The charged-particle-producing reactions of 15-MeV neutrons with structural materials have been studied with a new type of spectrometer based around a magnetic quadrupole lens. Significant improvements in signal-to-background have permitted the detection of protons with energies as low as 800 keV. In several materials charged particles emitted with energies well below the Coulomb barrier contribute significantly to the total hydrogen and helium production cross sections. The production mechanism for these low-energy particles is second-chance emission in (n,n'p) and (n,n'..cap alpha..) reactions. At the high-energy end of the proton and alpha-particle spectra and for the entire deuteron spectra, non-statistical reaction mechanisms are indicated. Results for Al, Ti, V, Fe, Ni, Cu, Nb, and stainless steel 316 are compared with data from other experimental methods

Grimes, S. M.

Haight, R. C.

UNT Digital Library

5^-w/«aas- -n PMMWNT UGRL- 80235 Lawrence Livermote Laboratory EXPERIMENTAL STUDIES OF (N|CHARGED PARTICLE) CROSS SECTIONS, AWULAR DISTRIBUTIONS AND SPECTRA UITH A MAGNETIC QUADRIPOLE SPECTROMETER R. C. m i g h t and S. H. Grimes December 1977 This paper was prepared for presentation a t the Special ist Neetlng on Neutron Data of Structural Materials for Fast Reactors, Gael Belgium, December 5 -8 . 1977 • - - ; ; ~ Thla i i a prvrlnl ot a paper intended lor publication In a lournat or procotflnga. since chenpoe may be maoe before publication, inn preprint la made available with the underilindirsj that It will not bo cited or raproductd without the permlealonol the author. DISTRIBUTION OF THIS DOCUMENT IS UNLIMITED / Experimental Studies of (n. charged particle) Cross Sections, Angular Distributions and Spectra with a Magnetic guadrupole Spectrometer R. C H«1pht and S. H. Grimes Uwrence Llvormore Laboratory, Llvermore, California 94550 The charged-partlcle-produclng reactions of 15-MeV neutrons with strucural materials have been studied with a new type of spectrometer based around a magnetic quadrupolt lens. Significant Improvements In s1gnsl-to-background have permitted the detection of protons with energies as low as 800 keV. In several materials charged particles emitted with energies well below the Coulomb harrier contribute signi­ficantly to the total hydrogen and helium production cross sections. The production mechanism for these low-energy particles Is second-chance emission 1n (n.n'p) and (n.n'6) reactions. At the high-energy and of the proton and alptia-partlclu spectra and for the entire deuteron spectra, non-statistical reaction mechanisms are Indicated. Results for A l , T l , V, Fe, h i . Cu, nb. and stainless steel 315 are compared with data from other experimental methods. »wo r k perfomeJ under the auspices of the U.S. Energy Department, Contract W-7«05-Enj-48. NOTICI • — W* noort w« frvtt u M oKOMt of «wl( tfOMono' kr tw I W M Sum Gwiflmwftl. NMhti * • Ulna HUM KM * f IMw4 I t M i IWfMINRl of Eonf*. IKK ujr of * * * maloyoM, Mr injr of M l coninKlon, NibnoffKlfn, or ihtk omfloyM,. ir»k« wr wontfily, upMH M ImWWo, « MUMM my MM IMttlv of mpoooMHIy for «w iMvrwy, ftonjomwoi c, (wrulMM or uy MrormikM, »tr*nliM. oraaiKi oc pioMii itekwHl, or nomfna ** t Hi w wovkt m InTtlna* orlwi»ty ownH HfhM. 2 I. Introduction In the past two years, neutron-Induced charged-partlcle-produc^ng reactions have been Investigated at the Lawrence Llvarmora laboratory with a new type of spectrometer. The traditional experimental arrange­ment his been to datect the charged particles (usually Just protons, deuterons or alpha particles) with a counter telescope or, in the o'rJer