Andersen KK, Andersen SL, Esberg J, Knudsen H, Mikkelsen R, Uggerhøj UI, Sona P, Mangiarotti A, Ketel TJ, Ballestrero S. Direct measurement of the formation length of photons.
PHYSICAL REVIEW LETTERS 2012;
108:071802. [PMID:
22401193 DOI:
10.1103/physrevlett.108.071802]
[Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/02/2011] [Indexed: 05/31/2023]
Abstract
We report the first observation of a shoulder in the radiation spectrum from GeV electrons in a structured target consisting of two thin and closely spaced foils. The position of the shoulder depends on the target spacing and is directly connected to the finite formation length of a low-energy photon emitted by an ultrarelativistic electron. With the present setup it is possible to control the separation of the foils on a μm scale and hence measure interference effects caused by the macroscopic dimensions of the formation length. Several theoretical groups have predicted this effect using different methods. Our observations have a preference for the modified theory by Blankenbecler but disagree with the results of Baier and Katkov.
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