Correction for decay during counting in gamma spectrometry.

A basic result in gamma spectrometry is the count rate of a relevant peak. Correction for decay during counting and expressing the count rate at the beginning of the measurement can be done by a multiplicative factor that is derived from integrating the count rate over time. The counting time substituted in this factor must be the live time, whereas the use of the real-time is an error that underestimates the count rate by about the dead-time (DT) (in percentage). This error of underestimation of the count rate is corroborated in the measurement of a nuclide with a high DT. The present methodology is not applicable in systems that include a zero DT correction function.

Calculation of the volume of an HPGe crystal.

An important characteristic of a high-purity germanium detector for gamma spectrometry is the volume of the crystal, which relates to the detection efficiency and to the measured background level. The crystal dimensions provided by the detector manufacturer were used to calculate the crystal volume. The calculation method accounts for the various sub-volumes of the crystal, in particular the contact hole in the back face and the rounding of the 90° right edge in the front face. The calculation was corroborated using the INVENTOR(®) 3D software. The crystal volume was calculated to be 419.7 cm(3) compared with the 413 cm(3) stated by the manufacturer.

Dating the age of a nuclear event by gamma spectrometry.

The age of a nuclear event can be determined by measuring the activity of two fission products. The event studied was a short irradiation, of a small sample of uranium, in a nuclear reactor. Two types of a clock were investigated: non-isobaric and isobaric parent-daughter fission products. Measurements of the source by gamma spectrometry yielded very good agreement between true and measured ages. The accuracy of each clock and the upper and lower age limits of applicability were studied.

New precision measurement of the 3He(4He,gamma) 7Be cross section.

The 3He(4He,gamma) 7Be reaction plays an important role in determining the high energy solar neutrino flux and in understanding the abundances of primordial 7Li. This Letter reports a new precision measurement of the cross sections of this direct capture reaction, determined by measuring the ensuing 7Be activity in the region of Ec.m.=420 to 950 keV. Various recent theoretical fits to our data result in a consistent extrapolated value of S34(0)=0.53(2)(1) keV b.

Precision measurement of the 7Be(p,gamma)8B cross section with an implanted 7Be target.

The 7Be(p,gamma)8B reaction plays a central role in the evaluation of solar neutrino fluxes. We report on a new precision measurement of the cross section of this reaction, following our previous experiment with an implanted 7Be target, a raster-scanned beam, and the elimination of the backscattering loss. The new measurement incorporates a more abundant 7Be target and a number of improvements in design and procedure. The point at E(lab)=991 keV was measured several times under varying experimental conditions, yielding a value of S17(E(c.m.)=850 keV)=24.0+/-0.5 eV b. Measurements were carried out at lower energies as well. Because of the precise knowledge of the implanted 7Be density profile, it was possible to reconstitute both the off- and on-resonance parts of the cross section and to obtain from the entire set of measurements an extrapolated value of S17(0)=21.2+/-0.7 eV b.

Minimum detectable activity in in situ gamma-ray spectrometry.

The algorithm used to calculate a minimum detectable activity (A(D)) of an environmental volume sample measured by gamma-ray spectrometry was modified and adapted for in situ measurements performed at the standard 1 m height above ground. AD values of target radionuclides were determined from the in situ spectra collected at two disparate sites. A linear relationship between the two sets of A(D)s was found and the ratio between these two sets is equal to the square root of the ratio of the two respective absorbed dose rates measured at the sites at 1 m height. Absorbed dose rates were calculated using the concentration of potassium, thorium and uranium in the soils at the sites and found to agree well with the measured values. A(D) values can be predicted easily in in situ gamma spectrometry applying a simple experimental procedure that is based on the linear relationship.

Angular response of a HPGe detector to gamma-rays in in situ measurements.

The angular response of a HPGe detector used in in situ measurements was determined empirically as a function of gamma-ray energy at two source-detector geometries--the standard 1 m and the non-standard 0.1 m heights above ground. Values calculated by a Monte Carlo code were found to agree well with the experimental values. The code is thus labor-saving, because it can spare many measurements required for the empirical determination of the angular response in an unknown experimental configuration.

Minimum detectable activity in gamma-ray spectrometry--statistical properties and limits of applicability.

Some statistical properties of the essential physical quantity "Minimum Detectable Activity" were analyzed, as well as the limits of its applicability in gamma-ray spectrometry. It was shown that the Minimum Detectable Activity is not linear with respect to the quantity of the measured sample and it is advisable not to calculate the Minimum Detectable Activity per unit mass or volume (in Bq kg-1 or Bq cm-3). The MDA has a clear decisive relevance to samples that have an identical geometry and chemical composition with that of the sample that was used to derive the MDA. The relative uncertainty of the Minimum Detectable Activity is approximately +/- 32%. It is recommended not to exclude from evaluations those results that are below minimum detectable activities.

In situ hot spot measurements with HPGe detector 0.1 M above ground.

The objective of this study was to examine the feasibility of an accurate in situ measurement of radioactive hot spots by a HPGe detector located at a height of 0.1 m above the surface, thus sparing the awkward use of a heavy collimator required at the standard 1 m height. Radioactivity concentrations of primordial naturally occurring 40K, 232Th, 238U and of anthropogenic 137Cs in a sand plot were measured in situ at the two heights. Independently, six fractions of a soil profile collected in the site were measured in the laboratory. Good agreement was found between the results of the two geometries and the soil samples. Good agreement between measured and predicted radioactivity concentration values was obtained for hot spots simulated by uniform planar sources of 40K, 60Co and 137Cs having a circular pattern 0.5 m in diameter. The study shows that an in situ measurement without a collimator but close to a hot spot can provide a reliable result.

Isotopic analysis of uranium in U3O8 by passive gamma-ray spectrometry

Passive gamma-ray spectrometry was applied to analyze the isotopic composition of uranium in U3O8. Depleted and enriched U3O8 standard reference materials were used to calibrate the system. An independent calibration was performed by standard gamma-ray point sources. U3O8 SRM samples of the 950 series were analyzed. The present results show that the isotopic abundances of 235U in SRMs 950, 950a and 950b are higher by +3.6, +0.9 and +0.9% (relative deviation) than the natural value 0.7200%, while relative precisions were +/-0.4, +/-0.7 and +/-0.3%, respectively.

The distribution of selenium in human blood samples of Israeli population--comparison between normal and breast cancer cases.

A preliminary study was carried out in order to compare the selenium concentration in breast cancer patients and healthy subjects (controls) in Israel. Blood serum samples were obtained from 32 breast cancer patients and 36 controls and were analyzed for selenium by the XRF method. A weighted mean of 0.076 +/- 0.014 ppm Se in the blood serum of breast cancer patients, as compared to 0.119 +/- 0.023 ppm Se for controls, was obtained. These results indicate that the concentration of selenium in breast cancer patients is significantly lower than in controls. The relationship between selenium concentration and malignancy stage shows an inverse dependence, i.e., the concentration decreases with stage number.