Superheavy element search in bismuthinite, Bi2S3

i inorg, nucl. (.'hem. VoL 41. pp. 613-61~ Pergamon Press Ltd, 1979 Printed in Great Britain

SUPERHEAVY ELEMENT SEARCH IN BISMUTHINITE, Bi2S3~ E. L. FIREMAN:~ Center for Astrophysics,Harvard CollegeObservatoryand SmithsonianAstrophysicalObservatory, Cambridge, MA 02138, U.S.A.

and BRUCE H. KETELLE and R. W. STOUGHTON Chemistry Division,Oak Ridge National Laboratory, Oak Ridge,TN 37830, U.S.A. (Received 19 July 1978; received for publication 14 November 1978)

Abstract--Samples of bismuthinite were examined by using a neutron multiplicitycounter capable of detecting spontaneous fission events in any element. No events characteristic of spontaneous fission decay of superheavy elements were found. If a superheavy element exists in the cosmic abundance ratio estimated from the carbonaceous chrondrite measurements of Flerov et al. then it has not (even weakly)followedbismuth through its geochemicalevolution into our bismuthinite samples.

INTRODUCTION

There have been many searches for superheavy elements (SHE) in nature stimulated by the theoretical prediction of an island of nuclear stability near element 114 with half-lives sufficient for survival in nature (2x 10ayr). Herrmann[1] wrote an excellent review of these studies. Practically all the searches for SHE yielded negative results. Occasionally positive results have been reported, which were soon proved to be wrong (e.g. the report[2] of superheavy elements in Madagascar monazite). Recently Flerov et a1.[3-5] measured the presence of a superheavy element in the carbonaceous chondrites A1lende, Saratov, Efrenovka, and in hot spring water from the Cheleken Peninsula. Their positive results are so small that they are exceedingly difficult to substantiate by repeating their measurements on similar materials. The superheavy element concentrations in the carbonaceous meteorites (3 × 10-'4 to 3 x 10- u g/g) required the detection of a few multiple neutron events in several months from many kilograms of meteoritic material. The work of Flerov et al. [3-5], although difficult to verify, encourages additional searches for materials with higher superheavy element abundances. Their work, in fact, stimulated our study of bismuthinite. If a long-lived superheavy element exists as a result of nucleosynthesis, it should be present not only in meteorites but also in terrestrial materials at some concentration level. In fact, Flerov[5] measured a higher concentration, 2 × 10-13gig, of a superheavy, multipleneutron emitter in minerals from the hot spring water of the Cheleken Peninsula than in the meteorites. Zvara et al.[4] investigated the chemical properties of the multiple-neutron emitter in Flerov's samples and found it to be more volatile than lead and to form a sulphide that is insoluble in dilute acids and alkalis. It had the properties expected [6] for an element with atomic number 112, 113, 114, or 115. Such elements are the homologs of Hg, T1, Pb and Bi. Although more abundant in carbonaceous

chondrites than in ordinary chondrites, the concentrations of Hg, TI, Pb and Bi in carbonaceous chondrites are quite low: Hg in carbonaceous chondrites is - 3 p p m ; TI is - 5 × 10-Sg]g; Pb is - 3 p p m ; and Bi is - 10-~ g/g [7]. The cosmic elemental abundances for most elements are simply their carbonaceous chondrite abundances. Flerov's carbonaceous chondrite results give the following superheavy element cosmic abundance ratiEos: SHE/Hg - 3 x 10-'°; SHE/T1 - 2 x 10 s; SHE/Pb - 3 x 10-1°; and SHE/Bi - 1 0 -8. Large terrestrial mineral deposits of HgS (cinnabar), PbS (galena), and BJi2S3 (bismuthinite) occur. If the superheavy element followed its homolog, Hg, Pb, or Bi, through its geochemical evolution into the sulphide mineral, galena and cinnabar would contain superheavy concentrations of approx. 3 × 10-1°g/g and bismuthinite would have a superheavy concentration of approx. 10 8 g/g. The search for superheavy elements in galena lind other lead ores with neutron detectors has been very extensive[8-11]. None of the investigated lead samples had SHE/Pb ratios greater than - 1 0 ,2 g/g. Evidently the superheavy element has not followed lead. A cinnabar ore, HgS[8] and mercury in air from a closed mercury mine and in silica gel from an air reduction plant[10] have been examined for superheavy elements with negative results. A sample of native bismuth, from Saxony, Germany, also gave negative results[10]. A combined sample[8] of native bismuth, bismuthinite, Bi2S3 and bismutite (BiO)2CO3, from a variety of places gave a larger number of high neutron multiplicity events than the cinnabar ore; however, the large number of events in the combined bismuth sample was attributed to the fact that some of the bismuth ores contained as much as - 150 ppm of uranium. Thallium sulphide minerals are rare and have not been studied by neutron multiplicity counting. A more extensive search for superheavy elements in bismuth and thallium terrestrial minerals appears to be necessary in view of Flerov's positive results.

