Measurement of 14.7 MeV neutron-activation cross sections for fusion

Ann. nucl. Energy, Vol. 14, No. 9, pp. 489-497, 1987

0306-4549/87 $3.00+0.00 Pergamon Journals Ltd

Printed in Great Britain

M E A S U R E M E N T OF 14.7 MeV N E U T R O N - A C T I V A T I O N CROSS SECTIONS F O R F U S I O N J. W. MEADOWS, D. L. SMITH, M. M. BRETSCHERand S. A. Cox Applied Physics Division, Argonne National Laboratory, Argonne, IL 60439, U.S.A. (Received 5 December 1986)

Abstract--The activation method has been used to measure 14.7 MeV cross sections for the following 21 reactions which are important for fusion-energy applications : 7Li(n, n't)4He, 27A1(n,p)27Mg, 27Al(n,~)24Na, Si(n,X)2SA1, Ti(n, X)46Sc, Ti(n, X)47Sc, Ti(n, X)4gSc, 5W(n,p)51Ti, 5W(n,ct)4gSc, Cr(n, X)s2V, 55Mn(n, 2n)54Mn, 54Fe(n,ct)SICr, Fe(n, X)56Mn, 59Co(n, p)59Fe, 59C0(n,2n)58C0, 59C0(n,~)56Mn, 5SNi(n,2n)57Ni,65Cu(n, p)65Ni, 65Cu(n, 2n)64Cu, Zn(n, X)e4Cu, and 64Zn(n,2n)63Zn. The results of this investigation are compared with corresponding recently evaluted cross-section values. Within the combined errors of the measured and evaluated results, the agreement is generally quite acceptable although some problem areas remain to be resolved. Significant improvements in the knowledge of the cross sections are achieved as a result of the present investigation for the reactions Ti(n,X)47Sc, S9Co(n,p)59Fe and 65Cu(n, p)65Ni. In the case of 7Li(n, n't)4He, the present measurement supports a recent evaluation in which a number of other n + 7Li processes were simultaneously considered in conjunction with neutron-induced tritium production. !. INTRODUCTION

It is generally accepted that the first generation of fusion-energy reactors will be based u p o n the T(d, n)4He reaction. Neutrons produced by d - T collisions in a fusion plasma have primary energies in the 14-15 MeV range. Consequently, knowledge of neutron-interaction cross sections at these energies is essential for fusion conceptual-design studies. The present investigation concentrates on the following 2 1 reactions : 7Li(n, n't)4He, 27Al(n, p)27Mg, 27Al(n, g)24Na, Si(n,X)28A1, Ti(n,X)46Sc, Ti(n,X)47Sc, Ti(n, X)48Sc, 5W(n, p)51Ti, 5IV(n, g)48Sc, Cr(n, X)sW, "Mn(n, 2n)S4Mn, 54Fe(n, ~)51Cr, Fe(n, X)56Mn, S9Co(n, p)59Fe, S9Co(n, 2n)58Co, ~9Co(n, ct)56Mn, SSNi(n, 2n)57Ni, 65Cu(n, p)65Ni, 6SCu(n, 2n)64Cu, Zn(n,X)64Cu, and 64Zn(n,2n)63Zn. These reactions are of considerable importance for tritium fuel breeding, radiation-damage and neutron-dosimetry applications. Cross-section requirements for these reactions have been compiled (e.g. W R E N D A 83/84, 1983 ; DOENDC, 1984 ; Cheng et al., 1984 ; 1985). A large body of relevant neutron-activation crosssection data in this energy range is available in the literature (e.g. CINDA-A, 1935--1986 ; CSISRS, 1986) due to the widespread availability of 14 MeV neutron generators. However, knowledge of the pertinent cross sections is often confounded by the presence of large discrepancies between the various reported results. It is the opinion of the authors that in order to

resolve this dilemma it is necessary to proceed in the following way : (i) The existing data have to be evaluated in a rigorous and consistent manner, taking into consideration contemporary knowledge of the relevant fundamental constants of the measurements (e.g. radioactive decay properties). (ii) New measurements need to be performed in order to test the predictions of these evaluations, and to refine the knowledge of the cross sections in those instances where the current knowledge, as reflected in contemporary evaluations, is clearly inadequate. F o r those reactions considered in the present investigation, the first requirement appears to have been satisfied as a result of recent efforts at this laboratory (Evain et al. 1985) and elsewhere (Hayes and Ryves, 1981 ; Young, 1981 ; Winkler and Ryves, 1983 ; Winkler et al., 1983). The present investigation has been conducted to satisfy the second requirement, namely the provision of new experimental data to compare with the corresponding evaluations. 2. EXPERIMENTAL PROCEDURES

