The lattice site of 10F implanted into diamond by TDPAD

Nuclear Instruments and Methods North-Holland, Amsterdam

THE LATTICE

in Physics

Research

423

B35 (1988) 423-430

SITE OF ‘OF IMPLANTED

INTO DIAMOND

S. CONNELL, K. BHARUTH-RAM and J.E. LOWTHER * * *

*, J.P.F. SELLSCHOP,

Schonland Research Centre for Nuclear Sciences, Republic of South Africa

University of the Witwatersrand,

BY TDPAD

M.C. STEMMET,

H. APPEL * *

P.O. Wits, 2050 Johannesburg

into diamond have been identified, using the technique of time Two distinct residence sites for 19F ions recoil-implanted differential perturbed angular distributions (TDPAD) applied to the 197 keV state (t = 128 ns, Q = 0.10 b) of 19F. The electric field gradient parameters and their temperature dependence correlated well with the site assignments of recent cluster model calculations and strong host-impurity chemical effects are evident.

1.

Introduction

Recently, diamonds have been doped successfully with boron to become p-type semiconducting material [l-3]. The achievement and understanding of this requires a study of ion implantation, the physical and electronic changes which are possible due to the controlled presence of defects and impurities, and the processes of damage creation and annealing. Perturbed angular correlation (PAC) studies can reveal information about impurities present in the host lattice [4]. The electric field gradient (efg) at the residence sites of radioactive probe impurity perturbs the angular distribution of the decay radiation. Measurement of the perturbation allows local charge distributions to be deduced, along with other information of a chemical and dynamic nature. Previous PAC studies by others of the residence sites taken up by the implanted impurities “iIn and lslHf in diamond [5,6] found that, at most, only a small fraction of the probe ions came to rest at characteristic lattice sites with well-defined quadrupole interaction frequencies and definite symmetries. This effect was attributed to radiation damage of the host matrix produced by the implantation of the high Z probe ions. Nevertheless, for the small nonrandom fraction, two residence sites were identified, each with the efg principal axis aligned along a (111) crystallographic direction. Kalish [7] found no annealing conditions

* Physics Department, University of Durban-Westville, P. Bag X54001, Durban 4000, South Africa and Honorary Research Associate of the University of the Witwatersrand. ** Visiting Ablett Fellow from the Universitat Karlsmhe, Postfach 3640, D 7500 Karlsruhe, FRG. * * *Physics Department, University of the Witwatersrand, South Africa. 0168-583X/88/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)

B.V.

which could drive In implants to substitutional sites in an undamaged environment of diamond. This article reports on similar studies using the technique of time differential, perturbed angular distributions (TDPAD) with the substantially lighter i9F nuclei as the recoilimplanted hyperfine probe.

2. Experimental procedure The samples used in these studies were single crystal wafers of the four different types of natural diamond (Ia, Ib, IIa, IIb). Measurements were also performed on some other allotropes of carbon (single crystal and polycrystalline graphite and vitreous and colloidal carbon). Sample dimensions were 5 x 10 x 2 mm. Single crystals were cut and polished very close to the (111) plane. A 30 pg cm-* layer of CaF, was evaporated onto the surface. The i9F probe nuclei were excited via the into the i9F(p, p’)t9F * reaction and recoil-implanted diamond host by a pulsed 4 MeV proton beam. The two 2.5 x 4.0 cm NaI(Tl) detectors were positioned at O” and 90° to the beam direction in a standard TDPAD setup [8]. A great advantage of the recoil-implantation technique is that very dilute impurity-host concentrations are sufficient: a typical experiment loads the lattice with less than 10e7 of a monolayer. The defect creation (vacancies) by both the pulsed proton beam and the recoiling 19F in the probed region is = 2 X 10’s defects/cm3 by the end of the TDPAD measurements, thus the i9F probes are most unlikely to interact with their own damage. The implantation is 5 orders of magnitude below the thermal spike context of 1 eV/atom energy deposition. The 19F probe takes about 1 ps to slow down, and it measures its surroundings from lo-500 ns. This amounts to a “snapshot” time V. APPLICATIONS

S. Connell et al. / Lattice site of 19F in diamond

424

window taken immediately after recoil implantation, since the diffusion time across one lattice constant is larger than 10 us [9]. In the first experiment, an orientation dependence measurement was made on the purest diamond, type IIa, in order to identify the residence sites of the 19F recoil-implanted impurity and to establish the orientation of the efg at these residence sites. In the second experiment TDPAD measurements were performed on all the four different diamond types, mounted in an optimized geometry which increased the sensitivity of the TDPAD technique to slight polycrystalline effects. Following this, a third experiment was carried out to investigate the temperature dependence of the efg parameters for the residence sites of the 19F probes. Four different measurements were made: at near-liquid nitrogen temperature, room temperature, 200 o C and 550 o C. Both vacancies and interstitials are immobile at the first temperature and only interstitials should migrate at the other three temperatures. Mobility of the vacancies is expected only above 1100 K. Finally, results for diamond were compared to those for other allotropes of carbon in a fourth experiment.

