Spherical electrostatic electron spectrometer

Nuclear Instruments and Methods 197 (1982) 545-556 North-Holland Publishing Company

545

SPHERICAL ELECTROSTATIC ELECTRON SPECTROMETER T-S. Y A N G , B. K O L K *, T. K A C H N O W S K I **, J. T R O O S T E R t an d N. B E N C Z E R - K O L L E R Department of Physics, Rutgers University ***, New Brunswick, New Jersey 08903, U.S.A. Received 27 April 1981

A high transmission, low energy spherical electrostatic electron spectrometer particularly suited to the geometry required for Mrssbauer-conversion electron spectroscopy was built. A transmission of 13% at an energy resolution of 2% was obtained with an 0.5 cm diameter source of 13.6 keV electrons. Applications to the study of hyperfine interactions of surfaces and interfaces are discussed.

1. Introduction. Conversion electron spectroscopy has been occasionally used for the investigation of electronic structure in solids. However, the combination of MOssbauer and conversion electron spectroscopies can provide considerable additonal information and, for example, constitutes the only tool capable of determining the microscopic details of core s-electron densities at the nuclear site. The identification of individual electronic contributions to hyperfine interactions is achieved through the special high electron energy resolution and spin discrimination inherent in Mrssbauer-conversion electron spectroscopy: the MiSssbauer effect is used to excite the sample and an electron spectrometer acts as an analyzer for the internal conversion electrons emitted in the de-excitation of the resonantly excited state. For example, variations in chemical composition of a sample with depth or structural differences at boundaries between different metals or compounds can be determined. Similarly the depth dependence of hyperfine interactions may be measured and related to surface structure studies. The instrument which combines conversion electron and MiSssbauer spectroscopies must optimize both the geometry required to excite an absorber by the recoilless radiation emitted from a T-ray source and the energy analysis of the conversion electrons emitted in the deexcitation of this absorber. For maximum excitation a rather small "c-ray source-to-absorber distance and a relatively large absorber are necessary, whereas high * Department of Physics, Boston University, Boston, Massachusetts, U.S.A. ** Burroughs Corporation, Flemington, New Jersey 08822, U.S.A. * Katholieke Universiteit, Nijmegen, The Netherlands (deceased). *** Supported in part by the National Science Foundation. 0167-5087/82/0000-0000/$02.75 © 1982 North-Holland

resolution electron spectroscopy usually requires a small electron source. Conventional magnetic/~-spectrometers are not well suited for M6ssbauer-conversion electron studies because they require either small or filamentary electron sources and their transmission characteristics are relatively poor [1,2]. On the other hand, in the low energy region ( < 50 keV), electrostatic electron spectrometers [3-6] exhibit higher transmission and better resolution. Furthermore, in this category, the spherical spectrometers are far superior to the cylindrical spectrometers in transmission albeit not in ultimate resolution. Therefore, a spherical electrostatic spectrometer which combines high transmission and large source area with good energy resolution was designed and constructed [7]. This instrument is similar to the "Keplertron" designed by Ritchie and Birkhoff [8] at Oak Ridge. It can be operated with an electron source in the form of a flat disc whose diameter is a large fraction of the diameter of the spectrometer inner sphere. Such an extended source has the ideal geometry required for an absorber in Mrssbauer-conversion electron spectroscopy.

2. Energy analyzer 2.1. Basic design parameters The spectrometer characteristics will be determined for the case of an idealized point source before considering the real case of a disc source.

2.1.1. Point source In a spherical electrostatic condenser, the inverse square electric field between two charged spheres is used to focus electrons which are emitted isotropically from a source situated on the inner sphere (fig. 1). The electrons travel along elliptical paths between the spheres

546

T-S. Yang et a L / Electrostatic electron spectrometer

The ratio of E o to V, the applied voltage between the spheres, is

AXIS OF SYMMETRY

Eo _

V

ro/rm

2 ( r e / r m -- 1)"

The transmission of the spectrometer is a function of & If the spectrometer accepts trajectories for a in the range a I ~< a ~
SLIT

where T(8) is normalized to unit isotropic emission from the point source. The detailed calculation shows that the transmission T and energy resolution R of the spectrometer are determined by the angle X. For a point source

"~

AE R-~ E

1 16 X ,

and Fig. 1. Sectional cut of the spherical electrostatic spectrometer with a point source located on the symmetry axis, showing the principal parameters determining the transmission profiles.

