A new concept is introduced for the classification of "unresolved problems" in the understanding of interactions in thick targets irradiated with relativistic ions: The centre-of-mass energy per nucleon of a hypothetical compound nucleus from a primary interaction, E CM /u, is calculated and correlated with experimental observations in thick target irradiations. One observes in various reactions of relativistic primary ions with thick targets that there appears to be a threshold energy for

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... ions leading to "unresolved problems" which lies around E CM /u ~ 150 MeV. All "unresolved problems" are exclusively observed above this threshold, whereas below this threshold no "unresolved problems" are found. A similar threshold at 158 ± 3 MeV exists for massive pion production in nuclear interactions. Hagedorn had proposed this threshold decades ago and it is known as the Hagedorn limit. In this paper we will only mention, but not elaborate on Hagedorn's theoretical concept any further. Some considerations will be presented and further studies in this field are suggested.