tResearch sponsored by the Division of Nuclear Sciences, SAMPLES U.S. Department of Energy, under contract W-7405-eng-26with the Union Carbide Corporation. Our best sample was a 435-g bismuthinite crystal from ~:E. L. Fireman, Smithsonian Astrophysical Observatory, 60 an old pegmatite formation in Kingsgate, N.S.W., ArtsGarden Street, Cambridge,MA 02138, U.S.A. tralia (Harvard Mineralogy sample No. 90673). Our other 613 JINC VO! 4L No 5--A

614

E. L. F I R E M A N

samples were a 35-g bismuthinite crystal from Haddam, Connecticut (Harvard Mineralogy sample No. 36), and a 35-g bismuthtantalite crystals (Harvard Mineralogy sample No. 96044). These samples had been stored in a museum drawer for several decades and could not .have been easily contaminated by debris from nuclear bomb tests. URANIUM MEASUREMENTS

The uranium contents of the samples were measured by y-ray counting with a GeLi detector and a 4000channel analyzer. The 351.9-, 609.3- and 1764-keV lines of the uranium series were used for the uranium determination. The line intensities from the Bi samples were compared with those from old uraninite, UO2, samples of similar sizes and with a NBS pitchblend standard. For the Kingsgate sample, we obtained 31 + 2ppm from the 351.9keV line, 36_+2ppm from the 609.3 keV line, and 28_+ 2 ppm from the 1764 keV line. The value 32 + 4 ppm, encompasses the three uranium determinations. The uranium contents of the other two samples were smaller than that of the Kingsgate bismuthinite. NEUTRON MULTIPLICITYMEASUREI~.,NTS The neutrons from the three bismuth samples were measured in the Oak Ridge Neutron multiplicity detector[12, 13], which contains thirty 3He proportional counters in a cylindrical array, embedded in a paraffinpolyethylene matrix surrounding a sample chamber 10 cm in diameter and 30 cm in length. This configuration has a single-neutron counting efficiency of 0.47 as determined with a 252Cf source. The system is operated in a mode such that the first neutron detected opens a time window of 400/~s to allow neutron thermalization and detection. The electronic system permits neutron multiplicities of 1-9 to be recorded together with their times of detection. The Oak Ridge detection system is physically very similar to the detection system used by Flerov et aL[3-5] and has a higher neutron detection efficiency, 0.47, compared to 0.30 of the Flerov system. However, it has a much higher background: 11.6 neutron doubles day -~ for an empty chamber compared zero neutron doubles in 200 days for the Flerov system. The very low background of the Flerov system is due to its underground location (1!00 m.w.e, depth) and to a higher purity in the construction materials, i.e. less uranium contaminants in the counter walls.

et ai.

The observations and corrections are summarized in Table 1. The first column gives the neutron multiplicites 1-7. The second column gives the counts day -I from the bismuthinite samples (505 g) obtained during 15.465 days of live-time. There is 9% dead-time resulting from an anticoincidence mantle over the system to reduce neutron bursts from cosmic-ray interactions. The third column gives the count rate with the empty sample chamber. There is little difference in the neutron single rates from the sample and empty chamber; however, the sample has a significantly higher doubles rate and a slightly higher triples rate. The fourth column gives the neutron double, triple, and quadruple rates estimated to arise from the uranium in the bismuthinite samples. The estimate is based on calibrations done with uranium-rich samples of known composition. The uranium can contribute only 10% of the neutron doubles and 20% of the neutron triples and quadruples from the sample. The fifth column gives the excess doubles, triples, and quadruples rates over the empty chamber rates when 406 g of (pure) Pb is counted. These events are caused by cosmic-ray interactions on the Pb. Cosmic-ray interactions can account for 1/3 of the doubles and all the triples and quadruples from the bismuthinite. The total background rate is obtained by summing counts from the empty system, the uranium and the cosmic-ray interactions with the sample. The total background for doubles is equal to that of the sample. The total background rates for triples and quadruples are slightly higher than from the sample. The errors shown in Table 1 are one standard deviation in the counting errors. The error in the uranium analysis of the bismuthinite samples is negligible compared to the counting errors and the possibility that the mock-up background lead has some uranium. The last column gives the net surplus of multiple neutron events from the sample. It is evident that there is no surplus. CONCLUSION

To compute the maximum amount of SHE present in the sample, the maximum count rate, C(N), of multiplicity N from superheavy element fissions is taken to be twice the error given in the last column of Table 1. If the average number of neutrons from the fission is P, then the probability for observing double, triple, and quadruple neutrons, P(2), P(3) and P(4) are calculated from a standard formula (13) and are given in Table 2 for P of 3 and 6. The number of atoms of the superheavy element,

Table I. Neutron count data for bismuthinite and background corrections Cosmic Ray Interactionst (Ct/day)

Total Background (Ct/day)

Bismuthinite* (Ct/day)

1

8220 + 23

2

19.7 ± 1.1

11.6 ± 0.98

1.78 ± 0.00

6.6 ± 0.6

19.98 ± 1.1

- 0 . 3 ± 1.6

3

1.6 ± 0.3

0.99 ± 0.29

0.37 ± 0.00

1.7±0.3

3.06 ± 0.4

-1.4 ± 0.5

4

0.26 ± 0.13

0.04±0.00

0.5±0.2

0.64±0.2

-0.4±0.2

5

0.13 ± 0.1

6

0.13 ± 0.1

7

0.1

Empty System (Ct/day)

Uranium (30 ppm)** (Ct/day)

Neutron Multiplicity

Net (Ct/day)

8143 +_ 32

± 0.1

*505 g sample material counted 15.4647 days live counting time. *'15 mg of uranium in equillbrium with decay products. t406 g Pb used as mock-up for cosmic ray interactions in bismuthinite.

