  Since the late '80s, CERN has pioneered the development of niobium thin film
radio-frequency (RF) cavities deposited on copper substrates for several
particle accelerator applications. However, niobium thin film cavities
historically feature a progressive performance degradation as the accelerating
field increases. In this study, we describe a temperature mapping system based
on contact thermometry, specially designed to obtain temperature maps of
niobium-coated copper cavities and, consequently, study the mechanisms
responsible for performance degradation. The first temperature maps on a
niobium/copper 1.3 GHz cavity are reported along with its RF performance. In
addition to some hotspots displayed in the temperature maps, we surprisingly
observed a wide cold area on the outer cavity surface that may shed new light
on understanding the heat dissipation of niobium thin film cavities in liquid
helium, which might be exploited to improve the RF cavity performance.
