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ATOMIC ENERGY COMMISSION
RADIOACTIVITIES PRODUCED IN NEUTRON IRRADIATION OF CHLORINE
Ralph T. Overman
RADIOACTIVITIES PRODUCED IN NEUTRON IRRADIATION OF CHLORINE
By Ralph T. Overman
Considerable interest has been shown in the use of tracers for the study of a great many
chemical reactions. Of the more important elements that have been used for the study of
both organic and inorganic chemical reactions, those that can be derived from the bom-
bardment of chlorine have shown themselves to be quite promising. For quite some time
cyclotron groups have produced C138 as a chlorine tracer and Sa5 to be used for following
sulfur reactions. However, the short half-life of the chlorine activity has made it quite
difficult to work with except at the cyclotron site,and the soft radiation from the sulfur
activity has made it difficult to follow. This was particularly true since the chemical pro-
cedures used in the past with the sulfur usually have involved the addition of isotopic
carrier. It has not been possible to produce large amounts of carrier-free sulfur tracer
before the advent of the chain reacting pile.
In conjunction with a program of research related to isotope production, C.P. potassium
chloride was placed in the Clinton pile and bombarded for long periods. The samples de-
scribed in this paper had irradiation times of from 12 to 18 months.
The first activities that were studied were the chlorine activities. Little work was done
with the 37 minute activity since its characteristics had been confirmed many times. The
other possible activity in the chlorine was Cl36. This had been sought for by O'Neal1
who set an upper limit of its half-life as about 103 years,since he did not find it in a nine
months irradiation in a Ra-Be source. The activity was found in minute quantities by
Graham and Walke2, who characterized the beta radiation and gxve some evidence for
positron and X-ray emission based on a few cloud chamber photographs. They found no
appreciable decay in a one year period and postulated that a half-life of the order of 103
years was probably correct. They obtained a beta energy of 0.64 Mev.
The procedure for the isolation of the chlorine activities from other radiochemical
impurities is given as follows:
To about 0.2 gram of the KC1 were added 20 mg of KH2PO4 and 20 mg of K2SO4 in 5 ml
of water. Sufficient AgNO3 was added,according to a standard method,to precipitate
the chloride as AgCl. The precipitate was then dissolved in NH40 after being filtered
from the original solution. The hold-back carriers were added again and the silver
chloride again precipitated. This cycle was repeated three times. The AgCI was then
fused with NagCO3 and the resulting sodium chloride solution distilled from phosphoric
acid solution. An aliquot of the chloride was then precipitated with silver nitrate, dried,
weighed as AgCland counted to determine the specific activity.
The published absorption cross-section of chlorine for slow neutrons is given as 33 barns.
The cross-sections of the competing reactions are known to total about 0.7 barns. The
cross-section was then assumed to be 32.3 barns for the Cl35 (n,y)C136. Using this value
a half-life of 1 x 106 years is found for the long-lived activity. No evidence for decay by
other than B "emission was found.
- 1 -
2 MDDC 857
An absorption curve was taken in aluminum from which we obtained a range as deter-
mined by the Feather method of 230 mg/cm2, This corresponds to an energy of 0.66 Mev.
No detectable decay has been noticed during several months. The practical specific activ-
ity obtainable in bombarding chlorides in a pile has been issued in the publications of the
Isotopes Branch, A. E. C.
Another quite interesting activity produced in the bombardment was the S35 from the
Cl35(n,p)S35 reaction. This has been studied rather extensively by Kamen 3 and Libby
and Lee4 The irradiation of chlorine containing compounds has already been considered
the best way of preparing this activity since it is a slow neutron reaction with a rather
high cross-section. Much higher specific activity can be made than in the direct irradia-
tion of sulfur.
Libby and Lee prepared the activity and determined the energy of the beta particle by a
specially designed magnetic counter tube which acted in a manner somewhat analogous to
a spectrometer. They reported an energy of 0.107 Mev. Kamen has done considerable
work on the radiochemistry of sulfur and reports absorption values in aluminum and cello-
phane which gave an energy of 0.120 Mev. More recently Henriques, et al.5, have dis-
cussed the measurement of the activity with a special electroscope and thin-walled window
We have separated the activity from chlorine containing compounds with several methods.
One of these was done with the aid of added inert carrier and the others yield carrier-free
The separation of the sulfur activity from the KC1 mentioned in the previous sections
was brought about as follows:
To about 0.2 grams of KCI were added 20 mr of K1SO4 in 5 ml of water and 1 ml cone.
HNO3. After boiling for a few minutes BaC12 was used to precipitate the sulfate. The
precipitate was centrifuged and washed. The BaSO4 was then mounted on platinum and
heated in a Fisher micro combustion furnace at a temperature carefully controlled
within 900-9500C. for 1 hour in an atmosphere of hydrogen. The resulting BaS was
treatediwith dilute HCI and the H2S thus formed was passed through lead acetate. The
resulting PbS was oxidized with 30%, H202 to PbSO4. The lead was then metathesized
to PbS in a slightly acid solution. The PbS was then centrifuged off and the solution
containing the sulfate activity brought back to neutral with a drop of dilute NaOH. This
will leave Na2SO4 as the compound containing the activity and a trace of NaCI in the
solution. The solution was made up to 25 ml and an aliquot of 0.1 ml was counted.
