Fluorescent x-ray analyzer and monitoring system for increasing operative life

An improved fluorescent x-ray instrument includes an x-ray tube for generating x-rays, with a control grid regulating the production of x-rays. An operator can set the voltage to be applied to the control grid, and a feedback system will set a desired voltage to the control grid. An operator will be provided an output signal representative of the monitor control grid voltage to enable the operator to determine the operative status of the x-ray tube.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a fluorescent x-ray analyzer and, more 
particularly, to an improved fluorescent x-ray analyzer that can monitor 
the status of the x-ray tube and thereby increase the effective life of 
the instrument, while ensuring accurate readings. 
2. Description of Related Art 
Fluorescent x-ray instruments have been utilized as analytical instruments. 
Reference can be made to FIG. 4 to disclose a schematic construction of 
one form of a fluorescent x-ray analyzer. In this regard, a sample can be 
held on a sample monitoring stage (not shown) and subjected to irradiation 
from primary x-rays 3 from an x-ray tube 2. As a result, fluorescent 
x-rays and scattered x-rays 6 are generated at the sample, and a filter 4 
is placed before an x-ray detector 5. An output signal from the x-ray 
detector is processed in a pulse height analyzer (not shown) after 
suitable amplification to conduct a predetermined analysis. 
FIG. 5 discloses one form of construction for controlling the output of the 
x-ray tube 2. The x-ray tube 7 supports a vacuum and contains a thermal 
cathode 10 that includes a filament 8 and a cathode 9 that is connected to 
an appropriate power source so as to generate thermal electrons 11. The 
cathode 9 is connected through a buffer amplifier 12 to the input terminal 
13a of a comparator 13. An x-ray tube electric current I.sub.x may flow 
through a detecting resistance 14 provided on the input side of the buffer 
amplifier 12, to thereby generate a voltage V.sub.x obtained by converting 
the x-ray tube electric current I.sub.x into a voltage value. This voltage 
V.sub.x is input as one signal to the comparator 13. 
A target 16 is mounted at the other end of the tube member 7 as an anode, 
and it is connected with a high-voltage power source 15. An x-ray 
transmissive window 17 made, for example, of beryllium is formed and 
provides an output from the tube 7 of the primary x-ray 3. A first grid 
member 18 is capable of regulating the quantity of thermal electrons 11 
that are permitted to collide with the target 16. The quantity of thermal 
electrons 11 is a function of the x-ray tube electric current I.sub.x, and 
the grid 18 can provide a constant value of control thermal electrons 11. 
A second grid member 19 is used for contracting thermal electrons before 
they collide with the target 16 so that the stream of electrons is not 
excessively expanded and are controlled to be arranged between the thermal 
cathode 10 and the target 16. 
A controlled set value for regulating the x-ray tube electric current 
I.sub.x can be input by the operator into the other input terminal 13b of 
the comparator 13 as the voltage signal V.sub.R. This voltage signal 
V.sub.R is compared with the voltage signal V.sub.x in the comparator 13 
to provide a feedback loop to apply a voltage to the first grid 18 through 
a level converter circuit 20. As a result, a controlled grid voltage of 
the first grid 18 can be desirably controlled so as to provide a 
predetermined x-ray tube electric current I.sub.x. 
A problem that can impact on the use of fluorescent x-ray instruments has 
been the stability and life of the x-ray tube 2. The inside of the tube 
member 7 can deteriorate in degree of vacuum where the thermal cathode 10 
can deteriorate to produce an emitting factor of the thermal electrons 11. 
As a result, the ability to provide constant current control deteriorates, 
and eventually can become impossible. In the conventional fluorescent 
x-ray analyzer, it becomes difficult to determine the specific time period 
in which an x-ray tube becomes inaccurate or its control current starts to 
deteriorate. As a result, erroneous readings can occur as the quantity of 
x-rays emitted by the x-ray tube 2 is reduced. As can be appreciated, when 
the x-ray tube 2 loses its ability to be controlled by the operator, it is 
necessary to exchange the x-ray tube 2. The life of the x-ray tube 2, 
however, cannot be readily determined. The prior art has frequently 
resorted to periodic changes of the x-ray tube 2 to guard against 
analytical errors. As can be appreciated, however, the life of an x-ray 
tube 2 could be extended beyond the periodic changing, since the 
maintenance schedule usually requires a safety factor to avoid erroneous 
readings. Thus, the cost of x-ray tubes 2 must be increased to cover the 
wasteful utilization of them in an analytical instrument. 
