Plutonium Biokinetics in Human Body A. Luciani - Kit-Bibliothek - FZK

2.3.2.2 Performances
Energy resolution of the HPGe and phoswich detecting systems are evaluated for γ
59.5 keV energy peak of 241 Am, the radionuclide of interest for the direct measurements
performed on the contaminated subject. The results for the energy resolution R of the two
detecting systems expressed as percentage are given in Table 2.3.2.
Table 2.3.2 Energy resolution (R) of each detector of the HPGe and Phoswich detecting
systems for 59.5 keV energy peak of 241 Am.
Detecting system Detector R [%]
HPGe
Phoswich
63
D 1 1.1
D 2 1.2
D 3 0.9
D 4 1.0
D 1 29
D 2 24
D 3 23
It can be pointed out the two detecting systems are characterized by significantly
different energy resolution performances. Therefore different energy calibration methods
were applied.
The energy calibration of HPGe detecting system in the low energy range of the
spectrum was carried out using two point sources of 241 Am and 57 Co of 40 and 1 kBq activity,
respectively. They were located at about 25 cm distance from the detectors on the symmetry
central axis of the whole detecting system. Three full energy peaks of 241 Am were considered:
13.9, 26.4 and 59.5 keV. For 57 Co the 121 keV peak was considered. The choice of such peaks
allowed to uniformly cover the energy range of interest. The spectrum from the energy
calibration sources was acquired using Silena IMCA Plus programme for spectroscopy [138]
and the peaks were visually located. The centroid of eack peak is calculated by the acquisition
programme on the basis of a Gaussian filter for the fitting of spectrum peaks. Each
Germanium detector was singularly calibrated using a 3 rd degree polynomial curve to fit the
considered energy peaks. A minimum least square fitting procedures was applied for this
purpose.
The poor energy resolution of phoswich detecting system doesn’t allow to individuate
each single peak as for the HPGe detectors. Therefore a proper energy calibration curve was
not calculated, but a spectrum for just 241 Am was acquired and the channels for delimiting the
region of interest for the 59.5 peak energy were fixed.
The efficiency calibration of both the detecting systems were carried out by acquiring
the spectra of Americium loaded calibration phantoms. In the actual conditions of
measurements, HPGe detecting system was used for thorax measurements, and phoswich
detectors for measurements of activity in lungs, liver, head and knee. In Figure 2.3.10 the
measurement positions of the detecting systems in respect of subject are presented. In case of
measurements on the skull with the phoswich detecting system (measurement position: Head),