and return to the spectrometer symmetry axis after having rotated through a central angle ~. An adjustable slit is located at this focal position. Beyond the slit the electrons proceed into a field-free region where an appropriate detector is located. The parameters needed to calculate the transmission profiles for a point source on the axis of symmetry of the spectrometer are defined in fig. 1. a is the electron's take-off angle from the plane tangent to the inner sphere at the point of emission. X = ~r - ~ is the exit angle which turns out to be the key element in the analysis of the transmission and resolution of the instrument, r m and re are the radii of the inner and outer spheres respectively. The exit slit is positioned between r m and a radius r 0. The theoretical analysis of the instrument performance was carried out in great detail in two papers by Ritchie, Birkhoff, and collaborators [9], in which they analyze the transmission properties and line profile for both a point source geometry and a disc source. Only the main ideas and the behavior of the various parameters will be reviewed here. The orbit equation for non-relativistic electrons in terms of the parameters which determine the geometry of the instrument is given by

r rm

cos 2~x (~)(1--cos~b)+cosotcos

(~-]-ot)

where 8 is an energy parameter, 6 = ( E / E o ) 1, E is the non-relativistic kinetic energy of emitted electrons, and Eo is the kinetic energy for which the electrons will move in a circular orbit.

T=¼X. Furthermore, the opening of the exit slit (d = r0 - rm) is given by d = ~X2rm . T h e characteristic dependence of R and T on the angle X is noteworthy. For the spherical electrostatic spectrometer, R = T 2. If the angle X is reduced, the resolution of the instrument improves as X 2 while the transmission is reduced but only linearly with X. Most magnetic spectrometers are characterized by R > 25T 2 which implies that a modest increase in transmission causes a drastic loss of resolution. The theoretical limit for a magnetic solenoid spectrometer is R = 8.9T 2. 2.1.2. Disc source Ritchie and Birkoff's [9] calculations show that no adverse effects would alter the transmission profile if the point source were moved a small distance perpendicular to the symmetry axis, as long as its radial separation from the inner sphere remained small. U n d e r these assumptions, the orbit equation governing an electron emitted from a point away from the symmetry axis is:

r

COS 2OrI

rm

( 1 / 1 + 8)(1 -- cos a ' ) + cos a ' cos(dp + a ' ) '

where a ' = ~ r - 0'. This equation has the same form as the orbit equation for a point source on the symmetry axis; however, 0' and ~' are measured in the rotated coordinate system defined in fig. 2. From this figure, the relation between 4, and ~' is seen to be: cos q, = cos ~7cos q / - sin ~ sin q,' cos ~', in which ~b' is measured relative to the POP' plane. The average transmission for the off-axis point source, Tp(8, 7) is found by integration over ~b':

T-S. Yang et al. / Electrostatic electron spectrometer SYMMETRY AXIS

for the case X = 0.52 rad, (a) for a point source on the axis of symmetry, (b) for a point source 0.5 cm from the axis of symmetry, and (c) for a disc source of radius 0.5 cm. Very little deterioration of the spectrum is observed. In fact, it was shown by Birkhoff et al. [9] that for a disc source of diameter as large as 10% of that of the inner sphere and for an angle X = 1.0 rad, the transmission is reduced from the value obtained for a point source by only 14% while the resolution is increased by 16%. It is instructive to compare the performance of the electrostatic spherical spectrometer with the results achieved with magnetic spectrometers exhibiting the same resolution. The most striking characteristic is displayed by the luminosity L defined as the product of the transmission T and the source area S: L = S- T. Table 1 displays the transmission and luminosity obtained for several magnetic spectrometers for which the energy resolution A E / E - - - 2 % . It is clear that with the exception of the "orange-type" spectrometer none of the magnetic spectrometers can achieve the luminosity characteristic of the spherical electrostatic condenser.

(-,'/,',r) I p L

POINT~

_ SO?__RC?

ELECTRON TRAJECTORY

/

,/

/

Fig. 2. Relationship between the primed and unprimed coordinate systems. The primed parameters define the spectrometer characteristics for a point source located off the symmetry axis.

