The spontaneous fission half-life of 240Pu

Journal of Nuchr

Energy. 1967b Vpl. 21.

pp.749

THE SPONTANEOUS

to 7~#q@iih6~‘~~

Phtod

h Noam

FISSION HALF-LIFE

Ir&nd

OF nroPu

P. FIELDHOUSE, D. S. MATHER and E. R. CULLIFORD Atomic Weapons Research Establishment, Aldermaston, Perks. (Received6 April 1967) Abstract--A value for the spontaneous fission half-life of *opU has been obtained from the measured neutron emission rates of four relatively large Y%t neutron sources (--106~lo”n/sec) and their known masses, compositions and multiplications. The four sources are the Aldermaston primary and two substandards and the Harweh standard, all of which have been recently calibrated to an accuracy of better than 1 per cent. The ‘best value’of the average number of neutrons per fission, Q, for 140Puis discussed in arriving at a final mean half-life of (1.170 f OG25) x 10” yr. The present determination is compared with other published measurements obtained by the more conventional technique of counting fission fragments from thin foils.

A SURVEY of the literature shows a considerable spread (-20%) in the published values of the spontaneous fission half-life of ?%t although the individually quoted errors are for the most part quite, small (~2%). Early measurements of (1.314 k 0.026) and (l-225 f O-030) x loll yr have been reported by KINDERMANN(1953) and BARCLAY et al. (1954) respectively; a further value of l-2 x 101l yr, with no error given, by CHAMBERLAIN et al. (1954), may be suspect (see WATT et al., 1962). MIKHEEV (1959) has obtained a half-life of 1.20 x 10n yr in apparent agreement with BARCLAY et al. whilst in more recent literature WATT et al. (1962) and MALKINet al. (1963) have published significantly higher values of (l-340 f O-015) and (1.45 f O-02) x 101i yr respectively. An additional unpublished value of (1.27 & O-05) x loll yr has recently been reported by WHITB (1967) in which the fission counter and foil (99.3 % s?u) were the same as described in earlier papers (PERKINei al., 1965 ; WHITE,1965). In all of these experiments fission fragments from thin foils of plutonium were counted either in simple ionization chambers (both 27r and 47r in the ease of WATTeiuZ.)or in gas scintillation counters as in the two Russian measurements. The plutonium samples contained relatively low enrichments ( G 14%) of MPu with the exception of WA’rr ei al. and WHITBwho used material with > 92 % of this isotope. The isotopic abundances were in all cases obtained from mass spectrometric analyses and the amounts of plutonium from low geometry a-assays (in the case of WA= et al. also by an independent chemical method). The explanation for the large divergence of these results is not clear since the techniques are basically the same. They all suffer to some extent from the inherent limitations of the method, viz. difficulties of accurately measuring small (G few milligrams) quantities of plutonium and of absolutely counting fissions from these with accompanying problems of self-absorption, scattering, alpha pile-up and deadtime losses etc. However, inadequate correction for these effects in some cases could account for part of the observed spread. The present paper describes a completely independent method of determining the half-life of 24opu by measuring the neutrons associated with &ion rather than by counting the fission pulses themselves. Each fission which occurs releases, on average, 4 neutrons; this number is known to high aeeuraoy ( G 1%) from many recent 749

P. FIELDHOUSE, D. S. MATHER and E. R. CULLIFORD

750

investigations. Provided the sample is large enough its total neutron output can be determined with similar precision by one of several well-established techniques and so, in principle, an accurate measure of the fission rate, and hence of the half-life, can be obtained from the ratio of these two numbers. In practice it is necessary to correct for other spontaneously fissile materials present in the sample, for spurious neutrons from, for example, (a, n) reactions in the light and medium-weight impurity elements always present in small traces, and for the body multiplication of the sample (additional self-induced fissions) which for large sample masses, in excess of 100 g or so, might be as large as 10-20 per cent. It is clearly advantageous, as a check on these corrections, if several samples of different compositions and sizes are available. In this work we consider four different a40Puneutron sources with neutron outputs ranging from about 10s to 105 n/set all of which have been calibrated to better than 1 per cent (FIELDHOUSEet al., 1967; 1966). These sources are the Aldermaston primary standard (sphere 3.2 cm diameter) and two sub-standards (sphere 2.10 cm diameter and cylinder 1-Ocm diameter, 1.6 cm length), and the recently revised Harwell standard (sphere 2.8 cm diameter). The three Aldermaston sources contain respective masses of 224.1, 67-O and 7.4 g of plutonium alloy stabilized in the deltaphase by the addition of 4.33 per cent by weight of cerium. From a recent (October 1966) spectrographic analysis of a small sample of the source material the isotopic composition is BVu

aaoPu

72.82 & 0.03 %

2plPu

23.85 * 0.02%

=Pu

2.50 f 0.004%

=Vll

0.73 0.10 f 0@04 % & 0.002 %

by weight.