This gave a sample which had a thickness of about 0.1 mg/cm2. This is a thickness
almost negligible for self-absorption corrections but these can be made if desired.
For quantitative measurement of the energy, the most satisfactory method is to obtain
carrier-free sulfur activity. The sulfur activity can be extracted most simply in a quali-
tative manner by the use of ion exchange resins. This method gives a very pure product,
with a minimum of effort,but does not allow one to make yield calculations since the chem-
ical yield for the column process is not known.
Carrier-iree sulfur has also been prepared by the bombardment of several highly chlo-
rinated hvdrocarbon. 2... of these,on which work has been doneis deca chloro biphenyl
which goes under the trade name of Aroclor 1271. This material was bombarded for a
suitable length of time and dissolved in benzene. A small amount of water and a few ml of
bromine were added and the solution refluxed overnight. The solid organic compound was
then precipitated by the addition of an equal quantity of water and filtered. After distilling
off the dioxane- and water from the supernatant solution a small amount of concentrated
nitric acid was added to oxidize any organic residue. The activity was taken up in
0.1 N HCI. If desired,the phosphate activity can be precipitated by the addition of phos-
phate ion and the precipitation of zirconium phosphate or iron phosphate.
Absorption curves can be taken with an ordinary end-window counter and fairly reliable
quantitative estimates of the amount of activity present made by extrapolation to zero ab-
sorber. In order to determine the quantity present in a given amount of chlorine-containing
compound,it was felt that an absolute measure of the yield would be desirable. To do this
in terms of millicuries it is, of course, necessary to use a counter rather than an electro-
scope. Henriques had used a counter with a 2 mg/cm2 window and had developed an elec-
troscope in which the sample was mounted directly in the chamber of the electroscope. It
was felt desirable to have our measurements made on the inside of a counter in a some-
what similar fashion.
To accomplish this end, use was made of a low absorption counter designed by
R. L. Butenhoff of our laboratories. In this counter the sample is mounted on a thin
Formvar film of about 60/1 g/cm2 thickness. The use of the Formvar is recommended for
thin backing of samples to reduce the backscattering. Formvar has been found to be many
times stronger than zapon films of the same thickness. The Formvar is also much more
resistant to chemical attack than are the zapon films. The absorbers are mounted on a
rotating many-sided wheel so that they may be rotated into position between the collecting
electrode and the source. The absorbers used here were beaten aluminum which had been
calibrated for thickness by alpha counting techniques. The volume of the chamber is about
a liter and is of such a size that it can be evacuated and filled in a few minutes.
An aliquot of a larger sample was mounted on the source holder inside the counter and
an absorption curve was taken. On this particular instrument are 12 absorbers below
10 mg/cm2 in thickness. The thinnest absorber is about 0.1 mg/cm2 which added to the
absorption due to the argon gas in the chamber amounting to about 0.6 mg/cm2. The geom-
etry of the counter was determined by taking a curve on Co60 which had been calibrated
by use of coincidence measurements made by W. C. Peacock of our Laboratories.
The sulfur source was then counted on a standard counter and an absorption curve taken
in the same way that will be done on isotopes released for distribution. The Co60 standard
was also taken in this way and the geometry thus determined for the standard counter. The
curves taken on the two instruments were normalized by using the relative geometry
factors as determined by the Co standard and a composite absorption curve drawn. About
20 points were taken on the two machines with 7 points overlapping on the two curves.
This composite curve was used then to determine the absolute amount of sulfur activity
present and also to determine the energy from the absorption data. From a Feather plot
of the absorption data,a range of 35 mg/cm2 was determined. This corresponds to an
energy for the sulfur beta particle of 0.17 Mev.
A very small amount of phosphorous activity was found which had been formed by the
- 3 -
Cl35(n, c)P32 reaction on the chlorine. This was separated by the addition of potassium
carrier to the solution and the addition of small amounts of potassium sulfate as hold-back
carrier. The usual precipitation with magnesia mixture was satisfactory for the separa-
tion of the activity from the bulk of the solution after three or four cycles. Additional sul-
fate hold-back carrier was added after each precipitation. The magnesia mixture had suf-
ficient chloride in it to act as a chlorine activity hold-back. The yield was so small that
no great amount of work was done on the reaction except to confirm the half-life of 14.7
days and the energy of 1.7 Mev. as determined by aluminum absorption.
These data are summarized in the following table: (Confirmed values in parentheses).
Reaction Half-life Beta Energy
Cl (n, y )C1 1 x 106 years 0.66
Cl (n, T )C1 (37 minutes) complex
Cl (n,p ) S (87 days) 0.17
Cl (n, y ) P (14.7 days) (1.7)
(1) O'Neal, Phys. Rev., 59, 109 (1941)
(2) Graham and Walke, ibid., 60, 909 (1941)
(3) Kamen, ibid., 60, 537 (1941)
(4) Libby and Lee, ibid., 55, 245 (1939)
(5) Henriques, Kistiakowsky, Margnetti, and Schneider, Ind. Eng. Chem., Anal. Ed., 18,
UNIVERSITY OF FLORIDA
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