effective capture cross-sections in the fast reactor zephyr

J. Nuclear Energy I. 1957, Vol. 5, pp. 4 to 15. Per&mm Press Ltd.. London

EFFECTIVE CAPTURE CROSS-SECTIONS IN THE FAST REACTOR ZEPHYR

H. ROSE Atomic Energy Research Establishment, Harwell, England

(Received 18 June 1956)

Abstract-The simple perturbation technique has been used to determine effective capture cross- sections for a large number of materials in the small experimental fast reactor ZEPHYR. Measure- ments have been made not onlywith the majority of the natural elements, but also with a considerable number of enriched isotopes. The results provide interesting information concerning the dependence of the fast-neutron capture cross-sections upon nucleon number and shell structure.

1. INTRODUCTION

AN extensive programme of perturbation measurements has beencarriedoutat Harwell in the small experimental fast reactor ZEPHYR. This system, which has been described in detail in the paper by HOLMES et al. (1954) consists essentially of a plutonium core surrounded by a natural uranium envelope. Although the reactor is operated at most at only a few watts of fission power, the corresponding neutron flux at the core centre is nearly IO9 n/cm2/sec. The energy spectrum of the neutrons in the core region is approximately constant, with an average energy about I.5 MeV, being only slightly degraded below that of a fission spectrum.

Perturbation studies have been performed at the centre of the ZEPHYR core with the majority of the natural elements, in order to investigate their capturing properties for the core neutrons. The results have already been reported briefly at the Geneva conference (HOLMES et al., 1955). Significant variations were observed for the measured capture cross-sections. Marked reductions occur for elements whose mean neutron numbers are close to the magic numbers, whilst, in general, odd-Z elements show larger capture cross-sections than corresponding even-Z elements. These conclusions are in qualitative agreement with those reported recently by OKRENT et al. for GODIVA which is a bare U235 fast reactor whose neutron-energy spectrum is similar to that in ZEPHYR, and by SNYDER (1955) for SAPL PPA-5, an intermediate system. Because of these interesting variations, the ZEPHYR program has been extended to include a considerable number of enriched isotopes, in an effort to determine the variation of fast-neutron capture as a function of nucleon number.

This article describes, in detail, the measurements which have been carried out with nonfissionable materials at the centre of the ZEPHYR core, both for the natural elements and the enriched isotopes. It is emphasized that the effective capture cross- sections which are reported here do not represent the effects of neutron absorption only. In that they show marked dependence upon proton and neutron number, however, they provide interesting information concerning odd-even and shell- structure variations.

2. THEORY

The energy spectrum of neutrons over the ZEPHYR core remains approximately constant. It seems reasonable, therefore, to use a simple one-group theory for an

4

Effective capture cross-sections in the fast reactor ZEPHYR 5

interpretation of the perturbation data, especially since such a theory gives a fairly clear picture of the physical processes involved. The one-group perturbation theory developed by FUCEI~ (1949) has been adopted for this purpose. The essential features of the theory are now summarized in sufficient detail to facilitate an understanding of the discussions which follow later.

We consider a small perturbation sample of matefial of volume 6V and atomic density N per cm 3. When the sample is introduced into a vacant space at some point (r) in a critical reactor, a (the average number of collisions per cm of neutron path in the sample) and p (the average number of secondary neutrons produced per cm of neutron path) are given by

(1)

where a, and (T, are the nuclear cross-sections of the sample material for fission and neutron capture, respectively, and cSt7 is detied as the scattering (elastic + inelastic) component of the total transport cross-section a/N. When the sample has been introduced, the reactor is no longer just critical. According to,the theory, the reactivity R of the new system is given by

R = [B - gWal[ WI2 aif SPVM’) m (2)

where &r’) and p(r’) are the neutron-flux and neutron-production rate respectively at (r’), and the integration is taken over all regions of the reactor where fission occurs. The so-called “perturbation function” g(r) weights neutrons lost to the sample according to the anisotropy of the neutron flux 4(r) at the position occupied by the sample.