exper1ments« with photographic emulsions ( r ig . l a ) . Our modifications are Indicated In F1g. lb. First we m>ve tiie detector telescope farther away from the neutron source and the irradiated target f o i l . Secondly, we Insert magnetic quadrupole lenses between the foi l and the detectors. Finally we add shielding (not shown In Fig. 1) between the neutron source and the detector. The details of the resulting spectrometer and Us operation have been described at length In several publi­cat ions. 1 " 4 Kith this spectrometer tha signal-to-background ratio has been Improved by more than a factor of 30 1n (n.chargtd particle) measure­ments at a neutron source energy of 14 to 15 MeV. A typical raw snectrum (Fig. 2) shows the good signal-to-background ratio. Other advantages Include the Increased lifetime of the silicon surface-barrier detectors used In the telescope and the simplicity of the spectrometer. To obtain confidence In the results obtained with this spectrometer. oiw must compare the cross section and Sf>--„ T« with those obtained by other methods. One purpose of this paper Is therefore to compare our results with many different types of dats Including activation cross sections, helium accumulation measurements, and other direct measurements of the charged particles. To faci l i tate comparisons the cross section data from our work with this spectrometer are summarized. A second purpose Is to discuss possible problems with the method especially 1f greater Accuracies were to be required. JTR1BUTION OF THIS DOCUMENT IS UNLIMITED 3 I I , Mvantagts and Problems of tht ExpirlMntal Method The advantages of tht magnetic quadruple sptctromttr derive from the great reduction In background, because of tht relatively low background, we have been able to employ tht LLL rotttlng-target neutron source5 et en Intensity of 3 • l o ' 2 n/sec Into 4ir steredlens. And because such en Intense neutron source can be used, the terget foils can be very thin. As we have found, 1t Is Importent to measure tht charged-ptrtlclt energy spectrum to energies well below tht Coulomb barrier to be certain that the total chargtd-partlcle-productlon cross section I t obtained. The problem essocleted with this spectrometer do not appear to Se limitations 1n the context of tht MtENDA request l i s t 6 for measure­ments near E„ • 14 HaV. The requested accuracy Is usuelly for ossess-ments of candidate mettrlels for fusion reactors and a 15 to 20* un-certelnty Is ecceptable. In tht futurt , however, greeter accuracy could be rtqutsted end some pecullerltlts of the spectrometer could be signi­ficant. Some problem tr ies could t r t s t I f one t r l t t to push the eccuracy much below NX. One of these potential problem areas 1s In the determination of the energy-dependent, particle-dependent ecceptences (see Fig. 2) of the spectrometer. The ecceptance 1s the product of the transmission of tht magnetic lens system end tht , tff ldency of the detector celt-scopt. At present, we determine the occeptence by measuring the known spectrum of protons (deuterons) knocked out of a thick CH2 (C0 2) sample. The spectral shapes are determined by the acceptance and by (dtvdx)" 1 for the particular particle type. Tht normalization 1s fixed by the dif ­ferential t ltstlc-scatterlng cross section, for overell accuracies much below l o t , the uncertainties In dE/dx could become significant. This dif ­f iculty would therefore epply to measurements of alt particle types. Also, I f the liiddent neutron spectrum Is degraded end not monoenergetlc, an error would be^lntroduced In the derived ecceptence. For deuterons 4 another diff iculty Is that