A new detector for the study of angular effects in 57Fe conversion electron Mössbauer spectroscopy

NUCLEAR

INSTRUMENTS

AND

METHODS

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NORTH-HOLLAND

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A NEW D E T E C T O R FOR T H E STUDY OF ANGULAR EFFECTS IN STFe CONVERSION E L E C T R O N MOSSBAUER S P E C T R O S C O P Y M . J . T R I C K E R , A G. F R E E M A N * ,

A.P. WINTERBOTTOM

a n d J. M. T H O M A S

Edward Dawes Chemical Laboratories, Umversity College of Wales, Aberystwyth, SY23 1SE, Great Britain Received 6 F e b r u a r y 1976 A H e / C H 4 flow proportional counter for use in conversion electron M o s s b a u e r spectroscopy is described. T h e counter Is constructed in such a way that the incident y-photon-to-sample angle can be varied. Thickness effects have been observed in highly enriched iron foils a n d m e t h o d s o f increasing the surface selectivity o f the technique investigated.

1. Introduction In addition to the usual transmission geometry 57Fe M6ssbauer spectra may also be accumulated by detecting the back-scattered 14.4 keV, y-, and 6.3 keV X-ray or 7.3 keV conversion (and subsequent 5.4 keV Auger) electrons which result from the decay of excited 57Fe nuclei within the absorber. As the reduction in count rate in a back-scatter geometry is often not compensated for by a sufficiently enhanced signal to noise ratio, espeically for samples containing natural abundances of 57Fe, most M6ssbauer spectra are accumulated by transmission methods. However, the use of transmission methods imposes the need to prepare " t h i n " absorbers and this often leads to the destruction of the sample. Where such preparation is undesirable or impossible 5 7 F e M6ssbauer information relating to the bulk of the specimen can be obtained by counting the back-scattered y- or X-rays, since the escape depth of these photons is relatively large (ca 3/~m). Alternatively if the back-scattered electrons are detected the probing depth is limited to within 3130-500 nm thus enhancing the surface selectivity of 57Fe M6ssbauer spectroscopyY,S). The increasing awareness of the usefulness of 57Fe conversion electron M6ssbauer spectroscopy (CEMS) in divers areas of study such as metallurgy9), corrosion s,1°,11) mineralogy 12) and ion implantation 13), together with the fact that surface effects can be monitored which would have gone undetected if transmission methods alone had been employed I°,12) has prompted a number of groups to develop methods whereby the back-scattered conversion electrons can be efficiently detected. To date most workers 7-1s) have employed He/CH 4 flow

proportional counters for such studies although some work has been performed with channel electron multipliers 19,20). Most He/CH4 flow proportional counters employed hitherto are constructed in such a way that the y-ray subtends a fixed, usually normal, incidence angle with the sample. Because of the additional information 21) which may be obtained from angular M6ssbauer studies we describe here the design and performance of an efficient He/CH4 proportional counter where the angle ct between the incident y-ray and the sample surface can be continuously varied from 90 ° to l0 °. By use of glancing incident angles enhanced count rates are obtained and thickness effects can be observed for enriched 57Fe absorbers. The counters discussed so far detect the total flux of backscattered electrons with only modest energy resolution. By use of a fl-ray spectrometer, with a momentum resolution of ca 5%, M6ssbauer spectra may be accumulated on selected energy bands of electrons and because electrons from deep within the solid have a large probability of losing energy compared to those originating in surface regions 22) the resulting spectra can be related to specific depths within the s o h d E 3 - 2 8 ) . However such experiments are unfortunately still in the developmental stage and confined to feasability studies of highly enriched 57Fe containing samples. Because of the importance of such depth resolved data in, for example, the study of zoning of corrosion productsa), we have explored ways of obtaining these data by increasing the surface selectivity of existing He/CH 4 counters.

