\title{MARS15 Simulation Studies in the CMS Detector of Some LHC Beam Accident Scenarios}

\begin{abstract} The CMS tracker, made of silicon strips and pixels and silicon-based electronics, is vulnerable to effects of radiation exposure during the LHC operation. Of much concern is the potential for damage from a high instantaneous dose to the pixel detectors and electronics located only a few centimeters from the beam in the event of a fast accidental beam loss. One of the worst case scenarios for such a beam loss is an unintended firing of an abort kicker module, referred to as the kicker pre-fire. MARS15 simulation studies of radiation loads in CMS for the kicker pre-fire scenario are described in this paper. It is found that, in a kicker pre-fire accident, in a time span of about 100 ns, the innermost pixel layer may see a radiation dose of about 0.02 Gy \-- equivalent to a fluence of $\sim 6\times 10^{7}$ MIPs/$cm^2$. No discernible damage to the pixel detectors or the electronics were seen at these levels of fluence in recent beam tests. We note that the dose is about 1000 times smaller than previously estimated before tertiary collimators were introduced in sector 5-6. We also present here a brief discussion of other accident scenarios. Our studies show that a mis-steered 450 GeV beam hitting a TAS absorber in CMS at $\sim$19 m from the interaction point can produce 0.5 Gy per pilot bunch of 10$^{11}$ protons. We strongly urge that procedures be put in place to eliminate the risk of this accident. \end{abstract}