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Radiation Hardness

In the GaAs forward tracker detector of the ATLAS experiment the fluence is dominated by pions at an estimated level of 1.2 1014 $\pi$/cm2 over the experiment`s ten year lifetime [14]. The neutron fluence is expected to be 3 to 10 times less that this, while that due to the other charged hadrons is small in comparison. The effects of particular radiation have therefore been investigated. Electrons and gamma rays have shown negligible effects. The increase in leakage current due to neutrons (n), protons (p), and pions ($\pi$) is small, typically at the 50% level after 1014 particles/cm2, and the current exhibits a slow decrease with time at room temperature after irradiation. The value of Vfd is typically half its pre-irradiation value after 3 1013 p/cm2 and remains unchanged at higher fluences.

The charge collection, however, falls with fluence, with the hole signal component being more affected. The reduction of CCE with fluence depends on the type and energy of the irradiating particle. For a 200$\mu$m thick LEC detector the CCE falls to 20% after 4 1014 n/cm2 (5300 electrons) or 1.5 1014 p/cm2 or only 6 1013 pions/cm2 [15]. The differences have been explained in term of the different non-ionising energy loss, NIEL, of the particles in the detector material, as shown in figure 4 [16].

Figure 4: The charge collection efficiency as a function of total NIEL for ISIS neutrons, 300MeV/c pions and 24GeV/c protons for 200$\mu$m thick SI-U LEC GaAs detectors

\resizebox {.7\textwidth}{!}{\includegraphics{{psdniel2.eps}}}\end{figure}

All LEC material, except for the low carbon material, appears to degrade to give a minimum ionising particle signal of approximately 4500 electrons after a fluence of 1.6 1014 p/cm2. The latter material may be slightly more radiation hard due to the high initial values of the charge carrier mean free drift lengths and gives a signal of 7000 electrons after the same fluence [17].

Diodes fabricated on Tomsk material have shown better radiation hardness [18], with a reduction of 50% in CCE after a fluence of 1.1 1013 (1GeV/c) p/cm2 at a flux of 5 1014 p/hour. A dependence of the degradation of CCE on the rate of irradiation has been reported. With a flux of only 5 1013 p/hour the 50% CCE reduction occurs after a fluence of 1.5 1014 p/cm2. Proton irradiations have been performed between -5oC and -10oC on LEC material. Preliminary results suggest that the reduction in CCE is slightly worse than with room temperature irradiations.

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Next: X-ray Detection Up: Recent results on GaAs Previous: Pixel Detectors

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