J. Nuclear Energy. 1954. Vol. 1. pp 144 to 149. Pergamon Press Ltd., London

THE THERMAL NEUTRON ABSORPTION CROSS-SECTION OF BORON

By A. GREEN, D. J. LITTLEK, E. E. LOCKETT, V. G. SMALL, and A. H. SPURWAY A.E.R.E., Harwell

and

E. BOWELL C.I., Chatham

(Received August 19.54)

Abstract-This paper summarizes measurements of the thermal neutron absorption cross-section of boron made at Harwell, in Sweden, and in the U.S.A. It also describes a new measurement made at Harwell. The agreement between the Harwell and Swedish values is good, but there is still some disagreement between these and the latest American values.

1. INTRODUCTION

BECAUSE it has a high thermal neutron absorption cross-section which varies inversely with velocity, boron has been used as a standard for absorption cross-section measure- ments made on the G.L.E.E.P. oscillator (COLMER and LITTLER, 1950). Until 1951, the 2,200 mjsec absorption cross-section was taken to be 710 & 21 barns, which is a mean value compiled by Ross and STORY (1949) from four American measurements. These were 708 barns (BACHER, BAKER, and MCDANIEL, 1946), 743 f 25 barns (RAINWATER and HAVENS, 1946), 732 + 9 barns (SUTTON et al., 1947), and 703 barns (FERMI, MARSHALL, and MARSHALL, 1947).

However, in 1951, in view of the spread in the American values and the fact that ‘THODE et al. (1948) had reported that the boron-10 content of boron varied according to its geographical origin, LITTLER and LOCKETT decided to reserve a stock of boric acid for use as a British standard and asked P. A. ECELSTAFF to measure absolutely the absorption cross-section of the boron in it. Before the boric acid was used it was chemically analyzed for high cross-section impurities, and no significant amounts of these were found.

The absolute measurement was made using boric oxide prepared by ignition of the boric acid, and dissolved in heavy water. Some heavy caustic soda (Na OD) was added to increase the solubility of the boric oxide, and the transmission of a silica cell containing this solution was compared with that of a cell containing only heavy water and heavy caustic soda. From these measurements EGELSTAFF (1953) obtained a value for the absorption cross-section at 2,200 mjsec for the Harwell boron of 781 & 6 barns.

At about the same time as EGELSTAFF’S measurements were being made, new measurements were undertaken in the U.S.A. on an American standard boron sample (which we will refer to as the Argonne-Brookhaven boron). These measure- ments, using a method similar to that of EGELSTAFF, were made by RINGO (HAMERMESH, RINGO, and WEXLER, 1953), who obtained a value of 755 & 3 barns, and by CARTER et al., (1953), who obtained a value of 749 & 4 barns. We may therefore take the American value for the Argonne-Brookhaven standard ?o be 752 & 3 barns.

144

The thermal neutron absorption cross-section of boron 14.5

In Sweden, VON DARDEL and WALTNER (1953) measured the mean lifetime of thermal neutrons in a tank of borax solution, and obtained a value for the cross- section of the Swedish boron of 708 _& 12 barns. Also SCOTT and THOMSON (1953), and SCOTT, THOMSON, and WRIGHT (1954), performing similar experiments in the U.S.A., reported values for the boron cross-section of 709 & 17 barns and 744 I_ 20 barns.

Samples of the Swedish borax and the Argonne-Brookhaven standard boric acid were obtained, and the cross-section:3 of the boron in them were compared with the Harwell standard boron cross-section by oscillator measurements in the G.L.E.E.P. In all cases the boron compounds were ignited at 900°C for at least twenty-four hours, and were then dissolved in heavy water. After correcting for the absorption due to sodium in the Swedish borax, the ratios found were:

Swedish boron = 0.983 -t OGO3

%&well boron

Argonn;;H”,;e;;;t;I; boron = Q.985 & 0.003

(The latter value is the mean of measurements made on two separate samples received from the U.S.A.)

Using these ratios and EGELSTAFF’S result, we obtained a value of 769 f 6 barns for the Argonne-Brookhaven boron to be compared with their value of 752 f 3, and 768 & 6 barns for the Swedish boron to be compared with their value of 708 & 12 barns.

It was in an attempt to resolve these discrepancies that the following experiment was carried out.