2. Experimental * D e p a r t m e n t o f Chemistry, Victoria University o f Wellington, New Zealand.

2.1. CONSTRUCTION OF THE He/CH4 COUNTER The percentage effect for a CEM spectrum as given

118

M.J. TRICKER et al

by Davies 19) lS:

NR

X 100,

NA+No+Nx where N R is the count-rate o f electrons due to resonance absorption, N g background electrons produced in the absorber, N O is the background due to electronic noise and N x the background due to electrons produced within the detector by scattering from the walls etc. In practice No is negligible and in designing a H e / C H 4 counter it is important to minimize Nx. This is achieved by g o o d collimation of the y-ray and by keeping the active volume of the counter small. A thin counter, essentially similar to that employed by Fenger 17) is m o u n t e d on a base plate so that the whole insert can be rotated in a brass tube (fig. 1). The y-ray enters through a mylar window, passes through a thin alumlnium (2 mil.) window and is incident on the sample. Exit windows are also provided. Three anodes, 1 cm apart, constructed of 2 mil. wire are mounted 2 m m in front o f the sample surface. The purpose o f the aluminium window is to reduce the flux of electrons produced by stray radiation from the brass walls of the counter reaching the anodes and to reduce the y-path length in the active region. Originally it was thought that count-rates could be doubled by counting conversion electrons from the rear o f the sample surface by mounting three further anodes behind the sample. In practice it transpired that for a 90% enriched Iron ~J

i

4e~

foil containing 2 mg 57Fe c m - 2 self inversion o f the resonances occurred causing a reduction in the observed percentage effects. The optimal operating conditions for the counter were 1200 V anode potential with a gas flow o f ca 1 cm3/s of 50/0 He in C H 4. 2.2. THE MOSSBAUER SPECTROMETER The Harwell spectrometer was of conventional design employing constant acceleration transducers. Two spectra could be simultaneously accumulated into two halves of an Ireland 512 channel multichannel analyser. The source used was 25 mCi of 57Co in a rhodium matrix and was supplied by the Radiochemical Centre, Amersham, U.K. Enriched foils were obtained from the Isotope Separation Unit at A E R E , Harwell, U.K.

3. Results 3.1. PERFORMANCE OF COUNTER The H e / C H 4 proportional counter was tested usmg a 2 cm square 90% 57Fe enriched iron foil in conjunction with a 25 mCi 57CoRh source. In order to eliminate cosine effects the 7-beam was well collimated by mounting the source l0 cm away from the sample and illuminating it through a 6 m m hole in a 2 m m thick lead plate attached to the counter window. The pulse-height spectrum obtained using the 90% enriched iron foil, with a lower discrimination setting against noise, is shown In fig. 2. The peak is mainly due to 7.3 keV internal conversion electrons and 5.4keV Auger electrons and unless otherwise stated all electrons with energies above X were used in the accumuY

i1!

U

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Fig. 1. Schematic diagram of He/CH, flow proportional counter (1) EHT lead-through, (2) gas inlet, (3) gasket, (4) perspex support, (5) earthed brass supports, (6) anodes, (7) earthed alummlum window, sample behind, (8) mylar window, (9) card shields, (10) tensioning screws, (11) gas outlet, (12) sample, (13) collimator.

CHANNEL NO

Fig. 2 Pulse height spectrum obtained by scanning through the resonances of a 90% 57Fe enriched ~ron foil. The maximum ~s mainly due to 7.3 keV converslon and 5.4 keV Auger electrons.

STFe C O N V E R S I O N

ELECTRON

lation of the C E M spectra. The count-rate above the energy X was f o u n d to increase by up to 70% as the angle ~ was varied, passing through a m a x i m u m o f ca 1.7 × 103 counts/s at 0t= 15 °. Conversion electron spectra were accumulated for 5 rain for varying cc The resulting spectra are shown in fig. 3. As the magnetic field in the sample is polarized in the plane o f the surface, hnes 2 and 5 are enhanced in the spectrum for ct = 9 0 ° and the intensities vary with angle. The variation in percentage effects for this sample are shown in fig. 4 and table 1. The m a x i m u m total percentage effects of ca 3500% at ~ = 70 ° are amongst the largest ever measured, however, this figure lS reduced as ~ decreases from 70 °. It can be seen from figs. 3 and 4 and table 1 that the ratio of areas o f peaks 1 and 6 to peaks 3 and 4, which should be independent

