Activation measurements of the cross section of the reaction 147Pm(n, γ)148gPm for reactor neutrons

J. inorg, nucl. Chem.. 1970, Vol. 32. pp. 3433 to 3440.

ACTIVATION SECTION OF

Pergamon Press.

Printed in Great Britain

MEASUREMENTS OF THE CROSS THE REACTION 147pm(n, y)148~pm F O R REACTOR NEUTRONS

M. J. C A B E L L Analytical Sciences Division, Atomic Energy Research Establishment, Harwell, Berkshire. England

(Received 1 May 1970) A b s t r a c t - T h e effective cross section of the reaction ~4rpm(n, y)t~"Pm has been measured for reactor neutrons. 197Au and 59Co monitors have been used to determine the 2200 m/sec neutron fluxes in the spectra employed, and cadmium ratio measurements for gold and the silver-cobalt activity ratio method have been used to determine the epithermal indices. 7"he amounts of ~48"Pmformed, and the amounts of ~4rPm from which they were formed, were both determined after purification of the irradiated material, y-Spectrometry was used for the former purpose, and the 4~-/3-y tracer method, with 46Sc as the tracer, was used for the latter. The cross section of the reaction for Maxwellian neutrons (~r,,g) and its reduced resonance integral (Y') were found to be 96.0 ± 1.8 barns (T = 60°C) and 1274 ± 66 barns respectively. INTRODUCTION

THE FISSION product reactor poison 147pm captures neutrons to produce both 5-4 d 14s-qpm and 43 d 148mpm.These (and subsequent) reactions are shown schematically in Fig. 1, and measurements of the cross sections are summarised in Table 1. It will be seen that there is good agreement between values for the total cross section for 2200 m/sec neutrons in which both isomers are formed (Table 1. column 2). By contrast there is almost no agreement between values for the capture cross sections for the formation of the individual isomers (columns 3-8), and this is true whether reactor neutrons (i.e. 6-), Maxwellian neutrons (i.e. o-og), or epithermal neutrons (i.e. E') are involved. The aim of the present experiments was to improve this situation. By using different reactor spectra new values of 6-, crog and E' have been obtained for the reaction ~47pm(n,y)148Upm. It is hoped to follow these eventually by similar measurements for the reaction 147pm(n, T)148mpm. 148m

O'SG

m

148 Sm

149pro

149Sm

Fig. I. Schematic diagram of the nuclides involved in the neutron irradiation of ]47pm, 3433

3434

M.J.

CABELL

.,-.7

~'~ O

e'~ r . u~,~

¢'~ r".r~,,, D

o

< .

.

.

,~

.

,6 I"..-

¢",1 ~

¢~1

¢'q

,-..7 ,b

.~

~5

t"q

41

÷1

41

41

+1

+~

E

E

~b

e~

L) ..d

+1

+1

÷1

+~

+1

+1

[.-

e~

+1 ,.. ,-¢


÷1

~D

,4 . = " " "-a~, ....

-~5 --e,i

Activation measurements of 14~Pm(n, y)14aoPm

3435

EXPERIMENTAL Nuclear data f o r Pm nuclides The results of the experiments are directly related to the half-lives assumed for 147pm and 14supra. Measurements of the former quantity are summarised in Table 2; a value of 2-623 +- 0.001 yr has been used in this work. The author's own value of 5.370---0-009 d has been used for the half-life of t~8oPm [31]. This agrees with the results obtained by other workers]12-14, 16, 17, 32] but is much more precise. Table 2. Published values of the half-life of ~47pm (yr) Ref. Reynolds et al., 1968 [11 ] Jordan, 1967 [ 12] Anspach et al., 1965 [ 13] Eichelberger et al., 1965 [14] Wheelwright et al., 1965 [ 15] Flynn et al., 1965 [ 16] Roberts et al., 1963 [17] Cali et al., 1959118] Merritt et al., 1957119] Schuman et al., 1956 [20] Melaika et al., 1955 [21] Schuman et al., 1951 [22]

