Bee oar So 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