This calculation can be done analytically by replacing the integral with the sum:

/) n

1 n+l

547

r (8,n,i), i

where i is a non-negative integer and n is large enough to assure convergence of the sum. For a specific i, T ' ( 8 , ,/, i) = ½(sin et2 -- sin a , ) , where a, and a 2 represent end points of accepted a value. The calculations show that the maximum transmission occurs for negative 8 and that the transmission profiles have a tail in the region of positive 8. Once the transmission profile for a point source off the axis of symmetry has been determined, it is possible to calculate the average transmission, ~(8, Ra) , for a disc source where Ra is the radius of the electron source. The integral can again be done analytically by dividing the source into concentric rings and calculating the ,/ value which corresponds to the mid-radius of the rings. Thus, the average transmission for the disc is a weighting of the point sources corresponding to a given ~/ value by their appropriate ring areas. Fig. 3 shows the calculated transmission and energy resolution profiles

2.2. Construction

A schematic of the spectrometer that was built is shown in fig 4 and a photograph of the actual instrument is reproduced in fig. 5. The diameters of the outer and inner spheres 2rc = 34 cm and 2r m -- 25 cm respectively, were chosen to provide an energy range up to 50 keV with an instrument of reasonable size. The position of the slit at X = 3 0 ° = 0.52 tad is determined by the transmission and energy resolution required for the planned experiments and corresponds to optimum transmission and energy resolution of 13.1% and 1.7% respectively. The source of electrons is located 0.6 cm above the inner sphere. Thus, the innermost trajectory is a circular orbit 0.6 cm above the inner sphere. This geometry was chosen to reduce the scattering of electrons following the inner trajectory without having to add antiscattering rings or ridges on the inner sphere. The outer sphere consists of a cage of 42 aluminum rods 0.3 cm diameter, bent into a spherical shape of the appropriate diameter and held at the source and detector ends by aluminum rings of 8.7 cm and 15.2 cm diameter, respectively. The top opening is required for easy access to the source and for positioning of a transducer for M~ssbauer-conversion electron spectroscopy operation. The discrete rod structure of the outer sphere was chosen to reduce electron scattering. The small non-uniformity in the electric field resulting from use of a rod structure rather than a smooth solid surface, contributes minimally to the deterioration of the transmission and resolution of the instrument in the present configuration. The inner sphere of the spectrometer is made of aluminum and has a smooth surface. Its core was re-

T-S. Yang et al. / Electrostatic electron spectrometer

548

13 12 II

II

10

IC

POINT SOURCE 0.5 cm OFF SYMMETRY AXIS

z.

b)

DISK SOURCE

12

c)

II I0

>. 9 I-8 Z

9

---6

6

8

7

hi

5

_> 5

4

2

|e i i i i , i 13.1 13.2 13.3 13.4 13.513.6 13.713.8 ENERGY (keV)

3 2

I

I

0

13.2 13.3 13.4 13.5 13.613.7 13.8 ENERGY ( k e V )

i

I

I

I

I

,I

13.2 13.313.4 13.5 13.613.7 13.8 ENERGY (keV)

Fig. 3. Calculated transmission profiles for X 0.52 rad. (a) For a point source on the axis of symmetry, (b) for a point source off the axis of symmetry, (c) for a disk source of radius r~ 0.5 cm and r~/rm =0.04.

m o v e d a n d filled w i t h lead. A funnel-like a l u m i n u m s t r u c t u r e d e t e r m i n e s the exit slit at the angle X, a n d allows those electrons in a c e r t a i n radial b a n d to p a s s into the d e t e c t i o n region. I n o p e r a t i n g the s p e c t r o m e t e r , a negative high voltage is a p p l i e d to the o u t e r s p h e r e

w h e r e a s the i n n e r s p h e r e is g r o u n d e d . T h e s p e c t r o m e t e r is p l a c e d in a large a l u m i n u m c a n held at b e t t e r t h a n 10 6 Torr. T h e high voltage o n the o u t e r s p h e r e is c o n t r o l l e d b y a digital circuit w h i c h can m a i n t a i n the voltage at a p a r t i c u l a r value or c a n sweep it a u t o m a t i -

Table 1 Transmission and luminosity characteristics of different types of spectrometers. Type

R = AE/E(%)

T(%)

S(cm 2)

L(104)

Flat-type spectrometers 180 ° [10] V/2~r [10] "orange" [11]

2 2 1.8

0.095 0.63 16.0

12.0 5.0 0.78

115. 315. 1248.

