Neutron activation cross sections at 14.7 MeV for isotopes of tungsten

2.A.1

I

Nuclear Physics A242 (1975) 317--322; (~) North.HollandPublishing Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher

N E U T R O N A C T I V A T I O N C R O S S S E C T I O N S AT 14.7 MeV F O R I S O T O P E S OF T U N G S T E N S. M. QAIM and C. G R A C A y

Institut fiir Nuklearchemie der Kernforschungsanlage Jiilich GmbH, D-517 Jiilich, Federal Republic of Germany Received 29 November 1974 Abstract: Cross sections for (n, 2n), (n, p), (n, ~) and [(n, n ' p ) + ( n , pn)] reactions at 14.7+0.3 MeV on enriched 1so. ~s2. ~s3, ~s4, is6 w isotopes have been measured by the activation technique using Ge(Li) detector y-ray spectroscopy. Some systematic trends in the cross-section

data have been analysed. The (n, 2n) reaction at this energy in this mass region is by far the most favoured reaction and accounts for > 80 % of the total inelastic cross section. NUCLEAR REACTIONS ~s°.ls2.1s6W(n, 2n), xs°.ts2.1s3.1s4.~s6w(n,p), ts3'la4'lS6W[(n, n'p)+(n, pn)], E=14.74-0.3 MeV, measured tr. Enriched targets; Ge(Li) detector. Investigated cross-section trends. Compared oR(exp) with or,,.

ls2'ls3"ts*'ts6w(n,~),

E

1. Introduction Tungsten is a potential structural material in fusion technology. An accurate knowledge o f the cross sections for 14 MeV neutron induced nuclear reactions on this element is thus of importance for calculating neutron multiplication, nuclear transmutation rates and radiation damage. In addition, an intercomparison of the crosssection data should be able to shed some light on the relative competition between evaporation of neutrons and charged particles from the heavier c o m p o u n d nuclei at an excitation energy of about 20 MeV. The available published information t - 6 ) , however, shows that for m a n y of the energetically possible nuclear reactions the cross-section data are incomplete. In this work we report several activation cross sections at 14.7 +0.3 MeV measured using enriched target isotopes and high-resolution Ge(Li) detector T-ray spectroscopy. The data are discussed in the f r a m e w o r k o f the cross-section systematics at this energy.

2. Experimental procedure Cross sections were measured by activation and identification of the radioactive reaction products. Enriched tungsten isotopes of the isotopic compositions given in table 1 were sealed in polyethylene bags and sandwiched either between two A1 foils

* Present address: Departamento de Fisica, Universidade de Santa Maria-RS, Santa Maria, Brazil. 317

318

S. M. QAIM A N D C. GRA(~A TABLE 1 Isotopic composition of the enriched tungsten isotopes

Enriched

isotope *) 1soW 1s2W 1saW a4W 186W

Isotopic analysis (~o) b)

1s 0w < < < <

6.93 0.05 0.1 0.1 0.02

~szW

1saw

1s,W

~saW

43.73 94.32 3.46 1.15 0.45

11.83 2.54 89.8 1.78 0.33

18.83 2.32 5.63 95.09 2.16

18.69 0.82 1.13 1.98 97.06

a) Chemical form o f the isotope was WOn. Spectrographic analysis showed no major impurities. b) Supplied by Oak Ridge National Laboratory.

or between two As20 a layers, and irradiated with 14.7_+0.3 MeV neutrons at flux densities of ( 1 - 3 ) x 109 cm - 2 - sec -1, which were determined via one of the following two monitor reactions: 27A1(n, ct)24Na, T~ = 15.0 h, a = 121_+6 mb; 7SAs(n, 2n)V4As, T~ = 17.9 d, a = 970_+80 mb. For measurements on short-lived isotopes, irradiations were performed in a fast pneumatic tube system and the neutron density was measured via an internal standard. Except for ~ssW¢ where a gas flow t-proportional counter was employed, the radioactivity of all the reaction products was measured by Ge(Li) detector y-ray spectroscopy. Details of the counting method and analysis o f the y-spectra have already been described 7, s). The half-lives (T~), the y-transition intensities (fd) and the internal conversion coefficients (a) were taken from the literature 9 - i s) and are given in table 2, together with some other data. In the case of 179W¢ and ls~W, due to the absence of suitable y-rays, K X-rays were counted. The fluorescence yield (cot) was taken from ref. 9) and the probability of the capture of an electron from the K-shell (PK) was estimated from the work of Brysk and Rose 16). Cross sections were calculated by applying the usual corrections like those for counting efficiency, geometry, absorption, etc. The maximum errors were estimated as described earlier 7, s) and are given in table 2. It may be mentioned that the errors due to uncertainties in decay schemes are not included. These should, however, be small because only strong and unambiguously identifiable y-rays with fairly wellknown branching ratios and conversion coefficients, as described in recent decay schemes, were used.

