An application of SSNTD to nuclear physics: Search for superheavy elements

Nuclear Instruments and Methods 173 (1980) 0 North-Holland Publishing Company

AN APPLICATION

121-125

OF SSNTD TO NUCLEAR PHYSICS: SEARCH FOR SUPERHEAVY

ELEMENTS

R. BRANDT, T. LUND, D. MOLZAHN, P. VATER, H. JUNGCLAS and A. MARINOV * Kernchemie,

F.B. 14, Philipps- Universitkt, Marburg, Germany

Experiments given.

are described

to look

for SHE using SSNTD.

First results

on the investigation

of the hot brine

Atlantis

II are

Fig. 1. The peninsula of stable nuclei and the predicted of superheavy elements (taken from ref. 12).

island

1. Introduction One of the present topics in nuclear science is the question: Do superheavy elements with 2 % 114 exist? The liquid drop model of nuclear matter does not predict any stability for nuclei in the region 2 > 110. About fifteen years ago it became clear that by a combination of the liquid drop model and the nuclear shell model one could understand many phenomena in nuclear physics-and one could predict a new region of relatively stable elements around 2 = 114 (or Z = 126, depending on the model used). Such unknown elements are called superheavy elements (SHE). Fig. 1 shows schematically the “peninsula of known nuclei” and the “possible island of superheavy elements” in a Z-VW diagram. The effect of nuclear shell structures on nuclei in the region of SHE is shown in fig. 2. The calculated and predicted properties of SHE are included. It should be pointed out that all these predictions are rather uncertain (a detailed review is given in ref. 1). A typical example is the calculated half-life T,,l of SHE: This can be in the range of 109a > Tllz 2 1 ns. If T,,, = 109a, then it might be possible to find SHE in nature. If T,,, 1 1 a, one could possibly produce them at heavy-ion accelerators in some quantity; but if T1,, < 1 ps, it might be even difficult to identify them with conventional chemical methods. However, one property of SHE might be predicted fairly certain: If SHE are observed around Z N 114, then their chemical behavior will be similar to their lighter homologues OS to PO. Their volatility appears to be high, their nobility might also be fairly high. If it is possible to obtain chloride-compound ions in solution, then it should be possible to coprecipitate SHE with H2S in slightly acidic solutions (PH v 1): SHE could be reduced to * GSI, Darmstadt

and Hebrew

University,

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N

f 2 k 5

Nuclear

deformotlon >

---___

0 Spontaneous

5

,

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llquld

drop

-

shell

effects

Calculated

properties

11

FEssion

barrier

2)

Spontaneous

31

Neutron

LI

Possibly.

121

mcluded

SHE.

d

fission

10 MeV

holflife

multlpliclty ternary

model

of EB

S $ 10

ns d

T

6 10’~ ‘0

fission

Fig. 2. Schematic representation SHE and some of its properties.

Jerusalem.

flsslon

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5

of the nuclear

IV. APPLICATIONS

stability

of

OF SSNTD

122

R. Brandt et al. / Superheavy

metallic atoms and adsorbed to the sulfide precipitate, or it could form an insoluble sulfide itself and be coprecipitated with the carrier element (i.e. Cd). Not only the decay properties of SHE are rather uncertain, in addition also the mechanism how to produce SHE is completely unknown. One could speculate on the production of SHE in the astrophysical r-process or in heavy-ion induced reactions, as described in ref. 1. In addition, ref. 2 contains a quite interesting proposal to produce SHE in the bombardment of a heavy spherical nucleus (i.e. Pb) with a heavy, deformed nucleus (i.e. U or heavier elements) in a so-called “instantaneous fission” process. In this paper, we will limit ourselves to a description of some typical experiments, where different types of SSNTD are used as one of the basic tools to detect the spontaneous fission decay of SHE.

