This work deals with full-energy peak efficiency for a counting array of two
NaI(Tl) scintillation detectors (2"x 2" and 3" x 3" with 7.5% and 8.5%
resolutions, respectively) and radioactive sources in the form of rectangular
parallelepipeds of various dimensions. Aqueous solutions containing the 152Eu
radionuclide were used; the latter exhibits a favourable multiline gamma
spectrum covering a wide energy range from121.78 keV up to 1408.03 keV. A new
mathematical and analytical approach to the problem is developed. The well
known, accurate and widely used efficiency transfer principle is applied,
together with detector efficiency calculations based on the effective solid
angle concept. The self-attenuation of the source matrix, attenuation by the
source container and detector housing materials, as well as a number of other
relevant details of the experimental set-up were duly accounted for. A
remarkable agreement between the measured and calculated efficiencies was
observed for a variety of source-to-detectors distances, confirming the
reliability of the method developed.
ANGLE software for semiconductor detector efficiency calculations - long
existing and widely accepted tool in quantitative gamma spectrometry - has
been recently extended to scintillation NaI detectors. The extension features
in the latest edition (ANGLE 4) and it is briefly outlined. Discretization of
reference efficiency curve, meaning possibility of using ANGLE 4 for
particular gamma energies without constructing the complete reference
efficiency curve, is particularly emphasized. This yields both in enhanced
practicality and higher accuracy, while reducing the potential for systematic
errors. The present work is primarily focussed on experimental verification
of ANGLE 4 for NaI detectors. Two detectors (2 ? 2 and 3 x 3 inches) were
employed in the experiment. Commercially calibrated gamma sources (in the
forms of quasi point and cylinder) and homemade solutions (diluted from
calibrated ones) were measured at various distances from the detector(s),
ranging 0 cm to 50 cm. Energy range observed was 59 keV to 1408 keV.
Versatility of counting conditions, in terms of detectors and sources used,
gamma energies observed, source detector separations, etc., was aimed at
creating as large experimental evidence as possible for verification
purposes. Experimentally obtained efficiencies are compared with those
calculated by ANGLE 4. Very good agreement is obtained - well within the
experimental uncertainties - thus proving the reliability of the software.
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