Well logging neutron generator control system

A pulsed neutron well logging system using a sealed off neutron generator tube is provided with a neutron output control system. The control system monitors the target beam current and compares a function of this current with a reference voltage level to develop a control signal. The control signal is used in a series regulator to control the replenisher current of the neutron generator tube.

BACKGROUND OF THE INVENTION 
This invention relates to pulsed neutron well logging and more particularly 
to means for controlling the neutron output of a neutron generator tube 
used in pulsed neutron well logging. 
In recent years pulsed neutron well logging has become a commercially 
important well logging technique. Pulsed neutron techniques have been 
utilized for measuring the thermal neutron lifetime or thermal neutron 
decay time of earth formations in the vicinity of a well borehole, for 
making activation analyses of elemental constituents of the earth 
formations in the vicinity of the well borehole, for making porosity 
measurements of the earth formations in the vicinity of the well borehole 
and for making inelastic neutron scattering measurements for fast 
neutrons. In each of these well logging techniques the pulsed neutron 
source used to generate neutron pulses for the physical measurements has 
typically been an evacuated tube, deuterium-tritium accelerator type 
source. Such sealed off or evacuated tube neutron sources generally 
comprise an outer envelope of glass, metal or some other vacuum 
encapsulation material, such as ceramic, which houses therein the elements 
of the neutron generator tube. The tube elements generally comprise a 
target which is electrically insulated at a high voltage potential, a 
source of ions which may be accelerated onto the target by its high 
voltage potential and a pressure regulator or replenisher element which 
may be used to stabilize or or control the amount of pressure of gas 
within the evacuated outer envelope. Gas pressures of about 10.sup.-2 mm 
Hg. are typical for the operation of these tubes. 
The replenisher or pressure regulator of neutron generator tubes generally 
comprises a heater element which is surrounded by a surface which is 
capable of absorbing or emitting gas molecules of the gas filling the 
evacuated tube envelope as a function of its temperature. The capability 
of such a surface for emitting or absorbing gases in the tube envelope is 
controlled by the temperature of a heating element associated with it. 
When the heating element is elevated in temperature, the surrounding gas 
impregnated surface is encouraged to dispel absorbed gases by thermal 
emission. When the heating element is cooled, the surrounding surfaces 
associated with it are encouraged to absorb gases from the atmosphere 
inside the evacuated tube envelope. The amount of gas present in the tube 
envelope controls the amount of gas present in the ion source and hence, 
the capability of the ion source to produce positively charged ions of gas 
for acceleration onto the target material. 
In a typical neutron generator tube operation, the gas present in the 
evacuated envelope may be either deuterium gas or a mixture of deuterium 
and tritium gas. The target material is impregnated with tritium. Thus 
when deuterium ions are formed in the ion source and accelerated onto the 
target by its high voltage potential, the electrostatic Coulomb repulsion 
between the ions being accelerated and the nuclei of the tritium atoms is 
overcome and nuclear fusion takes place. This produces the unstable 
isotope helium 5 which immediately decays by the emission of an 
approximately 14 MEV monoenergetic neutron characteristic of this decay. 
A problem which has been associated with the use of such neutron generator 
tubes in well logging has been that the output of the neutron generator 
falls off as a function of time as the tritium in the target material is 
effectively used up by the nuclear reactions and by heating of the target. 
Also high voltage power supply voltage variations, replenisher heater 
current variations and ion source emission capability can cause neutron 
output to vary. 
For most well logging operations it is highly desirable that during a given 
logging run the neutron output of the tube remain constant and also as 
high as possible. High output is desirable to promote the nuclear 
interactions sought to be measured by the well logging technique in use. 
Consistency of the neutron output is desirable to promote measurement 
consistency and to avoid systematic errors. 
BRIEF DESCRIPTION OF THE INVENTION 
The neutron output of a neutron generator tube is a function of the target 
current of the tube. The target current, in turn, is a function of the 
target high voltage, the ion source voltage and the replenisher heater 
current. In the present invention, the target high voltage is at a fixed 
value. The ion source voltage is pulsed at a given repetition rate and 
pulse width. By varying the replenisher heater current and hence its 
heater element temperature, the average neutron output of the tube is 
controlled. In a preferred embodiment of the present invention, the target 
beam current is monitored, converted to a voltage signal and compared to a 
reference voltage. An error voltage developed from this comparison is used 
to control the replenisher current in such a manner that the replenisher 
current is automatically adjusted to maintain a constant value of the 
average target beam current corresponding to the reference voltage. A 
circuit for accomplishing this is provided which may be described as a 
series regulation replenisher current control circuit. Such a circuit may 
be used to vary the replenisher heater current or even turn off the 
replenisher current completely. The circuit embodiments of the present 
invention provide advantages over prior art control circuits for 
controlling neutron generator tubes in that improved regulation of the 
replenisher current is accomplished and a relative simple circuit using 
few parts is required for this. The replenisher current regulating circuit 
of the present invention also has a smaller power consumption than those 
known in the prior art and is capable of operating at temperatures of up 
to 200.degree. C. An additional feature of the control circircuitry of the 
present invention is the remote turn on or off capability of the 
replenisher heater current with relatively lower power COSMOS logic level 
voltages. 