Lens-spectrometers [ 10] Solenoid Short lens Long lens Intermediate image

2 2 2 2

1.41 0.25 1.4 2.0

0.12 0.24 0.13 0.035

17.5 6.0 19. 7.0

Tokyo elliptical electrostatic spectrometer [12]

0.7

5.0

0.07

35.5

Stevens cylindrical electrostatic spectrometer [ 13] (design parameters)

0.47

2.0

0.12

24.0

Rutgers spherical electrostatic spectrometer

2

13.0

0.78

1140.

T-S. Yang et al. t

2r e =34cm

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in.er-,phere /

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2rm=25cm~"~

S o u r c e ~ Aluminum-..

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~

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outer-sphere (rods)

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549

radioactivity was evaporated on 50/~g/cm 2 aluminized formvar foils. The conversion electrons emitted from 57C0 and l l9Sn calibrated sources were used to probe parameters such as transmission, resolution, and the spectrometer constant, E o / I I . The detectors in the field-free region were chosen according to the energy of the electrons being detected.

3. Detectors Insulatincj~

Geiqer, scintillation ~

,,,

1

I {.

\

multiplier

The instrument has been operated with three different kinds of electron detectors: a Geiger counter, a scintillation counter, and a "channeltron" electron multiplier. 3.1. Geiger counter

Fig. 4. Schematic of the electrostatic spherical spectrometer.

cally to scan an energy region of interest. Radioactive isotopes were used as electron sources to test the properties of the spectrometer. In order to reduce back sattering of electrons from the sources, the

The basic design consists of a brass cylinder of 5 cm inner diameter, 4.5 cm deep, with a front end window (fig. 6a). A 5 cm diameter, 20-50/~g/cm 2 formvar foil is supported against vacuum by a nickel mesh (90% transmission). The fill gas is ethyl alcohol at a pressure of 1.7 cm Hg. This detector has the advantage of simplicity of construction and operation and provides a nearly delta function pulse height spectrum, constant efficiency, and a very low sensitivity to undesired background radiation; the natural room background is about 1 count/s. However, the counter has several important limitations: (a) the thin formvar window is very fragile, is subject to puncture by accidental electrical discharge, and is weakened by repeated cycling between vacuum and atmospheric pressure. Hence the window lifetime is short, from one week to several weeks under the best conditions. (b) Even for the thinnest foils, the effective thickness is a factor 2 or 3 larger than the actual thickness because of the shallow angle of incidence of incoming electrons; thus the counter is not really useful for the detection of electrons of energy less than about 10 keV. However, the spectra with the best resolution and lowest background were obtained with this detector for the 13.6 keV and 14.3 keV L and M electrons emitted from a 57Co source and the 19.4 keV and 23.0 keV L and M electrons from a 119Sn source. 3.2. Scintillation counters

Fig. 5. Photograph of the actual spectrometer.

The scintillation counter consists of a thin layer of scintillator bonded to a lucite cylinder optically coupled to an Amperex 56 DVP photomultiplier. Some tests were also carried out on an EMI 9789A photomultiplier which has a much smaller photocathode. Although the detection efficiency varies with electron energy and the

550

T-S. Yang et aL / Electrostatic electron spectrometer

counter is sensitive to light (so that all electrical and plumbing connections to the vacuum must be light tight), this type of detector has the advantage of being mechanically and electrically rugged and stable. Two different types of scintillators have been tested: NE-102 and anthracene. 3.2.1. NE-102

Thin (12.5-15 ~m) sheets of NE-102 plastic bonded either to the face of a 5 cm diameter by 0.6 cm thick lucite disc, or to the cylindrical surface of a I cm diameter by 2 cm long lucite rod, were tested. Such thin layers of scintillator are in principle very inefficient for y-rays while detecting electrons with close to 100% efficiency. NE-102 has relatively high light output, good optical properties, and is not affected by humidity or vacuum. Unfortunately, for low energy electrons the light output is so low that the signal from the photomultiplier is of the same order of magnitude as the dark current and a poor signal-to-noise ratio was obtained. 3.2.2. Anthracene