The integral in the denominator of equation (2) can be determined from fission- chamber measurements, provided of course that Y, the mean number of neutrons produced per fission, is known for all the isotopes contributing to the chain reaction in the reactor (Pumg, U238, and UB6 in the case of ZEPHYR). In principle therefore, equatioli (2) allows the nuclear parameters for the perturbation sample to be correlated in terms of the reactivity change R, since the latter 6an be derived by noting the control-rod movement necessary to regain criticality. In practice it is extremely difficult to obtain a very accurate calibration of the ZEPHYR control rod in terms of absolute reactivity, and it is considered that the calibration may be in error by several per cent (HOLMES et al., 1955). The necessity of an accurate reactivity calibration has been obviated by using instead a standard perturbation sample for which the nuclear parameters are reasonably well known. In ZEPHYR wiz have used as the standard for all measurements the reactivity change produced at the core centre by a small sample of PUBIS. We consider that the appropriate fission cross-section for this material can be determined fairly accurately, inasmuch as the cross-section remains reasonably constant over the range of neutron energies encountered in the ZEPHYR core.*

We are confining our attention in this article to perturbation measurements carried out at the centre of the ZEPHYR core; it is convenient for our purpose to rewrite equation (2) in the simplified form

R = constant x (@ - a) 6V

* Neutron Cross-Sections, BNL 325 (1955).

(3)

6 H. ROSE

since at the core centre the neutron flux is so nearly isotropic that g(r) is unity. Thus neutron scattering at the core centre has no effect upon R. We now define oz as the “perturbation cross-section” for the sample material at the core centre, given by

0, = (B - a)lN = (Y - 1) 0, - cr,. (4)

For our standard material, Pu 23g, the following values were assumed;

a, = 1*83 barns

cc = O-20 barns Y = 3.10.

It is clear that any inaccuracy in the estimate of ce will not affect o, significantly, since the fission term predominates for Pu 23s. With the above values, the resulting o, obtained for Pu23s is +3*64 barns. It is unlikely that this value can be in error by more than 5 %. A perturbation measurement at the core centre with the plutonium sample thus allows control-rod movement to be calibrated directly in terms of barns.

In general, the insertion of a non-fissionable perturbation sample to the centreof the ZEPHYR core produces a negative reactivity change, and it is evident from equation (4) that the perturbation cross-section --0, should then be identical with a,. Where it has been possible to measure values of cC directly by activation methods, it has been observed that these are smaller than the cross-sections o, obtained by the perturbation method. This discrepancy arises due to the limitation of the one-group theory, in that the latter does not allow for any change in the neutron-energy spectrum. Such a change will occur, for example, when neutrons are slowed down by inelastic scattering collisions in the perturbation sample. The cross-sections CJ~, ce, and cstr for the reactor materials vary with neutron energy. When a neutron experiences a loss of energy, therefore, the relative probabilities that it will undergo fission, radiative capture, or that it will escape from the system, are changed. This means that the neutron’s “effectiveness” X (i.e. its ability to maintain the chain reaction) will be altered. It was previously assumed that neutron scattering at the core centre produces no reactivity change; this assumption is only true provided X remains unchanged for the neutrons scattered by the perturbation material. If X is decreased, the reactivity change will be more negative than that predicted by the one-group theory. It requires a fall in X of only a few per cent to explain the majority of results which are obtained.

3. EXPERIMENTAL PROCEDURE

For a detailed description of ZEPHYR, the reader is referred to earlier papers (HOLMES et al., 1954,1955). The core matrix used for the perturbation studies consists of a solid cylindrical block of natural uranium approximately 15 cm in length and diameter, with vertical channels of &in. diameter into which are assembled the plutonium-fuel elements in a regular array. Those channels at the edge of the matrix which do not contain plutonium elements are filled with similar elements of natural uranium. The reactor envelope consists of about 8 tonnes of l-in-diameter natural uranium rods arranged in the form of a right cylinder symmetrically situated around the core. Sections of the envelope may be moved to uncover the core.

In these experiments, most samples were introduced through a thin-walled nickel tube (about 10 ft long) which extended from the concrete biological roof-shield,


through the uranium envelope and into the central channel of the core. The outside diameter of each specimen was limited, therefore, to about O-22 in.

