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A high pressure-temperature press is used to make target material for accelerator mass spectrometry. Graphite was produced from typical **1**4C samples including oxalic acid and carbonates. Beam strength of **1**2C was generally adequate, but random radioactive contamination by **1**4C made age measurements impractical.
A sonde has been built for high-resolution measurement of natural or neutron-induced gamma rays in boreholes. The sonde is 7.3 cm in diameter and about 2.2 m in length and weighs about 16 kg. The lithium-compensated germanium semiconductor detector is stabilized at −185 to −188°C for as much as ten hours by a cryostatic reservoir containing melting propane. During periods when the sonde is not in use the propane is kept frozen by a gravity-fed trickle of liquid nitrogen from a reservoir temporarily attached to the cryostat section. A 252 Cf source, shielded from the detector, may be placed in the bottom section of the sonde for anlysis by measurement of neutron-activation or neutron-capture gamma rays. Stability of the cryostat with changing hydrostatic pressure, absence of vibration, lack of need for power to the cryostat during operation, and freedom of orientation make the method desirable for borehole, undersea, space, and some laboratory applications.
Like sediment samples from Scofield Resevior in Utah were analyzed for 210Pb by the gamma-ray spectrometric method. The top 10 cm of surface sediment yielded excess 210Pb activity (excluding in situ 226Ra supported 210Pb) of 1.05 pCi/g dry weight and decreased to 0.25 pCi/g at a depth of 25 cm. Based on these data, sedimentation rate was approximately 0.49 cm/y for a total of 30 cm of lake sediment and a lake history of approximately 60 y. An alternative method of 210Pb measurements using wet chemical procedures followed by beta counting gave equivalent results. ?? 1982.
The linear combination technique, when applied to a gamma-ray spectrum, gives a single number indicative of the extent to which the spectral lines of a sought element are present in a complex spectrum. Spectra are taken of the sought element and of various other substances whose spectra interfere with that of the sought element. A weighting function is then computed for application to spectra of unknown materials. The technique was used to determine calcium by radiative-neutron-capture gamma-ray analysis in the presence of interfering elements, notably titanium, and the results were compared with those for two popular methods of peak area integration. Although linearity of response was similar for the methods, the linear combination technique was much better at rejecting interferences. For analyses involving mixtures of unknown composition the technique consequently offers improved sensitivity.
The recent availability of borehole logging sondes employing a source of neutrons and a Ge(Li) detector opens up the possibility of analyzing either decay or capture gamma rays. The most efficient method for a given element can be predicted by calculating the decay-to-capture count ratio for the most prominent peaks in the respective spectra. From a practical point of view such a calculation must be slanted toward short irradiation and count times at each station in a borehole. A simplified method of computation is shown, and the decay-to-capture count ratio has been calculated and tabulated for the optimum value in the decay mode irrespective of the irradiation time, and also for a ten minute irradiation time. Based on analysis of a single peak in each spectrum, the results indicate the preferred technique and the best decay or capture peak to observe for those elements of economic interest.
Both an accelerator and a 252 Cf neutron source have been used to induce characteristic gamma radiation from extended soil samples. To demonstrate the method, measurements of the neutron-induced radiative capture and activation gamma rays have been made with both Ge(Li) and NaI(Tl) detectors, Because of the possible application to space flight geochemical analysis, it is believed that NaI(Tl) detectors must be used. Analytical procedures have been developed to obtain both qualitative and semiquantitative results from an interpretation of the measured NaI(Tl) pulse-height spectrum. Experiment results and the analytic procedure are presented.
In situ -produced 10 Be in quartz is widely used to constrain exposure ages and denudation rates, traditionally measured in sand-sized grains. Here we report a new method for isolating fine-grained quartz from shale and demonstrate its reliability for grain sizes down to single microns. Sequential dissolution tests and analyses of grain size separates show that meteoric 10 Be is eliminated and that recoil losses during cosmogenic nuclide production are balanced by implantation from other mineral grains. High-temperature combustion leads to diffusion of meteoric 10 Be into fine-grained quartz and must be used with caution.
A method is presented for assaying radioactive sandstone deposits in situ by using a high-resolution borehole gamma-ray spectrometer. Gamma-ray photopeaks from the same spectrum acquired to analyze a sample are used to characterize gamma-ray attenuation properties, from which a calibration function is determined. Assay results are independent of differences between properties of field samples and those of laboratory or test-hole standards generally used to calibrate a borehole sonde. This assaying technique is also independent of the state of radioactive disequilibrium that usually exists in nature among members of the natural-decay chains. ?? 1985.
