. et nyt " TOFI ORNL P 1356 1 TEEEEEEE 191.25 1.1.4 .16 MICROCOPY RESOLUTION TEST CHART NATIONAL BUREAU OF STANDARCS -1963 . C Tiiu , ' L LEGAL NOTICE This report was prepared as an account of Government sponsored work. Neither the United States, nor the Commission, nor any person acting on behalf of the Commission: A. Makes any warranty or representa- tion, expressed or implied, with respect to the accuracy, completeness, or usefulness of the information contained in this report, or that the use of any information, appa- ratus, method, or process disclosed in this report may not infringe privately owned rights; or B. Assumes any liabilities with respect to the use of, or for damages resulting from the use of any information, apparatus, method, or process disclosed in this report. 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Malas wynsruly or nopron outoa, expressed or lapued, med respect to the serve racy, completeness, or wahulness of the laboration contained in wo roport, or that the wee of way lwormation, apparatu, method. or proces. dieclosed la to report way hot latring prinuly ond nidus; or D. Asmuss w labtuvos mu roopact to the wool, or for deres rendus trou then w olwy tuloruation, appunt, molkod, or procon disclound in who report. Ao ward in the store, "pornon orthug an bwall of the Counsetan" wideo way a. plugn os contractor of the Commisslon, or saplogue of mucha contractor, to the extent that much employn or contractor of the Counaslan, or suployw al mal couinctor propers, diomuinası, or provides suwe to, way walorastaa purnaal ho Wo employmeal or contract mucha Connoslon, or M. omployment with such contrscior. Session IV Intermediate States as Observed by Differential Scattering of Neutrons by Doubly Closed Shell Nuclei* C. H. Johnson and J. L. Fowler Oak Ridge National Laboratory Oak Ridge, Tennessee, USA * The recent development of the idea of intermediate states has brought a renewed interest to the spectroscopy of nuclei, such as to and 9Pb, which have one nucleon plus a doubly closed shell. For this reason we have measured the differential cross sections and have made phase shift analyses for neutrons scattered from "O and -Pb. Figure I shows the experimental arrangement for scattering from ºo. Neutrons are produced by the T(p,a) reaction in a gas target and the oº neutrons enter through a shaped collimator into a heavily shielded scattering chamber. Unscattered neutrons leave through a slot in the rear shield of the chamber. The scattering cavity is about a meter square and is shown in the figure with the upper shield removed. The "ºo scatterer, which is centered in the chamber, is liquid cxygen contained in a l-mil-wall Dewar flask designed to minimize background scattering. The inner wall is only I mil thick, and the outer wall, which is under atmospheric pressure, is 20 mils of stainless steel; it has been work-hardened to withstand the pressure. An Swrnod.vida innheimt 5*,- Research sponsored by the U.S. Atomic Energy Commission under contract. with the Union Carbide Corporation. s . PATENT. CLEARANCE OBTAINED. RELEASE TO THE PUBLIC IS APPROVED. PROCEDURES ARE ON EILE IN THE RECEIVING SECTION, sii identical Dewar flask can be rotated into the neutron beam in order to evaluate the background. Neutrons scattered from the sample are detected in a stilbene crystal from which pulses due to gamma rays are depressed by pulse-shape discrimination. We also measure the direct lux by moving the crystal into the Oº neutron beam; thus we are able to measure absolute differential cross sections. Our results have been corrected for the effects of spurious neutron backgrounds, self-attenuation and multiple scattering in the sample, finite angular resolution, and the energy dependence of the detector efficiency. Our measurements on <°°pb were made with a 99.75% 20°pb sample with the same apparatus as above except for the use of a Li(p,n) neutron source. For-ºo we found distributions at ten neutron energies from 3.2 to 4.2 MeV. Figure 1 shows some typical ones obtained near the center of the broad peak that appears at about 3.5 MeV in the published (1) total cross section. The total cross section curve, tilted on its side, is shown to the right in the figure. The solid lines through the angular distribution data are the results of least-squares fits which gave the phase shifts shown on the left-hand side. We took