All perturbation measurements were made with a flux at the centre of the core of 8 X lOa neutrons/cm2/sec, when the reactivity of the system was about -0.0003 %. When a perturbation sample was inserted, the control rod was moved so as to keep the reactor power-level constant. The change in reactivity due to the alteration of the control-rod position was then equal and opposite to that produced by the sample. For each specimen, the average was taken of three or more successive measurements of the necessary control-rod movement. The accuracy of setting of the control rod was about 5 X lW% in reactivity, and a movement of this amount from the correct balance point resulted in a power-level drift of about O-001 % per second.

The reactor power level was measured with the annular fission chamber which surrounds the core (HOLMES et al., 1954), counting at a rate of l-6 x IO6 c.p.s. at the power level used, and the trend in the counting rate was observed with a recorder driven by a linear ratemeter. The determination of the correct setting of the control rod for each balance point took 5 min or more. Since the power measurement in ZEPHYR is made using pulse counting, a minimum time for the determination of the balance point was set by the statistical fluctuations in the counting rate; In practice, however, the flux in the reactor is never quite steady and very small drifts and jumps of the power level occur, presumably due to temperature effects and small mechanical displacements in the reactor. Limitations set by the pulse-counting method of power- level measurement made it appear unlikely that a greater sensitivity could be obtained by an oscillator method. In any case, the sensitivity obtained by the control-rod balance method was adequate, giving a lower limit of detection corresponding to a cross-sectional area of about IO-4 cm2.

Preliminary measurements which were carried out established that, within the experimental limits, the presence or absence of material in the portions of channel above and below the perturbation sample made no measurable difference to the reactivity change observed. The majority of the natural elements were tested, therefore, in the form of small cylinders O-22 in. in diameter and O-8 in. long which were suspended in an otherwise empty central channel. Those specimens which could not be obtained as solid cylinders were examined as powders or liquids in small containers. Where possible, the elements themselves were used for measurements; but in the case of gaseous elements, or if the element itself was unobtainable (e.g. the rare earths), a compound, usually an oxide, was used. The small containers were made from magnesium or aluminium foil, and had a negligible effect at the core centre. The enriched isotopes were also examined in these containers. The samples or containers were suspended on fine tungsten wire, 0.004 in. in diameter. Since no materials have abnormally high capture cross-sections as in the case of thermal reactors, very high purity of the samples was not so important. However, the highest obtainable purity was used in every case, being rarely less than 99 % with the natural elements.

A. The natural elements 4. RESULTS

Over sixty of the natural elements have been examined in the core of ZEPHYR. The perturbation cross-sections which were measured for these materials at the core centre are listed in Table 1. Whereas the simple one-group theory predicts that these

8 H. ROSE

TABLE I.-EFFEcTIvECAPTURECROSS-SECTIONSMEASURED ATTHECENTREOF THEZEPHYRCORE. THENATURALELEMENTS

Element Z 3

H 1 Li 3 Be 4 B 5 C 6 N I 0 8 F 9 Na 11 Mg 12 Al 13 Si 14 P 15 S 16 Cl 17 K 19 Ca 20 SC 21 Ti 22 v 23 Cr 24 Mn 25 Fe 26 co 27 Ni 28 CU 29 Zn 30 Ga 31 Ge 32 As 33 Se 34 Br 35 Rb 37

1.0 0 6.9 3.9 9.0 5.0

10.8 5.8 12.0 6.0 14.0 7.0 16.0 8.0 19.0 10.0 23.0 12.0 24.3 12.3 27.0 14:o 28.1 14.1 31.0 16.0 32.1 16.1 35.5 18.5 39.1 20.1 40.1 20.1 45.1 24.1 47.9 25.9 51.0 28.0 52.0 28.0 54.9 29.9 55.9 29.9 58.9 31.9 58.7 30.7 63.6 34.6 65.4 35.4 69.7 38.7 72.6 40.6 74.9 41.9 79.0 45.0 79.9 44.9 85.5 48.5