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250% EFFECT

IZ

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MOSSBAUER

SPECTROSCOPY

119

o f incident angle, deviates from the expected value of 3:1 even at normal incidence and is further reduced as ct decreases. This effect is more p r o n o u n c e d if the ratio o f peak heights is taken (table 1). The percentage effects for an identical experiment using a natural iron sample are shown in fig. 4 and table I. A b r o a d m a x i m u m in total percentage effect was observed at ~ = 4 5 °, c o m p a r e d with ct = 70 ° for the enriched sample. The area and peak height ratios for the outermost to innermost lines remains constant at 3:1 (table 1). For angles up to 65 °, the percentage effects for the enriched sample are approximately 45 times as large as those for the natural iron. This factor is reduced for decreasing ~, for example it is 33 for ~ = 20 °. It emerged from the computer fits assuming Lorentzlan line-shapes that the line-width of lines 1, 2, 5 and 6 in the spectrum o f the enriched absorber increased (table 1 and fig. 5) with decreasing ~. This broadening was considerably less p r o n o u n c e d for the innermost lines 3 and 4 o f the enriched foil and absent for all lines o f the natural iron foil (table 1 and fig. 5). The deviations o f the area ratios o f lines 1 to 3 from 3:1, the increasing line width and the falling percentage effects as glancing incidence lS approached are due to the occurrence of saturation effects within the enriched iron foil. i

30 o_ I-

2.6

22

200q

VELOCITY M M S 70

F]g. 3. Conversion electron M 0 s s b a u e r spectra o f a 90% STFe enriched foil as a function o f angle. Note large percentage effects a n d variation o f the relative m t e n s m e s with angle. A typical b a c k g r o u n d c o u n t is 500.

50

30 ANGLE °

Fig. 4. Variation o f the total percentage effect o f enriched iron fo]l (C) a n d a natural ]ron fod (D), wlth angle ct. Also s h o w n is the variation o f the ratio o f peaks 1 a n d 6 to 3 a n d 4 o f the iron spectra as a function o f angle, (A) natural and (B) enriched iron.

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T R I C K E R et al.

TABLE 1 S u m m a r y o f full widths at half maxima (fwhm) and percentage effects of the iron foils studied as a function o f incident y-angle. Enriched sample Ratio if lines 1 + 6: 3 + 4 (q- 0.1) Percentage effect Area Peak height

Angle (degrees)

81 70 60 45 30 20 10

3.2.

2.9 2.7 2.8 2.6 2.5 2.3 22

2.5 2.5 24 2.3 2 1 2.0 1.7

3325 3561 3444 3181 2824 2566 1662

Natural iron Line widths (fwhm) 4- 0 01 m m / s Lines 1 and6

Lines Lines 2and 5 3 and4

0.26 0.25 0.25 0 27 0.28 0.29 0.33

0.25 0 25 0 25 0.26 0.25 0.26 0.28

SURFACE SELECTIVITY

In order to compare methods of preferentially detecting the signal from regions close to the surface a test absorber was prepared by oxidlsing a 2 x 1 cm z piece of 90% 57Fe enriched iron foil. A duplex oxide film of Fe30 4 and F e 2 0 3 (with FezO 3 outermost) ca 100 nm thick was formed by heating in air for 10 min at 350°C. Spectra (counting time 45 min) of the oxidised foil as a function of angle were accumulated. Because a significant fraction of the incident y-photons are attenuated by M6ssbauer events within the oxide layer this layer is increasingly enhanced relative to the substrate as parallel incidence is approached. This can be seen (fig. 6) by noting the diminution of the iron lines 4 and 7 (see bar-diagram for assignments) relative to the oxide lines 1, 2, 3, 5, 6 as ~ decreases.

Ratm of hnes 1 + 6: Average 3 + 4 ( 4- 0.1) Percentage hne width effect 4-0.01 m m / s Area Peak height

Angle

0.23 0.24 0.23 0.24 0.25 0.25 0.25

87 75 48 38 18

3.2 2 8 2 8 3.0 3.0

3.0 2.9 2.9 2.9 2.7

76 78 89 91 73

0.23 0.23 0.24 0.23 0 23

A second approach to increased surface sensitivity can be made by utilizing the inherent, albeit poor, FezO3

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80 6Q 50 ANGLE* Fig. 5. VariaUon o f the fwhm wzth angle o f (A) average o f all natural iron transitions, (B) average o f Am = 0 transition and (C) Am = ± 1 transitions o f a 90% STFe enriched iron foil. 1 channel ca 0.025 mm/s.

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Fig. 6. CEM 57Fe spectrum of an oxldlsed 90% S7Fe enriched ~ron foil at ct = 90 ° and ~t = 15 °. The oxide signal is increased relative to the substrate s~gnal at glancing modence.

