Amplification factors of a particle multiplier for multiply charged noble gas ions

Physica 32 749-76 1

Schram, B. L. Boerboom, A. J. H. Kleine, W. Kistemaker, J . 1966

AMPLIFICATION FACTORS OF A PARTICLE MULTIPLIER FOR MULTIPLY CHARGED NOBLE GAS IONS by B. L. SCHRAM, W. KLEINE F.O.M.-Laboratorium

A. J. H. BOERBOOM,

and J. KISTEMAKER

voor Massascheiding, Amsterdam, Nederland

synopsis Multiplier efficiencies were determined for multiply charged noble gas ions, by a direct comparison of the signals of a particle multiplier (EMI-9603) and a Faraday cage at the detector side of a mass spectrometer. The energy of the ions was varied between 3 and 90 keV. The ions involved were He+ and Hes+, Ne+ uo to Nea+, Ar+ up to Arhf, Krf up to Kr7+ and Xe+ up to Xes+. For neon the isotope effect was examined. The secondary emission factors show an almost exact linear dependence on the velocity of the ions, independent of their charges. The extrapolated straight lines all cut the velocity axis at 5.5 x 10s cm/s. The slopes of the lines are proportional to the square roots of the ion masses.

1. Introduction. Relatively few measurements have been done on the secondary emission factors for multiply charged ions on metal surfaces. Kaminskyi) gives an extensive review. Other reviews, specially for noble gas ions, have been given by Francis and Jenkinss), Massey and Burhops), Granowski4) and Little5). In the elder literature the experimental conditions as adsorbed surface layers and crystal structure of the target material,

often are not well defined. But also experiments

per-

formed with atomically clean surfaces and under severe precautions do not seem to provide well reproducible results. Existing theories differ very much in their explanation of the observed experimental behaviour of the secondary emission. From the point of view of the experimentalist, however, there is a constant demand for empirical values of the secondary emission factors, in connection with the increasing application of secondary electron multipliers. We investigated the amplification factors of a commercial type of particle multiplier for singly and multiply charged ions in the usual application region of 3-10 keV of ion energy per charge (the total energy being 3 to 90 keV). The voltage over the multiplier was kept constant. Variation of this -

749 -

750

B. L. SCHRAM,

voltage

A. J. H. BOERBOOM,

only results in a proportional

independent

AND J. KISTEMAKER

change of all amplification

of the energy and the kind of the impinging

2. Experimental. mounted

W. KLEINE

a Faraday

factors,

ions.

At the detector side of our mass spectrometer we cage collector and a particle multiplier beside each

other (figure 1).

Fig. 1. Collector

and multiplier

assembly.

The particle multiplier was an EM1 type, nr. 9603, with Cu-Be dynodes. The Faraday cage collector was placed 25 mm apart. It consisted of a long wedge shaped cage, behind a suppressor electrode. The meaning of the long cage is to have a black box for the ions to secure a 100% collection. A negative voltage on the suppressor plate had no effect on the measured collector current, so obviously the trapping of secondary electrons by this type of collector is effective, because of its small opening angle. By the same argument, we assume the amount of ions reflected from the collector to be negligible, and the collector current is believed to give the accurate value of the ion beam current passing through the collector slit. For both detectors the same slit width of 4 mm was used, as well as equal slit heights of 14 mm. Due to an inhomogeneous magnetic field analyzer, the resolution of the mass spectrometer still was 300. The ion beam was led from one detector to the other by a small alteration in magnet current. As the peak on both collectors showed the same shape, we believe that an equal number of ions per unit of time is passing through the respective slits. As the Faraday cage collector is believed to give the exact value of the ion beam current, the amplification factor of the multiplier can be directly found from the ratio of both collector signals. We define the amplification factor as the average number of secondary electrons arriving on the last dynode per impinging ion. So for a singly charged ion this factor is given by the ratio of the collector signals, whereas this ratio has to be multiplied by n for a multiply charged ion of charge n. The minimum current detected was 3 x lo-15 A over lo12 a. The maxi-

