AE-11 Cross sections and neutron yields for U233, U235 and Pu239 at 2200 mjsec. N.G. Sjostrand andJ.S. Story AKTIEBOLAGET ATOMENERGI STOCKHOLM • SWEDEN • I960 ADDENDUM to report AE - 11 233 Cross sections and neutron yields for U and Pu239 at 2200 m/sec. N. Go Sjostrand and J. S. Story This is a preliminary report written in February 1959 and not intended for wide distribution. No corrections or additions have been made for this new edition. AE-11 Cross sections and neutron yields for U233, U235 and Pu239 at 2200 m/sec. N.G. Sjostrand and J.S. Story Summary: The experimental information on the 2200 m/sec values for <r , , ™ 233 235 239 3-ds (T^, a, V, and rj for U , U and Pu has been collected and discussed. The values will later be used in an evaluation of a "best" set of data. In an appendix the isotopic abundances of the uranium isotopes are discussed and 239 also the alpha activities of the uranium isotopes and Pu . Printed April 1960 LIST OF CONTENTS Page The absorption cross-sections 2 Notes on data in Table 3 4 The fission cross section 5 The capture to fission ratio 9 The number of neutrons per fission 11 The number of fission neutrons per absorbed neutron 14 Appendix. Half-lives and atomic abundances of the isotopes in natural uranium 20 Notation 20 Experimental Results. Mass spectrometry 20 Alpha Counting Experiments 21 Assessment of Recommended Values 26 References 29 TABLES 1. g-factors according to WESTCOTT (1958) 1 2. 2 3.0". at 2200 m/sec 3 4„ cat 2200 m/sec 6, 7, 8 5. a at 2200 m/sec 10 6. V at thermal energy 13 7. T) at 2200 m/sec 16 8 . V 0"^ ratios at 2200 m/sec 17 9. (t] - 1) o"^g/o"g at 2200 m/sec 18, 19 A 1. Relative U abundances by mass spectrometry 21 A 2. Experimental data and best values 25 A 3. Recommended values of decay rates and isotopic abundances 28 233 ^235 239 Cross sections and neutron yields for U and Pu at 2200 m/sec. In this report we discuss the experimental information on tr a or , V and r| for the three most important fissionable nuclides under bombardment with neutrons of 2200 m/sec. As the experimental tech­ niques used in the measurements have been described thoroughly by others (see for instance HARVEY and SANDERS, 1956) we will limit ourselves to such comments as may be necessary. Unfortunately, many of the earlier measurements have not yet been described in the open literature, and we must still use some of them. In such cases we have often had to rely on secondhand information and may have misinterpreted it. In order to reduce all the measurements to 2200 m/sec values some assumptions must be made about the variation of the different parameters with energy and also about the neutron spectrum. We have adopted the scheme of WESTCOTT (1958) who gives the g-factors, listed in Table 1, for Maxwellian spectra at different neutron temperatures. Table 1, g-factors accordingto WESTCOTT (1958). T, °C 20 40 60 80 U233, gf 1.0112 1.0112 1.0112 1.0114 ®abs 1.0078 1.0073 1.0069 1.0068 gn 1.0034 1.0039 1.0043 1.0046 U235 g£ 0.9754 0.9702 0.9653 0.9607 ®abs 0.9749 0.9698 0.9649 0.9603 1.0005 1.0005 1.0005 1.0004 n p«239 sf 1.0499 1.0629 1.0777 1.0946 ®abs 1.0737 1.0920 1. H28 1.1362 gn 0.9778 0.9734 0.9685 0.9634 | 2 For measurements done inside reactors or in reactor beams we have assumed the neutron temperature was effectively 60°C except when a thermal column was used,for which we assumed 30°C ,or when a different temperature has been stated in the reference. When experimen­ tal details have been inadequately reported, we have assumed that corrections were made for the contributions of epithermal neutrons, by cadmium difference methods for example. In recalculating relative measurements the following standard cross sections and g-factors have been used (according to BNL-325, second ed. and WESTCOTT, 1958). Table 2. cr , abs g-factor barns Li 71.0 1.000 Li6 945 1.000 B 755 1.000 Na 0. 536 1.000 Mn 13.3 1,000 Au 98. 8 1.006 (at T = 300°C) The absorption cross-sections. Most of the measurements of the absorption cross-sections, o' , , have been made by the transmission method, which measures total cross-sections. The most reliable values of the scattering cross­ sections for subtraction seem to be the following: 233 + U 12.5 - 1.0 b OLEKSA (1958) U235 15 t 2.5 b FOOTE (1958) 239 Pu 10 b Calculated potential scattering. However, as HAVENS and MELKONIAN (1958) have observed, these values may be somewhat high at 2200 m/sec, as a result of mole­ cular and crystallic binding effects in the coherent scattering. The measured values of absorption cross-sections are listed in Table 3. Table 3 <r , at 2200 m/sec. abs ' —^33 ..u235 Pu-139 Reference Method Remarks MAY (1944) 687 t 15 Beam experiment, rel. to boron Original datum unknown. 