3. Results and discussion 3.1. CROSS-SECTION DATA

The nuclear reactions investigated, their Q-values (calculated from the binding energies given in ref. 9)) and the measured cross sections together with their maximum errors are given in table 2. Each cross-section value is based on three or more independent measurements. Since enriched isotopes were employed the contribution

W(n, x) CROSS SECTIONS

319

TABLE 2 Cross-section measurements on isotopes of tungsten Reaction

Q-value

(MeV)

Decay data of product nucleide *) r~

er

A

~,

Activation cross section (mb) this work literature

(keV) (n, 2n) reactions 1s°W(n,2n)lvgmw ls°W(n,2n)t79~W

--8.66 --8.44

6.7 min b) 38 min

221.5 Y,56.3-67.0 (K X-rays)

0.99 PE = 0.858c) toK = 0.94

10.2 b)

lSZW(n,2n)lSlW

--7.99

130 d

xa6W(n,2n)~ssmw

--7.58

1.6 rain

Y.56.3-67.0 (K X-rays) 131.5

Pg = 0.858 ¢) tog = 0.94 0.90 18.7 f)

xa6W(n,2n)lSssW

--7.21

74 d

xs°W(n, p)XS°mTa xs2W(n, p)X s2mTa

+0.06 -- 1.54

8.1 h 16 rain

laZW(n, p)t S2STa

--1.03

115 d

1saW(n, p)ZSaTa

--0.29

5.1 d

134W(n, p)la4Ta as6W(n, p)186Ta

--1.94 --2.96

8.7 h 10.5 rain

t - counting

4904-45 18664-176 (ctmaulative and total) 2162±140

642±60 2272 4- 250 (cumulative and total)

23004-200 e) 21604-120 e) 5404-80 *) 11524-110 a) 2290 4-230 e)

(n, p) reactions 93.3 147 172 1121 1222

0.27 0.88 0.88 0.35 0.28

4.85 l) 1.00 0.80 0.0034 0.0020

108 354 413 197

0.33 0.16 0.83 J) 0.77

2.06 0.115 0.00 0.04 t)

9.64-1.0 0.124-0.015 5.9±0.5 ~urnul~e andtotal) 4.1±0.5 2.94-0.3 1.4±0.2

4.754-1.0 k) 2.9 4-0.6 k) 2.0±0.4 m)

(n, ct) reactions a 82W(n, tx)179mHf xsSW(n, o0tS°mHf

+7.39 +7.78

18.6 sec 5.5 h

ls4W(n, ~t)lSlHf

+7.44

42.5 d

xa6W(n, ~)lSZHf

+6.21

63 min

183W(n, n'p)lS2Ta --7.21

115 d

134W(n,n'p)1aaTa

--7.71

5.1 d

xseW(n,n'p)135Ta

--8.43

49min

217 215 332 133 482 784

0.995 1.00 1.00 0.93 0.82 0.65 n)

(n, n ' p ) + (n, pn) reactions 1121 0.35 1222 0.28 108 0.33 354 0.16 177.6 0.837

0.050 0.120 0.050 0.99 0.032

0.124-0.02 0.22±0.03 1.15±0.15 0.55±0.07

0.0034 0.0020 2.06 0.115 0.89 f)

1.30±0.50 °) 0.65±0.15 o) 0.25±0.05 °)

") Decay data taken from ref. 9) unless otherwise stated.

b) Ref. io). ¢) Ref. 16). d) Ref. 1). e) Ref. 2). f) Ref. ix). g) Ref. a). i) Calculated using tables given in ref.x2). J) Ref. 13). n) Ref. is)

") Ref. 4).

k) Ref. 5). 1) Ref. 14). m) Ref. 16). o) This value describes a sum of (n, n'p) and (n, pn) cross sections.

