Lead silicate glass and a process for setting a reduced...

Details Lead silicate glass and a process for setting a reduced... Lead silicate glass and a process for setting a reduced...

2000-05-17

2002-07-16

Group, Karl (Department: 1755)

Compositions: ceramic

Ceramic compositions

Glass compositions, compositions containing glass other than...

C065S032100

Reexamination Certificate

active

06420291

ABSTRACT:

The invention relates to a lead silicate glass and to the use thereof for the production of secondary electron multipliers. The invention furthermore relates to a process for setting a reduced surface resistance of such lead silicate glasses according to the invention.
Secondary electron multipliers (SEMs) are, besides the photocathode, the main component of photomultipliers. The principle of operation of this electronic component is based on the outer photo effect, in which mobile charge carriers are generated through interaction of material (photocathode) with radiation (IR, visible and UV light, X-rays or radioactive radiation). Conventional photomultipliers consist of an evacuated glass tube (bulb) containing the photocathode and an anode. A number of electrodes, known as dynodes, are arranged between the cathode and the anode. The photoelectrons generated by irradiation with photons (or other particles of suitable energy) are accelerated by means of a voltage gradient between the electrodes and hit the dynode surface with a kinetic energy of about 100 eV and release a number of secondary electrons. Each electron produces a cascade of secondary electrons in this way. The photocurrent can thus be amplified by up to 10
9
times. The function of the discrete dynodes can be taken over by a continuous glass tube surface if the latter has a suitable surface resistance, for example through coating or reduction. A photomultiplier of this type is described in the U.S. Pat. No. 6,166,365.
Open SEVs (without a photocathode) are furthermore employed for detecting electrons and ions, for example in mass spectrometers and residual gas analysers.
With a low surface resistance, better current flow can be ensured, which in turn guarantees optimum subsequent delivery of the released secondary electrons and enables an increase in the dynamic range. However, if the surface resistance is too low and thus the electrical conductivity is too high, undesired warming of the material occurs or even destruction thereof, for example through thermal stresses.
Besides a high secondary electron yield and a high amplification factor, a low background noise of the photomultiplier is necessary in order, for example on use in a gamma-ray camera for medical purposes, to achieve good image resolution. The background noise is caused by naturally occurring radioactive isotopes, such as potassium
19
40
K and rubidium
37
87
Rb, which are used, for example, as a constituent of glasses and contribute towards undesired formation of secondary electrons.
Lead silicate glasses and the use thereof for secondary electron generation have been known for some time.
Thus, GB 1,239,687 describes a lead- and bismuth-containing silicate glass of the composition (in % by weight) SiO
2
30-70, PbO 6-30, Bi
2
O
3
2-45, Al
2
O
3
0.5-10, MgO 0.5-7, and the optional components B
2
O
3
=5, Na
2
O=6, K
2
O=10, CaO+SrO=8, As
2
O
3
+Sb
2
O
3
=2. Likewise described is a dynode containing a plurality of glass channels produced from the said glass. A striking feature of this glass composition, besides the relatively low PbO proportion, is the wide Bi
2
O
3
range with a relatively high upper limit.
PbO and Bi
2
O
3
affect the surface resistance of the glass after reduction in the same way. However, one component cannot be replaced by the other. The combination of PbO and Bi
2
O
3
within the abovementioned ranges results only in a limited range of the achievable surface resistance. Through the content of K
2
O, the glass is of only limited suitability for the production of very sensitive secondary electron multipliers. The glass examples given all contain K
2
O in the range from 0.6 to 7.5% by weight. Due to the background noise associated therewith, the sensitivity of the dynodes containing the glass is significantly reduced.
