In situ gamma measurements

It has previously been stated (Section 1.3) that an assumption sometimes made is that a single in situ measurement covers a circular area of approximately 10 m diameter, when the detector is positioned 1 m above the ground surface. There are also published methods that can be used to estimate the depth profile of activity in the groimd using multiple measurements or advanced methods of spectrum analysis, although these will not be discussed here". It is, however, a fact that the true extent and depth of an individual in situ gamma measurement cannot be known with certainty. This is because a high activity source at some distance or depth from the detector can induce the same number of counts as a low activity source that is a short distance away. 

Fong SH, Alvarez JL. 1997. Data quality objectives for surface-soil cleanup operation using in situ gamma spectrometry for concentration measurements. Health Phys 72 286-295. 

Miller KM, Shebell P, Kemic A. 1994. In situ gamma-ray spectrometry for the measurement of uranium in surface soils. Health Phys 67(2) 140-150. 

The operations carried out in the field included gamma dose rate measurements in situ gamma spectrometry and the collection of samples of grass, meat, milk, offal, vegetables and soil, as well as biological indicators such as animal bones, mushrooms and moss. 

A particular issue with the use of in situ measurements on land is that the area of interest is much larger than the size of the specimen that is analysed by individual measurements. Because a gamma detector counts radiation quanta that have arrived at the detector from any source, the size and shape of an individual specimen can never be fully known. This is in contrast to a laboratory measurement, where a specimen is removed from the target area and its mass and volume can be accurately determined. An assumption such as an area of 10 m diameter is clearly simplistic, because in reality the specimen size depends on several factors, e.g. the distributions of radionuclides and attenuation properties of the soil, also the intrinsic efficiency of the detector and the coimting time used. It would therefore be useful to be able to define more accurately the dimensions of an in situ specimen. This could assist at both the design and interpretation stages of an in situ gamma smvey. 

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 only in situ chemical data for asteroids are from the NEAR Shoemaker spacecraft, which orbited 433 Eros in 2000-1, and from the Japanese Hayabusa spacecraft, which visited 25143 Itokawa in 2003. NEAR obtained numerous measurements of the surface composition using X-ray fluorescence and gamma-ray spectrometers, and Hayabusa carried an XRF. The magnesium/silicon and aluminum/siUcon ratios for both asteroids are consistent with the compositions of chondrites. However, sulfur is depleted in Eros relative to chondritic compositions, possibly due to devolatilization by impacts or small degrees of melting. 

H. L. Beck, et al, In Situ Ge(Li) and Nal(Tl) Gamma-Ray Spectrometry, U.S. Department of Energy, Environmental Measurements Laboratory, HASL-258, September 1972. 

The Swedish National Defence Research Institute (FOA), which from 1978 until 1983 was connected to the SSI, runs a system of high-volume aerosol samplers that normally detects very small amounts of radionuclides by high-resolution gamma spectrometry. The FOA also has access to army airplanes and helicopters to take air samples at different heights, to record measurements from the air, and to transport equipment and personnel to remote areas quickly to perform in situ measurements with portable germanium detectors. At the SSI, routine measurements of milk were run before the Chernobyl accident. Routine programs concerning environmental surveillance of nuclear power plants currently exist. 

A monotonic relationship between intensity (i.e., count rate) and distance between the tracer and each detector was established by calibration. The density dependence of gamma ray attenuation through the bed made it necessary to calibrate in situ because of the inhomogeneity of the bubbling fluidized bed. The procedure involved positioning the tracer in a large number of distributed locations within the bed and then measuring the count rates of all detectors at each tracer location. 

Nuclear logging using gamma radiation, to measure the bulk density, and neutron radiation, to measure water content, are techniques used both at sea and ashore. Preiss (1968) and Richards and Chaney (1997) describe how these techniques may be used for marine sediments both on core samples and in situ. These in-situ methods are particularly apph-cable to near-surface sediments which can be extremely porous and tend to suffer the greatest amount of disturbance when sampled. It is difficult using these techniques to obtain accuracies better than 1%, due to the problems of cahbrating the instruments with specimens of different chemical and mineralogical compositions. 

Chace, Jr., A.B. 1985. Pressure release disturbance in simulated marine cores as indicated by a laboratory gamma-ray densitometer. In Strength Testing of Marine Sediments Laboratory and In Situ Measurements. ASTM STP 883, Chaney, R.C., and Demars, K.R., eds., American Society for Testing and Materials, Philadelphia, PA, pp. 154—165. 

Preiss, K. 1968. In-Situ measurement of marine sediment density by gamma radiation. Deep Sea Research, 15 637-641. 

For power measurements, gamma thermometers depend upon heating of the instrument sensing area by fission and fission product gamma rays. A central heater cable along the assembly is provided for in situ calibration. 

Material samples to prove falling below the mass-specific clearence level of 0.1 Bq/g were taken from the waste water duct and from several sumps in the turbine hall and the feedwater tank building. In other cases, proof was provided by means of gamma-spectrometric in-situ measurements. 

A total of 729 contamination measurements had been carried out. About 30 samples were tested with the gamma-ray spectrometer and 10 in-situ measurements were taken with a portable gamma-ray spectrometer. 

It is also possible to make measurements in situ, for example portable gamma detection equipment can be used if the target radionuclides can be characterised by key-line gamma energy levels. Considerations of the specimen size leads to an important difference between in situ investigations of chemically contaminated land and radioactively contaminated land. It is possible to make field measurements in situ of the chemical concentration of metals in soil by using Portable X-Ray Fluorescence (PXRF) equipment. In PXRF, the specimen size is of the order of a few cubic millimetres. It is therefore much smaller than a typical specimen that would be extracted for laboratory analysis. ... 

The advantages conferred by the remote detection of gamma-emitting radionuclides, combined with substantially lower costs of obtaining in situ measurements, potentially make a case for promoting the use of in situ measurements for the characterisation of land areas at decommissioning nuclear sites. Consequently, the objectives of this paper are to ... 

Systematic Component of Uncertainty. Laboratory detectors were calibrated using a geometry standard comprising a sample pot filled with dry soil, spiked with a certified mixed nuclide gamma standard. Systematic imcertainty (bias) of the means of the in situ measurements can be estimated by comparing these with the means of the ex situ measurements. 

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