Summary
The soft γ radiation which produces rather high background rates in unshielded scintillation counters has been studied at the earth’s surface and at aircraft altitudes. It is concluded that near the surface of the earth the radiation is primarily from the earth and building materials. At high aircraft altitudes it is primarily due to degraded cosmic radiation and at lower altitudes it is due to air radioactivity and cosmic radiation. Energy spectra at the earth’s surface and at aircraft altitudes are presented. Most of the radiation below 500 keV is included in the region from 40 keV to 200 keV with a prominent peak between 50 keV and 85 keV. Since most of the photons emitted from the various sources undergo several Compton interactions before detection, the location of the peak is nearly independent of the energy of the source. The peak location depends primarily on the photoelectric and Compton cross-sections of the medium (air, earth, or building materials) surrounding the detector. Theoretical calculations of the shape of the spectrum are given and are in good agreement with the experimental data. A calculation of expected underground intensity near the earth’s surface based on known average concentrations of radioactive materials in the earth’s crust is consistent with experimental results.
Riassunto
Si è studiata la radiazione γ molle che produce un fondo assai marcato nei contatori a scintillazione non schermati, alla superficie della terra ed alle altezze raggiunte dagli aerei. Si conclude che presso la superficie terrestre la radiazione proviene principalmente dal terreno e dai materiali degli edifici. Alle maggiori altitudini raggiunte dagli aerei è dovuta principalmente a radiazione cosmica degradata; ad altitudini minori invece è dovuta alla radioattività dell’aria ed alla radiazione cosmica. Si dànno gli spettri d’energia alla superficie terrestre ed alle altezze raggiunte dagli aerei. La maggior parte della radiazione al di sotto di 500 keV è inclusa nella regione fra 40 keV e 200 keV con un picco prominente fra 50 keV e 85 keV. Poichè la maggior parte dei fotoni emessi dalle varie sorgenti subiscono molte interazioni Compton prima della rivelazione, la posizione del picco è quasi indipendente dall’energia della sorgente. Essa invece dipende principalmente dalle sezioni d’urto fotoelettrica e Compton del mezzo (aria, terra, o materiali degli edifici) che circonda il rivelatore. Si espongono i calcoli teorici della forma dello spettro che sono in buon accordo con i dati sperimentali. Un calcolo dell’intensità sotterranea prevista presso la superficie basato sulle concentrazioni medie note dei materiali radioattivi nella crosta terrestre concorda con i dati sperimentali.
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This work has been supported by the National Science Foundation, the U.S. Atomic Energy Commission, and the Research Corporation.
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Hanson, R.J., Marker, D. Soft gamma background radiation at the earth’s surface and at aircraft altitudes. Nuovo Cim 32, 793–815 (1964). https://doi.org/10.1007/BF02733849
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DOI: https://doi.org/10.1007/BF02733849