Neutron

search for SHE using SSNTD

2.1. On the use of a ‘low background spontaneous fission detector” in investigations of the meteon’te Allende One requirement to look for spontaneous fission events due to SHE is a detection system with an extremely low background (Gl event/a). This can be achieved with a coincident registration of the different types of particles emitted during the fission process (heavy fragments, neutrons, gamma-rays). Such an instrument is described in ref. 3 and schematically shown in fig. 3. It is a highly complex electronic counter system, which can register the energy of a fission fragment in coincidence with the number of fission neutrons. However, at very low event-rates (Gl event/month) one cannot exclude, that spurious big electronic noise pulses trigger the counters and one registers artificial “fission” events. SSNTD are therefore used as an independent low-background detector. The san(ple to be investigated has to be relatively small (<60 mg) and is placed as a thin layer (<6 mg . cm-“) onto a preetched and annealed mica plate. Each fission event is registered simultaneously as an energetic fragment in the Si(Au)-detector electronically and as a track in mica. For example, this detector operated two years in Marburg with zero fission events observed in the electronic sy_stem and correspondingly zero tracks were seen in mica [4]. Then, during the investigation of an Allende-sample, we saw 5 electronic events in five months [5]; how-

detector

Fission

detector

4

k

100

cm

Lcm

WA4

detector

--_---__-Sclntlllator

Count and

2. Experimental

elements

tank

neutrons

background

Take

photo

L!st

fissioq

energy

Count

flsslon

independently

Fig. 3. Schematic representation of the low background spontaneous fission detector with four obtainable items of information (details see ref. 3).

ever, subsequent scanning of the mica plate revealed zero fission tracks. This shows, that the events seen electronically were not due to fission; the mica results are more trustworthy. Furthermore, it shows that SSNTD can play a decisive part in complex experiments, they are independent from electronic complications and failures. It is interesting to note, that so far no SHE could be detected with this technique in the meteorite Allende [6] - in contrast to other experimental results. 2.2. Ancient heavy element cosmic rays, registered tracks in olivine from the meteorite Marjalahti Perelygin and coworkers at Dubna have studied for many years long tracks in olivine crystals from the meteorite Marjalahti, which had an exposure age in outer space of 17.5 mega-years [7]. Here, the transiron component of the relativistic cosmic radiation has left long tracks in the olivine. By investigating such tracks in olivine crystals with a diameter over 2 mm, they found the track-length distribution as shown in fig. 4. Each track length is associated with a minimum 2 leaving such a track. For example, the longest track with I= 1110 pm requests a 2 > 96. Because no longer tracks are observed, there are no

123

R. Brandt et al. / Superheavy elements

120

8C

40

0 lo

400

600

800

1000

-L

Fig. 4. Distribution of track lengths observed in Marjalahti olivines [7]. (Tracks with I > 1400 pm would be due to SHE and are not observed.)

SHE found in this experiment (Z(SHE) > 1400 pm). However, this method is not very sensitive, quantitatively, as the total number of tracks is relatively small. 2.3. Present heavy element

cosmic rays, observed in

space-jlights

Price and coworkers have carried out many space experiments using SSNTD, in order to look for relativistic, very heavy nuclei in the cosmic radiation [8]. Their experimental set-up is a highly complex stack of different types of SSNTD [8]. It allows the investigation of relativistic heavy element cosmic rays, whose flux in outer space is extremely low ( 26). The summary of their results is given in fig. 5. With a Z-resolution of about two charge units they observe elements as heavy as actinides (90 < 2 < 98). The observations of heavy actinides in the relativistic component of cosmic rays show the existence of products from the astrophysical r-process in supernova explosions which had occurred not too long time ago. However, the total number of events observed is rather low and the production of SHE through any process should be considerably lower than the production of actinides. The non-observation of SHE in this experiment may be due to the limited sensitivity.