S7Fe CONVERSION

ELECTRON

energy resolution of the He/CH4 counter to detect only those electrons with energies close to 7.3 keV which have emerged from regions near to the sample surface with little energy loss. Although the importance of correct discriminator setting on the He/CH4 pulse height spectra has been realised in relationship to signal to noise ratio16), to our knowledge no data on the possibility of obtaining in depth measurements by altering these settings have been reported. Fig. 7 shows the simultaneously accumulated M6ssbauer spectra from the high and low energy regions of the pulse height spectrum (regions X to Y and Y and above In fig. I). Because of the difference in count-rate the high-energy electron M6ssbauer spectrum was continued till the statistics were comparable. It can be clearly seen in fig. 7 that a significant enhancement of the oxide especially Fe203 signal, relative to the iron

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substrate is achieved by preferentially detecting the faster electrons. The oxide to iron and Fe203 to Fe304 signals are both approximately doubled and this effect is emphasized in the difference spectrum, fig. 7c. lsozume has suggested 15) that the effective depth probed by CEMS can be limited to ca l0 nm by placing the counter wires 10 mm away from the sample surface. In this way he argues that only the high energy electrons are able to reach the counter wires and be amplified. Pulse height spectra with the wires l0 mm from the sample surface showed a large reduction in count-rate. CEM spectra of the oxldised iron foil with the counter wires 2 and 10 nm from the surface were accumulated and an enhancement of the outermost surface is certainly obtained although the effect is less pronounced than in fig. 7. 4. Discussion The number of M6ssbauer interactions per unit y-flux at a depth x from the absorber surface, in a layer of thickness dx, with the y-photons incident at an angle ~ to the plane of the surface* at resonance is:

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M(JSSBAUER SPECTROSCOPY

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~ba exp I-/sin~ (~b~+/te)l sindX' where /~e = electronic absorption coefficient for the y-radiation, and 4)=namfaao, where n a m = a t o m i c density of the M6ssbauer isotope, f . is the recoil free fraction of the absorber, and a 0 the resonance crosssection. The attenuation, assuming an exponential form, for an electron emitted at a depth x at an angle 0 to the surface normal is exp[-

0

xPc ],

cos OA

where /~c is the linear absorption coefficient for the electron. The number of conversion electrons emitted for unit flux of M6ssbauer y-photons from a solid of infinite thickness over an angle of 2n (appropriate to the counter described in this work) is given by:

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o ~kJo J o exp ~ ( q ) . + / ~ ) VELOCITY

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Fig. 7. C E M 57Fe M 0 s s b a u e r spectra o f a n oxidised 90% 57Fe enriched iron foil obtained with (A) low energy electrons (B) high energy electrons (C) &fference spectrum. This is m a r k e d e n h a n c e m e n t o f the Fe2Oa contribution m s p e c t r u m B. This Is clearly seen by noting the increase in mtenslty o f the Fe203 h n e at h~gh posittve velootles a n d in the difference spectrum.

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* T h e estimation o f m a x i m u m percentage effects given here is along the hnes o f Davis 19) although we m a k e a m o r e detailed allowance for the a t t e n u a t i o n o f the p h o t o n s a n d electrons involved.

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T R I C K E R et al.

TABLE 2 Parameters used in estlmatmn o f signal to background ratios of CEM spectra. Absorptzon coeffioents for the electrons are calculated from the emp,rlcal formulae o f Cosletta°).

Contnbutmn to

Type of p h o t o n

Signal

Background

Mossbauer 14.4 keV

Background Background

Mossbauer ),-ray 14 4 keV non-Mbssbauer 7-ray 14 4 keV X-ray 6.3 keV y-ray 122 keV

Background

7-ray 135 keV

Background

Intensity

0.65

Absorptmn coefficient ( c m - 1)

2.57 x 10 - i s

144 000 (90% enriched) 3 200 (natural abundance)

ls conversion (K) 2s conversion (L)

7.3 13.6

0.81 0.09

170 000 67 000

3s conversion (M) K L L Auger

14.3 5.4

0.01 0.57

62 200 266 000

8.45 x 10 -21

716

ls photoelectron

7.3

1

170 000

0 35

8.45 x 10 -21

716

ls photoelectron

7.3

1

170 000

537 2.2 2.2 1.9 1.9

photoelectron photoelectron C o m p t o n electron photoelectron C o m p t o n electron