Single-crystal

3. Data evaluation The efg is characterized by the orientation of its principal axis, (6, +), the magnitude of its principal component, V,, , its asymmetry, n, and its distribution in magnitude over many similar residence sites, 6. The values of these parameters were extracted from the TDPAD data by fitting a theoretical model to the experimental spin rotation spectrum generated by computing the ratio R,,,(t)

3

N(OO,1)-NPOO, t) N(O”,

where N(0 O, malized count theoretical fit the geometries

t)+2N(90”,

t)’

t) and N(90 O, t) are the observed norrates in the 0” and 90” detectors. The function applicable to single crystals for used in these experiments is [S]

This function allows one unique residence

for the population of more than site by the i9F probe ions each

orect~on of EFG

R(t) Poly-crystal

m

FT

R(t)

q

FT

Fig. 1. Theoretical simulations of the TDPAD spin-rotation function R(t) for polycrystalline targets and single crystal targets in two different

geometries.

Fourier

transforms

of each R(r) function

are included.

S. Connell et al. / Lattice site of 19F in diamond

425

with * Diamond

lib

. Diamond

* Poly-crystalline

100 Time

200

G;‘,’ = Pk (cos 0) G,, .

II0

Grophit

30(

(nsecs)

Fig. 2. Representative sets of experimental R(t) raw data and the computer fits. The single crystal diamonds were oriented as in position 2 of the text and fig. 1. Top: diamond IIb. Middle: diamond IIa. Bottom: polycrystalline graphite. with fraction 1,. The amplitude attenuation factor f(a, Si) corrects for the (Gaussian) distribution in the efg, 6, and the finite time resolution of the detection equipment, u. For a truly polycrystalline host matrix, the theoretical spin rotation function reduces to [lO,ll]

Table 1 Orientation-dependent

Position 1 Position 2

V,, is proportional to the TDPAD frequency woi. For consistency with the literature we quote only the quadrupole coupling frequency ~9~. The relations between these three parameters are described in ref. [S]. One should appreciate that the sum of the fractional site populations cannot be 100% due to some of the 19F stopping in symmetric or nonunique sites in the diamond or in the CaF, layer where no efg is expected. For a given efg orientation, the TDPAD single crystal R(t) spectra are strongly dependent on the crystal-detector geometry. This fact may be used to optimize the measurement capabilities to certain effects. In these experiments, two different crystal-detector geometries were used. In position 1, the beam and detectors were oriented along (100) crystallographic axes. Position 2 is obtained by two 45 ’ crystal rotations about axes parallel to the two (100) directions which were perpendicular to the beam in position 1. These two detection geometries anticipated the result that the efg would be oriented along a (111) crystallographic direction. Simulations showed [8] that for the efg parallel to (111) there is almost a maximum qualitative difference between the TDPAD spectra for single crystals in these two geometries and also between polycrystalline TDPAD spectra and single crystalline TDPAD spectra collected in position 2. A summary of these R(t) simulations and their Fourier transforms are displayed in fig. 1. Fig. 2 displays a representative set of R(t) raw data and the computer fits.

4. Results and conclusions The tabulated results for the four sets of 19F TDPAD experiments mentioned above are shown in tables 1 to 4. Essentially two residence sites for 19F probe ions in diamond and its allotropes were found. The first, which has been termed the principal site, has a quadrupole coupling frequency of = 60 MHz. It is well-defined (very little distribution in quadrupole frequency, S = 5%) and has near axial symmetry for the efg (n = 5%). The second residence site for the implants, termed the dif-

efg data for 19F in diamond IIb 8

Site

f

i

(8)

&-Iz)

@I

&I

KZ (10’8V/cm2)

1 2 1 2

53(3) 23(3) 5I(3) 44(5)

59(3) 27(3) 59(3) 23(3)