the differential elastic scattering n-d cross section Is not so well known £s for n-p elastic scattering. Uncertainties 1n the normalization of (n,xd) cross sections could then become Important for a very accurate measurement. Usually, however, there Is so much structure 1n the engular distribution of (n,xd) deuterons that the uncertainty In this structure dominates the error unless a great many angles are measured. For alpha particles, there Is no good calibration standard corresponding to elastic n-p or n-d scattering since n-a elastic scattering only gives alpha particles of energy at most 16/25 x E n and t l» raxlmum energy of the elpha particles from (n,a) on structural materials 1s about E n . Consequently for the alpha-particle acceptance we have had to use the acceptance for protons or for deuterons, suitably adjusted In energy. Tor more accurate measure­ments, the difference In detector efficiency for deuterons and alpha particles could be a problem. Finally the stabi l i ty of the magnetic lenses would require a more precise verification for mora accurate measurements. A second erea of potential di f f iculty for more accurate measurement 1s the uncertainty In the source-to-terget fo i l distance. For best signal-to-background, this distance must be small. The area of the target f o i l , however, should not be too small. We us a 2-cm diameter fo i l and a minimum source-to-target-foll-center distance of 5 cm. The variations of i / r 2 would become significant over the area of !!s fo i l 1n a more precise measurement. Also the centrold of the source position could vary and change the source-to-targot fo i l distance. At Incident neutron energies different from the 13-15 HeV range the advantages and many of the potential problems wil l remain. The additional problem Is the lower Intensity of avlalable neutron sources. This problem might be solved by a variation of the spectrometer that would Increase Its efficiency from Its present *a 2 msr. Detectors of large area hilp In this regard. 2 The other solution Is patience wlttt long maasur1r.il times. \ s IK. Ccupirlson of ftisults MsuKi from tht Mgnttlc quidrupolt spictronitir cin bi compirid with ditJ fro* sivtrtl dlffirint typis of misurtmnts. «ctintlon: For (n.xp) md (n.xd) cross net Ions, om cin "akt conslstincy chocks with icttvitlon dito In « f«w cuts. For < 8 T I , tht (n,p), (n.n'p) ind (n,d) chinntls >rt optn i t En • 15 HoV. From •vilwtid ictlvitlon dot"7 tht cross stctlons trt "rKn.p) 4BSc • 65.1 mb 4 eritn.n'p + d) 47Se • 15,1 Totil 00.2 «* i t 15 HtV. This totil should tquil tht following sin of rtsults3 from tht qmdrupolt spictroMttr: 4BT1(n.xp) • 86 116 1* 4BTt(n,xd) • 7 1 3 Totil 92 + 16 Bb t t 15 r*V Tht igrttmnt 1s within irrori, A sicond ixwpll 1s for S , N1. From ictlvitlon8 5SNt(n,F) - £48 Ml ^Kn.n'p + d) • 693 nib Totil Ml rt> Tin unmwsund s8(H(n,Jp) cross sictlon would Incrtiu this totil only slightly. The (unstittd) uncirtilnclos 1n thtst numoirs i n diflnltoly lirge enough so thit thin 1s no dlsignimnt outsldi of trrori of this totil with tht sun of our misurad villus 9: 5BN1(n.xp) -1002 + 150 ^Nlln.xd) • l « t « Totll 1016 • 150 For ilphi-pirtlcli production our rtiults cm bt conpind ixictly with ictlvitlon diti for only oiw tirgot, 5 , V. From iviluittd ictl-vitlon d i t i 1 0 6 5 , V(n,a) • 21.0 a* ' M n . n ' a ) • 8.5 Total 27.5 mb at IS HeV Our result I s " ; V(n.xo) » 17 + 3 «* There Is see* disagreement here, part of which could be due to discrepancies In the activation measurements for ' V(n.o). Prelimi­nary results of helium accumulation experiments by H. Farrar' 2 tend to support our