Liskien, H.

Paulsen, A.

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EUROPEAN ATOMIC ENERGY COMMUNITY - EURATOM CROSS-SECTIONS FOR THE REACTIONS Cu63(n, cx)Co60, Ni60(n,p) Co60, Ti46(n,p) Sc46 and Na23(n,2n)Na22 by H. LISKIEN and A. PAULSEN ® Joint Nuclear Research Center Geel Establishment - Belgium Central Bureau for Nuclear Measurements - CBNM Reprinted from NUCLEAR PHYSICS Vol. 63, No. 3 - 1965 • " LEGAL NOTICE This document was prepared under the sponsorship of the Commission of the European Atomic Energy Community (EURATOM). Neither the EURATOM Commission, its contractors nor any person acting on their behalf: 1° - Make any warranty or representation, express or implied, with respect to the accuracy, completeness, or usefulness of the information contained in this document, or that the use of any information, apparatus, method, or process disclosed in this document may not infringe privately owned rights; or 2° - Assume any liability with respect to the use of, or for damages resulting from the use of any information, apparatus, method or process disclosed in this document. This reprint is intended for restricted distribution only. It reproduces, by kind permission of the publisher, an article from "NUCLEAR PHYSICS", Vol. 63, No. 3 - i965, pp. 393-400. For further copies please apply to North-Holland Publishing Company, P.O.Box 103, Amsterdam (Netherlands). Dieser Sonderdruck ist fur eine beschriinkte Verteilung bestimmt. Die Wiedergabe des vorliegenden in ,,NUCLEAR PHYSICS" Vol. 63, No. 3 - 1965, pp. 393-400 erschienenen Aufsatzes erfolgt mit freund-licher Genehmigung des Herausgebers. Bestellungen weiterer Exem-plare sind an North-Holland Publishing Company, P.O.Box 103, Amsterdam (Netherlands) zu richten . Ce tire-a-part est exclusivement destine a une diffusion restreinte. Il reprend, avec l' aimable autorisation de l' editeur, un article publie dans « NUCLEAR PHYSICS», Vol. 63, No. 3 - 1965, pp. 393-400. Tout autre exemplaire de cet article doit etre demande a North-Hol-land Publishing Company, P.O.Box 103, Amsterdam (Netherlands). Questo estratto e destinato esclusivamente ad una diffusione limitata. Esso e stato riprodotto, per gentile concessione dell'Editore, da « NUCLEAR PHYSICS», Vol. 63, No. 3 - 1965, pp. 393-400. Ulteriori co pie dell' articolo debbono essere richieste a North-Holland Publishing Company, P.O.Box 103, Amsterdam (Netherlands). Deze overdruk is slechts voor beperkte verspreiding bestemd. Het artikel is met welwillende toestemming van de uitgever overgenomen uit ,,NUCLEAR PHYSICS", Vol. 63, No. 3 - 1965, pp. 393-400. Meer exemplaren kunnen besteld worden bii North-Holland Pu-blishing Company, P.O.Box 103, Amsterdam (Nederland) . ' . . . 12.A.1:l ~ Nuclear Physics 63 (1965) 393-400; © North-Holland Publishing Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher CROSS-SECTIONS FOR THE REACTIONS Cu63(n, oc)Co60, Ni60(n, p)Co60 , Ti46(n, p)Sc46 and Na23(n, 2n)Na22 H. LISKIEN and A. PAULSEN Central Bureau for Nuclear Measurements, EURATOM, Geel, Belgium Received 8 October 1964 Abstract: Absolute activation cross-sections for the reactions Cu63(n, °')Co60, Ni60(n, p)Co60, Ti46(n, p)Sc46 and Na23(n, 2n)Na22 were determined in the energy range 12.5 MeV to 16.5 MeV. Mono-energetic neutrons were obtained from the T(d, n)He4 reaction at 1 MeV deuteron ener-gy. The very low induced specific activities were detected by a coincidence counting technique. For comparison, statistical model calculations from 5 to 20 MeV neutron energy were per-formed taking into account second particle emission. E NUCLEAR REACTIONS Na23(n, 2n), Ti46(n, p), Ni60(n, p), Cu63(n, °'), E= 12.5-16.6 MeV; measured O"n, 2n(E), O"n, (E), O"n,a<E). Natural targets. 