751

AMPLIFICATION FACTORS OF A PARTICLE MULTIPLIER

mum multiplier

current

was kept below 2 x

lo-8 A to avoid saturation

effects in the multiplier. To produce multiply charged ions, the energy of the ionizing electrons must be taken high enoughs). Each peak was scanned several times to permit averaging and to get an idea about the precision of the measurements For the lower charged ions the reproducibility was within lo%, but for the highest charged ions the error was greater, due to weak signals on the Faraday cage collector. The voltage across the multiplier was kept constant at 2 kV, whereas the ion accelerating voltage was varied between 1 and 8 kV, in steps of 1 kV. So the final energy of the ions impinging on the first dynode of the multiplier varied from 3 to 10 keV per charge, which means an energy of maximum 90 keV for Xesf, for example. The multiplier did not get a special treatment. The background pressure in the mass spectrometer was 4 x 10-S torr, rising to about 2 x IO-7 torr, when the gas under investigation was introduced to the ion source.

d,

MULTIPLIER GAIN ION

ACCELERATING

VOLTAGE

7 kV

x

I*

0

2*

b 0

3+ 4,

P

5*

.

6*

.

7t

I

8,

I

s+

Fig. 2. Comparison of the amplification factqrs for the singly and multiply charged noble gas ions at 7 kV ion accelerating voltage.

For neon both isotopes 20 and 22 were measured; of the 22 isotope, however, only the singly charged ion was measured. For krypton we used the 86 isotope, instead of the most abundant 84 isotope, in order to avoid confusion of the 3+, 6+ and 7+ ions with the background peaks at masses 28, 14 and 12, respectively.

752

B. L. SCHRAM,

A. J. H. BOERBOOM,

W. KLEINE

TABLE MultiDlier anmlification

factors

AND

J. KISTEMAKER

I x lO-4 for noble eas ions 1

Ion

He+ He”+

I

z”Ne+ Z0Ne2+ ZoNeS+ Z0Ne4i-

3 kV

4kV

6 kV

7 kV

8 kV

9 k’

18.4

16.0 27.8

15.3 21.6

17.8 25.6

18.2 27.4

14.83 22.9 29.5 35.3 14.10

15.45 22.9 28.7

17.0 8 23.0 27.8

14.99

16.2 8

16.8

9.65 18.0

12.71 20.8 27.8

6.75

=Ne+

10 kV*) 17.8 22.6

6.72

9.50

12.61

Ar+ Ar2f Ar3f A++ Ar5+ Are+

5.50 11.4 15.9

8.25 14.6 21.5

11.3 19.3 23.9 29.1 35.6

12.9 20.8 26.6 31.5 36.5 42.0

14.5 22.5 28. i 32.1 37.9

17.0 25.6 35.1 48.7 51.8

Kr+ KG+ Kr3+ Kr“+ Krh+ Krc+ Kr7+ KrB+

3.78 7.27 10.8 16.7

6.05 10.1 15.0 20.7 25.0 26.0 32.0

8.48 15.3 19.6 23.2 28.4 30.9 34.4

9.44 16.6 21.5 26.4 30.0 32.6 37.6 40.5

10.1 17.6 22.4 28.4 31.5 35.0 39.0

11.7 20.2 25.0 32.0 35.2 37.8 43.6

2.78 5.18 8.14 11.4 14.6 17.8 20.0 25.0 30.5

4.3 1 8.68 12.7 16.5 20.4 24.6 29.0 32.1 35.3

6.99 13.9 18.8 24.6 25.5 27.3 31.9 36.7 37.2

8.03 15.3 21.2 25.0 29.0 29.8 35.6 39.6 41.6

8.67 15.9 21.9 27.3 28.8 31.1 32.8 41.5 44.7

xe+ xi++ xe3+ Xe4+ Xeb+ Xe6+ Xe7+ Xe*+ Xe9’

-

-

*) The total ion accelerating

voltage

-

9.3 12

I

9.67

23.1

23.7

33.1

32.0

39.8

42.9

41.6

51.9

is indicated.