The measurement and data-processing procedures employed in this work are extensively documented (Smith et al. 1984; Meadows and Smith, 1984 and other references cited therein) so they will be outlined briefly here, with emphasis on the provision of information of specific importance to the evaluation of cross-section results. Important properties of the nuclear reactions con-

489

490

J.W.

MEADOWS et al.

Table 1. Activation reactions considered in the present experiment Sample material

Activity

Lithium-7 Aluminum Silicon

Tritium 24Na 27Mg 28A1

Titanium

46Sc

47Sc

48Sc

Vanadium Chromium

48Sc 5LTi 52V

Manganese Iron

54Mn 51Cr 56Mn

Cobalt

56M n 58Co 59Fe STNi 64Cu ~SNi 63Zn ~Cu

Nickel Copper

Zinc

Reaction ~

7Li(n, n't)4He 27Al(n, ~)24Na 27Al(n, p)27Mg [ ZsSi(n, p)28All 29Si(n, d)28A1 29Si(n, np)28Al [46Ti(n, p)46Sc] 47Ti(n, d)46Sc 47Ti(n, np)46Sc 48Ti(n, t)46Sc [47Ti (n, p) 47Sc] 4~Ti (n, d)47Sc 48Ti(n, np)475c 49Ti (n, t)478c l~Ti (n, p) 48Sc] 4~)Ti(n, d)485c 49Ti(n, np)48Sc 5°Ti(n, t)48Sc 5~V(n, ~)48Sc 5tV(n, p)51Ti [52Cr(n, p)52V] ~3Cr(n, d)52V 53Cr(n, np)S2V ~Cr(n, t)52V 55Mn(n, 2n)54Mn ~4Fe(n, ct)5~Cr [56Fe(n, p)56M n] 57Fe(n, d)56Mn 57Fe(n, np)SrMn 58Fe(n, t)56Mn 59Co(n,~t)56Mn 59Co(n, 2n)58Co 59Co(n, p)59Fe 58Ni(n, 2n)57Ni 65Cu(n, 2n)64Cu 65Cu(n, p)6~Ni e4Zn(n, 2n)63Zn [64Zn(n, p)64Cu] 66Zn(n, t)64Cu

Isotopic b abundance (%)

Q" (MeV)

Not applicable d 100

-

92.23 4.67 8.2 7.4 73.7 7.4 73.7 5.4 73.7 5.4 5.2 99.75 83.79 9.5 2.36 100 5.8 91.8 2.15 0.29 100

68.3 30.8 48.6 27.9

2.468 3.132 1.827 3.861 - 10.111 - 12.335 - 1.585 - 8.240 - 10.464 - 13.611 + 0.181 -- 9.223 - 11.447 - 11.108 - 3.208 - 9.126 - 11.350 - 13.813 -2.055 - 1.684 -3.194 - 8.910 - 11.134 - 12.371 - 10.227 +0.843 - 2.914 - 8.336 - 10.560 - 12.123 + 0.327 - 10.453 -0.783 -- 12.219 - 9.908 - 1.356 - 11.861 + 0.204 - 10.354

aThe dominant reaction is marked with brackets [...] when more than one reaction contributes to production of the observed activity. bLederer e t al. (1978). CTuli (1985). aThe lithium material used in this experiment is enriched to > 99.9% in 7Li. In natural lithium the relative abundance of 6Li and 7Li has been observed to vary considerably from one batch of material to another.

sidered in this work are listed in Table 1. Except for 7Li(n, n't)4He, the measurements of this work have been performed using elemental samples. No attempt is made to distinguish the various contributions from isotopic-component reactions when more than one reaction is involved. For example, Ti(n, X)46Sc designates *6Sc production from neutrons on elemental titanium. It consists of contributions from the isotopic reactions 46Ti(n,p)46Sc (the dominant reaction), 47Ti(n, d)465c,

47Ti(n, np)46Sc

and

48Ti(n,t)46Sc.