The publication DE 33 177 78 A1 describes a glass which is suitable for the production of micro-channel plates which are used as secondary electron multipliers. A composition range (in mol %) of SiO
2
63-72, PbO 20-30, alkali metal oxides 3-7, alkaline earth metal oxides 1-3.5 and Al
2
O
3
, Bi
2
O
3
, Al
2
O
3
less than 1 is given for the glass. In this glass too, its surface resistance can only be set in a limited range. Due to the content of alkali metal oxides, the glass is of only limited suitability for the production of very sensitive secondary electron multipliers.
A similar situation applies to the glass claimed in the specification GB 2,218,982 A of the composition (in % by weight) SiO
2
30-35, PbO 50-57, Cs
2
O 2-10, MgO+CaO+SrO+BaO 0-5, Al
2
O
3
+ZrO
2
+TiO
2
+Nb
2
O
5
0.1-1, and a molar ratio between SiO
2
and PbO of 2.0-2.4. Particularly low surface resistance values cannot be achieved through the said composition. High Cs
2
O contents likewise increase the achievable surface resistance greatly, meaning that, in overall terms, the resultant current flow cannot ensure the electrode push in the SEM.
A further lead silicate glass which is suitable for the generation of secondary electrons is described by the specification SU 17 175 66 A1. Besides SiO
2
and PbO, the glass contains 1-15 mol % of BeO. The use of BeO is extremely questionable for toxicological reasons. Furthermore, the surface resistance of this glass can again only be set in a limited range.
Furthermore, the specification JP 88 166 735 discloses a lead silicate glass having the composition (in % by weight) SiO
2
15-65, PbO 15-75, Al
2
O
3
0-10, CS
2
O 0.1-50, Li
2
O 0-5, Na
2
O 0-10, K
2
O 0-20, Rb
2
O 0-30, Cs
2
O+Li
2
O+Na
2
O+K
2
O+Rb
2
O 0.1-50, B
2
O
3
0-20, Al
2
O
3
0-10, MgO 0-10, CaO 0-10, SrO 0-20, BaO 0-25, ZnO 0-15, CdO 0-10, MgO+CaO+SrO+BaO+ZnO+CdO 0-25, Bi
2
O
3
0-20, Sb
2
O
3
0-25, Tl
2
O 0-30, Bi
2
O
3
+Sb
2
O
3
+Tl
2
O 0-30, TiO
2
0-10, WO
3
0-7, As
2
O
3
0-2, T
2
0-10, where at least one or two of the oxides Bi
2
O
3
, Sb
2
O
3
, Tl
2
O, WO
3
or As
2
O
3
must be present.
The specification JP 91 295 828 describes a glass containing heavy-metal oxides and rare-earth oxides. The heavy-metal oxides are PbO, Bi
2
O
3
, CdO, Ga
2
O
3
, TeO
2
, Sb
2
O
3
, As
2
O
3
and GeO
2
. In the case of the two last-mentioned glasses, it is virtually impossible to foresee what effect the multiplicity of components mentioned will have on the surface resistance. In any case, targeted setting of the surface resistance is difficult. In addition, the use of a large number of components, which are frequently expensive, increases the production costs.
The object of the invention is to find a lead silicate glass. A glass should be simple to process and nevertheless have good heat stability. It should be possible to set a low surface resistance of the glass, and the glass should be suitable for the production of secondary electron multipliers having a stable secondary electron yield, a high amplification factor and low background noise. In addition, the resultant glass surface should be as smooth as possible in order substantially to avoid unevenness of the surface, which results in field emission and impairs the sensitivity of the secondary electron multipliers.
This object is achieved by a lead silicate glass, characterized by a composition by % weight, 15-35% SiO
2
, 35-55% PbO, >20-29 Bi
2
O
3
, 0-10 BaO, 1-10 Cs
2
O, 2-13 BaO+Cs
2
O, 0-10 CaO, 0-10 SrO, 0-10 CaO+SrO+BaO, and up to 1% by weight of fining agents, the use of said glass for production of secondary electron multipliers, and by a process for setting a reduced surface resistance of said glass, characterized in that the lead silicate glass is exposed to a reducing hydrogen atmosphere, during which a certain surface resistance is set under defined reduction conditions, depending on the composition of the lead silicate glass.
In contrast to conventional lead silicate glasses, the glass according to the invention is distinguished by a proportion of >20-29% by weight of Bi
2
O
3
. Owing to its good processing properties, it

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