H alf-life 2.62---_0-01 2.6234 ___0.0005 2-618--.0.007 2.6226 +- 0.002 2.620 +- 0-005 2.60 +- 0.02 2.67---0-06 2.7--+0.1 2.64 +- 0.02 2-66---0.02 2.52 +- 0.08 2.6---0.2

Of lesser importance are the values assumed for the half-life of 148"Pm and for the proportion of this nuclide which decays via the ground state (the "k'" of Fig. 1). Measurements of these quantities are summarised in Table 3. They give mean values of 43.1 + 1.1 d and 6.8+0.7% respectively, and these are the values used in this work. The cross sections o-r~t, o-8.~and o-st; (see Fig. 1) are also required in the calculations. For the first 1 I. S. A. Reynolds, J. F. Emery and E. 1. Wyatt, Nucl. Sci. Engng 32, 46 (1968). 12. K. C. Jordan, USA EC Rep. M LM- 1399 (1967). 13. S. C. Anspach, L. M. Cavallo, S. B. Garfinkel, J. M. R. Hutchinson and C. N. Smith, U.S. Nat. Bur. Stan. Misc. Pub. 260-9 (1965). 14. J. F. Eichelberger, G. R. Grove and L. V. Jones, USA EC Rep. M LM-1221 (1965). 15. E.J. Wheelwright, D, M. Fleming and F. P. Roberts, J. phys. Chem. 69, 1220 (1965). 16. K. F. Flynn, L. E. Glendenin and E. P. Steinberg, Nucl. Sci. Engng 22, 416 (1965). 17. F. P. Roberts, E. J. Wheelwright and W. Y. Matsumoto, USA EC Rep. HW-77296 (1963). 18. J. P. Cali and L. F. Lowe, Nucleonics 17, 89 (1959). 19. W. F. Merritt, P. J. Campion and R. C. Hawkins, (5'an. J. Phys. 35, 16 (1957). 20. R. P. Schuman, M. E. Jones and A. C. Miewhierter, J. inorg, nucl. Chem. 3, 160 (1956). 21. E. A. Melaika, M. J. Parker, J. A. Petruska and R. L. Tomlinson, Can. J. Chem. 33,830 (1955). 22. R. P. Schuman and A. Camilli, Phys. Rev. 84, 159 ( 195 t ). 23. C. V. K. Baba, G. T. Ewan andJ. F. Su~irez, Nucl. Phys. 43, 264 (1963). 24. C. W. Reich, R. P. Schuman, J. R. Berreth, M. K. Brice and R. L. Heath, Phys. Rev. 127, 192 (1962). 25. C. F. Schwerdtferger, E. G. Funk andJ. W. Mielich, Phys. Rev. 125, 1641 (1962). 26. J. S. Eldridge and W. S. Lyon, Nucl. Phys. 23, 131 ( 1961 ). 27. S. K. Bhattacherjee, B. Sahai and C. V. K. Baba, Nucl. Phys. 12 356 (1959). 28. R. L. Folger, P. C, Stevenson and G. T. Seaborg, Phys. Rev. 98, 107 t 1955). 29. J. K. Long and M. L. Pool, Phys. Rev. 85, 137 (1952). 30. V. Kistiakowsky, Phys. Rev. 87,859 (1952). 3 I. M.J. Cabell and M. Wilkins, J. inorg, nucl. ('hem. 32, 1409 (1970). 32. J. D. Ktwbatov and M. L. Pool, Phys. Rev. 63,463 (1943).

3436

M.J. CABELL Table 3. Half-life of~4smpm and percentage decaying via 14supm

Ref.

Half-life (days)

Baba et al., 1963 [23] Reich et al., 1962 [24] Schwerdtferger et al., 1962 [25] Eldridge et al., 1961 [26] Bhattacherjee et al.,1959127] Folger et al., 1955[28] Long et al., 1952 [29] Kistiakowsky, 1952 [30]

