2.1 - Introduction to Measuring Procedures
Although the methods and procedures described in this chapter have been carefully tested it is the responsibility of each laboratory to perform an individual validation procedure. Compliance with appropriate national and international safety rules and health practices for chemical and radioactive safety is obligatory. “Good Laboratory Practice” and the state of the art in analytical quality management is a prerequisite for the reliability of analytical results.
General basic procedures for equipment calibration are outlined in the first section 2.1. They are elementary for all following analytical methods and one is referred to these unless stated otherwise. In the following part the most important procedures both for natural radionuclides as well as fission and activation products have been selected for description. Each method is subdivided into
Materials and Equipment
In the “Introduction” part the main goal and objectives are summarized. A description of the method is given.
The part “Materials and Equipment” compiles items which are necessary for the analytical procedures. All chemical reagents listed in this part must be of analytical grade. Standard laboratory equipment like fume hood, balance, burner, LS equipment, as well as standard laboratory materials like beakers, pipettes, flasks, pH papers, ultra pure water, etc are not mentioned anymore. For planning and installation of a radiochemical laboratory in general the reader is referred to [Moebius 1988].
It has to be taken into account that unweighable amounts of radionuclides (at least for short lived radionuclides) are handled within the “Procedures”. Therefore, carriers and hold-back carriers have to be introduced for precipitation and/or to avoid adsorption effects.
The final sample prepared for LS counting has to be either a homogeneous solution or a clear gel. Opaque and milky solutions must be avoided as well as two phase regions. Be aware that the phase diagram of the cocktails is temperature related. Therefore even homogeneous cocktail samples might unmix during temperature control and measurement. Vials should generally be stored (minimum 20 to 30 min) in a cold and dark place before the measurement (best in LS equipment) in order to minimize luminescence unless TDCR is applied.
The procedure for data “Evaluation” follows the general scheme below. The measuring efficiency e is determined through an appropriate standard solution via
Thus, the unknown activity Ax of the sample is calculated by
For the sample volume, the added aliquot to the cocktail for measurement has to be taken into account as well as the ratio of volume aqueous sample to volume organic phase, when solvent extraction steps are involved (e.g. Rn extraction).
If the time difference between sampling and time of measurement cannot be neglected, the decay of a relatively short lived radionuclide has to be corrected according to
At = Ao * exp-(t1/ T1/2)* ln2)
Activity ingrowth e.g. for Rn extraction procedures or in the system 90Sr/90Y is calculated through
At = Ao * exp (1 – exp-(t2/T1/2)* ln2))
At = Time of measurement
Ao = Time of sampling
t1 = Storage time
t2 = Ingrowth time
Data on lower limits of detection, mostly reported as minimum detectable activity (MDA) in Bq/Volume in the evalution, refer to the background data in chapter 3.2. They are recorded, in α-PSD mode for Triathler device (HIDEX), and in β-counting mode for a typical low level instrument. A measuring time of one hour is applied unless stated otherwise. If suitable, the analysis uncertainty with respect to measurement is stated.
However, data might be transferred to any other instrument by taking into account the appropriate background and counting efficiency data.
The chemical yield of a procedure is determined either by
the ratio of amount of carrier added to amount of carrier in product or
isotope dilution of a radioactive standard solution added.
For a detailed evaluation in these cases, see chapter 2.3.9.
Möbius S. 1988: Experiments for training in nuclear and radiochemistry; Report KfK 3920, Kernforschungszentrum Karlsruhe, Karlsruhe 1988