-

- --oz -u,

(in mb) I 3ement z z (in mb)

-

I 1 ______

-189 + 15 55 * 10

-31 i 2 165 h 15

Ok-5 80 & 10 0 * 10 256

21 * 6 14 * 5 26 I- 5 22 * 4 61 i_ 6 85 + 20 67 & 8 59 + 5 76 & 10 40 * 13 31 *4 34 * 2 35 _c 5 23 + 5 55 -+ 6 51 15 93 * 5 74 + 5 83 * 5 81 +8 17 * 8

114 * 12 88 & 8

148 * 15 73 + 15

Sr 38 87.6 49.6 61 + 10 Y 39 88.9 49.9 <45 Zr 40 91.2 51.2 40 f 5 Nb 41 92.9 51.9 141 + 5 MO 42 96.0 : 54.0 105 i 5 RlJ 44 101.7 57.7 126 + 15 Rh 45 102.9 57.9 217 + 5 Pd 46 106.7 60.7 165 _C 5 Ag 47 107.9 60.9 250 + 5 Cd 48 112.4 64.4 153 18 In 49 114.8 65.8 305 + I Sn 50 118.7 68.7 102 + 7 Sb 51 121.8 70.8 171 & 15 Te 52 127.6 75.6 114 * 10 I 53 126.9 73.9 238 i 20 cs 55 132.9 77.9 215 + 30 Ba 56 137.4 81.4 75 + 30 La 57 138.9 81.9 90 zt 10 Ce 58 140.1 82.1 69 i 7 Eu 63 152.0 89.0 330 + 65 Hf 72 178.6 106.6 177 * 25 Ta 73 180.9 107.9 265 * 5 w 74 183.9 109.9 174 f 5 Re 75 186.3 111.3 310 & 10 OS 76 190.2 114.2 146 i 15 Ir 17 193.1 116.1 303 + 5 Pt 78 195.2 117.2 178 i 5 AU 79 197.2 118.2 220 + 5 Hg 80 200.6 120.6 120 * 8 Tl 81 204.4 123.4 95 zt 15 Pb 82 207.2 125.2 60 C 5 Bi 83 209.0 126.0 46 & 7

values represent the true neutron-capture cross-sections ge, the perturbation cross- sections are actually larger than the corresponding values obtained directly by activation methods. This is illustrated by Table 2, which gives comparative values of crz and crc for certain cases where only one isotope is involved. The effect is particularly marked for lighter elements, which are expected to have very little true neutron capture.

It was pointed out in Section 2 that the discrepancy can be explained in terms of a decrease in neutron effectiveness 22 resulting from inelastic scattering in the perturbation samples. In the plutonium core of ZEPHYR, it is probable that the decrease in C, is caused mainly by a reduction of o, Pu23g, since a reduction in this cross-section of only a few per cent is enough to explain the results. Recent data (Neutron Cross- sections, BNL 325, 1955) for u, Pu23g suggest that this reduction does occur. In general, therefore, the ZEPHYR measurements yield enhanced capture cross-sections


because inelastic scattering in a perturbation sample produces a negative reactivity change in addition to that caused by true neutron absorption. The perturbation values --o, may be termed “effective capture cross-sections” for the ZEPHYR core; they are compounded from the true radiative capture cross-section plus a function of

TABLE 2

Isotope

Naas AP V= MrP As= I’S?

Tale1 ALP’

I

-

, -

‘erturbation (-% mb)

21 h6 26 h 5 34 * 2 23 zt 5

114 * 12 238 + 20 265 * 5 220 + 5

- Activation

(0, mb)

0.53 * 0.04 3.0 + 0.2 4.0 * 0.4 5.3 * 0.5

78 rt7 158 f 16 228 zt 25 183 +8

the inelastic scattering cross-section for the sample in question. The latter effect predominates in the lighter element, and the former in the heavier elements.

An examination of Table 1 reveals that, for the heavier materials, the effective capture cross-sections of even-Z elements are systematically smaller than those for

500 -

_...