The foregoing as well as other features and advantages of the present 
invention are described with particularly in the appended claims. The 
invention is best understood by reference to the following detailed 
description thereof when taken in conjunction with the accompanying 
drawings in which:

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring initially to FIG. 1 a well logging system embodying the concepts 
of the present invention is illustrated schematically. A well borehole 11 
is lined with a steel casing 12 and filled with a borehole fluid 15. The 
steel casing 12 is cemented in place by a cement layer 13 and effectively 
seals off earth formations 14 from communication with the borehole 11 
except in instances where the steel casing and cement layer are perforated 
for oil production. A fluid tight, hollow body member or sonde 16 is 
suspended in the borehole 11 by a well logging cable 17 of the usual 
armored cable type known in the art. The logging cable 17 communicates 
electrical signals to and from the sonde to surface equipment. 
At the surface, the well logging cable 17 passes over a sheave wheel 18 
which is electrically or mechanically linked, as indicated by the dotted 
line 20, to a well logging recorder 19. This linkage enables measurements 
made by the downhole sonde 16 to be recorded as a function of borehole 
depth by the recorder 19. Signals from the well logging cable 17 are 
provided to surface data processing circuits 21 which process measurement 
data to provide information which is supplied to the recorder 19 for 
recording as a function of borehole depth. Power supply 22, located at the 
surface, supplies power for the operation of the downhole equipment on 
cable 17 conductors. 
In the downhole sonde 16 equipment is provided for making pulsed neutron 
measurements. While not shown in the schematic drawing of FIG. 1 it will 
be understood that appropriate power supplies in the downhole instrument 
convert power source 22, power supplied from the surface into the 
necessary operating voltages for the equipment in the downhole sonde 16. 
Control circuits 23, which will be described in more detail subsequently, 
provide control functions for a neutron generator tube 27 and a high 
voltage power supply 28 associated therewith and which are located near 
the lower end of the sonde. Neutron shielding material 29 which may 
consist of alternate layers of iron, paraffin, cadmium and borated foils 
or the like is provided to shield the neutron generator 27 from the 
remainder of the instrumentation within the downhole sonde 16. 
A gamma ray detector in the form of a scintillation crystal 24 of thallium 
activated sodium iodide or the like is optically coupled to a 
photomultiplier tube 25. This provides for detecting gamma radiation 
originating in the earth formations in the vicinity of the borehole and 
resultant from neutron bombardment by the neutron generator 27. As is well 
known in the art the impingement of gamma rays upon the detector crystal 
24 produces light flashes therein whose intensity is proportional to the 
energy of the gamma ray producing the scintillation. The photomultiplier 
tube 25 is optically coupled to the detector crystal 24 and amplifies the 
light flashes produced by the detector crystal 24 and converts them to 
electrical voltage pulses whose amplitude is proportional to the intensity 
of the light flashes. These electrical signals are further amplified in an 
amplifier 26 and conducted into the control circuit electronics 23 portion 
of the tool where they are appropriately supplied to conventional cable 
driving circuits (not shown) for transmission of the data processing 
circuits 21 located at the surface of the earth. 
The neutron output of a neutron generator tube 27 of FIG. 1 is illustrated 
in FIG. 3 as a function of time. In the illustration of FIG. 3 a pulsed 
neutron mode of operation of the neutron generator tube is contemplated. 
The high voltage power supply is on at all times in the system and bursts 
or pulses of neutrons are produced by applying voltage pulses illustrated 
at 46 of FIG. 2 to the ion source. The neutrons are produced by the 
neutron generator tube 27 as previously described. Voltage pulses 46 of a 
predetermined amplitude and duration are applied to the ion-source of the 
neutron generator tube 41. In this manner the neutron output of the tube 
may be made to vary as indicated in FIG. 3. That is to say, the neutron 
output will rapidly increase from essentially zero to N.sub.max, the 
maximum value of neutron output of which the generator tube is capable 
when a given magnitude voltage pulse is applied to the ion source. During 
this time an average value N.sub.avg will be emitted. In this manner a 
very rapid build up or pulse of neutron output from the generator tube 27 
is accomplished as a function of time. Removing the voltage pulse from the 
ion source returns the neutron generator tube to the quiescent value of 
neutron output which is essentially zero. 
For typical well logging operations the on-time of the neutron generator 
tube in pulsed mode will usually not exceed a duty cycle of approximately 
5-10% of its operating cycle. That is to say, the neutron generator tube 
will generally only be on from 5-10% of the time and the off periods of 
FIG. 3 will occupy approximately 90-95% of its time in a typical well 
logging operation. Neutron pulse durations of approximately 50 
microseconds duration and repitition rates of from 1000-10,000 pulses per 
second are typical for pulsed neutron well logging techniques. 
The neutron generator control system of the present invention operates to 
maintain the average value N.sub.avg of the neutron output at a constant 
or predetermined value for the duration of a well logging run. Obviously 
such a system cannnot replace tritium in the tube which is used up in 
generating the neutron output. Long term deterioration of the neutron 
output is unavoidable in generator tubes which provide only deuterium in 
the replenisher. Tubes having a deuterium and tritium mixture can avoid 
such long term neutron output deterioration. 