40.6± 0.4 45.5 ± 0.5 41.8±0-2 45.8___2.9 43 48 42 ± l

Percentage decaying by isomeric transition to ground state 6.9±0-7 6.5±1.7 8

5-10

7.4

two quantities the weighted means of the values given in Refs. [3] and [6] have been used, i.e. 83b and 2.3 x 104b for Maxwellian neutrons respectively, and 2000b has been assumed for trsr;[33]. The uncertainties in thse values have a negligible effect on the results. Preparation o f target materials and monitors A number of small sealed silica ampoules, each containing between 1.5 and 3 p,g 147pm dissolved in about 100pal M HCI, were prepared. A number of similar ampoules, containing accurately known amounts (about 100 rag, containing about l 0 ptg gold) of a standard solution of gold dissolved in diluted aqua regia, were also prepared. Weighed cobalt wire (l cm lengths, 0.005 in. dia.), cobalt-aluminium wire (2.5 mm lengths, 0.05 in. dia., 1.012% cobalt) and silver-aluminium wire (2 cm lengths, 0.025 in. dia., 1% silver) neutron flux monitors were prepared also. Irradiations Two different reactor positions were used in the irradiations. In the first five irradiations, ampoules containing 14rpm, together with ampoules containing 19~Au and cobalt wire flux monitors, were irradiated in a pneumatic carrier position in the D I D O reactor, for about 10hr. In the last three irradiations, ampoules containing 14rpm, together with cobalt wire, cobalt-aluminium alloy wire and silver-aluminium alloy wire monitors, were irradiated in high-flux positions in the same reactor for about I hr. The pneumatic carrier fitted into the 12HGR2 hole of the reactor, which runs through the graphite reflector under the heavy water tank. The irradiated samples and monitors were contained in hollow terylene-reinforced resin cylinders and were stopped at a position in the rig at which flux gradients were at a minimum. This position was almost directly below the core centre and its immediate environment was water-cooled to 60°C. Separate experiments with a number of cobalt wire monitors established that ( 1) the 2200 m/sec flux varied linearly along the axis of the cylinders at a rate of 0.57 _+0.21 per cent per cm, and (2) the flux also varied across the diameter of the cylinders at a rate which could not have exceeded 2-67_0.41 per cent per cm. Since the J4;pm ampoules were packed tightly up against the 197Au ampoules, the two received the same 2200 m/sec neutron dose to within _+ 1'5 per cent at worst. The cobalt wire monitors received a dose intermediate between those experienced by the two solutions. Additional experiments involving the irradiation ofgold-aluminium alloys, both with and without cadmium boxes, showed that the epithermal index, r TV'~-~o, was always within the range 0.0012 ___0.0001 at all points within the cylinders. The rig used for the high-flux irradiations fitted into the hollow fuel rod D4 and was water-cooled to 60°C. Samples were contained in small aluminium cans. Post-irradiation treatment o f promethium solutions After irradiation the promethium solutions were purified by cation exchange chromatography and obtained finally in 2 ml 4 M HNOa. The progress of a similar purification will be illustrated elsewhere 33. M. D. Goldberg, S. F. Mughabghab, S. N. Purohit, B. A. Magurno and V. M. May, US.4EC Rep. BNL-325, 2nd Edn. Suppl. No. 2 (1966).

