Operation of negative ion generators in three rooms of an occupied residence caused decreases in indoor concentrations of the radon decay products218Po,214Pb and214Bi, and of the thoron progeny212Pb. Activity levels of212Pb were lowered more than those of214Pb, resulting in a decrease in the potential alpha energy concentration ratio, PAEC(Tn:Rn). The change in PAEC(Tn:Rn) is a function of the halflives of214Pb and212Pb and their respective precursors and is similar to the effect brought about by increased room ventilation. Decay products are also removed from indoor air by plateout of charged particles on room walls. Plateout rates calculated for214Pb suggest that even in well-ventilated houses, the potential alpha energy concentration of radon is affected as much by wall plateout as by infiltration of outside air.
Alpha-counting of filters used to sample large volumes of air provides a convenient way of determining atmospheric concentrations of210Pb. Following decay of short-lived222Rn and220Rn progeny, alpha activity of the filters increases as210Pb decays to210Po. After transient equilibrium is reached at about 3 y, alpha activity diminishes with the 22.3 y halflife of210Pb. The degree of equilibrium between210Pb and210Po can be calculated subsequent to sampling, and the average concentration of210Pb in the air during the sampling period can be computed. Contributions to the total210Pb from ambient short-lived radon progeny are small, typically 2–4%. Using high volume air samplers with collection rates of 1.1–1.7 m3/min for 24 h periods, and using counting times of 2 h for 20 cm2 filter sections, we measured alpha counts ranging from 0.0100±0.0050 to 0.200±0.0200 dps. Periodic measurements on 100 of these filters over a 4 y period yielded mean210Pb levels with standard deviations less than ±15%. The method requires minimal sample preparation and can be used to determine past atmospheric210Pb concentrations on filters stored for up to 20 y and more.
Ingrowth and decay of210Po on air filters used for gross alpha-determination can lead to errors in estimation of long-lived airborne radioactivity. Interferences are present to some degree in all samples and change with time, making the gross alpha-technique quantitatively invalid. However, since the degree of equilibrium between210Po and its parent210Pb can be calculated at any time after filter collection, it should be possible to use the gross alpha-method to estimate past atmospheric concentrations of210Pb. Measurements on filters after elapsed times of up to 15 years show that such determinations are feasible.
Recent attention to international safeguards has stimulated interest in nondestructive analysis techniques. These NDA techniques include high- and low-resolution gamma-ray spectrometry, active and passive neutron counting, and physical measurements. Often, the NDA measurements are made abroad under field conditions, and in these cases, portability is important. In other cases, the measurements are made under laboratory conditions but no calibration materials are available. This paper describes several NDA applications in support of international safeguards projects, all involving international cooperation.
A new radiotracer method has been developed to measure the migration of trace elements from food contact packaging into four standard food simulants; acetic acid, ethanol, olive oil, deionised water. A sample of material is irradiated in a thermal neutron flux of 1016n·m–2·s–1 to activate the trace elements and produce a range of radionuclides. The sample is then placed in the food simulant and the migration of the radionuclides is monitored by performing -ray spectrometry on a sample of the simulant. Any radionuclides measured must be due entirely to the migration of the elements present in the plastic, since the simulant itself is not radioactive. Preliminary studies have shown that detection limits of around 0.2g·dm–2 (0.002 mg/kg) can be achieved for antimony in a sample of polyethylene terephthalate. This method can now been extended to measure migration into real foods. This will highlight any differences between the standard simulants currently used and real foods. Since the method only involves irradiation of the packaging material, any food matrix can be studied.
A neutron activation method has been developed for the analysis of high density polyethylene, low density polyethylene, polypropylene, polyethylene terephthalate and polystyrene. Samples weighing 2–5 g were irradiated in a thermal neutron flux of 1016 neutrons m–2 s–1 and measured with gamma ray spectrometry for 64 elements. With the method developed here over 50 elements can be detected at concentrations below 1 mg/kg. Correction factors were applied for neutron flux variation and counting geometry.The method was validated using reference material citrus leaves (NIST) for Na, Mg, Al, K, Ca, Mn, Cu, Sr and I, and a suite of in house standards doped with Al, Cr, Co, Mg, Zn and Sb confirmed repeatability of the method. The method was used to measure inorganic contaminants in the raw polymers and retail samples of plastic packaging used in contact with food.
