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Photodissociation of complex nuclei at energies between the mesonic threshold and 1150 Mev

Roos, Charles E. and Peterson, Vincent Z. (1961) Photodissociation of complex nuclei at energies between the mesonic threshold and 1150 Mev. Physical Review, 124 (5). pp. 1610-1622. ISSN 0031-899X. doi:10.1103/PhysRev.124.1610.

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The photodisintegration of complex nuclei by gamma rays up to 1150 Mev has been studied by exposing nuclear emulsion to bremsstrahlung from the CalTech electron synchrotron and observing the "photostars" produced. Exposures were made at 16 different peak energies between 250 and 1150 Mev. Nearly 10 000 photostars were analyzed for star frequency, prong number, angular distribution, and (at 1143 Mev) the visible energy release per star. The bremsstrahlung yield of multiprong (≥2 prong) stars increases abruptly as photons capable of producing pions are included. The cross section per photon, derived from the bremsstrahlung yield by the photon difference method, is essentially constant at 250 microbarns/nucleon at all energies above 300 Mev. A model for photostar production is given which involves photopion production followed by absorption or scattering of the pion and recoil nucleon. Experimental free-nucleon photopion cross sections are used, together with the Monte Carlo calculations of Metropolis et al., to determine the probability for star formation. Good agreement with both the shape and magnitude of the excitation curve is obtained if nuclear motion is included. Mean and maximum prong numbers for photostars are the same as for stars produced by pions or protons of equal available energy. The mean-free-paths in nuclear matter of pions and protons are short, so that photostar yields are a measure of the integrated total photomeson cross section. More than 95% of the multiprong stars made by 1-Bev bremsstrahlung are made by photons whose energies exceed the pion production threshold of 150 Mev. Most of the 1-prong events are produced by photons below 150 Mev, and the yield is consistent with giant resonance (γ, p) reactions plus "pseudodeuteron" photodisintegration, a process whose cross section rapidly decreases as the photon energy increases. Variation of mean prong number with energy and comparison with nuclear cascade calculations suggests that the excitation of the residual nucleus is nearly constant at 100 Mev over a wide range of incident photon energies. The visible energy release per photostar shows a linear dependence on prong number, and more than half of the photon energy is carried away by neutral particles.

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Additional Information:© 1961 by The American Physical Society. Received 18 July 1961. One of us (C.E.R.) wishes to thank the Research Corporation for its support of that part of the program carried out at the University of California at Riverside. Mr. George Van Buskirk, Mr. Kenneth Crowder, Miss Jean Coulson, and Mr. William Baskett were of great assistance in scanning the excitation function plates. The exposures were made at California Institute of Technology's 1.2-Bev electron synchrotron, as part of a program of high energy nuclear physics research supported by the U.S. Atomic Energy Commission. The support and encouragement of Dr. R.F. Bacher is gratefully acknowledged. We have had several illuminating conversations with Dr. R.F. Christy concerning points of theory. Additional scanning, cross checks, and the 1150-Mev stack investigation were done by the CalTech scanning team of Mrs. Elaine Motta, Mrs. Nancy Gross, Mrs. Judith Lundalius, Mrs. Laura Patterson, and Mrs. Cheryl Maloy. This work was supported in part by the U.S. Atomic Energy Commission, the Research Corporation, and the National Science Foundation.
Issue or Number:5
Record Number:CaltechAUTHORS:ROOpr61b
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:10469
Deposited By: Tony Diaz
Deposited On:09 May 2008
Last Modified:08 Nov 2021 21:08

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