GAIN 2.d

!

l.l$ '

1

1

0 Fig. 3. Comparison

5

of the amplification a function

Ion accelerating

factors

voltage in”kV

for singly

of the energy.

charged

noble gas ions as

AMPLIFICATION

FACTORS

OF A PARTICLE

753

MULTIPLIER

For xenon we took the 129 isotope. Table I lists our results, which also are represented

in the various graphs.

It is clear that for many experimentalists it is of vital importance to know the amplification factors, at least at one accelerating voltage (7 kV),. for singly and multiply charged noble gas ions. This case indicated in figure 2 is taken

as an example.

It is of obvious

importance

to see, e.g.

that the impact of an Ark+ ion having passed 7 keV accelerating voltage, gives a signal that is 2.4 times larger at the end of the particle multiplier than one given by an Ar+ ion of the same voltage. The trend, as a function of the energy is demonstrated for singly charged particles in figure 3. The real experimentally gained results for all noble gas ions are shown in the figures 4a, 5a, 6a, 7a and 8a. The amplification factors measured have been given as a function of the energy.

GAIN

x

He

o

He’+

l

3.105 0

0 0

_________ __

0

0 2.105. ___.._*Yy----x

1.10” I

0

Fig. 4~. Multiplier

0

amplification

5 Ion accelerating

factor

as a function

10 voltage in kV

of the ion accelerating

voltage

for helium ions.

GAIN

3 105j

x

He11

o

He

O6-.

60

60

10

SO

100

ION VELOCITY in 10’ cm/r

Fig. 4b. Multiplier

amplification

factor

as a function ions.

of the ion velocity

for helium

754

B. L. SCHRAM,

A. J. H. BOERBOOM,

W.

KLEINE

AND

J. KISTEMAKER

GAIN 5.105. x

No *

o

NC?+

A 0

Nd+ N&t‘*

4.105-

2.105.

1.105.

00 0

Fig. 5a. Multiplier amplification

5

Ion accelerating voltage tn kV

factor as a function of the ion accelerating for neon ions.

voltage

GAIN

L1051

3.105

2 lo5

l.105

I00 ION VELOCITY

Fig. 5b. Multiplier amplification

in 10scm/s

factor as a function of the ion velocity for neon ions.

3. Discussion. There are only a few measurements with which our results can be compared. In general, not enough systematic research on the subject has been done. Foxr7) did comparable experiments for Ar, Kr and Xe ions, but he did not mention the type of multiplier and used only a constant ion accelerating voltage of 3 kV. In the present work we took all noble gases in wide ranges of charged states, and various ion accelerating voltages corresponding to ion velocities

AMPLIFICATION

FACTORS

OF A PARTICLE

755

MULTIPLIER

GAIN 5.105.

4.105.

3.105.

2.105.

1.105

01 0 Fig. 6a. Multiplier

amplification

5

factor

Ion accelerating voltage%

as a function

kV

of the ion accelerating

voltage

for argon ions.

GAIN 5.105

x Ar o

I

.

v 0

Ar'+

d Ar'*

2.d

/ OO

__=

_.--x

10

20

30

~

40 ION VELOCITY

Fig. 6b. Multiplier

amplification

factor as a function

of the ion velocity

50 in 10Ecmls

for argon

of 6 x 106 cm/s (for Xef) to 100 x 106 cm/s (for He2+). These ions impinge on a Cu-Be electrode, which mainly contains Cu (98%). We thus got a set of gain factors, which are primarily representative for the khzetic effect in secondary electron emission from the first dynode. The potelztiul effect investigated by Hagst rum’) between 1953 and 1956

756

B. L. SCHRAM, A. J. H. BOERBOOM, W. KLEINE AND J. KISTEMAKER

GAIN 5.12.

4.105.

