In the 2019 book Wilhelm Conrad Röntgen: The Birth of Radiology by Gerd Rosenbusch and Annemarie de Knecht-van Eekelen, there is a table of all the doctoral and/or habilitation students supervised by Röntgen.
In this table, Walter Friedrich's dissertation is dated 24 July 1911 and carries the German title "Räumliche Intensitätsverteilung der X-Strahlen, die von einer Platina Antikathode ausgehen." My own translation would be "Spatial distribution of the intensity of x-rays emanating from a platinum target."
The table also mentions that the chief results of the dissertation were published in the following article in 1912: Ann. Physik, volume 344, pages 377-430.
Hope this helps.
Matthew
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