3. A SSNTD sandwich for analyzing large terrestrial samples, including hot brine material from Atlantis II Using mica as detector for spontaneous fission in combination with electronic counting equipment (see

01

I

/

I

70

80

90

100

Nuclear charOe

Fig. 5. The observed abundance of ultraheavy cosmic rays compared with calculated abundances of elements in the Iprocess and s-process. The observed actinoides (90 Q Z < 98) can only be produced by the r-process (from ref. 8).

sect. 2.1) the amount of material to be investigated is rather limited (i.e. 5 60 mg). We have developed a makrofol-mica sandwich, which enables us to increase the sample area (and amount of material) by a factor of about one hundred as compared with a single mica fission track experiment. The arrangement is shown in fig. 6. After exposure the makrofol is etched and fission tracks are visualized by the well-know “spark-jump” technique. The resulting holes in the aluminized plastic foil can then either be due to fission or due to background. The macrofol serves thus as a position detector for any fission track, which one must find afterwards in the low background mica detector. The scanning of mica with an optical microscope is thus reduced to a small area (a few mm”) per “spark-jump” hole. This technique has been applied to the search for SHE in samples from the hot brine Atlantis II in the Red Sea. Based on the general chemical assumption that SHE chemically are somewhat similar in behaviour to Tl, Pb, Bi, etc., Flerov and coworkers in Dubna, USSR, conceived the idea to look in special hot geothermal waters for SHE [9,10]: Volatile metals are enriched in the upper mantle of the earth. IV. APPLICATIONS

OF SSNTD

R. Brandt et al. / Superheavy

124

elements

Red

\

A

fotl

and

c

Cd, Pb. (SHE

I

/

Preapltoted Makrofol of

i3.

Sea

posltlon damage

“spark

Sample MCO The 0 the

The

vlsuollzed

by

mm2

of

opposite

volotlle

elements

(Atlantis

metal II

compounds

deep1

Fig. 7. Schematic representation of the Atlantis II deep in the Red Sea. (Much oversimplified picture!)

< 1 mg /cm2 track

the

posItton etchlng

”

thickness

bockground

sconnmg few

detector

- ,“mplng

wtth low

1s

mlco to

detector. IS the

reduced holes

to I”

mokrofol

Fig. 6. Mixed SSNTD sandwiches in use to look for rare spontaneous fission events in terrestrial material (up to 1 g in about 100 sandwiches).

When this upper mantle comes in contact with hot water, a solution is formed rich in volatile, halidecomplexed ions. Where such hot brines come through the crust of the earth, we have a place to look for SHE in nature. One example of such hot springs is located in Cheleken, USSR. Flerov et al. are studying these brines and have published some results on the observation of spontaneous fission activities in metal compounds extracted from the brines [9] (see table 1). They suppose that the spontaneous fission activity might be due to SHE in nature. Hot springs similar to the Cheleken ones are also found in the Atlantis II deep in the Red Sea [lO,l I]. However, in contrast to the Cheleken hot spring, this spring finds its way through the crust of the earth at the bottom of the Red Sea (at a depth of about 2000 m). Due to the geological structure of the Red Sea and to the high density of the salt-saturated hot

brine, it has resulted in a thick layer of precipitates (metal-hydroxydes, -carbonates, -sulfides) on the bottom of the Red Sea in an area called Atlantis II deep [ 1 I]. The geological origin and macro-composition of these precipitates are apparently the same as the dried metal compounds extracted from about 2000 m3 Cheleken-brines, where the reported spontaneous fission activity is about 5 decays per day per 1 kg material [9] (see table 1). We obtained about 0.1 m3 of the Atlantis II hot brine/hot brine precipitate and separated the water soluble part from the metal compound precipitates by washing with distilled water. We isolated about 6 kg dry metal compounds. About 200 g of this material were heated to 1050°C in an oven using alternatively air or Hz gas streams. The entire volatile fraction was collected, dissolved in HCl and we precipitated at pH - 1 with H,S-gas the sulfides. We obtained appr. 200 mg sulfides, mostly Cd and Pb, which no longer could be observed in the heat-residue. We investigated this fraction for spontaneous fission activities with various techniques, as shown in table 2. Among the techniques used the SSNTD-technique is a very prominent one.