4.2(3) 3I(3) 9.7(3) 3I(3)

I6(2) 0 I9(2) 0

2.44(12) l.ll(lO) 2.44(12) 0.95(12) V. APPLICATIONS

S. Connell et al. / Lattice site of 19F in diamond

426 Table 2 Evaluated

efg parameters

for the four types of diamond

6

?1

(W)

(So)

58(2) 14(l)

7(l) 59(l)

20(2) 1.0(4)

0.70

48(2) 3%2)

58(2) 15(2)

7(l) 45(2)

8(l) 9(l)

1.5

1 2 3

48(2) 13(l) 11(l)

58(2) 25(2) ill(3)

7(l) 31(2) 7(l)

16(2) 5(l) 2(l)

0.76

1 2 3

552)

60(2) 40(2) 114(3)

g(1) 89(2) 4(l)

8(l) 9(l) 3(l)

0.51

efg (nnn>

Site i

f

IIb

(111) (111)

1 2

49(3) 19(l)

Ib

(111) (111)

1 2

la

(111) (111) (111)

IIa

(111) (111) (111)

Diamond type

Table 3 Temperature-dependent

(W)

37(2) 28(l)

efg data for 19F in diamond

6

T

f @‘I

1

100 300 476 826

54(3) 55(3) 56(3) 55(3)

61(3) 61(3) 60(3) 61(3)

2

100 300 476 826

38(3) 37(3) 29(2) 0

3

100 300 476 826

28(2) 28(2) 21(2) 24(2)

Table 4 Efg data for i9F in various Host sample Single crystal graphite Polycrystalline graphite Vitreous carbon Colloidal carbon (Awadag) Diamond IIb

carbon

g&h)

(nnn>

9(l) 8(l) 7(l) 6(l)

9(l) 8(l) 7(l) 6(l)

(111) (111) (111) (111)

39(2) 40(2) 40(2)

89(2) 89(8) 89(8) 89(8)

g(2) g(2) g(2) _

(111) (111) (111)

116(4) 115(4) 117(4) 116(4)

4(l) 4(l) 4(l) 4(l)

3(l) 3(l) 3(l) 3(l)

(111) (111) 011) (111)

f @)

fixed but not measured

1 2 1 2 1 2 1 2 1 2

36(2) 8(l) 75(3) 25(2) 66(3) 34(2) 70(3) 28(2) 49(3) 19(l)

(111) (111)

6)

allotropes Site i

fixed but not measured

orientation ;%I

efg (nnn>

_

X2

IIa

WI

Site i

$Hz)

s

572) 6(l) W2) 14(l) 56(2) 17(2) 57(2) 15(l) 58(2) 14(l)

@)

II (W)

3(l) 30(3) 5(l) 10(l) 5(l) 38(3) 5(l) 30(3) 7(l) 59(3)

4(l) 10(l) 16(2) 4(l) 2(l) 4(l) 14(l) 20(2) 1.0(4)

l(1)

Theory

X2

type single crystal

0.70

PolYcrystalline

0.70

POIYcrystalline single crystal single crystal

0.89 0.91 0.70

427

S. Connell et al. / Lattice site of j9F in diamond

60-L rF;

5 -40

-

0

?

l

1st fraction

0

2nd fraction

? P

0

Lk 20

”

A -l

0

_

i

”

a

0

0 Ilb

lb

Fig. 3. The TDPAD

I0

II0

SC

frequencies

PC

Vit

and frequency

Coil

Ilb

lb

la

Ila

distributions for the two residence sites of 19F found diamond and in other carbon allotropes.

fuse site, has a lower quadrupole coupling frequency (= 27 MHz), but still near axial symmetry and is characterized by large spread in the efg (8 > 30%). These results are in qualitative agreement with those found for 19F in Ge and Si hosts by Bonde-Nielsen et al. [12,13]. The frequencies and Gaussian frequency distributions for the two residence sites are shown graphically for the various samples studied in figs. 3a and b. The coupling frequency for the principal site of 59 MHz is in excellent agreement with those determined for the quadrupole interaction at the F-site in CF, [14], (C,F,), [15] and in various tetrafluorides [16]. This fact formed the early evidence that chemical effects were dominant in the impurity-host interactions for 19F in diamond and indicated that this efg component may be associated with an almost undisturbed C-F bond. 4.1. Orientation-dependent