2. COMPARISON OF THE CROSS-SECTIONS OF GOLD AND HARWELL 130RON

The measurements described in this section were made in a new oscillator situated in the reflector of the B.E.P.O. The oscillator has been described by SMALL and SPURWAY (1954), and is essentially the same as that first described by HOOVER et al. (1948).

It was decided to compare the Harwell boron with gold to obtain an independent value for the cross-section of the Harwell boron. Gold is a single isotope element which can be obtained in a very pure state. Its absorption cross-section at 2,200 m;scc has been measured by CARTER et al. (I 953) and found to be 98.7 -5: 0.6 barns. EGELSTAFF measured the total cross-section of gold at very long neutron wavelengths in 1951 ; after applying the same correction for incoherent scattering as CAKIER et al., he deduced the 2,200 mjsec cross-section of gold to be 98.4 ~1: 0.9 barns (EC;EL.STA~-I;. 1954). These two results are in very good agreement, and wc shall take the 2.200 tn,‘sec cross-section of gold to be 98.6 II 0.6 barns.

Evidently the experiment had to be done in such a way that the effects of setf- screening in the samples was almost eliminated. One way of doing this is to use heavy water solutions of the two elements concerned, such that we have the same absorption areas in the same volumes. Boric oxide is easily dissolved in heavy water, but there is not a suitable gold compound to use as a solute. At the suggestion of J. F. RAFFLE, we therefore made use of silver nitrate as an intermediary, and compared silver

146 A. GREEN, D. J. LITTLER, E. E. LOCKETT, V. G. SMALL, A. H. SPIJRIVAY, and E. BOWELL

nitrate solution with boric oxide solution, and silver foils with gold foils. For the foil comparisons it was arranged that the foils were of the same surface area and had the same nuclear absorption area.

For the solution comparisons, the boron solutions were made by dissolving boric oxide in heavy water. The silver solutibns were prepared from some of the same silver as was used in the foil measurements, to ensure, as far as possible, that any impurities in the silver would not affect the gold-boron comparison; in any case, the silver was of 99.999 per cent purity. The silver was dissolved in pure nitric acid, and silver nitrate made by four crystallizations, the first two being made from light water, and the second two from heavy water. Finally, the silver nitrate was dissolved in heavy water after being dried over phosphorous pentoxide.

In this experiment, as well as in the G.L.E.E.P. comparison experiments and EGELSTAFF’S absolute determination of the Harwell boron cross-section, chemical analyses were made on the samples either before or after the experiments to check the amounts of silver, gold or boron which were being measured. In all cases very good agreement was found between the amounts of material weighed into the solutions and the amounts determined by chemical analysis.

Now, both gold and silver have resonances at about 5 eV, whereas boron is a l/v absorber. However, the cadmium ratio of thin gold inside the oscillator chamber was found to be about 440, so that we can ignore the effects of the capture of epi- cadmium neutrons in these measurements. Furthermore, although the cross-section variations of silver and gold are not strictly l/v below the cadmium cut-off, we have calculated that the comparisons in the oscillator are the same to within 0.1 per cent as for measurements made with neutrons of 2,200 m/set velocity. The calculation assumes that the boron detector chamber is a l/v absorber, and that the thermal neutron spectrum in it is Maxwellian.

Ten foil comparisons with different thicknesses of foil, and ten comparisons of silver nitrate and boric oxide solutions of different strengths, were made, and the mean results were found to be:

Cross-section of silver Cross-section of gold

= 0.646 I OGOl

Cross-section of silver nitrate Cross-section of Harwell boron

= 0.0836 i_ 09005

Taking the cross-section of gold as 98.6 f 0.6 barns, and the ratio

Cross-section of nitrogen Cross-section of boron

= oIxl25 f oxlOO

(COLMER and LITTLER, 1950), we have the following results.

Cross-section of silver at 2,200 m/set = 63.1 & 0.4 barns.

Cross-section of Harwell boron at 2,200 m/set = 785 f 8 barns.

POMERANCE (1951) reported the silver cross-section as 60 f 3 barns, assuming a value for gold of 95 barns. However, taking the gold cross-section as 98.6 barns, POMERANCE’S figure for silver becomes 62 & 3 barns, which is in good agreement with our value. SCOTT, THOMSON, and WRIGHT (1954) quote a value of 58 + 2 barns, which does not agree with our value.

The thermal neutron absorption cross-section of boron 147

Our boron figure of 785 f 8 barns is in excellent agreement with ENGELSTAFF’S value of 781 h 6 barns.

