Diamond Detectors for Ionizing Radiation - HEPHY

Chapter 7
Radiation Hardness
7.1 Radiation Defects
The properties of diamond may be aected by impurities in the lattice. Especially, the
charge collection distance strongly depends on the presence of inhomogeneities.
Atoms that do not t into the diamond lattice or lattice positions that are not occupied
are called defects in general. In the virgin state, CVD polycrystalline diamond has a
certain number of defects, depending on the growth parameters. In particular, there
are considerable nitrogen impurities. Additionally, the grain boundaries are suspected
to provide a signicant number of charge traps and recombination centers. The defects
introduce energy levels inside the band gap. As the carrier transition between valence
and conduction bands becomes more probable with the introduction of intermediate levels,
the intrinsic carrier density increases, resulting in a higher leakage current. However, as
diamond has a very large band gap, and the impurities in detector material are below the
ppm range, the bulk current remains negligible in practice. In fact, no signicant eect
has been observed on the leakage current before and after the irradiation experiments.
Additional defects are introduced with irradiation [6]. Depending on the incident
particle type and momentum, various defects may occur by atom displacement. With
low momentum particles, only simple defects are probable. These are vacancies, where
a lattice position is unoccupied and interstitials, where an atom is posed in between the
lattice. Due to the conservation of matter, these two always occur together, called Frenkel
defects. Heavy particles, especially ions, usually have a very short range in the order of
micrometers. They are stopped in the diamond lms, transfering their whole energy
and additionally placing themselves in the diamond lattice. For this reason, the damage
induced by ions, is by orders of magnitude higher than that of traversing particles.
All of these defects aect the charge collection eciency by the creation of trapping
and recombination centers, which decrease the carrier lifetime and thus the drift distance.
Considering the tightly bound, compact lattice, diamond has a reputation of being
quite insensitive to radiation. However, as theoretical prediction is dicult, experiments
have been carried out to observe the damage introduced by various kinds of particles.
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