Anthracene has a much higher light output than does NE-102. Therefore the signal-to-noise ratio of anthracene is better than that of NE-102. The detector was prepared by brushing anthracene dissolved in benzene over the face of a 5 cm diameter by I cm long lucite disc. A significant disadvantage of anthracene, however, is that it has a high vapor pressure. In order to prevent the anthracene from evaporating in the vacuum, the

jMESH

AND FORMVAR FOIL

.....:i------°

I em

BASEPLATE

TO ALCOHOL BATH

o)

scintillator must be covered by a thin formvar foil ( - 20 /~g / c m 2). Three types of scintillator-photomultiplier combinations were tested: (1) a disc NE-102 scintillator with an Amperex 56 DVP photomultiplier (5 cm diameter photocathode); (2) an anthracene scintillator on a lucite disc with the 56 DVP; (3) a rod of NE-102 scintillator on an EMI 9789A photomultiplier with a 1 cm diameter photocathode. Of the two forms, the disc scintillators gave by far the better signal, the rod having a much poorer geometry with respect to the optical path between scintillating surface and photocathode. In order to reduce the dark noise of the photomultipliers it was necessary to cool them by a stream of cold nitrogen gas obtained by evaporating liquid nitrogen. A seven-fold reduction in dark noise of the tube was thus achieved. Some improvements in resolution or counting rates were obtained with slight modifications of the basic detector assembly described above. For example, when the NE-102 scintillator was covered by a thin reflective coating of MgO, the counting rate and the resolution increased by about 20%. The unexpected change in the resolution is possibly a result of absorption of the low energy secondary electrons arising from scattering in the slit area by the MgO coating. To further improve the signal-to-noise ratio, the anthracene scintillator was deposited on the top surface on a cone-shaped light pipe of 5 cm upper diameter and

../MESH

L\

/

I cm

b)

Fig. 6. Schematicsof (a) the Geiger counter and (b) the channeltron assembly.

I~ :

PLASTIC

~:

BRASS

~

: CERAMIC

T-S. Yang et al.

/ Electrostatic electron spectrometer

with an unusually wide entrance cone, 5 cm in diameter. It was operated with the front end of the cone at ground potential. In order to shield the electric field within the cone, a 90% transmission nickel mesh was installed across the entrance to the cone. A small negative voltage ( - 5 V) was applied to the mesh to prevent secondary electrons from entering the counter. In this case, the background of the channeltron is very low, like that of the Geiger counter, namely about I count/s. Unlike the previous detectors whose efficiency decreases rapidly as the energy of electrons decreases, the efficiency of the channeltron remained constant over the low energy region (3-15 keV), but it is low (c-- 10-20%).

3 cm lower diameter. A small negative voltage (5 - 10 V) applied to a focus electrode in the phototube acted to reduce the effective area of the photocathode and hence the dark noise, without significantly affecting the signal. Although the formvar foil stops the very low energy electrons (especially below 10 keV), the efficiency of this counter at higher electron energies is higher than that of the other counters and was used for all high energy measurements. 3.3. Channeltron

This device is a type of solid state electron multiplier in which an electron cascade is produced within a special semiconductor material across a large potential gradient. The device has the appearance of a funnel (fig. 6b). The cone section acts as an electron multiplier while the straight section amplifies the signal. It is mechanically rigid and can be cycled indefinitely between atmospheric pressure and vacuum with no ill effect as long as it is kept clean and free of oil. It is insensitive to light and to v-rays. The particular device used in the spectrometer is a specially ordered unit [14]

IO0 8O

Source: 57C0 evaporated

!

on aluminized formvor

~I

op 0

rr

8

60

i

60 L ~':2.S%

40

z

KLL

2O

KLM

4. Electron energy spectra

Fig. 7 shows some of the electron spectra obtained with various sources and with several different detectors. The transmission of the spectrometer was evaluated from data obtained with the thinnest evaporated source and the anthracene scintillation counter. The source intensity was measured absolutely with proportional

Source: 57Co evaporated on aluminized formvor

L

c)

J

j',

Detector= Geiger counter

I

50

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Detector~ Geiger counter

K--

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z= 30

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zo

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80

Electron Energy (keY)

0 b)

on aluminized formvor

400 -

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o.

n~

c)o

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o

l

""

90J8 0 J-

•.z" "•

70}-

L~

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,i,

7.1

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d)

Detector= Geiqer counter

a =.~E= Z%

40ff

M

/

•

jso

-.,,"

.~.,t "-'a*"

s'o

I

• •

c~ 50~-

I00

I

Source= 119Sn evoporaled on glass

• " • •

60t.-

KLM

200

I

1

1 A~er ~... transitions

500

I

K

Detector, chonneltron

Z

I

120 125 130 135 14.0 14.5 I 0 Electron Energy (keV)

Source: 57Co evaporated

t

::

~

, ,,

20P

N+O

~'.ag.~,¢~.~

6'o

s'.o

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Electron Energy (keY)

Fig. 7. Electron spectra obtained for various sources and detectors.