The

convention is adopted here of reporting effective isotopic cross sections that correspond to those isotopes which contribute most to production of the observed activities, e.g. 46Ti in this example. This particular cross-section normalization convention is commonly

used in applications, especially in neutron dosimetry. Conversion of these isotopic cross sections to elemental ones is readily accomplished using the information provided in Table l, where the dominant reactions are clearly labelled. Knowledge of the decay properties of the reactionproduct nuclei is essential for the determination of activation cross sections. The pertinent decay parameters are listed in Table 2, and additional information required for estimating uncertainties is available in the references cited in this table. The samples used in this work were chemically pure so there was negligible interference from any contaminants. The lithium samples were disks of lithium metal sealed in thin-walled, air-tight aluminum cap-

Measurement of neutron-activation cross sections

491

Table 2. Nuclear-decay properties of the reaction products encountered in the present experiment Tritium a Half life: 12.35+0.04 yr Decay mode : fl minus emission (100%) 24Nab Half life: 15.030_+0.003 h Decay mode : fl minus emission (100%) y-ray emission : 1.369 MeV (100%) 2.754 MeV (100%)

27Mgb

Half life : 9.462 -+ 0.012 min Decay mode: fl minus emission (100%) y-ray emission : 0.844 MeV (73%) 1.014 MeV (29.1%) 0.171 MeV (0.9%) ~SAlb Half life : 2.2405 + 0.0003 min Decay mode : fl minus emission (100%) y-ray emission: 1.779 MeV (100%)

46Sch

Half life: 83.80-+0.03 day Decay mode : fl minus emission (100%) y-ray emission: 0.889 MeV (100%)

475cb

Half life : 3.422 -+ 0.004 day Decay mode : fl minus emission (100%) y-ray emission : 0.159 MeV (68.5%)

488cb

Half life : 43.67-+0.09 h Decay mode : / / m i n u s emission (100%) y-ray emission : 0.984 MeV (100%) 1.037 MeV (97.5%) 1.312 MeV (100%) SlTib Half life: 5.80-+0.03 min Decay mode : fl minus emission (100%) y-ray emission: 0.320 MeV (93.4)% 5,CrC Half life : 27.700 -+ 0.011 day Decay mode: electron capture (100%) y-ray emission : 0.320 MeV (9.83%)

52vb

Half life : 3.746 + 0.007 min Decay mode : fl minus emission (100%) y-ray emission : 1.434 MeV (100%)

~Mn b Half life : 312.20+0.07 day Decay mode: Electron capture (100%) y-ray emission : 0.835 MeV (100%) 56Mnb Half life : 2.5785 + 0.0006 h Decay mode : # minus emission (100%) y-ray emission : 0.847 MeV (98.87%) 57Nib Half life : 35.99_+0.12 h Decay mode : Electron capture (60%) Positron emission (40%) y-ray emission: 1.378 MeV (77.6%) 0.511 MeV annihilation radiation (0.8 photons per decay)

58Cob

Half life : 70.87_+0.07 day Decay mode : Electron capture (85%) Positron emission (15%) y-ray emission : 0.811 MeV (99.44%) 0.511 MeV annihilation radiation (0.30 photons per decay) 59Feb Half life : 44.56--+0.03 day Decay mode : fl minus emission (100%) y-ray emission: 1.099 MeV (56.5%) 1.292 MeV (43.5%) 63Znb Half life: 38.0-+0.1 m Decay mode : Electron capture (7%) Positron emission (93%) y-ray emission : 0.670 MeV (8.4%) 0.511 MeV annihilation radiation (1.86 photons per decay) 64Cud Half life: 12.698-+0.002 h Decay mode : fl minus emission (39.04%) Electron capture (43.10%) Positron emission (17.86%) y-ray emission : 0.511 MeV annihilation radiation (0.3572 photons per decay) 65Nib Half life : 2.520 -+ 0.002 h Decay mode : fl minus emission (100%) y-ray emission: 1.482 MeV (23.5%)

aSmith et al. (1984). bLederer et al. (1978). eHelmer (1978). dChristmas et al. (1983).