0

measurements

The measurement of the TDPAD spectrum for diamond type IIb in two different beam-detector-crystal geometries [8] (table 1) showed that the efg was along the (111) crystallographic direction for both the principal and diffuse site. The extracted efg parameters should be independent of crystal orientation, and this was indeed the case, except for fi, the fraction occupying the second unique site, where the large 6, value makes this parameter very sensitive to background subtraction effects. 4.2. Diamond type dependence measurements In the diamond type dependence study [17] (table 2) the detector-crystal geometry of position 2 was used. This geometry increased the sensitivity of the TDPAD method to polycrystalline effects by enhancing the contrast between the R(t) simulations for the single crystal and polycrystalline cases (see fig. 1). One should recall that our work above already established that the efg is

SC

PC

Vit

Coil

in the four types of natural

parallel to a (111) crystallographic axis. The principal difference for the two geometries lies in the greatly diminished strength of the contribution from the 2nd harmonic term for the single crystal case relative to the polycrystalline case. Polycrystalline effects were deduced from the appearance of a third fraction in the analysis with a frequency = 112 MHz which corresponds to the second harmonic of the fundamental frequency for the first (principal) site. This “third fraction” represents a supplement to the contribution of the second harmonic of the first site, rather than a distinct site. It occurs when a sample which has slight polycrystalline characteristics is analyzed with single crystal theory, which does not provide enough strength in the 2nd harmonic in describing them. A polycrystalline analysis is also insufficient, since the true situation is something in between. The amount of extra strength needed, as indicated by the percentage for the third fraction (or calculated from the Fourier spectrum), signified the deviation from the perfect lattice. This third fraction occurred for type ‘a’ diamonds. Two facts give further support to the conclusion that this third fraction is an artifact indicating slight polycrystalline effects rather than an extra site. Firstly, a quadrupole coupling frequency of this magnitude (112 MHz) requires a bond of double the strength of the covalent C-F bond [14-161, and therefore precludes this being considered as due to a distinct site. Secondly, the fact that the sum of the fractions is greater than 100% for diamond type IIa is well explained by this rationale. Results of these studies correlate with the already well-known variations of natural impurities and defects in diamond. Type IIb has the lowest nitrogen content, and no mosaic spread, and therefore the R(t) spectrum correlates best with the single crystal simulations. Type IIa also has a low nitrogen content, but has significant mosaic spread. In this case the sharp principal component of the quadrupole frequency in the R(t) spectrum V. APPLICATIONS

S. Connell et al. / Lattice site of 19F in diamond

428

reflects the short range structural order; the extra strength required in the second harmonic is evidence of polycrystalline features, due to the effect of the slight misorientation of neighbouring crystallites. Type Ia has no mosaic spread, but the largest concentration of nitrogen. We have suggested that the occurrence of the nitrogen in platelets or at least pairs imposes a strain on the lattice. The overall effect of this is to produce a similar distortion of the perfect lattice, as does the mosaic structure in diamond IIa, although not necessarily as severe. Thus the extra strength required here is not as much as in the case of diamond IIa. Type Ib also has a fairly large concentration of nitrogen, though here ESR results show that some of this occurs in single substitutional sites, and consequently the strain on the lattice is not as great as in the previous case.

0

0.1

0.2 dr

Fig. 5. Total energy of formation along a (111) crystallographic

0.3

0.4

(A) of cluster. direction.)

(The 19F is moved (Taken from ref.

1181.) 4.3. Cluster calculations Cluster calculations were performed by our collaborators [18,19] for various possible sites of the i9F probe. In these calculations, a suitably terminated cluster of atoms, including the impurity, is taken to represent the lattice. Quantum mechanical procedures are then used to evaluate the formation energy of the cluster for different positions of the impurity. These calculations showed total energy minima for i9F at off-centre positions at the tetrahedral interstitial site and the substitutional site [18]. Fig. 4 shows a diamond unit cell. The tetrahedral interstitial site is in the tetrahedron drawn onto the figure. The TDPAD frequencies (50 MHz and 25 MHz respectively) calculated from the charge distributions in the cluster for the two sites strongly suggested the tetrahedral interstitial site for the first (principal) fraction and the substitutional site for

the second (diffuse) fraction. Fig. 5 shows the total energy diagram and fig. 6 shows the computed TDPAD frequencies for these two sites. In addition the shape of the potential well in which the 19F finds itself was sharp for the first site and very broad for the second. These strong correlations between the data and the theoretical cluster calculations argue for the correctness of these site assignments. The off-centre distortion of the 19F in the interstitial site leading to the efg shows most probably the strong affinity of these two species to form a strong chemical bond due to the large difference in their electro-negativities. In fact cluster calculations show that at both sites the i9F became slightly charged, indicating again the importance of chemical effects in the host-impurity interactions. Notably, the depth of the potential well at the tetrahedral interstitial site corresponds well with the C-F bond energy of 5.08 eV.

