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

This quantity is normally expressed as a percentage. The main important factors
affecting the energy resolution are:
• the statistical fluctuations in the number of charge carriers (i.e. electron-hole pair)
produced in the detector;
• the electronic noise in the detector itself, the preamplifier and the amplifier;
• the incomplete collection of the charge carriers.
Such factors are generally present in any kind of detecting system. For a scintillation
detector further factors affect the energy resolution because the initial electric sign is firstly
converted into light and then converted again into an electric one. Such further factors are the
emission of the luminescence, the collection of photons on the photocathode and the emission
of photoelectrons.
The energy (w) spent by a radiation interacting with a detecting materials to create one
electron-hole pair (as in the semiconductor detectors) is actually greater than the energy gap
(E g) between valence and conduction bands. For instance for a Germanium detector the mean
energy required for creating a pair is 2.96 eV whereas the energy gap is only about 0.67 eV at
300 K. This difference between w and E g reveals that part of the energy is dissipated into
processes that don’t generate charge carriers. Therefore if the energy E is deposited in the
detector, the average number of charge carriers is given by E/w and in case of a pure Poisson
statistical process the standard deviation of the number of pairs should be just the square root
of such ratio. Yet, the experience has shown that the fluctuations are smaller. A factor, called
Fano factor (F), is therefore introduced and the standard deviation of the number of charged
carriers is more realistically given by:
σ = FE
w
54
equation 2.3.2
The two extreme values of F are 0 and 1. The former means there are no statistical
fluctuations and all the energy is spent for generating charged carriers. The latter means that
the number of produced pairs is governed by Poisson statistics. If the distribution of an energy
spectrum relating to a monoenergetic source with energy E is described with a Gaussian
function, the FWHM results as following [129]:
FWHM = 2 2ln(2)w = 2 2ln(2)wFE
equation 2.3.3
It results that the FWHM and so the energy resolution decreases with decreasing values of w.
On the basis of such considerations a semiconductor detector is expected to have the
best energy resolution, thanks to its low energy threshold for an electron-hole pair production.
Single full energy peaks even separated by few keV energies can be easily identified in the
spectrum. In case of scintillation detectors full energy peaks can be still located and the
centroid calculated, but the lower energy resolution doesn’t allow sometimes to distinguish
photons with energy closer than some ten keV.
When single full energy peaks are present energy calibration is easily performed. The
centroids of the full energy peaks are associated to the energy of the emitted radiations,
known from the decay characteristics of the radionuclides. A fitting is normally performed
using a polynomial function to calculate the energy value of each channel of the MCA. On the
basis of the energy calibration the energy of any further full energy peak can be evaluated and
the relative radionuclide identified.