When the earliest paper related to X-rays was published? 
SELECT ?publication_years
WHERE {
  ?papers rdf:type orkgc:Paper.
  ?papers rdfs:label ?papers_labels.
  FILTER(REGEX(?papers_labels, "X-ray", "i"))
  ?papers orkgp:P29 ?publication_years. 
}
ORDER BY ASC(?publication_years)
LIMIT 1