I

t

dimensions

in

in the tetrahedron

interstitial drawn onto the figure.

-

Interrtltlalsite

,’

dr (A)

hgstroms

Fig. 4. A diamond unit cell. The tetrahedral

I1

,’

site is

Fig. 6. Expected TDPAD frequencies, computed from the charge distribution in the cluster. (The 19F is moved along a (111) crystallographic direction.) (Taken from ref. [18].)

S. Connell et al. / Lattice site of 4.4. Temperature dependence measurements

Studies of the temperature dependence of the efg parameters for the two residence sites of the i9F ion in diamond IIa (table 3) corroborated the above results and also yielded new insights. Firstly, the behaviour of the “third fraction” followed closely that of the first, supporting the interpretation that it is associated with it and an indication of the deviation from a perfect single crystal lattice. Secondly, the fractional site population experimental results displayed in fig. 7 correlated well with the cluster model molecular orbital calculations and supported the site identifications based on these calculations. The fractional population of the tetrahedral interstitial site was extremely stable, as was expected from the depth of the potential well and the strength of the F-C bond. The diminishing fractional population of the substitutional site followed a Boltzmann driven depopulation process with an activation energy matching that of the depth of the potential well for this site (E = - 0.14 ev). However, this is not interpreted as t9F leaving the site, as there is not enough time for this process. A charge transfer mechanism is preferred as the electrons are more mobile during the measurement time window. Cluster calculations [19] showed that the 19F attracted about 0.4 e charge from the lattice. When an electron was deliberately added to the cluster, the 19F developed the electronic structure of neon. This resulted in the 19F moving away from its off-centre position and back to the centre of the cluster, where the efg is zero due to symmetry. The disappearance of the diffuse (substitutional) site with temperature need thus not be due to the physical removal of the t9F p robe from that site, since the i9F can simply become transparent to the TDPAD technique as the efg has become symmetric as a result of charge transfer. Note that this implies that the substitutional 19F forms a “hole” gap state 0.14 eV above

“F in diamond

429

the valence band. This explanation has the advantage over the previous one involving the thermally driven depopulation of the substitutions site, since it explains why the “missing fraction” does not reappear at another site with an efg, such as the surface. Thirdly, the quadrupole coupling constants for all fractions over the entire temperature range investigated were remarkably stable. The current understanding of the efg in se~conductors is that its temperature dependence is related to the concentration of conduction electrons [20]. A high band gap or insulating material like diamond would not be expected to show a temperature dependence of the efg in this temperature range, as the thermally driven change in the conduction electron density will not be large. It is interesting that such a large fraction of the implants (40%) occupied the substitutional site, when it was argued above that the ballistic processes created so few vacancies. This large substitutional fraction has to arise from a replacement collision in the final part of the cascade. It is possible that this replacement is not permanent as other experimenters [21] have found that the i9F readily diffuses back to the surface. Within the TDPAD time window, however, this process has not yet occurred. 4.5. measurements

on other carbon allotropes

Since essentially the same two sites for the other allotropes of carbon have been found [table 41, this strongly suggests that the residence sites are locally very similar. This argues that chemical effects are playing a major role in determining the charge distributions surrounding the probe ions. It is therefore reasoned that the efg for semiconductors and insulators is substantially more determined by the electronic charge distributions than is the efg in metals. It seems that in these cases the formation of strong chemical bonds may override first order information on the residence sites, rendering the efg for 19F in all allotropic forms of carbon locally very similar.

60 -+

p+----+P+_

5. Summary

I

0

200

400 Temperature

600 (K)

h

800

Fig. 7. Temperature dependence of the population fractions for the two characteristic residence sites in diamond.

