No abstract

The absolute energy spectrum of prompt photons emitted from the fission of 235U by thermal neutrons was measured in the range from 0.01 to 10 MeV by using single-crystal, Compton, and pair NaI|'Tl) scintillation spectrometers. Each was operated in 69-nsec coincidence with a fission chamber exposed to thermal neutrons from a reactor. The pulse-height response functions of the spectrometers were constructed in detail by exposing the spectrometers to radioactive sources of known disintegration rates. These data were used to "unfold" the measured pulse-height spectra to give the absolute differential energy spectrum and its random uncertainties. A careful analysis of systematic uncertainties was also performed.The average number of photons per fission is 8.13+0.35 and the average photon energy release per fission is 7.25 + 0.26 MeV, both over the energy region from 10 keV to 10.5 MeV.The results obtained here are in approximate agreement with the recent measurement by Verbinski and Sund in the energy region above 140 keV. From 1.5 to 4 MeV the calculation of Zommer, Savel'er, and Prokofiev gives results which are close to the measurements. The observed total energy release in photon emission per fissionhas been predicted by two published calculations which treated statistical evaporation theory in different ways to enhance the emission of photons. The K-shell x-ray intensities for the lightand heavy-fragment groups were found to be 0.08+ 0.02 and 0.23+ 0.02 photons/fission, respectively. The x-ray intensities are consistent with internal conversion of the observed y-ray spectrum for various assumed mixtures of E1, E2, and M2 transitions, including roughly equal E1-82 mixtures; the intensities are not consistent with all the transitions having any single multipolarity.

I. INTRODUC'ITON Although many properties of the f i s s i o n fragments and fast neutrons from U-235 f i s s i o n have been measured i n d e t a i l (l) , corresponding studies f o r fission-associated gamma radiation have not been reported. describes experiments designed t o determine as a function of time after f i s s i o n t h e absolute energy spectra of the gamma radiation emitted with the thermal f i s s i o n of U-235*. This paper The energy release i n the form of gamma radiation from U-235 fission, (1 3. 1 + 1. 3) MeV per f i s s i o n , represents a significant f r a c t i o n (about 6$) of t h e t o k energy release per fission. Moreover, of t h e f i s s i o n radiations, only neutrons compete with gamma, rays f o r the position of greatest importance t o the design of nuclear reactor shields (2). In order t o design an e f f i c i e n t reactor shield, the absolute energy spectrum of the emitted gamma radiation must be known, since t h e cross sections pertinent t o the attenuation processes are strongly energy dependent. For a shut-dam reactor or f o r used reactor f i e l , the shielding requirements are determined almost e n t i r e l y by the presence of gamma rays emitted by the radioactive f i s s i o n products. An additional. reactor-design problem i s the removal of heat produced by gamma-ray interactions both in the reactor core and i n the surrounding regions. pressure shell, and shield regions of a reactor, and f o r experimental apparatus used with r e s e a x h reactors. Efficient design of these cooling systems i s dependent upon a knowledge of t h e energy spectra of the sources of gamma radiation associated with f i s s i o n (3). Special cooling systems must frequently be provided f o r r e f l e c t o r , Predictions of t h e fission-associated gamma-ray spectra have not been Therefore, experiments attempted on the basis of the theories of fission. provide the only access t o these spectra. ray spectrum i n time coincidence with fission used for a source the U-235 Experiments t o determine the gamma-* Oak Ridge National Laboratory, operated by Union Carbide Corporation f o r the U. S. Atomic Energy Commission. Throughout this paper, references t o f i s s i o n imply f i s s i o n of U-235 by thermal neutron capture.

The nuclear radiations from I 124 (4 days) and I m (13 days) have been measured. I 124 decays to Te 124 with the emission of three positron groups with end points at 2.20zfc0.01, 1.50±0.01, and 0.67±0.05 Mev, possibly accompanied by some iT-electron capture. Gamma-rays of energy 0.603±0.002, 0.73d=0.01, 1.72±0.02, and 1.95±0.05 are given off in the disintegration. I 126 decays to Xe m with the emission of two beta-ray groups of energies 1.268±0.010 and 0.85=1=0.01 Mev, together with a gamma-ray of energy 0.395±0.005 Mev. An active state of I, of 13.0±0.5 hr. half-life, which decays by ^"-capture and the emission of an internally converted gamma-ray of energy 0.159 Mev, has been found. This activity is assigned to I 123 .* Note added in proof: Drs. I. Perlman and L. Marquez have informed us that they obtained this activity from the reaction Sb m (a,2n)I m . This isotope probably decays by iT-electron capture to Te m which emits the 159 kev gamma-ray. [See R. D. Hill, Phys. Rev. 76, 186A (1949)].

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