Activation measurements of ~47pm(n, y)~4sopm

3437

[34]. Aliquots were then taken from the purified solutions as follows. Three weighed aliquots, of about 0.2 g each, were evaporated to dryness as near-point sources on thin plastic films and their ~4s~pm contents were determined by y-ray spectrometry. Another weighed aliquot, of about l g, was diluted approx. 50-fold and a portion of this diluted solution was spiked with a known quantity of 46Sc. The ~47pm contents of aliquots of this final solution (containing about l0 -5 gg ~47pm) were then determined using a 47rB-T coincidence counter (see next section). The ),-counting equipment was specially calibrated for the absolute determination of ~48ypmusing a novel method which combined 4~rB-T coincidence counting with weighed least mean squares analyses of the gross B--decay curves obtained from mixtures of ~47pm, '4s~Pm, 148mpm and '4'~pm. A full description of the method used will be given elsewhere[34]. Having obtained the area of the 1-465 MeV y-photopeak at the time of counting, this was corrected for contributions due to the growth of 148gpm via ~48"pm from the end of the irradiation to the time of counting, and for the effect of sum peaks [31] (--~0.2 per cent of the peak area). An adjustment was then made for the decay of the initial amount of ~48~pm present from the end of the irradiation to the time of counting (~ 15 per cent) and a further correction was then applied to take account of the amount of ~48°pm formed via ~48"Pm during the irradiation (~ 0.02 per cent). The corrected photo-peak area was then related to the absolute disintegration rate of the ~48oPm in the source via the efficiency of the counter (known to +_0-8 per cent), and the amount of 148ypm formed by the reaction ~47pm(n. y)~48°Pm could then be obtained. Absolute determination o f ~47Pm Campion et a/.[35] have shown that, when two B--emitting nuclides are present together in the same physical and chemical form in a source, if the spectral shapes and end-point energies of the/3-particles are similar, there is a linear relationship between their B--counting efficiencies. This fact can be put to practical use to eliminate self-absorption errors in the 4~rB-counting of pure/3 -emitters. As might be expected, the method is particularly valuable if, as with ~47pm, the B--particle is a weak one. 83.9 d ~Sc decays 99-996 per cent by emission of a B--particle with an allowed spectrum and an end-point energy of 357 keV. This is sufficiently close to 224 keV, which is the end-point energy of the B--particle emitted in 100 per cent of 147pm disintegrations, for the efficiency tracing technique to be applied. (The B--spectrum of 147pm is first forbidden but very close to an allowed shape [36]). To establish the method a series of mixed t47pm-46Sc sources of different thicknesses was prepared. Weighed aliquots (~ 100 mg) from stock solutions of 147pm and 46Sc were delivered on to insulintreated VYNS films which had been gold-coated on their reverse sides. Various amounts of ScCI:~ carrier were added and then excess HF. The solutions were evaporated to dryness under an i.r. lamp and the sources so prepared were each covered by a second gold-coated VYNS film. These "sandwiches" were then counted in a 41r/3-y coincidence counter, with biases set to record both 4~Sc yphotopeaks. The absolute 46Sc content of the 46Sc stock solution was determined in a similar way in separate experiments at the same time. Since the amount of 46Sc added to each mixed source was now known, and since the 4zrB-counting efficiency for 46Sc, ¢n.sc, had been determined for each source also, the ~47pm count-rates for each source could be obtained by difference. These quantities could then be related to the count-rates per g of stock 147pm solution. The variation of count-rate per g of 147pm stock with ¢~,s,: was analysed by the method of least squares, taking errors in both variables into account, and (as expected) a linear relationship was found. Normalising this to the condition that ¢o.pm: 1 when E,,s~.= 1 gave the expression Et3,pm=

(

1-768 +- 0"025)e~.sc -- (0.768 + 0.021 ).

(l)

This relationship is shown in Fig. 2 (solid line); the rectangular boxes in this figure show some of the experimental data. The 147Pro contents of the irradiated solutions from the main experiments were determined by spiking aliquots with known amounts of 468c (see above) and counting sources prepared in the way.just described. Expression (1) was used to relate measured values of ~a,sc to the corresponding ~a,m- Since 34, M.J. Cabell and M. Wilkins, J. inorg, nucl. Chem. To be published ( 1971). 3 5, P.J. Campion, J. G. V. Taylor and J. S. Merritt, Int. J. appl. R adiat. Isotopes 8, 8 ( 1960). 36. H. M. Mahoud and E. J. Konopinski, Phys. Rev. 88, 1266 (1952).

3438

M. J. CABELL

I

>" 1 - 0 0 ~ zC)

I

I

u. ,a 0'80

-

0

_z

~0 '70 -:3 0 U

o0o -

_~0"50 I I

1,00

0.90

I

0.80

I

0.70

46Sc 4,~ COUNTING EFFICIENCY Fig. 2. The 47r/3 counting efficiencies of 465c and 147pm in mixed sources. at least 30 days was allowed to elapse from the end of the irradiation until these measurements were made, no correction was necessary for the small amounts of 14suPm and 148mpm remaining in the solution by that time. (The sum of the fl--activities of these was less than 0-1 per cent of that due to 14rpm, and their y-activities were largely "biased-off.' from interfering in the determination of Ca,so.) [Note. Since this work was completed Mowatt and Merritt [37] have described a similar method which they have used to determine ~47pm absolutely.]

