A new system for depth-selective conversion electron mössbauer spectroscopy

Nuclear Instruments and Methods m Physics Research 221 (1984) 577-581 North-Holland, Amsterdam

A NEW SYSTEM

FOR DEPTH-SELECTIVE

CONVERSION

577

ELECTRON

MOSSBAUER

SPECTROSCOPY S C PANCHOLI

*, H

DE WAARD,

J.L.W. PETERSEN

a n d A. V A N D E R W I J K

Laboratorlurn voor Algernene Natuurkunde, University of Grontngen, The Netherlandv J. V A N K L I N K E N Kernfvst~h Versneller lnstttuut. Umverstty of Grontngen, The Netherlands Received 12 September 1983 A conversion electron Mossbauer spectrometer has been constructed in which a mini-orange magnetic filter focuser conversion electrons emitted by a moving absorber onto a than window silicon surface barrier detector Thas detector serves as an electron spectrometer A specml multi-scaler is used to record a number of Mossbauer spectra, corresponding to different conversion electron energy groups, simultaneously A large reduction in counting time compared to other methods of energy differential conversion electron Mossbauer spectroscopy is achieved by the relatively hagh transmission of the mira-orange ( - 2%). the high efficiency of the detector and the feature of simultaneous recording Depth selectivity is mainly limited by the resolution (600-900 eV)) of the Sl-detector The system has been tested w~th a sandwich absorber of 57Fe evaporated onto stainless steel enriched in s7Fe irradiated by a 57CoRh source

1. Introduction Integral conversion-electron M o s s b a u e r spectroscopy (CEMS), mostly using 57Fe a n d ll9Sn, has been used for a n u m b e r of different studies, concerning surface oxadatton and corrosion of iron and steel, surface treatment, surface stress and Ion-Implantation [1]. W h e n the c o n v e r s m n electrons that are emitted from a sample after recodless resonance absorption of gamma-rays are analyzed in energy and the Mossbauer spectra are recorded for a n u m b e r of different electron energies, the m e t h o d is called energy differential CEMS. By the use of this m e t h o d M o s s b a u e r spectra can be m e a s u r e d for one particular electron shell (K or L, etc.) as a f u n c t m n of electron energy as well as for different electron shells In the first type of investigations one measures the electron energy loss spectrum for K or L shell conversion electrons Ttus gives information a b o u t the n u m b e r of conversion electrons cormng from different depths u n d e r the sample surface. Ttus t e c h m q u e xs called depth-selective conversion M o s s b a u e r spectrometry (DSCEMS). The m e a s u r e m e n t of Mossbauer spectra for different electron shells may yield partial spin densities for different electron shells. Using energy differential C E M S techmques, studies have been m a d e which include the m e a s u r e m e n t of partial 2s a n d 3s electron spin denslnes m Iron metal [2], surface-spin orientation * Present address Department of Physics and Astrophysics,

Umverslty of Delhi, Deltu-ll0007, India 0 1 6 7 - 5 0 8 7 / 8 4 / $ 0 3 . 0 0 © Elsevier Science P u b h s h e r s B.V ( N o r t h - H o l l a n d Physics P u b h s t u n g Division)

m rare-earth Iron garnet films [3], a n d d e p t h - d e p e n d e n t hyperfine mteractxons [4-6] Different types of spectrometer [7-12] have been used for the m e a s u r e m e n t of low-energy conversion electron spectra. The basic requirements for an electron spectrometer suatable for energy dlfferennal C E M S are a large transm~ssmn (several percent at least) and a reasonably good energy resolution (2-5%) All systems used so far for differential C E M S use magnetic or electrostatic spectrometers, which generally implies that M o s s b a u e r spectra must be measured separately for each energy setting. Measurements with such systems tend to be time consurmng Exceptmns are the spectrometer d e s c n b e d by Parallada et al. [9] in which three separate detectors can be installed that accept electrons of different energies and, very recently, the use of a reslstwe wire p o s m o n sensitive p r o p o m o n a l counter in a double focusing spectrometer by T o n y a m a et al [13] Experimental techniques for C E M S have recently been reviewed by Sawlcl~ and Sawlcka [14] We have constructed a n d tested an energy dlfferentml C E M S system capable of recording several Mossb a u e r spectra simultaneously for different electron energies, using a special SI-detector as the spectrometer Ttus reduces the total m e a s u n n g tame very considerably, b u t the energy resolution ~s less good than with electrostatic or m a g n e n c spectrometers. O n the other hand, the use of the more energetic L electrons (13 6 keV) instead of the K electrons (7.3 keV) allows the p r o b i n g of larger d e p t h s (up to 250 n m instead of 50 n m )

