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Patentecl Sept. 1, 1946
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.
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)
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
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
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
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,
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
55 radioactive material deposited on the carrier pa
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
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
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