The neutron capture cross-sections of 149Sm and 150Sm

J. Inorg. Nu¢l. Chem,, 1961, Vol. 17. t-~. 6 to 11. PergamonPress Ltd.

THE

NEUTRON CAPTURE O F 149Sm A N D

CROSS-SECTIONS 15°Sm

K . L. AITKEN a n d F. W . CORNISH* Atomic Energy Research Establishment, Harwell, Didcot, Bcrks. (Received 7 August 1960)

Abstract~The effective neutron cross-section of t'gSm has been determined in three different reactor positions by measuring mass speetrometrically the amount of xs°Sm formed by neutron capture. Analysis of the results shows that the effective cross-section of x'sSm in a well-thermalized neutron spectrum of known temperature can be calculated to within 4-2 per cent. The cross section of is°Sin in a therrnalized spectrum at 50°C with r = 0-0015 is found to be 102 4- 5 barns.

Trm stable nuclide m S m is an important fission product poison and the exact calculation of its neutron absorption under any given conditions is of interest in connection with reactivity changes. Several workers have determined 'pile neutron' cross-sections ~x-s~but such results are of limited value since the cross-section is highly dependent on neutron temperature. In the present work the cross-section has been measured in three well-eharacterised reactor spectra; the results are compared with other data obtained in known spectra at the end of the paper. EXPERIMENTAL The determinations were based on the mass-spectrometric measurement of the xs°Sm produced on irradiating "foils" of samarium electromagnetically enriched in the 149 isotope. If X"No, XS°N0,I"N~ and XS°N1 represent the original and final numbers of the appropriate atoms, then with Ro = tS°No/ ~'SN0 and R I = XS°N~p'SN1 it can easily be shown that (RI + 1)/(Ro + 1) = exp i~

(1)

where i is the integrated neutron flux and ~ is the effective cross-section of " ' S m . This equation is simple because capture by x'sSm and 15°Sm is negligible compared with captule by x4'Sm.m The factor i was obtained by the simultaneous irradiation of cobalt monitors. Three separate irradiations were performed, the samarium and cobalt being contained in aluminium cans 7.5 cm high and 2.5 cm diameter. In experiment 1 samples of samarium and cobalt were contained in a 0"036" thick cadmium 'pill' box (½0 × ½" diameter) at the bottom of the can; two other cobalt wires and another samarium sample were also placed in the can, being separated from the cadmium box and from each other bY cylinders of graphite (see Table 1). The irradiation was carried out in the core of BEPO for 14 days with a flux of N10 as neutrons/era -s sec-L No cadmium was used in .experiments 2 and 3; alternate samples of samarium and cobalt were separated by cylinders of graphite. Both these irradiations were performed in the graphite reflector of BEPO; experiment 2 took 36 days in a flux of ,~5 x 10xxneutrons era-* sec -1, and experiment 3 took 180 days in a flux of ~,10 u neutrons c m -s Sec -1. In order to simplify the chemistry and thus avoid any possible contamination after irradiation, the samarium "foils" were prepared by impregnating 1-2 c m diameter disks of low ash filterpaper. * Present address: Atomic Weapons Research Establishment, Aldermaston, Berks. cx~E. A. Met~agA, M. J. PARKER, J. A. PETRUSZ,A and R. H. TOMLmSON, Canad. J. Chem. 33, 830 (1955). csJ D. J. LrrrLEa, Proceedings o f the International Conference on the Peaceful Uses o f Atomic Energy, Geneva, 1955, A/CONF. 151P/II, United Nations (1956). es~ K. L. AITKEN,D. J. Ln'rLEa, E. E. Locrdea-r and G. H. PALMER,J. Nucl. Energy, 4, 33 (1957). 6

T h e n e u t r o n capture cross-sections o f : " S i n and xS~Sm

"-7.~

7

o.r

If ~O

.2 6 . tl

6 6

6 ~r qq

~o 0~ ¢~ ~. X

X

X ¢q

q. oo O0

~b

<

~D

z

1_

¢b

6

6

6

6 ¢q

I

I I

II

I

I

I

I

I

I ~~

).

o

r/)

I. a~

<

t-

ga., , .

g, 6

~

~

~

¢b~

6

"

J~

I

6

"~o

0

0

I

I~

"