2'2 2'3 2'4

Electron Energy (keY)

552

T-S. Yang et a L / Electrostatic electron ~v~ectrometer

and NaI(T1) counters of known geometry and efficiency. The transmission of the formvar foil covering the anthracene scintillator was calculated from the semiempirical formula presented by Subba Rao [15]. This formula was further checked by comparing the transmission of 7.3 keV and 13.6 keV electrons through the same foil. The formvar foil thickness (20 ~ g / c m 2) was obtained by weighing several foils. F r o m these measurements a spectrometer transmission of T = 13% was derived, close to the expected maximum value calculated from the spectrometer design characteristics. The observed energy resolution A E / E o = 2% at 13.6 keV is slightly larger than the calculated value (1.7%). The increase arises partly from the intrinsic thickness of the source and its backing, and partly from the nonuniformity of the electric field between the spheres. The fringe effects resulting from the discontinuities in the electric field in the source and the exit slit regions, and from the discrete nature of the outersphere rods, are difficult to calculate. It was found that the energy resolution was degraded from 2% to 2.5% when the outer sphere was off-center by 1 mm. Surprisingly, the

VRETARDING: 0 V

a)

K

observed energy resolution seemed to depend on the type of detector used. This effect is probably caused by scattered secondary low energy electrons as discussed above. As described previously, the non-negligible thickness of the Geiger counter window insures that these scattered electrons are not counted. Similarly, a small negative voltage on the mesh of the channeltron and the MgO cover on the scintillator can prevent the secondary electrons from getting into the counter, and in fact, improved resolutions were obtained with all detector systems when care was taken to eliminate low energy electrons. F r o m these spectra the calibration constant of the spectrometer was determined, E o / V = 1.650. This result compares well with the design value E o / V = 1.603.

5. Resolution enhancement by means of retarding potentials The transmission and energy resolution of the spectrometer are fixed for any given spectrometer geometry

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5.0

5.5

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G.O 6.5

7.0

ENERGY (keV)

7.5

•

i

8.0

0 0.5

1.0 ENERGY (keY)

6 VRETARDIN G =

I-(¢ l 4 W n-"

K ~

4kV AUGER

,:o

I KLc:~

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b)

t

KL2L3 KiLIIL~,% ~~

1.5

ab ' ENERGY (keV)

/ [

3:0

4.0

Fig. 8. Spectra of the K and Auger lines observed with a 57Co evaporated on aluminized formvar with a retarded voltage applied to the source of (a) 0 kV, (b) 4 kV. and (c) 6 kV, respectively.

553

T-S. Yang et al. / Electrostatic electron spectrometer

since these parameters depend solely on the angle X. In principle, in order to improve the resolution, the angle X must be reduced, which involves constructing a new outer shpere or extending the present outer sphere inward. However, with the correct geometry the overall transmission would be reduced as X is decreased. An alternative method to artificially improve the observed resolution for any given structure of the apparatus involves applying a retarding potential to the electron source to reduce the energy of the electrons before they enter the spectrometer. Since the spectrometer geometry has not changed, the intrinsic resolution R = A E o / E o has not changed. However, with a retarding potential V on the source, the electrons emerge with an energy E ' = E o - 8 E and the effective line width becomes A E = R E ' , hence A E < A E0" In the above expression, 8 E = e S V where e is the electronic charge. The ratio of the effective line widths with and without the retarded potential is given by

apparent resolution becomes A E / E o = 20/(7.3 × 10 3) ~ 0.27%, roughly a factor of seven better than the intrinsic resolution. Figs. 8 and 9 show the spectra obtained for the Auger, K, L, and M conversion lines in 57Fe as a function of the applied retarding voltage with such a scheme. The linewidth indeed decreases with the increasing retarding voltage, although the ultimate resolution is never achieved. At these low electron energies the limitations are caused by source thickness effects and, more important, by the non-uniformity of the electric field in the neighborhood of the outer sphere rods. Fig. 10a displays the line width of the K conversion line as a function of the retarding voltage for an evaporated 57Co source. At a retarding voltage of 7.3 kV the line width should in principle vanish. The residual width of 72 eV is attributed to scattering within the source material and b a d d n g i-tslef. This effective width

>=

A E/A E o = 1 - (6E/Eo).