result. Helium Accumulation; Our (n,xn! data may be compared with (n,x He) data from mass spectrowtrtc measurements (Table 1). Our data appears to be 1n general agreement with the helium accumulation measure--*r iU although there 1s perhaps a slight systematic difference of about lOt with our results being lower. Charged Particle Measurements; The situation for 2 7 Al(n,xp) 1u s n a r l i e d In Table Z. Tin disagreements can be traced mostly to the fact thati 1n a l l previous measurements, the lower energy end of the protoi; spectrum was not measured well. This problem (s typical of most other targets. Theory; The charged-partlcle spectra can be compared with theoretical calculations to yield Information unavailable from act i ­vation or Helium accumulation data. Two such calculations ere shown in Figs. 3 and 4. The dramatic effect of second chance proton emission In 4 5 TI(n,xp) and Its absence 1n , 8 T(n,xp) are evident. IV. Summary of Results The results from the magnetic quadrupole charged particle spectro­meter are summarized 1n Table 3. These data were a l l taken at 15-KeV Incident neutron energy because of the Importance of neutrons 1n this range for fuston reactors. The target materials are those most l ikely to be used for structural elements In these devices. 7 For applications to fast fission reactors, the data In Table 3 and tht chargad-partlclt spectra published elsewhere should provide useful normalization points for nuclear reaction model calculations. With continuing Improvements In the acceptance of the spectrometer, we hope to extend these measurements directly into the energy range of fast fission reactors. NOTICE "Tit* NMft WM PMUIB M «(l MCOUIII Of M i l l I M M N l k¥ UM UftSMd SUM OomnMMl. NtMMi IN UitlM S U M iwr ih* Uniud Sunt DMWtntwt •* hmvt- KM «W of i tur •mHoy***. • M W t d (Mr «MUtcion, ttihtoniririori, or t, DMkM » y wtniHiy. wpm* or •ik-ihty (ar ih i iccutity, comil i i _. iMtrulMH trf Mir InfomHiM. tntui iu i , ftatuti or f r M M •iidBinJi or rafntMii tai l i n UH WIWM HM uninn privivtly-ownM tl|Mi," Rtrtrcnct to u company or product name d o t i not imply approval or recommendation of the product by the University o f California or the U.5. Department of I'tierj;)' m ihe nc lu t ion of o i h m thut may be luitabrc. e Tablt 1 Comparison of he)turn-production cross stctlons with (n.xa) cross sections nw«surcd with the magnetic quadrupole charged par t ic le spectrometer. The Incident neutron energy 1n a l l cases 1s near 15 MeV. Helium'*^ Material n.xq Production Al tt.i * 25 nib 140 mb T1 34 + 7 < b ) 39 V . 17 + 3 15 Fe 413 + 7 46 Hi 97 ± 1 6 97 Cu 4 2 ± 7 ( b * 5 1 , 5 4 + s ' e ) Nb 14 + 3 17 SS-316 48 ± 7 56 (a) Ref. 12 (preliminary results). (b) Inferred from Isotoplc data. (c) Ref. 13. Table 2 Comptrlson of data for 2 ' A l ( n , *P) at E n a 15 HeV & Zi mln Terset Thickness ing/cm* o(n,xp) Reference Hethod 14.1 2.2 7.5 140 16 Emulsion 14.1 2.0 6.9 191 ± 25 17 Telescope* 14.8 2.0 7.1 273 + 20 \0 Tetatopt 1 15.3 0.7 0.4 to 2.4 405 • 60 3 Quadrupole h Spectrometer e. Telescope consisted of two proportional aE counters and a CsI(T«) scinti l lator. b. Telescope consisted of two surface-barrier detectors 10 Tible 3 Sumriry of cross sections «t E_ * 15 HtV mtisurtd with the magnetic quidrupolt iptctruwter. The vilues are 1n units of rob. (n. 0 xp) An (n, 0 ,xd) is ( n , 0 _4p_ Kef. Al 405 60 19 8 121 25 3 «n 669 90 9 4 98 18 3 48, , 85 16 7 3 28 6 3 n a t T , l 132 21 7 3 34 7 . V 91 14 7 3 17 3 11 M r , 896 107 10 4 80 13 9 M F . 