¢'q

~do ¢'q

"~

ob r,,I

~

~

oo

%O

~O

~D

~b

~

6

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8

K . L . AITKEN and F. W. C o s ~

The enriched samarium nitrate solution supplied by the Electromagnetic Separation Group was diluted to 300/~g/ml; the disks were wetted with 20 #1 of this solution and were immediately cooled and vacuum freeze-dried. This procedure ensured a reasonably even distribution of --,6 pg Sm/cm*, a "foil" thickness too small to give rise to appreciable neutron self-shielding. For irradiation each loaded disk was sandwiched between two plain fiRer paper disks and the whole combination was wrapped in 0.0002 in. aluminium foil. After irradiation the slightly carbonised, brittle disks were ashed at 600°C in a platinum crucible; the ash was dissolved in 0.1 ml of 50 per cent nitric acid, evaporated almost to dryness and the resulting residue was dissolved in 50 #1 of distilled water. Mass spectrometric analysis was performed on 5 pl of this solution. Analysis of the solution resulting from the application of the above treatment to an unirradiated loaded disk gave the percentage abundances shown in Table 2. The errors quoted are standard deviations from the mean. TABLE M u s number

2.~Pr~cENTAO ISOTOPIC COMIN3SITION OF SAMARIUM SAMPLES

144

147

148

Original solution

0'041

1"98

0"365

Solution recovered from tmirradiated

0"042

1"98

0"363

disk

Natural s a m ~ . um

-4- 0-001

4- 0"001 3"1

-4- 0"01

4- 0"01 15-0

± 0-008 4- 0'008

11"2

149 96"58 4- 0"03

96'56 4- 0"03

150

151

152

154

0.430 4- 0"006

0.000

0.397 ± 0"005

0.202 -4- 0'004

0"433 + 0'006

0.000

0'396 ± 0'005

0'203 ± 0'004

0

26"8

22-7

13"8

7'4

I t

The mass spectrometer was a prototype for the present Metropolitan-Vickers M.S.5 model, with 12 in. radius and a mass resolution of I in 800. The ion source was a multiple filament thermal ionization type; t4) one filament was held at --01800°K and the other, holding the sample, was warmed until evaporation occurred with subsequent ionisation at the hotter filament. The ion beams were detected with an ll-stage Allen type electron multiplierc6) which was stable for long periods and linear over the range used. Currents were never less than 10-is A. Independent experiments supported the conclusion of PLOCH and WALCHERcs) that the response was inversely proportional to ion velocity; a correction factor ofthe form ~/(ms/ml) was therefore applied in this work. Fresh filaments were cleaned at 2000°K in vacuum for at least an hour before loading 5 pl of samarium solution (,--1 pg $m). The source was rapidly heated in the spectrometer until ionized samarium was just detectable; the source was left in this condition for 24 hr to remove more easily evaporated materials before the analysis. In some experiments traces of neodymium, apparently of natural isotopic abundance, were detected. The least abundant samarium isotope in the present work was l"Sm (,,,.0.04 per cent); no spectrogram was used inwhich any of the neodymium mass peaks at 142, 143, 145 and 146 exceeded 10 per cent of this value; e v e n so, appropriate corrections for the presence of neodymium were made. The samples were not exhausted after several hours of full-scale running, which was ample time to take up to forty spectrograms. One sample in experiment 3 was carefully examined for mass 151. The peak at this mass was compared with the peak for mass 152 at a greatlyincreased ion emission; peaks at masses 152 and 150 were compared at normal emission, the product giving the mass ratio 151/150. Appropriate "background" corrections were made when comparing peaks at masses 151 and 152. Johnson Mattbey's "Specpure" cobalt wire, diameter 0.005 in. and mass ,-,1-1.6 mgm, was used as the flux monitor. The same wire was also used both free and wrapped in cadmium to determine before irradiation the ratio of epithernml to thermal neutrons in the irradiation positions used. Similar wires of diameter 0.001, 0.005, 0.010 and 0-020 in. were used in earlier experiments to measure the extent of neutron self-shielding; this was determined by extrapolating to zero diameter the curves relating activity per unit weight and diameter for different wires irradiatedtogether. The mass of each wire was measured on a quartz fibre ultra=microbaiancec,) using platinum weights which were calibrated against an NPL 50 mg weight. Each wire was wrapped in two layers of 0.0002 in. aluminium foil before irradiation. After irradiation the monitors were allowed to decay for at least a week to ensure that no significant activity arose from the ahiminium foil; without removing the foil ~4) M. O. I N O H ~ and W. A. CHUrKA,Rev. Scl. Ig;trum. 24, 518 (1953). (S) J. $. ALLEN, Rev. Scl. lnstrum. 18, 739 (1957). re) W . PLOCH a n d W. WALCHER, Rgv. Sc[. Instr. 22, 1028 (1951). {7} H . CAitMICHAEL, Canad. ?. Phys. 30, 524 (1952).