Post-irradiation treatment of monitors The irradiated 19rAu solutions were transferred quantitatively to volumetric flasks, diluted to a known weight with M HCI, and weighed aliquots were taken for y-spectrometric analysis. The areas of the laSAu photopeaks at 412 keV were determined by a method which has already been described [38], and the corresponding counts per sec of ~98Au produced at the end of the irradiation per p~glayAu irradiated were calculated using 2.4946_+ 0.0010 d for the half-life of 198Au[31]. Replicate determinations of these quantities agreed to within better than __+0.5 per cent. From the amounts and the known efficiency of the counter for la8Au y-photons[38], the amount of ~9~Au produced at the end of the irradiation per unit weight of 1ayAu irradiated was obtained. The activation equation then led to the 2200 m/sec neutron doses, i.e. (nvo)t, experienced by the monitors. (The 2200 m/sec neutron capture cross sections of ~97Au and ~aSAu were assumed to be 98.7 _+0.2b [39] and 25,102 -+ 371 b [38] respectively.) The activities of the cobalt wire monitors were determined by direct comparison of their gross y-activities (y-energies from 0.4 to 1.8 MeV) with that of an IAEA standard source containing about 10/xCi6°Co (known to -+-0.7 per cent). The activation equation then led to values of (nvo)t. (The halflife of 6°Co and the 2200 m/sec neutron absorption cross section of 59Co were taken to be 5.260-_+ 0.003 yr [40] and 37-37 --+0-11 b [41 ] respectively; other data applicable to the monitors are given in Ref. [42]). 37. R. S. Mowatt and J. S. Merritt, Can. J. Phys. 4 8 , 453 (1970). 38. M.J. Cabell and M. Wilkins, J. inorg, nucl. Chem. 31, 1299 (1969). 39. G. C. Hanna, C. H. Westcott, H. D. Lemmel, B. R. Leonard, J. S. S,tory and P. M. Attree,Atom. Energy Rev, 7, No. 4, p. 3 (1969). 40. J. S. Story, UKAEA Rep. AEEW-R 597 (1968). 41. M. G. Silk, B. O. Wade and J. S. Williams, UKAEA Rep. AERE-R 6059 (1969). 42. M.J. Cabell and M. Wilkins, UKAEA Rep, AERE-R 4866 (1965).

Activation m e a s u r e m e n t s of ~47pm(n, yp48upm

3439

The activities of the silver-aluminium and c o b a l t - a l u m i n i u m monitors used in the high-flux irradiations were also determined by gross ",/-counting. T h e activity ratio normalisation factor, P[43], was determined for the conditions used from separate irradiations in the pneumatic carrier. An accuracy of ± 0 , 5 per cent was achieved. The program S I L C O [ 4 3 ] , re-written to run on the Harwell IBM 360/75 computer, was used to calculate epithermal indices. RESULTS

Results from the monitor m e a s u r e m e n t s are s u m m a r i s e d in Table 4. It will be seen that where a comparison can be made between the neutron doses obtained by the two monitoring methods, i.e. '97Au and 59C0 monitoring, they are in agreement well within the limits of error. (197Au monitor solutions could not be included in the high-flux irradiations due to lack of space). There is only marginal evidence for a systematic difference between the results given by the two methods. Table 4. Results of neutron dose and epithermal index m e a s u r e m e n t s

(nvo)t x 10 ,7

Irradiation No.

T i m e of irradiation t (sec)

Epithermal index r TV~,,

1 2 3 4 5 6 7 8

36,360 36,360 36,360 36,540 36,540 3660 3720 3600

0.0012+0.0001 0.0012±0.0001 0.0012 _ 0.0001 0.0012 ± 0.0001 0-0012±0.0001 0-0981 _+ 0.0055 0.1004±0-0056 0.0989 ± 0.0056

F r o m 1~47Atl monitors 5.309±0.079 5.348_+0.079 5.364 ± 0.079 5.158 _+ 0.078 5.193_+0.077