578

S ( Panchoh et al / Depth-selectwe CEMS

2. T h e instrument

In fig. 1 a schematic drawing is shown of the system The central part consists of a mira-orange magnetic fdter [15] containing six wedge shaped Phihps "ferroxdur" pieces magnetized to such a strength that energies from 3 to about 20 keV may be transmitted, depending on the source-to-resonant absorber and the resonant absorber-to-detector distances An energy band ranging m width from about 3 keV at E,~ = 7 keV to 6 keV at E~l = 14 keV ~s transmitted (E¢I ~s the center energy of the electrons for which the filter is adjusted). The measured transmission of the mira-orange m these ranges varies from 1.5 to 2%. The resonant absorber to be investigated is lrradmted by a source mounted inside the central lead absorber of the mini-orange filter as shown in fig. 1. The gamma-rays are collimated by a conical lead piece. On the front of this a 10/Lm thick mylar foil is glued to prevent conversion electrons from the source and secondary electrons generated in the collimator from reaching the absorber. For a 57CoRh source and a thin resonant absorber this foil causes a background reduction by a factor of 3. The resonant absorber is mounted on an alunumum tube with a wall thickness of 0 1 mm, that reduces a possible background contribution by scattered gamma-rays. The alumimum tube ~s attached to a rod that is connected at its other end to a Mossbauer drive system via a bellows vacuum feed through that allows easy movement of the dnve rod. For electron detection a special n-St surface barrier detector of 8 mm diameter with good energy resolution and very thin dead layer was used. This detector was developed by one of us (J.L.W.P.); it is more fully described in another paper [16]. It is cooled to liquid nitrogen temperature together with a FET preamplifier. An electron spectrum measured with this detector when a 2 #Cl S7Co source implanted in A1 at a depth of 5 nm replaces the resonant absorber is shown m fig. 2 The resolution obtained in this case, e.g. 800 eV for the

" MINI-ORANGE " SOURCE MOVING

ELECTRON~

RESONAN' /'

% 2

3

t,

Co All

1000 ~,,

0

J 2

I ~,

t

J

5 8 ELECTRON

t .---x--- I "~._ t I0 ENERGY

~2 IkeV)

It.

_j

16

F~g. 2 Conversion electron spectrum of very thin 2/~Cl 57Co source implanted m A1 at a depth of 5 nrn

L-conversion hne, is less good than that obtained when the mini-orange is removed ( - 6 0 0 eV). This loss of resolution is ascribed to the spread m angle of incrdence of the electrons that are collected onto the detector when the rmm-orange is m place. This spread causes a spread in energy loss of the electron m the (thin) surface Au-layer (20-40 # g / c m 2) of the detector The spectrometer is evacuated by a zeolite pump. The vacuum space must be kept very clean to prevent deterioration of performance in the long run, due to the budd-up of a condensate on the surface of the cooled detector. The amphfied pulses from the detector are applied to four single channel analyzers adjusted to various parts of the conversion electron spectrum. Their output pulses are fed to four groups of 1000 channels each of a modified D I D A C 4000 multiscaler. These groups are scanned synchronously with the aid of one address scaler from the output pulses of which a triangular wave form is derived that is applied to the Mossbauer drive umt in the normal way.

3. P e r f o r m a n c e S)-DETECTOR

- FERRITE WEOGE I

AUGER~I i

LEAD DIAPHRAGM

/,

ABSORBER

1500t

5 CM

Fig. 1. SchemaUc drawing of conversion electron Mossbauer spectrometer for depth-selective measurements

The performance of the instrument was tested with a 3 mCl 57CoRh source and several absorbers containing 57Fe. A n essential feature of any Mossbauex spectrum is the resonant peak to background ratio. This ratio determines to a high degree what statistical accuracy or number of counts per channel is needed for meaningful results. In integral conversion electron MiSssbauer spectroscoppy w3th 57Fe, peak to background ratios in excess of 10 have been achieved.

