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Measurement control software designed for LP-4900B analyzer allows:
- independently start and stop each of 7 ISC' measurement;
- collect spectrometry information of 4096 channels per spectrum for 7 spectra in amplitude analysis live-time mode;
- display spectrometry and control information in real time;
- user input of information necessary for measurement identification;
- input and output measurement results from/to IBM formatted floppy.
Sample measurement time necessary for representative spectrum and determined with its activity,
type, radionuclide content and solved task is usually 2 – 60 hours. To ensure spectrometry information will be
kept safe during long-term measurement the partial spectra are stored on the floppy each 1-4 hours (depends on
the whole measurement duration). Beside that, such redundancy gives the operator opportunity (without influence
to or stopping current measurement) to perform earlier measured spectra summation providing common energy
calibration. That is equal to ISC transformation factor periodic correction.
Resulting spectrum accumulated
during sample measurement time (or obtained by operator with corrected or not partial spectra manual summation)
is provided with sample name, date and measurement time information. This total spectrum is stored on floppy disk
and transferred to PC for next processing. As well spectrum can be transmitted from analyzer to PC by communication
line using parallel port.
The software designed for quantitative analysis of the spectrometry results includes 3 parts:
- interactive spectra processing;
- sample radionuclide content calculation using gamma-spectrometry analysis results;
- data set preparation necessary for analysis.
1. Interactive spectra processing. Spectrum processing objective is count rate and
other parameters measurement and their errors evaluation for full energy absorption peak. Processing starting
data are measured spectrum, sample and its measurement condition information, spectrometry characteristics,
radionuclides and their gamma-radiation data library. The processing is interactive. Processing executing
operator is guided by spectrum view and gamma-radiation lines table with software help. Operator can control
view type, choose single peaks or peak groups for analysis, give or correct peaks and peaks groups limits
chosen by software, identify peaks or correct the peak identification provided. The algorithms used in program
give opportunity to measure single peaks areas and FWHM, execute multiplets resolution with position and area
components measurement, estimate peak area upper limit (for peaks included in the table, but not found in spectrum).
During the processing energy calibration is tuned automatically to the given spectrum. For all measured
parameters both statistic and systematic errors are estimated.
2.Sample radionuclide content calculation. The given list sample radionuclide content
measurement is executed with using peaks count rates measured during spectrum processing. After background count
rates subtraction and sample density correction using spectrometer sensitivity (peak count rate divided by sample
activity) values library for each gamma-radiation line radionuclide activity or its upper limit estimation are
calculated using all lines averaging. Using received results measurement protocol is prepared, where sample
characteristics and measurement condition are described and sample radionuclide specific activity values are
given in units of Bk/kg for sampling date. If the sample is characterized by definite surface, then specific
activity values in units of Bk/m2 are given, if by volume – then in units mBk/cm3.
For soil depth layer-by-layer selection sample measurement results analysis the program has been designed
that approximate depth specific activity distribution with one or two exponential curves measuring distribution
parameters and soil radionuclide reserve activity. Radionuclides activity errors are calculated by used data
systematic errors (sensitivity and efficiency calibration sample sources characteristics, samples own uptake
correction parameters etc.) and by spectrum statistics and are between 5 and 15%. Processing results protocol
examples are on pictures 4, 5.
3.Analysis data set preparation. For above described operations execution the following
data are necessary:
- background count rates for gamma-lines to be analyzed
- these lines sensitivity values,
- FWHM peak dependence on energy for multiplet resolution,
- source own uptake correction parameters.
Auxiliary software is designed for these values calculation and their introduction in data
processing.
3.1. Background count rates are measured by experimental background spectra measured at regular
intervals for each ISC using processing program measurement control software and put down in corresponding data
set for each ISC.
3.2. Each "detector + sample container" combination efficiency calibration is executed for
sensitivity values measurement. The calibration includes determination of values and errors for full energy
absorption peaks efficiency analytical dependence on gamma-quantum energy using sample sources measurement results.
For coincided with used sample sources list radionuclides (137Cs, 134Cs, 226Ra,
232Th, 40K, 57Ñî, 54Mn, 22Na, 65Zn,
139Ce, 113Sn, 152Eu) sensitivity values are measured by sample sources
measurement results averaging, for other radionuclides – by measured efficiency (using sized curve) for
each gamma-line quantum yield.
3.3. FWHM dependence on energy is measured using the same sample sources measurements
and approximated by linear function.
3.4. To adjust efficiency dependence on sample density and gamma-quantum energy correction
coefficient analytical dependence is used. This dependence's parameters are measured by least squares technique
for each "detector + container" combination using 226Ra spectra from samples with density
between 0.2 and 2.5 g/cm3.
Received by this strategy data are kept in individual file for each "detector+shield+container"
set and used during spectrum processing and sample radionuclide content measurement.
To ensure accumulation technique and data processing accuracy the test
samples 137Cs, 232Th and 152Eu content measurement was executed.
These samples were provided by NIKIET in the context of "RF Ministry of Nuclear Power factories gamma-spectrometry
metrology expert examination" program. Specific activities received estimations to their certificate value
ratio are:
137Cs: 1.01 ± 0.05; 232Th: 0.95 ± 0.09; 152Eu: 1.02 ± 0.10
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