186 112 8 3 41 7 9 " " r e " 227 27 8 3 43 7 -" " F t 223 30 7.8 3 43 7 b S B N I 1002 120 14 6 106 17 9 M N 1 325 39 11 4 76 12 b " 'V 787 94 13 5 96 15 . nit,,, 794 95 13 5 97 16 b "<M 320 45 9 4 56 10 15 «CU 44 5 9.8 4 13.5 2.6 IS "«Cu* 237 28 9.6 4 42 7 -Nb 51 8 8 3 14 3 u SS 316 252 38 e 2 48 7 14 i . Inferred from the Isotopic do t o . 11 References 1. R. C. Hilght, S. H. Grimes, B. J. Tuckey and 0. D. Anderson, "ACharged-Fartlcle Kegnetlc-Quedrupole Spectrometer for Neutron-Induced Reactions," UCRl-77151, Lawrence Llvermore Laboratory report, (1975). 2. K. R. Alvar, 'H. H. Barschall, R. R. Borchers, S. H. Grimes and R. C. Height, Nucl. Instr. and Hath. (In press). 3. S. N. Grimes, R. C. Height and J. 0. Anderson, Had, Scl . Eng. S , 187 (1977*. 4. R. C. Height. S. H. grimes and 0. 0. Anderson, Rhys. Rev. C16, 97 (1977). 5. R. Booth and H. H. Berschsll , Nucl. Instr. and Hath. 99, 1 (1972). 6. R. H. Lessltr , e d . , WRENOA 76/77. Uorld Newest H i t for Unclear Data. iNDC (SEC)-SS/UR5F, IAEA (Vienna) 1976. 7. B, A. Hagurno, e d . , EHDF/B-IV Dosimetry File BNI-NCS-50M6, Brookhivan National Laboratory (T97S). 8. P. Guenther, A. Smith, D. Smith, J. Uhalen and R. Howerton, "Measured and Evaluated Fast Neutron Cross Sections of Elemental Nickel," ANL/NDM-11 Argonne Nettonel Laboratory report (1975). 9. K. R. Alver, R. R. Borchtrs, S. H. Brines and R. C. Height, Bull. Am. Phys. Sue. 22 , 992 (1977). 10. P. Guanther. D. Havel, R. Htwerton, F. Hann, D. Smith, A. Smith end J. Uhalen, "Fast Neutron Cross Sections of Vanadium and an Evaluated Neutranlc Fi le ,* ANl/NDH-24, Argonne National Laboratory (1977). 11. S. H. Crimes, R. C. Height an. . . D. Anderson, Phys. Rev. c ( In press). 12. H. Ferrer (prlvete communication). 13. J. B. Holt, 0. U. Hosmer, and R. A. Van Konynenburg, Int. Conf. on Rad1et1o* Effects and Tritium Technology, GatHnfcurg, Ttnn., Conf. 750989, vol . I I , 280 (1975). 12 14. R. C, Height, 5. H. Grimes and J . 0. Anderson, (fuel. Set. t n j . 63, 200 (1977). 15. S. H. Crimes, P.. C. Height, J . 0. Anderson, K. B, Alvir end R. R. Borchers, M l . Am. Phys. Soc. 22, 631 (1977). 16. 0. L. Allen, Proe. Phys. Soc. (London) A70, 19S (1957). 17. K. R. Alver, Unci. Phys. A195, 289 (1972), 18. R. N. Glover ind E. Udgold, Nuel. Phyt. 24, 630 (1961). Figures 1 . Two methods of measuring (n.charged particle! reactions: (« ! traditional ipproich with a datictor telescope! (b) new method empToyfnp i magnetic quadrupole muttlplet 1ms to transport the charged particles from tht target fo i l to the detector. 2. Raw dace for protons from i 2.4 mg/cm aluminum fo i l (circles) compared with target-out bickground (» 's ) . Ihe dita hivi not btan corrected for the energy-depindent icceptince of the spectrometer at this particular magnet setting. 3. Angle-ewraged proton spectrum from bombardment of * T1 with 15-MeV neutrons. 4. Angle-averaged proton spectrum from bombardment of 4*T1 with 15-HeV neutrons. Ntutron lourot Tirfltt Moll Chargtd pirticlM Dittclor ' llltKOpf Migmtlo quadrupolt trlptit lini FI9. 1 1.0 1.5 2.0 E -MeV Al (n.Xp) B = 45° or Acceptance 0 g ISSS^f— x-^-x-^^^xn^iif x'^ic'^P^'- • I I •25 -20 15 10 20 40 60 Channel number 80 I FtJ. 2 200 100 T — i — i — i — r ~ l 6T1 (n. xp) 10 12 14 Fig. 3 10 -1 1 1 1 1 1 _ _ - _ _ W H (n. «P)-Jr ^\f" 1 ^ ^ l ™ •X — ___ 1 \ "*— ,„, 1 ~ \ ~ 0.1 1 1 1 1 1 1 0 2 4 6 10 12 14 Fig- 4 

Experimental studies of (n, charged particle) cross sections, angular distributions and spectra with a magnetic quadrupole spectrometer

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Experimental studies of (n, charged particle) cross sections, angular distributions and spectra with a magnetic quadrupole spectrometer

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