The prior art is still seeking an improved fluorescent x-ray instrument for 
analytical use. 
SUMMARY OF THE INVENTION 
An improved fluorescent x-ray instrument utilizes an x-ray tube capable of 
generating primary x-rays with a control grid that can be regulated by the 
operator to control the production of the primary x-rays. Voltage is 
applied to the control grid, and this voltage can be monitored by 
providing an output signal representative of the monitor control grid 
voltage to the operator, to thereby enable the operator to determine the 
operative status of the x-ray tube. 
In one embodiment of the invention, an output signal representative of the 
monitor control grid voltage can be compared with a predetermined 
reference voltage to determine the operative life of the x-ray tube by 
providing an indication of such a comparison directly to an operator, for 
example, through an appropriate warning control light or an alarm. 
The present invention therefore provides a fluorescent x-ray tube analyzer 
that is capable of determining the degree of deterioration and defining a 
specific exchange time or maintenance cycle of an x-ray tube without 
affecting the readings of the analytical instrument. By monitoring the 
control grid voltage, both the degree of deterioration and the exchange 
time period of the x-ray tube can be easily managed by the operator to 
increase the effective life of the x-ray tube and lower the operating cost 
of the instrument. It is possible to provide an appropriate monitoring 
signal to define a first warning period wherein the x-ray tube should be 
replaced but is still operative, and a second warning period in which the 
life cycle of the x-ray tube has deteriorated to a point where it can no 
longer be reliably utilized in an analytical measurement.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The following description is provided to enable any person skilled in the 
art to make and use the invention and sets forth the best modes 
contemplated by the inventors of carrying out their invention. Various 
modifications, however, will remain readily apparent to those skilled in 
the art, since the generic principles of the present invention have been 
defined herein specifically to provide an improved fluorescent x-ray 
analyzer and monitoring system. 
To appreciate an application of the features of the present invention, 
reference is made to FIG. 6, which discloses mutual conversion 
characteristics (hereinafter referred to as G.sub.1 -I.sub.x 
characteristics) between a grid control voltage G.sub.1 of a first grid 
member 18 and an x-ray tube electric current I.sub.x. Referring to FIG. 6, 
an axis of abscissa designates the voltage G.sub.1 of the first grid 18, 
while an axis of ordinate designates the x-ray tube electric current 
I.sub.x. As can be appreciated, the common elements of the x-ray tube are 
identified with the same reference numbers as shown, for example, in FIGS. 
1 and 5. The G.sub.1 -I.sub.x characteristic curve is expressed by a curve 
shown by the full line in FIG. 6 during the time period when an x-ray tube 
2 is new and fresh and is operated as per its original specifications. 
After a substantial period of use, the x-ray tube 2 can deteriorate in 
degree of vacuum, or the thermal cathode 10 can deteriorate to reduce the 
emitting factor of the thermal electrons 11. These factors, alone or in 
combination, can deteriorate the output of the x-ray tube 2, and will 
result in shifting the curve A shown in FIG. 6 in the direction shown by 
the arrow D in FIG. 6. As a result, a constant current control can be 
conducted so that the x-ray tube electric current I.sub.x may be equal to 
the setting electric current I.sub.1, so that the grid control voltage 
G.sub.1 is changed to -V.sub.1, -V.sub.1 ', and -V.sub.1 ", to thereby 
gradually approach a zero voltage. As can be appreciated, the constant 
current control will become impossible over this progressive 
deterioration. 
Referring to FIG. 1, a schematic drawing of a construction of the present 
invention for a fluorescent x-ray tube analyzer is disclosed. In this 
regard, the fluorescent x-ray analyzer is specifically designed to 
continually monitor the control grid voltage of the grid 18. This can be 
accomplished in a number of different methods. For example, as shown in 
FIG. 1, the control grid voltage G.sub.1 of the first grid 18 can, through 
an appropriate I/O circuit (not shown) be converted from an analog to a 
digital value by an A/D converter 21. The output signal can then be 
monitored by a CPU or microprocessor-based system 22. In a fluorescent 
x-ray analyzer having such a construction, the x-ray tube electric current 
I.sub.x that flows through a detecting resistance 14 from the cathode 9 
will generate a detecting voltage V.sub.x across the resistance 14. This 
voltage signal V.sub.x can be compared with the setting voltage V.sub.R in 
the comparator 13. The obtained result is fed back to the first grid 18 
through a level converter circuit 20. 