The neutron capture cross-sections of a"Sm and xS°Sm

9

each monitor was carefully compared with a radium source in a pressurized argon ionization chamber. ~s~ The radium source had previously been compared with active cobalt wires which were subsequently dissolved and assayed for radioactivity by ~-~,-coincidencecounting,c'~ RESULTS The measured activity of each cobalt wire monitor was subject to two corrections. Firstly, correction was made for decay over the period from cessation of irradiation to actual measurement (1-4 weeks). The half life of e°Co has been taken as 5-26 =k 0.03 years. ~°~ Secondly, correction was made for "self-shielding" in the 0.005 in. diameter cobalt wires; the correction factors, derived from earlier experiments, were 1.046, 1.026 and 1.022 for experiments 1, 2 and 3 respectively. The factor for cobalt wires irradiated under 0.036 in. cadmium was 1-285. The integrated neutron flux i, was then calculated from the normal irradiation equation

I

specific activity of monitor = i2tO 1

( i ~ + 20 + (2t) 2 + (2t)(i~)6 + (i~)~

]

(2)

by successive approximation. The quantity t is the total time in the reactor. The effective cross section ~ was derived from the equation

: Cro(gq- rs)

(3)

where % is the 2200 m/sec cross-section, and g and s are functions of neutron temperature depending on departure from the 1Iv law. (11) For cobalt g is unity and

s

/ ( 4iq ao ~/ \rrI10/

57" 1 barns representing the non-1/v part of the resonance integral;(19) a0 was taken as 36-6 =k 0-4 barns.el3) The parameter r was obtained from cadmium ratio measurements with cobalt monitors in the same irradiation positions as in the three experiments, using the equation (I + rs)/rRcd ---=s + Tt/KTo I (4) where Rcd is the cadmium ratio, and K was taken as 2"25. ~lx) The "neutron temperature," T, was assumed to be 130 °, 80° and 50°C (14) in the three experiments respectively since the moderator temperature at the irradiation positions were 80 °, 50° and 30°C. txS~ Many uncertainties which are not easily subject to evaluation enter into the calculation of i; these include errors in the absolute standardization and half-life of e°Co, the cross-section of 59Co and the experimental measurements themselves. It seems likely that the standard error on values of i is ,~-[-2-5 per cent. The error introduced in the calculation of ~ for 1495m by uncertainties in the mass analysis leads to an overall standard error o f , ~ i 3 per cent. ts) K. L. AITKEN and F. W. CoRNtsa, U.K.A.E.A. Report AERE/C/R 2627 (1958). ~') B. SLAOE, Private communication (1959). ~0~ W. E. PERRY. Private communication (1959). ~xx~C. H. WESTCOTT, W. H. WALKER and T. K. ALeXANDeR, Proceedings of the Second International Conference on the Peaceful Uses of Atomic Energy, Geneva, 1958, A/Conf. 15/P/202. United Nations (1959). ~ts) T. A. EAsrwooo and R. D. WERrq~R, Canadian Atomic Energy Report, CI-207 (1959). ~ta~ C. H. WESTCOTT, Canadian Atomic Energy Report, CRRP-787 (1958). ix4) H. Ros~. Private communication (1959). ~xb~D. J~FF~RSON-LowoAv. Private communication (1959)

10

K . L . Arrl~N and F. W. CORmSH DISCUSSION

As expected, the effective cross-section of ~°Sm (#) varies considerably from one irradiation position to another. That this is due to temperature effects rather than to variation in epi-thermal flux is shown by the results obtained after irradiation under 0.036 ~ cadmium (experiment l). The total resonance integral for 149Sm, based on 72.3 barns as the resonance integral of cobalt, (1~ is 4,400 barns. The influence of temperature is again brought out in the variation of effective cross-section ~ under different TABLE 3 . - - E ~ c T t ~