5.6 100 100 100 100

1 1 1 1 1

253 000 700 700 700 700

5.6 9.0 10

6.34 x 12.2 x 14.2 x 8.5 x 14.5 x

10 T M 10 -2`* 10 -2'* 10 -2`* 10 -2`*

\IAk+A/

where A = (4~ +/to)/sln c~. The off-resonance count is mainly due to photoelectrons as the Compton effect is negligible for low energy X- and ?-photons. The background is proportional to:

t,J,k

Energy (keV)

0.65

where ~k lS the normalised internal conversion coefficient of the kth shell and/~ck is the appropriate absorption coefficient.

sln~ LA 2

Type of electron

Absorptmn Probabdlty coefficient of for electron productmn ( c m - J)

Crosssectmn (cm2/atom)

eters used) for pure iron samples indicate that in principle very large percentage effects should be possible. For example the calculated effect (at normal 7-ray incidence) for the outer hne of a natural iron TABLE 3 Relauve contributions and percentage effects to the s,gnal and background in natural and enriched iron fod. Case 1, ls converston and photoelectrons. Case 2, all y-ray conversion and photoelectrons and K L L Auger electrons. Case 3, as case 2 together with iron L shell photoelectrons produced by 6.3 keV X-ray S,gnal from

n,N j F(ff,j k, [.lljk),

Abundance of 57Fe 2% 90%

where n, is the density of the ith atomic species, Nj is the normahsed rate of incidence of thejth photons and ¢r,jk the cross-section of the kth shell of the ith species for the jth photon. The function F(C%k, It,k) is similar to eq. (1) and allows for the attenuation of the photons and subsequent photoelectrons within the solid. Calculation* (see table 2 for a summary of the param-

ls conversion electrons

0.0023

0.082

ls conversion electrons

0 00069

0.019

3s conversion electrons

0.000075

0 0021

K L L Auger electrons

0.0010

0.040

ls photoelectrons (Mossbauer yray)

0 0014

0.0011

ls photoelectrons (non-Mossbauer y-ray)

0 00074

0 00074

* It is assumed in these calculatmns that the recod-free y-photons are annlhdated by the Mossbauer absorption-process, but this is only true for 90% o f events. For the remamlng 10% yphotons are produced and 80% o f these are recod-free. In addition there are 27 6.4 keV K L L X-ray-photons produced for every 100 Mossbauer events and these are also excluded from the calculaUon.

X-ray photoelectrons

0.0059

0.0059

108% 194o 51o

4508% 7847% 1850%

Percentage effect Case 1 Case 2 Case 3

123

57Fe C O N V E R S I O N E L E C T R O N MOSSBAUER S P E C T R O S C O P Y

sample assuming a 3:2:1:1:2:3 ratio of the cross section is 108% if the signal is made up of iron conversion electrons and the background iron 7.3 keV photoelectrons produced by the 14.4 y-photons. Inclusion of the 5.6 keV Auger electrons increases this to 194 per cent but adding the contribution of the ca 5.5 keV L-shell iron photoelectrons produced by the 6.3 keV X-ray to the background reduces the effect to 51%. The percentage effects will also be further reduced by the photo- and Compton electrons produced by the high energy 122keV and 136 keV 7-photons. These electrons are likely to be of high energy, ca 100 keV, and consequently will have long escape-depths, although the cross-section for their production is small. I n practice these fast electrons may not themselves be detected, unless they have lost considerable energy, but they will produce low-energy secondary electrons which will be detected and further increase the background. This increase in background was not included in the simple model, described here, and consequently the calculated percentage effects will be systematically too large. However, they do indicate the experimental trends and form a useful framework for discussion. The values for iron enriched by a factor of 45 are 4500%, 7847% and 1850% for the three cases described above and are respectively 42, 40 and 37 times the values for the unenriched cases (see table 4). This is less than the enrichment factor of 45, because of saturation effects caused by the more rapid attenuation of 14.4keV M6ssbauer 7-photons in the enriched absorber. For the case where only 7-photons contribute to the background, the reduction in these photoelectrons largely compensates for this effect. However, addition of the background due to X-ray photoelectrons, which is the same for both the natural and enriched samples, causes a reduction in the percentage