The need for short term control of neutron output arises from the 
relationship between the replenisher current and the neutron output, which 
is a complicated function. Very small changes in the replenisher current 
can cause very large changes in the neutron output. By monitoring the 
target current (which is related to the neutron output) and correcting the 
replenisher current to hold the target current constant, the neutron 
output may be stabilized for short term variations such as could occur 
during a well logging job. 
Alternatively, it should be noted that the neutron generator 27 could be 
operated continuously for certain types of well logs. In such a case the 
values N.sub.avg and N.sub.max would be the same value. In this instance 
the control system of the present invention would maintain approximately a 
constant neutron output from the neutron generator 27 for the duration of 
a well logging job. 
Referring now to FIG. 2 a portion of the control circuitry 23 of FIG. 1 
having to do with the control of the neutron output from a neutron 
generator tube is illustrated in more detail, but still schematically. 
Point A of the circuit is connected to the low side of the target high 
voltage power supply (which may typically be negative 125 kilovolts). The 
neutron generator tube 41 target beam current (which is sampled at Point 
A) flows to ground through resistor R.sub.1 generating a voltage V.sub.B 
at Point A which is related to the neutron output of the generator tube. 
The sampled voltage at Point A is used to regulate the drain current 
I.sub.D of the VMOS power field effect transistor 45 (labelled FET1). This 
current is also the replenisher current of the neutron generator tube 41, 
and is sampled at point 47. 
A variable resistor VR1 establishes a reference voltage for controlling the 
average neutron output of the generator tube 41. The setting of variable 
resistor VR1 is determined by the transfer characteristics of the VMOS 
power FET 45 and the relationship between the replenisher current and the 
neutron output of the generator tube. In general, the transfer 
characteristics will vary with each FET and generator tube. For the 
purpose of this description it will be assumed that the desired neutron 
output is obtained when the average target beam current is 100 
microamperes and the replenisher current is 3 amperes. These are typical 
values encountered in the operation of neutron generator tubes in well 
logging usage. In this case, the variable resistor VR1 is adjusted until 
the replenisher current is 3 amperes and the target current is 100 
microamperes giving a reference voltage V.sub.B at Point A, of 3 volts. 
This voltage establishes the average operating point of the neutron 
generator tube 41. 
Operational amplifier 44 is connected as an inverting voltage gain circuit 
with the gain determined by the ratio R3/R.sub.2. The output voltage 
V.sub.G of the operational amplifier 44 is applied to the gate of the 
field effect transistor 45. This gate voltage controls the drain current 
I.sub.D of the field effect transistor 45 which is supplied from a 5 
ampere current supply 42. This drain current is sampled at point 47 and 
fed back through resistor R3 to establish the operating conditions of the 
operational amplifier 44 as previously described. The non-inverting input 
of the operational amplifier 44 is connected to the voltage setting 
provided by variable resistor VR1 through resistor R.sub.4. This voltage 
applied to the non-inverting input of the operational amplifier 44 plus 
the voltage developed across R.sub.1 determines the output voltage of the 
operational amplifier 44. 
If the average value of the neutron output N.sub.avg begins to decrease 
below the operating value as determined by the setting of VR1, the target 
beam current will decrease. This will cause the voltage V.sub.B to 
decrease. When V.sub.B decreases, this causes the output voltage V.sub.G 
of the operational amplifier 44 to increase. The increased voltage output 
V.sub.G of the operational amplifier 44 causes the replenisher current 
I.sub.D to increase. The increase in replenisher current I.sub.D tends to 
increase the target beam current as sampled at Point A. 
If the neutron output of the generator tube 41 begins to increase above 
predetermined average operating value N.sub.avg, the target beam current 
will increase. This causes the voltage V.sub.B across resistor R.sub.1 to 
increase. When V.sub.B increases, this causes the output voltage V.sub.G 
of the operational amplifier 44 to decrease. The decrease of V.sub.G, the 
gate voltage on field effect transistor 45, causes the replenisher current 
I.sub.D, as sample at point 47, to be reduced. This in turn reduces the 
output of the neutron generator tube 41 by cooling the replenisher heater 
element. 
A solid state switching integrated circuit 43 is used to apply and remove a 
control voltage source (0-15 volts) to variable resistor VR.sub.1. When 
the control voltage source is removed from the variable resistor VR.sub.1, 
the voltage applied to the non-inverting input of operational amplifier 44 
goes to zero volts. When zero volts is applied to the non-inverting input 
of the operational amplifier 44, the voltage output of the operational 
amplifier is reduced sufficiently to insure that the field effect 
transistor 45 is completely turned off. This interrupts the replenisher 
current I.sub.D completely and effectively reduces the output of the 
neutron generator tube 41 to zero. 
The foregoing descriptions may make other alternative embodiments of the 
invention apparent to those skilled in the art. It is therefore the aim of 
the appended claims to cover all such changes and modifications as fall 
within the true spirit and scope of the invention.