For example, for an intrinsic resolution R = 2%, and a 7.3 keV electron which is retarded by an energy ~E = 6.3 keV, E ' = l k e V , AE=0.02×IkeV=20 eV and the

a)

E3 2

0

0

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Z L

a)

"7

bJ I00

800

z

600

It. b.J

t'

0 400

~,.¢^°

200

°o

I

soo ,60 760 860 9M t.

W

2

b)

6

7

RETARDING VOLTAGE

3

4

5

(kV)

1.2

(x 6 0 0 Iz

z

~) 400

¢

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///

1.0

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"

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•

b)

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200 "

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600

700

800

3O0

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900 I000

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lw °.

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Fig. 9. Spectra of the L and M conversion lines observed with a 57Co source evaporated on aluminized formvar with a retarding voltage applied to the source of (a) 0 V, (b) 4 kV, and (c) 7 kV, respectively.

0 RETARDING VOLTAGE

(kV)

Fig. 10. (a) Effect of the applied retarding voltage on the linewidth of the 7.3 keV K conversion line observed with a 57Co source evaporated on aluminized formvar. (b) Effect of the retarding voltage on the relative transmission of the spectrometer. The data are normalized to the maximum transmission obtained at zero retarding voltage. The dotted line is drawn to guide the eye.

554

T-S. Yang et al. / Electrostatic electron spectrometer

depends, of course, on the source preparation. The "shoulder" observed on the high energy side of the line profile could not be eliminated in several attempts to reduce the electron scattering both at and beyond the exit slit and is due to the intrinsic focusing properties of the spectrometer. Similar shoulders were present in the original Ritchie and Birkhoff [9] spectrometer, and appear as well in the theoretical calculations of the line shape. They are not apparent with poor resolution but must be contended with at high resolution. Fig. 10b shows the effect of the applied retarding voltage on the relative transmission of the spectrometer. The data are normalized to the maximum transmission obtained with zero retarding voltage.

TO PHOTOTRANSISTOR

6. Application to M6ssbauer-conversion electrion spectroscopy For M6ssbauer conversion electron spectroscopy, the electron source is replaced by an absorber which must be excited by recoilless v-rays. A strong source of recoilless radiation is attached to a velocity transducer situated above the absorber (fig. 11), and is collimated by a lead shield. To prevent electrons emitted from the -/-ray source from entering the spectrometer, the opening in the collimator is covered with a 13.7 m g / c m 2 thick aluminum foil. The requirement that the M6ssbauer -/-ray source be close to the absorber poses a problem for the operation of the electrostatic spectrometer. To be sufficiently close, the -/-ray source must be between the spheres, but then the electric field in the absorber region is considerably disturbed by the presence of the -/-ray source shield and the cylinder which connects it to the transducer. If the collimator is grounded, almost no electrons are transmitted. If the collimator is left floating, electrons are detected, but the counting rate is unstable. Therefore, a suitable high voltage on the collimator is necessary to compensate the disturbance in the electric field. This voltage must be scaled to the high voltage on the outer sphere and must be adjusted for any aprticular position of the collimator. At present, the distance between the collimator and the absorber is 1 cm and the ratio of the high voltage on the collimator to the high voltage on the sphere is 0.4. Fig. 12 shows the conversion electron spectra for a 100 /~g/cm 2 thick, 1 cm diameter a-F%O 3 absorber deposited on a sapphire disc and a 25 # g / c m 2 thick, 1 cm diameter 57Fe foil evaporated in a mica plate. Figs. 12a and b were obtained with the sources [50-80 mCi 57Co(Rh)]moving ON- (open circles) and OFF- (filled circles) resonances, while the data of figs. 12c and d were obtained after subtracting the OFF-resonance data from the ON-resonance curves. Similar spectra obtained with the magnetic solenoid spectromer [1] at much

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lower resolution are shown in the insert. Fig. 13 shows the resonance spectrum obtained with the electron spectrometer set on the peak of the 7.3 keV K conversion electrons. The main contribution to the background is due to non-resonance external conversion electrons. The background at zero voltage was 5 counts/s.

