February 1, 2021
      
                  
        
  Author(s)
  Joseph  Fowler,   Galen  O'Neil,   Bradley K. Alpert,   Douglas  Bennett,   Edward V. Denison,   William  Doriese,   Gene  Hilton,   Lawrence T. Hudson,   Young I. Joe,   Kelsey  Morgan,   Daniel  Schmidt,   Daniel  Swetz,   Csilla I. Szabo-Foster,   Joel  Ullom
 
       
            
    
    
        We use an array of transition-edge sensors, cryogenic microcalorimeters with 4 eV energy resolution, to measure the x-ray emission-line profiles of four elements of the lanthanide series: praseodymium, neodymium, terbium, and holmium. The spectrometer also