The suppression of resonance absorption in indium antimonide

International

Journal

of Applied

Radiation

and Isotopes,

1962, Vol.

13, pp. 247-251.

Pergamon

Press Ltd.

Printed

in Northern

Ireland

The Suppression of Resonance Absorption Indium Antimonide L. G. PENHALE Physics Branch,

Royal

Military

in

and D. E. VAUGHAN College

of Science,

Shrivenham,

Berks.

(First received 30 January 1962 and in final form 12 March 1962) High-resonance neutron absorption both in antimony and indium makes it impossible to achieve uniform neutron irradiati0.n of thick samples of indium antimonide by simple insertion in a neutron flux. A technique has been devised in which the fluxes of resonance neutrons are attenuated by surrounding the specimen with suitable thickness of indium and antimony. LA

SUPPRESSION

DE L’ABSORPTION A RESONNANCE L’ANTIMONURE D’INDIUM

DANS

L’absorption neutronique a haute resonnance et dans l’antimoine et dans l’indium rend impossible la procuration d’une irradiation neutronique uniforme de gros tchantillons d’antimonure d’indium par l’insertion simple dans un flux de neutrons. On a combine une technique oh les flux des neutrons a rtsonnance sont atttnues en entourant I’tchantillon d’tpaisseurs convenables d’indium et d’antimoine. YCTPAHEHHE B Cnnbxoe

nornomerrne

HeB03MOZWSIM IlyTeM

paBHOMepHO3

IIPOCTOI’O

pe3OIElHCHbE

DIE

pesonancnbrx o6nyseuue

IlpHMt?HeHHH H&TPOHbI

noMeuraeMrrMn nonpyr

PE30HAHCHOI’O AHTHMOHHJ(E

H&TPOHHOrO

IIOrJIO~EUOTC~

netirponoa

sari

HefiTpOHaMIl llyYK3. IfHAIleM

IIOI’JIOlIIEHIIR IIHJHIH cypbnioti, ran 06paanoa

TOJlCTbIX P33p36OTaH

M

CYPbMOti

Mt?TOA,

n un~nere, EtHTMOHIl~Et COrJlaCHO

COOTBeTCTByIOI@

nenaer IiHfiIlR

KOTOPOMY TOJIIQHHbI,

obpasna.

UNTERDRUCKUNG

VON RESONANZABSORPTION ANTIMONID

IN

INDIUM

Hochgradige Resonanzabsorption von Neutronen in Antimon sowie Indium macht es unmtjglich eine gleichmiissige Bestrahlung von dicken Proben aus Indium Antimonid mittels einfacher Einfiihrung in den Neutronenfluss au erzielen. Eine Methode wurde entwickelt mittels derer die Resonanzneutronenfltisse durch eine Umhtillung der Probe mit Indium und Antimon angemessener Dicke abgeschwlcht werden.

INTRODUCTION

and closer together as the atomic number increases and the energy decreases. The elements in the middle of the table have their resonances predominantly in the lower epithermal region (l-100 eV) and in particular ,sInns has a pronounced resonance at 1.45 eV. This fact. has led to the widespread use of indium foil for neutron flux determinations. The consequence of resonance absorption in

the curves(l) of the variation of neutron cross section with energy for the elements of the periodic table show, in general, a smooth transition from fast to thermal neutron energies, there are resonance absorption peaks in several cases. Such resonances are small and widely spaced for elements of low atomic number at the higher energies, becoming larger ALTHOUGH

247

248

L. G. Penhale and D. E. Vaughan

thick specimens is the phenomenon known as self-shielding, whereby the interior of the specimen experiences a greatly reduced neutron flux due to the severe adsorption in the surface layers. Thus the irradiation is extremely inhomogeneous so that radiation effects in the bulk of the material are swamped by those in the surface: In cases where uniform irradiation is required it is essential to provide a means of suppressing the resonances.

GENERAL In dealing with indium antimonide it is necessary to consider the isotopic abundances, as well as the resonances, in formulating the conditions required for uniform irradiation. This information, together with other relevant data is presented in Table 1. A cursory TABLE 1 Principal resonance data

Thermal neutron Neutron .activation cross Neutron cross Relative section Isotope abundance section energy (barns) (eV) (barns) (%) 491n113