For example, the level converter circuit 20 can regulate the control grid 
voltage G.sub.1 to the--side when V.sub.x &gt;V.sub.R, and to the + side when 
V.sub.x &lt;V.sub.R. The thermal electrons 11 will come into collision with 
the target 16 as a result of regulating a control grid voltage G.sub.1 in 
the above-described manner to generate the primary x-rays 3, when can then 
be applied to a sample 1 to conduct the desired analysis. 
In operation, the control grid voltage G.sub.1 of the first grid 18 is 
constantly monitored, and a value representative of that voltage is input 
into the CPU 22 through the A/D converter 21. This value of the control 
grid voltage G.sub.1 can be displayed to an operator in charge of the 
analysis. Alternatively, if it arrives at a predetermined value such as 
-V.sub.1 ' in FIG. 6, an x-ray tube exchange alarm or monitoring warning 
alarm can be output directly to the operator. If the control grid voltage 
G.sub.1 arrives at a value -V.sub.1 " as shown in FIG. 6, a life-ending 
alarm can be output and the system can be rendered inoperative to avoid 
any false readings. 
Although the x-ray tube 2 disclosed is a tetrode transmission type in the 
above-described preferred embodiment of FIG. 1, it may also be a triode 
transmission-type tube without a second grid 19, or a reflection-type tube 
as shown in FIG. 2. Referring to FIG. 2, an alternative embodiment of the 
present invention can be utilized wherein a filament 23 serves as the 
thermal cathode and a Wenert's electrode serves as the grid 24. In FIG. 2, 
the target 25 is positioned adjacent an x-ray transmissive window 26, and 
a high-voltage power source 27 is applied to the target. 
Referring to FIG. 3, an alternative embodiment of the present invention can 
be utilized wherein the control grid voltage G.sub.1 is monitored by an 
analog circuit having two separate comparator circuits 28 and 29, to each 
output a separate alarm. In FIG. 3, reference numbers 30 and 31 are 
directed to a standard voltage source, while reference numbers 32 and 33 
refer to an LED monitoring light. Reference numbers 32 and 33 refer to 
resistance values. In this embodiment, if the control grid voltage G.sub.1 
becomes less than a value determined by the standard voltage source 30, an 
"x-ray tube exchange alarm" indicator is provided by the LED 32. Further, 
if the control grid voltage G.sub.1 becomes less than a value determined 
by the standard voltage source 31, a life termination signal for the x-ray 
tube can be output. 
As can be readily appreciated, a number of alarms can be optionally 
selected to accommodate variations in the above-described embodiments. In 
addition, in the preferred embodiment shown in FIG. 3, the passive LED 
alarms 32 and 33 can instead be input to a CPU to provide an on/off signal 
for the driving of a display device such as a CRT or a liquid crystal 
display. 
Thus, according to the present invention, the degree of deterioration in an 
accurate exchange maintenance time period for the x-ray tube can be 
achieved, since the life cycle of the x-ray tube can be readily monitored. 
Additionally, the x-ray tube can be fully utilized throughout its useful 
life. Therefore, the costly periodic change to avoid even the possibility 
of erroneous readings in the analytical measurements can be eliminated. 
Thus, the quantity of x-rays that are utilized in an analytical 
measurement can be guaranteed by the utilization of the present invention. 
In operation, an improved fluorescent x-ray instrument can monitor the 
control grid voltage to a specific x-ray tube. The specific type of x-ray 
tube will have a predetermined grid control voltage and x-ray tube 
electric current relationship that can be empirically established for a 
type of x-ray tube. As shown in FIG. 3, a corresponding voltage value can 
be set as a reference. When the grid voltage reaches that value, an 
appropriate alarm or warning can be issued to the operator. Thus, an 
initial alarm can indicate that an x-ray tube is approaching the end of 
its life, and a subsequent alarm can indicate that the grid voltage has 
reached a value wherein the quantity of x-rays being produced by the x-ray 
tube cannot be dependably controlled to meet the needs of the analyzer 
instrument. 
Those skilled in the art will appreciate that various adaptations and 
modifications of the just-described preferred embodiment can be configured 
without departing from the scope and spirit of the invention. Therefore, 
it is to be understood that, within the scope of the appended claims, the 
invention may be practiced other than as specifically described herein.