10 0 20 40 60 80 100 120 140

Mean neutron number i FIG. 1 .-Effective capture cross-sections in ZEPHYR : Even-Z elements.

odd-Z elements. Even the latter, however, have values which are small in comparison with the fission cross-sections of the nuclear fuels. Fig. 1 plots the value of a, as a function of mean neutron number R for each even-Z element (Z > 10); Fig. 2 is the corresponding plot for odd-Z elements. In both cases, the cross-sections lie on a series of smooth curves which show marked depressions near the magic numbers of neutrons #= 28, 50,82, and 126. The general variation of effective capture cross-section with neutron number is very similar, in fact, to that obtained by HUGHES et al.’ for true

10 H. ROSE

neutron-capture cross-sections with fission neutrons. The depressions in o, near the magic numbers are expected ; a nucleus whose neutron number is magic is particularly stable and has an unusually low binding energy for the incident neutron, so that true neutron capture and inelastic scattering should be small. No depression is evident, however, at R = 20; note o,-Ca and o,-K.

The relatively low values for Sn and Sb indicate that the possession of the magic proton number 50 gives extra stability to the nucleus. Ge and Ga have relatively small effects also, which suggest that N = 40 may perhaps be semimagic. These elements will be considered further in the next section.

For light elements, (n,p,) and (n,cr) reactions sometimes make a significant

1’ ~ t ll - I - + Magic numbers

IO 1 ! 0 20 40 60 80 100 120 140

Mean neutron number hj FIG. 2.-Effective capture cross-sections in ZEPHYR: Odd-Z elements.

contribution to gr. In particular, the effect of boron (see Table 1) can be attributed entirely to the BlO(n,a)Li’ reaction, whilst that of lithium is due mainly to the Li6(n,a)H3 reaction. The (n,p) and (n,ar) reactions in N14 probably account for the relatively large perturbation cross-section measured for nitrogen.

Although neutrons in general probably suffer only one collision in the perturbation samples, elastic scattering by hydrogen nuclei is sufficient to moderate the energy of some neutrons to very low values where o,-Pu 23g has increased appreciably. Their probability of escape from the core is then reduced, whilst their X value is increased considerably. The effective perturbation cross-section for hydrogen has, therefore, the large positive value given in Table 1. This peculiar moderating effect appears to disappear quickly as the size of the target nucleus increases; the deuteron exhibits only a small positive o, (see Table 3), and no change in reactivity can be observed when a carbon sample is inserted to the centre of the ZEPHYR core. It is probable, therefore, that the small positive effect produced by beryllium occurs as a result of the Be9(n,2n)Bea process, and not as a result of neutron moderation due to elastic scattering by the beryllium nuclei.

B. The enriched isotopes Odd-Z elements possess only one or two stable isotopes each, and these are almost

invariably of even N. A large number of the points in Fig. 2 represent directly,


therefore, the effects of individual odd-Z-even-N nucleides. Even-Z elements, however, contain a selection of both even-N and odd-N stable isotopes. Thus it is uncertain whether the larger cross-sections of the odd-2 elements arise because they possess odd numbers of protons, or because their total nucleon numbers are odd. To investigate further the effect of the odd nucleon upon fast-neutron capture, a‘ variety of enriched isotopes have also been examined. These isotopes were produced in the electromagnetic separator at Harwell. The amount of material available in

TABLE 3.-Odd-Z

Isotope - uz (in mb) Isotope

-

_

-us (in mb)

1H2 ,LP zB10 SB’l

,N14 80 u c I33 eo u c es 3,Rb35

-29 + 5 585 f 30 786 i 4

19 * 19*

80& 10 81 & 7” 56 f 3

122 * 30

s,Rba’ <35* &glo’ 250 f 5 4&=0s 253 f 8 10 I n 113 388 $ 60

&P 302 & 9* SISb’= 146 i 30 61Sb123 213 & 30

* Obtained by inference from data with the other isotope and the natural element.

each case was generally small, being of order several hundred milligrams, and so the accuracy of measurement was usually less than that obtained with the natural elements.