0.21 ±0.05 m)

320

S . M . QAIM A N D C. GRA(~A

from interfering reactions was either negligible or could be estimated. Most of the reactions have been investigated for the first time. In those cases where some literature data do exist, the values have been listed in the last column of table 2. Except for a few cases, the agreement between our values and the literature data is fairly good. It may be pointed out that the activation technique used in this work does not allow a distinction between (n, d), (n, n'p) and (n, pn) reactions, all of which lead to the same product nucleus. Our preliminary Hauser-Feshbach calculations, however, show that the contribution from the (n, d) reaction is relatively small, the dominating processes being (n, n'p) and (n, pn). The question regarding the sequence of neutron and proton evaporation, however, cannot be answered without neutron-proton angular correlation work. 3.2. T R E N D S IN CROSS SECTIONS

The measured total reaction cross sections are plotted against the asymmetry parameter (N-Z)/A in fig. 1A. This parameter is of special interest here because Z is constant. We chose this parameter because it seems to be directly related to the separation energy of the last neutron in the target nucleus 17), and because it has been found to play a very important role in establishing correlations ?' is, 29). It should be noted that the partial cross sections, i.e., the cross sections of reactions

'[

104 (n,2n)

(B)

1.0

103 v b

n-l~

(A)

O' ne.

=.

: ~ = ~l...~ ~

.5 "6

o

$

c

== 10~

\o \

b

n\=

10-3

._o

o

0(n,2n) i

~emO

\=\

\ ~o

.O (n,p) O'ne

\o \o

\

\

(n,p) 10

o. o'(n,o.)

10-~

\--e-~

\ O" [(n,n'p] + (n,pnl] fine

[(n,n'p) * In,phil 0.1

I

0.16

0.18

I

i 0.20

\

i

i0 -s

022

Asymmetry

=

0.16

parameter

,

= 0.18

i

= 0.20

) 0.22

(N-Z}IA

Fig. 1. Trends in (n, 2n), (n, p), (n, c() and [(n, n ' p ) + ( n , pn)] reaction cross sections for isotopes of tungsten. (A) Cross sections (~ versus (N--Z)/A. (B) Fractional cross sections ~a/(~ne versus

(N--Z)/A.

W(n, x) CROSS SECTIONS

32I

leading to only one isomeric state, cannot be treated on the same footing as the total cross sections. Such partial cross sections are therefore not shown in fig. 1. As can be seen in fig. 1A, the (n, 2n) cross section in this mass region increases only slowly with the increasing relative neutron excess, (N-Z)/A, of the target nucleus; the (n,p), (n, 0t) and [(n,n'p)+(n, pn)] reaction cross sections on the other hand decrease sharply. The straight lines give the best fit to the data. Using these trends it is possible to predict the total cross sections with errors of about 15 Yo for the following cases which, due to stable or very long-lived reaction products, cannot be studied with the activation technique:

iS°w:

tr(n, p) 12.5 mb; a(n, ~t) 6.3 mb; a[(n, n'p)+ (n, pn)] 6.2 mb;

182W: a(n, ~) 2.6 mb; a[(n, n'p)+(n, pn)] 2.0 mb; 183W: a(n, 2n) 2150 mb; a(n, ct) 1.8 mb; la4W:

a(n, 2n) 2180 mb.

It may be mentioned that the trends in (n, 2n), (n, p) and (n, ~) cross sections described here comprise only a small section of the gross systematics discussed by us recently 7, 2o, 21). The advantage of treating the data for a series of isotopes (constant Z) in a small mass region is that the isotope effect.(Q-value effect) can be well investigated and the cross sections for unknown reactions can be predicted with accuracies higher than from the gross systematics. This is amply demonstrated by the correlations shown in fig. 1A. The [(n,n'p)+(n, pn)] reactions, it may be emphasized, have been only sparsely investigated 22). Recently we showed 23) that the contributions of these reactions are by no means negligible as far as radiation damage is concerned. The correlation for [(n, n'p)+(n, pn)] data shown in fig. 1A has been established for the first time. The reaction cross sections (aR) as fractions of the total inelastic cross section (a.e) are shown in fig. 1B as a function of (N-Z)/A. For this purpose the trne value for each target nucleide for 14.5 MeV neutrons was calculated from the optical model using the formulation described by Lindner 24). One finds that a(n, 2n)/tr.e is > 0.80 for all the nucleides. This shows that in this mass region the (n, 2n) reaction is by far the most favoured reaction, accounting for > 80 % for the total inelastic cross section. As discussed earlier 8) the remaining part of tr.~ is constituted almost entirely by the (n, n'7) process. The ratios tr(n, p)/a,., tr(n, ~)/o-~ and tr[(n, n ' p ) + (n, pn)]tr,~ are quite small and decrease with the increasing relative neutron excess of the target nucleide, confirming thereby the known fact that charged particle emission from the heavier compound nuclei atan excitation energy of about 20 MeV is a very unfavoured phenomenon. We thank Prof. G. Strcklin for his interest and encouragement, and H. Ollig, D. yon Ameln and A. Delvoigt for experimental assistance.

322

S . M . QAIM A N D C. G R A ~ A

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