This is very similar to a previous analysis (April 1961) reported by FIELDHOUSE et al. (1967) where, however, the accuracy was not as good and consequently showed no sssPu. Chemical analysis yields l-22 % mAm and 0.03 % =‘Np by weight with respect to the Pu; the 241Am is produced by the p-decay (13.2 yr) of 241Pu, the respective amounts of these two nuclides given by the analyses being consistent with no 241Am being present immediately after the material was separatedin 1959. Light and mediumweight impurity elements constitute about O-2 per cent of the total mass. The Harwell source contains 163-Og of alpha-plutonium with the following composition (RICHMONDand GARDNER,1957; SOWERBY,1966) =9Pl.I 90.60 f 0.10%

mPu 8.46 f 0.08 %

=Vtl 0.84 f 0.04%

=Pu 0.07 f 0.02 %

by weight

referred to January 1956. Impurity elements amount to about 0.2 per cent of the total mass. The total neutron output of each source can be represented by

Qp, = Q(spont) + Q(a, n) where Q(spont) is the spontaneous fission neutron yield, of which more than 94 per cent is due to the NPu, and Q(a, n) is the yield from (a, n) reactions contributing about 3 per cent of the total. Calculation of &(a, n) and its growth rate (due to the p-decay of WPn into 841Am) has been described elsewhere (FIELDHOUSE et al., 1966; 1967). The spontaneous fission disintegration constant of aqoPu is thus

The spontaneous fission half-life of aro~

751

given by 1”

=

[Qm

-

Q(a, dl - MB M,CN

z:G,“N,,

(1)

where MB is the body multiplication of the source and where, for 24opu, the average number of neutrons per fission is denoted by Q, the fission disintegration constant by 1” and the number of atoms by N (here assumed constant although actually decreasing at a very slow rate due largely to a-decay). The subscript n refers to the corresponding quantities for the n other spontaneously fissile nuclides present, the most important of these being 238Pu and MFu. The total proportion of these two isotopes in the plutonium material was less than 1 per cent (see earlier) and corrections for them are given later. Other nuclides with spontaneous fission half-lives shorter than 24opu and which may be present in small traces are =%I, 244pu, 242Cmand 244Cm. The amounts of the two plutonium isotopes ought to be very much below that of 238puand their effect on the neutron yield is estimated to be very small ( 20 ppm by weight follows on the assumption that the limit of detection was G 1% of the !%!rn peak. Two independent estimates of the decontamination from curium have been obtained (MONK, 1967) using, firstly, the a-pulse analysis data for the various stages of the actual process which showed no detectable trace of curium beyond the first aqueous raffiate and, secondly, from consideration of the decontamination achieved in the process from rare-earth fission products e.g. l%e which have similar chemistry to curium. Both approaches lead to an estimated decontamination factor in the case of curium of about lo6 per cycle i.e. ~10~~ for the overall process and so one can say that the curium content in the finally separated plutonium ought to be
752

I’. -HOD,

IL S. ,Mmsm

and B. R CuLLIpoRD

include the effects of (n, 2n), (n, 7) etc. reactions in the Pu. No allowance, however, is made for (n, 2n), (y, n), (n, y) and (a, n) reactions in the impurity elements but only the latter is significant and this is treated separately. None of these reactions occurs to a sign&ant extent in the source claddings or the cerium stabilizer. Results of the S,, calculations are given in Table 1. The numbers of atoms were derived directly from the known masses and compositions of the sources and values of 1,” and F,, were taken from the published literature. Arrival at the ‘best value’ of 9 for “Pu involved a certain amount of data selection and renormalization worthy of mention. Firstly, it was necessary to decide upon a value of prompt 4 for spontaneous fission of %*Cfsince all of the relevant WPu + data are referred either directly or indirectly to this standard. The value chosen in the present paper +pS2Cf(spontaneous) = 3.733 f O-018 is a weighted average of the four post-1961 values discussed in earlier papers (FIELDHOUSE et al., 1966; COLV~N et al., 1966) and two more recent determinations of 3,741 f 0.028 (DE VOLPIand PORGES,1966) and 3.789 f O-033 (Wm and AXTON,1966). After renormalizing to this standard value, but excluding its error, the most recently published values of +#Blopu (spontaneous) become 2 150 f 0.022 (MOATet al., 1961), 2.167 f 0.026 (HOPKINSand DIVEN, 1963), 2.116 f 0.018 (ASPLUND-NILSON et al., 1963), and 2.129 f O-015(CALVINand S~WWBY, 1965). These lead to a final weighted mean which, including the error in the standard and corrected for OXlOdelayed neutrons (Cox et al., 1958), is GWPu(spontaneous) = 2.143 f O-015. In a similar manner 0 values of 2.17 f O-17and 2.19 f 0.06 can be obtained for the isotopes *Pu and mPu from the experimental measurements reported by HICKSet al. (1956) and CRANSet al. (1956). The spontaneous fission half-lives of these two isotopes were taken to be (7.04 f O-15) x lOi yr and (5-Of O-3) x lOi yr respectively being averages of the published values summarized by Hn>a (1964) but also including three additional =Pu measurements of 6.7 x lOi yr (HIJIZENGA,1954; PE~RZHAK,1957; DRUINet al., 1961) and a 898pumeasurement of (5.2 f 06) x l@Oyr by DRUINet al. (1961). No further measurements after 1964have appeared in the literature. Table 1 summarizes values of the a4opudecay constant Y given by expression (1) for each of the four neutron sources considered; also shown are the individual source strengths Qpu, their calculated multiplications MB, and the percentage corrections for (a, n) reactions and other isotopes (i.e. Bs8puand Spapu). Errors in the half-life given in the last column of the table comprise individual uncertainties of 0+0*8 per cent in the source strengths, 19-05 per cent in the multiplications (typically &lo% in [MB - l]), O-7 per cent in li for Bpopu,and l-5 per cent and 04-O~l per cent in the corrections for (a, n) reactions and other isotopes respectively. The total systematic component of these errors common to all four measurements is estimated to be ~2% [due to MB, B and (a, n)]. Uncertainties in the numbers of atoms are negligible ( 60.3 “/,) except for the small Aldermaston source where the error is 2.8 per cent due mainly to a possible error of 4.2 g in the mass compared with 4.1 g for the other sources.