1. Odd-Z-odd-N. The values for a, obtained with odd;Zisotopes are listed in Table 3. The small positive effect of the deuteron has already been attributed to a small increase in C for neutrons which are elastically scattered by the Hz nuclei (Section 4). Apart from the deuteron and N14 the only other odd-Z-odd-N nucleides measured in ZEPHYR were Lis and BlO, which are the largest neutron absorbers examined in the ZEPHYR core. When the published (Neutron Cross-sections BNL 325, 1955) variation with neutron energy of the @,a) cross-section of either of these two isotopes is is integrated over the ZEPHYR core spectrum of neutrons, a value is obtained which in excellent agreement with the perturbation result of Table 3. For these two isotopes, therefore, the effects of neutron scattering, whether elastic or inelastic, are negligible in comparison with their true neutron absorption.

2. Odd-Z-even-N. Fig. 3 shows the variation of a, with neutron number for all the odd-Z-even-N nucleides measured in ZEPHYR. The information available above N = 82 is slight, because of a scarcity of available materials (mainly the rare earths), and the curve dotted in this region of Fig. 3 is that obtained for the odd-2 elements.

As the magic number of 50 or 82 neutrons is reached, effective capture falls sharply. This suggests that a nucleus containing a number of neutrons just less than magic has a large probability of capturing a surplus neutron. Nevertheless, the falloff in o, cannot be so rapid near N = 126, since otherwise thallium would have a larger cross-section than that shown in Fig. 2. Excluding magic effects, the effective capture cross-section increases with increasing N for the lighter nuclei : for total nucleon number greater than 100, however, o, in almost all cases remains between 200 and 300 mb. In this region the contribution of inelastic scattering to a, is relatively small, and the measured

12 H. ROSE

values should rarely exceed the true radiative capture cross-sections by more than about 50 mb.

In Section 4A, the relatively low value for antimony was attributed to a proton number Z just greater than magic. The isotopic data illustrate that a secondary dip in the odd-Z-even-N curve occurs for 51Sb121 at N = 70, whilst o, for 51Sb123 is similar in magnitude to the values for 531127 and &s~~. It may be, therefore, that N = 70 is partially magic also. A similar phenomenon at N = 40 was suggested to explain the

1000 r I I , 1

500

t /

I I I I I .._ I I I I I 1

0

.c

20 40 60 80 100 120 Neutron number N

FIG. 3.-Effective capture cross-sections in ZEPHYR: odd-Z-even-N isotopes.

low value for gallium. Although no gallium isotopes were available for experiment, the low value for CUDS provides some evidence for extra stability somewhere between N = 36 and N = 42.

3. Even-Z-even-N. Although an appreciable number of even-Z isotopes were examined, these comprised a variety of both even-N and odd-N nuclei, so that neither the even-Z-even-N nor the even-Z-odd-N pattern was as clearly resolved as that for odd-Z-even-N nuclei. Nevertheless, the ZEPHYR results do provide some interesting information about odd-even behaviour.

The results with the even-Z isotopes are listed in Table 4 and the even-Z-even-N data are illustrated in Fig. 4, where a direct comparison can be made between the effects for even-Z-even-N and odd-Z-even-N nuclei.

Below N = 50, the data appear more random in the even-Z case, Although in this region most cross-sections are comparable in magnitude with those for neighbouring odd-Z-even-N nuclei, a few are appreciably larger. Notable in this respect


Isotope

-

-

-

- ua (in mb)

30 f 10 80 f 20

108 f 9 41 f 7

llO& 5 60 & 8 60 f 35 99 & 20

168 i 40 150f45 170 $40 35 + 20

TABLE 4.-Even-Z

- a, (in mb)

15f25 115 * 20 60 rt 20

llOrt20 16Of60 250 h 60 365 & 70 175 f 55 163 & 15 80 & 20

160 & 25 70 f 25 47 & 22

-

I --a, (in mb)

210 i: 30 270 f 40 2OOIt35 195 f 45 20 If 20

110 f 30 60 f 35

are FeM and Ni5*. For these nuclei, inelastic scattering effects may be pronounced, either because their total inelastic scattering cross-sections are large, or because the neutrons which they scatter inelastically are reduced in energy to regions of unusually low Z. Above N = 50, all the effective capture cross-sections measured in ZEPHYR for even-Z-even-l\r isotopes are. definitely smaller than those of their odd-Z-even-IV neighbours. Away from the magic numbers, the addition of an extra proton to a nucleus appears to increase effective capture by roughly 100 to 150 mb, so that in some cases it is more than doubled.

