Radiation Damage

The total fluences of photons, neutrons and charged hadrons expected in the CMS experiment over the scheduled 10 years of LHC operation is shown in fig. . $z$ is the distance from the vertex along the beam axis, while the parameter is the radius $r$. In the region of the CMS Tracker, charged hadrons are dominant, most of which are pions with a momentum below $1\,\rm GeV/c$. The innermost layer of the strip tracker ( $r=27.7\,\rm cm$) has to deal with a total fluence of less than $\rm 2\cdot 10^{14}\,particles\,cm^{-2}$.

Figure: The expected radiation fluences of photons, neutrons and charged hadrons in the CMS experiment over 10 years of operation as a function of the distance $z$ from the collision point along the beam axis and the radius $r$.

Radiation effects in silicon are usually normalized to an equivalent fluence of $1\,\rm MeV$ neutrons. With few exceptions, the damage of pions, protons and neutrons at various energies can be scaled using the Non Ionizing Energy Loss (NIEL) values [21].

Intense irradiation causes three effects in silicon, which have been studied extensively by the CERN RD48 collaboration [22]. The doping concentration changes, the leakage current increases proportional to the fluence and the charge collection efficiency decreases due to charge trapping.