Electron multiplier with replaceable rear section

A segmented electron multiplier is disclosed with front and rear sections. The sections are specially designed so that the length of the rear section compared to the length of the front section is no less than 4:1. This permits multiple replacements of the rear section, after the multiplier wears out, without any unsatisfactory drop in the overall electrical gain produced by the repaired device. In the preferred embodiment, the front portion is a funnel having a tubular stem, and the rear portion is a straight tube with a cylindrical helical inner channel. The length-to-length split is 5:1, which theoretically permits up to six or seven replacements of the rear section before unsatisfactory gain occurs.

BACKGROUND OF THE INVENTION 
This invention relates to continuous dynode electron multipliers ("CDEMs"). 
More particularly, it deals with replacing such multipliers, when they 
wear out. 
As described more fully in U.S. Pat. No. 3665497 to Deradorian et al., 
electron multipliers have been used for years to increase ion, electron, 
neutral or photon signals. The increase generally ranges from the order of 
10.sup.4 to 10.sup.8, depending upon the structure involved. 
The Deradorian structure is shown in this application's FIG. 1. It 
comprises a flared inlet 2 with a stem 4--known collectively in the trade 
as a funnel. The stem is connected, by electrically conductive adhesive, 
to a series of spiraled tubes 6. These tubes 6 are made of a lead-glass 
compound and each tube has an inner channel (not shown) that is coated 
with a secondary electron emissive surface. 
CDEMs have many different configurations. Some have flared inlets, while 
others do not. To avoid feedback, many are either spiraled or bent, and 
some are even straight tubes with their inner channels spiraled instead. 
Nonetheless, each multiplier tube is made of a lead-glass compound like 
Deradorian's; and each has an inner channel that is coated with a 
secondary-emissive layer. 
Electrical contacts (not shown) are deposited onto Deradorian's inlet 2 and 
the outlet end 8 of tubes 6. This allows good electrical contact between 
an external voltage source and the CDEM. This voltage source serves a dual 
purpose: it charges the secondary-emissive surface, inside the channel; 
and it draws the electrons through the channel, accelerating them along 
the way. 
Electrons enter Deradorian's flared inlet 2, where they are directed to the 
tubes 6, by the applied voltage. As they hit the secondary-emissive wall, 
each electron breaks off a new counterpart, and each pair continues to 
multiply by factors, typically greater than one, as they travel 
downstream. 
It has been proved that CDEMs produce high gains at low voltage, with 
little accompanying electrical noise. In addition, they are compact, with 
this application's FIG. 2 sketches being larger than their real-life 
counterparts. 
Due to these characteristics, CDEMs have achieved widespread use in 
scientific and medical instruments. In almost all cases, the internal 
structures of these instruments are quite compact, especially when 
available space is a limited commodity. 
CDEMs work well, but like all parts they eventually wear out. Most CDEMs 
last about one year. After they are exhausted, electron multipliers 
usually can be replaced. However, due to the compact nature of the 
equipment involved, this is often a tedious and delicate task. 
Most times, the entire multiplier has to be replaced. However, there are 
some multipliers that are segmented, with front and rear portions. Such 
devices are shown in Deradorian's aforementioned patent and U.S. Pat. No. 
3312857 to Farnsworth. In both types, the front section is approximately 
equal in length to the rear section; and the rear section could possibly 
be replaced once before unsatisfactory gains occur. 
Accordingly, it is a primary object of the present invention to provide a 
specially segmented CDEM, which allows for multiple replacement of its 
rear section before unsatisfactory gain degradation occurs. 
It is another object to provide a segmented CDEM with a removable rear 
section, wherein the CDEM is extremely simple in design and easy to 
repair. 
It is yet another object to provide a CDEM, commensurate with the 
above-listed objects, which is highly reliable during use. 
SUMMARY OF THE INVENTION 
Applicant has determined that the degradation of the CDEM's 
secondary-emissive surface (and the resulting life of the device) is 
directly related to the number of electron (ion) bombardments. During the 
electron multiplication process, the density of these electrons (ions) is 
continually increasing and reaches a maximum at the output end of the 
CDEM. As a result, the output of the CDEM will become unusable long before 
the input end. 
The input end of the CDEM is, like in Deradorian, usually funneled. It is 
the most expensive part of the CDEM to manufacture. Accordingly, the 
present invention deals with a specially designed CDEM, in which the rear 
section can be replaced up to six or seven times before unsatisfactory 
gain degradation occurs. With the present invention, this is accomplished 
by manufacturing two separate sections (see FIG. 2). 
The front section includes a flared input section attached to a short 
cylindrical stem section--collectively known as a funnel. The rear portion 
consists of only a cylindrical rear section; however, this rear 
cylindrical section is much longer than the cylindrical stem section 
attached to the funnel. Although the front stem section and the rear 
section differ in length, they have equivalent inner and outer diameters. 
These two sections are removably attached by any suitable means, such as a 
standard fuse clip. 
In order to maximize the number of replacements of the rear section that 
can be made before the entire multiplier must be thrown away, Applicant 
has determined an appropriate ratio entitled the "overall" ratio. 
Typically, in the field, this overall ratio would be composed of yet two 
more ratios: (i) the front cylindrical stem section 
length-to-inner-diameter (hereinafter "front stem length-to-diameter") and 
(ii) the rear cylindrical length-to-inner-diameter (hereinafter "rear 
section length-to-diameter"). The overall ratio is then the ratio of the 
rear section length-to-diameter to the front stem length-to-diameter. 