Considerable advances have been made in the underimpurity in distanding of the 19F recoil-implanted amond. Two unique residence sites are found. There is a strong body of evidence that these are the tetrahedral interstitial and substitutional sites. The tetrahedral interstitial site is characterized by a covalent F-C bond which leads to the observed sharp TDPAD frequency of 60 MHz. This site exhibits thermal stability as expected. The high percentage (40%) of i9F on substitutional sites was unexpected and interesting. It is normally very difficult to achieve substitutional residence sites for V. APPLICATIONS

430

S. Connell et al. / Lattice site of “Fin

impurities. In diamond, this had not been done in a controlled manner before the selective temperature predamage technique of Prim (11 and the high-dosefollowed-by-annealing method of Braunstein et al. [2]. A high concentration of impurities on substitutions sites is important in the context of electrically active dopants. In the i9F case, the origin of this high percentage is not yet fully understood, and may not be permanent. It is not likely to be associated with predamage by the proton beam, but is more probably due to chemical effects which last until the 19F becomes ionized and leaves the site. In addition, if the charge transfer model explaining this disappearance of the TDPAD signal from the substitutional site is correct, then the creation of a hole state 0.14 eV above the valence band is extremely interesting and warrants further corroboration. The authors record with appr~iation the support of the Foundation for Research Development, and support and the provision of sample materials from Messrs de Beers Industrial Diamonds (Pty) Ltd. The skillful contributions of M. Rebak to the preparation of the diamond targets are gratefully acknowledged.

References

111J.F. Prim, Phys. Rev. B38 (1988). @I G. Braunstein

and R. Kalish, Appl. Phys. Lett. 38 (1981) 416. and R. Kaiish, J. Appl. Phys. 54 (1983) [31 G. Braunstein 2106. on Material [41 R. Vianden, in: Nuclear Physics Applications Science, eds. E. Recknagel and J.C. Soares (Kluwer Academic Publishers, 1988) p. 239. [51H. Appel, J. Raudies, W.G. Thies, A. Hanser and J.P.F. Sellschop, Hyperfine Interactions 10 (1981) 735.

diamond

[6] J.H. Raudies, H. Appel, G.M. Then, W.G. Thies, K. Frietag, J.P.F. Sellschop and M.C. Stemmet, Hyperfine Interactions 15/16 (1983) 487. f7] R. Kalish, M. Deicher, E. Recknagel and Th. Wickert, J. Appl. Phys. 50 (1983) 6870. [8] S.H. Connell, K. Bha~th-Ram, H. Appel, J.P.F. Sellschop and M.C. Stemmet, Hyperfine Interactions 36 (1987) 185. [9] D.W. Palmer, Ph.D. Thesis, University of Reading (1961). [lo] H. Frauenfelder and R.M. Steffan, in: Alpha-, Beta- and Gamma-Ray Spectroscopy, vol. 2, ed. K. Siegbahn (North-Holland, Amsterdam, 1965) p. 997. [ll] R. Beraud, I. Berkes, J. Daniere, G. Marest and R. Rougny, Nucl. Instr. and Meth. 69 (1969) 41. [12] K. Bonde-Nielsen, H.K. Schon, T. Lauritsen, G. Weyer, I. Stensgard, J.W. Petersen and S. Damgaard, J. Phys. Cl7 (1982) 3519. [13] K. Bonde-Nielsen, S. Damgaard, J.W. Petersen, H.K. Schou, I. Stensgard and G. Weyer, AIP Conf. Proc. 73 (1981) 111. [14] H. Barfuss, G. Bohiein, G. Gradl, H. Hohenstein, W. Kriesche, H. Niedrig and A. Reimer, J, Chem. Phys. 76 (1982) 5103. [15] K. Bonde-Nielsen and B. Toft, Hyperfine Interactions 10 (1981) 747. [16] M. Frank, F. Gubitz, W. Kreische, A. Labahn, C. Ott, B. R&her, F. Schwab and G. Weeske, Proc. Int. Conf. on Hyperfine Interactions, Bangalore (1986), Hyperfine Interactions 37/38 (1987) 193. [17] K. Bharuth-Ram, S. Connell, J.P.F. Sellschop, M.C. Stemmet and H. Appel, to be published. [lS] J.E. Lowther, S. Connell, K. Bharuth-Ram, H. Appel, J.P.F. Sellschop and M.C. Stemmet, to be published. 1191 W. Verwoerd, these Proceedings (IBA ‘88) Nucl. Instr. and Meth. 835 (1988) 509. 1201 W. Witth~n, Hyperfine Interactions 24 (1985) 547. f21] T.E. Deny, J.F. Prim, C.C.P. Madiba, J. Ennis, R.A. Spits and J.P.F. Sellschop, these Proceedings (IBA’88) Nud. Instr. and Meth. B35 (1988) 431.