a FORTRAN code, written to compute differential cross sections for neutron scattering from 0-spin nuclei as a function of phase shifts, and in- corporated it into a search routine. The resulting code, which averages the theoretical curves over the energy spread of the experimental measurements, adjusts several phase shifts simultaneously in order to arrive at a minimum in the sum of the weighted squares of the deviations between experimental points and theoretical values. In order to avoid false minima, we required that the phase shifts change smoothly with energy and be continuous with the earlier results [2] at lower energies. We estimated the uncertainties in the phase shifts at a given energy by taking cross section values from the smooth curve and altering them. in a random fashion but with the same standard errors. Thus we obtained a set of points to be fitted with the original code. This procedure was repeated a number of times and the standard deviations were found from the resulting variations in the phase shifts. The distribution at the 3.77-MeV resonance in Fig. 1 is of particular interest apart from the subject of intermediate states. There is an falo resonance in 100 proton scattering (3] that arises from a t'F level whose excitation energy corresponds closely to the to level which is associated with the 3.77 MeV neutroa resonance. Fossan et al. (1) found from their total cross --2- section work that the to state has J = 5/2 with r = 25 kev. We have re- measured the total cross section in this region in good geometry with 5 kev resolution by use of a Beo scatterer and a matching Be background scatterer, and we find the resonance has J = 5/2 with r = 20 kev, in essential agreement with Fossan et al. The best fit to our angular distribution was obtained for an fal assignment as shown in Fig. 2. The level width is about 10% of the single particle limit. Figure 3 shows the results of the analysis from 0.5 to 4 MeV. We have omitted the narrow resonances which would cause abrupt 180° changes in resonant phase angles. Typical uncertainties determined in the manner described above are indicated at some points. The analysis shows a dais resonance at 3.40 MeV approximately 500 keV wide, another da la resonance at 3.82 MeV 50 keV wide, and a Palo resonance about 500 keV wide. The broad dalo resonance corresponds to ~1/2 of the single particle limit, and the Palo rescnance is about 1/5 of the single particle limit. This large width of the 3/2* resonance at 3.40 MeV is interesting because at 1 MeV there is already“ a broad 3/2* state which contains a major part of the dalo single particle limit. It seems reasonable that the observed 3/2* resonance.results from a state having rotational character. This is consistent with Brown's (4) conclusion that the ground state wave function has an appreciable contribution of the distorted 2-partició 2-hole configuration. The resonance states observed in the total neutron cross section of ZUPb (5) were the first to be compared with theoretical expectation for intermediate states. Shakir. [6] calculated the widths of 1/2* resonances under the assumption they arise from one-hole two-particle excitations of Pb and compared the widths with those of the J = 1/2 resonances observed at 1.204, 1.318, 1.354, 1.632, 1.715 and 1.872. He found order of magnitude agreement. of course, only J values are generally deduced from total cross sections, and angular distributions are necessary to assign & values and parities. Figure 3 shows three angular distributions at the upper energy range of a set of 29 measurements made between 0.7 and 1.8 MeV. Beside each curve are listed the set of phase shifts resulting from the least squares search routine code. Attempts to fit the two upper resonances (particularly the 1.761 peak) as Salo resonances were unsuccessfùl; they both secs to be fryla resonances. The difference in their appearances arises from the rapidly changing s, lo phase shift which is going through a resonance in this energy region. This latter - 3- . fact indicates the broad peak centered around 1,715 MeV is an 8, lo resonance. At 1.702 MeV there is an Ialo resonance. Listed below are the resonance parameter assignments for 21 resonances from .723 to 1.761 MeV. The resonant energy in MeV is followed by the informa- tion in parenthesis which includes the J value, parity, and reduced widths in units of 3n