Because both the front and rear cylindrical sections contain both a 
constant inner and outer diameter (in the illustrated embodiment), the 
same result found by using the overall ratio, however, may be found by 
simply comparing the length of rear cylindrical sections to the length of 
the front cylindrical stem. For purposes of this application, applicant 
will now use this simple rear cylindrical section length to front 
cylindrical stem length (hereinafter "length-to-length") ratio. 
In more complicated situations, like divergent channels (not shown), one 
may be forced to actually determine the ratio of the length-to-inner 
diameter of the rear cylindrical section and compare it to the 
length-to-inner diameter of the cylindrical front stem. But, as Applicant 
has shown, that computation is unnecessary when referring to the 
illustrated embodiment because this embodiment shows both a constant inner 
and outer diameter. 
Applicant has discovered that the key to satisfactory multiple replacements 
is to have the tubular rear section be vastly "electrically longer" than 
the front, or usually funneled, section of the multiplier. If there is 
approximately a 3:1 length-to-length split between the stem of the front 
section and the tubular stem of the front section a one-time replacement 
of the rear section is marginally worthwhile. But, if the split is no less 
than 4:1 (that is, the length of the rear section is at least four times 
greater than the length of the front stem), as in the preferred 
embodiment, the rear section can be replaced multiple times, with 
satisfactory gains still being achieved after each replacement. 
The above and other objects and advantages of this invention will become 
more readily apparent when the following description is read in 
conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIGS. 2-3, a segmented CDEM or detector is shown and generally 
designated by the reference numeral 10. This preferred embodiment 
comprises a funneled front portion 12; a tubular rear portion 14; and a 
metal fuse clip or other connecting means 16 for removably connecting the 
front and rear portions together. 
Front portion 12 includes a flared inlet 18. It leads to a tubular stem 20 
having a central throughbore or channel (not shown). This stem is made of 
any standard lead-bismuth glass compound, and its inner channel is coated 
with a standard secondary-emissive layer. 
In the preferred embodiment, the outer diameter of the stem 20 is 
approximately 0.195 inches. Its inner diameter is approximately 0.035 
inches. 
Rear section 14 has the same or matching inner and outer diameters as the 
stem 20. While the stem 20 has a straight inner channel, the rear 
section's channel is a cylindrical helix (not shown) inside the tube 14, 
to prevent ion feedback. As an alternative, the rear section's channel 
does not need to be helical; instead, a tube itself can be bent to achieve 
the same result. 
As best shown in FIG. 2, clip or other connecting means 16 resembles a 
standard metal fuse clip. It includes a flat base 22 and two aligned 
horseshoe-shaped clip springs 24, 26--one in the front and one in the 
back. 
To assemble the detector, stem 20 is slipped into the front clip spring 24. 
Then, the detector's rear tube 14 is inserted into the back clip spring 
26; and the front and rear sections 12, 14 are slid together. Adhesive 
can, but need not be applied. 
The clip or other connecting means 16 serves three purposes: it properly 
aligns the inner channels of stem 20 and rear section 14; it provides a 
metal contact between the front and rear sections of the detector; and it 
allows for quick replacement of the rear section, after it becomes worn 
out. 
In operation, when the detector becomes unsatisfactory, the rear section is 
removed and replaced. Due to the clip or other connecting means 
configuration, this is an easy procedure that minimizes equipment 
downtime. Also, it can be performed in tight working spaces. 
At first glance, the segmented detector 10 (shown in FIGS. 2-3) looks just 
like the Deradorian detector shown in FIG. 1. However, upon closer 
inspection, the reader will see that the stem 20 of funnel 12 is much 
shorter than Deradorian's; and the present invention's rear portion 14 is 
much longer. 
The length-to-length split in Applicant's segmented detector 10 is 
approximately 5:1 (that is, the length of the rear section 14 is 
approximately five times greater than the length of the front tubular stem 
section 20). This design provides a detector in which the vast majority of 
the gain occurs in the rear section 14. 
Through testing, Applicant has determined that the 5:1 length-to-length 
split permits the rear section 14 to be replaced, at least four times 
before unsatisfactory results occur; and it is believed that, 
theoretically, the replacement can occur up to six or seven times--given 
optimum manufacturing conditions. Each time the rear section is replaced, 
the entire detector or multiplier 10 works about 90% as effective as the 
"generation" before. (This 10% dropoff is caused by the continuing decay 
of the stem 20.) Theoretically, after seven replacements, the multiplier 
would work at about 50% of its original efficiency. Anything below 50% is 
considered commercially unacceptable by Applicant. 
FIG. 4 demonstrates another standard that Applicant uses to determine when 
a multiplier, or multiplier replacement, becomes defective. For practical 
purposes, Applicant believes that a detector becomes unsatisfactory when 
the voltage needed to run it increases to over 3,000 volts. 
As a multiplier starts to degrade, it requires higher and higher voltage to 
maintain the same electrical gain. And, when the voltage required exceeds 
3,000 volts, a replacement is warranted. FIG. 4 shows the lifetime that 
will occur for the original multiplier 10 and the relative lifetimes for 
subsequent replacements of its rear section 14. 
While a 5:1 length-to-length split is preferred, Applicant has determined 
that the cutoff for multiple replacements is a 4:1 ratio. Anything smaller 
typically gives less than a 50% gain, after more than one replacement. 
It should be understood by those skilled in the art that obvious structural 
modifications can be made without departing from the spirit or scope of 
the invention. Accordingly, reference should be made primarily to the 
accompanying claims, rather than the foregoing specification, to determine 
the scope of the invention.