3.105-

2.105i

01

0

5

Fig. 7a. Multiplier amplification

GAIN L.105

3.10s

10 Ion accelerating voltage in kV

factor as a function of the ion accelerating for krypton ions.

x

Kr*

,',.I I'

o

Kr"

.

A 0

Kr'* I. Kr

o

Kr5+

.

Kr"

A

Kr'+

voltage

2.105

Fig. 7b. Multiplier amplification factor as a function of the ion velocity for krypton ions

was more recently investigated by Arifov e.a.8) and by Medved e.a.9). Their measurements show that below 1000 eV kinetic energy of the impinging projectile potential effects play a big role. Above 1000 eV, however, the kinetically induced secondary electron emission dominates. In our energy range (3 to 90 keV) we will observe kinetically induced emission. As a comparison we can only point to Tel’kovskiiia), who bombarded Zr and MO targets with Ar+, A++ and Ars+ in the velocity range from 1 to 8 x 107 cm/s.

AMPLIFICATION

FACTORS

OF A PARTICLE

MULTIPLIER

757

GAIN

.3.105.

2.105.

1.105.

I,,,,..,

OO

ion

Fig. 8a. Multiplier amplification

GAIN

10

5

Y

accelerating voLtage inkV

factor as a function of the ion accelerating for xenon ions.

XI*

voltage

.

5.105

L.105

10

Fig. 8b. Multiplier amplification

20

__~____~~~

30 ION VELOCITY

in

factor as a function of the ion velocity for xenon ions.

The most interesting result of the measurements shown in the figures 4b to 8b is that all gain factors for He, Ne, Ar, Kr and Xe ions, independent of the charge of the ions, satisfy a linear dependence on the velocity of the ions. The same was found by Tel’kovskii for Ar ions. Moreover, all our curves show a point of intersection with the velocity

758

B. L. SCHRAM,

A. J. H. BOERBOOM,

W. KLEINE

AND

J. KISTEMAKER

axis at about 5.5 x 106 cm/s. A straight line through the helium data has a low slope. The points scatter too much to obtain sufficiently accurate the intersection

with the velocity

the line has been

drawn

this speed corresponds

axis. Therefore,

through

the point

with a threshold

in case of He (figure 4b)

5.5 x 106 cm/s. Apparently

velocity

characteristic

for the target

material, independent of mass or charge of the impinging ion. We emphasize that it is a threshold velocity and not a threshold energy. The relative energies are quite different, because of the big range in masses (m = 4 to 129). (See tabel II). TABLE

II

Points of intersection and slopes of the secondary electron enlission curves, holding for fast noble gas ions Gas

vtllr in 10Bcm/s

He Ne Ar Kr Xe

(5.5) 5.8 5.5 5.5 5.2

average values:

5.5 * .3

C Ethr = frnvth~~ in lo-lo ergs in 10-S cm-$ (1.01) 5.6 10.1 22 29

C’ in lo8 ergs+

3.0 7.4 10.2 13.3 16.4

C’ in lo4 (keV)-*

1.64 1.81 1.77 1.57 1.58

6.6 7.3 7.1 6.3 6.3

1.67 * 0.1

6.7 f 0.3

5.105-

4.105.

3.1O5

2.105

1. lo5

0

50 ION

Fig. 9. Comparison

of the multiplier function

vth,) = F

where v is the velocity,

in

amplification factors of the ion velocities.

In figure 9 we see the gain factors point, but having different slopes. The gain factors can be represented Gain = C(TJ -

VELOCITY

100 lo6 cm/s

for all noble gas ions, as a

all passing nearly

through

the same

by

(1/E -

z/E,,,)

vthr is the threshold

= C/(1/E -

velocity.

1/&r)

m is the mass of the

AMPLIFICATION

ion, E being its kinetic

FACTORS

OF A PARTICLE

energy in general,

MULTIPLIER

and Ethr being its kinetic

759 energy

at threshold. The values ot C and C’ are listed in table II. It appears that the value of C’ is almost constant for all noble gases. For neon the gain factors of the 22 isotope lie a few percents above the straight

line through the 20Ne points.

