Spectral gamma measurement

If a spectral gamma measurement is available, the use of thorium and potassium for shale content derivation is recommended. Fertl (1983) noted that the thorium curve of a spectral gammalog allows a quantitative clay volume estimate despite the presence of varying amounts of uranium and potassium... 

Figure 5.4 shows three typical indications from a spectral gamma measurement. 

On the other hand, Table 5.9 shows that different minerals have nearly the same PE level. For example, chlorite, biotite and glauconite are in the order of PE 6 b/e (red line in Fig. 5.11). A combination with a spectral gamma measurement can result in a clear answer if at same formation a ratio Th/K=2 was observed (blue Une), the cross corresponds to a biotite. 

In situ measurement of the concentration of radioactive tracers in the different phases requires that the phases are separated and arranged according to density difference over the measurement cross section in a horizontal pipe. In general, the measurements are performed with two spectral gamma radiation detectors placed on top and bottom of the pipe respectively. 

The gamma rays are detected today with sodium iodide crystals scintillation counters. The counters, 6 to 12 in. long (15 to 30 cm) are shock mounted and housed in the drill collars. Several types of measurements can be made total gamma rays, direction-focused gamma rays, spectral gamma rays. 

Spectral Gamma Ray Log. This log makes use of a very efficient tool that records the individual response to the different radioactive minerals. These minerals include potassium-40 and the elements in the uranium family as well as those in the thorium family. The GR spectrum emitted by each element is made up of easily identifiable lines. As the result of the Compton effect, the counter records a continuous spectrum. The presence of potassium, uranium and thorium can be quantitatively evaluated only with the help of a computer that calculates in real time the amounts present. The counter consists of a crystal optically coupled to a photomultiplier. The radiation level is measured in several energy windows. 

Radionuclides are confirmed by applying redundant processes. A radionuclide is identified by gamma-ray spectral analysis and checked by chemical separation followed by a second spectral analysis measurement. Different analysts are assigned to analyzing the same sample by different chemical separations. The expected absence of gamma rays is confirmed by gamma-ray spectral analysis after the usual measurement for alpha- or beta-particle activity. Measurements are repeated to determine the half-life or a parent-daughter relation. Tabulations of... 

In the geophysical field practice, two techniques of gamma measurement are applied spectral measurement and integral measurement. 

The Lunar Prospector orbiter carried a gamma-ray/neutron spectrometer (GRS) that made precise measurements of the concentration and distribution of thorium (Lawrence et al., 1998) and hydrogen (Feldman et al., 2001). Subsequent spectral deconvolutions (Prettyman et al., 2006) have produced analyses of iron, titanium, potassium, magnesium, aluminum, calcium, and silicon. The principles of these analytical techniques are explained in Box 13.1. 

In some cases, thermal neutrons can also be used to measure the absolute abundances of other elements. Transforming the neutron spectrum into elemental abundances can be quite involved. For example, to determine the titanium abundances in lunar spectra, Elphic et at. (2002) first had to obtain FeO estimates from Clementine spectral reflectances and Th abundances from gamma-ray data, and then estimate the abundances of the rare earth elements gadolinium and samarium from their correlations with thorium. They then estimated the absorption of neutrons by major elements using the FeO data and further absorption effects by gadolinium and samarium, which have particularly large neutron cross-sections. After making these corrections, the residual neutron absorptions were inferred to be due to titanium alone. 

Gamma-ray spectral interferences. This is important in cases where the 0.511 MeV annihilation radiation from the product radionuclide is measured in the determination. Obviously, all other reactions yielding positron emitters would provide an interference. In the determination of oxygen a spectral interference is produced if the sample contains an appreciable amount of boron. The uBe produced by the 11B( , )11Be decays with the emission of an 11 MeV negatron and also gamma-rays... 

Earlier methods used in the analysis of radium isotopes in water required labor-intensive radiochemical separations and subsequent measurement of alpha particles for 226Ra and beta particles for 228Ra. The method used in this experiment applies simpler gamma-ray spectral analysis of the progeny of both 226Ra and 228Ra. 

Proper preparation of biological solids for radiochemical analysis is essential for obtaining valid radioanalytical chemistry results. The samples often must be large because the radioactivity levels are low. Gamma-ray spectral analysis is the preferred method of radiation measurement because it requires little preparation. If gamma-ray spectral analysis of the untreated sample is not feasible because few or no gamma rays are emitted, the sample must be dissolved. Dissolution is almost always required for alpha- and beta-particle analysis. The first step usually reduces the mass of the solid sample and prepares it for dissolution. 

Most insidious are systematic errors that come from deficiencies in the application of interference corrections and from problems in getting proper backgrounds. We have internal checks in our analytical system for 12 elements. Each of these is measured in two or more independent ways. For example, different gamma rays from the same isotope provide results that are completely independent because the problems of spectral... 

The 200 to 400 nm spectral Irradlance Inside the CER was measured using a Gamma Scientific Spectroradlometer. A selenium photovoltaic cell and Corning 7-45 ultraviolet filter was used to monitor the UV Irradlance. A 1 x 2 cm silicon solar cell was used to measure the near-IR Irradlance. CER photon flux was also calibrated by using 0-nltrobenzaldehyde (0-NBA) as an actlnometer( ). Outdoor photon flux was measured by dispersing 0-NBA In thin films (25 urn) of polymethyl methacrylate. These films were then exposed at the outdoor site behind a neutral density filter and were examined on a weekly basis. Outdoor weekly UV photon flux was calculated based on the conversion rate of the 0-NBA. 

Milk samples should be refrigerated and delivered to the laboratory within 1-2 days. For longer storage, the milk sample should be frozen or a preservative added such as methiolate, formaldehyde, or thimerosal (DOE 1987, EPA 1984), as indicated in Table 5.1. Preservatives may interfere with radioiodine separation (Murthy and Campbell 1960), but can be added if radioiodine is measured directly by gamma-ray spectral analysis. 

Radiobioassay is the determination of the kind, quantity, and location of radionuclides in the body by direct measurement (in vivo) or by in vitro analysis of material excreted from the body. Spectral analysis permits rapid analysis of radionuclides that emit gamma rays. Computer systems with data analysis in terms of metabolic models support routine use of bioassay procedures for assessing internally deposited radionuclides. The whole-body counter is an example of an in vivo procedure. 

Air filters and gummed deposition collection films first are measured by gross alpha- and beta-particle counting and gamma-ray spectral analysis. Usually, 1/2 of the sample then is dissolved to perform radiochemical analysis of the deposited radionuclides. The filter can be dry ashed, and then totally dissolved with an HNO3-HF treatment. 

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