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Introduction

 

                A gamma-ray spectrometer system was constructed for the quantitative analysis of the naturally occurring radioelements potassium (⁴⁰K), uranium (²³⁸U), and thorium (²³²Th) in geologic reference samples, rocks and soils.  If the gamma-ray measuring system is properly calibrated (Grasty and Darnley, 1971), the gamma-ray data can be expressed in terms of the estimated concentrations of the radioactive elements. Potassium concentrations are usually expressed in units of percent (percent K); uranium concentrations are expressed as parts per million equivalent uranium (ppm eU); and thorium concentrations are expressed as parts per million equivalent thorium (ppm eTh).  The term “equivalent” is used because the technique measures gamma-rays from the decay series of bismuth (²¹⁴Bi), which is a decay product of uranium (²³⁸U), and thallium (²⁰⁸Tl), which is a decay product of thorium (²³²Th).  Radioactive disequilibria in the decay series of uranium and thorium may make the equivalent concentrations differ from the actual concentrations of these elements present in rock or soil (Adams and Gasparini, 1970).  Further discussions of the geochemistry of the three radioelements can be found in (Hansen, 1980) and (Faul, 1954).

 

                This gamma-ray spectrometer system is located at the Physics Building on the grounds of the U.S. Geological Survey in Reston, VA.  It consists of a gamma-ray spectrometer sensor (a crystal of thallium-activated sodium iodide) attached to a photomultiplier tube, which is housed inside a lead chamber (also known as a "castle") with walls 8" thick. The photomultiplier tube is connected by means of a cable to a PC running computer software to collect the data. The castle is made using lead (low-level radioactivity) bricks measuring 8" by 4" by 2".  An opening in one side of the castle allows placement of the sample directly beneath the detector by means of an aluminum sample holder. This opening is then sealed by rolling a dolly carrying lead bricks up against the side of the castle.  The sensor is held in place above the sample by means of a support assembly made from sheets of aluminum.  Another sheet of aluminum is placed on top of the castle opening and three layers of lead bricks are placed on top of this aluminum sheet.  To allow the passage of the cable from the photomultiplier tube to the PC, a hole was drilled through the aluminum sheet and the three layers of lead bricks above it. The lead bricks were drilled in such a way that the cable zig-zags through the layers of lead bricks, thus preventing any direct path into the castle chamber.  A ¹³⁷Cs source is used as a reference for making measurements and is located within the lead castle.  A small sheet of 1/4" thick lead is placed between the ¹³⁷Cs source and the detector to block some of the gamma-rays.  The design and construction of these components are given in more detail by clicking on the links below.  For a further discussion of laboratory instrumentation and similar systems, see Adams and Gasparini, 1970, pp. 68-88.

 

 

Sample Requirements

 

               Samples to be measured can vary in weight, particle size and composition. However, due to the dimensions of the opening in the castle, along with the sample holder, the largest sample container that will fit into the system is 6" in diameter and 1.5" deep. The sample holders used in this system are made from a clear polystyrene (or similar) plastic and are sealed with vinyl electrical tape to prevent the escape of radon and moisture from the sample.  Depending on density, typical samples range in weight from about 400 to 900 grams.  Geological reference standards are generally 600 grams. The samples don't require any special preparation. 

 

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This report is preliminary and has not been reviewed for conformity with the U.S. Geological Survey editorial standards. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

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