1. Introduction Measurements of activation cross-sections for reactions leading to product nuclei of long half-life are very rare and inaccurate. This is due to the low fluxes of mono-energetic neutrons available by accelerators which induce only very low activities. These low activities can hardly be distinguished from background with normal counters. On the other hand, the determination of these cross-sections by direct de-tection of the produced particles is in most cases associated with large uncertainties due to the plurality of the different secondary particles and their continuous energy distributions. In all cases where two characteristic radiations per disintegration are emitted the signal-to-noise ratio can be increased considerably by application of coincidence counting technique. The reactions Cu63 (n, et)Co60, Ni60 (n, p)Co60, Ti46(n, p)Sc46 and Na23 (n, 2n)Na22 were studied by means of this technique. 2. Experimental Procedure 2.1. IRRADIATION The general arrangement for neutron irradiation is shown in fig. 1. Neutrons between 12.6 and 16.6 MeV energy were obtained by bombarding a T-Ti target with a 25 µA beam of 1 MeV deuterons and using the angular range 20° to 155°. The sam-ples were suspended from a graduated circle of about 82 mm radius. This circle was 393 394 H. LISKIEN AND A. PAULSEN adjusted with its centre above the geometrical centre of the deuteron beam spot by means of a mirror system. The geometrical size of the neutron source is defined by an aperture to a width of 3 mm and a height of 8 mm. The specimens were mounted at both sides with respect to the deuteron beam direction to determine a possible shift of the beam centre. In addition to the TD neutrons a certain number of OD neutrons (produced by self-target formation) were observed. However, the thresholds of all the examined reactions are high enough to avoid interference due to these neutrons. d ----c[] ra - aperture 0.3cm width deuteron energy 1 MeV r-n-rarget 1.0 or 2.5 mglcm2 Fig. 1. Experimental arrangement for sample irradiation. recoil proton telescope The tritium target holder and the facilities for sample mounting were kept as light as possible to keep flux attenuation low. The neutron flux was determined absolutely by means of a proton recoil telescope counter 1) positioned in 0° direction at 25 cm TABLE 1 Some data about irradiation and activity determination Reaction Cu63(n, a)Co60 Ni60(n, p)Co60 Ti46(n, p )Sc'6 Na23(n, 2n)Na22 Half-life of the product 5.265 5.265 0.230 2.583 nucleus (y) Tritium-titanium target 2.5 2.5 1.0 1.0 used (mg/cm2) Sample material Cu Ni Ti NaCl Sample size (mm) 300 X8 300 x8 200 x5 200 x5 Typical specific sample 3 3 30 14 activity (pCur/g) Efficiency of the coincidence 1.1 1.1 1.1 5.7 counter (%) Background of the coincidence 18 18 38 90 counter (c/h) Typical foreground-back- 13 7 8 4 ground ratio CROSS SECTIONS 395 distance from the target. Irradiation time for each reaction was about 32 h. Informa-tion about sample material, sample dimensions and T-Ti target thickness can be obtained from table 1. 2.2. ACTIVITY DETERMINATION Activity measurements were started about two weeks after irradiation for Cu, Ni and NaCl samples. For the Ti samples however, a waiting time of four weeks was necessary due to a 1.8 d activity from Ti48 (n, p)Sc48 • The induced specific activity of the samples of the order of pCur/g were determined in a y-y coincidence spectro-meter which consists of two scintillation counters (7.6 cm x 10.2 cm 0 NaI(Tl) crystals coupled to type 54A VP photomultipliers ). Only pulses due to a photoeffect were fed into the coincidence unit (resolution time 2 µsec). The efficiency of the y-y coincidence spectrometer was determined with absolutely calibrated sources and was checked several times per day. To eliminate differences in geometry and y-self-ab-sorption