Table 1 Some results from the measurement of the mineral fraction extracted from the Cheleken geothermal hot brines. The experiment was carried out with a big volume 3Hecounter. The concentration of the spontaneous fission activity (possibly due to SHE) is (2-4.5) X 10-l 3 g/g (details see ref. 9). Sample

Neutron detection efficiency

Measuring time (d)

Neutron multiplicity n=2

n=3

n=4

n=5

16-70 I -

0 28 190

0 16 30

0 3 2

0 0 0

(%)

SiOz, FeOz background 6 kg mineral fraction 2 3 % (calibration)

2-l 40 38

R. Brandt et al. / Superheavy

Table 2 Preliminary results on the search for SHE in Atlantis II (see text).

Sample 1 b Sample 2 a Sample 3 b

Method

Concentration limit of SHE Wg)

proportional counter ’ Si(Au) detector ’ SSNTD, as shown in fig. 6 ’

<4 x 10-13 <3 x 10-13 <3 X lo-t3

a Volatile sulfides obtained by heating in Hz and air gas flow (details see text). b Volatile sulfides obtained by heating in air gas flow (details see text). ’ No effect above background was observed.

Our preliminary results, as given in table 2, are based on the conventional assumptions of SHE-properties (Tr,* = lo9 a, m = 300 amu) andless than 3 events are observed [6]. Our preliminary limits are: (SHE)/ (Atlantis II metal compounds) < 3 X lo-l3 g/g. Our upper concentration limit has the same order of magnitude as the concentration of a spontaneous fission activity given by Flerov et al. [93. Therefore we continue our experiment.

4. Conclusions It has been shown that SSNTD can be used in quite a competitive way, as compared to electronic

elements

125

counting systems, in such an interesting search for SHE presently is. This work was supported

field, as the

by the BMFT (Bonn).

References [l] G. Herrman, International review of science; inorganic chemistry, Ser. 2, vol. 8 (London, Butterworth, 1975) p. 221. [2] H.H. Deubler, K. Lekkas, P. Sperr and K. Dietrich, Z. Physik A284 (1978) 237. [3] H.-J. Becker, R. Brandt, H. Jungclas, T. Lund and D. Molzahn, Nucl. Instr. and Meth. 159 (1979) 75. [4] H.-J. Becker, D. Molzahn and R. Brandt, Inorg. Nucl. Chem. Lett. 13 (1977) 643. [5] R. Brandt, Proc. Int. Symp. on Superheavy elements, Texas Tech. University, Lubbock, USA (Pergamon Press, Oxford, 1978) p. 103. [6] T. Lund, H.-J. Becker, H. Jungclas, D. Molzahn, P. Vater and R. Brandt, Inorg. Nucl. Chem. Lett. 15 (1979) 413. [7] V.P. Perelygin, S.G. Stetsenko, D. Lhagvasuren, 0. Otgonsuren, P. Pellas and B. Jakupi, Joint Inst. Nucl. Research, Dubna, Preprint E-7-10667 (1977). [8] P.B. Price, E.K. Shirk, W.Z. Osborne and L.S. Pinski, Phys. Rev. D 18 (1978) 1382. [9] G.N. Flerov et al., Joint Inst. Nucl. Research, Dubna, Preprint D-7-11724 (1978). [lo] Yu.T. Chuburkov and L.M. Lebedev, Radiokhimiya 16 (1974) 524. [II] H. Backer, Geol. Jb. D17 (Hannover, 1976) p. 151. [12] C.E. Bemis, Jr. and J.R. Nix, Comm. Nucl. Part. Phys. 7 (1977) 65.

IV. APPLICATIONS OF SSNTD