sules. The lithium material was enriched in 7Li to 7Li : 6Li isotopic ratio of 1511 + 5. Details on these lithium samples appear in Smith et al. (1981). All the other samples used in this experiment were elemental metal disks with natural isotopic abundances. Neutrons with average energy of 14.74_+0.02 MeV a

were produced

by bombarding

a TiT target with deu-

from a Texas Nuclear Generator (Model 9400) operated at 150+10 kV. The neutron-energy resolution (FWHM) was estimated to be 0.324 MeV. Individual samples were attached to a low-mass fission chamber which contained a depleted-uranium (essentially 100% 23sU) deposit. This assembly was placed close to the target (sample distances in the range 4-9 cm) at zero degrees for the irradiations, as terons

shown in Fig. 1. The deuteron beam from the generator was not analyzed before impinging on the TiT target, and consequently it consisted of unknown mixtures of D + and D-2 + ions. However, various tests were conducted, as described by Smith et al. (1984), in order to determine the effect of the D-2 + ions. The results indicate that the D-2 + component contributed negligibly to the neutron production in this experiment. Likewise, the effect of neutron production from deuterons impinging on deuterium which builds up in the target was also negligible. Tritium produced in the neutron-irradiated lithium samples was extracted in a vacuum furnace and then combined with oxygen to form water, along with pure hydrogen carrier gas, according to an isotopic-

492

J.W. MEADOWSet al. ALUMINUM TARGET CASING

TARGET

AIR IN

GAS IN

TO pREAHP

\

NEUTRONS

f CENTERING PLATE

L

NYLON NUT

COPPER STEEL RETAINER

BACKING

AIR OUT

TARGET ASSEMBLY

GAS OUT

FISSION CHAMBER

Fig. 1. Schematicdiagram of the TiT target assemblyand fission-detectorapparatus used for irradiations in the present experiment.

dilution method described by Smith et al. (1981). The tritium beta activity was then measured by a liquidscintillation counting method described by Smith et al. (1981; 1984), using the NBS tritiated-water standard for calibration purposes (Unterweger et al., 1980). All other types of samples were 7-ray counted according to procedures described in detail by Meadows and Smith (1984). The experimental data were corrected for various effects including geometry factors, neutron absorption, neutron multiple scattering and details of the neutron emission from the source, as described by Smith et al. 1984). The quantities actually measured in this experiment are activation cross-section ratios relative to 23sU neutron fission. In order to relate the present results to the better known 235U neutronfission cross-section standard, a fission-rate ratio measurement was made for the present depleted-uranium deposit relative to a very-well-calibrated uranium deposit enriched in 2~5U (Poenitz et al., 1979), according to the method described by Smith et al. (1981). This approach offered the additional advantage of

avoiding difficulties associated with determining the absolute mass of the depleted-uranium deposit. The entire experimental process was examined in detail in order to identify the principal sources of experimental error. These errors were grouped under the categories described in Table 3. Estimated values of specific error components for all the reactions are listed in Table 4. Total experimental errors are obtained by combining all these components in quadrature. These errors are given in Table 5. The systematic error components are correlated between reactions to varying degrees. Information on these correlations is provided in the footnotes to Table 3. A correlation matrix for the uncertainties of the entire experimental data set can be readily generated using the information available in Tables 3 and 4, according to methods described by Smith (1981). The result of this analysis is provided in Table 6. 3. R E S U L T S AND C O N C L U S I O N S

The results of the present experiment are presented in Table 5. These experimental cross-section values

Measurement o f neutron-activation cross sections Table 3. Sources of experimental error Source

Magnitude (%)

Random errors Sample properties Geometric effects Activity measurements

Na-1.0 0.2 0.1-4.4

Systematic errors b Decay half-lifec Decay branching factors c Calibration of activity-measurement apparatus a Fission counting statistics° Fission-monitor calibration Standard 2~U neutron-fission cross-section Neutron-source properties Neutron absorption and multiple scatteringf Geometric effectsr

N~).5 N-4.0 0.6-1.8 0.1-1.3 1.4 4.0 0.2 0.2-1.8 0.2-1.0

aN = negligible. bUnless otherwise indicated the uncertainties in each systematic category are 100%-correlated for all the reactions. Cl00%-correlated for reactions yielding the same product and uncorrelated otherwise (see Table 1). aNo correlation for reaction 1 (see Table 4); 50%-correlated for reactions 4, 5, 7, 11, 12, 14, 15, 17, 18 and 20; 75%-correlated otherwise. °100%--correlated for reactions (5, 6 and 7), (14 and 15), (18 and 19), (20 and 21) (see Table 4) ; uncorrelated otherwise. f75%-correlated for reactions involving the same sample material or the same reaction product ; 50%-correlated otherwise.

are based on the ENDF/B-V (1979) value of 2101 ( _ 4%) mb for the 235U neutron-fission cross-section standard. Comparison is also made in Table 5 between the measured values and recent evaluated results.