between these sources and the samples, each calibrated source had the size of a normal sample and consisted of four active foils sandwiched between blocks of inactive material. These calibrated sources were delivered by the Radioactivity Group of the CBNM with an accuracy of ±0.5 % for the P-activity. In the case of Na22, a value of 0.898 p+ per disintegration was used 2). The efficiencies and natural backgrounds of the coincidence counting equipment for the different window settings and the specific sample activities together with the resulting foreground-background ratios are listed in table 1. 3. Results 3.1. DATA EVALUATION For activation measurements in which the irradiation time is short with respect to the half-life of the product nucleus the reaction cross-section <1 is given by Ao (1 = ' N1N2l where A 0 is the sample activity at the end of irradiation and). is the decay constant of the product nucleus. The number of neutrons per cm2 at sample position N1 was calculated from the proton recoil telescope measurement at 0°, taking into account the distance target-sample and the angular distribution of the TD reaction. Mean values from all avail-able data for this angular distribution, as compared in ref. 3), were used. Corrections for neutron absorption and scattering within the sample were applied. The number of sample nuclei of interest N 2 is given by the sample weight, isotopic abundance and the chemical purity of the sample material. All the final results are listed in tables 2 and 3. The mean neutron energies were calculated from the kinematics of the TD reaction taking into account the deuteron 396 H. LISKIEN AND A, PAULSEN energy, the irradiation angle and the deuteron energy loss in the target. The deu-teron energy was determined with the aid of a calibrated magnetic resonance system. The given neutron energy spread is due to the finite solid angle of the samples and the target thickness. The quoted values are the half-widths of the neutron spectra seen by each sample. TABLE 2 Experimental results for the reactions Cu63(n, cx)Co60 and Ni60(n, p)Co60 En(MeV) Cu63(n, cx)Co60 Niso(n, p)Coso a(mb) a(mb) 12.67±0.17 39.1±2.7 141±10 12.86±0.22 38.1±2.7 141±10 13.24±0.29 38.2±2.7 138±10 13.54±0.33 37.8±2.6 135± 9 14.05±0.37 39.1±2.7 122± 9 14.42±0.39 35.7±2.5 118± 8 14.99±0.39 31.6±2.2 109± 8 15.37±0.37 30.2±2.1 102± 7 15.88±0.36 27.2±1.9 90± 6 16.18±0.41 26.8±1.9 91± 6 16.52±0.46 23.3±1.6 85± 6 TABLE 3 Experimental results for the reactions Ti46(n, p)Sc46 and Na23(n, 2n)Na22 Ti4s(n, p )Sc46 En(MeV) a(mb) 12.60±0.11 12.76±0.14 12.98±0.17 13.10±0.18 13.38±0.20 13.54±0.22 13.88±0.24 14.05±0.25 14.42±0.26 14.61±0.26 14.99±0.27 15.18±0.26 15.55±0.24 15.71±0.23 16.03±0.21 16.31±0.19 16.52±0.19 3.2. ACCURACY 300±21 307±21 296±21 297±21 292±20 303±21 305±21 299±21 295±21 293±21 285±20 290±20 289±20 284±20 288±20 285±20 296±20 Na23(n, 2n)Na22 En(MeV) a(mb) 12.63±0.11 12.81 ±0,15 12.98±0.17 13.24±0.20 13.46±0.22 13.79±0.23 14.05±0.25 14.42±0.26 14.71±0.27 15.09±0.26 15.37±0.25 15.72±0.23 15.99±0.21 16.25±0.20 16.42±0.19 16.61±0.20 0.9~~:~ 1.1±1.l 3.3±1.7 3.4±1.2 8.0±1.2 14.2±1.4 28.0±2.1 44.0±3.1 65.8±4.6 84.8±5.9 87.3±6.1 108±8 113±8 122±9 133±9 141±10 The experimental errors on the resulting cross-sections are given in tables 2 and 3. They were calculated by compounding quadratically the uncertainties of the different factors involved. CROSS SECTIONS 397 The largest uncertainty of ±5.7 % is due to the number of neutrons per cm2 at sample position N 1. This uncertainty contains the errors of the telescope measure-ment (±4 %), the irradiation geometry (±3 %), the neutron angular distribution (±2.5 %) and the neutron absorption and scattering correction (±0.5 %). The un-certainty in the sample activity at the end of irradiation A 0 amounts to ±2.7 % and contains statistical errors ( ± 2.0 %) and the error due to efficiency and background variations of the coincidence counter (± 1.7 %). · All other uncertainties are small compared with the abovementioned ones. Com-bining all the uncertainties one gets errors between ± 6 % and ± 7 %. Only the cross-sections of the reaction Na23 (n, 2n)Na22 near threshold have larger uncertainties due to the poor foreground-background ratio. 