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 alpha-to-beta 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%.

Radioactivity in Trinitite six decades later

“…GA and GB measurements were made using a model XLB5 a/b gas-proportional counting system by Canberra, Inc., which has a typical background of 0.04 cpm for a and 0.5 cpm for b (Semkow and Parekh, 2001).…”

Modeling of alpha mass-efficiency curve

“…As discussed in Section 2.1, m t is a sum of m s and any contributions due to absorbers above the sample. The thickness of the absorbers are 1:29 mg cm À2 for air, 0:08 mg cm À2 for the detector window, and an estimated 0:22 mg cm À2 for the detector threshold [20], for a total of 1:59 mg cm À2 : Addition of this value to m s from Table 2 (first divided by 20 cm 2 ; an apparent surface area of the sample planchette) results in m t values, listed in the last column. These values can be compared with the values of R in Table 1.…”

“…Due to the physical limitations of the detector and sample sizes, one can expect edge effects. Frequently, both the detector and sample are of circular shapes having cylindrical symmetries [20]. The exact efficiency can be calculated by integrating over all angles imposed by the dimensions of the detector and sample, or by a Monte Carlo simulation (Pomme´et al [21], and references therein).…”

Comparison of two different methods for gross alpha and beta activity determination in water samples

“…Permissible activity concentrations in drinking water are recommended in Guidelines for Drinking Water Quality (World Health Organization) [3]. Contribution to the total alpha activity comes primarily from Ra, Th, U, Rn and its decay product, while contribution to the total beta activity comes from 40 K, natural or artificial tritium, artificial 90 Sr and artificial 137 Cs [4]. The most frequently applied methods for the determination of gross alpha and beta activities in drinking water are EPA method [5][6][7][8] and ISO methods [9][10][11][12].…”