Aging Study with High-Level Radiation Sources for the CMS Forward RPCs

Abstract
An aging study with intensive radiation sources was performed for the forward resistive plate chambers (RPCs) of the Compact Muon Solenoid (CMS) detector in the Large Hadron Collider experiment (LHC). The expected rate of the radiation background to be induced in the forward RPCs ranges from a few tens of Hz/cm${^2}$ to as much as $sim$1 kHz/cm${^2}$. Therefore, the radiation hardness for the forward RPCs should be sufficient to ensure the safe operation of the CMS and successful achievement of the LHC physics. Two oil-coated and one plain double-gap RPCs were manufactured and tested with the neutron beam provided by the MC50 cyclotron of the Korea Institute of Radiological and Medical Science (KIRAMS) and with gamma rays from a 200-mCi $^{137}$Cs source. The estimated radiation dose induced by the fast neutrons was approximately 50 Gy. No significant aging effect was found to be a consequence of the neutron irradiation, However, a degradation of the RPC performance was observed after intense gamma irradiation for a total of 32.5 days. The mean gamma-signal rate drawn in each RPC was 4.0 kHz/cm${^2}$, which was approximately two times the maximum rate expected in the highest $eta$-region covering the particle trigger. Increases in the noise rates and the currents accounted for the degradation in the efficiencies for the RPCs. As a result, safe operation of the forward RPCs in the avalanche mode was ensured for signal rates up to 2 kHz/cm${^2}$.
Keywords: Compact Muon Solenoid, Large Hadron Collider, Forward RPC, Muon trigger detector, Neutron irradiation, Linseed-oil treatment

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