Context: Ultraviolet radiation (UV) influences the physics and chemistry of star-forming regions, but its properties and significance in the immediate surroundings of low-mass protostars are still poorly understood. Aims: We aim to extend the use of the CN/HCN ratio, already established for high-mass protostars, to the low-mass regime to trace and characterize the UV field around low-mass protostars on $sim 0.6times0.6$ pc scales. Methods: We present $5times5$ maps of the Serpens Main Cloud encompassing 10 protostars observed with the EMIR receiver at the IRAM 30 m telescope in CN 1-0, HCN 1-0, CS 3-2, and some of their isotopologues. The radiative-transfer code RADEX and the chemical model Nahoon are used to determine column densities of molecules, gas temperature and density, and the UV field strength, $G_mathrm{0}$. Results: The spatial distribution of HCN and CS are well-correlated with CO 6-5 emission that traces outflows. The CN emission is extended from the central protostars to their immediate surroundings also tracing outflows, likely as a product of HCN photodissociation. The ratio of CN to HCN total column densities ranges from $sim$1 to 12 corresponding to G$_0$ $approx$ $10^{1}-10^{3}$ for gas densities and temperatures typical for outflows of low-mass protostars. Conclusions: UV radiation associated with protostars and their outflows is indirectly identified in a significant part of the Serpens Main low-mass star-forming region. Its strength is consistent with the values obtained from the OH and H$_2$O ratios observed with Herschel and compared with models of UV-illuminated shocks. From a chemical viewpoint, the CN to HCN ratio is an excellent tracer of UV fields around low- and intermediate-mass star-forming regions.
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