as writing is carried, being completely invisible and under normal conditions being incapable of detection chemically or elec- 4 trically. 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 car rier may be used. While I have described my invention as prefer ably including the direct application of the radio active material to the carrier in intelligible form, I also deem my invention to include the spread ing 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 trans fers may be made in proper order to give a true negative on the ?nal ?lm radiograph so that 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 solu tion 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 photo graphs 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 photo graphs which comprises forming a gelatin matrix from a photograph, charging said matrix with a colorless aqueous solution of radioactive mate rial, pressing said matrix on a carrier, and expos ing said carrier to a radiation sensitive surface to form a visible image of said photograph thereon. CHARLES PECHER.