3.3. COMPARISON WITH EXISTING DATA No previous activation measurement is known for the reaction cross-section of Cu63 (n, tX)Co60• A determination of the a1pha particle yield at 55° ± 55° resulted in a differential cross-section of 5.65±0.6 mb · sr- 1 at 14.8 MeV neutron energy 4 ). Assumption of isotropy would lead to a reaction cross-section of 71.2 mb which is considerably higher than these results. Furthermore there exists a measurement 5 ) of the partial cross-section for Cu63 (n, tX)Co60m leading to the isomeric state of Co60(Tt = 10.5 min). This partial cross-section was reported to be 23±3 mb at 14,70±0.15 MeV. For the Ni60 (n, p )Co60 reaction only one earlier activation measurement 6 ) resulting in 150±20 mb at 14.5 MeV neutron energy is known. However, there are some further measurements carried out by direct proton detection. Due to the plurality of different secondary particles and their continual spectra occurring in the fast neutron induced reactions, these measurements are associated with larger uncertainties. The following cross-sections have been reported for the su111 of compound and direct interaction part: 155±25 mb at 13.5±0.1 MeV (ref. 7)), 240±48 mb at 14 MeV (ref. 8)) and 158±32 mb at 14.1 MeV (ref. 9 )); and for the compound nucleus re-action part only: 134±28 mb at 14 MeV (ref. 10)). The agreement of all results may be considered as satisfying taking into account that the cross-section of 240 ± 48 mb is surely exaggerated by observation at 34 ° ± 20° and assumption of isotropy as stated in the original paper. The cross-sections for the reaction Ti4 6(n, p)Sc46 can be compared with a rough activation measurement 11 ) giving ~ 520 mb at 14.8 ± 0.9 Me V. This result is in-dicated in the original paper as a tentative figure. For the reaction Na23{n, 2n)Na22 the previous measurements of Prestwood 12) resulted in 13.8 mb at 14.1 ±0.2 MeV. The quoted uncertainty of ±2.2 mb is twice the standard deviation for six measurements showing only the reproducibility. The measurement of Picard and Williamson 13) resulted in considerably smaller cross-sections than the present work. 398 H, LISKIEN AND A, PAULSEN 4. Statistical Model Calculations For comparison statistical model calculations from 5 to 20 MeV were performed taking into account second particle emission. Cross-section formulae as given in ref. 14) were used. For the decay of the compound nucleus after neutron absorption emission of neu-tron, proton, deuteron, tX-particle and y-quanta was considered. Neutrons, protons and gammas were taken into account as second emitted particles. The reaction Q values were taken from ref. 15). For the dependence of level density p on excitation energy e the following expres-sion was used 16): p(e) ~ a-*(e-<>+t(e)t 514 exp[4a(e-<5)]-i, with the nuclear temperature given by (e-J)t t(e) = -a- (1 + [4a(e-<>)rt + [8a(e-<>)r 1). The pairing energies <5 were taken from ref. 17 ). The parameter a can be written as 18) to include shell effects. The effective single-particle angular momentum quantum numbers iz and iN were taken from ref. 18), while for the constant tX, a value 16 ) of 0.0374 was used. The values of a recently derived from experimental data in ref. 20 ) are in good agreement with tX = 0.0374. Other tX values evaluated from experimental data are 0.031 (ref. 18)) and 0.028 (ref. 