695 t 20 Slow chopper See note 1 below. ANDERSON et al. (1944) 1057 Time-of-flight measurement with cyclotron PALEVSKY & MTJETHER (1954) 585 t 10 Slow chopper PALEVSKY et al. (1954) 6 85 t 5 Slow chopper EGELSTAFF (1954) (Revised) 709 - 15 Slow chopper ZIMMERMAN & PALEVS] Y (1955) 1020 - 10 Slow chopper PATTENDEN (1956) 595 - 15 1005 t 30 Slow chopper See note 2 below. NIKITIN et al. (1956) 570 - 20 695 t 20 1030 - 30 Time-of-flight measurements with cyclotron KUKAVADSE et al. (1956) 618 t 30 M^ss spectrometric, relative to Original datum Li , in a thermal spectrum °"abg(u) + --------r----- = 0.66 - 0.03 'Li" GREEN et al. (1957) 578 - 17 Oscillator, relative to gold Original datum same. Corrections to 2200 m/sec. and new standard cross sec­ tion does not change the result. SCHWARTZ ( 1958) 6 83 t 6 Crystal spectrometer Unpublished work at Colum­ bia and Argonne 681 - 6 Chopper BOLLINGER et al. (1958) 988 - 10 Chopper u> 4 Note 8 on data in Table 3, (1) MAY 's (1944) measurement was made with the Argonne slow chopper. At that time rather low values were obtained with the same instruments for other cross-sections: cr ^ at 2200 m/sec. ANL 1944-47 1959 data B 705 759 Au 100 106 U235 660 t 16 It is plausible to assume that the energy calibration of the instrument was erroneous and that values quoted for neutrons of 2200 m/sec. were really measured at 2350 - 20 m/sec. Then May's datum would imply <r ,p(U233) = 710 - 18 at 2200 m/sec or <r = 695 - 18 b. ( 2) PATTENDEN's (1956) data shows a discontinuity in 233 cr rp(U ) at 0. 025 eV. This might be due to a very weak level in that region but it has not been observed by others and does not appear in measurements of the functions cr ^ and a. So we have smoothed Pattenden ys data locally to get <r ,p = 60 8 b or cr^g = 595 b. Alternatively if one should accept Pattenden's data in detail it would be necessary to revise the corresponding g-factors for reactor calculations! 5 The fission cross section. Direct measurement of fission cross sections, (r^, usually involves: 1) Calibration of the neutron flux incident on the fissile sample. Thi s is usually done by using a BF^ chamber or by an activation of gold or manganese foils, and the fission cross section measurement is then only comparative. 2) Observation of the fission rate in a fission chamber or with photographic emulsions. 3) Assay of the thickness of the fission foil. 4) Assessment of counter efficiencies. Various methods have been used to assay the fission foil. They rely basically on a) absolute alpha-counting b) weighing but the procedure is usually rather complex. We refer the reader to the various papers cited below, and especially to FLEMING (1952), 235 When the measured cross section is that of U in natural 235 uranium we give the result for pure U , assuming that the isotopic 235 + abundance of U in natural uranium is 0. 7200 - 0,0005 per cent (see the Appendix). The data are listed in Table 4. Table 4 cr at 2200 m/sec. f Pu23^ p^239 P eference u233 u235 u233 Method Remarks -^35 DeWIRE et al. 1.27 Original datum unknown. (1944) - 0. 05 Corrected by EGELSTAFF et al. (1955). 540 t 30 DEUTSCH & Relative to gold and man­ Original data unknown.Correc­ LINENBERGER 575 - 30 ganese . ted by HARVEY & SANDERS (1944) (1956) 587 ± 13 235 MAY (1944 ai.d 1945) 0. 83 U relative to boron Original data unknown. 10.03 Corrected to cr = 755. BISWAS & PATRO 561 Fission counter and boron Original datum 526 t 10, cor­ (1949) : is counter in paraffin block. rected for boron cross sec­ RaBe source, and Cd diffe­ tion, ^ abundance,and rence measurements. g-factor FACCINI & GATTI 598 Ion chambers in paraffin. Original datum corrected (1950) t 15 Relative to Li. RaBe s Durce according to COHEN et al. and Cd difference measure­ (1952) and to 2200 m/sec. ments. TUNNICLIFFE (1951) 536 648 Fission counter and boron Original data unknown. 1 16 t 20 counter with monocromatic Boron cross section neutrons from a crystal corrected.