The gain of an open electron secondary

emission

coefficient

first dynode. So we find experimentally

is directly

proportional

to the

ion bombardment

of the

for noble gas ions on a Be-Cu

dynode

y due to positive that

YDepending

multiplier

(z/E -

Z/J%hr)

on

different behaviour as a function of kinetic energy will be found. A survey of theories has been given by Kaminskyl). According to Von Roosil) y - E2, Sternglasslz) y - E-*, Parilis C.S. 13) y N (E* - Et,,), which hold for different energy ranges, whereas Harrison gives a very complicated relation. The theory by Parilis and Kishinevskii fits rather well to our experiments. It has the great merit to point to a threshold energy independent of charge and mass of the ions, like we found. They ascribe the electron emission to ionizing collisions of the projectiles with the target atoms, followed by a recombination of a conduction electron with the open shell place accompanied by transfer of energy to another conduction electron (Auger recombination). This theory predicts yield curves vs velocity which asymptotically approach straight lines, the extension of which intersects the velocity axis in one point for different projectiles. The intersection point is 1.05 x 107 cm/s for MO and W. According to this theory, the slope of y versus v curves should be proportional to z1+ (

d/z,+%%

22

2 >

where 21 is the atomic number of the ion and 22 the atomic number of the target atoms. This condition holds for $ < (Zi/Zs) < 4. The Be-Cu dynodes consist almost entirely of copper. So we can use for 2s the atomic number of copper. This gives a rather good agreement with the experimental results for Ne, Ar, Kr and Xe. The slope C fits within 10%. For a better comparison their collision integral should be calculated numerically. An alternative explanation might be given by an interaction between an interaction between fast particles and metal electrons, as if they were a continuous medium, a conducting plasma. Lindhard c.s.15) have given a

760

B. L. SCHRAM,

description fast

of the generalized

particle

nuclear

A. J. H. BOERBOOM,

through

stopping

ates sputtering

KLEINE

energy dissipation

matter.

dominates

W.

In our energy

the electronic

and electronic

AND

J. KISTEMAKER

during the stopping region,

stopping.

Nuclear

stopping creates secondary

Electronic stopping heats up the electrons cording to dE (6)dr

with

of a

v < es/h, the stopping

electron

in various conduction

cre-

emission. bands ac-

Vion

glcctr. -

Ycrit

dE

is the energy dissipated in the electron plasma, per unit of dr electr. (&> path length, per conducting electron. There is an apparent analogy between y and (dE/dr).,,,,,,. Considering the electron heating by an interaction mechanism between a beam of fast charged particles and a plasma, the threshold velocity obl served gets a very special meaning. It corresponds with the average thermal electron gas in the metal during bomvelocity ctth,+ in the conducting

bardment . It is well known from plasma theoryi6) only takes place if: vbeem >

that heating

of the electron

gas

%h,e

which might explain in principle the existance of a threshold velocity. The with observed value of vbeitm at threshold 5.5 x 106 cm/s, corresponds of the L,e = 8.5 x 10-s eV. This would mean an electron temperature order of lOO”K, which low value might be due to the presence of the POtential secondary electron emission mechanism decreasing the apparent

E threshold

Of the

ions’

Acknowledgements. We thank Dr P. K. Rol for valuable criticism. This work is part of the research program of the Stichting voor Fundamenteel Onderzoek der Materie (Foundation for Fundamental Research on Matter), and was made possible by financial support from the Nederlandse Organisatie voor Zuiver-Wetenschappelijk Onderzoek (Dutch Organization for Pure Scientific Research). Received

15-9-65

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AMPLIFICATION

3)

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8) Arifov,

OF A PARTICLE

761

MULTIPLIER

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FACTORS

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- Centre d’Etudes

Nucleaires

a Fontenay-aux-