49In115

$blfl

slSblas

4.3

95.7

57.25

42.75

75 92 38 30,000 a50 1000

2.4 14.5 25.0 1.45 1.8 9.0

1300 a00 110

6.2 11.0 30.0

1200 25 la

22-o 50.0 100.0

56 f 12 1 145 f 15 I 6.8 * 1.5

antimony resonances in addition to those of indium. The simplest, and most obvious, means of obtaining resonance suppression is to surround the specimen with suitable thicknesses of the appropriate materials. For indium, because of the magnitude of the resonance cross sections, a thin foil (approx. 0.25 mm thick) is more than sufficient to ensure complete suppression(3). In the case of antimony a calculation, based on the assumption that the thickness quoted for indium is correct, indicates that a linear array of 2.2 x 10’ atoms of Sblss would be required. Antimony metal possesses approximately 3 x 10’ atoms of Sblss per centimetre, so that an enclosure of pure antimony 7 mm thick is theoretically sufficient. The metallurgical properties of pure antimony render fabrication difficult and so the various alloy phases of antimony were considered and a 12 per cent antimony-lead alloy was selected as being the most suitable. The required linear array of atoms of Sbl* for suppressionis provided by a thickness of 1.7 cm of this alloy. In practice the standard irradiation cans used with the A.E.I. Research Laboratories MERLIN reactor restrict the maximum thickness of alloy that can be used to 1 cm, either side of an indium antimonide sample of 0.5 mm thickness. However, it seems likely that the thickness of indium foil employed is more than adequate by at least a factor of two and, as the indium thickness is the basis of the antimony calculation, the 1 cm thickness of alloy ought to provide the desired suppression.

I 2.5 A 6.5 1

inspection of the resonance cross sections suggests that the only significant resonance is that of ,IsP at l-45 eV. However, resonances having smaller cross sections cannot be ignored, since consideration of the resultant decay schemes(2) reveals that slSWZQ, with a 60 day half-life, is an equally significant factor in the long-term Conactivity of the irradiated specimens. sequently it is highly desirable to suppress the

ENCLOSURE

DESIGN

The physical properties of the antimony alloy listed in Table 2 enabled calculations to be made on the heat generation and transfer conditions TABLE 2 Physical properties of the eutectic alloy (12 per cent) Sb-Pb’5) Density Specific heat Thermal conductivity Melting point

10.5 g. crnp3 0.032 Cal. g-l. ‘C-l 0.24 W. ‘YF1. cm-r 247°C

The suppression of resonance absorption in indium antimonide LEAD ANTIMONY

L4

lNDIUM L

At4Ttt40Nl~Eshwts

249

ALLOY

I /

FIG. 1. Exploded diagram of lead-antimony enclosure.

to be expected in MERLIN, for an irradiation position at the edge of the reactor core at a power rating of 4 MW. The calculated equilibrium values aret : temp. temp. temp. temp.

of of of of

water can lead-antimony lead-antimony

alloy surface alloy centre

40°C 45°C 149°C 151%.

These favourable thermal data, ease of milling and the high self-absorption in the alloy were the selection criteria. The physical form of the enclosures is illustrated in Fig. 1. It was prepared by casting two half-cylinders, in one of which a rectangular recess was milled to accommodate the indium antimonide sample in its indium foil wrapping. The two halves were held together by high purity aluminium wire as shown in the figure. ACTIVATION

OF MATERIALS

Calculations have been carried out of the expected activities of the components in the irradiation can in terms of the specific activity induced per megawatt hour for irradiation positions at the reactor core edge in MERLIN, and these are summarized below.

A. Activation of tlu sample (i) Initial activity due to IrP is 19.7 c/g sample/ MWh. The half-life is 54 min, hence the daily decay factor is approximately 10’. Thii means that the activity has decayed to less than 1.4 PC/g sample/MWh by the end of the first day. (ii) Initial activity due to InU4mis 0.3 me/g sample/ MWh. The half-life is 50 days. (iii) Initial activity due to Sbm is 3.7 me/g sample/ MWh. The half-life is 2.8 days, hence the weekly decay factor is approximately 8. (iv) Initial activity due to Sbrwis 0.18 mc/gsample/ MWh. The half-life is 60 days. B. Activation of indiamwra@er (i) Initial activity due to IrP is 230 c/g indium/ MWh. This estimate is extreme, since it is baaed upon the assumption that all the epithermal neutrons are concentrated at the 1.45 eV peak. A more reasonable estimate is 2.3 c/g indium/MWh. (ii) Initial activity due to InU*m is 0.95 me/g indium/MWh. C. Activation of lead-antimony enclosure (i) Initial activity due to SP is 0.7 c/g alloy/ MWh. (ii) Initial activity due to Sbw is 30 me/g alloy/ MWh. Again, these estimates are extreme, being based upon the assumption that all the epithermal

250

L. G. Penhale and D. E. Vaughan

neutrons are concentrated at the respective major resonance peaks of 6.2 eV for Sb*rand22 eVforSb1s3. More reasonable estimates are 7 me/g alloy/MWh and 0.3 me/g alloy/MWh, respectively. An independent calculation@‘, on a more accurate basis, places the

initial Sbl% activity at 0.13 me/g alloy/MWh.

hazard. The dose rates were determined using Ecko Radiation Monitor Type N569. From the expected specific activity figures, dose-rate estimations, after 1 week’s decay, were made assuming that, because of its small dimensions, the sample could be regarded, effectively, as a point y-emitter at a distance of 1 ft. A. Sample activity

