NON-DESTRUCTIVE DETERMINATION OF 224Ra, 226Ra AND 228Ra CONCENTRATIONS IN DRINKING WATER BY GAMMA SPECTROSCOPY

Abstract

The U.S. Environmental Protection Agency mandates that drinking water showing gross alpha-activity greater than 0.19 Bq L(-1) should be analyzed for radium, a known human carcinogen. The recommended testing methods are intricate and laborious. The method reported in this paper is a direct, non-destructive gamma-spectroscopic method for the determination of 224Ra, 226Ra, and 228Ra, the three radium isotopes of environmental concern in drinking water. Large-volume Marinelli beakers (4.1-L capacity), especially designed for measuring radioactive gases, in conjunction with a low-background, high-efficiency (131%) germanium detector were used in this work. It was first established that radon, the gaseous decay product of radium, and its progeny are quantitatively retained in this Marinelli beaker. The 224Ra, 226Ra, and 228Ra activity concentrations are determined from the equilibrium activities of their progeny: 212Pb, 214Pb (214Bi), and 228Ac; and the gamma-lines used in the analysis are 238.6, 351.9 (and 609.2), and 911.2 keV, respectively. The 224Ra activity is determined from the first 1,000-min measurement performed after expulsion of radon from the sample. The 226Ra activity is determined from the second, 2,400-min measurement, made 3 to 5 d later, and the 228Ra activity is determined from either the first or the second measurement, depending on its concentration level. The method's minimum detectable activities are 0.017 Bq L(-1), 0.020 Bq L(-1), and 0.027 Bq L(-1) for 224Ra, 226Ra, and 228Ra, respectively, when measured under radioactive equilibrium. These limits are well within the National Primary Drinking Water Regulations required limit of 0.037 Bq L(-1) for 226Ra and for 228Ra. The precision and accuracy of the method, evaluated using the U.S. Environmental Protection Agency and the Environmental Resource Associates' quality control samples, were found to be within acceptable limits.

Critical comparison of radiometric and mass spectrometric methods for the determination of radionuclides in environmental, biological and nuclear waste samples

“…Although the resolution of semiconductors for α−spectrometry is good, the relatively small difference in alpha particle energy between some alpha emitters makes it difficult to spectrometrically separate the peaks. This is usually the case with isotopes such as 241 Am and 238 Pu, and 237 Np, 233 U, 234 U and 230 Th. It is therefore that a good chemical separation of radionuclide of interest from the matrix is required; otherwise, the self-absorption of α particles in the source will not only reduce the counting efficiency, but also worsen the energy resolution of the α spectrum.…”

“…The major problem in determination of radioisotopes using ICP-MS is the appearance of isobaric interferences of other elements at the same mass, such as 129 Xe interfering with 129 I and 238 U with 238 Pu. Another important interference is from polyatomic ions, such as 238 U 1 H and 204 Pb 35 Cl which interferes with 239 Pu.…”

“…RIMS as a highly selective and sensitive mass spectrometric technique for ultratrace and isotope analysis has been used for the analysis of many radionuclides such as 41 Ca, 90 Sr, 99 Tc, 135 Cs, 210 Pb, 236 U, 238 Pu, 239 Pu, 240 Pu, 242 Pu, and 244 Pu in environmental, biological and waste samples [33,34,[94][95][96][97][98][99][100][101][102][103][104][105]. In RIMS the solid or liquid samples are vaporized and atomized by an atomic beam source (e.g.…”

“…For ICP-MS instruments the most critical parameter is the abundance sensitivity which determines whether the intermediate half-life isotopes 234 U and 230 Th can be measured without significant influence of tailing from the more abundant 235 U, 238 U and 232 Th isotopes. In alpha spectrometry this is never a problem.…”

“…The most interest isotopes of Pu in environmental and waste sample are 238 Pu, 239 Pu, 240 Pu, and 241 Pu. 241 Pu is a beta emitter with maximum energy of 20.8 keV, while the other isotopes are alpha emitters (Table 2).…”