Solid-phase extraction disks produced by 3M and Eichrom were evaluated for routine use in supporting the Hanford Groundwater Monitoring Project. Both disk formats contain Sr- or Tc-selective extractants, bound in a filter support, that act to preconcentrate and isolate the isotope of interest. The 3M Empore Sr Rad Disks and Tc Rad Disks were tested with respect to precision, accuracy, radiochemical yields, interferences, and volume-load variation. The Empore and Eichrom solid-phase extraction disks were applied to the 90Sr and/or 99Tc determination in representative Hanford groundwater samples with varying chemical and isotopic compositions. Results were compared to standard analytical methods. Both the Empore and Eichrom Tc extraction disks produced consistently higher radiochemical yields, lower detection limits, and greater accuracy than the standard analysis method. The Empore Sr extraction disks produced comparable radiochemical yields, detection limit, and accuracy relative to the standard method; however, total uncertainties were lower.
Authors:Diana Thompson, Susan Parry, and R. Benzing
Validation of a method for the determination of trace elements in plastic using neutron activation analysis (NAA) required evaluation of nuclear effects. Since plastic is a moderator of neutrons, it is possible that the neutron flux may be affected in such a way as to effectively increase or decrease the measured activity of radionuclides within the plastic. Such effects are likely to depend on the thickness of the sample and so thicknesses of plastic ranging from 1 to 20 mm were used to quantify the effects. A standard containing vanaldium (a thermal neutron absorber) and silver (with epithermal neutron resonances) was used in a sandwich of the plastic. The sandwich was then irradiated and the standard counted. The activity of each radionuclide for each thickness was evaluated. The results showed that a variation of only 10% maximum was observed for thicknesses up to 20 mm. An increase in measured activity was observed for both the thermal and epithermal absorbers.
A neutron activation method has been developed for the analysis of high density polyethylene, low density polyethylene, polypropylene, polyethylene terephthalate and polystyrene food contact plastics. The method provides determination of over 50 elements at concentrations below 1 mg kg–1. This technique has now been extended to study migration from food contact materials into standard food simulants (olive oil, acetic acid, ethanol and water). Samples of plastic are irradiated in a thermal neutron flux to produce radionuclides of the elements present in the plastic. Over a period of time the radionuclides of these elements may travel from the plastic into the food simulants, and hence the migration can be determined. Gamma ray spectrometry is performed on the simulants at the end of the test to quantify the migration. Any activity present must be due only to the migration of radionuclides of elements in the plastic and nothing else. This eliminates the need for a blank determination, which is required with existing migration methods. Preliminary studies have shown that detection limits of around 0.002 mg kg–1 can be achieved for Sb in a retail polyethylene terephthalate (PET) bottle. This can be compared to levels of 0.005
Authors:D. Thompson, S. Parry, R. Benzing, J. Williams, and K. Jarvis
The use of paint as a coating for toys intended for sale on the European market is controlled by a European Standard (EN 71-3:
1994-European Standard for the Safety of Toys, Part 3—Migration of Elements). This work is the result of a preliminary study
organised by the European Commission to produce a new paint reference material which can be used to validate the test methods
given in the European Standard. The migration of elements from painted panels and comminuted paint produced by the Laboratory
of the Government Chemist, (U.K.) was studied. Several methods were used in this certification exercise and this work reports
the recults of neutron activation analysis and inductively coupled plasma mass spectrometry to determine the migration of
As, Ba, Cd, Cr, Hg, Pb, Se and Sb into a simulated stomach environment. Replicate extracts obtained from paint samples gave
a precision of within 10% for most elements by both analytical techniques for the paint panels and for the comminuted paint
sample. This preliminary study has shown that the standard method can provide reproducible results for each of the paint materials
studied and indicates that the study should be continued to produce a fully certified paint reference material.