19)). Absorption cross-sections for the formation of a compound nucleus were mainly taken from refs. 21• 22). However, since in these papers absorption cross-sections are only presented for certain Zand A values also refs. 23 - 25) were consulted. All these cross-sections calculated by different authors are based on nearly the same optical potentials for one kind of incoming particles. The results of this computation and the experiments are shown in fig. 2. Discrep-ancies between theory and experiment are of the order of a factor 2 to 4. It turns out that the statistical model is still not a reliable help for extrapolating experimental results. It should be noted that the theoretical cross-sections of three of the four in-vestigated reactions are larger than the measured values. The authors wish to thank Dr. J. Spaepen and Mr. K. H. Bockhoff for their kind interest in this work. Many thanks are due to Dr. H. Horstmann for carrying out all the computer work in connection with the statistical model calculations and to Mr. R. Widera for his continuous help during experiment and data evaluation. Thanks are also due to the staff of the 3 MeV electrostatic accelerator of the CBNM, es-pecially to Messrs. J. Leonard and R. Duchez. . CROSS SECTIONS 399 100 )! Cu 63(n, «) Co60 • this work 75 o lrfan and Jack f 50 it!t ff 25 ii II I 5 10 15 En (MeV) 20 200 )! 150 H il 1 I100 ! ! o Cross f Hi o March and Morton 50 • Allan • Storey • this work 5 10 15 En (Me VJ 20 300 )i Ti 46(n,p)Sc 46 I\nHinmuiI • this work 225 150 75 5 10 500 Na23(n,2n)Na 22 1~ !. a Prestwood 375 b o Picard et Williamson • this work 250 125 2 2 Fig. 2. Statistical model calculations of the excitation functions together with the experimental cross-sections. The dashed line represents the theoretical cross-sections without regarding second particle emission. 400 H. LISKIEN AND A. PAULSEN References 1) S. J. Bame, Jr., Eugene Haddad, J.E. Perry, Jr. and R. K. Smith, Rev. Sci. Instr. 28 (1957) 997 2) Nuclear Data Sheets, National Academy of Sciences, Washington 3) A. Paulsen and H. Liskien, Nuclear Physics 56 (1964) 394 4) M. lrfan and W. Jack, Proc. Phys. Soc. 81 (1963) 808 5) J. Kantele and D. G. Gardner, Nuclear Physics 35 (1962) 353 6) W. G. Cross and R. L. Clarke, Progress Report AECL-1542/PR-P-53 (1962) 7) P. V. March and W. T. Morton, Phil. Mag. 3 (1958) 577 8) D. L. Allan, Proc. Phys. Soc. A70 (1957) 195 9) R. S. Storey, W. Jack and A. Ward, Proc. Phys. Soc. A75 (1960) 526 10) D. L. Allan, Nuclear Physics 24 (1961) 274 11) A. Poularikas and R. W. Fink, Phys. Rev. 115 (1959) 989 12) R. J. Prestwood, Phys. Rev. 98 (1955) 47 13) J. Picard and C. Williamson, J. Phys. 24 (1963) 813 14) H. Biittner, A. Lindner and H. Meldner, Nuclear Physics (1964) 15) Nuclear Data Tables, Part 1 (1960), ed. by USAEC 16) D. W. Lang, Nuclear Physics 26 (1961) 434 17) A. G. W. Cameron, Can. J. Phys. 36 (1958) 1040 18) T. D. Newton, Can. J. Phys. 34 (1956) 804 19) D. B. Thomson, thesis (1960) LASL 20) A. V. Malyshev, JETP (Soviet Physics) 18 (1964) 221 21) H. Meldner and A. Lindner, Z. Phys. 180 (1964) 362 22) H. Meldner, Diplomarbeit (1964) Universitiit Frankfurt 23) C. M. Perey and F. G. Perey, Phys. Rev. 132 (1963) 755 24) L. F. Hansen and R. D. Albert, Phys. Rev. 128 (1962) 291 25) J. R. Huizenga and G. Igo, Nuclear Physicf 29 (1962) 462 ... .. .,11i,i mri1!11t~ m: !H~ m 11~. ~~ 1~1, .. :::::::::::::::::::::::::::::::::: To disseminate knowledge is to disseminate prosperity - I mean :::::::::::::::::::::::::::::::::::::: general prosperity and not individual riches - and with prosperity =::~:;:'::::;:::::::;:::f ~;::• great~ part of the e~l whlch is our:::~:: ,I .~ . .. t .. EURATOM - C.I.D. 51-53, rue Belliard Bruxelles (Belgique) '1 

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