493

Experimental values from the literature (CINDA, 1935-1986) are not generally discussed here because the volume of information is very large and it is considered to be well represented by the listed evaluated results. As indicated in Section 1, the objective of the present experiment was the provision of a new set of measured values to be compared with contemporary knowledge of the cross sections, as embodied in recent evaluations. In order for such comparisons to be meaningful, there should be a quantitative measure of the degree of agreement or disagreement. For this purpose, ratios of the measured-to-evaluated (M/E) results have calculated. It is assumed that for a particular reaction all of the M/E values which might have been obtained in this work are distributed according to a normal distribution which is centered on unity and possesses a standard deviation (la) formed by combining the experiment error and evaluation error in quadrature. The results of this analysis appear in Table 5. Figure 2 summarizes the information from Table 5 in a readily-comprehended graphical form. The measurement precision of the present experiment is of moderate proportions so it could not be expected that the results of a single new measurement would have much impact on refining the knowledge of the cross section in any instance where the cross section was considered a priori to be quite well known,

Table 4. Specific error components

Reaction 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.

7Li(n, n't)4He 27Al(n, p)27Mg 27Al(n, ~t)UNa Si(n, X)~A1 Ti(n, X)'~Sc Ti(n, X)47Sc Ti(n, X)~Sc 5IV(n, p)51Ti 5IV(n, ~t)4SSc Cr(n, X)52V 55Mn(n, 2n)~Mn ~4Fe(n, ct)SICr Fe(n, X)~Mn 59Co(n, p)59Fe 59Co(n, 2n)SSCo 59Co(n, ct)~Mn 5SNi(n, 2n)57Ni 65Cu(n, p)65Ni 65Cu(n, 2n)~Cu Zn(n,X)UCu 64Zn(n, 2n)63Zn

Random" error (%)

S1

$2

$3

Systematic errors (%)b $4 $5 $6

$7

$8

$9

1.2 2.6 3.2 1.4 1.8 2.0 3.8 2.2 0.8 2.9 0.5 1.5 4.5 0.6 0.6 1.1 3.0 1.4 1.5 1.9 1.5

0.3 0.1 0.2 N N 0.1 0.2 0.5 0.2 0.2 N N N N 0.1 N 0.3 0.1 0.1 0.1 0.3

NAc 1.4 Nd N N 4.0 0.3 1.0 0.3 N N 1.4 N 2.6 N N 1.0 0.3 0.8 0.8 0.8

0.7 0.6 0.6 1.3 1.8 0.6 0.7 0.6 0.6 0.6 0.8 1.1 0.6 0.8 0.8 0.6 0.9 1.0 0.6 1.6 1.6

0.1 0.9 0.4 l.l 0.1 0.1 0.1 0.7 0.2 1.3 0.2 0.3 0.5 0.2 0.2 0.4 0.2 0.1 0.1 0.4 0.4

0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2

0.5 0.2 1.0 0.2 0.6 0.2 0.9 1.0 1.4 0.6 1.3 1.2 1.0 1.0 1.6 1.2 1.8 0.3 1.5 0.5 1.7

1.0 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.5 0.5 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2

aAll sources of random error from Table 3 are combined in quadrature. bColumn headings correspond to the categories listed in the same order in Table 3. °NA = Not applicable. aN = Negligible ( < 0 . 1 % ) .

1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4

4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0

494

J. W. MEADOWS e t al. Table 5 Results Measurement (M) a Cross Error section (mb) (%)

Reaction 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.

