Thomas M. Semkow

Recoil-emanation theory applied to radon release from mineral grains

Abstract In this paper the α-recoil emanation theory is presented as a major mechanism for Rn release from solid materials. The basic theory is developed for Rn emanation from geometrical shapes: spheres and cylinders having variable thickness of precursor-Ra containing material. Subsequently, a sphere-pore model of emanation from mineral grains (like those in rocks, soils, etc.) is developed. In this model a surface of the grain is constructed as a superposition of in- and out-directed spherical caps and cylindrical pores. The model incorporates processes like Rn embedding, pore overlap, water effect and emanation from the edges. Using such a geometrical model it is shown how the Rn emanating-power measurements can give a clue to the Ra distribution and surface properties of the samples. In particular, it is shown that for grains reasonably larger than the recoil range and for surface Ra distribution, the emanating power is independent of the grain size but the total Ra concentration varies with the reciprocal of the grain size. The opposite is true for Ra distributed uniformly throughout the volume of the grain. It is shown how the Rn emanating power is related to the surface porosity of the grain. Also, implications are made with respect to U transport in nature, and with respect to low emanating power from lunar soil. Finally, a failure of the emanation theory is discussed to reproduce emanation curves for the samples having high specific surface areas and Ra distributed throughout the volume.

Coincidence summing in gamma-ray spectroscopy

Abstract A new technique has been developed to calculate coincidence-summing corrections in γ-ray spectroscopy. In this technique the general coincidence-summing equations were derived in matrix notation, which allowed extracting either the first-order correction (combinations of only two coincident γ-rays) or a full correction (all possible combinations of emitted γ-rays). Subsequently, it is shown how the technique can be applied to the determination of the source disintegration rate, γ-ray emission rates or peak efficiencies in the presence of coincidence summing. In particular, the technique has been applied to the determination of peak efficiencies of a germanium detector. The peak efficiencies were iterated self-consistently using the coincidence-summing equations. The above calculation showed that, in general, the full correction is necessary for complicated decay schemes. In addition, a method has been developed to determine the peak-to-total ratio for a germanium detector in the presence of an interfering γ-ray.

Principles of gross alpha and beta radioactivity detection in water.

Abstract—A simultaneous detection of gross alpha and beta radioactivity was studied using gas proportional counting. This measurement is a part of a method mandated by US Environmental Protection Agency to screen for alpha and beta radioactivity in drinking water. Responses of a gas proportional detector to alpha and beta particles from several radionuclides were determined in drop and electroplated geometries. It is shown that, while the alpha radioactivity can be measured accurately in the presence of beta radioactivity, the opposite is not typically true due to &agr;-to-&bgr; crosstalk. The crosstalk, originating from the emission of conversion and Auger electrons as well as x rays, is shown to be dependent primarily on the particular alpha-decay scheme while the dependence on alpha energy is small but negligible. It was measured at 28–35% for 241Am, 22–24% for 230Th, and 4.9–6.5% for 239Pu. For 210Po, the crosstalk of 1.2–1.6% was observed mostly due to energy retardation. A method of reducing the crosstalk to a <3% level is proposed by absorbing the atomic electrons in a 6.2 mg cm−2 Al absorber, at the same time decreasing the beta efficiency by 16–31%.

Efficiency of the Lucas scintillation cell

Abstract The Lucas scintillation cell (LC) and its modifications are among the most sensitive instruments to detect low levels of radon. In this work we describe a comprehensive investigation of the counting efficiency of the LC. A known quantity of 222 Rn was transferred into the LC and the decay and ingrowth of Rn and its daughters was followed in an automated measurement with counting intervals ranging from 5 s to 12 h. A total of 7014 measurements were taken in a period exceeding 5 months. The measurements taken during Rn transfer to the LC indicated that the separation was not ideal and that small amounts of daughters were also transferred to the LC. A numerical model was then developed and fitted to the data. The model includes kinetics of Rn transfer to the LC, a matrix-algebra method for description of decay and growth of Rn and its daughters, individual counting efficiencies, as well as several small corrections. Individual α counting efficiencies (counts/particle) were determined: 0.76(2) for 222 Rn, 0.77(3) for 218 Po, 0.91(1) for 214 Po, and 0.8283(6) for 210 Po. A contribution from electrons to the efficiency was found to be insignificant. The work is concluded by providing an overall counting efficiency of LC as a function of decay and counting times, which is of practical interest to radon measurements.

Rapid alpha spectroscopy of evaporated liquid residues for emergency response.

A new method for &agr; spectroscopy of evaporated water residues was developed, consisting of evaporation of drinking water, flaming of the planchets, and &agr;-spectroscopic measurements using a grid ionization chamber. The method can identify and quantify radioactivity concentrations ≥3 mBq L−1 in a matter of several hours, whereas determination of sub-mBq L−1 levels is achievable in 1 day. Detailed investigations of flaming of the planchets, the humidity effect, and &agr; spectroscopy of thick sources are described. A three-dimensional calibration of the method was performed using standards containing 238U, 230Th, 239Pu, 241Am, and 244Cm radionuclides. In addition to its application to evaporated drinking water, this calibration is common for any environmental sample that can be prepared as a uniform layer, such as the residues from surface water, acidic washing or leaching from materials, as well as biological fluids such as urine. The developed method serves as a fast identifying or screening technique for emergency response involving &agr; radioactivity.

Experimental verification of overdispersion in radioassay data.

Abstract— New evidence is provided suggesting that radioassay data are frequently overdispersed with respect to the Poisson distribution. Twelve cases of radioassay data were measured using commonly available detection systems. The data were analyzed using a limited version of the overdispersion model developed earlier. In that limit, the relationships between three overdispersed distributions were derived and discussed: beta-Poisson, negative binomial, and overdispersed Gaussian. Out of a total of 13 cases studied (12 measured plus one from the literature), 4 were consistent with the Poisson statistics at 90% confidence level while the remaining 9 were found overdispersed. This shows that the overdispersion is rather prevalent in radioassay. All three overdispersed distributions fitted the data very well. The overdispersion was attributed mostly to the excess fluctuations of the detection systems or, in 2 cases, sequential radioactive decay.

Statistical deconvolution of beta spectra: Implications for neutrino mass detection

Abstract We performed a Monte-Carlo simulation study of β− spectrum of 35S. A statistical deconvolution of the spectrum was then developed to address a priori detectability of heavy neutrino (such as the much-discussed 17 keV) in calorimetric β− measurements. We described the influence of detector energy resolution and other response parameters on the distortions in the β− spectrum and possible neutrino mass detection. A design of a Xe gas-scintillation proportional detector for precision β− spectroscopy is described.

Fractal approach to the solid-particle environmental radioactivity

Abstract A model of solid particle, based on fractal geometry of surfaces, was developed to describe the distribution of radioactivity in the natural environment. Two-fold distribution of natural radionuclides was assumed: internal and in the surface layer of the particle. The fractal-particle model was fitted to two sets of literature data on radioactivity distribution in coal fly ash particles and three sets of data for soil particles. In all cases studied, it was possible to determine the fractal dimension of the particle surface as a measure of its roughness. The values ranged from 2.40 to 2.79 for coal fly ash and from 2.50 to 2.73 for soil. Internal radioactivity concentrations were also determined. In some cases, it was possible to determine the surface concentrations of radioactive elements as well as an average thickness of the surface layer. Most of the radionuclides were found to be predominant on the surfaces of the particles, whereas in some cases, 40 K, 226, 228 Ra showed a depletion on the surfaces. The criteria for experimental data best suitable for the model are described, and new experiments to study roughness of environmental particles using radioactive tracers are proposed.