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

produced. The impurity is called donor when it creates energy levels located very close to
conduction band. Semiconductor with donor atoms has a large number of electrons and a
small number of holes. Therefore its conductivity is mainly due to the electrons and the
semiconductor is called n-type. On the other hand when the impurity creates energy levels
located very close to valence band, such impurity is called acceptor. Semiconductor with
acceptor atoms has a large number of holes and a small number of electrons and consequently
it is called p-type. For instance Phosphorus impurity in Silicon crystal creates donor states,
whereas Boron creates acceptor states.
When n-type and p-type semiconductors are joined together (n-p junction ) electrons
and holes undergo to migration because of two reasons: for diffusion, moving from areas of
high concentrations to areas of low concentrations; under the influence of the electric field
that will move them because of their charge in opposite direction. When the concentration of
charge carriers has reached equilibrium, an imbalance charge is determined over a region,
called depletion region that extends into both the p and the n sides of the junction. The
depleted region exhibits some very attractive properties as a medium for the detection of
radiation. The electric field of this area causes any electrons created in or near the junction to
be swept toward the n-type part, and any holes toward to p-type part. Yet, this junction
without any applied external electric field will be a detector with very poor performance
because of recombination processes. But if the p side of the junction is made negative with
respect to the n side, the junction is reversibly biased. Now the potential difference from one
side of the junction to the other is enhanced and the depletion region is significantly
expanded. Under this circumstances the charge carriers created by a radiation interaction
within the depleted region are quickly and efficiently collected: this configuration of the
semiconductor becomes a useful device for ionizing radiation detecting purposes. Yet,
because a semiconductor has a relatively low band gap, they must be cooled in order to
reduce thermal generation of charge carriers (leakage current). Otherwise, noise induced by
the leakage current would degrade the resolution performance of the detector.
Different techniques are used to produce detectors based on n-p junction principle
[128]. Examples are surface barrier detectors, and diffused junction detectors. These detectors
are characterized by a sensitive region useful for detecting purposes with a depth upper limit
of about 2 mm. This limitation affects the maximum energy of radiation that can be measured.
Depletion region can be significantly increased if the semiconductor is doped with an ion
drifting process. Starting from a p-type material, donor atoms are added to generate the
desired compensation. Alkali metals such as Lithium, Sodium and Potassium tend to form
interstitial donors in crystals of Germanium and Silicon. Actually only Lithium can be
introduced into Silicon or Germanium in sufficient concentration to serve as a practical
compensating impurity. Typical examples are the so-called Lithium drifted Silicon or
Germanium detectors, Si(Li) and Ge(Li) respectively. A practical disadvantage of Lithium
drifted detectors is that Lithium spatial distribution becomes unstable at room temperature.
This phenomenon is particularly important for Ge(Li) because Lithium has a higher mobility
in Germanium at room temperature determining an undesirable redistribution of the Lithium
at such temperature. Therefore these detectors are not only used, but also continuously stored
at reduced temperature, typically by means of a bulky liquid nitrogen dewar.
The use of a hyperpure Germanium detector (HPGe) represents the last important step
in the development of semiconductor detector for detection purposes. This is based on the
present availability of Germanium in a very high state of purity. In such material impurity
concentration can be reduced to a very low level. Therefore the resistivity of the material and
consequently the depth of the depleted area is increased [129]. It is possible to obtain active
regions that are comparable with those for Ge(Li), for instance, but without any Lithium
compensation. The resulting detector can be stored at room temperature, whereas it is cooled
only in the measurement phase to prevent excessive leakage current.
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