effect and this is more marked in the enriched case (table 3). The advantages of filtering out the 6.3 keV X-ray from the source has been demonstrated in the above calculation, but it can only be achieved at the expense of count-rate. In the experiments recorded here the ratio of the 14.4 keV 7- to the 6.3 keV X-photon was increased from 1:4 to 2:1 by a combination of a perspex cap on the source and the mylar and aluminium windows of the counter3). The calculated effects using this ratio and assuming 3:2:1:1:2:3 transition probabilities at all angles are given in table 4. For the natural iron sample the total percentage is 620% and remains constant over the angular range. Moreover the intensity ratios are close to 3:2:1:1:2:3 and therefore the sample is effectively thin over the whole range. The total percentage effect for the enriched absorber is 25 000% at normal incidence and it can be seen that it reduces rapidly at glancing incidence. In addition the intensity ratios deviate from 3:2:1:1:2:3. These effects are a factor of 41 and 33 larger than the unenriched case for normal incidence and ~ = 20 ° respectively. Examination of the effects for the individual hnes (table 4) shows that the fall of percentage effect is more pronounced for the transitions with large resonance cross-sections. The increased background at glancing incidence occurs because photoelectrons which would have been produced beyond the escape depth at normal incidence are released closer to the surface and can escape into the detector. This increase outweighs the increase in conversion electrons because a very large fraction of the M6ssbauer y-photons are attenuated withm the escape depth of the conversion electrons even at normal incidence in an enriched foil. Clearly the observed experimental fall off of percentage effects especially pronounced for the A m = +_ 1

"]'ABLE 4 Estimated percentage effects (neglecting high-energy ),-photons and with 14.4 keV y to 6.3 keV X-ray ratio 2:1) for natural and enriched iron fods as a function o f angle The ratios o f the cross-sections are 3 : 2 : 1 : 2 . 3 at all angles. Percentage effects for natural iron (2% 57Fe) Angle

90 79 67 56 44 33 21 10

Line 1 and 6 Lane 2 and 5 Line 3 and 4

155 155 155 155 154 154 153 151

104 103 103 103 103 103 103 102

51.5 51.5 51.5 51.5 51.5 51.4 51.3 50.9

Percentage effects for enriched sample (90% 57Fe)

Total for 6 hnes 621 620 620 620 617 617 615 608

Line 1 and 6 Line 2 and 5 Line 3 and 4

6085 6070 6025 5940 5798 5560 5124 4105

4236 4228 4205 4160 4085 3957 3718 3127

2207 2205 2198 2184 2161 2120 2040 1830

Total for 6 hnes 25060 25030 24860 24560 24090 23270 21760 18120

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M . J . T R I C K E R et al.

transition of the enriched foil is due to thickness effects. However, these effects are only observed under extreme conditions of enrichment and for most purposes, especially with samples containing natural abundances of iron, the absorbers can be taken to be effectively thin. The experimental observed maximum in percentage effect is not reproduced by the calculation. It is probable that this occurs because of the variation of the X-ray flux incident at the sample surface because the aluminium window thickness effectively increases with decreasing ~. An additional complication in making absolute comparison of the observed and calculated effects is the unknown sensitivity of the counter to electrons of different energies. However, the observed effects are considerably lower than the calculated effects. This is in part due to p h o t o - a n d Compton-electrons (produced by the high energy y's) from the absorber but also indicates that it is worthwhile trying to further eliminate background electrons from the counter. A considerable number of these electrons must be forward scattered from the counter window for example. A source of background electrons impossible to eliminate in He/CH4 counters is photoelectrons from iron and from atoms other than iron in the sample itself. This seriously limits the ultimate sensitivity as not only are conversion electrons lost as iron atoms are replaced more background photoelectrons are produced. This makes the accumulation of CEM spectra of samples containing small quantities of natural iron ca < 10% difficult at the present stage of development. All three methods of increasing the surface selectivity of CEMS were successful. However, the glancing incidence method is confined to enriched samples and in view of saturation effects is probably of limited value. Surface selectivity can be obtained in H e / C H , counters by preferentially detecting the electrons with energy close to 7.3 keV either by suitable discriminator settings or sample anode distances. Counting rates are diminished because the quantity of sample probed is now only of the order of l0 nm rather than 100 nm if all the electrons are detected. The depth resolution is not as good as that obtainable with a fl-ray spectrometer but in wew of the slmphcity of the method it is expected to be of value in such areas as zoning of corrosion products. It is now possible by use of CEMS backscattered X-ray and transmission y-ray methods to examine the outermost l0 nm, l02 nm, l03 nm and the bulk of a specimen by M6ssbauer spectroscopy. T. E. Cranshaw of A E R E Harwell is thanked for

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