7Li(n, n't)4He 27Al(n, p)27Mg 27Al(n, ~)24Na Si(n, X)2SAI Ti(n, X)46Sc Ti(n, X)*7Sc Ti(n, X)~Sc SIV(n, p)51Ti 5IV(n, ~)~Sc Cr(n, X)52V S~Mn(n, 2n)~Mn ~Fe(n, ~)SICr Fe(n, X)56Mn 59Co(n, p)59Fe 59Co(n, 2n)58Co SgCo(n, ~)~6Mn 58Ni(n, 2n)57Ni 65Cu(n, p)65Ni 65Cu(n, 2n)64Cu Zn(n, X)64Cu 64Zn(n, 2n)63Zn

295.0 63.88 109.4 237.7 g 273.4 g 260.8 g 58.89 g 28.10 16.68 74.46 g 765.1 92.44 109.9g 44.80 754.3 30.81 38.35 19.16 924.0 164.8g 175.8

4.6 5.3 5.5 4.8 5.0 6.2 5.8 5.1 4.6 5.4 4.5 5.0 6.3 5.2 4.6 4.6 5.7 4.6 4.9 5.0 5.2

Evaluation (E)b Cross Error section (mb) (%)

M/E ~

Probability d of M/E (%)

292.0 e 70.46 113.4f 257.3 g 294.8 j 223.18 60.61 g 31.87 15.89 72.49 g 816.7 87.86 107.2r'~ 56.60 747.7 30.17 38.80 h 20.46 967.8 r 165.4g 162.2

1.010 0.907 0.965 0.924 0.927 1.169 0.972 0.882 1.050 1.027 0.937 1.052 1.025 0.792 1.009 1.021 0.988 0.936 0.955 0.996 1.084

>50 10 > 50 16 18 36 >50 8 34 >50 23 36 >50 22 > 50 >50 >50 45 37 >50 14

4.4 ~ 2.0 0.6 f 2.5 2.3 17.2 2.4 4.4 2.4 4.4 2.6 2.7 0.6 f 16.2 2.4 1.4 1.7h 7.0 1.2f 2.6 2.4

~Present results. bThe evaluated results are from Evain et al. (1985) except where otherwise indicated. CM/E is the ratio of measured-to-evaluated cross-section results based on values from this table. dStatistical probability of observing an M/E which deviates from unity by at least the indicated amount, assuming that the possible values of M/E are normally distributed around unity in a manner consistent with a standard deviation (one sigma) which is formed by combining the experiment error and evaluation error in quadrature (Hodgman et al., 1959). Large probability implies good M/E consistency. eYoung (1981). fHayes and Ryves (1981) and Winkler and Ryves (1983). 8Cross section is quoted as an isotopic cross section, corresponding to that isotope which contributes most to the observed reaction-product yield, as indicated in Table 1. hWinkler et al. (1983).

Table 6. Data correlation matrixa

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

1 0.76 0.74 0.85 0.83 0.65 0.70 0.79 0.88 0.75 0.90 0.83 0.64 0.79 0.88 0.88 0.73 0.88 0.84 0.82 0.81

1 0.64 0.74 0.72 0.56 0.60 0.68 0.76 0.65 0.77 0.70 0.55 0.67 0.75 0.76 0.62 0.76 0.72 0.71 0.69

1 0.71 0.70 0.54 0.59 0.67 0.76 0.63 0.77 0.70 0.54 0.67 0.75 0.75 0.62 0.74 0.72 0.69 0.69

1 0.81 0.63 0.67 0.77 0.85 0.72 0.86 0.79 0.62 0.75 0.84 0.85 0.69 0.85 0.81 0.79 0.80

1 0.61 0.66 0.75 0.84 0.71 0.84 0.77 0.61 0.74 0.83 0.84 0.68 0.83 0.80 0.78 0.80

1 0.51 0.58 0.65 0.55 0.66 0.60 0.47 0.58 0.64 0.64 0.53 0.65 0.61 0.60 0.59

1 0.63 0.72 0.59 0.71 0.65 0.51 0.62 0.70 0.70 0.58 0.69 0.67 0.64 0.65

1 0.83 0.68 0.82 0.75 0.58 0.71 0.81 0.80 0.67 0.79 0.77 0.74 0.74

1 0.76 0.92 0.84 0.65 0.80 0.91 0.90 0.75 0.88 0.87 0.83 0.84

1 0.77 0.70 0.55 0.67 0.75 0.75 0.62 0.75 0.72 0.70 0.69

1 0.84 0.66 0.81 0.92 0.92 0.76 0.89 0.88 0.83 0.86

1 0.62 0.73 0.83 0.83 0.69 0.81 0.80 0.76 0.79

1 0.58 0.65 0.66 0.54 0.64 0.62 0.60 0.60

1 0.81 0.81 0.66 0.78 0.76 0.73 0.74

1 0.93 0.75 0.87 0.87 0.81 0.85

1 0.75 0.88 0.86 0.82 0.83

alndices coincide with reaction labels used in Tables 4 and 5.