90Sr (T1/2 = 28.5 years) is a long-lived radionuclide produced in nuclear fission. Fast radiochemical detection of 90Sr in environmental samples is not feasible using current analytical methods. Accelerator Mass Spectrometry (AMS) measurements of 90Sr were made with the Rehovot 14UD Pelletron accelerator at a terminal voltage of 11 or 12 MV using our standard detection system. Injection of hydride ions (SrH3-) was chosen owing to high beam intensity and low Coulomb explosion effects. 90Sr ions were identified and discriminated from isobaric 90Zr by measuring time of flight, total energy and three independent energy-loss signals in an ionization chamber. A reference sample and a ground-water sample were successfully measured. The detection limit determined for a laboratory blank by the residual counts in the 90Sr region is 90Sr/Sr = 3 ?? 10-13, corresponding in practice to (2-4) ?? 10790Sr atoms or about 0.5-1 pCi/L in environmental water samples.
From experimental studies of gamma rays from fast and thermal neutron reactions in hydrogeneous and non-hydrogeneous, semi-infinite samples and from Monte Carlo calculations on soil of a composition which might typically be encountered on planetary surfaces, it is found that gamma rays from fast or inelastic scattering reactions would dominate the observed spectra. With the exception of gamma rays formed by inelastically scattered neutrons on oxygen, useful spectra would be limited to energies below 3 MeV. Other experiments were performed which show that if a gamma-ray detector were placed within 6 m of an isotopic neutron source in a spacecraft, it would be rendered useless for gamma-ray spectrometry below 3 MeV because of internal activation produced by neutron exposure during space travel. Adequate shielding is not practicable because of the size and weight constraints for planetary missions. Thus, it is required that the source be turned off or removed to a safe distance during non-measurement periods. In view of these results an accelerator or an off-on isotopic source would be desirable for practical gamma-ray spectral analysis on planetary surfaces containing but minor amounts of hydrogen.
In situ capture gamma-ray analysis in a coal seam using a high resolution gamma-ray spectrometer in a close-fitting borehole has been reported previously. In order to check the accuracy of the method under adverse conditions, similar measurements were made by means of a small-diameter sonde in an oversize borehole in the Pittsburgh seam, Greene County, Pennsylvania. The hole was 5 times the diameter of the sonde, a ratio that substantially increased the contribution of water (hydrogen) to the total spectral count and reduced the size of the sample measured by the detector. The total natural count, the 40K,count, and the intensities of capture gamma rays from Si, Ca, H, and Al were determined as a function of depth above, through, and below the coal seam. From these logs, the depth and width of the coal seam and its partings were determined. Spectra were accumulated in the seam for 1 h periods by using neutron sources of different strengths. From the spectra obtained by means of several 252Cf neutron sources of different sizes, the ultimate elemental analysis and ash content were determined. The results were not as good as those obtained previously in a close-fitting borehole. However, the results did improve with successively larger source-to-detector distances, i.e.,as the count contribution due to hydrogen in the water decreased. It was concluded that in situ borehole analyses should be made in relatively close-fitting boreholes. ?? 1983.
A feasibility study has been made to operate by remote control an unshielded portable positive-ion accelerator type neutron source to induce activities in the ground or rock by “in situ” neutron irradiation. Selective activation techniques make it possible to detect some thirty or more elements by irradiating the ground for periods of a few minutes with either 3-MeV or 14-MeV neutrons. The depth of penetration of neutrons, the effect of water content of the soil on neutron moderation, gamma ray attenuation in the soil and other problems are considered. The analysis shows that, when exploring for most elements of economic interest, the reaction 2 H(d,n) 3 He yielding ∼ 3-MeV neutrons is most practical to produce a relatively uniform flux of neutrons of less than 1 keV to a depth of 19″–20″. Irradiation with high energy neutrons (∼ 14 MeV) can also be used and may be better suited for certain problems. However, due to higher background and lower sensitivity for the heavy minerals, it is not a recommended neutron source for general exploration use. Preliminary experiments have been made which indicate that neutron activation in situ is feasible for a mineral exploration or qualititative soil analysis.