7. Conclusion The high transmission and good energy resolution of the spherical electrostatic electron spectrometer qualify it as a most convenient instrument for MOssbauerconversion electron spectroscopy. The background is low and the signal-to-background ratio is in excess of 800%. Six-line STFe resonance spectra with excellent statistics can be accumulated within a day or even within several hours with 50-100 mCi MiSssbauer sources. The instrument has been used in a study of the dependence of the magnetic hyperfine interaction as a function of depth from the surface into the bulk of a Fe203 film [16]. Differences in hyperfine fields of less than 100 G (or 0.02%) have been determined, and subtle changes in the quadrupole interactions at the surface of such foils have been observed. In a more critical application, the spin density of s-electrons at the 57Fe nuclei have been remeasured with considerably higher accu-

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556

T-S. Yang et aZ / Electrostatic electron spectrometer

racy than possible heretofore [17]. The i n s t r u m e n t has thus been proved a unique tool for the study of small effects in electronic properties of solids. We wish to t h a n k Drs. R.D. Birkhoff and R,H. Ritchie for their t h o r o u g h advice in the early stages of design of this instrument.

[6]

[7]

[8]

References [9] [1] C.J. Song, J. Trooster, N. Benczer-Koller and G.M. Rothberg, Phys. Rev. Lett. 29 (1972) 1165; C.J. Song, J. Trooster and N. Benczer-Koller, Phys. Rev. B9 (1974) 3854. [2] U. BS.verstam, C. Bohm, T. Ekdahl, D. Liljequist and B. Ringstr~,m, MSssbauer effect methodology (Plenum Press, New York, 1974) vol. 9, p. 259; U. B~iverstam, T. Ekdahl, C. Bohm, B. RingstrOm, V. Stafansson and D. Liljequist, Nucl. Instr. and Meth. 115 (1974) 373; U. Bgverstam, T. Ekdahl, C. Bohm, D. Liljequist and B. RingstrSm, Nucl. Instr. and Meth. 118 (1974) 113; U. B~tverstam, C. Bohm, B. RingstriSm and T. Ekdahl, Nucl. Instr. and Meth. 108 (1973) 439. [3] T. Toriyama, K. Saneyashi and K. Hisatake, J. Phys. (Paris) Coll. C2 (1979) 14. [4] T. Shigematsu, H.-D, Pfannes and W. Keune, Proc. ACS Meeting, Houston, U.S.A. (1980), and Phys. Rev. Lett. 45 (1980) 1206. [5] M. Domke, B. Kyvelos and G. Kaindl, Proc. 5th Int. Conf.

[10] [1 I] [12] [13] [14] [15] [16]

[17]

on Hyperfine interactions, West Berlin, Germany (1980) Hyperfine Interactions 9/10 (1981) 1137. M.R. Polcari, J. Parellada, K. Burin and G.M. Rothberg, Brit. Inst. Phys. Conf. Set. 39, Transition Metals 1977 (1978) 584. N. Benczer-Koller and B. Kolk, in Workshop on New directions in M6ssbauer spectroscopy (1977) ed., G.J. Perlow, AIP Conf. Proc. No. 38 (American Institute of Physics, N.Y., 1977) vol. 38, p. 107. H.H. Hubbell, Jr., W.J. McConnell and R.D. Birkhoff, Nucl. Instr. and Meth. 31 (1964) 18. R.H. Ritchie, J.S. Cheka and R.D. Birkhoff, Nucl. Instr. and Meth. 6 (1960) 157; R.D. Birkhoff, J.M. Kohn, H.B. Eldridge and R.H. Ritchie, Nucl. Instr. and Meth. 8 (1960) 313. C.S. Wu and C. Geoffrion, Nuclear spectroscopy, ed., F. Ajzenberg-Selove (Academic Press, New York, 1960). M.S. Freedman, F. Wagner, Jr., F.T. Porter, J. Terandy and P.P. Day, Nucl. Instr. and Meth. 8 (1960) 225. K. Hisatake, private communication (March 1980). J. Parellada, M.R. Polcari, K. Burin and G.M. Rothberg, Nucl. Instr. and Meth. 179 (1981) 113. Galileo Electro-Optics Corp., Sturbridge, Mass. B.N. Subba Rao, Nucl. Instr. and Meth. 44 (1966) 155. T. Yang, J. Trooster, T. Kachnowski and N. BenczerKoller, Proc. 5th Int. Conf. on Hyperfine interactions, West Berlin, Germany (I 980) Hyperfine Interactions 9 / I 0 (1981) 795. T. Yang, J. Trooster and N. Benczer-Koller, Bull. Am. Phys. Soc. 26 (1981) 29.