17

18

19

20

1 0.72 1 0.72 0.84 1 0.67 0.82 0.82 1 0.70 0.82 0.80 0.80

21

1

Measurement of neutron-activation cross sections

0.7

0.8

0.9

I

I

I

1.0

t.1

1.2

1.3

I

I

I

I

L i - 7(n,n't) He-4

495

i

AL-27(n,p) Mg-27

~

I~

I

AL-27(n,atpho) N0-24

J.,

I

Si(n,X) AI.-28

~

I

TI (n,X)Sc-46

~

I

Ti(n,X) Sc-47 Ti(n,X)Sc-48

~

V-51 (n,p) T1-51

I

~ll

I

II

V-51(n,aLpha ) Sc-48

/

i

I

~/

Cr (n,X) V-52

!

Mn-55(n,2n) Mn-54 Fe --54(n,atpho) Cr-51

lJJlI

I

1

Fe (n,X)Mn-56

I

/

~

I

I

I

/

Co-59(n,p) Fe-59 ~

I / -i,,,

Co-59(n,2n) Co-58 Co-59(n,ol.pha) Mn-56

I

/

I

Ni--58(n,2n) Ni-57

I "L

I

Cu-65(n,p) NI-65 Cu-65(n,2n) Cu-64

/

Zn (n,X) Cu-64

I

Zn-64 (n,2n) Zn-63 I

ll~

/

I ~ I

I

/

I I

I I

I

Fig. 2. Graphical comparison of the measured and evaluated neutron-activation cross sections listed in Table 5. The solid bars represent evaluated results while the open ones designate experimental values. Uncertainties in these quantities are indicated by the widths of the bars. All results are plotted as ratios to corresponding evaluated values. Consequently, the solid bars are all centered on unity. The vertical lines in the center of the open bars thus indicate the ratios (M/E) of measured-to-evaluated cross sections found in the present work.

and consequently where the evaluated result bears a small error. The following five reactions appear to fall into this category, which is designated here as Category A : 27Al(n,0024Na, Fe(n, X)56Mn, 59Co(n,ct)56Mn, 5SNi(n,2n)57Ni and 65Cu(n, 2n)64Cu. F o r each of these reactions, the present results are found to be very consistent with the corresponding

evaluated values, as indicated by the statistical tests on M/E. This in turn supports the contention that there are no serious systematic errors present in either this experiment or in the standard cross section upon which the present cross-section results are based. The next class of reactions, designated as Category B, consists of those reactions where the evaluated

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values are moderately-well known, and they agree reasonably well with the present experimental results. The following seven reactions are considered to fall into this category : 7Li(n, n't)aHe, Ti(n, X)48Sc, 51V(n, ~t)48Sc, Cr(n, X)52V, 54Fe(n, ~t)S~Cr, 59C0(n, 2n)SSCo and Zn(n, X)64Cu. For these reactions, the present experimental results add substantial support to the conclusions of the corresponding previous evaluations. Because it is of particular importance to fusionenergy technology, and it has been the subject of much controversy during the past decade, some discussion of the 7Li(n, n't)4He reaction is in order here. Smith et al. (1984) review the situation prior to the present investigation. The present experimental cross-section result has been found to be in very good agreement with the evaluated result of Young (1981). This is a significant finding since that evaluation simultaneously considered a number of reaction channels for 7Li. This procedure avoided excessive dependence of the evaluated results on the data base for 7Li(n, n't)4He, a collection of rather discrepant information when viewed alone. The experimental result reported here for this reaction corresponds to the same measurement reported by Smith et al. (1984). The difference in the final values is due to a change of the neutron-fission cross-section standard from 238U to 235U. Recently, Goldberg et al. 0985) reported a new cross-section value of 302+ 18 mb at 14.94 MeV for this reaction, a result which is in quite good agreement with the present work. It would appear that the long-standing controversy over this reaction is close to being resolved. The following three reactions form a group which is designated here as Category C : Ti(n, X)47Sc, 59Co(n,p)S9Fe and 65Cu(n,p)65Ni. The evaluated results in this category carry large uncertainties owing either to sparse data bases or to the existence of serious discrepancies. Here it is believed that the present results contribute substantially to improving the knowledge of the cross sections. Nevertheless, further experimental work will be required in order to resolve the discrepancies and thereby reduce the cross-section uncertainties. The remaining six reactions comprise Category D : 27Al(n, p)27Mg, Si(n, X)28AI, Ti(n, X)46Sc, 51V(n, p)5~Ti, 55Mn(n, 2n)54Mn and 64Zn(n, 2n)63Zn. For these, the results of the present work appear to be at odds, to varying degrees, with corresponding evaluated results of good accuracy, although in no instance does any M/E value deviate from unity by as much as two standard deviations (2~r). For this category, then, the outcome of the present investigation appears to be inconclusive. Further study of