5

IO

The estimated dose rates at a distance of 1 ft, due to Inu4m, Sblss and Sbm were 0.036, 0.13 and 0.084 mr/hr, respectively, giving a total value at this distance of 0.25 mr/hr. The observed total dose rate at 1 ft was 0.47 mr/hr. The reason for this apparent discrepancy will be discussed later. The y-spectrum of the sample was examined using a lg in. diameter, sodium iodide (thallium activated) crystal spectrometer, I.D.L. pattern, unshielded, Type 663, and employing a single-channel pulse-height discriminator. The energy calibration of the discriminator bias was achieved by using the 0.66 MeV peak of a standard caesium-137 y-emitter. In this way the peaks labelled A, B and C were identified as appertaining to Sb122, whilst peak D relates both to Sb12s and Sbl%. No identifiable peaks attributable to In114m were detected, probably due to its lower abundance. I5

20

25

30

Discriminator volts

FIG. 2. Gamma spectrograph of indiumantimonide sample. D. Activation of lead The only significant nuclear reaction involving slow neutrons is PbBos (n, y) PbsOs -+ Biaov (half-life 3.2 hr, reaction cross section 6 x lo4 barns) but, in view of the low cross section and short half-life, activities due to this reaction will be negligible compared to those of antimony. Similar considerations apply to fast neutron activation, with the possible exception of the 47 day half-life product of the reaction Pbsog (n, a) Hg 203. No cross-section information appears to be available for this reaction, although it is known that the threshold is about 2 MeV. However, the activity due to this cause is not likely to be significant, particularly if considered in relation to the high y- and /?- self-absorption in lead.

EXPERIMENTAL CONFIRMATION OF ACTIVITIES These measurements were made 1 week after the irradiation and were confined to determination of y-dose rates, since in this case it is y-activity which constitutes the major radioactive

B. Foil wrapper activity The estimated y-dose rate due to In114m was 0.38 mr/hr at 1 ft, whilst the observed dose rate was 0,345 mr/hr at 1 ft. This represents excellent agreement between observed and estimated values. The y-spectrum of the foil, which is shown in Fig. 3, was investigated to check whether any particles of the sample had adhered to it; in fact no peaks due to Sb122 or SblS4 were discernable. The peaks labelled E and F were identified as appertaining to In114m and the peak labelled G is probably due to the small amount of Inn4 which is expected to be present’s’. The remaining peak could not be identified positively. C. Enclosure activity The estimated y-dose rate due to Sblan and SbrM was 46.5 mr/hr at 1 ft whilst the observed dose rate was 35 mr/hr at 1 ft. This shows very good agreement when account is taken of the significant selfabsorption of y-radiation which takes place due to the lead content of the enclosure.

CONCLUSIONS In view of theverygood agreement between the observed and estimated dose rates for the indium foil wrapping, the discrepancy between these

The suppression of resonance absorption in indium antimonide

251

times greater than those observed. Since the observed activity is only twice that calculated for complete suppression, it may be concluded that the provision of I.7 cm of antimony alloy would have resulted in effectively complete suppression. The results summarized above indicate that the proposed method for the suppression of resonance absorption in thick specimens is sound in this particular case. Consideration of the neutron cross-section data(l) shows that the method should be susceptible of more general application to elements with atomic numbers within the range 15430, where most resonance peaks occur.

Discriminator

volts

FIG. 3. Gamma spectrograph of indium foil wrapping. values for the indium antimonide is almost certainly due to discrepancies in the antimony activities. This can be attributed to the fact that, whereas it was possible to provide a sufficient thickness of indium foil to suppress resonance absorption in the indium of the sample, the restriction imposed by the use of standard MERLIN irradiation cans to a thickness of 1 cm of antimony alloy instead of the 1.7 cm calculated has resulted in a less complete suppression of resonance absorption in the antimony of the sample. However, calculation of the expected antimony activities, in the absence of suppression, give values which are six

Acknowledgments-The authors wish to thank DR. A. J. SALMON and his reactor staff for making available They the irradiation facilities of the MERLIN reactor. are also indebted to MR. B. E. RICHARDS and DR. K. G. STEPHENS for general discussion and assistance with some of the calculations, and to MR. A. HARE of the Metallurgy Branch, Royal Military College of Science, for the preparation of the indium foil and the lead-antimony alloy.

REFERENCES 1. HUGHES D. J. and HARVEY J. A. Neutron Cross-Sections, U.S. Atomic Energy Commission, McGrawHill, New York (1955). 2. STROMINGERD., HOLLANDER J. M. and SEABORG G. T. Rev. mod. Phys. 30, (2)) 585 (1958). 3. STEPHENS K. G. A.E.I. Research Laboratories. Private communication ( 196 1) . 4. Rrcrmnns B. E. A.E.I. Research Laboratories. Private communication (1961). 5. Metals Handbook, pp. 153341 (1939).