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210
ANALYSIS OF ALPHA EMITTERS IN THE CORAL, FAVITES VIRENS
sections (30 «wm thick) of the coral it is pos-
the concentrations of selected alpha emitters
in the 1954-1955 coral-growth increment. The
values were either determined by radiochemical
or
analytical
techniques
or
are
representative background values expected
for some alpha emitters obtained from the
literature. Background uranium and thorium
effects are of concern when the concentrations of alpha emitters being measured are in
the range of 1 pCi/g or less. Accordingly, at
concentrations measured in hot spots in the
coral, Favites virens, the uranium and
thorium backgroundis negligible.
METHODS
Calibration of cellulose nitrate with respect to
alpha-particle detection
It is important to establish the efficiency of
the alpha detector in each matrix to be
analyzed. Kodak LR-115 was calibrated by
using two standards to measure the efficiency
of registering alpha particles in two matrices,
polycarbonate and calcite. The standards
sible to establish the concentration and distribution of the alpha emitters in relation to
the textural and structural features of the
coral. The resolution of the method is on the
order of 5 4m (FI75) consequently it is possible to determine whether the alpha-emitters
are concentrated in the coral skeleton, in void
filling cements, or in other organisms (algae
or bryozoa). These results should be of importance in understanding the biogeochemical
processes influencing the distribution of alpha
emitting nuclides in the marine environment.
Alpha particle detection in natural materials
using solid-state track detectors
A particular advantage of using solid-state
track detectors for analysis is the very high
resolution in determining the location of the
element being detected. With the solid state
track detector technique it is possible to
measure alpha emitter concentrations in | pg
of calcite
50 pCi/g.
if the concentration is
above
Careful microscopic analysis, both of the
solid-state track detector and the sample
being analyzed, is required to take advantage
of this very high resolution. Consequently,
most analyses are done at 625X magnification
using a high-quality research microscope and
diligently recording the data in a manner to
allow correlation between the alpha-emitter
distributions and the structural features of
the sample understudy.
A disadvantage of using solid-state track
detectors for alpha-emitter analysis is the inability to distinguish the different alpha-particle energies and consequently the inability
to detect which isotope has produced the
observed track. To be more specific, the data
reported in this study are based on total
alpha-emitter concentration which means the
observed tracks are due to any alpha emitter
contained in the sample *Pu, Pu, “Pu,
“44m, 7°Po and others that might exist in the
environment. Consequently in our analyses
of the coral samples a certain proportion of
the tracks detected are due to the naturally
occurring uranium and thorium isotopes and
their decay products. Listed in Table | are
used are: (a) a 0.015 pCi Pu source elec-
troplated on to a stainless stee! planchet to
give a homogeneous and thin (Ra75) source
of plutonium and (b) an analyzed powdered
coral sample containing the alpha-emitting
radionuclides “*Pu "Pu and “Am.
(a) Calibration of the Kodak film by use of
the electroplated Pu source. Kodak film LR115, Type H is sensitive to alpha particles
having energies of less than 4 MeV while the
alpha particle energies of the plutonium and
americium isotopes are in the range of 55.5 MeV. Alpha particles emitted from a surface of material (the thickness of which
exceeds the range of the alpha energy con-
sidered) within which the radioactive isotopes
are uniformly distributed will have energies
Table 1. Alpha emitting radionuclides in 1954-1955 coral growth section
Radionuclide
Concentration
(pCi/g)
Comments
239Py
21.6
240Py
17.4
238Pu
210Pp (210Po}
238, 235U (U)
2.18
1.34
2.4
Determined by alpha spectrometry
Determined by alpha spectrometry
Determined by neutron activation
22Th
0.0008
230Th
228Th
226Ra
0.0006
0.005
0.027
Expected coral background levels
from Dodge and Thomsen (1974)
As above
As above
As above
247 Am
17.6
Determined by alpha and mass
spectrometry following radiochemical separation
As above
Determined by gamma spectrometry
and fission track detectors