3. DISCUSSION OF RESULTS

Since the measurements on the B.E.P.O. oscillator were completed, we have heard from Sweden (VON DARDEL and SJ~~STRAND, 1954) that the Swedes have repeated their measurements of the boron cross-section, using a modified method. They have made measurements both on the Swedish boron and on the Argonne-Brookhaven boron, and obtain:

Cross-section of Swedish boron = 763 I;r 3 barns.

Cross-section of Argonne-Brookhaven boron .= 764 i 3 barns.

They now discard the results published by VON DARDEL and WALTNER (1953) in favour of these later measurements. As a tentative explanation of the different result they suggest that, in their earlier experiment they erroneously assumed that the background counting-rate during the decay-time of the thermal neutrons in the borax solution was constant, whereas it in fact changed with time. If this explanation be correct, it may possibly explain the low values obtained for boron and silver by SCOTT and THOMSON (1953) and SCOTT, THOMSON, and WRIGHT (1954), since their experiments were somewhat similar to that of VON DARDEL and WALTNER.

We have also heard that the ratios of Swedish to American boron, and Harwell to American boron, have been measured in the U.S.A. by a pile-oscillator method (RINGO, 1953). These ratios are:

Cross-section of Harwell boron Cross-section of Argonne-Brookhaven boron

:= 1.009 & oGO.5

Cross-section of Swedish boron Cross-section of Argonne-Brookhaven boron

:= 0.991 * 0.005

We may therefore compile the following comparison table. In it we have taken the mean of 785 & 8 barns and 781 f 6 barns as our value for the Harwell standard boron.

TABLE 1

Argonne-Brookhaven boron

Swedish boron

-_

Harwell value 783 & 5 i 771 i 6 770 h 6 Swedish value ) - 764 & 3 763 & 3 American value 759 i: 5 752 c 3 745 * 5

It will be seen that the Harwell and Swedish results are in statistical agreement, whereas the American results are not in statistical agreement with either the Harwell or the Swedish ones. It is known that the discrepancies are not due to errors in the determination of the boron content of the samples used, since the American and Swedish samples have been checked by chemical analysis in England, and the Swedish sample has been checked in the U.S.A., all the measurements being in good agreement.

Jf we consider the earlier American results quoted in Section 1, it appears that at

148 A. GREEN, D. J. LIITLER, E. E. LOCKETT, V. G. SMALL, A. SPURWAY, and E. BOWELL

least two of the values, namely those obtained by FERMI, MARSHALL, and MARSHALL, and BACHER, BAKER, and MCDANIEL, must have been in error. Although the geo- graphical origin of the boron used by these experimentalists is unknown, it is unlikely that the cross-sections of their boron samples differed from that of the Harwell standard boron by more than 34 per cent, since this is the total range of variation found by THODE et al. in samples collected from all over the world.

4. CONCLUSION

The two measurements made at Harwell of the cross-section of the Harwell boron standard are in very good agreement. The British measurements also agree well with those made in Sweden. This is very gratifying, since three different techniques were used.

The disagreement between the British and American results is surprising, since there is no disagreement between the Harwell and Brookhaven velocity selector measurements of the gold cross-section. Furthermore, the mean of these two gold cross-section values was used in the oscillator determination of the Harwell boron cross-section. This tends to suggest that the two American measurements on the Argonne-Brookhaven boron sample, though agreeing with each other, are both rather low.

Two further checks are to be made on the cross-section of the Harwell boron standard. These are:

(a) An absolute cross-section measurement by ECELSTAFF on a solid sample of borax. This is being done to cover the possibility suggested by VON DARDEL that, in EGELSTAFF’S original experiment, the scattering cross-section per molecule of the D,O may have been slightly changed by the addition of the boric oxide.

The borax to be used has already been compared with the Harwell standard boron, and the ratio was found to be:

Cross-section of boron in Harwell borax Cross-section of Harwell standard boron

== 0.998 _& 0.003

(b) A comparison, by absolute counting, of the activity acquired by gold and sodium when irradiated in the same thermal neutron flux, followed by a comparison of the sodium and Harwell boron cross-sections in the G.L.E.E.P.

If these two measurements agree with those already done at Harwell, then we must conclude that the American measurements on the Argonne-Brookhaven standard are too low.

In any case it appears that some of the earlier American measurements of the boron cross-section must have been in error, and should be discarded.

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