579

S C Panchoh et al / Depth-sele¢twe CEMS

A Gate on

K-hne

5

~3 z 2

>~ o~ 3

-3

2

-I 0 VELOC[TY

1 (rnm/s)

2

3

Fig 3 Conversion electron Mossbauer spectra for 1 ,am thick 310 stainless steel fod (95% 57Fe)

3O

8 7 - ) 0 4 keV

25

2O

15

25

20

)04-121 keY

Table 1 Raaos of hne areas of SS-310 and Fe components m the DSCEMS spectra of a double layer absorber consisting of 70 nm ~TFe evaporated on a 1 #m thick stainless steel foil enriched to 95% m 57Fe

0

2

If

-6

i

i

-4

-2

i

In our case, b a c k g r o u n d c o m p o n e n t s in the spectrum of the S~-detector can be caused by 123 keV gamma-rays from the 5VCoRh source that are scattered from the walls and from the absorber and by C o m p t o n electrons produced by the gamma-rays Source holder a n d colh m a t o r construction were chosen so as to hmlt these b a c k g r o u n d contributions to the a l u m m m m walls remote from the detector In fig 3 M o s s b a u e r spectra are shown that were o b t a i n e d with 1 # m thick stainless steel absorber enriched to 95% in 57Fe for channels set on the K a n d on the L + M conversion lines. For the K-lane the peak to b a c k g r o u n d ratio is a b o u t 7, for the L + M-line a b o u t 2, these values are quite satisfactory. The measuring time for these spectra was 24 h. The d e p t h selectivity of the system was tested using a resonant scatterer consisting of a 70 n m iron layer (95% 5VFe) evaporated onto a 1 ~ m thick 310 stainless steel foil (95% 57Fe) In the L-line region four windows were set covering electron energy ranges 7-8.7 keV, 8 7-10.4 keV, 10.4-12 1 keV a n d 12.1-13.3 keV The last three spectra are shown m fig. 4 In the first two of these, the central line due to the stainless steel backing dominates, while in the last spectrum the total intensity of the 6 hne spectrum of ferromagnetic surface layer ~s 3 × larger than that of the central line In table 1 line area ratios for all four spectra are given, clearly showing the d e p t h selectivity If one wishes to correlate the energy channels with d e p t h s of electron emission, use should be m a d e of energy loss d I s m b u t i o n s . The m e t h o d to be followed has been extensively treated by Llljequxst et al. [17], who have also developed a computer program for practical purposes A rough estimate of the average depth can be o b t a i n e d by interpolation of the energy loss data of Itoh et al. [18] a n d of Bonchev et al. [19], taking into account the average emission angle which leads to an average increase by a factor 1 3 of the &stance traversed by the electrons in the foil relative to their depth of generation The d e p t h estimates are also given m table 1 These are

233,8,

J

J

~

0 2 VELOCITY (ram/s)

4

6

1

Fig 4 L-conversion electron Mossbauer spectra for 1 ,am thick SS-310 fod (95% 57Fe) covered with 50 nm 57Fe (95%) Different energy groups of L-conversion line, showing depth select]v~t2¢

Electron energy (keV)

Area ratio (SS/Fe)

Estimate of average depth (ntn)

7 0- 8 7 8 7-10 4 104-12 1 12.1-13 1

L-hne 2 84(17) 2.51(15) 1 11(7) 0 35(2)

270 190 90 20

5 0- 8 0

K-hne 0 21(2)

25

580

S C Panchoh et al / Depth-selectwe C E M S

~20 ~r

~d

*,t<,

~zlO

I

t

-6

-Z,

I

I

I

-2 0 2 VELOCITY (mm/s)

~

t_

4

6

l

F~g. 5. K-conversion electron Mossbauer spectrum for ] ~tm thick SS-310 foil (95% 57Fe) covered with 50 nm 57Fe (95%) Channel set to accept energy range from 5-8 keV Ratm of SS-310 and 57Fe areas 0.21 (1)

confirmed by an analysis according to the procedure of Ldjequist et al. [15]. It is seen that the depth range probed by the L electrons extends to about 250 nm. It is limited by the K electron contribution for L electron energy losses exceeding 6 keV. The usable K-line energy region is much smaller due to the closeness of the Auger electron groups. These hrmt the maximum depth probed by K-electrons to about 50 nm. Due to our detector resolution we can hardly aclueve any depth selectivity in this region. However, the much larger conversion coefficient of the K-line allows us to achieve good statistical accuracy for a 50 nm thick surface layer in a much shorter time. This is demonstrated in fig. 5, where a K-conversion electron MOssbauer spectrum is shown. This spectrum was obtained in 24 h. The result suggests the possibility of performing depth selective measurements by setting some windows in the L-hne region to probe larger depths ( d = 50-250 nm) and one gate on the K-line for the surface region ( d ~< 50 nm). A disadvantage of using a solid state detector as a spectrometer compared to magnetic or electrostatic analyzers is the unavoidable flat taft that appears on the low energy side of any electron line because a part of the electrons are backscattered out of the detector. These electrons only lose a fraction of their energy in the detector. The tail contains 15-25% of the incident electrons (fig. 2). Its effect is that electrons emitted closer to the surface contribute to spectra obtained for larger average depths. When the detector line shape is known, however, such contributions can be "peeled off", at least in principle. 4. Conclusions It has been demonstrated that depth-selective conversion electron M0ssbauer spectroscopy can be carried