From ~Co monitors

Weighted mean value

5.185--+0.065 5.330±0-069 5-308 + 0.067 5.085 +_ 0.066 5.178±0.067 5.498 ± 0.085 5.765±0.088 4.874 _+ 0.073

5-235 ± 0.050 5.338+--0.052 5.331 +0.051 5.115±0.050 5.184 +0.051

The cross-section results were c o m p u t e d by iteration from the activation equation, which in this case has the form Ns6

(exp ( - &7(;i) -- exp {- 8-~;i -- ,ks.t) )

N7 = d'Tci

( 6"~;i + ks.t-- Or7t;i)

(2)

in which Ns{: atoms of 148-"pm are formed from N 7 atoms of 147pm in an irradiation of i = (nvo)t neutrons/cm 2. The circumflexes indicate that reactor neutrons are involved and the other symbols refer to Fig. 1. Values of Nsa/N7 are listed in column 3 of Table 5 and the corresponding values of 8-7{; which fit the activation equation are given in column 4 of the same table. If these values of &7~;are fitted to the expression 0-7(; = 0"0,7(; (g'~- r X/T/T,, so,z{:)

{3)

derived by Westcott et a1.[44], it will be found that a%.7{;g= 96-0__ 1.8b and that So,TJg= 14.98-+0.83. Since, m o r e o v e r , So,7(;~2~'7<;/g'~o-o,7(;, it follows that 43. M, J. Cabell and M. Wilkins, UKAEA Rep. A E R E - R 5122 (1966). 44. C. H. Westcott, W. H. Walker and T. K. Alexander, Pro{'. 2nd UN Int. Cor¢/i P U A E 16, 70 (1958).

3440

M. J. C A B E L L Table 5. Neutron capture cross sections for the reaction 147pm(n, y)148~pm and reactor neutrons (this work)

Irradiation No.

r X/T/To

Nsc/N7 × 105

Effective capture cross section (barns) O-z~;

1 2 3 4 5 6 7 8

0.0012 0.0012 0.0012 0"0012 0'0012 0.0981 0.1004 0.0989

4.968+0.177 5.102+0.225 5.017+_+0-196 4.901 +0.167 4.897+0.211 13-049+0.461 13.718-+0.506 11.602-+0.418

97.6±3.6 98.3+4.4 96.7--+3.9 98.5-+3.5 97-1 -+4.3 238.1±9.2 238.8+9.6 238.8-+9.3

]! |/ ~" / |! J

Mean effective cross section

97.7+1.7

E'Ta = 1274_66b. Thus the cross section of the reaction 147pm(n,y)148°pm for Maxwellian neutrons at a temperature of 60°C is 96.0+ 1.8b, and its reduced resonance integral is 1274 + 66b. DISCUSSION

Neither the cross section for Maxweilian neutrons, i.e. 96.0__+ 1.8b, nor the reduced resonance integral, i.e. 1274 + 66b, obtained in this work agree with the results of previous measurements (see Table 1). It is not evident why this should be so and there are no apparent flaws in the present method. Almost nothing has been taken for granted in the present experiments. A new and apparently well-substantiated method has been developed for the determination of 147pm and since this has been applied after the irradiation and purification of the target material, possible losses in the purification are of no account. The half-life of ~48Upm has been re-measured[31,34] and consistent and precise results have been obtained. Moreover, the value adopted agrees with earlier measurements. Although a value has been assumed for the half-life of ~47pm, several independent measurements have confirmed its reliability. In addition, no assumptions needed to be made regarding the intensities of the y-photons involved in the decay of 14SOpm, since the y-counter used for the determination of ~48°Pm was calibrated by a method which did not require such assumptions [34]. Any corrections to the counting data which were needed to take account of the presence of ~48"pm were small and could be made with confidence. Two independent methods have been used to measure the neutron doses received by the targets during the low-flux irradiations and these have given results which are in close agreement. Finally, the neutron spectra employed in the irradiations were well suited for the purpose and have been well characterised. It is clear that, if the method described is giving inaccurate results, this can only be due to some unidentified systematic error, since there is good agreement between the results obtained under similar irradiation conditions. Acknowledgement-The author is grateful to Mr. M. Wilkins for some of the experimental work described.