The

spont.aneous fission half-life of Uopu TABLZL-S UMMARY

Neutron source and calibration date

QF-U x lo(nlsec

753

OF RESULTS

XofQm MB

Q(a, n)t

SpOlltiUlCOU8

MBIZi&“N.

x

&lx yr-’

lission half-life

xl(r’yr

Aldermaston (January 1964) &Zy 1 Secondary 2 Harweli (January 1956) Primary

6562 & 0.029 1.847 f 0.011 0.1981 & 00X6

1.184 1.113 1.050

2*9* 2.9* 2.9+

1991 f 0.008

1.234

3.0

;:; 5.5

0.5887 0.5897 0.6108

1.177 f 0.028 1.175 & 0925 1.135 f 0.047

1.3

0.5961

1.163 & 0.033

* Inclusion of 0.10 per cent by weight of *-Pu in the present calculations leads to an increase of @5 per cent in Q(a, n) over the figure of 2.4 per cent given in an earlier paper (Fieklhouse et al., 1967). The growth rate, however, is not altered signi6cantly from O-1per cent per year. t The (a, n) contribution from the plutonium material itself (including the cerim stabilizer in the case of the AWRE sources) is insignhicant compared with the yield from the light and mediumweight impurity elements present.

After taking a mean of the four values of half-life given in Table 1, weighted with the random errors and then including the 2 per cent systematic error, one obtains a value of: sVu (spont. half-life) = O-6932/1”= (1.170 f 0.025) x 10rl yr. Previously published measurements of the spontaneous fission half-life of s%r, discussed earlier, fall roughly into three groups-three measurements of about 1.2 x 10rl yr, two at about l-3 x 10rl yr and one above 1.4 x 101lyr, with the further unpublished measurement by WHMZfalling roughly between the fust two groups. The present work appears to favour the tist group. One could only reconcile it with the higher values by presupposing that the measured source strengths contain a further uncorrected contribution of about 10-20 per cent from spurious neutrons. Allowance has already been made for the other spontaneously fissile nuclides present and the (a, n) reactions taking place in the impurity elements; consideration of other possible reactions e.g. (y, n) and (n, 2n) shows both of these to be negligible. Acknow1e&me&s--This work was prompted by a suggestion of Dr. R J. TUTXU of Atomics International. The authors would also like to thank the stag of the Chemistry Division and the Physical Measurements Grou , AWRE, for the analyses, the staff of the Theoretical Physica Division for the S, calculations, an % Dr. R. G. MONK and Mr. L. P. O'CONNOR for valuable discussion of the separation and isotopic composition of the material. REFERENCES L., CoNDd H. and STARF~LT N. (1963) Nucl. Sci. Etgtg 15,213. BARCLAY F. R, GALBRAITH W., GLOVERK. M., H.UL G. R. and Wnrrsrr~~ss W. J. (1954) Proc. phys. Sot. A67, 646. ~ERLAIN O., FARWELLG. W. and Ssoti E. (1954) Phys. Rev. 94,156. COLVIND. W. and Sowsany M. G. (1965) Proceedings of the Symposium on the Physics and Chemistry of Fission, Salzburg, Vol. 2, p. 25, IAEA, Vienna. COLWN D. W., SOWERBY M. G. and IUcDow R I. (1966) Proceeditgs of the Conference on ASPLUND-NILSSON

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754

P. FIELDHOUSE, D. S. MATHER

and E. R

CULLIFORD

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