out with the combination of a special Si-surface barrier detector and a mira-orange filter for low energy electrons Its advantages are a large reducuon in measuring time and the posslblhty to probe larger depths by using L-conversion electrons. The easy adjustment of position and width of the windows set on the electron spectrum allows adaptation to optimum experimental conditions such as prescribed, for instance, by the energy distribution of electrons from a certain depth. Disadvantages are the lower energy resolution that has been achieved ,~o far and the occurrence of a low energy tail in the spectra due to electrons scattered out of the detector. A special feature of the new method is the simultaneous probing of the near surface region by K-electrons and the deeper region by L-electrons The authors wish to thank K Post and B.A. Sawlcka for their skllful assistance m fitting the spectra and S.P Steendam for his contributions in the construction of the spectrometer. Useful comments of J,S. Sawicka and B A Sawlcka have had a beneficial effect on the paper One of us (S.C.P.) gratefully acknowledges the kind hospitality of the Laboratonum voor Algemene Natuurkunde of the University of Gronmgen. Tlus investigatmn was financed by the Stichtmg voor Fundamenteel Onderzoek der Materie (FOM) subsidized through the Dutch Orgamzauon for Pure Research (ZWO).

References [1] M J. Tricker, Symp. on Mossbauer spectroscopy and chermcal apphcatlons, eds., J.G Stevens and G.K Sheno:y (American Chermcal Society, 1981) p. 63. [2] Cheng-Jyi Song, J. Trooster, N. Bcnczer-Koller and G M. Rothberg, Phys. Lett 29 (1972) 1165 [3] K Saneyosht, Y Yoshtda, J Itoh, T Tonyama, K Hlsatake and S. Clukazumi, Proc lnt Conf on Mossbauer effect and its appheation, Jaapur, In&a (Indian National Science Academy, New Deltu, 1982) p 222 [4] T Slugematsu, H.-D. Pfannes and W. Keune, Phys. Rev. Lett 45 (1980) 1206. [5] T Yang, T Trooster, T Kachnowslo and N. BenczerKoller, Hyperfine Interactions 10 (1981) 795 [6] T Yang, A. Krishnan, N Benczer-Kotler and G Bayreuther, Phys Rev Left. 48 (1982) I292. [7] U Baverstam, B. Bodlund-RingstriSm, C. Bohm, T Ekdahl and D Liljequlst, Nud. Instr. and Meth 154 (1978) 401 [8] T. Toriyama, K. Sancyoshi and K. Hisatake, J Phys (PARIS), Coll. C-2 40 (1979) 14 [9] J Parellada, M.R. Polcan, K. Burin and G.M Rothberg, Nucl Instr. and Meth. 179 (1981) 113 [10] M. Domke, B. Kyvelos and (3. Kaindl, Hyperfine Interactions 10 (1981) 1137. [11] T. Shigematau, H.-D. Pfaanes and W Keune, Syrup on Mt~ssbauer spectroscopy and chotmeal applications, eds., J.G Stevens and G.K Shenoy (Amertcan Chenucal Soctety, 1981) p. 125

S C Panchoh et al / Depth-selective C E M S

[12] T-S Yang, B Kolk, T Kachnowskl, J Trooster and N Benczer-Koller, Nucl In~tr and Meth 197 (1982) 545 [13] T Tonyama, H Mlyasaka, Y Fujita and K. Hisatake, Proc 6th lnt Conf on Hyperfme interactions (to be pubhshed) [14] J A Sawlckl and B D Sawlcka, Hyperfme Interactions 13 (1983) 199 [15] J van Khnken and K Wlsshak, Nucl Instr and Meth 98 (1972) 1, J van Khnken, S J Feenstra, K Wlsshak and H Faust, Nucl Instr and Meth 130 (1975) 427

581

[16] J W. Petersen, Nuc[ Instr and Meth this issue, p 582 [17] D Llljequlst, T Ekdahl and U Baverstam, Nucl. lnstr and Meth. 155 (1978) 529, D Llljeqmst and B BodlundRmgstrom, Nucl lnstr and Meth 160 (1979) 131 [18] J Itoh, T. Torlyama, K Saneyoshl and K Hlsatake, Nucl Instr and Meth 205 (1983) 279 [19] Tsv Bonchev, A Mlnkova, G. Kushev and M. Grozdanov, Nucl Instr and Meth 147 (1977) 491