Table 7. Examples of some cross-section data requests which are pertinent to the present investigationa

Reaction 7Li(n, n't)4He 27Al(n, p)27Mg 27Al(n, ct)24Na Si(n, X)28A1 Ti(n, X)46Sc

Ti(n, X)475c Ti(n, X)4SSc 5IV(n, p)51Ti 5IV(n, ct)4SSc Cr(n, X)52V 55Mn(n, 2n)~Mn ~Fe(n, ~t)51Cr Fe(n, X)56Mn 59Co(n, p)59Fe 59Co(n, 2n)SSCo 59Co(n, ~)56Mn 5SNi(n, 2n)57Ni 65Cu(n, p)65Ni 65Cu(n, 2n)~Cu Zn(n, X)64Cu ~Zn(n, 2n)63Zn

Requested accuracies (%)b WRENDA 83/84c DOENDC d Cheng~ 5 10 10 20 20 5 20 10 15 10 5 5 10 5 -20 10 15 20 5 --

3 10 10 5 20 20 20 20 20 10 -10 20 20 -10 10 20 -5 --

3f NS NS NS NS NS NS --~ NS -NS NS NS NS NS NS NS NS NS NS NS

"Requests for 14-MeV cross-section data which either correspond to or are closely related to the quantities considered in the present work. bWhen multiple requests are documented, the indicated accuracy request is the most stringent one. CWRENDA 83/84 (1983) dDOENDC (1984). "Cheng et aL (1984). fCheng et al. (1985). gSymbol " - - " indicates that no request is documented for this reaction.

some of these "problem" reactions thus seems to be justified. A question of crucial importance to the formulation of future research policy in this area emerges from this investigation: To what extent is it necessary to invest further measurement and evaluation effort in order to refine the knowledge of the 14 MeV cross sections considered in this work, and others of a similar nature? This is a complex question which is not likely to be answered in an entirely objective manner. Attainment of cross-section accuracy objectives has in the past proved to be quite elusive because the requirements for applications seem to change periodically toward higher precision. They always seem to range well ahead of what has already been achieved or what appears to be attainable in the near future. If, for example, it were decided at present that a blanket 10%-accuracy level were adequate, then from Table 5 and Fig. 2 it is evident that the current state of affairs is satisfactory for all but three of the reactions considered in this work, namely Ti(n,X)47Sc, 5IV(n, p)5~Ti and 59Co(n, p)59Fe. Turning to the crosssection requirements stated in W R E N D A 83/84

Measurement of neutron-activation cross sections (1983), D O E N D C (1984) and Cheng et al. (1984; 1985), one finds the situation which is summarized in Table 7. Cheng et al. (1984; 1985) do not indicate desired accuracies. However, based on the other two request compilations, it appears that the requests have been satisfied, or nearly so, for fifteen out of 21 o f the reactions investigated here (about 70%). It should be emphasized that the specific purposes for the requested data have to be kept in mind when discussing the accuracies. The accuracies required for dosimetry purposes are generally greater than those for radiation-damage and component-activation purposes. Finally, Table 7 by no means reflects all of the requests for nuclear data which have been expressed by the nuclear-data community. It is clear from the present investigation that there is a need for further study of 14 MeV activation reactions. However, investigators who intend to embark upon a research program in this area of work ought to examine the existing data bases very carefully, consider the applied needs for the data they intend to measure, and then carefully assess their ability to provide new data of sufficiently-good accuracy to make an impact. Acknowledgements--This work was supported by the U.S. Department of Energy, Nuclear Energy Programs, under Contract W-31-109-Eng-38.

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