Concentration of 131I in the air can be calculated using the following approximate expression:
1 N 8 - q - py t - V Where
C = concentration of 131I in the air at the mid-point of sampling time [Bq/m3] tv = sampling time [h]
q = adsorption efficiency of charcoal filter for 131I V = air volume that passed through charcoal filter [m3].
Accuracy is estimated to be ±20%.
Step 12
Record all the results and appropriate measuring parameters in Worksheet D2.
Key features in gamma spectra are peak positions and peak intensities. Peak position data are used to identify the energy of the gamma radiation, which can be done if the channel-energy correspondence is known, i.e. the system is calibrated for energy.
The channel - energy correspondence is fairly stable and can easily be determined by
calibration measurements. The relationship is close to linear in the energy range of interest in gamma spectrometry (50-2000 keV). Most up-to-date spectroscopic systems provide built-in functions for automatically performing the calibration (corrections for the departure from linearity is often provided by fitting higher order polynomials to calibration measurement data). Despite the wide variety of realizations they all are based on the same simple procedure which can be performed by any user lacking the support of sophisticated instrumentation and software.
Equipment/Supplies
> Spectrometer
> Certified gamma emitting point sources
Step 1
Select sources of known radionuclide content, with gamma energies in the range of interest (50-2000 keV) so that at least two sufficiently separated energies (one below 200 keV and one above 1000 keV) are easily measurable.
Step 2
Put the sources to a position where the expectable count rate is high enough to collect a well defined peak (at least 1000 counts) during a reasonable time (100-1000s), for each of the lines to be used for energy calibration.
Energy calibration Procedure D2a, Pg. 2 of 2
Step 3
Start data acquisition for a time needed to collect sufficient number of counts (at least 1000) in the peaks of interest.
Step 4
Determine the position of the peak maximum with an accuracy of a tenth of a channel either manually or using the function provided by the spectrometric system.
Manual method
Set the limits of the region of interest around the peak. Let a and b note the lower and upper limit channels, respectively. Let N; the number of counts in the channel /. To get the peak position/? calculate the following expression:
StepS
Plot the corresponding peak positions vs. energy obtained from nuclide libraries on linear scale diagram and fit a straight Line to the data.
Step 6
Record data and calibration function in the spectrometer logbook.
The basic points to consider in performing the counting efficiency are: (a) sample counting configuration (geometry), (b) calibration method, (c) calibration sources, and (d) analytical efficiency expressions [16].
Sample counting configuration (geometry)
For routine, reproducible sample analyses, the containers used for counting must be selected taking into account both the quantity of sample material available and the sensitivity acquired by the geometry of the sample in the container. In practice, it is recommended that several containers with practical geometries be selected in accordance with the sample matrices to be measured. Some examples of sample containers are scalable plastic bags for filters, Marinelli beakers for both liquids and solids, cylindrical plastic containers with screw caps (bottles and jars), petri dishes and aluminum containers of various dimensions for small volumes of soils
and ashed materials etc. In general, the dimensions of the container should be well suited to the dimensions of the detector and the shield, e.g. not too tall or too small.
Calibration method
Several theoretical efficiency calibration methods are in use today. It is recommended, however, that efficiency calibration be determined experimentally for environmental measurements even though this method takes more effort and time. Practical calibration standards must be prepared for each counting configuration from appropriate certified standard calibration source or a standard stock solution traceable to a national standard. The composition of these laboratory standards should approximate as closely as possible, with respect to density and matrix, to the actual samples to be analyzed. The volume and/or height within each configuration must be the same for standards and samples.
Calibration sources
Appropriate radionuclides must be selected for use as standards in efficiency calibration.
Solutions of certified mixed radionuclides with reasonably long half-lives are available from several reputable suppliers. Accurate absolute gamma ray emission rates should be stated in the certificates supplied with the standards. The certificate should also state the following characteristics of the standard:
i. uncertainty associated with the activity, ii. reference date,
iii. purity,
iv. mass or volume, v. chemical composition, vi. values of half-lives,
vii. emission probabilities for all modes of decay.
Efficiency calibration Procedure D2b, Pg. 2 of 5
If radionuclides, such as 60Co, 88Y, 133Ba, 152Eu, that decay by cascade transitions and produce multilined spectra, are used in the efficiency calibration, great care must be taken to correct for the counting losses (or gains) created by coincidence summing effects.
In Table Dl a list of radionuclides often used for efficiency calibration is shown. By the proper selection and combination of these radionuclides, an efficiency curve can be
determined over the energy range of interest (usually 50-2000 keV). Calibration points must adequately cover the energy range so that interpolation between the points is accurate.
Extrapolation below the lowest energy must be done with great care, as efficiency changes rapidly at low energies (see Figure D4). For measurements of radionuclides with energy below those of the calibration range performed, specific calibration with those radionuclides is recommended.
TABLE D1. LIST OF RADIONUCLIDES OFTEN USED FOR EFFICIENCY CALIBRATION [16] Remark: The half-life is given in days (d) andyears(a); one year = 365.25 days
Analytical efficiency expressions
Once a sufficient number of data are acquired experimentally in the energy region of interest, a means of representing the efficiency as a function of energy should be chosen. A graphic log-log plot of energy on the X-axis and efficiency on the Y-axis can be used, although it is frequently more useful and desirable to express the efficiency in some analytical
mathematical form as a function of gamma ray energy. An expression of this type can be readily programmed and is adaptable to automatic data analyses.