Neutron number N FIG, 4.-Effective capture cross-sections in ZEPHYR: even-Z-even-l\risotopes.

14 H. ROSE

The three Sn isotopes of even-N are of especial interest; each exhibits an unusually small cross-section, comparable with that of the magic Ce140. This evidence suggests that the extra stability is caused by the possession of magic 2 alone. Unlike the data with the odd-2 Sb isotopes, the results do not indicate any stabilizing effect for N = 70, so that this may only occur in the odd-Z(odd-nucleon) case. The large cross-section for Ge70 (N = 38), when compared with the much smaller effect for the natural element, points to small values of o, for Ge72 (N = 40) and Ge74 (N = 42). Thus both the odd-Z-even-N and the even-Z-even-N data indicate added stability in the region near 40 neutrons. It is interesting to note therefore, that, according to the shell model of MAYER,(~) N = 40 occurs at the closing of all terms in shell IV except

500

10 0 20 40 60 80 100 120 140

Neutron number N FIG. 5.-Effective capture cross-sections in ZEPHYR: even-Z-odd-N isotopes.

the last (gs,s); similarly, N = 70 occurs in shell V when all terms are filled with the exception of the last (hir,s).

4. Even-Z-odd-N. Only ten nuclei of this type were examined in ZEPHYR. All three measured isotopes in the region below N = 50 gave cross-sections which were relatively large. As with Fe54 and Ni 58, the explanation may lie in anomalous inelastic scattering effects. Both Sn117 and Snn9 have small cross-sections, which can be attributed to their magic proton number Z = 50. They are still significantly larger, however, than the cross-sections for the Sn isotopes of even N.

An examination of Table 4 shows quite clearly that for even-Z nuclei all measured odd-N isotopes have values of o, which are appreciably larger than their even-N neighbours. In addition, Fig. 5 illustrates that, above N = 50, fast-neutron capture cross-sections are the same, within the accuracy of measurement for both odd-Z- even-N and even-Z-odd-N nuclei. Thus the evidence obtained with the enriched isotopes suggests that odd-Z elements exhibit larger cross-sections than neighbouring even-Z elements because of their odd nucleon number, and not because their proton number is odd.

Acknowledgements-The author wishes to acknowledge the assistance he has received in these experiments from members of the ZEPHYR Group, in particular from Dr.


R. D. SMITH; he appreciates also the many helpful discussions with Dr. L. R. SHEPHERD and Dr. J. E. R. HOLMES. He is indebted to the Electromagnetic Separation Group, General Physics Division, Harwell, for the supply of the enriched isotopes.

REFERENCES FUCHS K. (1949) Proc. Phys. Sot. A62,791. HOLMES J. E. R., MCVICAR D. D., ROSE H., SHEPHERD L. R., SMITH R. D., and SMITH A. M. (1954)

J. Nucl. Energy 1,47-52. HOLMES J. E. R., MCVICAR D. D., ROSE H., SMITH R. D., and SHEPHERD L. R. (1955) Proceedings

of the International Conference on the Peaceful Uses of Atomic Energy, A/Cod 8/p/404. HUGH= D. J. (1953) Pile Neutron Research p. 113. Addison-Wesley, Cambridge, Mass. MAYER M. G. (1950) Phys. Reo. 78, 16. OKRENT D., AVERY R., and HUMMEL H. H. (1955) Proceedings of the International Conference on

the Peaceful Uses of Atomic Energy, A/Cod 8/p/609. SNYDER T. M. (1955) Proceedings of the International Conference on the Peaceful Uses of Atom&

Energy, A/Cod 8/p/602.

effective capture cross-sections in the fast reactor zephyr

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