Superheavy element search in bismuthinite, Bi2S3

615

Table 2. Count rates, C(N), and detecting probability, P(N), for N coincident neutrons from spontaneous fissions in No. SHE atoms with 109 yr halflife and 1; average fission neutrons ~=

3

N 2

(3.2

0.292

3


0.116

4

<0.4

0,0245

<8.6 × 1012

P(N)

#SHE (atoms)

P(N)

#SHE (atoms)

<5.8 × 1012

0.255

<2.1 , 1012

<4,-5 × 1012

0,273

<1.9 , 1012

0,186

<1.1 " 1012

(No. SHE), is related to the count rate, C(N), and the probability, P(N), by C(N)

V=6

C(N). (day - i )

(No. SHE)P(N), T

where r is the mean life for superheavy element fission. Values for No. SHE in the bismuthinite are given in Table 2 for a fission half-life of 109 yr. There are 406 g of Bi in the bismuthinite. If the atomic weight of SHE is assumed to be 300, the maximum concentrations of the superheavy element in the bismuthinite are 5.6x 10-'2gig Bi, if # = 3 ; and l a x 10-'2gig Bi, if z7=6. According to Flerov et al. [3-5], the average number of neutrons from superheavy element fissions that they have detected is between 3 and 6. From their carbonaceous chondrite data, the cosmic abundance ratio of SHE/Bi is -10-Sg/g; this ratio is approximately four orders of magnitude larger than the ratio we measure in bismuthinite. The superheavy element, if it exists, did not follow Bi during its geochemical evolution into our bismuthinite samples. Acknowledgements--We wish to thank Prof. C. Frondel and Carl Francis for lending us the bismuthinite and bismuthtantalite samples. REFERENCES 1. G. Herrmann, In International Review o[ Science, Inorganic Chemistry, Ser. 2, Vol. 8, Radiochemistry (Edited by A. G. Maddock), p. 221. Butterworths, London (1975).

2. R. V. Gentry, T. A. Cahill, N. R. Fletcher, H. C. Kaufrnann, L. R. Medsker, J. W. Nelson and R. G Flocchini, Phys. Rev. Lett. 37, 11 (1976). 3. G. N. Flerov, G. M. Ter-Akopyan, A. G. Popeko, 13. V Fefilov and V. G. Subbotin, J.LN.R. preprint P6-10581, Dubna (1977). 4. I. Zvara, G. N. Flerov, B. L. Zhuikov, T. Reetz, M. R. Shalayevski and N. K. Skobelev, J.LN.R. preprint P6-10589, Dubna (1977). 5. G. N. Flerov, Proc. Internat. Con[. on Nuclii Far .from Stability, Cargese, France, CERN 76-13, p. 542, Geneva (1976). 6. O. L. Keller, Radiochimica 17, 607 (1975). 7. B. Mason (Ed.), Handbook o[ Elemental Abundances in Meteorites, Vol. I. Gordon & Breach, New York (1971). 8. E. Cheifetz, R. C. Jared, E. R. Giusti and S. G. Thompson, Phys. Rev. 6, 1348 (1972). 9. R. W. Stoughton, J. S. Drury, R. J. Silva, M. H, Lietzke, J. Halperin, R. C. Jared, S. G. Thompson, E. R. Giusti arid E. Cheifetz, In 1t. H. Hyman Memorial Volume (Edited by J. J. Katz and I. Sheft), p. 215. J. Inorg. Nucl. Chem. Suppl. Pergamon, Oxford (1976). 10. R. W. Stoughton, J. Halperin, J. S. Drury, E G. Perey, R. L Macklin, R. V. Gentry, C. B. Moore, J. E Noakes, R. M. Milton, J. H. McCarthy and D. W. Sherwood, Nature Phys. Sci. 246, 26 (1973). II. G. N. Flerov, G. M. Ter-Akopian, L. S. Getskin, G. N. Goncharov, A. G. Popeka, N. K. Sobelev, B. A Gvozdev and P. P. Tsyb, Yad. Fiz 20, 639 (1974). 12. R. L. Macklin, F. M Glass, J. Halperin, R. T. Roseberry, H. W. Schmitt, R. W. Stoughton and M. Tobias, NucL Instrum. Methods 102, 181 (1972). 13. B. H. Ketelle, G. D. O'Kelley, R. W. Stoughton and J Halperin, Phys. Rev. Lett. 37, 1734 (1976).