CROSS-SECTIONOF xttSm UNDERVARIOUSCONDmONSAND

CALCULATEDVALUESOF (7o 0"o

T Author

#

(°c)

Using WEs'rco~'s

Using PATI'ENDEN'S

8" & s data (ls~

g & a data(")

TATTERSALL et al. ~17)

20

0

68,200

42,100 (±700)

~,5~(±7~)

Present work

50

0.0015

73,620

42,600 (±1200)

4o,9o0 (±1200)

70

0"042

72,200

41,600 (±1000)

39,100 (-t- 1000)

et al. (s)

70

0'042

74,500

42,900 (±1000)

4o,4oo(4-1ooo)

Present work

80

0.0136

76,430

42,200 (~1200)

40,000 (+1200)

et al. c1'~

120

0"018

80,500

42,200 (-4-1000)

39,700 ( ± 1000)

Present work

130

0.0365

79,920

42,100 (±1200)

39,300 (-4-1200)

137

0"0098

83,800

42,600 (±1000)

4O,lOO(+100o)

42,300

40,000

42,030 . s )

39,900 c1.~

TATrERSALL et al. (1~)

ArrKEN

BIDINOS'n

BIDINOSTI et al."s~

Mean (70 Value of ~0 used for calculating g and s

conditions (Table 3). Different r values account for the apparent inversion of # for 120 ° and 130% Values of % calculated from equation (3) using the two independent sets of g and s values given by W~TCOTT (18) and PArrer~EN (m are listed in Table 3. The constancy of %, using any one set of g and s data is extremely good; this is particularly interesting, since cross-sections given by TATTERSALLe t al. u " and by AITKEN e t al. ~8~ a r e based on "Harwell Standard Boron" (766.6 -4- 3.5 barns), while (m N. J. PATTENDEN, Proceedings of the Second International Conference on the Peaceful Uses o f Atomic Energy, Geneva, 1958, A/Conf. 15/P/ll, United Nations (1959).

t17) R. B. TATT~nSALL,W. A. COOpeR, D. Jowrrr and S. K. PATTENDeN,U.K.A.E.A. Report, AERE/R/R 2459 (1958).

The neutron capture cross-sections of l " S m and 16°Sm

11

the present work and that of BIDINOSTIe t al. ~18~ is based on tr0 for 5aCo being 36.6 and 36.5 barns respectively. It is clear that the reaction rate of 149Sm in neutron spectra within the range covered by Table 3 is adequately accounted for by using an effective cross-section calculated from either set of g and s values. Integral cross-section measurements can throw no light on the accuracy of the two values of o 0. The integrated neutron flux was somewhat higher in experiment 3 (Table 1) and it was felt worthwhile to assay the 15tSm content as described earlier. At this particular irradiation level the quantity ofl51Sm remaining is only very slightly dependent on the cross sections of both 149Sm and 15XSm; it is mainly dependent on the 15°Sm crosssection. By using the measured final atom ratio 151/150 ---- 0.000250 + 0.000005, and taking ~ for 14aSm as 73,620 barns (Table 1) and t~ for l~lSm as 12,800 barns t18~ the effective cross-section oflS°Sm in experiment 3 was found to be 102 i 5 barns. The expression used for the calculation was based on the normal irradiation equations; it is too cumbersome to be quoted. It should be noted however, that a change of I0 per cent in the cross-sections of 149Sm or 151Sm leads to less than 1 per cent change in the calculated cross-section of 15°Sm. Acknowledgements--We wish to thank Mr. B. SLADE of the Isotopes Division, Harwell for the absolute standardization of cobalt sources and Dr. M. L. SMITH of the Electromagnetic Separator Group, Harwell, for supplying the enriched samarium. Thanks are also due to Mr. W. E. PERRY of the National Physical Laboratory, Teddington for recent data on the half-life of 6°Co, to Mr. D. JEFFEP,~ON-LOVEDAY,Reactor Division, Harwell, for the measurement of moderator temperatures and Dr. H. ROSE of the Physics Division, Harwell, for advice and discussion. ~xs; D. R. BIDINOST[,H. R. FtCKEL and R. H. TOMLINSON,Proceedings of the Secondlnternational Conference on the Peaceful Usesof Atomic Energy, Geneva, 1958, A/Conf. 15/P/201. United Nations 0959).