Reinvestigation of the α-Activity of Conway Granite
Nature, vol. 273, no. 5659, pp. 217-218, May 18, 1978.
ADAMS et al.1,2 reported evidence for an unidentified
4.4 MeV α-activity in certain core sections taken from Conway granite in New
Hampshire. A similar α-activity has also been reported by
Cherdyntsev et al.3 and by Brukl et al.4
in different materials, but in
neither case was it ever confirmed. We report here our
reinvestigation of this phenomenon, and that we were unable to
confirm the evidence of a 4.4 MeV α-activity
in the Conway granite.
Because it was thought that a previous failure5 to confirm the existence
of this activity in the Conway granite might have been due to subtle differences
in sample preparation, considerable effort was
made to obtain the same core material and to follow the same
preparation techniques used by Adams et al.1,2.
On the other hand,
because no record was kept of the depth of the particular core section
which yielded evidence for the 4.4 MeV α-emitter,
it is not known
whether the Conway granite core sections used in these experiments
were from the same depth as the core section in question. Other
pieces of granite from this same general area were also obtained
from T. P. Kohman, and were given the same experimental treatment
as the original cores obtained by Adams.
The experimental treatment, with minor variations from time to
time, consisted of first crushing the 30.5 cm long, 2.5 cm diameter
cores to about 1 mm size, followed by magnetic separation of the
biotite in a standard Franz isodynamic magnetic separator. The
biotite flakes obtained from this magnetic separation were then
crushed using a mortar and pestle. To extract the small radioactive
inclusions from this crushed biotite, a heavy liquid separation was
carried out using methylene iodide (p = 3.4 g cm−3). The crushed
biotite was left in the methylene iodide for several hours to allow
ample time for the small (10−200 μm) radioactive mineral
inclusions (p > 3.5 g cm−3) to separate from the less dense biotite (p
3 g cm−3). The methylene iodide residue was subsequently removed
from the inclusions using an acetone rinse.
After drying, the inclusions were sprinkled on to the surface of 1
× 3 inch Eastman Kodak NTA emulsion track plates. These NTA
plates had an emulsion thickness of 25 μm and were sensitive
particles without recording α rays. The plates with inclusions were
then placed in a light tight box under refrigeration for periods
ranging from three weeks to three months. Subsequent development
and microscopic scanning of these plates revealed
the α-activities of
the various individual inclusions on the plate. At this point
microscopic techniques were utilized to pick out only the inclusions
which exhibited the greatest cluster of α tracks. These highly
radioactive inclusions, which varied in size from about 10 to 200 μm,
were then placed either singly or in groups of up to 25 on to Pt or
stainless steel disks. The initial experiments involved dissolution of
these inclusions on the Pt disks with drops of concentrated HF and
HNO8, the Pt disk itself serving as the source for
It was soon found that better energy resolution could be obtained by
subsequent crushing of the dissolved residue, and this technique was
followed until it was found that the acid dissolution could be
dispensed with entirely. From then on, the higher activity inclusions
were mounted on stainless steel disks, crushed, and then counted for
periods ranging from about one day to a week.
The measurements were made on an α-spectrometer with a 2 cm
diameter, gold-covered surface barrier Si-detector of 300-μm depth
mounted in a vacuum chamber. Sample disks were placed in the
bottom of a polyethylene cap which fitted over the detector so that
the sample was approximately 1 mm from the face of the Si-diode. A
preamp was mounted directly on the base of the detector (bias ~ 100
V) and its output was fed to an amplifier. The pulse-height spectrum
was measured with a multichannel analyzer. Although the system
exhibited a resolution of 30−35 keV (FWHM) for a very thin
source, samples prepared by the above technique exhibited a typical
resolution of 50−60 keV. Measurements generally spanned the
range of ~2 to 10 MeV over 2,048 channels. Energy drift and
background were negligible.
About 50 different α-spectra were
obtained on the various cores
and samples from the Conway granite. The α-spectra from the
multichannel analyzer were recorded on punched tape, which then,
by means of a computer program, generated an accurate plot of
counts per channel against channel number. Energy calibration was
accomplished by means of a pulser and
The α-spectra generally showed
evidence of both the 238U and
232Th α-decay chains,
and in some cases the abundance ratios of the
two elements and their respective daughters were noted to vary from
inclusion to inclusion. This was not surprising, for the inclusion
selection process was not specific for a particular mineral, hence
different U/Th ratios were to be expected as dissimilar mineral
inclusions were analyzed. In some cases disequilibrium in the U−Th
chains was observed in the α-spectra
but no attempt was made to
determine whether this condition arose from slight variations in the
sample preparation procedure or from an inherent disequilibrium
within the sample before crushing. The reason for this disequilibrium
condition was not pursued because the main point of the experiments
was to determine whether any 4.4 MeV α-activity could be detected.
With the possible exception of one sample, which exhibited poor
statistics because of a relatively weak source, the experiments did
not reveal any evidence for the 4.4 MeV α-emitter reported by
Adams et al.1. A redetermination of that particular sample
subsequently gave no evidence whatsoever for the 4.4 MeV activity.
Thus we were unable to find any confirmatory data for the existence
of a 4.4 MeV α-activity in the Conway granite.
This research was supported by Union Carbide Corporation under
contract with the Division of Basic Energy Sciences of the
Department of Energy. Separation of the biotite from the crushed
granite was carried out using the magnetic separator located in the
Geology Department of the University of Tennessee, Knoxville. Mr.
Mirza Beg and later Dr. Otto Kopp supervised this phase of the
|R. V. GENTRY|
J. H. HALPERIN
B. H. KETELLE
G. D. O'KELLEY
R. W. STOUGHTON
Oak Ridge National Laboratory,
Oak Ridge, Tennessee 37830
|J. A. S. ADAMS|
Houston, Texas 77001
Received 11 January; accepted 9 March 1978.
Adams, J. A. S. & Rogers, J. J. W. TID−20506(1964).
Cherry, R. D. Richardson, K. A. & Adams, J. A. S. Nature 202, 639 (1964).
Cherdyntsev. V. V., Zvenev, V. L., Kuptsov, V. M. & Kislitsina, G. I. Geokhimia
4, 395 (1968).
Brukl, A., Hennegger, F. & Hilbert, H. Oesterr. Akad. Wis. .Abt. 2A 160, 129 (1951).
Petrzhak, K. A., Yakunin. M. 1. & Ter-Akop'yan, G. M. Atomn. Energ. 32, 179 (1972).