A table of the analytical sensitivities of the principal lines in the thermal-neutron-capture gamma ray spectrum has been compiled for most of the elements. In addition a second table of the full-energy, single-escape, and double-escape peaks has been compiled according to energy for all significant lines above 3 MeV. Lines that contrast well with adjacent lines are noted as prominent. The tables are useful for spectral interpretation and calibration.
A table of the analytical sensitivities of the principal lines in the thermal neutron capture gamma-ray spectrum from 0 to 3 MeV has been compiled for most of the elements. A tabulation of the full-energy, single-escape, and double-escape peaks has also been made according to energy. The tables are useful for spectral interpretation and calibration.
High resolution intrinsic and lithium-drifted germanium gamma-ray detectors operate at about 77–90 K. A cryostat for borehole and marine applications has been designed that makes use of prefrozen propane canisters. Uses of such canisters simplifies cryostat construction, and the rapid exchange of canisters greatly reduces the time required to restore the detector to full holding-time capability and enhances the safety of a field operation where high-intensity 252 Cf or other isotopic sources are used. A holding time of 6 h at 86 K was achieved in the laboratory in a simulated borehole probe in which a canister 3.7 cm diameter by 57 cm long was used. Longer holding times can be achieved by larger volume canisters in marine probes.
A borehole sonde (~1.7 m long; 7.3 cm diameter) using a 200 mm 2 planar intrinsic germanium detector, mounted in a cryostat cooled by removable canisters of frozen propane, has been constructed and tested. The sonde is especially useful in measuring X- and low-energy gamma-ray spectra (40–400 keV). Laboratory tests in an artificial borehole facility indicate its potential for in-situ uranium analyses in boreholes irrespective of the state of equilibrium in the uranium series. Both natural gamma-ray and neutron-activation gamma-ray spectra have been measured with the sonde. Although the neutron-activation technique yields greater sensitivity, improvements being made in the resolution and efficiency of intrinsic germanium detectors suggest that it will soon be possible to use a similar sonde in the passive mode for measurement of uranium in a borehole down to about 0.1% with acceptable accuracy. Using a similar detector and neutron activation, the sonde can be used to measure uranium down to 0.01%.
Trace elements in coal can occur as components of either the organic constituents (macerals) or the inorganic constituents (minerals). Studies of the concentrations and distribution of the trace elements are vital to understanding the geochemical millieu in which the coal was formed and in evaluating the attempts to recover rare but technologically valuable metals. In addition, information on the trace element concentrations is important in predicting the environmental impact of burning particular coals, as many countries move toward greater utilization of coal reserves for energy production. Traditionally, the optical and the electron microscopes and more recently the electron microprobe have been used in studying the components of coal. The proton-induced X-ray emission (PIXE) microprobe offers a new complementary approach with an order of magnitude or more better minimum detection limit. We present the first measurements with a PIXE microprobe of the trace element concentrations of bituminous coal samples. Elemental analyses of the coal macerals-vitrinite, exinite, and inertinite-are discussed for three coal samples from the Eastern U.S.A., three samples from the Western U.S.A., and one sample from the Peoples Republic of China. ?? 1981.
A neutron generator pulsed at 100 s-1 was suspended in an artificial borehole containing a 7.7 metric ton mixture of sand, aragonite, magnetite, sulfur, and salt. Two Ge(HP) gamma-ray detectors were used: one in a borehole sonde, and one at the outside wall of the sample tank opposite the neutron generator target. Gamma-ray spectra were collected by the outside detector during each of 10 discrete time windows during the 10 ms period following the onset of gamma-ray build-up after each neutron burst. The sample was measured first when dry and then when saturated with water. In the dry sample, gamma rays due to inelastic neutron scattering, neutron capture, and decay were counted during the first (150 ??s) time window. Subsequently only capture and decay gamma rays were observed. In the wet sample, only neutron capture and decay gamma rays were observed. Neutron capture gamma rays dominated the spectrum during the period from 150 to 400 ??s after the neutron burst in both samples, but decreased with time much more rapidly in the wet sample. A signal-to-noise-ratio (S/N) analysis indicates that optimum conditions for neutron capture analysis occurred in the 350-800 ??s window. A poor S/N in the first 100-150 ??s is due to a large background continuum during the first time interval. Time gating can be used to enhance gamma-ray spectra, depending on the nuclides in the target material and the reactions needed to produce them, and should improve the sensitivity of in situ well logging. ?? 1984.